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).

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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).

The post Difference Between SMAW, GMAW and GTAW Welding Processes appeared first on Difference Box.

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.

The post Difference Between Orthogonal Rake and Normal Rake of Turning Tool appeared first on Difference Box.

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.

The post Difference Between SMAW and TIG Welding Processes appeared first on Difference Box.

On the basis of temperature at which protruding portion of rivets are hammered, riveting can be grouped into two categories—hot riveting and cold riveting. In hot riveting, the rivet end is heated by some external means (like flame heating) before hammering. Heating temperature lies around 2/3^rd of melting point of the rivet material. Due to such heating, the material becomes soft and plastic and thus lower upsetting force is required. So when rivet material is hard, like stainless steel, hot riveting is preferred as lower force is required. It is also favorable for large diameter rivets, usually diameter larger than 10mm. Thermal expansion of rivet due to heating also has important role in gripping strength.

On the contrary, cold riveting is performed at room temperature only. Here rivet is not heated and thus hammering is carried out at room temperature. So comparatively higher force is required for upsetting; however, no heat source is desired for heating rivets. Heating time is also not associated with it, so the process is comparatively faster. However, if rivet diameter is large or it is made of stronger material, then large amount of hammering force is desired. Various differences between hot riveting and cold riveting are given here in table form.

Table: Differences between hot riveting and cold riveting

Heating the rivets and heat source: Hot riveting is performed by heating the protruding end of rivets to an elevated temperature (50 – 70% of melting point of rivet material) and thereafter upsetting that end to make another head. Thus it requires a pointed and high energy density heat source to quickly apply heat. Usually fuel or gas flame is utilized for such purpose. Cold riveting, however, does not require any heating for hammering and thus heat source is irrelevant to it.

Riveting time: Hot riveting takes more time due to heating of rivets just prior to hammering (upsetting). Cold riveting is performed at room temperature, so heating time is not associated to it. Thus it is faster, economic and more productive.

Stress on rivets, tightness and leakage: Rivet joint always experiences shear stress as force is usually applied along the components or perpendicular to the rivet axis. It is irrespective of the way riveting is performed (hot or cold). In fact, shear failure of rivets is important design criteria while designing rivet locations. Apart from shear stress, every rivet is also subjected to tensile stress when hot riveting is performed. This tensile stress is independent of external load on components. Like every material, rivets material also undergoes volumetric expansion during heating. Since free end of rivet is hammered at hot condition, so free contraction is restricted and thus tensile stress develops when it cools down. This tensile stress helps gripping components tightly in a leak-proof way. However, no such tensile stress is induced within rivets when cold riveting is performed as no heating is performed and thus no volumetric expansion or contraction is associated with it.

Hammering force: Engineering materials can be transformed from elastic state to plastic state either by applying sufficient force or by increasing temperature to certain level. A solid material in plastic state becomes very ductile and requires significantly smaller force (stress) for strain (change in dimension). In hot riveting, the rivet tail becomes plastic due to heating at elevated temperature and thus smaller hammering force is required. In cold riveting, the rivet material remains in elastic state as no heating is performed and thus comparatively higher hammering force is desired to bulge the tail.

When hot riveting is desired? If rivet material is hard, like stainless steel, then hot riveting is preferred otherwise large hammering force has to be applied. Consequently when rivet material is soft, like aluminum or brass, then cold riveting can be performed. From dimension point of view, if rivet diameter is larger than 10mm then hot riveting is recommended as it will require lesser force during upsetting. For smaller diameter rivets, cold riveting can be carried out.

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

• Difference Between Hot Riveting and Cold Riveting by difference.minaprem.com.
• Introduction to Machine Design by V. B. Bhandari (2017, McGraw Hill Education India Private Limited).

The post Difference Between Hot Riveting and Cold Riveting appeared first on Difference Box.

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.

In order to stabilize the arc and also to protect the hot molten metal pool beneath this arc from oxidation and other contamination, proper shielding gas is used to shield or cover entire welding zone surrounding this arc. This shielding gas can be chemically inert one or can contribute in many relevant properties by actively participating in the welding process. Accordingly GMAW can be classified into two groups—Metal active gas (MAG) and Metal inert gas (MIG) welding. So MIG and MAG both are basically variants of GMAW process; only difference lies on the shielding gas used in those processes.

As the name suggest, Metal Inert Gas (MIG) welding process utilizes suitable inert gas for shielding purpose during welding. Such gas is mainly argon or helium, or a mixture of these two in different proportions. Since these gases are chemically inert thus they remain stable even at extreme arc heat. Therefore they do not contribute in altering any weld characteristics apart from protecting the weld bead and electric arc from any external influence.

