fundamentals of modern manufacturing 4th edition by mikell p groover 2 851

They form the three dimensions of the machining process, and for certain operations e.g., most single-point tooloperations they can be used to calculate the material removal rate for the

Part VI Material Removal

21.3.1 Forces in Metal Cutting21.3.2 The Merchant Equation21.4 Power and Energy Relationships in Machining21.5 Cutting Temperature

21.5.1 Analytical Methods to ComputeCutting Temperatures

21.5.2 Measurement of Cutting Temperature

The material removal processes are a family of shapingoperations (Figure 1.4) in which excess material is removedfrom a starting workpart so that what remains is the desiredfinal geometry The‘‘family tree’’ is shown in Figure 21.1.The most important branch of the family is conventionalmachining, in which a sharp cutting tool is used to me-chanically cut the material to achieve the desired geometry.The three principal machining processes are turning, dril-ling, and milling The ‘‘other machining operations’’ inFigure 21.1 include shaping, planing, broaching, and saw-ing This chapter begins our coverage of machining, whichruns through Chapter 24

Another group of material removal processes is theabrasive processes, which mechanically remove material bythe action of hard, abrasive particles This process group,which includes grinding, is covered in Chapter 25 The

‘‘other abrasive processes’’ in Figure 21.1 include honing,lapping, and superfinishing Finally, there are the non-traditional processes, which use various energy forms otherthan a sharp cutting tool or abrasive particles to removematerial The energy forms include mechanical, electro-chemical, thermal, and chemical.1The nontraditional pro-cesses are discussed in Chapter 26

Machining is a manufacturing process in which asharp cutting tool is used to cut away material to leave the

1 Some of the mechanical energy forms in the nontraditional processes involve the use of abrasive particles, and so they overlap with the abrasive processes in Chapter 25.

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desired part shape The predominant cutting action in machining involves shear mation of the work material to form a chip; as the chip is removed, a new surface isexposed Machining is most frequently applied to shape metals The process is illustrated

defor-in the diagram of Figure 21.2

Machining is one of the most important manufacturing processes The IndustrialRevolution and the growth of the manufacturing-based economies of the world can betraced largely to the development of the various machining operations (Historical Note22.1) Machining is important commercially and technologically for several reasons:

FIGURE 21.1Classification of materialremoval processes

Abrasive processes

Material removal processes

Nontraditional machining

operations

Other abrasive processes Mechanical energy processes Electrochemical machining Thermal energy processes Chemical machining

Grinding operations

FIGURE 21.2 (a) A cross-sectional view of the machining process (b) Tool with negative rake angle; compare withpositive rake angle in (a)

å Variety of work materials Machining can be applied to a wide variety of workmaterials Virtually all solid metals can be machined Plastics and plastic compositescan also be cut by machining Ceramics pose difficulties because of their highhardness and brittleness; however, most ceramics can be successfully cut by theabrasive machining processes discussed in Chapter 25.

å Variety of part shapes and geometric features Machining can be used to create anyregular geometries, such as flat planes, round holes, and cylinders By introducingvariations in tool shapes and tool paths, irregular geometries can be created, such asscrew threads and T-slots By combining several machining operations in sequence,shapes of almost unlimited complexity and variety can be produced

å Dimensional accuracy Machining can produce dimensions to very close tolerances.Some machining processes can achieve tolerances of
0.025 mm (
0.001 in), muchmore accurate than most other processes

å Good surface finishes Machining is capable of creating very smooth surface finishes.Roughness values less than 0.4 microns (16m-in.) can be achieved in conventionalmachining operations Some abrasive processes can achieve even better finishes

On the other hand, certain disadvantages are associated with machining and othermaterial removal processes:

å Wasteful of material Machining is inherently wasteful of material The chipsgenerated in a machining operation are wasted material Although these chipscan usually be recycled, they represent waste in terms of the unit operation

å Time consuming A machining operation generally takes more time to shape a givenpart than alternative shaping processes such as casting or forging

Machining is generally performed after other manufacturing processes such ascasting or bulk deformation (e.g., forging, bar drawing) The other processes create thegeneral shape of the starting workpart, and machining provides the final geometry,dimensions, and finish

21.1 OVERVIEW OF MACHINING TECHNOLOGY

Machining is not just one process; it is a group of processes The common feature is theuse of a cutting tool to form a chip that is removed from the workpart To perform theoperation, relative motion is required between the tool and work This relative motion isachieved in most machining operations by means of a primary motion, called the cuttingspeed, and a secondary motion, called the feed The shape of the tool and its penetrationinto the work surface, combined with these motions, produces the desired geometry ofthe resulting work surface

Types of Machining Operations There are many kinds of machining operations, each

of which is capable of generating a certain part geometry and surface texture We discussthese operations in considerable detail in Chapter 22, but for now it is appropriate toidentify and define the three most common types: turning, drilling, and milling, illustrated

in Figure 21.3

In turning, a cutting tool with a single cutting edge is used to remove material from arotating workpiece to generate a cylindrical shape, as in Figure 21.3(a) The speed motion inturning is provided by the rotating workpart, and the feed motion is achieved by the cuttingtool moving slowly in a direction parallel to the axis of rotation of the workpiece Drilling isused to create a round hole It is accomplished by a rotating tool that typically has two

cutting edges The tool is fed in a direction parallel to its axis of rotation into the workpart toform the round hole, as in Figure 21.3(b) In milling, a rotating tool with multiple cuttingedges is fed slowly across the work material to generate a plane or straight surface Thedirection of the feed motion is perpendicular to the tool’s axis of rotation The speed motion

is provided by the rotating milling cutter The two basic forms of milling are peripheralmilling and face milling, as in Figure 21.3(c) and (d)

Other conventional machining operations include shaping, planing, broaching, andsawing (Section 22.6) Also, grinding and similar abrasive operations are often includedwithin the category of machining These processes commonly follow the conventionalmachining operations and are used to achieve a superior surface finish on the workpart

The Cutting Tool A cutting tool has one or more sharp cutting edges and is made of amaterial that is harder than the work material The cutting edge serves to separate a chipfrom the parent work material, as in Figure 21.2 Connected to the cutting edge are twosurfaces of the tool: the rake face and the flank The rake face, which directs the flow of thenewly formed chip, is oriented at a certain angle called the rake angle a It is measuredrelative to a plane perpendicular to the work surface The rake angle can be positive, as inFigure 21.2(a), or negative as in (b) The flank of the tool provides a clearance between thetool and the newly generated work surface, thus protecting the surface from abrasion, whichwould degrade the finish This flank surface is oriented at an angle called the relief angle.Most cutting tools in practice have more complex geometries than those in Figure 21.2.There are two basic types, examples of which are illustrated in Figure 21.4: (a) single-pointtools and (b) multiple-cutting-edge tools A single-point tool has one cutting edge and is usedfor operations such as turning In addition to the tool features shown in Figure 21.2, there isone tool point from which the name of this cutting tool is derived During machining, thepoint of the tool penetrates below the original work surface of the part The point is usuallyrounded to a certain radius, called the nose radius Multiple-cutting-edge tools have more

FIGURE 21.3 The three

most common types of

machining processes:

(a) turning, (b) drilling, and

two forms of milling:

(c) peripheral milling, and

(d) face milling

Cutting tool

Feed motion (tool)

(c)

Feed motion (work)

Work

Rotation Milling cutter

New surface

than one cutting edge and usually achieve their motion relative to the workpart by rotating.Drilling and milling use rotating multiple-cutting-edge tools Figure 21.4(b) shows a helicalmilling cutter used in peripheral milling Although the shape is quite different from a single-point tool, many elements of tool geometry are similar Single-point and multiple-cutting-edge tools and the materials used in them are discussed in more detail in Chapter 23.

Cutting Conditions Relative motion is required between the tool and work to perform

a machining operation The primary motion is accomplished at a certain cutting speed v

In addition, the tool must be moved laterally across the work This is a much slowermotion, called the feed f The remaining dimension of the cut is the penetration of thecutting tool below the original work surface, called the depth of cut d Collectively, speed,feed, and depth of cut are called the cutting conditions They form the three dimensions

of the machining process, and for certain operations (e.g., most single-point tooloperations) they can be used to calculate the material removal rate for the process:

where RMR¼ material removal rate, mm3/s (in3/min); v¼ cutting speed, m/s (ft/min), whichmust be converted to mm/s (in/min); f¼ feed, mm (in); and d ¼ depth of cut, mm (in).The cutting conditions for a turning operation are depicted in Figure 21.5 Typicalunits used for cutting speed are m/s (ft/min) Feed in turning is expressed in mm/rev

FIGURE21.4 (a) A single-point tool showing rake face, flank, and tool point; and (b) a helical milling cutter, representative

of tools with multiple cutting edges

FIGURE 21.5 Cuttingspeed, feed, and depth ofcut for a turning operation

achieve the final dimensions, tolerances, and surface finish In production machining jobs,one or more roughing cuts are usually performed on the work, followed by one or twofinishing cuts Roughing operations are performed at high feeds and depths—feeds of 0.4

to 1.25 mm/rev (0.015–0.050 in/rev) and depths of 2.5 to 20 mm (0.100–0.750 in) aretypical Finishing operations are carried out at low feeds and depths—feeds of 0.125 to 0.4

mm (0.005–0.015 in/rev) and depths of 0.75 to 2.0 mm (0.030–0.075 in) are typical Cuttingspeeds are lower in roughing than in finishing

