There exist four basic type of gears—spur gear, helical gear, bevel gear and worm gear. Spur and helical gears are used for parallel shafts; however, spur gear has straight teeth parallel to the gear axis; while, helical gear has teeth in helical form that are cut on the pitch cylinder. This helical form of teeth offers certain advantages such as gradual pick up of load on the tooth, less vibration, less noise, less wear, etc. A pair of bevel gear can be applied for transmitting motion and power between intersecting shafts (usually perpendicular); while, worm gear arrangement is applicable for non-parallel and non-intersecting shafts. There are different types of bevel gears, such as straight bevel gear, skew bevel gear, face gear, spiral gear, hypoid gear, crown gear, zerol gear, etc. Each of them has different feature and are suitable for particular type of applications.

Teeth of the bevel gear can have either straight or spiral profile; however, both are applicable for intersecting shafts only. As the name suggests, in straight teeth bevel gear, teeth are straight like spur gear but are cut along the gear axis maintaining an inclination. Due to sudden contact between teeth of two mating gears, tooth experiences impact load. This results in vibration and noise. Face bevel gear and skew bevel gear also have straight teeth. Spiral teeth helical gears offer similar benefits over straight teeth bevel gears as the helical gears provide over spur gears. Here teeth have a spiral profile and thus gradually come in contact during mating. Thus it offers smooth, vibration-free and quiet operation. However, it exerts high thrust load on the bearings. Hypoid gear also have spiral teeth. Various differences between straight bevel gear and spiral bevel gear are given below in table format.

Table: Differences between straight bevel gear and spiral bevel gear

Teeth orientation: Bevel gears may have either straight or spiral teeth. When teeth of a bevel gear are straight but inclined in such a way that they converge into a common apex point, then that gear is called straight bevel gear. Two such gears should be mounted on intersecting shafts only. A variant of straight bevel gear, called skew bevel gear, can be employed for non-parallel and non-intersecting shafts. On the other hand, when teeth are cut in spiral form on the conical gear blank in such a way that spiral curves converge in a single apex point, then it is termed as spiral bevel gear. A pair of mating spiral bevel gears should be mounted on intersecting shafts; alternatively hypoid gear (also has spiral teeth) can be employed for non-parallel and non-intersecting shafts.

Contact scenario: Gear drive is one type of rigid engagement drive that indicates teeth of two mating gears come in direct contact to transmit motion and power from one shaft to another. Irrespective of gear type, teeth that are parallel to gear axis come in sudden contact. This happens in case of spur gears as well as straight bevel gears. Here the contact is always a line of length equals to face width of the teeth. On the other hand, teeth of two mating spiral bevel gears come in contact gradually, as it happens in helical gears. Here engagement initiates with a point and gradually becomes a line and finally disengages as point.

Impact load and its consequences: Due to sudden contact, teeth of the straight bevel gears experience impact or shock loading. It also results in considerable noise and vibration. This vibration sometimes imposes limit on maximum speed of operation and also power transmission capacity. On the other hand, teeth of the spiral bevel gears experience gradual loading, which is far less deleterious. Their operation is thus smooth and quiet, even at high speed.

Cost comparison: Complicated manufacturing leads to high price for spiral bevel gears. For same material, size and tolerance, spiral gears are 1.2 – 1.5 times costly than straight bevel gears. Cost difference increases when gear size is small due to difficulty in manufacturing. High finishing requirement may also lead to broader price difference.

Thrust force: Two mating gears always exert force on the bearings that mount the shafts. Based on teeth orientation, this force can be either radial or axial or both. For example, straight teeth that are parallel to gear axis (as in spur gear) produce only radial load; whereas, helical teeth (as in helical gear) produces both type of force. Straight bevel gears as well as spiral bevel gears produce both radial and thrust load; however, thrust load (axial) in spiral bevel gear is comparatively higher as it induces from two sources.

Scientific comparison among parallel helical gear and crossed helical gear is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Design of Machine Elements by V. B. Bhandari (Fourth edition; McGraw Hill Education).
2. Machine Design by R. L. Norton (Fifth edition; Pearson Education).
3. A Textbook of Machine Design by R. S. Khurmi and J. K. Gupta (S. Chand; 2014).

The post Difference Between Straight Bevel Gear and Spiral Bevel Gear appeared first on Difference Box.

Driver and driven shafts may have three mutual orientations, namely (i) parallel shafts, (ii) intersecting shafts and (iii) non-parallel non-intersecting shafts. There exist four basic types of gears and a suitable gear should be selected based on the mutual orientation of the driver and driven shafts. Spur gear and helical gear are applicable for parallel shafts. Bevel gear can be applied for two intersecting shafts, which may not necessarily be perpendicular. Worm gear arrangement is used for the third category (non-parallel non-intersecting shafts). Unlike spur gears that have straight teeth parallel to the gear axis, helical gears have teeth in helical form that are cut on the pitch cylinder. Although helical gears are commonly used for parallel shafts like spur gears, it can also be used for perpendicular but non-intersecting shafts.

Accordingly there are two types of helical gears—parallel and crossed. Parallel helical gears, the common one, is used to for power transmission between parallel shafts. Two mating parallel helical gears should have same module, same pressure angle but opposite hand of helix. They offer vibration-free and quiet operation and can transmit heavy load. On the other hand, crossed helical gears are used for non-intersecting but perpendicular shafts. Two mating crossed helical gears (also called screw gears) should have same module, same pressure angle and either same or opposite hand of helix. This type of gear has application similar to worm gear; however, worm gear is preferred for steep speed reduction (1:15 to 1:100), whereas crossed helical gears cannot provide speed reduction beyond 1:2. Various differences between parallel helical gear and crossed helical gear are given below in table format.

Table: Differences between parallel helical gear and crossed helical gear

Power transmission and orientation of shafts: Except quarter-turn belt, gear drive is only mechanical drive that can be advantageously employed to transmit power and motion at any angle at any plane. Different types of gear are preferred for varying orientations of driver and driven shafts. Similar to spur gears, parallel helical gears can transmit motion and power between parallel shafts only. Therefore they should be mounted on parallel shafts with proper locational and angular alignment. On the other hand, crossed helical gears or screw gears can transmit motion and power between perpendicular shafts and thus they should be mounted on perpendicular but non-intersecting shafts.

Hand of helix: Irrespective of parallel or cross arrangement, two mating helical gears should have same module, same pressure angle and same helix angle. Pitch circle diameter of two mating gears should also touch at a single point. Now, if the mating helical gears have parallel arrangement, then one gear should have right hand helix and other gear should mandatorily have left hand helix. Thus, two mating parallel helical gears must have opposite hand of helix. However, in case of crossed arrangement, one helical gear with right hand helix can mate with another helical gear having either left or right hand helix. Thus, two mating crossed helical gears may have same or opposite hand of helix.

Contact between teeth: Based on the teeth profile and gear orientation, contact between teeth of two mating gears occurs in several fashions. For example, teeth of two spur gears come in sudden contact and their contact length always remains a line of length equals to face width of teeth. Such sudden contact is replaced by gradual contact in helical gear. In case of parallel helical gears, the mating of two teeth initiates with a point and gradually becomes a line and then they disengage as a point. However, in case of crossed helical gears, contact between two teeth always remains a single point. At the beginning of engagement, this point lies at one end of the teeth face and gradually the contact point moves over the face. At disengagement, the contact point lies on the other end of the teeth face.

Power transmission capacity: Basic purpose of every mechanical drive is to transmit power from driver shaft to driven shaft. Being an engagement type rigid drive, gear drive has the maximum transmission capacity among all mechanical drives. However, different types of gear drive offer varying level of power transmission capacity. A pair of parallel helical gears can transmit comparatively higher power as compared to similar size spur gear of same material. Based on the size and material, parallel helical gears can transmit from only few watt to few mega-watt power. On the other hand, crossed helical gears are preferred for small power transmission, usually limited to 100kW.

Areas of application: Due to high power transmission capacity, parallel helical gears are advantageously employed in many fields. It can be successfully employed for low speed as well as high speed applications. Typical areas of application include gear box of machine, automobile, oil mill, marine drive, etc. On the other hand, crossed helical gears are suitable for low speed and low duty applications such as roving machine of textile industry, oil pump of internal combustion engine, etc.

Scientific comparison among parallel helical gear and crossed helical gear is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Design of Machine Elements by V. B. Bhandari (Fourth edition; McGraw Hill Education).
2. Machine Design by R. L. Norton (Fifth edition; Pearson Education).
3. A Textbook of Machine Design by R. S. Khurmi and J. K. Gupta (S. Chand; 2014).

The post Difference Between Parallel Helical Gear and Crossed Helical Gear appeared first on Difference Box.

Gears can be classified into four basic groups—spur gear, helical gear, bevel gear and worm gear. Each of these gears has specific features and can offer certain benefits over others. A spur gear is the simplest one that is used to transmit motion and power between parallel driver and driven shafts. It has straight teeth parallel to the gear axis. Another similar gear, called helical gear, is also used for parallel shafts; however, its teeth are cut in the form of a helix over the cylindrical blank. On the other hand, bevel gear is used for intersecting shafts, which may or may not be perpendicular. It may have teeth along the gear axis (straight teeth bevel gear) or in spiral form (spiral teeth bevel gear). The worm gear is used for perpendicular but non-intersecting shafts.

Although both spur gear and helical gear are meant for parallel shafts, each has different teeth orientation and is suitable for different applications. In case of spur gears, teeth of mating gears come in sudden contact. Thus the tooth experiences impact or shock loading. It causes vibration and also reduces service life of the gear. Helical form of teeth can eliminate this problem. In two mating helical gears, teeth come in contact gradually and thus it is free from impact or shock loading. It can carry comparatively higher load. It also offers higher speed reduction. Contrary to spur gear that imposes only radial load, helical gear imposes both radial and axial loads on the bearings. Various differences between spur gear and helical gear are given below in table format.

Table: Differences between spur gear and helical gear

Configuration of gear teeth: Apparent difference between spur gear and helical gear is the form of their teeth. Spur gear has straight teeth that are parallel to the axis of the gear. It is worth mentioning that straight bevel gears also have straight teeth orientated along the gear axis but they are not parallel to the gear axis (they are inclined and thus intersecting). As the name suggests, helical gear has teeth in helix form that are cut on the pitch cylinder. A helical gear can have either left hand helix or right hand helix. Helix angle can vary form one gear to another. Typically, it lies in between 15 – 25°; however, a higher helix angle (up to 45°) can be employed for double helical or herringbone gears.

Nature of load and corresponding bearings: During mating of two gears, the bearings experience torque and forces, which are ultimately transferred to the ground. Direction and intensity of such force vary based on the speed of rotation and teeth configuration. Since some bearings can efficiently handle only radial load, some can handle only thrust load and few can handle both loads, so suitable bearing is required to select based on nature and intensity of such forces. Because of the straight teeth, spur gears impose only radial load on the bearings. Thus a cheaper cylindrical roller bearing can be used. On the other hand, helical gears impose significant radial load and thrust load on the bearings. So deep groove ball bearings, angular contact bearings and taper roller bearing can be employed as these bearings can handle both axial and radial loads.

Orientation of driver and driven shafts: One basic advantage of gear drive over other mechanical drives (like belt or chain drive) is its possibility to use for non-parallel shafts. However, several types of gear are suitable for varying orientations of driver and driven shafts. Both spur gear and helical gears are overwhelmingly used for parallel shafts; whereas, bevel gears can be used for intersecting shafts and worm gear can be used for perpendicular non-intersecting shafts. There exists a particular type of helical gear, called crossed helical gear, which can be employed for transmitting power between perpendicular shafts. This is quite similar to worm gear; however, crossed helical gear cannot provide high velocity reduction. Typically, it is suitable for 1:1 to 1:2 speed ratio (as compared to 1:15 to 1:100 in worm gear). Its application is also limited because of many limitations.

Contact scenario between mating teeth: Spur gears have straight teeth parallel to gear axis. Two mating gears are also mounted in parallel shafts. Thus teeth of two mating spur gears come in sudden contact and the contact is always a line of length equals to teeth face width. On the contrary, helical gears have helical teeth and they are mounted on parallel shafts. So teeth of two mating helical gears come in gradual contact. Their engagement starts with a point and becomes a line and then gradually disengages as a point. So contact length does not remain constant.

Impact load, vibration and noise: Since teeth of two mating spur gears comes in sudden contact, so they experience a shock or impact load. This also produces significant vibration and noise, which sometimes impose limit on maximum permissible speed of operation. On the contrary, gradual contact between mating teeth results a gradual load on the teeth and lower vibration and noise. Thus helical gears can be employed at higher speed without much problem.

Cutting gear teeth: Cutting straight teeth is comparatively easier than cutting helical teeth. Gear milling or gear hobbing can be used to cut teeth of spur and helical gears. In milling, only two simultaneous motions are desired to cut teeth of spur gears; however, three simultaneous motions are required for cutting teeth of helical gear.

Scientific comparison among spur gear and helical gear is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Design of Machine Elements by V. B. Bhandari (Fourth edition; McGraw Hill Education).
2. Machine Design by R. L. Norton (Fifth edition; Pearson Education).
3. A Textbook of Machine Design by R. S. Khurmi and J. K. Gupta (S. Chand; 2014).

The post Difference Between Spur Gear and Helical Gear appeared first on Difference Box.

A gear drive is one engagement type rigid mechanical drive that is suitable for small distance power transmission. It can transmit heavy power without any slip (positive drive). Based on the relative orientation of driver and driven shafts and teeth profile, gear drive can be classified into four groups—spur gear, helical gear, bevel gear and worm gear. Spur gear is the simplest type of gear having straight teeth parallel to the gear axis and can transmit power between parallel shafts only. Although helical gear is also used for parallel shafts, teeth are not parallel to the gear axis. Here teeth are cut in helical form on the gear blank maintaining same helix angle. Bevel gear may have straight or spiral teeth and is used for intersecting shafts; while, worm gear is used for perpendicular but non-intersecting shafts.

Although helical gears offer certain benefits over spur gear, it imposes axial thrust load on the bearing. This thrust load is detrimental and limits the power transmission capability. Moreover it requires costly and bulky bearings to withstand under radial and axial loadings. However, this thrust load can be eliminated by utilizing herringbone or double helical gear. In herringbone gear, teeth are cut in two halves of the gear blank maintaining same module, number of teeth and helix angle but opposite hand of helix. Thus thrust force produces by each half of the herringbone gear is equal and opposite and thus eliminates each other. Moreover, it can transmit comparatively higher power without much problem. Various differences between helical gear and herringbone gear are given below in table format.