On the other hand, Metal Active Gas (MAG) welding utilizes an active gas mixture as shielding gas. For example, a suitable mixture of carbon di-oxide (CO₂) and oxygen (O₂) along with other comparatively stable gases like argon, helium, nitrogen, etc. Besides fulfilling basic requirement of shielding gas, such active gases can break down due to arc heating and subsequently induce various chemical elements on weld bead that can enhance joint properties. It also contributes in stabilizing arc, reducing spatter level, etc. Various differences between MIG welding and MAG welding are given below in table format.

Table: Differences between MIG welding and MAG welding

Characteristics of shielding gas: As the name suggests, metal inert gas (MIG) welding utilizes only inert gas like argon or helium. Such gases remain stable even at extreme arc temperature. Metal active gas (MAG) welding utilizes a mixture of inert gas and active gases as shielding gas. Such active gas mainly includes oxygen and carbon di-oxide. The mixing proportion between inert and active gases can significantly vary based on many parameters like base metal and its thickness, filler metal, root gap, welding polarity, intended properties of weld bead, etc. Sometimes environmental condition also governs this proportion.

Capability of altering properties of weld bead: Inert shielding gases remain stable during welding and thus do not induce any chemical elements on weld bead. However, active gases can break down under extreme arc temperature and can subsequently induce relevant chemical elements on weld bead. This leads to change in chemical and mechanical properties of joint. For example, while joining low carbon steel (like mild steel) using carbon di-oxide rich shielding gas, carbon inclusion may occur and this can enhance surface hardness of joint. Therefore, MIG welding cannot alter weld bead properties; while, MAG can do the same.

Cost of shielding gas and application area: In GMAW, shielding gas flow rate in the order of 10 – 20L/min is utilized. Cylinders, filled with industrially pure shielding gas, is comparatively costlier and thus MIG welding is one costlier process. It is primarily used for welding non-ferrous materials like aluminum. Oxygen based active gas is not favored when parent materials are non-ferrous because of high chances of oxidation. In this sense, MAG is economical and preferred for welding of ferrous metals, especially stainless steel.

Scientific comparison among metal active gas (MAG) and metal inert gas (MIG) welding is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

• Metal Inert Gas (MIG) / Metal Active Gas (MAG) Welding by linde-gas.com.
• What is the difference between MIG and MAG by twi-global.com.

The post Difference Between Metal Inert Gas and Metal Active Gas Welding appeared first on Difference Box.

Gas metal arc welding (GMAW) is one economic joining process in which coalescence is formed by melting faying surfaces and filler metal by means of electric arc constituted between consumable electrode and conductive parent metal. Since electrode is a consumable one so it is continuously fed at a constant rate by means of a mechanized wire feeder. So it cannot be performed in autogenous mode as consumable electrode is one integral part of the process. Suitable shielding gas (inert or active) can be used for protecting the weld bed from contamination. However, filler metal can be deposited on the weld bead at a faster rate and thus this process is productive and economic. If performed properly using optimum set of parameters, it can produce a sound and reliable joint.

Gas tungsten arc welding (GTAW), popularly known as tungsten inert gas (TIG) welding, is one sophisticated fusion welding process where joining is realized by coalescence formation due to fusion of faying surfaces. Here electric arc is constituted between conductive base material and non-consumable tungsten electrode. Since electrode is non-consumable one, filler material is required to apply separately when sufficient root gap exists. It is also favorable for autogenous mode of welding where no filler is applied. Shielding gas, preferably inert gas like argon, is also applied for protecting the hot metal pool during welding from atmospheric oxygen. Although is the process is comparatively slower, its capability of joining diverse metals and producing strong and reliable joints makes it one favorable process in many applications. Splendid weld bead appearance and defect-free joint are also two important advantages. GMAW differs from GTAW in many aspects and their differences are given below in table form.

Table: Differences between GMAW and GTAW welding processes

Consumable and non-consumable electrode: During arc welding, the electric arc is established between an electrode and conductive work materials. High heat density of this arc melts down faying surfaces of the parent components as well as filler metal if applied. When an electrode fuses and consequently deposits on weld bead during welding, it is termed as consumable electrode. In other word, when electrode is consumed for providing filler, it is called consumable electrode. In gas metal arc welding (GMAW), the electrode is consumable type as it melts down due to arc heating and subsequently deposits on weld bead. Contrary to this, in gas tungsten arc welding process (GTAW) process, electrode remains intact under intense arc heating. Since it does not melts down to deposit necessary filler metal, so it is non-consumable type. Consequently, life of GTAW electrode is higher than that for GMAW electrode.

Composition of electrode material: In GMAW, metallurgical composition of electrode (or filler) is more or less same with that of parent components, which are to be joined. GTAW always utilizes a pointed tungsten electrode, irrespective of the composition of parent material. However, few alloying elements (for example thorium, lanthanum oxide, cerium oxide, zirconia, etc.) are also added with tungsten for improving various welding characteristics like electron emissivity, electrode erosion, etc.