A cutting fluid is often applied to the machining operation to cool and lubricate thecutting tool (cutting fluids are discussed in Section 23.4) Determining whether a cuttingfluid should be used, and, if so, choosing the proper cutting fluid, is usually included withinthe scope of cutting conditions Given the work material and tooling, the selection of theseconditions is very influential in determining the success of a machining operation

Machine Tools A machine tool is used to hold the workpart, position the tool relative

to the work, and provide power for the machining process at the speed, feed, and depththat have been set By controlling the tool, work, and cutting conditions, machine toolspermit parts to be made with great accuracy and repeatability, to tolerances of 0.025 mm(0.001 in) and better The term machine tool applies to any power-driven machine thatperforms a machining operation, including grinding The term is also applied to machinesthat perform metal forming and pressworking operations (Chapters 19 and 20).The traditional machine tools used to perform turning, drilling, and milling arelathes, drill presses, and milling machines, respectively Conventional machine tools areusually tended by a human operator, who loads and unloads the workparts, changescutting tools, and sets the cutting conditions Many modern machine tools are designed toaccomplish their operations with a form of automation called computer numericalcontrol (Section 38.3)

21.2 THEORY OF CHIP FORMATION IN METAL MACHINING

The geometry of most practical machining operations is somewhat complex A simplifiedmodel of machining is available that neglects many of the geometric complexities, yetdescribes the mechanics of the process quite well It is called the orthogonal cutting model,Figure 21.6 Although an actual machining process is three-dimensional, the orthogonalmodel has only two dimensions that play active roles in the analysis

21.2.1 THE ORTHOGONAL CUTTING MODEL

By definition, orthogonal cutting uses a wedge-shaped tool in which the cutting edge isperpendicular to the direction of cutting speed As the tool is forced into the material, thechip is formed by shear deformation along a plane called the shear plane, which isoriented at an angle f with the surface of the work Only at the sharp cutting edge of thetool does failure of the material occur, resulting in separation of the chip from the parent

material Along the shear plane, where the bulk of the mechanical energy is consumed inmachining, the material is plastically deformed.

The tool in orthogonal cutting has only two elements of geometry: (1) rake angle and(2) clearance angle As indicated previously, the rake angle a determines the direction thatthe chip flows as it is formed from the workpart; and the clearance angle provides a smallclearance between the tool flank and the newly generated work surface

During cutting, the cutting edge of the tool is positioned a certain distance belowthe original work surface This corresponds to the thickness of the chip prior to chipformation, to As the chip is formed along the shear plane, its thickness increases to tc Theratio of toto tcis called the chip thickness ratio (or simply the chip ratio) r:

lsbe the length of the shear plane We can make the substitutions: to¼ lssinf, and tc¼ lscos(f a) Thus,

r ¼l lssin f

scos (f  a)¼

sin fcos (f  a)This can be rearranged to determine f as follows:

tan f¼ r cos a

The shear strain that occurs along the shear plane can be estimated by examiningFigure 21.7 Part (a) shows shear deformation approximated by a series of parallel platessliding against one another to form the chip Consistent with our definition of shear strain

FIGURE 21.6 Orthogonal cutting: (a) as a three-dimensional process, and (b) how it reduces to two dimensions inthe side view

(Section 3.1.4), each plate experiences the shear strain shown in Figure 21.7(b) Referring topart (c), this can be expressed as

g¼ACBD ¼AD þ DCBDwhich can be reduced to the following definition of shear strain in metal cutting:

Solution: The chip thickness ratio can be determined from Eq (21.2):

r ¼ 0:50

1:125¼ 0:444The shear plane angle is given by Eq (21.3):

Finally, the shear strain is calculated from Eq (21.4):

g¼ tan (25:4  10) þ cot 25:4

21.2.2 ACTUAL CHIP FORMATION

We should note that there are differences between the orthogonal model and an actualmachining process First, the shear deformation process does not occur along a plane, butwithin a zone If shearing were to take place across a plane of zero thickness, it would implythat the shearing action must occur instantaneously as it passes through the plane, ratherthan over some finite (although brief) time period For the material to behave in a realisticway, the shear deformation must occur within a thin shear zone This more realistic model ofthe shear deformation process in machining is illustrated in Figure 21.8 Metal-cuttingexperiments have indicated that the thickness of the shear zone is only a few thousandths of

an inch Since the shear zone is so thin, there is not a great loss of accuracy in most cases byreferring to it as a plane

Second, in addition to shear deformation that occurs in the shear zone, anothershearing action occurs in the chip after it has been formed This additional shear isreferred to as secondary shear to distinguish it from primary shear Secondary shearresults from friction between the chip and the tool as the chip slides along the rake face

of the tool Its effect increases with increased friction between the tool and chip Theprimary and secondary shear zones can be seen in Figure 21.8

Third, formation of the chip depends on the type of material being machined andthe cutting conditions of the operation Four basic types of chip can be distinguished,illustrated in Figure 21.9:

å Discontinuous chip When relatively brittle materials (e.g., cast irons) are machined

at low cutting speeds, the chips often form into separate segments (sometimes thesegments are loosely attached) This tends to impart an irregular texture to themachined surface High tool–chip friction and large feed and depth of cut promotethe formation of this chip type

å Continuous chip When ductile work materials are cut at high speeds and relativelysmall feeds and depths, long continuous chips are formed A good surface finishtypically results when this chip type is formed A sharp cutting edge on the tool and

FIGURE 21.8 Morerealistic view of chipformation, showing shearzone rather than shearplane Also shown is thesecondary shear zoneresulting from tool–chipfriction

Chip

Tool

Primary shear zone

Secondary shear zone Effective

low tool–chip friction encourage the formation of continuous chips Long, continuouschips (as in turning) can cause problems with regard to chip disposal and/or tanglingabout the tool To solve these problems, turning tools are often equipped with chipbreakers (Section 23.3.1).

å Continuous chip with built-up edge When machining ductile materials at medium cutting speeds, friction between tool and chip tends to cause portions of thework material to adhere to the rake face of the tool near the cutting edge Thisformation is called a built-up edge (BUE) The formation of a BUE is cyclical; itforms and grows, then becomes unstable and breaks off Much of the detached BUE

low-to-is carried away with the chip, sometimes taking portions of the tool rake face with it,which reduces the life of the cutting tool Portions of the detached BUE that are notcarried off with the chip become imbedded in the newly created work surface,causing the surface to become rough

The preceding chip types were first classified by Ernst in the late 1930s [13] Sincethen, the available metals used in machining, cutting tool materials, and cutting speedshave all increased, and a fourth chip type has been identified:

å Serrated chips (the term shear-localized is also used for this fourth chip type) Thesechips are semi-continuous in the sense that they possess a saw-tooth appearance that

is produced by a cyclical chip formation of alternating high shear strain followed bylow shear strain This fourth type of chip is most closely associated with certaindifficult-to-machine metals such as titanium alloys, nickel-base superalloys, andaustenitic stainless steels when they are machined at higher cutting speeds However,the phenomenon is also found with more common work metals (e.g., steels) whenthey are cut at high speeds [13].2

21.3 FORCE RELATIONSHIPS AND THE MERCHANT EQUATION

Several forces can be defined relative to the orthogonal cutting model Based on theseforces, shear stress, coefficient of friction, and certain other relationships can bedefined

Irregular surface due

21.3.1 FORCES IN METAL CUTTING

Consider the forces acting on the chip during orthogonal cutting in Figure 21.10(a) The forcesapplied against the chip by the tool can be separated into two mutually perpendicularcomponents: friction force and normal force to friction The friction force F is the frictionalforce resisting the flow of the chip along the rake face of the tool The normal force to friction N

is perpendicular to the friction force These two components can be used to define thecoefficient of friction between the tool and the chip:

The friction force and its normal force can be added vectorially to form a resultantforce R, which is oriented at an angle b, called the friction angle The friction angle isrelated to the coefficient of friction as

In addition to the tool forces acting on the chip, there are two force components applied

by the workpiece on the chip: shear force and normal force to shear The shear force Fsis theforce that causes shear deformation to occur in the shear plane, and the normal force to shear

Fnis perpendicular to the shear force Based on the shear force, we can define the shear stressthat acts along the shear plane between the work and the chip:

Vector addition of the two force components Fsand Fnyields the resultant force R0

In order for the forces acting on the chip to be in balance, this resultant R0must be equal

in magnitude, opposite in direction, and collinear with the resultant R

FIGURE 21.10 Forces in metal cutting: (a) forces acting on the chip in orthogonal cutting, and (b) forces acting onthe tool that can be measured

None of the four force components F, N, Fs, and Fncan be directly measured in amachining operation, because the directions in which they are applied vary with differenttool geometries and cutting conditions However, it is possible for the cutting tool to beinstrumented using a force measuring device called a dynamometer, so that two additionalforce components acting against the tool can be directly measured: cutting force and thrustforce The cutting force Fcis in the direction of cutting, the same direction as the cuttingspeed v, and the thrust force Ftis perpendicular to the cutting force and is associated with thechip thickness before the cut to The cutting force and thrust force are shown in Figure 21.10(b) together with their resultant force R00 The respective directions of these forces areknown, so the force transducers in the dynamometer can be aligned accordingly.Equations can be derived to relate the four force components that cannot

be measured to the two forces that can be measured Using the force diagram inFigure 21.11, the following trigonometric relationships can be derived:

Note that in the special case of orthogonal cutting when the rake angle a¼ 0, Eqs (21.9)and (21.10) reduce to F¼ Ftand N¼ Fc, respectively Thus, in this special case, friction forceand its normal force could be directly measured by the dynamometer

Solution: From Example 21.1, rake angle a¼ 10, and shear plane angle f¼ 25.4 Shear

force can be computed from Eq (21.11):

Fs¼ 1559 cos 25:4  1271 sin 25:4 ¼ 863 N

FIGURE 21.11 Force diagram showinggeometric relationships between F, N,

Fs, Fn, Fc, and Ft

The shear plane area is given by Eq (21.8):

As¼(0:5)(3:0)sin 25:4 ¼ 3:497 mm2Thus the shear stress, which equals the shear strength of the work material, is

Fc¼ Stow cos (b  a)sin f cos(f þ b  a)¼

Fscos (b a)cos(fþ b  a) ð21:13Þand

Ft¼ Stw sin (b  a)sin f cos(fþ b  a)¼

Fssin (b a)cos (fþ b  a) ð21:14ÞThese equations allow one to estimate cutting force and thrust force in an orthogonalcutting operation if the shear strength of the work material is known

21.3.2 THE MERCHANT EQUATION

One of the important relationships in metal cutting was derived by Eugene Merchant[10] Its derivation was based on the assumption of orthogonal cutting, but its generalvalidity extends to three-dimensional machining operations Merchant started with thedefinition of shear stress expressed in the form of the following relationship derived bycombining Eqs (21.7), (21.8), and (21.11):

t¼Fccos f Ftsin f

Merchant reasoned that, out of all the possible angles emanating from the cuttingedge of the tool at which shear deformation could occur, there is one angle f thatpredominates This is the angle at which shear stress is just equal to the shear strength ofthe work material, and so shear deformation occurs at this angle For all other possibleshear angles, the shear stress is less than the shear strength, so chip formation cannotoccur at these other angles In effect, the work material will select a shear plane angle thatminimizes energy This angle can be determined by taking the derivative of the shearstress S in Eq (21.15) with respect to f and setting the derivative to zero Solving for f, weget the relationship named after Merchant:

The importance of increasing the shear plane angle can be seen in Figure 21.12 If allother factors remain the same, a higher shear plane angle results in a smaller shear planearea Since the shear strength is applied across this area, the shear force required to formthe chip will decrease when the shear plane area is reduced A greater shear plane angleresults in lower cutting energy, lower power requirements, and lower cutting temperature.These are good reasons to try to make the shear plane angle as large as possible duringmachining.

Approximation of Turning by Orthogonal Cutting The orthogonal model can be used

to approximate turning and certain other single-point machining operations so long as thefeed in these operations is small relative to depth of cut Thus, most of the cutting will takeplace in the direction of the feed, and cutting on the point of the tool will be negligible.Figure 21.13 indicates the conversion from one cutting situation to the other

FIGURE 21.12 Effect of shear plane angle f: (a) higher f with a resulting lower shear plane area;(b) smaller f with a corresponding larger shear plane area Note that the rake angle is larger in (a), whichtends to increase shear angle according to the Merchant equation

The interpretation of cutting conditions is different in the two cases The chipthickness before the cut toin orthogonal cutting corresponds to the feed f in turning, andthe width of cut w in orthogonal cutting corresponds to the depth of cut d in turning Inaddition, the thrust force Ftin the orthogonal model corresponds to the feed force Ffinturning Cutting speed and cutting force have the same meanings in the two cases.Table 21.1 summarizes the conversions.

21.4 POWER AND ENERGY RELATIONSHIPS IN MACHINING

A machining operation requires power The cutting force in a production machiningoperation might exceed 1000 N (several hundred pounds), as suggested by Example 21.2.Typical cutting speeds are several hundred m/min The product of cutting force and speedgives the power (energy per unit time) required to perform a machining operation:

Turning Operation Orthogonal Cutting Model

Feed f¼ Chip thickness before cut to

Depth d¼ Width of cut wCutting speed v¼ Cutting speed vCutting force Fc¼ Cutting force Fc

Feed force Ff¼ Thrust force Ft

FIGURE 21.13Approximation of turning

by the orthogonal model:

(a) turning; and (b) thecorresponding orthogo-nal cutting

Pg¼PEc or HPg¼HPEc ð21:19Þwhere Pg¼ gross power of the machine tool motor, W; HPg¼ gross horsepower; and E ¼mechanical efficiency of the machine tool Typical values of E for machine tools arearound 90%.

It is often useful to convert power into power per unit volume rate of metal cut This

is called the unit power, Pu(or unit horsepower, HPu), defined:

Solution: From Eq (21.18), power in the operation is

Pc¼ (1557 N)(100 m/min) ¼ 155; 700 N  m/min ¼ 155; 700 J/min ¼ 2595 J/s ¼ 2595 WSpecific energy is calculated from Eq (21.21):

U ¼ 155; 700100(103)(3:0)(0:5)¼

155; 700

150; 000¼ 1:038 N-m/min3 nUnit power and specific energy provide a useful measure of how much power (orenergy) is required to remove a unit volume of metal during machining Using thismeasure, different work materials can be compared in terms of their power and energyrequirements Table 21.2 presents a listing of unit horsepower and specific energy valuesfor selected work materials

The values in Table 21.2 are based on two assumptions: (1) the cutting tool is sharp,and (2) the chip thickness before the cut to¼ 0.25 mm (0.010 in) If these assumptions arenot met, some adjustments must be made For worn tools, the power required to performthe cut is greater, and this is reflected in higher specific energy and unit horsepower values

As an approximate guide, the values in the table should be multiplied by a factor between1.00 and 1.25 depending on the degree of dullness of the tool For sharp tools, the factor is

1.00 For tools in a finishing operation that are nearly worn out, the factor is around 1.10,and for tools in a roughing operation that are nearly worn out, the factor is 1.25.Chip thickness before the cut toalso affects the specific energy and unit horsepowervalues As tois reduced, unit power requirements increase This relationship is referred to asthe size effect For example, grinding, in which the chips are extremely small by comparison tomost other machining operations, requires very high specific energy values The U and HPu

values in Table 21.2 can still be used to estimate horsepower and energy for situations in which

tois not equal to 0.25 mm (0.010 in) by applying a correction factor to account for anydifference in chip thickness before the cut Figure 21.14 provides values of this correction

TABLE 21.2 Values of unit horsepower and specific energy for selected workmaterials using sharp cutting tools and chip thickness before the cut to= 0.25 mm(0.010 in)

Specific Energy U orUnit Power Pu

Material HardnessBrinell N-m/mm3 in-lb/in3 Unit Horsepower

HPuhp/(in3/min)Carbon steel 150–200 1.6 240,000 0.6

201–250 2.2 320,000 0.8251–300 2.8 400,000 1.0Alloy steels 200–250 2.2 320,000 0.8

251–300 2.8 400,000 1.0301–350 3.6 520,000 1.3351–400 4.4 640,000 1.6Cast irons 125–175 1.1 160,000 0.4

175–250 1.6 240,000 0.6Stainless steel 150–250 2.8 400,000 1.0Aluminum 50–100 0.7 100,000 0.25Aluminum alloys 100–150 0.8 120,000 0.3

0.125

1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.005

0.25 0.010 0.015 0.020 0.025 0.030 0.040 0.050

0.38 0.50 0.63 Chip thickness before cut t (mm)

Chip thickness before cut to (in.)

0.75 0.88 0.1 1.25

21.5 CUTTING TEMPERATURE

Of the total energy consumed in machining, nearly all of it (98%) is converted into heat.This heat can cause temperatures to be very high at the tool–chip interface—over 600C(1100F) is not unusual The remaining energy (2%) is retained as elastic energy in the chip.Cutting temperatures are important because high temperatures (1) reduce tool life,(2) produce hot chips that pose safety hazards to the machine operator, and (3) can causeinaccuracies in workpart dimensions due to thermal expansion of the work material In thissection, we discuss the methods of calculating and measuring temperatures in machiningoperations

21.5.1 ANALYTICAL METHODS TO COMPUTE CUTTING TEMPERATURES

There are several analytical methods to calculate estimates of cutting temperature.References [3], [5], [9], and [15] present some of these approaches We describe themethod by Cook [5], which was derived using experimental data for a variety of workmaterials to establish parameter values for the resulting equation The equation can beused to predict the increase in temperature at the tool–chip interface during machining:

DT ¼0rC:4U vtKo 0:333 ð21:22ÞwhereDT ¼ mean temperature rise at the tool–chip interface, C(F); U¼ specific energy

in the operation, N-m/mm3or J/mm3(in-lb/in3); v¼ cutting speed, m/s (in/sec); to¼ chipthickness before the cut, m (in); rC¼ volumetric specific heat of the work material, J/mm3-

C (in-lb/in3-F); K¼ thermal diffusivity of the work material, m2/s (in2/sec)