Table: Difference between helical gear and herringbone gear

Teeth profile: Teeth of the helical gear are cut in the form of helix on the pitch cylinder. A particular helical gear consists of either left hand helix teeth or right hand helix teeth. At the time of engagement, a helical gear with left hand helix can only mate with a helical gear with right hand helix. On the other hand, a herringbone gear consists of both hands of helix in a single gear unit. Each herringbone gear has two distinct halves—one halve must have left hand helix teeth and the other halve should have right hand helix teeth. Other features such as pitch circle diameter, module, number of teeth, helix angle and width or thickness will be same in both the halves; the only difference is their hand of helix.

Thrust force: Helical gear offers many benefits over spur gear, such as gradual loading on teeth, less vibration, more loading capacity, higher service life, etc. Main drawback is the thrust load. Since teeth of spur gear ate straight and parallel to the gear axis, so a pair of mating spur gears induces only radial load. Due to helical form of teeth, a pair of mating helical gears induces both radial load and axial load. Thus stronger bearings are required to sustain both types of load simultaneously. Herringbone gear can offer similar benefits as the helical gear and at the same time eliminates the thrust force. Here thrust force produces by each halve is equal and opposite (because of opposite hand of helix) and thus the resultant thrust force becomes zero. So a pair of mating herringbone gears induces no axial thrust force on the bearings (only radial force exists).

Helix angle: Thrust force increases with helix angle of the teeth; however, a higher helix angle can significantly reduce vibration and tooth wear and increase load carrying capacity. This thrust force limits the maximum helix angle in case of helical gear. Typically it is kept in between 20 – 25°. However, absence of thrust force in herringbone gear facilities use of higher helix angle, even up to 45°.

Bearings for mounting the shafts: Driver and driven shafts are mounted at both ends using appropriate bearings. Apart from supporting the shafts and maintaining accurate position, bearings also talk away the vibration and load, and subsequently transmit them to the ground via frame. Typically rolling contact bearings are employed in gear units, unless the rotational speed is very high. There are various types of rolling contact bearings, some of them are suitable for radial load only, some are suitable for axial load only and few can handle radial and thrust load. For example, deep groove ball bearings, angular contact bearings and taper roller bearings can handle both radial load and thrust load. Thus these bearings can be employed for helical gear unit. On the other hand, cylindrical roller bearings can handle heavy radial load and thus can be utilized with herringbone gear unit.

Power transmission capacity: Basic function of every mechanical drive is to transmit mechanical power from driver shaft to driven shaft. Different drives offer varying level of transmission capacity based on size, material and other features. Although transmission capacity of helical gear is comparatively higher than that of similar spur gear, it is generally not used for heavy duty applications. The thrust load and availability of bearings impose the limit in heavy duty applications. Herringbone gear and double helical gears are preferred choice for such areas. For example, herringbone planetary gear train (HPGT) can be observed in heavy machineries like coal cutter, aerospace engine, wind turbine, etc.

Gear fabrication: Cutting helical teeth on the cylindrical gear blank is not difficult; however, problem arises when clearance does not exist at the end of teeth. Like spur gear, helical gear teeth can also be cut by pinion-type gear shaping cutter. Hobbing can also be used for cutting helical gears. But in herringbone gear, no relief gap exist between left and right halves. Thus cutting teeth at this junction is quite complicated as there exists a risk of overstepping to the other halve. It requires specialized hobbing cutter having a grove matching with the herringbone gear. High degree of locational and angular alignment are also required to maintain.

Scientific comparison among helical gear and herringbone gear is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Design of Machine Elements by V. B. Bhandari (Fourth edition; McGraw Hill Education).
2. Machine Design by R. L. Norton (Fifth edition; Pearson Education).
3. A Textbook of Machine Design by R. S. Khurmi and J. K. Gupta (S. Chand; 2014).

The post Difference Between Helical Gear and Herringbone Gear appeared first on Difference Box.

Problems associated with impact loading can be eliminated by utilizing helical gear. Like spur gear, helical gear is also used for parallel shafts; however, teeth are cut in the form of helix on the cylindrical gear blank. The helical teeth of two mating gears gradually come in contact, resulting a gradual loading on the teeth (instead of impact loading as in case of spur gear). This also increases power transmission capability and at the same time reduces vibration. However, helical profile of the tooth induces axial thrust load on the bearing, which is sometimes detrimental and limits the maximum allowable speed of application. In order to eliminate the thrust force maintaining the helical form of teeth intact, either herringbone gear or double helical gear can be employed.

In both the cases, teeth are cut in two halves of the gear blank maintaining same module, number of teeth and helix angle but opposite hand of helix. Thus thrust force produces by each half of the gear is equal and opposite and thus eliminates each other. Although herringbone gear or double helical gear is free from axial load, there are few differences between them in terms of constructional feature and their manufacturing. In double helical gear, small relief gap is provided between two halves. So teeth of left hand helix do not physically touch the teeth of right hand helix. However, in case of herringbone gear, no such gap is provided and thus teeth having left hand helix touch the teeth having right hand helix. Various differences between herringbone gear and double helical gear are given below in table format.

Table: Difference between herringbone gear and double helical gear

Presence of relief gap: A relief gap between the left hand helix teeth and right hand helix teeth is the main differentiating factor. In herringbone gear, no gap is provided and thus left hand helix teeth remain in physical contact with right hand helix teeth. In double helical gear, a small gap (2 – 10cm based on size) is maintained between left hand helix teeth and right hand helix teeth.

Difficulty in manufacturing the gears: While cutting the gear teeth for herringbone gear, the cutter is not allowed to move beyond the teeth face in one side (where junction exists). Any overstepping will create undesired groove on other half as intermediate groove or space does not exist. This makes the manufacturing complicated and a dedicated machine is required to cut such gear teeth. In case of double helical gear, the small relief gap allows the cutter to move freely beyond the teeth face. Due to difficulty in manufacturing, cost of herringbone gear is marginally higher than the cost of similar double helical gear.

Axial width: Width or thickness of the gear actually determines tooth strength and power transmission capability. Due to presence of small relief gap in double helical gear, tooth face width is not equivalent to the gear width. For same gear width and other features, teeth face width of herringbone gear will be larger and consequently its transmission capability will be higher. Inversely, for specified power transmission, a thinner herringbone gear can be employed. This is particularly suitable for designing a compact machine unit having limited available space.

Scientific comparison among herringbone gear and double helical gear is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Design of Machine Elements by V. B. Bhandari (Fourth edition; McGraw Hill Education).
2. Machine Design by R. L. Norton (Fifth edition; Pearson Education).
3. A Textbook of Machine Design by R. S. Khurmi and J. K. Gupta (S. Chand; 2014).

The post Difference Between Herringbone Gear and Double Helical Gear appeared first on Difference Box.

Gear drive is preferably suitable for heavy power transmission over short distance. There are four basis types of gears, namely spur gear, helical gear, bevel gear and worm gear—each of them has different features and provides unique advantages over others. Differences between these four types of gear based on various factors are explained below.

1. Orientation of gear teeth

Gear drive is one engagement type mechanical drive, which indicates motion and power are transmitted directly from driver shaft to one or more driven shafts by means of successive engagement and disengagement of teeth. Different types of gear have different tooth arrangement. During manufacturing of gear, teeth are cut on a cylindrical blank by precisely maintaining several features, one such feature is orientation of teeth with respect to gear axis. Tooth for four types of gear are provided below.

Spur gear— Teeth are straight and parallel to the axis of the gear.

Helical gear— Teeth are neither straight nor parallel to the axis of the gear. They are cut in the form of helix on pitch cylinder; so they are inclined at helix angle with the gear axis; however, never coincide.

Bevel gear— Teeth are straight but not parallel to the gear axis; however, teeth are orientated along the gear axis (in case of straight teeth bevel gears). An extended teeth profile will coincide with gear axis.

Worm gear— Here worm and worm wheel have different teeth profile. Worm is similar to threaded screw (a cylindrical shaft on which threads are cut maintaining constant pitch); whereas, worm wheel has teeth similar to spur gear (straight and parallel to the gear axis).

2. Orientation of driven and driven shaft

Power transmission either between two parallel shafts or between two intersecting shafts or between two perpendicular non-intersecting shafts may be desired in various cases. A particular type of gear is specifically suitable for any one arrangement between driven and driven shaft. For example, spur gear or helical gear are suitable when driver and driven shafts are parallel to each other. Suitability of four types of gear based on relative orientation of driver and driven shafts are discussed below.

Spur gear— It is used to transmit motion and power between two parallel shafts.

Helical gear— It is used to transmit motion and power between two parallel shafts. Crossed helical gears can be employed for non-parallel shafts (usually perpendicular); however, are rarely used.

Bevel gear— It is used to transmit motion and power between two intersecting shafts (not necessarily be perpendicular).

Worm gear— It is used to transmit motion and power between two non-intersecting shafts, which are usually perpendicular to each other.

3. Radial and thrust force on bearings

Mating of two gears imposes force on bearings. These bearings provide support to the driver and driven shafts and ensure correct position. They take up the forces that are acting on the shaft and subsequently transmit them to the ground via frame or casing. A suitable bearing is selected based on numerous factors, notably type and magnitude of force acting on it and rotational speed. Gear drive can impose radial force (acts radially towards centre from the mating point) and thrust force (acts along gear axis).

Spur gear— Since the teeth are parallel to the gear axis, it imposes only radial force to the bearings. No axial thrust force is produced here. However, gear teeth are subjected to a tangential force.

Helical gear— Since the teeth are inclined at helical angle to the gear axis, it imposes both radial force and axial thrust force on the bearings. However, double helical gear is free from thrust force. Tangential force acts on the gear teeth as usual.

Bevel gear— It also produces both radial force and axial thrust force. Additionally tangential force acts on the gear tooth.

Worm gear— Even though the teeth of the worm wheel are straight and parallel to the gear axis (similar to spur gear), both worm and worm wheel impose radial force and thrust force on the bearings. Tangential force acts on the teeth as usual. It is worth mentioning that friction force is significant in worm gear drive because of sliding contact between teeth (unlike rolling contact in other gear drives). Friction force changes radial force and thrust force substantially.

4. Engagement of gear teeth

Since gear is an engagement drive, so teeth of two mating gear successively mesh in a particular fashion. This meshing style has great impact on overall performance of gear drive. It can influence a number of factors including force, vibration, noise, heat generation, tooth wear, service life, and efficiency. Meshing style primarily depends on the orientation of teeth with respect to gear axis, as discussed below for each of the four basic gear types.

Spur gear— Teeth of two mating spur gears, having straight tooth parallel to the gear axis and mounted on parallel shafts, come in sudden contact over the entire face width. Contact between two meshing teeth is always a straight line of length equals to gear teeth width. Sudden contact imposes impact or shock load on teeth.

Helical gear— Here contact between teeth of two mating gears occurs gradually. Initially engagement starts with a point and gradually it becomes a line and thereafter it starts disengaging in reverse way. A gradual load acts on teeth.

Bevel gear— Based on gear teeth type, engagement condition and load varies. In case of straight teeth bevel gear (most common) the mating condition is similar to spur gear (i.e., sudden contact of full length); whereas, in case of spiral teeth bevel gear, mating scenario is similar to helical gear (i.e., gradual contact that starts with a point).

Worm gear— Here teeth of the worm wheel comes in contact with the worm (threaded screw). Although the contact is a line contact (higher pair), the line slides over the surface (instead of rolling as in other types of gear).

5. Power transmission capacity

Gears are used to transmit motion, torque and mechanical power from one place to another and also and alter the same as per requirement. Power transmission capacity of a gear primarily relies on gear size, gear tooth dimensions, number of tooth in contact at a time and gear material. Different types of gears have varying level of capability, as discussed below.

Spur gear— It can be employed for a wide range of power transmission requirements, typically from few watt (for example, watch or toy) to 500 kW (for example, power plant). Size of the gear also varies accordingly.

Helical gear— It can also transmit similar power as spur gear; however, it is preferred over spur gear for high speed applications. Double helical gears and herringbone gears can be used for much higher power transmission range, usually up to 1MW.

Bevel gear— Straight teeth bevel gear can be used for power transmission in the order of 300kW. Spiral bevel gears can, however, transmit more power because of increased contact ratio.

Worm gear— It has very limited power transmission capacity, usually up to 100kW.

6. Vibration and noise

In belt, chain and rope drives, an intermediate flexible element (such as belt, chain or rope) exists in between pulleys or sprockets of driver and driven shafts. This flexible element absorbs vibration arises in driven shaft and thereby protects the prime mover (such as an electric motor, the driver element). No such intermediate flexible element exist in gear drive and thus vibration is transmitted from driven shaft to driver shaft, which is very harmful for prime mover when allowable limit is exceeded. On the basis of how the teeth are mating, different gear drives tends to generate different levels of vibration and noise, as discussed below.

Spur gear— Due to sudden contact between mating teeth, sufficient vibration and noise generate in spur gear arrangement. Moreover, vibration level increases with speed and thus this type of gear is not suitable for high speed applications.

Helical gear— Here teeth of two mating gears gradually come in contact. No impact force exist here. Accordingly, generated vibration and noise levels are comparatively lower.

Bevel gear— Based on the type of teeth, bevel gears may produce similar effects as discussed above. Straight teeth bevel gears will produce effects similar to spur gears (i.e., significant vibration and noise); while, spiral teeth bevel gears will produce effects similar to helical gear (i.e., reduced vibration and noise).

Worm gear— Operation of worm gear is usually smooth and silent as it is used in low speed and low power applications.

7. Achievable velocity reduction

In usual cases, the prime mover (like electric motor or hydraulic turbine) rotates at comparatively higher speed than it is desired in machinery. This requires a reduction in rotational speed between the driver shaft and driven shaft, which can be easily achieved by utilizing different size gears. However, each type of gear arrangement is preferred for a range in speed reduction, as given below. It is worth mentioning that higher reduction can also be achieved by utilizing multi-stage gears.

Spur gear— Spur gears are not suitable for higher speed reduction. It can provide velocity ratio ranging from 1:1 to 4:1.

Helical gear— It can provide little higher reduction as compared to spur gear. It can be applied for velocity ratio in the order of 1:1 to 6:1. However, up to 9:1 speed reduction can also be achieved in single stage for low speed applications if sufficient space is available (higher speed reduction requires larger driver gear).

Bevel gear— It is specifically suitable for 1:1 speed reduction. In certain occasions, it can be applied up to 3:1 reduction.