Possibility to perform in autogenous mode: An autogenous mode of welding is performed without applying any filler metal. Here root gap is maintained minimum, usually zero. During welding, only faying surfaces are fused by heating via electric arc and are allowed to cool down. The fused material of the two surfaces gets mixed and upon cooling it creates the weld bead. GMAW process inherently deposits filler metal on weld bead as it utilizes a consumable electrode. So it cannot be performed in autogenous mode. Since TIG welding utilizes a non-consumable electrode, so it can be advantageously performed in autogenous mode. In fact, it is one preferred welding process for joining in autogenous mode.

Application of filler metal externally: No filler metal is required to apply from external source in GMAW process as the electrode itself acts as filler metal. In TIG welding, however, filler can be applied separately if desired. Although TIG is preferred for autogenous mode joining, it can also be performed in homogeneous mode or heterogeneous mode. Filler in the form of small diameter rod can be fed to the weld zone beneath the electric arc at a constant pre-determined rate. This filler rod melts down due to arc heating and deposits necessary filler. Composition of this filler rod can be similar to that of work material or can be different; however, it must be compatible with parent metal otherwise defective joint will be produced.

Inert and active shielding gas: Shielding gas is used to protect hot weld bead from atmospheric oxygen by creating a protecting layer surrounding the entire weld zone. It also, directly or indirectly, help reducing spatter level, stabilizing arc and improving weld bead properties. This shielding gas can be inert or an active gas also. An active shielding gas can show better capability in certain situations. Such gases can also induce chemical elements on weld bead to improve various mechanical properties of the joint. GMAW process can utilize both types of shielding gas; and accordingly it can be classified into two groups—metal inert gas (MIG) and metal active gas (MAG). MIG utilizes only inert gas like argon, helium, etc. MAG utilizes active gas like carbon di-oxide, oxygen, etc., usually mixed with inert gas in different proportions. On the other hand, GTAW or TIG welding process uses inert gas only, predominantly argon.

GMAW is faster process compared to TIG: In GMAW process, electrode in the form of small diameter wire and wound in a pool is continuously fed by a proper mechanized arrangement. So filler can be deposited at a faster rate and consequently this welding process is highly productive as compared to TIG welding.

Spatter level and appearance: Spatter is small droplets of molten filer metal that is produced due to scattering of arc and come 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. Many arc welding processes produce spatter including GMAW. It cannot be performed in spatter-free way even if optimum set of process parameters are used and proper welding technique is employed. TIG welding usually does not produce spatter unless the work material surface is not clean. Weld bead produced by TIG welding is smooth and superficially attractive.

Proficiency of welder: GMAW is quite easier to perform as most motions are automated. Even the movement of torch can be automated using suitable arrangement. Establishing arc is also easier. Compared to this, TIG welding is one sophisticated process and thus it requires experienced welder to carry out welding smoothly without arc blow or undesired arc termination. Establishing arc is quite crucial as tungsten electrode may stick to the work surface leading to tungsten inclusion defect.

Scientific comparison among 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.

• Tungsten inert gas (TIG or GTA) welding by TWI-Global.com.
• 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).

The post Difference Between GMAW and GTAW Welding Processes appeared first on Difference Box.

In arc welding, an electric arc is established in between parent metal and electrode. This arc is prime source of heat to melt down faying surfaces of base metal to form coalescence. Since base metals are required to fuse to achieve joining, so all arc welding processes are basically fusion welding. There are several arc welding processes such as manual metal arc welding (MMAW), gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), submerged arc welding (SAW), flux core arc welding (FCAW), electro slag welding (ESW), etc. All of them are based on same principle in terms of joining technique, but their capability and process are different. Each of them offers certain advantages over others.

Gas metal arc welding (GMAW) is one highly productive fusion welding process where consumable electrode is continuously fed to the welding zone from a wire spool using an automated system. Arc, constituted between electrode and base metals under the presence of sufficient potential difference, fuses electrode at a faster rate and subsequently deposits on root gap to perpetuate the process. Proper shielding gas is also supplied to protect high temperature arc and surrounding areas from oxidation. Based on shielding gas property, GMAW can be of two types—metal active gas and metal inert gas. In metal inert gas (MIG) welding, a chemically inert gas (like argon, helium, etc.) is used for shielding purpose; while in metal active gas welding, a chemically active gas (like carbon di-oxide or oxygen) is mixed with inert gas to use for shielding purpose.

Gas tungsten arc welding (GTAW), popularly known as tungsten inert gas (TIG) welding, is a versatile and reliable fusion welding process that utilizes a non-consumable tungsten electrode to constitute the electric arc. Filler metal, if needed, can also be supplied externally by feeding a filer rod into welding zone. Unlike MIG welding, TIG welding is not suitable for high filler deposition rate; however, the quality of joint and appearance of weld bead is much better. Therefore both MIG and TIG welding are fusion welding processes where heat is supplied by an electric arc and both of them utilize inert gas as shielding gas. However, they are different in certain ways including the process and capability. Various differences between MIG welding and TIG welding are provided below in table format.