Solution: Cutting speed must be converted to mm/s: v¼ (100 m/min)(103mm/m)/(60 s/min)¼ 1667 mm/s Eq (21.22) can now be used to compute the mean temperature rise:

DT ¼0:4(1:038)

3:0(103)

C 1667(0:5)50

 0 :333

¼ (138:4)(2:552) ¼ 353C

n

21.5.2 MEASUREMENT OF CUTTING TEMPERATURE

Experimental methods have been developed to measure temperatures in machining.The most frequently used measuring technique is the tool–chip thermocouple Thisthermocouple consists of the tool and the chip as the two dissimilar metals forming the

thermocouple junction By properly connecting electrical leads to the tool and part (which is connected to the chip), the voltage generated at the tool–chip interfaceduring cutting can be monitored using a recording potentiometer or other appropriatedata-collection device The voltage output of the tool–chip thermocouple (measured inmV) can be converted into the corresponding temperature value by means of calibra-tion equations for the particular tool–work combination.

work-The tool–chip thermocouple has been utilized by researchers to investigate therelationship between temperature and cutting conditions such as speed and feed Trigger[14] determined the speed–temperature relationship to be of the following general form:

where T ¼ measured tool–chip interface temperature and v ¼ cutting speed Theparameters K and m depend on cutting conditions (other than v) and work material.Figure 21.15 plots temperature versus cutting speed for several work materials, withequations of the form of Eq (21.23) determined for each material A similar relationshipexists between cutting temperature and feed; however, the effect of feed on temperature

is not as strong as cutting speed These empirical results tend to support the generalvalidity of the Cook equation: Eq (21.22)

REFERENCES[1] ASM Handbook, Vol 16, Machining ASM Inter-national, Materials Park, Ohio, 1989

[2] Black, J, and Kohser, R DeGarmo’s Materials andProcesses in Manufacturing, 10th ed John Wiley &

Sons, Inc., Hoboken, New Jersey, 2008

[3] Boothroyd, G., and Knight, W A Fundamentals ofMetal Machining and Machine Tools, 3rd ed CRCTaylor and Francis, Boca Raton, Florida, 2006.[4] Chao, B T., and Trigger, K J.‘‘Temperature Distri-bution at the Tool-Chip Interface in Metal

FIGURE 21.15Experimentally measuredcutting temperaturesplotted against speedfor three work materials,indicating generalagreement with

Eq (21.23) (Based ondata in [9].)3

3 The units reported in the Loewen and Shaw ASME paper [9] wereF for cutting temperature and ft/min for cutting speed We have retained those units in the plots and equations of our figure.

[7] Kalpakjian, S., and Schmid, R Manufacturing

Pro-cesses for Engineering Materials, 4th ed Prentice

Hall/Pearson, Upper Saddle River, New Jersey, 2003

[8] Lindberg, R A Processes and Materials of

Manu-facture, 4th ed Allyn and Bacon, Inc., Boston, 1990

[9] Loewen, E G., and Shaw, M C.‘‘On the Analysis of

Cutting Tool Temperatures,’’ ASME Transactions,

Vol 76, No 2, February 1954, pp 217–225

[13] Trent, E M., and Wright, P K Metal Cutting, 4th ed.Butterworth Heinemann, Boston, 2000

[14] Trigger, K J.‘‘Progress Report No 2 on Tool–ChipInterface Temperatures,’’ ASME Transactions,Vol 71, No 2, February 1949, pp 163–174.[15] Trigger, K J., and Chao, B T.‘‘An Analytical Eval-uation of Metal Cutting Temperatures,’’ ASMETransactions, Vol 73, No 1, January 1951, pp 57–68

21.3 Identify some of the reasons why machining is

commercially and technologically important

21.4 Name the three most common machining

processes

21.5 What are the two basic categories of cutting tools in

machining? Give two examples of machining

op-erations that use each of the tooling types

21.6 What are the parameters of a machining operation

that are included within the scope of cutting

conditions?

21.7 Explain the difference between roughing and

fin-ishing operations in machining

21.8 What is a machine tool?

21.9 What is an orthogonal cutting operation?

21.10 Why is the orthogonal cutting model useful in theanalysis of metal machining?

21.11 Name and briefly describe the four types of chipsthat occur in metal cutting

21.12 Identify the four forces that act upon the chip in theorthogonal metal cutting model but cannot bemeasured directly in an operation

21.13 Identify the two forces that can be measured in theorthogonal metal cutting model

21.14 What is the relationship between the coefficient offriction and the friction angle in the orthogonalcutting model?

21.15 Describein words what the Merchant equation tells us.21.16 How is the power required in a cutting operationrelated to the cutting force?

21.17 What is the specific energy in metal machining?21.18 What does the term size effect mean in metal cutting?21.19 What is a tool–chip thermocouple?

MULTIPLE CHOICE QUIZ

There are 17 correct answers in the following multiple choice questions (some questions have multiple answers that arecorrect) To attain a perfect score on the quiz, all correct answers must be given Each correct answer is worth 1 point Eachomitted answer or wrong answer reduces the score by 1 point, and each additional answer beyond the correct number ofanswers reduces the score by 1 point Percentage score on the quiz is based on the total number of correct answers.21.1 Which of the following manufacturing processes

are classified as material removal processes (two

correct answers): (a) casting, (b) drawing, (c)

extru-sion, (d) forging, (e) grinding, (f) machining,

(g) molding, (h) pressworking, and (i) spinning?

21.2 A lathe is used to perform which one of thefollowing manufacturing operations: (a) broaching,(b) drilling, (c) lapping, (d) milling, or (e) turning?21.3 With which one of the following geometric forms isthe drilling operation most closely associated:

(a) external cylinder, (b) flat plane, (c) round hole,(d) screw threads, or (e) sphere?

21.4 If the cutting conditions in a turning operation arecutting speed¼ 300 ft/min, feed ¼ 0.010 in/rev, anddepth of cut¼ 0.100 in, which one of the following

is the material removal rate: (a) 0.025 in3/min,(b) 0.3 in3/min, (c) 3.0 in3/min, or (d) 3.6 in3/min?

21.5 A roughing operation generally involves which one

of the following combinations of cutting tions: (a) high v, f, and d; (b) high v, low f and d;

condi-(c) low v, high f and d; or (d) low v, f, and d, where v¼cutting speed, f¼ feed, and d ¼ depth?

21.6 Which of the following are characteristics of theorthogonal cutting model (three best answers):

(a) a circular cutting edge is used, (b) a cutting-edge tool is used, (c) a single-point tool isused, (d) only two dimensions play an active role inthe analysis, (e) the cutting edge is parallel to thedirection of cutting speed, (f) the cutting edge isperpendicular to the direction of cutting speed, and(g) the two elements of tool geometry are rake andrelief angle?

multiple-21.7 The chip thickness ratio is which one of the following:

(a) tc/to, (b) to/tc, (c) f/d, or (d) to/w, where tc¼ chipthickness after the cut, to¼ chip thickness beforethe cut, f¼ feed, d ¼ depth, and w ¼ width of cut?

21.8 Which one of the four types of chip would beexpected in a turning operation conducted at low

cutting speed on a brittle work material: (a) tinuous, (b) continuous with built-up edge,(c) discontinuous, or (d) serrated?

con-21.9 According to the Merchant equation, an increase

in rake angle would have which of the followingresults, all other factors remaining the same (twobest answers): (a) decrease in friction angle,(b) decrease in power requirements, (c) decrease

in shear plane angle, (d) increase in cutting perature, and (e) increase in shear plane angle?21.10 In using the orthogonal cutting model to approxi-mate a turning operation, the chip thickness beforethe cut tocorresponds to which one of the followingcutting conditions in turning: (a) depth of cut d,(b) feed f, or (c) speed v?

tem-21.11 Which one of the following metals would usuallyhave the lowest unit horsepower in a machiningoperation: (a) aluminum, (b) brass, (c) cast iron, or(d) steel?