Worm gear— The biggest advantage of worm gear is its capability to provide steep speed reduction. It can be employed to achieve speed reduction ranging from 15:1 to 60:1; however, a single pair of worm gear can also provide speed reduction up to 100:1.

Scientific comparison among spur gear, helical gear, bevel gear and worm gear is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Design of Machine Elements by V. B. Bhandari (Fourth edition; McGraw Hill Education).
2. Machine Design by R. L. Norton (Fifth edition; Pearson Education).
3. A Textbook of Machine Design by R. S. Khurmi and J. K. Gupta (S. Chand; 2014).

The post Difference Between Spur Gear, Helical Gear, Bevel Gear and Worm Gear appeared first on Difference Box.

There are different types of springs made of different materials to fulfil varying requirements. Helical spring, leaf spring, spiral spring, Belleville spring, volute spring, disk spring, etc. are notable one. Among them, helical spring has wide range of applications. Specialized springs are also frequently used to suite the intended purpose. In its service life, a spring may experience axial loading (tensile or compressive), torsion, bending, or a mixed type of loading. A particular type of spring is selected on the basis of certain parameters such as intended purpose, nature of force, durability requirement, available apace, working condition, etc. Both coil spring and leaf spring are suitable for vehicle suspension system and railway buffer system; however, no one can fulfil each and every requirement for such purpose.

A coil spring, also called helical spring, consists of a small diameter wire bent in the form of helix. Apart from material, wire diameter, helix angle, gap between two adjacent coils of the helix, and also end type of the spring can vary one spring to another. Such springs are simple in construction, have light weight, and requires minimum space for installation. When installed in vehicle, they offer jerk-free motion but they cannot withstand under heavy load. On the other hand, leaf spring (also called laminated spring) consists of a number of flat semi-elliptical plates of varying length clamped together. They can endure heavy load but show poor vibration damping capability. Various differences between coil spring and leaf spring with respect to vehicle suspension are given below in table format.

Table: Differences between coil spring and leaf spring

How the springs are made? A leaf spring consists of multiple flat plates of varying length clamped or laminated together using rebound clips and bolts. Each flat plate is termed as leaf. The leaf length gradually decreases from top to bottom. The top most leaf, the longest one, is called master leaf and it is bent at both ends. Each leaf is of semi-elliptic shape and the overall spring also have similar shape. On the contrary, helical spring is made by winding a constant diameter coil or wire in the form of helix. Although the wire remains in the form of helix, overall helical spring occupy a hollow cylindrical shape.

Transverse flexibility: Small amount (usually up to 10°) of deflection of the suspension system in cross or transverse direction is necessary to drive the vehicle in rough roads. Such deflection is also useful while taking sharp turns. Coil spring can provide such flexibility in cross-movement. Thus this type of spring is suitable for such cars which are meant for running in rough, hilly and village road conditions. Leaf spring provides no such cross-deflection of the suspension system.

Load bearing capacity: Leaf spring totally dominates coil spring when load carrying capacity is the matter of interest. Heavy industrial trucks are fitted with leaf spring. Moreover, speed barkers (especially steep bumps) of the road exert enormous reactive force when vehicles, even in slow speed, run over it. Leaf spring can comfortably endure such jerk without much problem. Coil spring may fail by buckling if load exceeds certain limit.

Comfortable journey: Coil spring offers a comfortable and jerk free journey even when the vehicle is driving on in rough road. Thus it is preferred for passenger vehicles. However, in such road, leaf spring tends to lift up the vehicle body, especially when the vehicle is unloaded. So it is preferred in heavy trucks where comfortability is not prime factor.

Weight and cost: Leaf spring is heavy and costly, although it is simple in construction and installation. It also requires larger space for installation. On the other hand, coil spring is of light weight and in majority of the cases it is cheaper.

Scientific comparison among coil spring and leaf spring in terms of vehicle suspension is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Design of Machine Elements by V. B. Bhandari (Fourth edition; McGraw Hill Education).
2. What’s the Difference Between a Leaf Spring and a Coil Spring? By generalspringkc.com.

The post Difference Between Coil Spring and Leaf Spring for Vehicle Suspension appeared first on Difference Box.

There are different types of spring—each one is suitable for a particular type of application. During its service life, a spring can experience axial force (tensile or compressive), torsion, bending moment, or a mixed type force. Accordingly, several types of springs are also available, which include helical spring, leaf spring, spiral spring, Belleville spring, volute spring, and disk spring. Specialized springs are also frequently used to fulfil a particular requirement. Among these, helical spring is more popular as it can be used for low-duty applications (like toys, door hinges, ball-point pens, etc.) as well as heavy-duty applications (like suspension spring of transport vehicles, railway buffer system, vibration dampers, etc.).

A helical spring consists of a fixed diameter wire bent in the form of helix. Helix angle as well as the gap between two adjacent coils of the helix can be controlled during its manufacturing. Sufficient gap between two adjacent coils of a helical spring is necessary if the spring is intended to compress under axial loading. Such helical springs, which are intended to shorten its length under the action of axial compressive loading, are called compression spring. A compression spring is sometime termed as open coiled spring as the coil of the helix is not wound tightly. On the contrary, if the helical spring is made by tightly winding the coil keeping no visible gap between two adjacent coils, then it is called close coiled spring. Such a spring cannot undergo compression; instead, they are made to experience tensile loading or torsion. This type of spring is called extension spring or tension spring. Various differences between compression spring and extension spring are given below in table format.

Table: Differences between compression spring and extension spring

What are compression and extension springs? Every spring is a flexible mechanical element and should undergo large amount of elastic deflection. This deflection may be either elongation or reduction in length of the spring. Both compression and extension springs are basically helical springs (coil springs); however, they are made to serve two different purposes. In case of helical compression spring, the external force tends to compress the spring. Thus length of such spring reduces when subjected to external loading. On the other hand, extension spring undergoes increment in length when subjected to external loading.

Pulling or pushing the structural members: One of the crucial purposes of every spring is to store mechanical energy when it is elastically deformed (elongated or compressed) by external loading and subsequently release that energy by regaining original shape upon removal of external load. Under the action of external load, a compression spring will shorten its length storing energy within it. Such loaded spring will continuously push the connecting members to expand its own length to revert back to its original position by releasing stored energy. On the contrary, an extension spring will store energy when it is elongated by the action of external load. So it will continuously pull the connecting members to regain its original length by releasing stored energy.

Gap between adjacent coils: Actual length of any helical spring in unloaded condition is called free length. Length of any helical spring corresponding to full compression is called solid length and this indicates minimum possible length of any helical spring. However, solid length may go beyond elastic limit based on spring material, wire diameter, helix angle and other parameters. Sufficient gap must be provided between adjacent coils of a compression spring to facilitate compression in axial length under the action of external load. Such a spring can be compressed until adjacent coils touch teach other (that is until solid length is achieved). On the contrary, extension springs are not meant for compression and thus gap between adjacent coils are not necessary. In fact, this gap will increase when such coils are subjected to tensile loading. Thus extension springs can be wound tightly to minimize longitudinal space requirement without sacrificing spring stiffness.

Applications of compression and extension springs: A common application of compression spring is in spring-operated ball-point pen. When the head is pressed to take out the nib, the internal spring gets compressed and energy is stored in it.  This energy is released as the spring returns back to actual shape when the head is pressed again to bring in the nib. Such springs are also frequently used in mechanically controlled cam-follower mechanisms, certain valves, etc. Based on the size, a number of extension springs are used in the trampoline that provide necessary rebounding force. When someone jumps on it, the springs are extended and energy is stored in it. Springs then release this energy automatically by reducing its length to regain its original shape. This, in turn, exerts an upward force to help the person jump higher. However, spring-free trampolines, usually of smaller size, are also readily available in market. The car garage door lifting mechanism is also integrated with a pair of torsion spring or extension spring to reduce the effort required for lifting the heavy door.

Scientific comparison among compression spring and extension spring is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Design of Machine Elements by V. B. Bhandari (Fourth edition; McGraw Hill Education).
2. Machine Design by R. L. Norton (Fifth edition; Pearson Education).
3. A Textbook of Machine Design by R. S. Khurmi and J. K. Gupta (S. Chand; 2014).

The post Difference Between Compression Spring and Extension Spring appeared first on Difference Box.

There are different types of springs, which are designed to suite a particular type of application. Spring is usually designed on the basis of type of force that the spring is expected to endure throughout its service life. Several types of force that can act on a spring include axial force (tensile and compressive), torsion, bending, or a mixed type of force. Various types of springs that are commonly employed in mechanical systems are helical or coil spring, leaf spring, spiral spring, Belleville spring, and disk spring. Among these, helical spring can fulfil the requirement of diverse fields ranging from low duty (like ball-point pen) to heavy duty (like railway buffer). Custom made springs are also frequently utilized to better suite the intended requirement.

Helical spring is made by winding a small diameter coil or wire in the form of helix. Gap between two adjacent coils of the helix can be varied according to the desired property. Based on this gap, helical springs can be classified as close coiled helical spring and open coiled helical spring. While making a helical spring, if coil is wound tightly providing no visible gap between two adjacent coils, then the same is called close coiled spring or extension spring. On the other hand, when sufficient gap is provided between two adjacent coils, then the same is called open coiled spring or compression spring. Therefore, both are helical springs, but with varying space between two adjacent coils. Various differences between close coiled and open coiled helical springs are given below in table format.

Table: Differences between close coiled and open coiled helical spring

Winding of coil: Every helical spring is made of wire or coil having constant cross-section and is wound in the form of helix. Gap between two adjacent coils of the helix can be altered to obtain desired effect. Length of the spring and its capability to deflect under external loading greatly rely on this gap. When the coils are wound tightly in such a way that there exist no noticeable gap between two adjacent coils then the corresponding spring is termed as close coiled helical spring. On the contrary, when sufficient gap is provided between two adjacent coils then corresponding spring is termed as open coiled helical spring.

Helix angle and pitch: Helix angle of the spring is usually used to differentiate the two. Helix angle of the close coiled helical spring is limited to 10° maximum. Apparently, the plane containing each coil of such spring is perpendicular to the spring axis. Moreover, small helix angle also reflects smaller pitch. Pitch of a helical spring is defined as the axial distance between two similar points on the adjacent coils. On the other hand, springs with helix angle larger than 10° is called open coiled helical spring. Consequently, such springs have longer pitch.

Resisting deflection: Solid length indicates length of the spring along its longitudinal axis when the spring is compressed in such a way that there exist no noticeable gap between two adjacent coils. Whereas, original length of the spring in absence of external load is called free length. In case of close coiled helical spring, the solid length and free length are almost same. Thus no further compression is possible; however, they can elongate under tensile loading. In fact, these coils are designed to resist stretching and twisting. Helical torsion spring, which is used to transmit torque, is one type of close coiled spring. On the contrary, open coiled springs can undergo significant elastic elongation as well as compression under the action of external axial loading. However, they cannot resist twisting.

Applications: A close coil spring is suitable if it experiences either axial tensile loading or torsion during its service life. Most helical torsion springs are close coiled. Typical applications of such springs include garage doors, trampolines, vice-grip pliers, self-closing door hinges, bike or cycle stand spring, etc. On the other hand, open coil spring is suitable if it experiences axial compressive or tensile forces or both in certain cases. Every vibration absorber (including vehicle buffer and damping equipment) utilizes this type of spring. Such springs are also used in automobile brakes, clutches, valves, etc. Another well-known example include spring-operated ball point pen.

Scientific comparison among close coiled helical spring and open coiled helical spring is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Design of Machine Elements by V. B. Bhandari (Fourth edition; McGraw Hill Education).
2. Machine Design by R. L. Norton (Fifth edition; Pearson Education).
3. A Textbook of Machine Design by R. S. Khurmi and J. K. Gupta (S. Chand; 2014).

The post Difference Between Close Coiled and Open Coiled Helical Spring appeared first on Difference Box.

Material, orientation and geometry are three crucial parameters that influence machining performance and capability. A compatible cutter material should be selected based on work material and machining operation. Cutter should also be mounted on machine tool in proper orientation to continue machining uninterruptedly for longer duration. Cutter geometry refers to various angles and radius associated with cutting tool. It includes, but is not limited to, rake angles, clearance angles, cutting angles, nose radius, and edge radius. All these features, directly or indirectly, influence various machining characteristics in different scales. Some of these features are also displayed in the tool signature in a standardized manner.

There exist quite a few tool designation systems—each of them independently displays corresponding tool signature. American Standards Association (ASA) system, Orthogonal Rake System (ORS), Normal Rake System (NRS), and Maximum Rake System (MRS) are commonly used single point turning tool (SPTT) designation systems. Different tool designation systems utilize different imaginary planes for reference purpose. ORS system of tool designation displays inclination angle, orthogonal rake angle, orthogonal clearance angle, auxiliary orthogonal clearance angle, auxiliary cutting edge angle, principal cutting edge angle, and nose radius. NRS system of tool designation displays inclination angle, normal rake angle, normal clearance angle, auxiliary normal clearance angle, auxiliary cutting edge angle, principal cutting edge angle, and nose radius. Various differences between ORS system and NRS system are given below in table format.

Table: Differences between ORS system and NRS system

Full form of abbreviations: There are several cutting tool designation systems readily available and also commonly used. ASA, ORS and NRS are popular among them. ORS is the abbreviated form of Orthogonal Rake System. It is also frequently termed as Orthogonal Reference System. Both are correct. Similarly NRS is the abbreviated form of Normal Rake System or Normal Reference System.

Imaginary planes for referencing: Every tool designation system employs few imaginary flat planes in different directions and orientations. Various features of the cutting tool including angles and radius are measured with respect to these standard planes in order to maintain uniformity. ORS system employs three planes namely Reference Plane (P_R), Orthogonal Plane (P_O), and Cutting Plane (P_C). NRS system also employs three plain planes namely Reference Plane (P_R), Normal Plane (P_N), and Cutting Plane (P_C). Thus normal plane is used in NRS in place of orthogonal plane of ORS; the other two planes remain same in both the cases.