Table: Differences between MIG welding and TIG welding

Consumable and non-consumable electrode: A conductive electrode is mandatory in every arc welding process to constitute the arc. Sometimes this electrode itself deposits molten metal in root gap between base plates. A consumable electrode is one which can melt down by arc heat to deposit filler during welding. On the contrary, a non-consumable electrode is not expected melt down during welding and thus filler metal is required to supply externally whenever intended. In MIG welding, the consumable electrode melts down by arc heat and subsequently supplies filler metal. Thus electrode is continuously fed into welding zone at a pre-defined rate. TIG welding is performed using a non-consumable electrode and thus it does not melt down to supply filler. In case filler is desired, the same is supplied additionally by feeding a small diameter filler rod beneath the arc.

Electrode material: Filler material should be compatible with parent material, otherwise defective welding will be produced. With MIG welding, filler metal (same as electrode metal) can be chosen according to base metal. So electrode can be made of wide variety of metals, each one is usually suitable for a small group of base metals. In TIG welding, electrode is always made of tungsten because of its strength, high melting temperature and good shape retention capability. Sometimes few alloying elements are also added to (for example thorium, lanthanum oxide, cerium oxide, zirconia, etc.) with tungsten for improving various welding characteristics like electron emissivity, electrode erosion, etc.

Filler deposition rate and productivity: In MIG welding process, electrode in the form of small diameter wire that is wound in a pool is continuously fed by a proper mechanized arrangement. So filler can be deposited at a faster rate and consequently this welding process is highly productive as compared to TIG welding. Thus MIG welding is suitable when edge is prepared in V or U shaped or root gap is more.

Spatter level and appearance: Spatter is small droplets of molten filer metal that is produced due to scattering of arc and subsequently emerge out from the welding zone. This spatter causes loss of filler metal that leads to non-uniform filler deposition rate. It also destabilize the arc. Abruptly deposited molten metal droplets also hamper appearance and sometimes require grinding for its removal. Many arc welding processes produce spatter including GMAW (both MIG and MAG). Although MIG tends to produce low level spatter, it cannot be performed in spatter-free way even if optimum set of process parameters and proper welding technique are employed. TIG welding usually does not produce spatter unless the work material surface is not clean. Weld bead produced by TIG welding is clean, smooth and attractive.

Autogenous, homogenous and heterogeneous modes: On the basis of filler metal application and its composition, welding can be classified as autogenous, homogenous and heterogeneous modes. An autogenous mode of welding is performed without applying any filler metal. When root gap is virtually zero or very small then filler does not require. In homogeneous mode of welding, filler is applied and composition of filler is more or less same with that for parent metal. Filler is also applied in heterogeneous welding mode but composition of filler differs substantially from that of parent metal. Since consumable electrode is inherent to MIG welding, so it cannot be performed in autogenous mode. Contrary to this, TIG is suitable and preferred for such purpose. TIG can be also applied advantageously for homogenous and heterogeneous modes with optimum set of parameters.

Possibility for overhead joining: Welding position includes down-hand, inclined, overhead, etc. Overhead joining position is very difficult to execute as filler is required to deposit against to gravity. Molten metal pool always has a tendency to fall down and it can even injure welder. Thus gravitational force alone is not suitable for proper filler deposition; in fact, it imposes restriction. Lorentz force helps in such situation. TIG welding with optimum set of parameters can be employed for overhead welding. However, it requires experienced welder to achieve good penetration and welding quality.

Scientific comparison among metal inert gas (MIG) welding and tungsten inert gas (TIG) welding is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

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

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What is joining? Why it is required?

Joining is a part and parcel to every manufacturing process as it facilitates easy, efficient and economic fabrication of products. Complex shaped objects or high quality objects cannot be produced directly by casting. For research purposes or prototype development costly casting methods (like investment casting) can be employed to directly obtain desired product; but such a strategy will not be economical in industrial aspect where large scale production is required. To maintain production cost minimum, a basic shaped object is primarily made by casting and further processing are performed on it to impart intended features and functionalities as well as to improve upon surface finish and dimensional tolerance. Joining is one such post-processing technique which is employed to assemble individual components or structures to ultimately build a larger one.

By definition, joining is one of the manufacturing processes that is used to assemble two or more solid components either temporarily or permanently. Joining can be realized by utilizing additional elements (such as filler metal, rivet, strap, nut-bolt, screw, cotter, key, etc.) and/or taking assistance of external input (such as heat, pressure, shock wave, etc.). Not only metals, joining can be carried out for plastic, ceramic and composite also. To join a wide variety of materials in several ways, there exist quite a few joining processes, which can be broadly classified into two categories—temporary joining and permanent joining.

Temporary joints—As the name suggests, such joints are not meant to keep fixed. Such a joint allows easy and quick dismantling of assembled structures without breaking them. It is beneficial for repairing, maintenance and inspection purposes. There exist few joining processes that offer temporary joints, which include fastening, press fit, key joint, cotter joint, knuckle joint and few adhesive joints.