21.12 For which one of the following values of chip ness before the cut towould you expect the specificenergy in machining to be the greatest:(a) 0.010 in,(b) 0.025 in, (c) 0.12 mm, or (d) 0.50 mm?21.13 Which of the following cutting conditions has thestrongest effect on cutting temperature: (a) feed or(b) speed?

thick-PROBLEMS

Chip Formation and Forces in Machining

21.1 In an orthogonal cutting operation, the tool has arake angle¼ 15 The chip thickness before the cut¼0.30 mm and the cut yields a deformed chip thick-ness¼ 0.65 mm Calculate (a) the shear plane angleand (b) the shear strain for the operation

21.2 In Problem 21.1, suppose the rake angle werechanged to 0 Assuming that the friction angleremains the same, determine (a) the shear planeangle, (b) the chip thickness, and (c) the shearstrain for the operation

21.3 In an orthogonal cutting operation, the 0.25-inwide tool has a rake angle of 5 The lathe is set

so the chip thickness before the cut is 0.010 in

After the cut, the deformed chip thickness is ured to be 0.027 in Calculate (a) the shear planeangle and (b) the shear strain for the operation

meas-21.4 In a turning operation, spindle speed is set to provide

a cutting speed of 1.8 m/s The feed and depth of cut

of cut are 0.30 mm and 2.6 mm, respectively The toolrake angle is 8 After the cut, the deformed chip

thickness is measured to be 0.49 mm Determine (a)shear plane angle, (b) shear strain, and (c) materialremoval rate Use the orthogonal cutting model as

an approximation of the turning process

21.5 The cutting force and thrust force in an orthogonalcutting operation are 1470 N and 1589 N, respec-tively The rake angle¼ 5, the width of the cut¼5.0 mm, the chip thickness before the cut¼ 0.6, andthe chip thickness ratio¼ 0.38 Determine (a) theshear strength of the work material and (b) thecoefficient of friction in the operation

21.6 The cutting force and thrust force have beenmeasured in an orthogonal cutting operation

to be 300 lb and 291 lb, respectively The rakeangle¼ 10, width of cut¼ 0.200 in, chip thicknessbefore the cut¼ 0.015, and chip thickness ratio ¼0.4 Determine (a) the shear strength of the workmaterial and (b) the coefficient of friction in theoperation

shear strength is 40,000 lb/in2 Based on your answers

to the previous problem, compute(a) the shear force,

(b) cutting force, (c) thrust force, and (d) friction

force

21.9 In an orthogonal cutting operation, the rake angle¼

5, chip thickness before the cut¼ 0.2 mm and

width of cut¼ 4.0 mm The chip ratio ¼ 0.4

Deter-mine (a) the chip thickness after the cut, (b) shear

angle, (c) friction angle, (d) coefficient of friction,

and (e) shear strain

21.10 The shear strength of a certain work material ¼

50,000 lb/in2 An orthogonal cutting operation is

performed using a tool with a rake angle¼ 20at

the following cutting conditions: cutting speed¼

100 ft/min, chip thickness before the cut¼ 0.015 in,

and width of cut ¼ 0.150 in The resulting chip

thickness ratio ¼ 0.50 Determine (a) the shear

plane angle, (b) shear force, (c) cutting force and

thrust force, and (d) friction force

21.11 Consider the data in Problem 21.10 except that

rake angle is a variable, and its effect on the forces

in parts (b), (c), and (d) is to be evaluated

(a) Using a spreadsheet calculator, compute the

values of shear force, cutting force, thrust force, and

friction force as a function of rake angle over a

range of rake angles between the high value of 20

in Problem 21.10 and a low value of10 Use

intervals of 5between these limits The chip

thick-ness ratio decreases as rake angle is reduced and

can be approximated by the following relationship:

r ¼ 0.38 þ 0.006a, where r ¼ chip thickness and a ¼

The feed is 0.011 in/rev and the depth of cut is0.120 in The rake angle on the tool in the direction

of chip flow is 13 The cutting conditions result in achip ratio of 0.52 Using the orthogonal model as anapproximation of turning, determine (a) the shearplane angle, (b) shear force, (c) cutting force andfeed force, and (d) coefficient of friction betweenthe tool and chip

21.14 Low carbon steel having a tensile strength of

300 MPa and a shear strength of 220 MPa is cut

in a turning operation with a cutting speed of 3.0 m/s.The feed is 0.20 mm/rev and the depth of cut is3.0 mm The rake angle of the tool is 5 in thedirection of chip flow The resulting chip ratio is0.45 Using the orthogonal model as an approxima-tion of turning, determine (a) the shear plane angle,(b) shear force, (c) cutting force and feed force.21.15 A turning operation is made with a rake angle of

10, a feed of 0.010 in/rev and a depth of cut¼ 0.100

in The shear strength of the work material isknown to be 50,000 lb/in2, and the chip thicknessratio is measured after the cut to be 0.40 Deter-mine the cutting force and the feed force Use theorthogonal cutting model as an approximation ofthe turning process

21.16 Show how Eq (21.3) is derived from the definition

of chip ratio, Eq (21.2), and Figure 21.5(b).21.17 Show how Eq (21.4) is derived from Figure 21.6.21.18 Derive the force equations for F, N, Fs, and Fn

(Eqs (21.9) through (21.12) in the text) using theforce diagram of Figure 21.11

Power and Energy in Machining

21.19 In a turning operation on stainless steel with

hard-ness¼ 200 HB, the cutting speed ¼ 200 m/min,

feed¼ 0.25 mm/rev, and depth of cut ¼ 7.5 mm

How much power will the lathe draw in performing

this operation if its mechanical efficiency¼ 90%

Use Table 21.2 to obtain the appropriate specific

energy value

21.20 In Problem 21.18, compute the lathe power

re-quirements if feed¼ 0.50 mm/rev

21.21 In a turning operation on aluminum, cutting

speed¼ 900 ft/min, feed ¼ 0.020 in/rev, and depth

of cut¼ 0.250 in What horsepower is required of

the drive motor, if the lathe has a mechanicalefficiency ¼ 87%? Use Table 21.2 to obtain theappropriate unit horsepower value

21.22 In a turning operation on plain carbon steel whoseBrinell hardness ¼ 275 HB, the cutting speed isset at 200 m/min and depth of cut¼ 6.0 mm Thelathe motor is rated at 25 kW, and its mechanicalefficiency¼ 90% Using the appropriate specificenergy value from Table 21.2, determine the maxi-mum feed that can be set for this operation Use of

a spreadsheet calculator is recommended for theiterative calculations required in this problem

21.23 A turning operation is to be performed on a 20 hplathe that has an 87% efficiency rating The rough-ing cut is made on alloy steel whose hardness is inthe range 325 to 335 HB The cutting speed is 375 ft/

min, feed is 0.030 in/rev, and depth of cut is 0.150 in

Based on these values, can the job be performed onthe 20 hp lathe? Use Table 21.2 to obtain theappropriate unit horsepower value

21.24 Suppose the cutting speed in Problems 21.7 and21.8 is 200 ft/min From your answers to thoseproblems, find (a) the horsepower consumed inthe operation, (b) metal removal rate in in3/min,(c) unit horsepower (hp-min/in3), and (d) the spe-cific energy (in-lb/in3)

21.25 For Problem 21.12, the lathe has a mechanicalefficiency ¼ 0.83 Determine (a) the horsepowerconsumed by the turning operation; (b) horsepowerthat must be generated by the lathe; (c) unit horse-power and specific energy for the work material inthis operation

21.26 In a turning operation on low carbon steel (175BHN), cutting speed¼ 400 ft/min, feed ¼ 0.010 in/

rev, and depth of cut¼ 0.075 in The lathe has amechanical efficiency ¼ 0.85 Based on the unithorsepower values in Table 21.2, determine (a) thehorsepower consumed by the turning operationand (b) the horsepower that must be generated

by the lathe

21.27 Solve Problem 21.25 except that the feed¼ 0.0075 in/

rev and the work material is stainless steel (Brinellhardness¼ 240 HB)

21.28 A turning operation is carried out on aluminum (100BHN) Cutting speed¼ 5.6 m/s, feed ¼ 0.25 mm/

rev, and depth of cut¼ 2.0 mm The lathe has amechanical efficiency¼ 0.85 Based on the specificenergy values in Table 21.2, determine (a) the cut-ting power and (b) gross power in the turningoperation, in Watts

21.29 Solve Problem 21.27 but with the following changes:cutting speed¼ 1.3 m/s, feed ¼ 0.75 mm/rev, anddepth¼ 4.0 mm Note that although the power used

in this operation is only about 10% greater than inthe previous problem, the metal removal rate isabout 40% greater

21.30 A turning operation is performed on an enginelathe using a tool with zero rake angle in thedirection of chip flow The work material is analloy steel with hardness¼ 325 Brinell hardness.The feed is 0.015 in/rev, depth of cut is 0.125 in andcutting speed is 300 ft/min After the cut, the chipthickness ratio is measured to be 0.45 (a) Using theappropriate value of specific energy from Table21.2, compute the horsepower at the drive motor, ifthe lathe has an efficiency ¼ 85% (b) Based onhorsepower, compute your best estimate of thecutting force for this turning operation Use theorthogonal cutting model as an approximation ofthe turning process

21.31 A lathe performs a turning operation on a piece of 6.0 in diameter The shear strength of thework is 40,000 lb/in2 and the tensile strength is60,000 lb/in2 The rake angle of the tool is 6 Thecutting speed¼ 700 ft/min, feed ¼ 0.015 in/rev, anddepth¼ 0.090 in The chip thickness after the cut is0.025 in Determine (a) the horsepower required inthe operation, (b) unit horsepower for this materialunder these conditions, and (c) unit horsepower as

work-it would be listed in Table 21.2 for a toof 0.010 in.Use the orthogonal cutting model as an approxi-mation of the turning process

21.32 In a turning operation on an aluminum alloy piece, the feed¼ 0.020 in/rev, and depth of cut ¼0.250 in The motor horsepower of the lathe is 20 hpand it has a mechanical efficiency¼ 92% The unithorsepower value¼ 0.25 hp/(in3

work-/min) for this minum grade What is the maximum cutting speedthat can be used on this job?

alu-Cutting Temperature

21.33 Orthogonal cutting is performed on a metal whosemass specific heat¼ 1.0 J/g-C, density ¼ 2.9 g/cm3,and thermal diffusivity ¼ 0.8 cm2

/s The cuttingspeed is 4.5 m/s, uncut chip thickness is 0.25 mm,and width of cut is 2.2 mm The cutting force ismeasured at 1170 N Using Cook’s equation, deter-mine the cutting temperature if the ambient tem-perature¼ 22C

21.34 Consider a turning operation performed on steelwhose hardness¼ 225 HB at a speed ¼ 3.0 m/s,feed¼ 0.25 mm, and depth ¼ 4.0 mm Using values

of thermal properties found in the tables anddefinitions of Section 4.1 and the appropriate

specific energy value from Table 21.2, compute

an estimate of cutting temperature using theCook equation Assume ambient temperature ¼

20C

21.35 An orthogonal cutting operation is performed on acertain metal whose volumetric specific heat¼ 110in-lb/in3-F, and thermal diffusivity¼ 0.140 in2

/sec.The cutting speed¼ 350 ft/min, chip thickness be-fore the cut¼ 0.008 in, and width of cut ¼ 0.100 in.The cutting force is measured at 200 lb UsingCook’s equation, determine the cutting tempera-ture if the ambient temperature¼ 70F

Assume ambient temperature¼ 88F.