Relative positions of imaginary planes: Reference plane (P_R) is a plane perpendicular to cutting velocity vector (V_c). Cutting plane (P_C) is one plane that contains principal cutting edge of the cutter and is perpendicular to reference plane (P_R). Orthogonal plane (P_O) is oriented in third perpendicular direction and thus it is perpendicular to both reference plane and cutting plane. That means three imaginary planes of ORS system are always mutually perpendicular. This is not the case for NRS. The normal plane (P_N) is perpendicular to principal cutting edge of the cutter. So if the principal cutting edge is not parallel to reference plane then normal plane will not be perpendicular to reference plane. The angle between principal cutting edge and reference plane (when measured on cutting plane) is called inclination angle (λ). Thus the angle between orthogonal plane and normal plane is also inclination angle. Therefore, three planes of NRS system are not mutually perpendicular unless inclination angle becomes zero. In such case (with λ=0°), NRS system and ORS system will be exactly same.

Orthogonal rake and normal rake: Rake angle is defined as the angle of inclination of the tools rake surface from reference plane and measured on some other plane. When it is measured by projecting on orthogonal plane, it is termed as orthogonal rake angle and the same is displayed in ORS tool designation system. Similarly when the angle is measured by projecting on normal plane, it is termed as normal rake angle and the same is displayed in NRS tool designation system. These two angles are widely used for various analysis relevant to machining and its performance.

Tool sharpening by grinding: Every cutter undergoes gradual wear during machining. Wear makes the cutting edges blunt (less sharp), which leads to undesirable increase in cutting force and surface roughness. Uncoated cutting tools can be sharpened by grinding in a regular interval to improve sharpness (reducing edge roundness). Single point turning tool can be ground by mounting it on a 3-D vice in proper fashion. It requires setting up multiple angles for each face of cutter. Such setting angles are calculated from the known tool signature. Use of ORS system for this calculation purpose leads to error; and thus requires additional calculations for compensation. Such limitation can be eliminated by employing features of NRS system during tool setting on 3-D vice. It is worth mentioning that use of MRS system for this purpose is best as it requires only two angle settings for each face; instead of 3 as in case of other systems.

ISO system: Earlier ORS system was international standard for turning tool designation. However, this system cannot reveal true picture of cutting tool. Now NRS system is overwhelmingly accepted international standard for turning tool designation.

Scientific comparison among ORS system and NRS system of turning tool is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Machining and Machine Tools by A. B. Chattopadhyay (1^st edition, Wiley).
2. Manufacturing Engineering and Technology: SI Edition by S. Kalpakjian and S. R. Schmid (7^th edition, Pearson Ed Asia).

The post Difference Between ORS System and NRS System of Tool Designation appeared first on Difference Box.

All of these are subtractive manufacturing processes, which indicates layer by layer material is removed from a solid workpiece to obtain desired three dimensional features; however, they follow varying principles of material removal and thereby possess varying capability in terms of machinable materials, stock removal rate, surface quality, production rate and cost, etc. Most NTM processes and micro-precision machining processes are not suitable for removing bulk volume of material; instead, they can generate fine features with high accuracy. Conventional machining processes are suitable for high stock removal as well as imparting reasonably good surface quality. However, achieving both in a single pass is not possible. Thus machining is usually carried out in two steps with varying process parameters (cutting velocity, feed rate and depth of cut).

In first step, bulk amount of material is quickly removed from workpiece as per required feature. Higher feed rate and depth of cut are employed for this step so that high stock removal rate is obtained. This step is called rough cut or roughing pass. It cannot provide good surface finish and close tolerance. After rough cut, a finish cut or finishing pass is carried out to improve surface finish, dimensional accuracy and tolerance level. Here very low feed rate and depth of cut are employed. So stock removal rate reduces in finish pass but surface quality improves. Various differences between roughing and finishing in conventional machining processes are given below in table format.

Table: Differences between roughing and finishing

Objectives of rough cut and finish cut: Rough cut is carried out to quickly impart a basic shape according to desired feature. Here surface roughness is not important factor; instead, removing maximum unwanted material is ultimate objective. Contrary to this, finish pass is carried out to improve surface finish, dimensional accuracy and tolerance of the desired feature. Stock removal rate has no importance in case of finish pass.

Process parameters and MRR: Cutting velocity (V_c), feed rate (s or f) and depth of cut (t or a) are three process parameters for every conventional machining process. These parameters greatly influence overall machining action and capability. Higher velocity, feed and depth of cut can increase material removal rate (MRR) but with the sacrifice of surface finish. MRR is proportional to velocity, feed and depth of cut and thus can be mathematically expressed by the multiplication of velocity, feed and depth of cut with a positive constant for unit conversion. During machining, velocity is normally maintained unchanged as it is selected on the basis of work and tool material, machine tool capability, vibration level and other important factors. To fulfil the basic objective, higher feed and depth of cut are employed in rough pass and as a consequence MRR increases. On the other hand, low feed and depth of cut are employed in finish pass and thus MRR reduces.

Surface finish and dimensional accuracy: Presence of scallop marks or feed marks on the finished surface is inherent to every conventional machining process due to feed velocity. Such saw-tooth alike scallop marks cause primary surface roughness. Apart from cutting tool geometry, surface roughness directly relies on feed rate. Higher feed rate can lead to poor surface finish. Higher depth of cut also tends to degrade surface finish and machining accuracy. In rough cut, higher feed and depth of cut are utilized and thus poor surface finish is obtained. It also fails to provide high dimensional accuracy and close tolerance. On the other hand, finish pass can improve finish, accuracy and tolerance as very low feed and depth of cut are employed.

Usage of old cutter: An old cutter may have less sharp edges (i.e., higher edge radius and nose radius) as it has already worn out during machining. Edge and nose sharpness limit the achievable surface finish in the process. A sharp edge cannot take high chip load but is mandatory to obtain better finish and accuracy. So an old cutter can be utilized in rough pass without noticeable problem as surface quality does not matter. However, a sharp tool should be used in finish pass so that better finish, accuracy and tolerance is achieved. Here feed and depth of cut remain low, so chip load possesses no detectable problem in tool breakage or edge chipping.

Scientific comparison among roughing and finishing in machining processes is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Machining and Machine Tools by A. B. Chattopadhyay (1^st edition, Wiley).
2. Manufacturing Engineering and Technology: SI Edition by S. Kalpakjian and S. R. Schmid (7^th edition, Pearson Ed Asia).

The post Difference Between Roughing and Finishing in Machining appeared first on Difference Box.

During machining, the cutting tool forcefully compresses a thin layer of material to gradually shear it off. For uninterrupted removal of material from workpiece, certain relative motions between cutter and workpiece are indispensably necessary. Such motions can be broadly classified into two categories—formative motions and auxiliary motions. Simultaneous action of these two motions help obtaining various three dimensional features. Cutting motion and feed motions come under formative motions; while secondary feed motion, depth of cut, indexing motion, relieving motion, etc. come under auxiliary motions. Cutting motion (CM) can be imparted by rotating or oscillating the workpiece or cutter. This CM basically imparts necessary cutting velocity; while feed motion (FM) imparts necessary feed velocity.

Cutting velocity is most important process parameter in every machining process as it influences a large number of machining responses including cutting force, power consumption, material removal rate, cutting temperature, tool life, etc. It is also one important parameter for analytically estimating all such responses. On the other hand feed rate helps covering up a large area of work surface during machining. A synchronous action of cutting velocity and feed rate helps continuously removing material from a larger area. Feed rate has only little influence on force, MRR, temperature, etc.; however, it directly influences machining time and finish or roughness of finished surface. Various differences between cutting velocity and feed rate are given below in table format.

Table: Differences between cutting velocity and feed rate

Interaction of cutting velocity and feed rate: During machining, the wedge shaped cutting tool compresses a thin layer of work material and gradually shears it off in the form of chip. A relative velocity between workpiece and cutter is desired to impart the intended compressive force. Cutting velocity actually supplies the primary relative velocity and thus helps in realizing material removal. However, to realize such material removal from entire work surface, the cutter or workpiece (different for different machining operations) must be provided another synchronous motion along the intended direction. This is known as feed motion. Simultaneous action of cutting velocity and feed rate along with other auxiliary motions fulfils basic machining requirement.

Generatrix and directrix: Basic function of machining is to produce various geometrical surfaces with higher accuracy and better finish. Generation of any geometrical surface requires use of generatrix and directrix. These are two line vectors—together they generate intended geometrical surface (flat or curved). These are provided by formative motions between cutter and workpiece. Usually, cutting motion provides necessary generatrix; while feed motion provides necessary directrix.

Units of measurement: Although both express velocity (a derived physical parameter), they are measured in different ways and so their units are different. Cutting velocity is commonly measured in meters per minute (m/min). Sometime, especially with high speed machining, it is also expressed in meters per second (m/s). Cutting velocity differs from cutting speed, which indicates rotational speed of cutter or workpiece and is measured in revolutions per minute (rpm). On the other hand, feed rate is measured in various units for different machining operations. In lathe operations feed is measured in millimetres per revolution (mm/rev); in milling processes mm/flute; in shaping mm/stroke; in drilling mm/rev; and so on. In CNC operations, feed is expressed in mm/min, which is obtained by multiplying the original feed with cutting speed and other parameters (such as number of flutes).

Impact on cutting force and power consumption: These are two important factors in every machining operation. Starting from cutter design, cutter selection, and machine tool performance to material removal rate (MRR), surface quality, and machinability—all depend (directly or indirectly) on those two factors. While machining a particular material using a specific cutter under given conditions, with increase in cutting velocity, cutting force and power consumption also increase. Feed rate also has influence on the same; however, in lesser scale.

Chip deviation angle: Majority of machining analysis is performed assuming orthogonal cutting that is chip flows on the orthogonal plane. However, in most cases, chip does not flow in the orthogonal plane; instead it flows along oblique plane. Restricted cutting effect, cutter nose radius, and inclination angle of the tool separately cause chip deviation from orthogonal direction. Although the deviation angle is small, feed velocity has an impact on the chip deviation caused by restricted cutting effect. On the other hand, cutting velocity has no direct role in deviating chips from orthogonal plane.

Cutting temperature and tool life: Cutting temperature is another crucial factor in machining as higher temperature can severely hamper finished surface quality, can accelerate tool wear and can steeply reduce tool life. Cutting temperature can be controlled by employing optimum set of machining parameters and using lubricant or coolant. Cutting velocity has greater impact in increasing temperature and thus it can be controlled by reducing velocity. However, that may lead to degraded productivity. Substantial portion of heat generates due to intense rubbing of flowing chips with rake surface of cutter. Reduction of velocity will also reduce chip production rate and thus lower heat will generate. High feed rate also causes high cutting temperature; however, its impact is lower as compared to cutting velocity. Life of cutting tool relies heavily on cutting velocity; and also on feed rate and depth of cut in lower scale (can be attributed to Modified Taylor’s tool life formula).

Scallop marks and surface roughness: Scallop marks or feed marks are inherent to machining and is found on finished surface that leads to primary surface roughness. Such undesired marks are produced between two successive passes due to presence of tiny unremoved material. Higher feed rate imparts larger marks and thus surface roughness increases. Thus surface finish can be improved by lowering the feed rate; however, that may lead to reduced MRR and productivity. On the other hand, cutting velocity has no role on generating scallop marks and thus does not contribute on surface finish.

Scientific comparison among cutting velocity and feed rate in machining is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Machining and Machine Tools by A. B. Chattopadhyay (1^st edition, Wiley).
2. Manufacturing Engineering and Technology: SI Edition by S. Kalpakjian and S. R. Schmid (7^th edition, Pearson Ed Asia).

The post Difference Between Cutting Velocity and Feed Rate in Machining appeared first on Difference Box.

For uninterrupted removal of material from workpiece, relative motion in particular direction between cutter and job is necessary. Such motions can be of two types—formative motions and auxiliary motions. Cutting motion (CM) and feed motion (FM) are formative motions; while depth of cut, relieving motion, etc. are auxiliary motions. Cutting motion imparts necessary cutting velocity, while feed motion imparts feed rate. Simultaneous action of cutting velocity and feed velocity helps removing material from the entire work surface. In some machining operations, either workpiece or cutting tool rotates; while in case of others, either workpiece or cutting tool reciprocates. Such rotation or oscillation provides intended cutting velocity and feed velocity.

With reference to rotating type operations, there exist two different terms to indicate rotational characteristics. First one is the number of rotations completed per unit time (usually minute), which is technically termed as cutting speed and is expressed in rpm (revolutions per minute). Another alternative can be the tangential velocity of the rotating body, which gives cutting velocity and is expressed in m/min (meters per minute). It is worth mentioning that either cutter or workpiece can rotate (based on operation); correspondingly speed or velocity can be associated with cutter or workpiece (whichever is rotating). With reciprocating type machining operations, linear velocity of the workpiece or cutter (whichever is reciprocating to provide CM) is termed as cutting velocity. In such cases cutting speed does not exist due to absence of rotating body. Various differences between cutting speed and cutting velocity are given below in table format.

Table: Differences between cutting speed and cutting velocity

Relation between cutting speed and cutting velocity: Cutting speed indicates the rotational speed of either workpiece or cutter (whichever is rotating). Tangential velocity at a point on the periphery of such rotating body is termed as cutting velocity. Thus one can be converted into another provided that diameter of rotating body is known. The following simple formula can be used for the sake of conversion in case of turning. Note that the formula is written based on the unit of each parameter as specified there.

Unit and notation: Cutting speed is commonly expressed in revolutions per minute (rpm). However, in certain cases, especially with high speed machining, speed can also be specified in revolutions per second (rps). Notation N (in capital) is used to denote speed in rpm; while n (in small) is used to denote speed in rps. Sometimes S is also used to denote speed, especially while writing CNC programme. On the other hand, cutting velocity is commonly expressed in meters per minute (m/min) and is denoted by V_c. Rarely it is expressed in meters per second (m/s).

Cutting speed is scalar but cutting velocity is vector: A scalar quantity is one that has only magnitude but no direction; while a vector quantity has magnitude along with direction. In order to fully describe the cutting speed, only magnitude with corresponding unit is sufficient. To specify cutting velocity, direction should be specified apart from magnitude and corresponding unit. However, for every machining operation, direction of cutting velocity is fixed based on the orientation of cutter and workpiece. Thus mentioning direction is not required while specifying it.

Rotating type and reciprocating type operations: Different machining operations are carried out in different ways. In some cases, either cutting tool or workpiece rotates; while in other cases, either cutting tool or workpiece reciprocates to impart cutting motion. A classification of machining operations on this ground along with typical examples is depicted below. Note that such a classification is primarily based on cutting motion (CM) and does not take care of feed motion (FM) or other auxiliary motions. Since cutting speed denotes the rotational speed, so it is relevant only in such machining operations where either cutting tool or workpiece rotates. In reciprocating type operations, cutting speed term is irrelevant. However, cutting velocity exists in all cases. With rotating type operations, the tangential velocity of cutter or workpiece on its periphery indicates cutting velocity; while in reciprocating type operations, the linear velocity of cutter or worktable indicates cutting velocity.