Permanent joints—Such joints are meant to stay fixed for longer duration and don’t require frequent disassembly. Any dismantling of assembled structures can only be realized by rupturing the components. Thus it imposes restriction for repairing, maintenance and inspection. However, such joints are reliable, strong and leak-proof. They offer high load carrying capacity and longer performance. Joining processes that offer permanent joints include welding, soldering, brazing, riveting, coupling, most adhesives, etc.

What is fastening? It is a temporary joining process

Fastening is one temporary joining process that employs additional elements to mechanically assemble or attach two or more parts together. Additional mechanical elements that are used for fastening is called fasteners. However, threaded elements like nut-bolt, screw, etc. are commonly considered as fasteners as it can provide reliable and sound joint. The term fastening, however, encompasses a larger group of fasteners. Apart from threaded fasteners, clamp, clip, ring, button, chain, pin, nail, metallic latch, hook, lobster, rope, cable, hose, flange, buckle, etc. are also fastening elements.

How fastening differs from joining?

Joining is a generic term that indicates assembly of solid parts by any means. As mentioned earlier, there are various joining processes and each of them has different capabilities. Fastening is one temporary joining process that is overwhelmingly employed for non-permanent joining of metals and wood components. In conclusion it can be said that fastening is one type of joining process, others being welding, riveting, coupling, adhesive, etc.

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Temporary joining processes are all those joining processes that allow easy dismantling of joined components without rupturing them. It facilitates assembly and disassembly of solid structures as and when required without harming them. All fasteners basically provide temporary joints. Such joining is beneficial for inspection, maintenance and repair purposes as joint can be dismantled easily without breaking parts. However, they are prone to failure under vibration because of loosening of joining elements. Also strength of joint is not very high and thus these processes are not suitable for heavy load applications.

Such problems can be eliminated by utilizing a permanent joining process. By definition, a permanent joint is one that does not allow disassembly of joined components without rupturing them. Welding, riveting, coupling, etc. are common examples of such process. It can provide a sound, reliable, leak-proof and sufficiently strong joint and thus can be safely used for heavy load applications. However, it possesses problem during inspection and maintenance. Various differences between temporary joining and permanent joining are given below in table format.

Table: Differences between temporary joining and permanent joining

Possibility of dismantling assembled parts: As the name suggests, a temporary joint allow easy and quick disassembly of joined components without breaking them. Contrary to this, permanent joints are intended to stay in fixed position for longer duration. Such joints are permanent in nature and cannot be dismantled without breaking assembled parts.

Suitability: Since temporary joint allows disassembly without damaging parts, so it is preferred where frequent assembly and disassembly are desired. This makes replacing damaged parts easier. Interchangeability is one of the most crucial features offered by this joining. Permanent joining is suitable in those applications that require a strong, reliable and leak-proof joint and at the same time the joint is expected to stay in assembled condition for longer duration; for example window frame.

Strength of joint: Most temporary joining processes provide weaker joint that cannot bear higher load under service condition. Usually joining strength is considered to be lower than that of the parent components. Permanent joining processes usually provide sound, reliable and stronger joint. Joint strength is comparable with the strength of parent components and mostly these are equal.

Leak-proof joining: Permanent joining processes, if carried out properly using optimum process parameters, can provide leak-proof joints. This feature is very useful in many applications including pipe joining, pressure vessel joining, joining  of structural parts that are intended to stay within water as in case of ships, etc. However, the capability of sustaining pressure or force during service life varies greatly on different factors including joining process employed. Temporary joining processes do not usually provide leak-proof joints.

Biggest advantage of temporary joint: Since assembled parts can be dismantled without breaking them, so inspection, repair and maintenance are easier and at the same time cost and time efficient. Non-destructive testing (NDT) can be employed for inspection purposes. So no parts get damaged during testing or maintenance service. No permanent joint offers this advantage and thus destructive testing method is frequently employed for inspection and maintenance purposes. So parts get damaged and thus scrap rate increases and as a consequence overall cost of production also increases.

Examples of these joining processes: Fasteners are the fast and foremost example for temporary joints. It includes, but not limited to, threaded elements like nut, bolt, screw, etc., clips, buttons, clamps, cables, nails, hooks, rings, bands, staples, etc. Apart from fasteners, joining by press fit, cotter joint, knuckle joint, etc. are also temporary joining elements. On the other hand, welding is one overwhelmingly accepted permanent joining process. Except this, rivet joint, coupling, soldering, brazing, etc. are considered as permanent joining processes. Most adhesive bonding also affix parts permanently.