21.37 An orthogonal machining operation removes

metal at 1.8 in3/min The cutting force in the

process¼ 300 lb The work material has a thermal

diffusivity¼ 0.18 in2

/sec and a volumetric specificheat¼ 124 in-lb/in3-F If the feed f¼ to¼ 0.010 in

and width of cut¼ 0.100 in, use the Cook formula

to compute the cutting temperature in the

opera-tion given that ambient temperature¼ 70F

couple was used to measure cutting temperature.The following temperature data were collectedduring the cuts at three different cutting speeds(feed and depth were held constant): (1) v¼ 100 m/min, T¼ 505C, (2) v¼ 130 m/min, T ¼ 552C,(3) v ¼ 160 m/min, T ¼ 592C Determine anequation for temperature as a function of cuttingspeed that is in the form of the Trigger equation,

Eq (21.23)

22 MACHINING OPERATIONS AND

MACHINE TOOLS

Chapter Contents

22.1 Machining and Part Geometry22.2 Turning and Related Operations22.2.1 Cutting Conditions in Turning22.2.2 Operations Related to Turning22.2.3 The Engine Lathe

22.2.4 Other Lathes and Turning Machines22.2.5 Boring Machines

22.3 Drilling and Related Operations22.3.1 Cutting Conditions in Drilling22.3.2 Operations Related to Drilling22.3.3 Drill Presses

22.4 Milling22.4.1 Types of Milling Operations22.4.2 Cutting Conditions in Milling22.4.3 Milling Machines

22.5 Machining Centers and Turning Centers22.6 Other Machining Operations

22.6.1 Shaping and Planing22.6.2 Broaching

22.6.3 Sawing22.7 Machining Operations for Special Geometries22.7.1 Screw Threads

22.7.2 Gears22.8 High-Speed Machining

Machining is the most versatile and accurate of all ufacturing processes in its capability to produce a diversity

man-of part geometries and geometric features Casting can alsoproduce a variety of shapes, but it lacks the precision andaccuracy of machining In this chapter, we describe theimportant machining operations and the machine toolsused to perform them Historical Note 22.1 provides a briefnarrative of the development of machine tool technology

22.1 MACHINING AND PART

GEOMETRY

To introduce our topic in this chapter, let us provide anoverview of the creation of part geometry by machining.Machined parts can be classified as rotational or nonrota-tional (Figure 22.1) A rotational workpart has a cylindrical ordisk-like shape The characteristic operation that producesthis geometry is one in which a cutting tool removes materialfrom a rotating workpart Examples include turning andboring Drilling is closely related except that an internalcylindrical shape is created and the tool rotates (ratherthan the work) in most drilling operations A nonrotational(also called prismatic) workpart is block-like or plate-like, as

in Figure 22.1(b) This geometry is achieved by linear motions

of the workpart, combined with either rotating or linear toolmotions Operations in this category include milling, shaping,planing, and sawing

Each machining operation produces a characteristicgeometry due to two factors: (1) the relative motions be-tween the tool and the workpart and (2) the shape of thecutting tool We classify these operations by which partshape is created as generating and forming In generating,the geometry of the workpart is determined by the feedtrajectory of the cutting tool The path followed by the toolduring its feed motion is imparted to the work surface inorder to create shape Examples of generating the work

507

shape in machining include straight turning, taper turning, contour turning, peripheralmilling, and profile milling, all illustrated in Figure 22.2 In each of these operations,material removal is accomplished by the speed motion in the operation, but part shape isdetermined by the feed motion The feed trajectory may involve variations in depth orwidth of cut during the operation For example, in the contour turning and profile millingoperations shown in our figure, the feed motion results in changes in depth and width,respectively, as cutting proceeds.

In forming, the shape of the part is created by the geometry of the cutting tool Ineffect, the cutting edge of the tool has the reverse of the shape to be produced on the partsurface Form turning, drilling, and broaching are examples of this case In theseoperations, illustrated in Figure 22.3, the shape of the cutting tool is imparted to thework in order to create part geometry The cutting conditions in forming usually includethe primary speed motion combined with a feeding motion that is directed into the work

FIGURE 22.1 Machined parts are classified as (a) rotational, or (b) nonrotational, shown here by blockand flat parts

to drill holes

Development of modern machine tools is closely

related to the Industrial Revolution When James Watt

designed his steam engine in England around 1763, one

of the technical problems he faced was to make the bore

of the cylinder sufficiently accurate to prevent steam

from escaping around the piston John Wilkinson built a

water-wheel powered boring machine around 1775,

which permitted Watt to build his steam engine

This boring machine is often recognized as the first

machine tool

Another Englishman, Henry Maudsley, developed the

first screw-cutting lathe around 1800 Although the

turning of wood had been accomplished for many

centuries, Maudsley’s machine added a mechanized tool

Development of the planer and shaper occurred inEngland between 1800 and 1835, in response to theneed to make components for the steam engine, textileequipment, and other machines associated with theIndustrial Revolution The powered drill press wasdeveloped by James Nasmyth around 1846, whichpermitted drilling of accurate holes in metal

Most of the conventional boring machines, lathes,milling machines, planers, shapers, and drill presses usedtoday have the same basic designs as the early versionsdeveloped during the last two centuries Modernmachining centers—machine tools capable ofperforming more than one type of cutting operation—were introduced in the late 1950s, after numericalcontrol had been developed (Historical Note 38.1)

FIGURE 22.2 Generating shape in machining: (a) straight turning, (b) taper turning, (c) contour turning, (d) plainmilling, and (e) profile milling.

FIGURE 22.3 Forming to create shape in machining: (a) form turning, (b) drilling, and (c) broaching

Depth of cut in this category of machining usually refers to the final penetration into thework after the feed motion has been completed.

Forming and generating are sometimes combined in one operation, as illustrated inFigure 22.4 for thread cutting on a lathe and slotting on a milling machine In threadcutting, the pointed shape of the cutting tool determines the form of the threads, but thelarge feed rate generates the threads In slotting (also called slot milling), the width of thecutter determines the width of the slot, but the feed motion creates the slot

Machining is classified as a secondary process In general, secondary processesfollow basic processes, whose purpose is to establish the initial shape of a workpiece.Examples of basic processes include casting, forging, and bar rolling (to produce rod andbar stock) The shapes produced by these processes usually require refinement bysecondary processes Machining operations serve to transform the starting shapes intothe final geometries specified by the part designer For example, bar stock is the initialshape, but the final geometry after a series of machining operations is a shaft We discussbasic and secondary processes in more detail and provide additional examples in Section40.1.1 on process planning

22.2 TURNING AND RELATED OPERATIONS

Turning is a machining process in which a single-point tool removes material from thesurface of a rotating workpiece The tool is fed linearly in a direction parallel to the axis ofrotation to generate a cylindrical geometry, as illustrated in Figures 22.2(a) and 22.5 Single-point tools used in turning and other machining operations are discussed in Section 23.3.1.Turning is traditionally carried out on a machine tool called a lathe, which provides power

to turn the part at a given rotational speed and to feed the tool at a specified rate and depth

of cut Included on the DVD that accompanies this text is a video clip on turning

and generating to create

shape: (a) thread cutting

on a lathe, and (b) slot

milling

22.2.1 CUTTING CONDITIONS IN TURNING

The rotational speed in turning is related to the desired cutting speed at the surface of thecylindrical workpiece by the equation

N ¼pDv

where N ¼ rotational speed, rev/min; v ¼ cutting speed, m/min (ft/min); and Do ¼original diameter of the part, m (ft)

The turning operation reduces the diameter of the work from its original diameter

Doto a final diameter Df, as determined by the depth of cut d:

The feed in turning is generally expressed in mm/rev (in/rev) This feed can be converted

to a linear travel rate in mm/min (in/min) by the formula

where fr¼ feed rate, mm/min (in/min); and f ¼ feed, mm/rev (in/rev)

The time to machine from one end of a cylindrical workpart to the other is given by

FIGURE 22.5 Turningoperation

22.2.2 OPERATIONS RELATED TO TURNING

A variety of other machining operations can be performed on a lathe in addition toturning; these include the following, illustrated in Figure 22.6:

FIGURE 22.6 Machining operations other than turning that are performed on a lathe: (a) facing, (b) taper turning,(c) contour turning, (d) form turning, (e) chamfering, (f) cutoff, (g) threading, (h) boring, (i) drilling, and (j) knurling

(a) Facing The tool is fed radially into the rotating work on one end to create a flatsurface on the end.