Usage: Machine tool mostly works on cutting speed. However, for its selection cutting velocity is first required to select judiciously and then speed is calculated using diameter of cutter or job. Cutting velocity is one of the three process parameters (other two are feed rate and depth of cut) and has great impact on machining performance. Due to its influence on a larger number of factors, it is used to analytically estimate cutting force, power consumption, machining time, cutting temperature, tool life, machinability, etc.

Scientific comparison among cutting speed and cutting velocity in machining is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Machining and Machine Tools by A. B. Chattopadhyay (1^st edition, Wiley).
2. Manufacturing Engineering and Technology: SI Edition by S. Kalpakjian and S. R. Schmid (7^th edition, Pearson Ed Asia).

The post Difference Between Cutting Speed and Cutting Velocity appeared first on Difference Box.

There exist few technical terms to characterize the quality and topography of a solid surface. Although many of such characteristic parameters are application related, there exist few guidelines for judging surface quality of manufactured product. Introduced by Field and Kahles in 1964, surface integrity (SI) encompasses all such parameters, as shown below. Surface finish is widely used element of integrity; in fact, it is most important aspect of assessing quality of as machined surface and sub-surface. In the context of machining, two terms—surface finish and surface roughness are commonly used to characterize the surface quality. Although they are used interchangeably, they are not scientifically same; however, they indicate same surface characteristics. Minimum, standard and extended surface integrity (SI) data set for a machined surface is depicted below.

Surface roughness is the measure of macro and micro asperities and irregularities present on a finished surface after machining. It is the quantitative approach to find out height of peaks and depth of valleys of surface asperities. There are various instruments to facilitate such measurement. On the other hand, surface finish is the qualitative approach to designate the surface either by observing the surface or from the roughness value. Various attributes like bad, poor, good, glossy, fine, etc. are used to designate the finish quality. It does not fetch any numerical value and thus it is affected by human bias. Various differences between surface roughness and surface finish are given below in table format.

Table: Differences between surface roughness and surface finish

Qualitative and quantitative value: Every physical parameter is measurable and the measured value is expressed using numeric and corresponding unit. A quantitative measurement always fetch numerical value along with unit. Surface roughness is one quantifiable parameter. For example, average roughness of micro-milled surface is 325 nanometre. On the other hand, finish is one qualitative parameter and cannot be expressed using numerical value. Only attributes like good, bad, poor, rough, etc. are used for the this purpose.

Subjective parameter and human bias: Attributes used for surface finish is subjective and depend on application or intended result. A milled surface used for casing may have good finish; however, if similar milled surface is intended to use for ball-bearing, then it can be considered as poor finish. However, the roughness will remain same (may be around 5µm). So roughness is independent of application. It is also not affected by human bias. Humans are incommensurable; so a good finished surface to one person may not appear good to another person. Thus attributes used for specifying surface finish has limited application in scientific context.

Measurement by instrument: Surface roughness can be directly measured using suitable instrument. Now-a-days several instruments (contact or non-contact type) are readily available in the market—each having varying features and capability. Irrespective of measuring instrument, roughness is basically the measure of height of peaks and depth of valleys of the surface asperities. Surface finish cannot be measured using instruments; instead, value of surface roughness is utilized to indirectly specify finish.

Estimation using other parameters: Different manufacturing processes provide varying level of surface finish. Usually surface generation (machining) and surface modification (heat treatment, coating and coloring) processes tend to provide lower roughness. However, various newly developed or sophisticated processes like powder metallurgy and investment casting can also provide better quality surface. A list of typical roughness obtained in various processes is provided below. Roughness can be estimated using various process parameters and relevant features of instrument employed in the process. For example, in machining using a sharp tool, estimated roughness can be expressed using feed rate, principal and auxiliary cutting edge angles. However, surface finish cannot be estimated in such way.

Table: Typical surface roughness obtained in various processes

Scientific comparison among surface roughness and surface finish is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Surface Integrity in Machining edited by J. P. Davim (2010, Springer Science & Business Media).
2. Surfaces and their Measurement by D. J. Whitehouse (2004, Elsevier).
3. Nonconventional Machining by P. K. Mishra (2007, Narosa Publishing House).

The post Difference Between Surface Roughness and Surface Finish appeared first on Difference Box.

These are three terms, namely surface roughness, surface finish and surface integrity, which are commonly used to define the quality of the surface of manufactured product. Sometimes they are used interchangeably, but they are not exactly same. Roughness and finish imply the same thing but in different way; whereas integrity has a broad domain and encompasses the former two. Their differences with respect to manufactured products are discussed in following sections.

What is surface finish? How it is related with surface roughness?

Smoothness of a surface is determined by surface finish. It indirectly indicates height and depth of asperities of the solid surface. No surface is perfectly smooth; even if it is perfectly smooth, it cannot be determined as every measuring instrument has a range of its capability. Looking on a surface in bare eye may not reveal asperities; however, a magnified view of solid surface will reveal such peaks and valleys, as shown below for a surface after grinding (abrasive finishing).

Surface roughness is the quantitative approach of determining smoothness of a surface by measuring height of such peaks and depth of such valleys over a fixed length. There are various principles of measuring roughness; however, all follow the aforementioned basic approach. For example, average surface roughness (R_a), 10 point average (R₁₀), root means square (R_z), etc. Whatever be the case, roughness can quantitatively determine the profile of a solid surface and is one crucial parameter of surface texture. A particular manufacturing process has the capability to provide certain level of roughness and therefore machining (surface modification) process should be selected based on the intended roughness. Usually secondary manufacturing processes, such as machining, tend to provide a smoother surface. There also exist many processes (like lapping, buffing, superfinishing, ion beam machining, etc.) that are exclusively used to reduce surface roughness of a component rather than bulk removal of material.

Unlike surface roughness, surface finish is one qualitative approach of indicating smoothness of a surface. In fact, roughness and finish are inversely proportional. Thus lower the roughness, better is the finish. Attributes like poor, bad, good, fine, smooth, etc. are used to express surface finish rather than an exact numerical value (like 100nm). Thus it is affected by human bias. It is worth mentioning that roughness and finish are localized factors and don’t take care of entire surface. There are many other surface profile parameters such as waviness, texture, etc. for the same.

What is surface integrity? Which parameters come under this?

Surface integrity encompasses all of the elements that describe all conditions of existing solid surface. It takes care of not only surface topography but also metallurgical aspects of the surface and sub-surface. The concept of surface integrity was introduced by Field and Kahles in 1964 and they defined surface integrity as the inherent or enhanced condition of a surface produced in machining or other surface generation operation. Every surface generation or modification process is associated with the alteration of various properties including roughness, plastic deformation, micro-cracking, phase transformations, micro-hardness, residual stress, etc. Thus surface integrity takes care of such alteration.

There exist a standard dataset for surface integrity (SI) prepared by Field et al. (1972) as given below. Surface finish is one important factor that come under this data set. In fact, finish is first and foremost parameter in integrity data set.

Set-1: Minimum SI data set (MDS)

• Surface finish
• Macrostructure (10X or less)
  • Macrocracks
  • Macroetch indications
• Microstructure
  • Microcracks
  • Plastic deformation
  • Phase transformation
  • Intergranular attack
  • Pits, tears, laps, protrusions
  • Built-up edge
  • Melted and redeposited layers
  • Selective etching
• Microhardness

Set-2: Standard SI data set (SDS)

• Minimum SI data set
• Fatigue tests (screening)
• Stress corrosion tests
• Residual stress and distortion

Set-3: Extended SI data set (EDS)

• Standard SI data set
• Fatigue tests (extended to obtain design data)
• Additional mechanical tests
• Tensile
• Stress rupture
• Creep
• Other specific tests (e.g., bearing performance, sliding friction evaluation, sealing properties of surfaces)

How surface finish differs from surface integrity?

Surface finish is one parameter of surface topography that indicates smoothness of a solid surface. It is one qualitative approach of designating quality of a surface of manufactured product. On the other hand, surface integrity has a broad domain and consists of all surface topography parameters as well as surface and sub-surface metallurgical parameters. Integrity encompasses surface finish, micro-crack, plastic deformation, phase transformation, melted and redeposited layers, microhardness, residual stress, distortion, creep, etc. Therefore, surface finish is just one element of surface integrity set.

Scientific comparison among surface finish and surface integrity of machined products is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Kaynak et al. (2014); Cryogenic Machining-Induced Surface Integrity: A Review and Comparison with Dry, MQL, and Flood-Cooled Machining; Machining Science and Technology.
2. Bellows and D. N. Tishler (1970); Introduction to surface integrity; General Electric.

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Accordingly, all arc welding, gas welding, resistance welding and intense energy beam welding processes fall within the envelop of fusion welding. In arc welding, an electric arc is constituted between a pointed electrode and the conductive base metals. This arc is the prime source of heat for melting faying surfaces and filler metals. There exist quite a few such processes—all of them follow the same basic principle but substantially vary with respect to procedure, benefits, limitations and feasible areas of application. Shielded metal arc welding (SMAW), gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW) are three such arc welding processes—each of them offers certain advantages over others. Differences between SMAW, GMAW and GTAW welding processes are discussed below.

Consumable and non-consumable electrode: Electrode is integral to every arc welding process for establishing and maintaining electric arc. Such electrodes can be two types—consumable and non-consumable. A consumable electrode melts down during welding due to arc heating and subsequently deposits on weld bead that finally becomes an integral part of the joint. Contrary to this, a non-consumable electrode does not melt down during welding and remains intact. A particular arc welding process utilize only one type. With respect to consumable and non-consumable nature of electrode, comparison among SMAW, GMAW and GTAW welding processes is enlisted below.

• SMAW—Consumable electrode.
• GMAW—Consumable electrode.
• GTAW—Non-consumable electrode.

Application of filler metal: Filler metal is required to supply for filling up the root gap. When root gap is substantially larger (>2mm) and/or edge is prepared then filler should be applied. With consumable electrode, the electrode itself melts down and deposits on weld bead. Thus no additional filler is required to supply. Such electrode behaves as filler and feeding rate of electrode can be controlled to manipulate filler deposition rate. On the contrary, non-consumable electrode supplies no filler on weld bead. Thus if filler is required then it must be supplied separately.

• SMAW—No additional filler is required. Electrode acts as filler.
• GMAW—No additional filler is required. Electrode acts as filler.
• GTAW—Electrode is non-consumable. So additional filler in the form of small diameter rod is supplied, only when it is required, by constantly feeding it below the arc column.

Continuous nature of electrode/filler: The filler material, either in the form of electrode or a separate one, continuously melts down and deposits on weld bead. Thus its length gradually shortens with welding time. If this filler length is short then it has to be replaced frequently by a new one. This reduces rate of production and interrupts the process. On the other hand, a long filler can be continuously fed to the welding zone for longer duration without any interruption. Such method is productive but requires bulk storage of costly filler.

• SMAW—Filler-cum-electrode is in the form of small diameter straight rod of length 2 – 3ft. Thus it requires frequent changing and interruption of process.
• GMAW—Filler-cum-electrode is in the form of small diameter wire that is wound on a wire pool. Quite long wire electrode is stored in that pool and thus welding can be carried out continuously for longer duration without intermediate stoppage for electrode changing. This electrode is continuously fed by means of mechanized arrangements.
• GTAW—Electrode is non-consumable. Additional filler usually comes in the form of short, small diameter rod and thus requires frequent stoppage for changing the filler. However, welding can be carried out continuously if no filler is utilized.

Preferable welding mode: Arc welding can be performed in three different modes. As mentioned earlier, filler is not necessary to supply when root gap is very small or base materials are thin. When welding is carried out without applying any filler, it is termed as autogenous mode. However, if filler is applied and metallurgical composition of filler is similar to that of parent component, then it is termed as homogenous mode. If metallurgical composition of filler substantially differs from that of parent component, it is termed as heterogeneous mode. Different welding processes are suitable for different modes.

• SMAW—Filler is inherent to this process because of consumable electrode. So autogenous mode is not possible. It is suitable for mainly homogenous welding.
• GMAW—Similar to SMAW, filler is inherent to this process because of consumable electrode. So autogenous mode is not possible. It is suitable for homogenous and heterogeneous welding.
• GTAW—Electrode is non-consumable. So autogenous mode is feasible; in fact, TIG welding is suitable for this mode only. However, it can also be applied for homogenous and heterogeneous welding by utilizing optimum set of process parameters.

Electrode material: Electrode material of every arc welding process must possesses few basic characteristics like good electrically conductive, good electron emissivity, desired melting point, etc. It is worth mentioning that filler metal must be compatible with parent metal otherwise they will not mix up properly leading defective welding. Thus with consumable electrode, electrode material should be chosen based on compatibility with base metal. With non-consumable electrode, filler material should be chosen based on compatibility with base metal, whereas electrode should be made of such material with high melting.

• SMAW—Electrode is mostly made of ferrous materials. It has only few variety in terms of electrode material. Thus it is suitable for homogeneous joining of ferrous components only.
• GMAW—Wide variety of electrode materials are available in market. Although most electrodes are ferrous, their metallurgical composition can be varied to harness desired result.
• GTAW—This electrode is made of tungsten only. This is irrespective of base metal or filler metal as electrode is non-consumable. Tungsten has highest melting point (3422°C). Other desired properties can also be manipulated by adding alloying elements in small proportions. For example thorium, lanthanum oxide, cerium oxide, zirconia, etc. are added with tungsten for improving various welding characteristics like electron emissivity, electrode erosion, etc.

Coated or bare electrode: Electrode can be coated to protect it from oxidation or atmospheric contamination. Apart from protection against oxidation, coating also provides other advantages such as supplying shielding gas, reducing spatter, stabilizing arc, inducing chemical elements into weld bead, etc. However, a coated electrode is costly and prone to damage with time. Different processes utilize different types of coating, each having desired function.

• SMAW—Utilizes thick flux coated electrode. Apart from protecting the electrode, this flux supplies shielding gas.
• GMAW—No flux coating is available on electrode. However, a thin coating of stable material is applied to protect electrode material from oxidation.
• GTAW—Utilizes bare tungsten electrode. No coating is applied on electrode.

Shielding gas supply: Shielding gas is supplied in arc welding to dispense the oxygen from welding zone and create an envelope of inert gases surrounding the weld bead. Its primary function is to protect hot weld bead from oxidation. Such shielding gas can be supplied directly from a gas cylinder or indirectly by disintegrating other chemical elements during welding.