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

1. Difference Between Temporary Joint and Permanent Joint by difference.minaprem.com.

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Fusion welding processes are those where faying surfaces of base metal are fused by applying heat to form coalescence for realization of joining; whereas, no such melting talks place in solid state welding processes. All arc welding, gas welding, resistance welding, and intense energy welding processes are basically fusion processes. In arc welding, an electric arc is constituted in between parent components and electrode by supplying sufficient potential difference in between them. This arc is the prime source of heat (thermal energy) to melt down base plates and filler. Once again there are various arc welding processes; for example, MMAW or SMAW, GMAW (MIG and MAG), GTAW or TIG, SAW, FCAW, ESW, etc. Each of them offer certain benefits over others.

Manual metal arc welding (MMAW), also called shielded metal arc welding (SMAW), is one fusion welding process where electric arc is constituted between a electrode and base plates. This welding is mostly carried out manually and thus the name. The consumable electrode is covered with suitable flux that disintegrates during welding to produce shielding gas and slag layer, which help protecting the arc and molten metal pool from oxidation or contamination. Thus it does not require application of shielding gas separately. Gas metal arc welding (GMAW) is also one fusion welding process where the arc is constituted between consumable electrode and parent components. The electrode, in the form of wire, is continuously fed to the welding zone from a wire spool using a mechanized system and at the same time appropriate shielding gas is supplied from external source to protect the arc and surrounding regions. GMAW is very fast process with high filler deposition rate. Various differences between manual metal arc welding (MMAW) and gas metal arc welding (GMAW) processes are given below in table format.

Table: Differences between MMAW and GMAW welding processes

Intermittent and continuous process: A small diameter (0.5 – 2.0mm) rod of length 30 – 50cm and coated with suitable flux is used as electrode in manual metal arc welding. Since this electrode is consumable so its length shortens with welding time because of its deposition on weld bead. Thus after certain interval (when flux coated part ended), electrode is required to be replaced by a new one to carry out welding. Thus MMAW requires frequent stoppage and is one intermittent process. On the contrary, a consumable electrode (in the form of wire) is supplied continuously from a wire pool in gas metal arc welding. This wire pool can store sufficient length of wire (it is usually measured by its weight). Thus GMAW can be carried out for longer duration without any pause for electrode changing.

Sources of shielding gas: Shielding gas is indispensably necessary in arc welding processes to protect the arc and molten metal pool from oxidation or other contamination. During arc welding, a thick layer of shielding gas encloses the entire welding zone and restricts atmospheric air to come in contact with weld bead and surrounding hot areas. In MMAW process, the electrode is coated with a flux which disintegrates by the welding heat and produces enough shielding gas to cover up heated areas. So no shielding gas is required to supply additionally. But in GMAW, no such flux coating exists on the electrode. Thus shielding gas is supplied from additional sources (like gas cylinder) using proper delivery pipeline and nozzle.

Slag deposition and its removal: MMAW utilizes a flux coated electrode and this flux actually produces intended shielding gas during welding. Flux also produces slag that deposits at the top surface of weld bead and protects it from contamination. But this slag layer is required to remove after welding is over in order to improve appearance. Usually grinding is adopted for such removal. However, if slag remains trapped within the weld bead and fail to float out at surface then defects like slag inclusion are observed. Such defects can reduce load carrying capacity, joint strength and make it vulnerable under corrosion—all of these ultimately reduce service life. GMAW is free from slag as no flux coated electrode is used. Thus it eliminates defects related to slag and also requirement of post-processing for slag removal.

Productivity: MMAW is not highly productive. In multi-pass welding, slag deposited on weld bead is required to remove completely after every pass so as to avoid slag inclusion defect. Moreover, electrode is required to change frequently. So it is not suitable if high volume of molten metal is required to deposit on weld bead. Thus it is not productive for multi-pass welding. GMAW is free from slag and also no electrode changing is desired. So it can deposit large volume of filler at short period of time. Thus it is perfect choice when root gap is more, edge is prepared in U or V shaped and/or plate thickness is more. Moreover, GMAW electrode diameter is smaller than that for MMAW, which increases arc current density and thus filler deposition rate.

Flexibility of welding: Flexibility indicates ability of a welding process to accommodate various shapes to join in diverse ways in different conditions. Indirectly it refers capability and feasibility to apply a particular process under certain conditions. GMAW requires shielding gas cylinder and pipelines and accessories for its controlled delivery. So it is not suitable for outdoor small scale applications. MMAW can be applied virtually for every location in all positions within the reach of electrode; however, its performance may not be at same level in all scenarios. Even though MMAW is not productive, it is highly flexible and has diverse applications.

Welding quality and dependency on welder: As the name suggests, manual metal arc welding is mostly carried out by human operators. So welding quality relies on the skill and experience of welder. It is also susceptible to human error including random and chance errors. Contrary to this, GMAW can be automated and requires little intervention of welder. So it can provide better quality joints if appropriate parameters are employed.

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

1. Arc welding processes by magmaweld.com.
2. Shielded Metal Arc Welding by W. L. Ballis (2011, Xulon Press).
3. Gas Metal Arc Welding Handbook by W. H. Minnick (2007, Goodheart Willcox).