(b) Taper turning Instead of feeding the tool parallel to the axis of rotation of the work,the tool is fed at an angle, thus creating a tapered cylinder or conical shape.(c) Contour turning Instead of feeding the tool along a straight line parallel to the axis ofrotation as in turning, the tool follows a contour that is other than straight, thuscreating a contoured form in the turned part

(d) Form turning In this operation, sometimes called forming, the tool has a shape that isimparted to the work by plunging the tool radially into the work

(e) Chamfering The cutting edge of the tool is used to cut an angle on the corner of thecylinder, forming what is called a‘‘chamfer.’’

(f) Cutoff The tool is fed radially into the rotating work at some location along its length

to cut off the end of the part This operation is sometimes referred to as parting.(g) Threading A pointed tool is fed linearly across the outside surface of the rotatingworkpart in a direction parallel to the axis of rotation at a large effective feed rate, thuscreating threads in the cylinder Methods of machining screw threads are discussed ingreater detail in Section 22.7.1

(h) Boring A single-point tool is fed linearly, parallel to the axis of rotation, on the insidediameter of an existing hole in the part

(i) Drilling Drilling can be performed on a lathe by feeding the drill into the rotatingwork along its axis Reaming can be performed in a similar way

(j) Knurling This is not a machining operation because it does not involve cutting ofmaterial Instead, it is a metal forming operation used to produce a regular cross-hatched pattern in the work surface

Most lathe operations use single-point tools, which we discuss in Section 23.3.1.Turning, facing, taper turning, contour turning, chamfering, and boring are all performedwith single-point tools A threading operation is accomplished using a single-point tooldesigned with a geometry that shapes the thread Certain operations require tools otherthan single-point Form turning is performed with a specially designed tool called a formtool The profile shape ground into the tool establishes the shape of the workpart Acutoff tool is basically a form tool Drilling is accomplished by a drill bit (Section 23.3.2).Knurling is performed by a knurling tool, consisting of two hardened forming rolls, eachmounted between centers The forming rolls have the desired knurling pattern on theirsurfaces To perform knurling, the tool is pressed against the rotating workpart withsufficient pressure to impress the pattern onto the work surface

22.2.3 THE ENGINE LATHE

The basic lathe used for turning and related operations is an engine lathe It is a versatilemachine tool, manually operated, and widely used in low and medium production The termengine dates from the time when these machines were driven by steam engines

Engine Lathe Technology Figure 22.7 is a sketch of an engine lathe showing itsprincipal components The headstock contains the drive unit to rotate the spindle, whichrotates the work Opposite the headstock is the tailstock, in which a center is mounted tosupport the other end of the workpiece

The cutting tool is held in a tool post fastened to the cross-slide, which is assembled to thecarriage The carriage is designed to slide along the ways of the lathe in order to feed the toolparallel to the axis of rotation The ways are like tracks along which the carriage rides, and they

are madewithgreatprecisiontoachievea highdegreeofparallelism relative to thespindleaxis.The ways are built into the bed of the lathe, providing a rigid frame for the machine tool.The carriage is driven by a leadscrew that rotates at the proper speed to obtain thedesired feed rate The cross-slide is designed to feed in a direction perpendicular to thecarriage movement Thus, by moving the carriage, the tool can be fed parallel to the workaxis to perform straight turning; or by moving the cross-slide, the tool can be fed radiallyinto the work to perform facing, form turning, or cutoff operations.

The conventional engine lathe and most other machines described in this section arehorizontal turning machines; that is, the spindle axis is horizontal This is appropriate forthe majority of turning jobs, in which the length is greater than the diameter For jobs inwhich the diameter is large relative to length and the work is heavy, it is more convenient toorient the work so that it rotates about a vertical axis; these are vertical turning machines.The size of a lathe is designated by swing and maximum distance between centers.The swing is the maximum workpart diameter that can be rotated in the spindle, deter-mined as twice the distance between the centerline of the spindle and the ways of themachine The actual maximum size of a cylindrical workpiece that can be accommodated

on the lathe is smaller than the swing because the carriage and cross-slide assembly are inthe way The maximum distance between centers indicates the maximum length of aworkpiece that can be mounted between headstock and tailstock centers For example, a

350 mm 1.2 m (14 in  48 in) lathe designates that the swing is 350 mm (14 in) and themaximum distance between centers is 1.2 m (48 in)

Methods of Holding the Work in a Lathe There are four common methods used to holdworkparts in turning These workholding methods consist of various mechanisms to graspthe work, center and support it in position along the spindle axis, and rotate it The methods,illustrated in Figure 22.8, are (a) mounting the work between centers, (b) chuck, (c) collet,and (d) face plate Our video clip on workholding illustrates the various aspects offixturing for turning and other machining operations

VIDEO CLIP

Introduction to Workholding This clip contains four segments: (1) workholding of parts,(2) principles of workholding, (3) 3-2-1 locational workholding method, and (4) work-piece reclamping

FIGURE 22.7 Diagram

of an engine lathe,

indicating its principal

components

Holding the work between centers refers to the use of two centers, one in theheadstock and the other in the tailstock, as in Figure 22.8(a) This method is appropriatefor parts with large length-to-diameter ratios At the headstock center, a device called a dog

is attached to the outside of the work and is used to drive the rotation from the spindle Thetailstock center has a cone-shaped point which is inserted into a tapered hole in the end ofthe work The tailstock center is either a‘‘live’’ center or a ‘‘dead’’ center A live centerrotates in a bearing in the tailstock, so that there is no relative rotation between the workand the live center, hence, no friction between the center and the workpiece In contrast, adead center is fixed to the tailstock, so that it does not rotate; instead, the workpiece rotatesabout it Because of friction and the heat buildup that results, this setup is normally used atlower rotational speeds The live center can be used at higher speeds

The chuck, Figure 22.8(b), is available in several designs, with three or four jaws tograsp the cylindrical workpart on its outside diameter The jaws are often designed so theycan also grasp the inside diameter of a tubular part A self-centering chuck has a mechanism

to move the jaws in or out simultaneously, thus centering the work at the spindle axis Otherchucks allow independent operation of each jaw Chucks can be used with or without atailstock center For parts with low length-to-diameter ratios, holding the part in the chuck

in a cantilever fashion is usually sufficient to withstand the cutting forces For longworkbars, the tailstock center is needed for support

A collet consists of a tubular bushing with longitudinal slits running over half itslength and equally spaced around its circumference, as in Figure 22.8(c) The insidediameter of the collet is used to hold cylindrical work such as barstock Owing to the slits,one end of the collet can be squeezed to reduce its diameter and provide a secure graspingpressure against the work Because there is a limit to the reduction obtainable in a collet

FIGURE 22.8 Four workholding methods used in lathes: (a) mounting the work between centers using a dog,(b) three-jaw chuck, (c) collet, and (d) faceplate for noncylindrical workparts

22.2.4 OTHER LATHES AND TURNING MACHINES

In addition to the engine lathe, other turning machines have been developed to satisfyparticular functions or to automate the turning process Among these machines are(1) toolroom lathe, (2) speed lathe, (3) turret lathe, (4) chucking machine, (5) automaticscrew machine, and (6) numerically controlled lathe

The toolroom lathe and speed lathe are closely related to the engine lathe Thetoolroom lathe is smaller and has a wider available range of speeds and feeds It is alsobuilt for higher accuracy, consistent with its purpose of fabricating components for tools,fixtures, and other high-precision devices

The speed lathe is simpler in construction than the engine lathe It has no carriage andcross-slide assembly, and therefore no leadscrew to drive the carriage The cutting tool isheld by the operator using a rest attached to the lathe for support The speeds are higher on

a speed lathe, but the number of speed settings is limited Applications of this machine typeinclude wood turning, metal spinning, and polishing operations

A turret lathe is a manually operated lathe in which the tailstock is replaced by a turretthat holds up to six cutting tools These tools can be rapidly brought into action against the workone by one by indexing the turret In addition, the conventional tool post used on an enginelathe is replaced by a four-sided turret that is capable of indexing up to four tools into position.Hence, because of the capacity to quickly change from one cutting tool to the next, the turretlathe is used for high-production work that requires a sequence of cuts to be made on the part

As the name suggests, a chucking machine (nicknamed chucker) uses a chuck in itsspindle to hold the workpart The tailstock is absent on a chucker, so parts cannot bemounted between centers This restricts the use of a chucking machine to short, light-weight parts The setup and operation are similar to a turret lathe except that the feedingactions of the cutting tools are controlled automatically rather than by a human operator.The function of the operator is to load and unload the parts

A bar machine is similar to a chucking machine except that a collet is used (instead of

a chuck), which permits long bar stock to be fed through the headstock into position At theend of each machining cycle, a cutoff operation separates the new part The bar stock is thenindexed forward to present stock for the next part Feeding the stock as well as indexing andfeeding the cutting tools is accomplished automatically Owing to its high level of automaticoperation, it is often called an automatic bar machine One of its important applications is

in the production of screws and similar small hardware items; the name automatic screwmachine is frequently used for machines used in these applications

Bar machines can be classified as single spindle or multiple spindle A single spindlebar machine has one spindle that normally allows only one cutting tool to be used at a time

on the single workpart being machined Thus, while each tool is cutting the work, the othertools are idle (Turret lathes and chucking machines are also limited by this sequential,rather than simultaneous, tool operation) To increase cutting tool utilization and produc-tion rate, multiple spindle bar machines are available These machines have more than onespindle, so multiple parts are machined simultaneously by multiple tools For example, a six-spindle automatic bar machine works on six parts at a time, as in Figure 22.9 At the end ofeach machining cycle, the spindles (including collets and workbars) are indexed (rotated) tothe next position In our illustration, each part is cut sequentially by five sets of cutting tools,

which takes six cycles (position 1 is for advancing the bar stock to a‘‘stop’’) With thisarrangement, a part is completed at the end of each cycle As a result, a six-spindleautomatic screw machine has a very high production rate.