• SMAW—Flux coating of electrode disintegrates during welding and produces shielding gas. No separate shielding gas is applied separately.
• GMAW—Shielding gas (inert or active) is supplies from gas cylinder.
• GTAW—Inert shielding gas is supplied from gas cylinder.

Spatter problem: Spatter is small droplets of molten filer metal that is produced due to scattering of arc and comes out from the welding zone. This spatter causes loss of filler metal and thus non-uniform filler deposition rate that sometime leads to various welding defects including negative reinforcement and dimensional inaccuracy. It also hampers appearance and requires grinding after welding for its removal.

• SMAW—Produces excessive spatter even with optimum set of process parameters.
• GMAW—It also produces spatter; however, can be reduced by utilizing optimum set of process parameters.
• GTAW—It is mostly free from spatter.

Manual and automation: Shielded metal arc welding is carried out manually and thus it is also called manual metal arc welding (MMAW). Gas metal arc welding can be automated easily where electrode wire is continuously fed rom the spool using mechanised arrangement and at the same time torch is moved by another automatic vehicle. Gas tungsten arc welding is commonly performed manually; however, can be automated also, especially the movement of torch. An automated process is fast and more productive; but manual process is more flexible and virtually possess no restriction of location for its application.

Productivity and quality issues: SMAW does not offer good quality joint. Thus it is carried out mostly for household and general industrial requirements. Frequent changing of electrodes causes interruption in process and thus it is not suitable for longer welding requirement. GMAW is highly productive and can be carried out continuously for long duration. It can be automated easily. Its volume deposition rate is also very high. Thus it is suitable where wide root gap exists, edges are prepared in U or V shapes, longer joining requirements or even for cladding. Although it is less prone to defects, its joint quality is not very good. Spatter also hampers weld bead appearance. In terms of quality, GTAW is best among the three. It provides superior joint with splendid appearance. It is less prone to defects, but deposition rate or welding rate is comparatively low.

Scientific comparison among shielded metal arc welding (SMAW), gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW) is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

• Gas Metal Arc Welding Handbook by W. H. Minnick (2007, Goodheart Willcox).
• Basic TIG & MIG Welding (GTAW & GMAW) by I. H. Griffin, E. M. Roden and C. W. Briggs (3^rd edition, Delmar Cengage Learning).
• Shielded Metal Arc Welding by W. L. Ballis (2011, Xulon Press).

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Apart from three crucial cutting parameters namely cutting velocity, feed rate, and depth of cut, machining performance also relies on a large number of factors. Cutter geometry, orientation and material are three notable parameters associated with cutting tool that can influence overall machining performance. Cutter geometry encompasses various geometrical features of the tool, which includes, but is not limited to, rake angle, clearance angle, inclination angle, cutting angles, nose radius, and edge radius. Judicial selection of these parameters is indispensably necessary to obtain best result for specific material under given conditions.

What is rake angle of SPTT? Why different names?

It is one important element of cutter geometry that indicates inclination of rake surface of the cutter. Chip produced during machining flows over the rake surface until it goes out of the machining zone; so rake angle indicates chip flow direction. By definition, rake angle is the angle of inclination of tool’s rake surface from reference plane and measured on some other plane. Reference plane is the plane perpendicular to cutting velocity vector.

Rake surface may be plane surface or curved surface, based on type of cutter. For example, rake surface of turning tool is plane but that for drill is curved one. Although rake surface of turning tool is a plane, it is not parallel to reference plane. It has a three dimensional orientation with respect to reference plane. On the other hand reference plane is a fixed plane and its angle with rake surface is termed as rake angle. Consequently rake angle will vary based on the direction or plane on which this angle is measured. Rake angle is given name based on the plane on which it is measured. For example: side rake, back rake, orthogonal rake, normal rake, maximum rake, etc.

What are orthogonal rake and normal rake?

Both these rake angles indicate inclination of rake surface from standard reference plane; however, they are measured in two different directions or planes. Orthogonal rake angle is measured on orthogonal plane; while, normal rake is measured in normal plane.

A cutting plane is one that is perpendicular to reference plane and contains principal cutting edge of the turning tool. Another imaginary plane, which is perpendicular to both reference plane and cutting plane, is called orthogonal plane. Therefore, cutting plane, reference plane, and orthogonal plane are mutually perpendicular. Rake angle measured on this orthogonal plane is called orthogonal rake angle. It is one feature of Orthogonal Rake System (ORS) of turning tool designation. It is commonly employed in different analysis. Measuring this angle is comparatively easy. However, use of this angle may pose restriction in certain cases (like tool grinding) if inclination angle is not zero.

Such limitations can be eliminated by using normal rake angle. This angle is measured on normal plane. Normal plane is one plane that is perpendicular to both cutting edge and cutting plane. It may not be perpendicular to reference plane. Therefore, normal plane, cutting plane and reference plane are not mutually perpendicular. They will be mutually perpendicular only if inclination angle is zero. Normal rake is one feature displayed in Normal Rake System (NRS) of turning tool designation. Measuring this angle is quite difficult but use of this angle is beneficial and error-free in many cases.

Orthogonal rake is larger than normal rake

For non-zero (either positive or negative) inclination angle, orthogonal rake angle will be comparatively larger than normal rake angle. This can be attributed to the conversion formula: Ratio of tangents of normal rake and orthogonal rake is equal to cosine of inclination angle.

When they are equal? One is measured on orthogonal plane, while other is measured on normal plane. Angle between orthogonal and normal plane is basically inclination angle. Therefore, if inclination angle becomes zero then normal plane will merge with orthogonal plane. In such a case cutting edge will be parallel to reference plane and consequently orthogonal rake and normal rake will be same.

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Once again, arc welding encompasses a large group of welding processes, each having varying capability in terms of joint quality, workable material, feasible position, welding time and productivity. Notable examples include manual metal arc welding, gas metal arc welding, gas tungsten arc welding, flux core arc welding, submerged arc welding, etc. In every arc welding process an electric arc is constituted between the electrode and conductive parent components under the presence of sufficient potential difference. This arc is the prime source of heat for fusing faying surfaces of components. SMAW and TIG both are arc welding processes and follow the same basic principle; however, each has different capability and applications.

Shielded metal arc welding (SMAW), also called manual metal arc welding (MMAW), is one arc welding process in which arc is established between a consumable electrode and base metals. It is mostly carried out manually. The consumable electrode supplies filler to fill root gap, thus no additional filler is required. Electrode is coated with suitable flux for protection against corrosion. This flux also disintegrates during welding to produce shielding gas. Tungsten inert gas (TIG) welding, formally known as gas tungsten arc welding (GTAW), is also one arc welding process where arc is established between a non-consumable electrode and base metals. Thus filler, if required, can be supplied additionally by feeding a filler rod at welding zone. Inert shielding gas is also supplied from additional source to protect weld bead. While SMAW is more flexible and has diverse applications, TIG can fetch sound, reliable, defect-free and apparently good joints. Various differences between SMAW and TIG welding processes are given below in table format.

Table: Differences between SMAW and TIG welding processes

Consumable and non-consumable electrode and filler metal: A consumable electrode is one that melts down during arc welding and supply necessary filler metal to fill root gap between parent components. Thus no additional filler metal is required to supply. Contrary to this, a non-consumable electrode does not melt down during welding and thus filler is required to supply additionally if required. Filler may not require if root gap is too small or base plates are thin. Shielded metal arc welding utilizes a consumable electrode and thus the electrode supplies filler. Tungsten inert gas welding is carried out with non-consumable electrode and therefore filler is supplied by feeding additional filler rod at the welding zone whenever required.

Electrode material: Every arc welding electrode must be electrically conductive so as to establish arc by the avalanche of flow of electrons between electrode tip and base plate surface. SMAW electrode comes in the form of small diameter rod of length 2 – 3ft. Composition of this electrode metal is more or less similar to that for parent metals. Therefore SMAW electrode comes in different materials and the compatible one can be selected primarily based on parent material. Contrary to this, TIG welding electrode is independent of work material as it does not fuse and deposit on weld bead. TIG electrode is made of titanium (highest melting point), with some alloying elements like thorium, lanthanum oxide, cerium oxide, and zirconia in order to improve various welding characteristics such as electron emissivity, electrode erosion, etc.

Three modes of welding: As mentioned earlier, filler material is not necessarily required during welding, especially when root gap is very small or base plates are thin. However, if filler is applied, its metallurgical composition must be compatible to that of base materials. Arc welding can be carried out in three different modes—autogenous, homogenous and heterogeneous modes. When joining is carried out without filler, it is termed as autogenous mode. When filler is applied and filler composition is almost similar to base metal composition then it is termed as homogenous mode. If filler composition substantially differs from base metal composition then it is termed as heterogeneous mode. Since filler is inherent to SMAW process (consumable electrode), so it cannot be carried out in autogenous mode. It is particularly suitable for homogenous mode; while, TIG is particularly suitable for autogenous mode. However, TIG can also be satisfactorily applied for other two modes; however, compatible filler is to be supplied additionally at a constant rate (process will be complicated).

Electrode coating and shielding gas: Arc welding electrodes can be either coated or bare. Coating is usually provided to protect electrode from corrosion. Flux coated electrode offers another benefit. During welding, this flux disintegrates to produce enough inert gas which can displace oxygen from welding zone and subsequently protects hot weld bead from oxidation or other contamination. Such shielding is indispensably necessary to obtain defect-free welding. If electrode is bare then shielding gas is required to supply from external source (such as gas cylinder) with the help of suitable accessories. SMAW utilizes a flux-coated electrode and thus flux supplies shielding gas during welding. Contrary to this, TIG welding employs a bare tungsten electrode and thus shielding gas is required to supply externally.

Welding spatter: In context of arc welding, spatter is the tiny droplets of molten filler metal that come out of the welding zone mostly due to scattering of electric arc. This spatter causes loss of costly filler metal as well as reduces filler deposition rate. It also tend to make the arc unstable causing difficulty in maintaining it. Unwanted deposition of metal droplets surrounding the weld bead also hampers appearance and sometimes requires additional post-processing like grinding. Certain arc welding processes are inherent to spatter formation, even with optimum set of parameters. Shielded metal arc welding is one such method that is always associated with spatter. However, spatter level can be minimized using optimum process parameters, properly cleaning parent metal surface and controlling environment in a desirable way. TIG welding is usually free from spatter and spatter related defects.

Manual and automatic operation and joint quality: Shielded metal arc welding is carried out manually and thus it is also called manual metal arc welding. Therefore maintaining constant arc length, weld direction, feed rate etc. are quite difficult; but such parameters influence on quality and strength of joint. Accordingly weld quality depends on experience and capability of welder. However, initiation and maintaining the arc is quite easy. No filler is also required to supply additionally. Thus the process is comparatively easier. Tungsten inert gas welding can be carried out manually else an automated system can be employed. Feeding filler metal at a constant rate is a bit difficult; but maintaining the arc is more complicated. Although process is quite difficult and requires experienced welder, TIG welding can easily produce sound, reliable and defect-free joint.

Scientific comparison among SMAW and TIG welding processes is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

• Tungsten inert gas (TIG or GTA) welding by TWI-Global.com.
• Gas Metal Arc Welding Handbook by W. H. Minnick (2007, Goodheart Willcox).
• SMAW, GMAW, and TIG Welding Comparison by weldingschool.com.

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Cutting tool, also called cutter, is a wedge shaped and sharp edged device that compresses a thin layer of workpiece material to shear it off the form of chips during machining. During machining, the machine tool actually holds the cutter and job, and at the same time it imparts desired relative motions (speed, feed, depth of cut). Therefore, cutter is a part and parcel of every conventional machining operation; however, its shape and size may vary depending on the feature to be produced and operation employed. Whatever be the geometry of cutter, it must consists of a sharp cutting edge to swimmingly remove material requiring minimum effort. A cutting edge is basically one straight or curved edge produced by intersection of any two tool point surfaces (rake surface, principal flank surface and auxiliary flank surface). Based on the number of cutting edge present, cutters can be classified as single point cutter and multi point cutter.

A single point cutter contains only one main cutting edge that can actively participate in material removal action during machining. Thus one cutting edge removes entire volume of material in a single pass. Contrary to this, a multi-point cutter consists of at least two cutting edges and all of them can equally participate in material removal action in a single pass. Thus chip load per cutting edge reduces significantly. Turning, shaping, planning, boring, fly cutting, etc. operations are performed using single point cutter; while, milling, drilling, knurling, reaming, etc. utilize multi-point cutter. A multi-point cutter can contain from two (drill or end-mill) to hundreds of cutting edges (abrasives of grinding wheel). Various differences between single point cutter and multi point cutter are given below in table format.

Table: Differences between single point and multi point cutters

Number of cutting edges: As the name suggests, single point cutting tools consist of only one main cutting edge. This edge participates in material removal action throughout the pass during machining. In the next pass, either the same cutting edge can be utilized or it can be replaced with a new sharper one. It is worth noting that such cutters may contain more than one cutting edges at a time on the tool; however, only one will engage during machining (for example consider insert based turning tool that usually contains 3 or 4 cutting edges present on the tool body at a time but only one partakes in cutting). Contrary to this, multi-point cutters contain more than one cutting edges and all (or most of them) actively participate in shearing action in a single pass.

Contact between cutter and workpiece and its consequence: When machining is carried out using single point cutter, only one cutting edge remains in continuous contact with the workpiece. This causes sharp increase of tool temperature and the result is thermal damages of cutter like enhanced rate of wear, plastic deformation, degraded tool life, etc. Thus proper precautions (like smaller speed, feed and depth of cut, application of coolant, insulated coating on cutter, etc.) must be taken to get rid of such damages. On the other hand, in multi-point cutter either all (example drilling, reaming, etc.) or only few engage with workpiece (example hobbing, broaching, etc.). Usually cutters with higher number of edges, remove material by simultaneous engagement and disengagement of cutting edges. This protects the cutter from overheating by allowing sufficient time to dissipate heat during disengagement period. However, intermittent cutting can increase vibration and unbalanced forces.

Chip load: It is axiomatic that during machining the cutter compresses a thin layer of work material and gradually shears if off. So in every instant, cutter movement is restricted by an area of work material, which is expected to be removed. This area of workpiece material just ahead of the cutter at a particular instant is called chip load. Mathematically it can be expressed by the multiplication of feed and depth of cut (s×t) and can be expressed either in terms of per unit time or per revolution. In case of single point cutter, entire chip load is borne by only one cutting edge; while, in multi-point cutter entire chip load is distributed among all cutting edges and thus each edge is subjected to significantly smaller chip load.