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As a consequence of extensive development throughout the last few decades, TIG welding has emerged as one promising and reliable welding technique for permanently joining two or more metallic components. It can be performed in autogenous mode; however, filler material can also be applied as and when intended (both homogenous and heterogeneous modes are possible). Sumptuous appearance of weld bead, higher arc efficiency, lesser chance of defect and minimum spatter level made this process a favorable fabrication technique in a wide range of industrial applications including construction, automobile and aerospace arenas.

In TIG welding, electric arc is stuck between the non-consumable electrode (made of tungsten with small alloying elements) and the conductive workpiece. This arc heat melts down the faying surfaces of the parent components, which ultimately produces coalescence. Filler metal, if applied, also deposits on the root gap in molten state due to arc heating. Edge preparation can also be carried out if plate thickness is more than 4 – 5mm. In spite of many advantages, TIG welding is limited by the achievable penetration, which is around 3 – 3.5mm based on many relevant parameters. Achieving depth of penetration more than 3.5mm in a single pass is practically difficult with TIG welding, if not impossible.

This limitation triggers palpable research interest and as a consequence many variants have emerged that provide unique advantages over conventional TIG welding process. Activated and flux-bound TIG welding are two notable variants. In Activated Tungsten Inert Gas (A-TIG) welding a thin layer of activating flux is applied on the faying surfaces and surrounding region of the parent components prior to welding. This shows a promising result by enhancing depth of penetration by 3 times or even more as compared to conventional TIG welding with similar process parameters. So achieving penetration of 7 – 11mm is feasible with A-TIG welding, which ultimately results in remarkable improvement of productivity in entire manufacturing. Various differences between Tungsten Inert Gas (TIG) welding and Activated Tungsten Inert Gas (A-TIG) welding are given below in table format. It is worth mentioning that both processes are performed in same set up and in same way except the application of flux in A-TIG welding.

Table: Differences between TIG welding and A-TIG welding

Usage of activating flux: This is the prime difference between TIG welding and A-TIG welding as activating flux is used only in the later one. Such activating flux include a large number of oxides and halides of metal such as titanium oxide (TiO₂), silica (SiO₂), chromium oxide (Cr₂O₃), magnesium carbonate (MgCO₃), magnesium oxide (MgO), manganese dioxide (MnO₂), calcium oxide (CaO), aluminum oxide (Al₂O₃), zirconium dioxide (ZrO₂), etc. A homogeneous mixture of such fluxes in different proportions are also used. Such flux is first mixed with suitable solvent to prepare a semi-solid paste and the same is applied on the faying surfaces and surrounding regions of the parent component. It has to be applied prior to welding and must be allowed to dry before establishing the arc. Flux can be applied manually or with the assistance of mechanized system; however, thickness of this flux layer has to uniform in order to achieve defect-free joint. Usually this thickness varies from 30 – 75µm based on a number of relevant parameters.

Penetration depth, weld bead width and HAZ: Various researches have clearly shown that a depth of 7 – 11mm is achievable in single pass without any edge preparation but with the usage of suitable flux; as compared to common TIG welding that can provide maximum 3.5mm penetration under similar conditions. Such remarkable improvement in penetration is attributed to the reversal of Marangoni Effect when flux is applied. Use of activating flux also leads to arc constriction, which subsequently increases heat density of electric arc. A constricted arc results in narrower weld bead and also narrower heat affected zone (HAZ) as lower rate of heat input is desired in a particular area.

Establishing arc between electrodes: Every arc welding process requires an electric arc to be established in between the electrode and conductive work materials. In fact, this arc is the prime source of heat for melting faying surfaces of parent material. In TIG welding, constituting arc between pointed tungsten electrode and conductive parent metal is not problematic. However, with A-TIG welding, because of presence insulating layer on work metal surfaces, flow of electrons is restricted and thus establishing arc is a bit difficult. Often additional flux-free supporting plate is used at the entry of joint to facilitate this purpose. It also requires a little larger closed circuit voltage for maintaining the arc throughout the process.

Thin sheet and tick sheet joining: A-TIG welding inherently provides deeper penetration and thus it is not economical to use it for joining thin sheets or plates having thickness below 4mm. Even if it is used for such cases, then excess penetration, dimensional inaccuracy and high deformation will be observed. However, for joining thicker components, A-TIG is preferred as it can give penetration of 7 – 11mm in a single pass and that too without any edge preparation. On the contrary, TIG welding can be advantageously used for both thin and thick component joining following necessary technique.

Edge preparation, multiple passes and productivity: Joining thick plates (thickness>3.5mm) by TIG welding requires proper edge preparation and multiple passes to properly fill entire root gap. Multiple pass welding also increases level of heat input in a particular area and thus HAZ width, deformation, etc. also increases, which are usually undesirable. This requires large volume of costly filler metal as well as considerable amount of time. In fact, TIG welding is not suitable when large volume of filler metal is required to deposit; gas metal arc welding (GMAW) is preferred choice in such scenario. However, A-TIG welding can also be advantageously used for such purposes without requiring edge preparation or multiple pass welding.