The sequencing and actuation of the motions on screw machines and chuckingmachines have traditionally been controlled by cams and other mechanical devices Themodern form of control is computer numerical control (CNC), in which the machine tooloperations are controlled by a‘‘program of instructions’’ consisting of alphanumeric code(Section 38.3) CNC provides a more sophisticated and versatile means of control thanmechanical devices This has led to the development of machine tools capable of morecomplex machining cycles and part geometries, and a higher level of automated operationthan conventional screw machines and chucking machines The CNC lathe is an example ofthese machines in turning It is especially useful for contour turning operations and closetolerance work Today, automatic chuckers and bar machines are implemented by CNC

22.2.5 BORING MACHINES

Boring is similar to turning It uses a single-point tool against a rotating workpart Thedifference is that boring is performed on the inside diameter of an existing hole rather thanthe outside diameter of an existing cylinder In effect, boring is an internal turning operation.Machine tools used to perform boring operations are called boring machines (also boringmills) One might expect that boring machines would have features in common with turningmachines; indeed, as previously indicated, lathes are sometimes used to accomplish boring.Boring mills can be horizontal or vertical The designation refers to the orientation ofthe axis of rotation of the machine spindle or workpart In a horizontal boring operation,the setup can be arranged in either of two ways The first setup is one in which the work isfixtured to a rotating spindle, and the tool is attached to a cantilevered boring bar that feeds

FIGURE22.9 (a) Type of part produced on a six-spindle automatic bar machine; and (b) sequence of operations

to produce the part: (1) feed stock to stop, (2) turn main diameter, (3) form second diameter and spotface, (4) drill,(5) chamfer, and (6) cutoff

into the work, as illustrated in Figure 22.10(a) The boring bar in this setup must be very stiff

to avoid deflection and vibration during cutting To achieve high stiffness, boring bars areoften made of cemented carbide, whose modulus of elasticity approaches 620 103MPa(90 106lb/in2) Figure 22.11 shows a carbide boring bar

The second possible setup is one in which the tool is mounted to a boring bar, andthe boring bar is supported and rotated between centers The work is fastened to afeeding mechanism that feeds it past the tool This setup, Figure 22.10(b), can be used toperform a boring operation on a conventional engine lathe

A vertical boring machine is used for large, heavy workparts with large diameters;usually the workpart diameter is greater than its length As in Figure 22.12, the part isclamped to a worktable that rotates relative to the machine base Worktables up to 40 ft indiameter are available The typical boring machine can position and feed several cutting

FIGURE 22.10 Two forms of horizontal boring: (a) boring bar is fed into a rotating workpart, and (b) work is fed past arotating boring bar

FIGURE 22.11 Boring

bar made of cemented

carbide (WC–Co) that

uses indexable cemented

carbide inserts (Courtesy

of Kennametal Inc.,

Latrobe, Pennsylvania.)

tools simultaneously The tools are mounted on tool heads that can be fed horizontally andvertically relative to the worktable One or two heads are mounted on a horizontal cross-railassembled to the machine tool housing above the worktable The cutting tools mountedabove the work can be used for facing and boring In addition to the tools on the cross-rail,one or two additional tool heads can be mounted on the side columns of the housing toenable turning on the outside diameter of the work.

The tool heads used on a vertical boring machine often include turrets toaccommodate several cutting tools This results in a loss of distinction between thismachine and a vertical turret lathe Some machine tool builders make the distinction thatthe vertical turret lathe is used for work diameters up to 2.5 m (100 in), while the verticalboring machine is used for larger diameters [7] Also, vertical boring mills are oftenapplied to one-of-a-kind jobs, while vertical turret lathes are used for batch production

22.3 DRILLING AND RELATED OPERATIONS

Drilling, Figure 22.3(b), is a machining operation used to create a round hole in aworkpart This contrasts with boring, which can only be used to enlarge an existing hole.Drilling is usually performed with a rotating cylindrical tool that has two cutting edges

on its working end The tool is called a drill or drill bit (described in Section 23.3.2) Themost common drill bit is the twist drill, described in Section 23.3.2 The rotating drillfeeds into the stationary workpart to form a hole whose diameter is equal to the drilldiameter Drilling is customarily performed on a drill press, although other machinetools also perform this operation The video clip on hole making illustrates the drillingoperation

pDwhere v¼ cutting speed, mm/min (in/min); and D ¼ the drill diameter, mm (in) In somedrilling operations, the workpiece is rotated about a stationary tool, but the same formulaapplies.

Feed f in drilling is specified in mm/rev (in/rev) Recommended feeds are roughlyproportional to drill diameter; higher feeds are used with larger diameter drills Sincethere are (usually) two cutting edges at the drill point, the uncut chip thickness (chipload) taken by each cutting edge is half the feed Feed can be converted to feed rate usingthe same equation as for turning:

where fr¼ feed rate, mm/min (in/min)

Drilled holes are either through holes or blind holes, Figure 22.13 In through holes,the drill exits the opposite side of the work; in blind holes, it does not The machiningtime required to drill a through hole can be determined by the following formula:

Tm¼t þ Af

where Tm¼ machining (drilling) time, min; t ¼ work thickness, mm (in); fr¼ feed rate,mm/min (in/min); and A¼ an approach allowance that accounts for the drill point angle,representing the distance the drill must feed into the work before reaching full diameter,Figure 22.10(a) This allowance is given by

A ¼ 0:5 D tan 90 u

2

ð22:10Þwhere A¼ approach allowance, mm (in); and u ¼ drill point angle In drilling a throughhole, the feed motion usually proceeds slightly beyond the opposite side of the work,

FIGURE 22.13 Two

hole types: (a) through

hole and (b) blind hole

thus making the actual duration of the cut greater than Tm in Eq (22.9) by a smallamount.

In a blind-hole, hole depth d is defined as the distance from the work surface to thedepth of the full diameter, Figure 22.13(b) Thus, for a blind hole, machining time is given by

Tm¼d þ Af

where A¼ the approach allowance by Eq (22.10)

The rate of metal removal in drilling is determined as the product of the drill sectional area and the feed rate:

22.3.2 OPERATIONS RELATED TO DRILLING

Several operations are related to drilling These are illustrated in Figure 22.14 and described

in this section Most of the operations follow drilling; a hole must be made first by drilling,and then the hole is modified by one of the other operations Centering and spot facing areexceptions to this rule All of the operations use rotating tools

(a) Reaming Reaming is used to slightly enlarge a hole, to provide a better tolerance onits diameter, and to improve its surface finish The tool is called a reamer, and it usuallyhas straight flutes

(b) Tapping This operation is performed by a tap and is used to provide internal screwthreads on an existing hole Tapping is discussed in more detail in Section 22.7.1

FIGURE 22.14Machining operationsrelated to drilling:

(a) reaming, (b) tapping,(c) counterboring,(d) countersinking,(e) center drilling, and(f) spot facing

surface on the workpart in a localized area.

22.3.3 DRILL PRESSES

The standard machine tool for drilling is the drill press There are various types of drill press,the most basic of which is the upright drill, Figure 22.15 The upright drill stands on the floorand consists of a table for holding the workpart, a drilling head with powered spindle for thedrill bit, and a base and column for support A similar drill press, but smaller, is the benchdrill, which is mounted on a table or bench rather than the floor

The radial drill, Figure 22.16, is a large drill press designed to cut holes in largeparts It has a radial arm along which the drilling head can be moved and clamped Thehead therefore can be positioned along the arm at locations that are a significant distancefrom the column to accommodate large work The radial arm can also be swiveled aboutthe column to drill parts on either side of the worktable

The gang drill is a drill press consisting basically of two to six upright drills connectedtogether in an in-line arrangement Each spindle is powered and operated independently,and they share a common worktable, so that a series of drilling and related operations can

be accomplished in sequence (e.g., centering, drilling, reaming, tapping) simply by slidingthe workpart along the worktable from one spindle to the next A related machine is themultiple-spindle drill, in which several drill spindles are connected together to drillmultiple holes simultaneously into the workpart

In addition, CNC drill presses are available to control the positioning of the holes inthe workparts These drill presses are often equipped with turrets to hold multiple tools thatcan be indexed under control of the CNC program The term CNC turret drill is used forthese machine tools

Workholding on a drill press is accomplished by clamping the part in a vise, fixture,

or jig A vise is a general-purpose workholding device possessing two jaws that grasp the

FIGURE 22.15 Upr ightdrill press