Provision for higher speed, feed and depth of cut: With single point cutter, if higher speed, feed and depth of cut are employed then cutting edge will subject to higher chip load and thus tool may fail prematurely by catastrophic breakage. With multi-point cutter, higher speed, feed and depth of cut can be employed without any palpable problem. Since material removal rate (MRR) is proportional to cutting velocity, feed rate and depth of cut (MRR = 1000V.s.t), so such cutters can provide higher MRR, which consequently help in improving productivity.

Breakage of cutting edge: While machining with single point cutter, in case of unplanned breakage of one cutting edge due to certain inexorable reason (like inhomogeneity in work material, vibration, chip clogging, malfunctioning of machine, etc.), the operation is mandatorily required to stop to replace the cutter with a new one. However, in similar case with multi point cutters (i.e., breakage of one edge), the operation can be carried out without much problem with the help of other intact cutting edges. However, it may not be possible in all scenarios, especially when tool consists of fewer cutting edges.

Design and fabrication: Due to simple geometry, single point cutters are easy to design and can also be fabricated easily as compared to multi point cutters.

Scientific comparison among single point cutters and multi point cutters is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

• Machining and Machine Tools by A. B. Chattopadhyay (1^st edition, Wiley).
• Manufacturing Engineering and Technology: SI Edition by S. Kalpakjian and S. R. Schmid (7^th edition, Pearson Ed Asia).

The post Difference Between Single Point Cutter and Multi Point Cutter appeared first on Difference Box.

Conventional machining processes mandatorily utilize a wedge-shaped cutting tool (also called cutter) to remove material in the form of chip from workpiece by shearing. Geometry, orientation and material are three important factors associated with every cutter that directly influence overall machining performance. For uninterrupted removal of material, cutter material has to be sufficiently harder than work material. Geometry of cutter, another crucial factor, encompasses various features such as tool point surfaces and their inclinations, location of cutting edges, sharpness of cutting edge and nose, etc. Every cutter consists of at least two tool point surfaces—rake surface and flank surface.

Rake surface is the chip flowing surface. Chips, which are produced during machining, continuously flow over the rake surface before leaving the cutting zone. Thus severe rubbing takes place between the chip face and rake face and as a result intense heat generates in that zone (called secondary deformation zone). Its inclination from reference plane, measured by rake angle, is influences many relevant parameters like shear deformation, chip thickness, cutting force, power consumption, etc. Apart from rake surface, every cutting tool should have at least one flank surface. Intersection of rake surface and flank surface produces cutting edge. Unlike rake surface, which remains in intimate contact with the flowing chips, flank surface remains open in adjacent with finished surface. However, due to presence of nose radius and edge radius, a tiny contact between machined surface and flank surface may occur. Various differences between rake surface and flank surface of a cutting tool are given below in table format.

Table: Differences between rake surface and flank surface

Contact with chip or finished surface: Rake surface remains in physical contact with the flowing chips during machining; however, it does not touch the finished or machined surface. On the other hand, chip does not touch the flank surface but finished surface touches the flank surface in a tiny portion due to presence of nose radius and edge radius. Such contact helps in smoothening scallop or feed marks; however, longer contact between machined surface and flank surface can hamper finishing quality. Thus sufficient gap (provided by clearance angle) has to be maintained mandatorily in between them to avoid rubbing.

Rake angle and clearance angle: These indicate inclination of tool point surfaces from standard plane or direction. By definition, rake angle is the angle of inclination of cutter’s rake surface from reference plane and measured on some other plane. Reference plane is a plane which is perpendicular to cutting velocity vector. On the basis of orientation of rake surface with respect to reference plane, rake angle may be wither positive, or negative or zero. Similarly, clearance angle is the measure of cutter’s flank surface from cutting velocity vector and measured on some other plane. However, clearance angle cannot be zero or negative, it must have a positive value. In both the cases, angle value may be different based on the plane on which it is measured.

Flank face remains exposed: As mentioned earlier, chips flow over the rake surface and thus it remains in close contact with flowing chips. However, flank surface always remain open as it neither touches chip nor touches finished surface (except a tiny portion at tip). However, flank surface may touch machined surface if cutter has worn out (flank wear), and in such scenario quality of machined surface will degrade steeply because of rubbing.

Flattening the feed marks: Scallop marks, which develops on finished surface due to presence of feed velocity, increase surface roughness and subsequently reduce finishing quality. Higher feed rate results rough surface; however, feed cannot be made zero because it is one of the two formative motions (other one being cutting velocity) indispensably necessary for every machining operation. A tiny contact between the flank surface and finished surface at tool tip due to presence of nose radius and edge radius inherently help in smoothening the surface by compressing scallop marks. Higher is the nose radius, better will be surface finish (i.e., low height of scallop marks); however, it may adversely impact other parameters. Rake surface has no direct role in improving surface finish.

Heat generation: As chips flow over rake surface, intense heat generation occurs due to rubbing. About 60 – 70% of total cutting heat arises due to this rubbing at secondary deformation zone. However, majority of generated heat is carried away from cutting zone by the moving chip. This protects the cutter as well as workpiece from overheating and other thermal damages. Contrary to this, only a fraction (below 5%) of total cutting heat generates due to tiny contact at tip. However, this heat partially flows inside the workpiece via finished surface and rest flows into cutter. Thus heat generated at tertiary rubbing zone can lead to thermal damages it exceeds certain limit.

Wear and its impact: Continuous rubbing accelerates abrasive wear rate of rake surface and thus crater wear occurs quickly. Although it changes chip flow direction and influences cutting force and other relevant parameters, a small crater wear is tolerable. However, flank wear directly influence accuracy of machined component and a tiny amount of wear can also lead to inaccuracy machining. Thus cutter life is conventionally determined by the allowable limit of flank wear (commonly it is limited to 0.3mm as per Taylor’s tool life formula).

Scientific comparison among rake surface and flank surface is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Machining and Machine Tools by A. B. Chattopadhyay (1^st edition, Wiley).
2. What is Clearance Angle in Cutting Tool? Its Derivative, Value and Function by minaprem.com.
3. Image source: minaprem.com.

The post Difference Between Rake Surface and Flank Surface of Cutting Tool appeared first on Difference Box.

Geometry of a cutter indicates inclination or orientation of various tool point surfaces. Cutting tool consists of three tool point surfaces—rake surface, primary flank surface and auxiliary flank surface. Various angles are used to indicate inclination of such surfaces in different directions. There also exist several national and international standards that clearly define various features of a cutter including various angles. Such information is gathered in a particular fashion to present in the form of tool signature. Several planes are also used to assist unambiguous measurement of these angles. In general, inclination of rake surface is indicated by rake angle; while inclination of flank surface is indicated by clearance angle.

By definition, rake angle is the angle of orientation of the rake surface of cutter from reference plane and measured on another plane. It may have positive, negative or even zero value; however, usually varies in between +15° to –15°. It is one crucial angle that determines strength of tool tip, cutting force, power consumption, shear deformation, and also machinability. On the other hand, clearance angle is the angle of inclination of the flank surface of cutter from velocity vector and measured on another plane. It must have a positive value and usually ranges between +3° to +15°. Various differences between rake angle and clearance angle of the cutting tool are given below in table form.

Table: Differences between rake angle and clearance angle

Basic purpose: Rake angle displays the inclination of rake surface of the cutting tool from reference plane. Since rake surface is the chip flowing surface, so rake angle also indicates chip flow direction. On the other hand, clearance angle displays the inclination of flank surface of the cutting tool from cutting velocity vector. This angle has no direct influence on chip flow. However, both the angle can be measured on different planes (directions) to get clear ides about inclination of corresponding surfaces.

Value of these angle: A cutter may have positive, negative or zero rake angle, each has special significance and advantages on machining performance. For example, a positive rake offers sharp cutting edge and thus shearing will occur smoothly requiring minimum effort. Alternatively a negative rake offers stronger tool tip and thus tool can resist higher cutting force. However, clearance angle of the cutter cannot be negative or even zero as, in such scenarios, flank surface of tool will rub with the finished surface of the product. Typically rake angle varies in between +15° to –15° and clearance angle varies in between +3° to +15°.

Role on chip deviation, product quality and accuracy: Rake angle directly influences chip flow direction and shear deformation of chips. A negative rake will increase shear deformation and thus chip thickness will increase. Chip reduction coefficient, abbreviated as CRC, will also be higher with negative rake. However, it has insignificant role on quality and dimensional accuracy of the machined component. Clearance angle plays crucial role in such factors. Lower clearance angle can severely hamper surface quality due to extreme rubbing between finished surface of job and flank surface of cutter.

Mechanics of machining and role of rake angle: Rake angle directly or indirectly influence a large number of factors including shear strain, cutting force, machining torque or thrust, power consumption, type and color of chips, etc. In fact it is one important parameter in entire mechanics of machining. Clearance angle plays insignificant role in such analysis.

Built up edge (BUE) and its effect: While machining ductile materials with longer chip-tool contact, an embryo of work or chip material may adhere to the tip of tool at most favorable location and subsequently grows until it becomes larger and carried away the flowing chips. Such unwanted presence of material on tool tip changes rake angle to highly negative one. As a result shear deformation and cutting force may increase, which are usually undesirable. However, clearance angle remains unaltered by BUE.

Scientific comparison among rake angle and clearance angle is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

• Machining and Machine Tools by A. B. Chattopadhyay (1^st edition, Wiley).
• What is Rake Angle in Cutting Tool? Names, Effects, Functions & Values by minaprem.com.
• Image source: www.minaprem.com.

The post Difference Between Rake Angle and Clearance Angle of Cutting Tool appeared first on Difference Box.

Rake angle of a cutter basically indicates inclination of rake surface. Since rake surface is the chip flowing surface, so rake angle also indicates chip flow direction (in orthogonal cutting). This is one crucial parameter as it directly or indirectly influences shear deformation, chip flow direction, cutting force, power consumption, machinability, etc. By definition, rake angle is the angle between cutter’s rake surface and reference plane and measured on some other plane. Based on the plane on which this angle is measured, it may have different names, such as orthogonal rake (measured on orthogonal plane), side rake (measured on machine longitudinal plane), back rake (measured on machine transverse plane), maximum rake (measured on a plane perpendicular to master line for rake), etc.

Irrespective of the plane or direction it is measured, rake angle can be either positive or negative or even zero. This concept evolved from relative orientation of rake surface with respect to fixed reference plane—in one direction it is considered as positive, in other direction it is considered as negative and when they merge together rake angle is considered as zero. A positive rake occurs when sum of wedge angle and clearance angle is below 90° on a particular plane. It offers a sharp cutting edge and thus can efficiently shear off material from workpiece requiring less force. Now if sum of wedge angle and clearance angle becomes equal to 90° on a particular plane, then rake angle becomes zero. Similarly, when sum of wedge angle and clearance angle is more than 90° on a particular plane, rake angle becomes negative. Negative rake offers stronger tool tip and thus enhanced tool life. Various differences between positive rake and negative rake are given below in table format.

Table: Differences between positive rake and negative rake

Concepts of flank angle, wedge angle and rake angle: Sectional view of the wedge of a cutting tool displays these three different angles; however, none of them has fixed value. Different cutting tools may have different values of these angles and the same plays key role on machining performance. Reference plane is assumed to be perpendicular to cutting velocity vector. Angle between cutting velocity vector and flank surface of the cutting tool is called flank angle or clearance angle. Wedge is produced due to intersection of the rake surface and flank surface along the cutting edge; and the corresponding angle is termed as wedge angle. Now the angle between rake surface and reference plane is termed as rake angle. With respect to the reference plane, when rake surface is oriented towards cutting velocity vector then corresponding angle is considered as positive; when rake surface is oriented opposite to cutting velocity vector then corresponding angle is considered as negative; and when rake surface merges with reference plane then corresponding angle is considered as zero.

Wedge thickness and rake angle: Since algebraic sum of rake angle, flank angle and wedge angle has to be zero, so for constant flank angle, wedge angle will be less with positive rake as compared to negative rake. Thus negative rake offers thicker wedge, which is capable of sustaining higher cutting force.

Shear deformation and chip thickness: During machining, cutter compresses a thin layer of material and gradually shears it off. Consequently uncut chip plastically deforms to produce chip and then flows over the rake surface before leaving machining zone. Shear deformation also increases thickness of the chip from its uncut value. This increment is measured by one coefficient, called Chip Reduction Coefficient (CRC), which is defined as the ratio of chip thickness to uncut chip thickness. CRC is always greater than 1; however, lower value is always desirable. Cutter with negative rake deforms the chip to a great extent resulting higher chip thickness and higher CRC. However, cutter with positive rake allows smooth flow of chip over rake surface that ultimately results in comparatively lower shear deformation and lower CRC. However, in no case CRC can be lower than 1, unless the cutting is in micro-scale or nano-scale.

Cutting force and power consumption: The lower the cutting power consumed during machining a particular material, the better will be the machining performance. Although force and power depend on many factors, rake angle also has meagre influence. Due to lower shear deformation and lower CRC with positive rake, cutting force required for removing a particular area of material is much lower than that with a cutter having negative rake when all other machining conditions remain unaltered.

Machinability aspect: Machinability is the indication of how easily a work material can be machined using a particular cutting tool under specified conditions. It cannot be reliably measured quantitatively; can only be judged qualitatively based on other parameters including shear deformation, CRC, cutting force and power, cutting temperature, chip form and color, BUE formation tendency, machined surface condition, etc. Overall, a positive rake tends to offer better machinability.

Strength of cutting edge and chip load: A negative rake emerges better in this case as it comes with thicker wedge. Increased wedge thickness improves strength of cutting edge and thus it can sustain higher load without failure. Thinner wedge, associated with positive rake, may break if chip load increases above certain limit; and as a result, larger volume of material cannot be removed in a single pass (lower material removal rate or productivity). It may also possess a risk of catastrophic failure under machine vibration or impact loading, which frequently occurs while machining rotationally non-symmetric parts or cutting workpiece having inhomogeneous material.

Suitable work materials: Since negative rake can endure higher cutting force and impact loading, so it can be safely applied while cutting hard and brittle material like tool steel, stainless steel, titanium, etc. When material composition is non-homogeneous or there is a chance of impact loading then negative rake is better choice. Positive rake works well while cutting softer and ductile materials like capper, aluminum, etc.