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

• Babu et al. (2016); Development of flux bounded tungsten inert gas welding process to join aluminum alloys; American Journal of Mechanical and Industrial Engineering; Vol. 1 (3); pp. 58-63.
• Saha et al. (2018); Investigation on the effect of activating flux on tungsten inert gas welding of austenitic stainless steel using ac polarity; Indian Welding Journal; Vol. 51 (2).

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Variants of tungsten inert gas (TIG) welding

Limited penetration sparks research interest on this topic and the result is emergence of few variants of TIG welding that shows remarkable improvement in achievable penetration depth. Activated tungsten inert gas (A-TIG) welding and Flux bound tungsten inert gas (FB-TIG) welding are two such variants that utilize suitable activating flux to improve upon various characteristics of conventional TIG welding. These processes are also called flux assisted TIG welding as they mandatorily require a layer of activating flux on the components to be joined.

Activating flux and its application on metal surface

In both the cases a thin layer (thickness usually below 50µm) of activating flux is applied on the surface of parent material prior to welding. Such activating flux include a large number of oxides and halides of metal such as titanium oxide (TiO₂), silica (SiO₂), chromium oxide (Cr₂O₃), magnesium carbonate (MgCO₃), magnesium oxide (MgO), manganese dioxide (MnO₂), calcium oxide (CaO), aluminum oxide (Al₂O₃), zirconium dioxide (ZrO₂), etc. A homogeneous mixture of such fluxes in different proportions are also used.

Such flux is first mixed with acetone to form a paste like solution and then applied on the surface of parent metal either manually using soft brush or automatically using a mechanized system. Mixing ratio is not important as acetone is highly volatile and thus this ratio will not remain constant even during application of flux layer on metal. However, maintaining uniformity in flux coating thickness is crucial factor in obtaining defect-free welding. After applying flux coating, welding is carried out exactly in same way as in case of conventional TIG welding. All process parameters remain same, except that the closed circuit voltage increase slightly in some cases (usually this change is negligible).

Activated tungsten inert gas (A-TIG) welding

Although A-TIG and FB-TIG follow same aforementioned principle, they differ on the position of application of activating flux on parent metals. In activated tungsten inert gas (A-TIG) welding, flux is applied at the faying surface of the parent metal and surrounding it. Usually flus is applied up to a width of about 4mm from faying surface across the root gap in each side. So here flux lies just below the arc during welding. A typical layer of flux on parent metal for A-TIG welding is schematically shown here.

Flux bound tungsten inert gas (FB-TIG) welding

In flux bound tungsten inert gas (FB-TIG) welding, no flux is applied on faying surface and surrounding it; instead, it is applied on the top surface of parent metal maintaining small space after root gap. So here flux does not lie just below electric arc during welding. Activating flux, method of its application on metal surface and the welding procedure remain exactly same with A-TIG welding. The only difference lies on the position where flux is applied. However both exhibit palpable improvement in achievable depth of penetration. A typical layer of flux on parent metal for FB-TIG is schematically shown here.

Advantages offered by A-TIG and FB-TIG over conventional TIG

Various investigations have revealed that use of such flux can fetch numerous advantages as compared to conventional TIG welding. Both A-TIG and FB-TIG provides similar results and thus their advantages are also same when compared with conventional one. Their advantages are enlisted below.

1. Enhanced depth of penetration: Use of activating flux leads to constricted arc that increases heat density of the arc. Many proponents claimed that reversal of Marangoni Effect causes such increase in penetration. Flux assisted TIG welding usually gives penetration in the range of 6 – 9mm; however, with use of optimum parameters as deep as 11mm penetration can also be achieved in a single pass. This indicates about 3 times increase in penetration as compared to conventional TIG welding process.
2. Narrow weld bead: Constricted arc also results in narrow weld bead. This has certain indirect advantages like lower deformation, less heat affected zone (HAZ), etc. HAZ is considered as weak point in weld joint as its metallurgical properties are severely affected by arc heating and narrower HAZ is always desirable.

Scientific comparison among activated tungsten inert gas (A-TIG) welding and flux bound tungsten inert gas (FB-TIG) welding is provided in this article. The author also suggests you to go through the following references for better understanding of the topic.

• Babu et al. (2016); Development of flux bounded tungsten inert gas welding process to join aluminum alloys; American Journal of Mechanical and Industrial Engineering; Vol. 1 (3); pp. 58-63.
• Saha et al. (2018); Investigation on the effect of activating flux on tungsten inert gas welding of austenitic stainless steel using ac polarity; Indian Welding Journal; Vol. 51 (2).

The post Difference Between A-TIG Welding and FB-TIG Welding appeared first on Difference Box.