Scientific comparison among positive rake angle and negative rake angle is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

• Machining and Machine Tools by A. B. Chattopadhyay (1^st edition, Wiley).
• Difference Between Positive Rake and Negative Rake by difference.minaprem.com.
• Image source: minaprem.com.

The post Difference Between Positive Rake Angle and Negative Rake Angle appeared first on Difference Box.

Therefore, turning and milling both are conventional machining operations; however, each of them has different capabilities and thus is suitable for fabricating different features by removing material from workpiece. Turning is extensively used for generating cylindrical or conical surface. Here the primary blank (pre-machined raw material) can be of any shape (based on available fixture and machine capability). It employs a single point turning tool to shear of a thin layer of material in the form of continuous or discontinuous chips. This operation is performed on a well-known and versatile machine tool, called lathe. Based on the machined features, turning can be sun-grouped as straight turning, taper turning, internal turning, stepped turning, etc. However in all cases generated surface is either cylindrical or conical.

Although milling is also a machining operation like turning, its process technique, capability and shape of generated feature differ from turning. While turning produces cylindrical or conical surface, milling is useful for generating flat or sculptured surfaces. It employs a multi-point cutting tool that may contain 2 – 150 cutting edges. Thus milling is a faster process that results higher productivity. The operation is performed on a milling machine that can have either horizontal spindle or vertical spindle. Based on generated features and technique, milling can also be sub-grouped as face milling, side milling, end milling, etc. Various differences between turning and milling are given below in table format.

Table: Differences between turning and milling

Generated surface and feature: Turning is predominantly used for generating cylindrical or conical surfaces, irrespective of the shape of raw material. Any feature generated by this process must be rotationally symmetric. Typically it can be employed for fabricating axisymmetric parts like cylindrical or conical shafts, stepped shafts, tapered objects, increasing diameter of internal holes (cannot originate a hole), cutting cylindrical groves, etc. Contrary to this, milling can be employed to generate flat surface of any orientation as well as contour surfaces. Features like slots, channels, key holes, pockets, walls, fins, webs, etc. are commonly encountered in milling.

Machine tools employed for operations: In metal cutting domain, machine tool is a specific category of machine that imparts all necessary motions to the job and/or cutter as well as provides facilities for mounting them. Different machining operation is performed in different machine tools; however, certain machine tool is also capable of performing a number of similar operations. Turning is performed in a versatile machine tool called lathe. This is primarily horizontal axis machine tool (spindle is horizontal); however, vertical axis lathe is also available, especially table top one. Milling is carried out in milling machine, which can have either vertical axis or horizontal axis (both are common).

Single point and multi-point cutter: A cutting tool (or cutter) may contain only one or more number of active cutting edges that can participate in material removal action during machining. Based on the number of cutting edges, cutters can be classified as single point or multi-point cutters. Turning operation is performed using a single point cutter, called SPTT (single point turning tool). So it has only one main cutting edge that can actively participate in shearing or removing of materials. Contrary to this a milling cutter may contain 2 – 150 cutting edges, sometimes even more. Obviously it is one multi-point cutter as more than one cutting edge simultaneously participate in shearing or removing of materials.

Cutting motion and feed motion: There are two relative motions between the workpiece and cutter whose simultaneous action causes shearing of a layer from work material. These two are called primary motions as majority of cutting power is absorbed by it; however, there may be other secondary motions based on feature or surface to be produced. It is the function of machine tool to impart such relative motions in intended direction at pre-determined rate. In turning, rotation of workpiece at a fixed cutting speed actually provides required cutting motion, while movement of tool carriage (on which cutter is mounted firmly) provides required feed motion. In milling it is completely opposite—cutting motion is provided by rotation of cutter at a fixed rpm while feed motion is provided by moving the work table (on which workpiece is mounted).

Continuous and discontinuous cutting and corresponding chips: During turning operation, cutter continuously remains in close contact with the workpiece and thus results in continuous chips. Although type of chip relies on many predictable and unpredictable factors, turning has the capability of producing continuous chips under favorable cutting conditions. Milling inherently produces discontinuous chips because no single cutting edge remains in contact with the workpiece; instead, cutting edges repeatedly engage and disengage throughout the operation.

Chip load and productivity issues: Area of the work material ahead the cutting tool at any instance is termed as chip load. It is proportional to the material removal rate (MRR). In milling, due to participation of more number of cutting edges in every revolution, higher chip load can be utilized because entire chip load will be divided equally among all the cutting edges. Thus higher cutting parameters, especially feed and depth of cut, can be employed in a single pass, and thus larger volume of material can be removed in a specific time. In turning, values of cutting parameter cannot be increased after certain level because of changes of tool failure. Thus milling is highly productive as compared to turning when bulk removal of material is primary concern.

Scientific comparison among turning and milling is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

• Machining and Machine Tools by A. B. Chattopadhyay (1^st edition, Wiley).
• Manufacturing Engineering and Technology: SI Edition by S. Kalpakjian and S. R. Schmid (7^th edition, Pearson Ed Asia).

The post Difference Between Turning Process and Milling Process appeared first on Difference Box.

Conventional machining processes utilizes a wedge shaped cutting tool (or cutter) to gradually remove excess material from workpiece in the form of chips. Such processes include turning, threading, facing, drilling, tapping, boring, milling, shaping, planing, reaming, knurling, etc. Metallic cutter was prominent in early days; but now many non-metallic cutters are readily available. Function of a cutter is to compress a layer of material ahead of it and gradually shear it off to realize material removal. Usually the workpiece and cutter are rigidly mounted on the concerned machine tool and necessary relative velocities (speed, feed and depth of cut) are imparted on them to facilitate material removal. Every machining operation is carried out using a specific type of cutter; however, several features of a particular cutter may vary within certain limit. Cutters should have a defined geometry and material.

Abrasive cutting processes also perform similar task, that is removing material in the form of chips, but no metallic cutter is employed. Abrasive cutters are primarily made of refractory ceramic materials, mainly abrasives. These tiny sharp abrasives actually remove material from workpiece. Although such cutter as a whole has certain specifications, abrasives have random geometry. Typical examples of such processes include grinding, honing, lapping, etc. All these processes are suitable for finishing rather than bulk removal. Grinding utilizes a cylindrical wheel, called grinding wheel, made of abrasives as its cutting tool. Abrasive material, abrasive size, binding material, and other relevant parameters of grinding wheel are pre-defined; however, geometry of abrasives is not defined. Various differences between cutting tool and grinding wheel are given below in table format.

Table: Differences between cutting tool and grinding wheel

Single point and multi-point cutter: A cutting tool may consists of one or more active cutting edges within the cutter body. A single point cutting tool is one that has only one main cutting edge to participate in material removal action at a time. If more than one cutting edge simultaneously participate in cutting action, then that cutter is termed as multi-point cutter. A single point cutter is cheaper and easy to fabricate but cannot take high chip load and thus process becomes less productive. Turning, shaping, planing, boring, fly cutting, etc. utilizes single point cutter; while, milling, drilling, reaming, broaching, hobbing, grinding, etc. employ multi-point cutter. Therefore, a cutting tool may be single point or multi-point; but grinding wheel is always multi-point cutter.

Material of cutters: A cutting tool can be made of various materials like carbon steel, high speed steel (HSS), tungsten carbide, ceramics, diamond, cubic boron nitride (cBN), etc. Different materials display varying capability and thus a compatible one is usually selected based on work material, machining process and other concerned parameters to achieve best performance. Grinding wheel is made of abrasives like alumina, silica and diamond. Such abrasive grits are bound in another soft or hard medium like resin, metal, etc. It is worth mentioning that material removal is realized by abrasive particles only; however, only few number of abrasives (about 1% among all that are exposed on periphery of wheel) participate in material shearing action in a pass.

Cutter geometry: Geometry, orientation and material are three important parameters that governs overall machining performance in every machining. Cutting tool should have a well-defined geometry; in fact, cutter is selected on the basis of such geometry. Geometrical features include rake angles, clearance angles, cutting angles, nose radius, etc. Grinding wheel usually has defined specification, but abrasives don’t have any specific geometry. Various tool angles are random and varies greatly among abrasives. Such random geometry sometimes affect machining performance (like increase in force or temperature).

Scientific comparison among cutting tool and grinding wheel is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

• Machining and Machine Tools by A. B. Chattopadhyay (1^st edition, Wiley).
• Manufacturing Engineering and Technology: SI Edition by S. Kalpakjian and S. R. Schmid (7^th edition, Pearson Ed Asia).

The post Difference Between Cutting Tool and Grinding Wheel appeared first on Difference Box.

By definition, machining or metal cutting is one of the secondary manufacturing processes by which layer by layer material is gradually removed in the form of chips from a preformed blank in order to obtain desired shape, size and finish. To fulfill this demand, there exist various machining processes like turning, threading, tapering, chamfering, spinning, tapping, necking, filleting, facing, grooving, parting, knurling, drilling, milling, shaping, planning, slotting, boring, hobbing, broaching, etc. Such processes can provide higher material removal rate (MRR) and thus are suitable, productive and economic for bulk removal of work material. A hard and sharp cutting tool or cutter is mandatorily employed for removing material by shearing. This cutting tool should also possess a defined specification and compatible material for uninterrupted and efficient machining. Most of these conventional processes can machine a large variety of materials; however, certain work materials don’t provide acceptable machinability and therefore other material removal processes (such as abrasive cutting or NTM) are recommended in those cases.

Grinding, one type of abrasive cutting process, can fulfill various limitations of conventional machining. Here a grinding wheel is utilized instead of so called cutting tool. The wheel is basically made of small size and harder abrasive particles, like alumina, silica, diamond, etc., which are bonded in a suitable medium. Such abrasives have arbitrary shape and thus they lack a definite geometry; although the wheel itself has specific configuration. Although material is removed in the form of chips, here the chips are micro-sized. This process is not suitable for bulk removal of materials; instead, it is preferred for finishing the surface in micron level (0.5 – 2.0µm). It can also efficiently ground hard and tough materials. Various differences between conventional machining and grinding are given below in table form.

Table: Differences between machining and grinding

Bulk removal vs. finishing process: Material removal rate (MRR) is defined as the rate of volume of work material removed from work surface during any cutting process. Most conventional machining processes (except few like knurling) are used for removing larger amount of material to impart a basic shape and size. It provides higher MRR and thus more productive. It can also semi-finish the product surface at the level of 1 – 50µm based on operation and corresponding process parameters. Grinding, on the other hand, is primarily employed for finishing the surface is much better level. A surface roughness in the order of 0.5 – 2µm is easily feasible with grinding. So it can provide high dimensional accuracy and close tolerance.

Cutting tool – its material and geometry: Every conventional machining process mandatorily employs a cutting tool (also called cutter) having specific geometry and material. This cutter contains one or more sharp cutting edges to efficiently shear off material from work surface. Material of this cutter is also important factor to decide machining performance. Most importantly hardness of the cutter material must be significantly higher than that of work material. Starting from high speed steel (HSS), carbide and ceramic, now-a-days cubic boron nitrate (cBN) and diamond cutters are readily available. Grinding utilizes the wheel instead of cutter and the abrasives on the wheel actually removes material. Unlike cutter, such abrasives have no defined geometry (various angles, edge radius, nose radius, length of cutting edge, etc. vary arbitrarily); however, abrasive material might be fixed like alumina, silica or diamond.

Rake angle and clearance angle: Rake angle of a cutting tool indicates inclination of rake surface from reference plane. It is one important factor that influences shear deformation, chip flow direction, chip thickness, cutting force, shear strain, power consumption, etc. Cutters, used in conventional machining, may have positive, negative or even zero rake angle and its value usually varies in between +15° to –15°. Abrasives of the grinding wheel have abrupt rake angle and can vary from +60° to –60°, sometimes even beyond this limit. Such high rake angle is usually undesirable because it causes imbalance in cutting and increases either cutting power consumption (high positive) or tool failure (high negative). Unlike rake angle, clearance angle of a cutter cannot be negative or zero—it should have a positive value. Quality and tolerance of finished surface depends on this angle. For cutters clearance angle usually lies in between +3° to +15°; while for abrasives it can be arbitrary (it can be zero or as high as +90°).

Shearing and participation of cutting edges: A cutter may consists one or more cutting edges and accordingly they can be either single point cutter or multi point cutter. Irrespective of the number, each of the cutting edges equally participate in material removal action. Moreover, in machining, material is removed by shearing of a thin layer of material when sufficient compressive force is applied by the cutter. In grinding wheel, only a few exposed abrasives (sometime even below 1%) participate in material removal action. Rest of the abrasives either do not touch the work surface (note that infeed is very low, even below 10µm) or they cause scratching, ploughing or rubbing instead of shearing. However, material is removed only by shearing; others just increase normal force undesirably.

Specific energy consumption: Cutting energy required to remove unit volume of material is termed as specific energy, measured in J/mm³. Mathematically power divided by MRR gives specific energy. Conventional machining provides high material removal rate (MRR) and thus specific energy is comparatively low. On the other hand, in grinding, MRR is low and most of the energy is wasted due to scratching, ploughing or rubbing instead of shearing. Accordingly specific energy increases sharply; even it can be 5 – 20 times higher.

Surface damage by heat generation: In conventional machining, majority of the cutting heat is generated in secondary deformation zone where intense rubbing between chip and rake surface takes place. Primary shear zone also contribute to some extent. However, most of the generated heat (70 – 80%) is carried away by the continuously flowing chips and only tiny portion goes inside the cutter or workpiece. Thus thermal damage of workpiece and cutter is usually insignificant, especially when appropriate cutting fluid is applied. In grinding, heat generation occurs mainly due to scratching, ploughing and rubbing. Such heat accumulates within the workpiece because abrasives are thermally insulator and very small volume of micro-chip forms. Extreme heat accumulation may lead to various thermal damages to the finished surface including surface burning, changes in mechanical properties, dimensional inaccuracy due to thermal expansion, etc.

Machining hard and tough materials: Such materials possess lot of challenges when processed by conventional machining processes like high tool wear, fragmented chips, etc., which ultimately lead to poor machinability. Grinding can be advantageously applied irrespective of toughness and hardness of the work material.

Scientific comparison among conventional machining and grinding is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

• Machining and Machine Tools by A. B. Chattopadhyay (1^st edition, Wiley).
• Manufacturing Engineering and Technology: SI Edition by S. Kalpakjian and S. R. Schmid (7^th edition, Pearson Ed Asia).
• Difference Between Machining and Grinding by difference.minaprem.com.

The post Difference Between Machining and Grinding appeared first on Difference Box.