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Chapter 8

Material-Removal Processes: Cutting

Questions

8.1 Explain why the cutting force, Fc, increases

with increasing depth of cut and decreasing

material being removed per unit time.

Thus, all other parameters remaining con-

stant, the cutting force has to increase lin-

8.2 What are the effects of performing a cutting

operation with a dull tool tip? A very sharp

tip?

the heat generated by the dull tool tip rubbing

against this surface. Dull tools also increase

8.3 Describe the trends that you observe in Tables

8.1 and 8.2.

By the student. A review of Tables 8.1 and 8.2

8.4 To what factors would you attribute the large

difference in the specific energies within each

group of materials shown in Table 8.3?

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8.5 Describe the effects of cutting fluids on chip for-

mation. Explain why and how they influence

8.6 Under what conditions would you discourage

the use of cutting fluids? Explain.

be discouraged under the following conditions:

(a) If the cutting fluid has any adverse ef-

8.7 Give reasons that pure aluminum and copper

are generally rated as easy to machine.

the middle of the tool-chip interface. The chip

reaches high temperatures in the primary shear

zone; the temperature would decrease from

temperature is highest somewhere in between

8.9 State whether or not the following statements

are true for orthogonal cutting, explaining your

reasons: (a) For the same shear angle, there

(a) To show that for the same shear angle

there are two rake angles and given the

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p. 430.

able to allow temperatures to rise excessively in

The consequences of allowing temperatures to

rise to high levels in cutting include:

cutting zone may induce metallurgical

changes and cause thermal damage to the

machined surface, thus affecting surface

8.11 Explain the reasons that the same tool life may

be obtained at two different cutting speeds.

8.12 Inspect Table 8.6 and identify tool materials

that would not be particularly suitable for in-

subjected to thermal cycling. It is thus desir-

able to utilize materials with low coefficients of

8.13 Explain the possible disadvantages of a machin–

8.2.1. Note that:

(a) The forces will continuously vary, possibly

8.14 It has been noted that tool life can be almost

infinite at low cutting speeds. Would you then

recommend that all machining be done at low

speeds? Explain.

life can be almost infinite at very low cutting

speeds, but this reason alone would not al-

carbides?

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improves is toughness and transverse-rupture

1997.

200

1000

85.7

e

r

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e

–

r

u

p

t

u

r

e

s

t

r

e

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h

8.16 Explain why studying the types of chips pro-

duced is important in understanding machining

sociated with good surface finish. Built-up-edge

chips usually result in poor surface finish. Ser-

8.18 Wood is a highly anisotropic material; that is,

When cutting a highly anisotropic material

direction would produce long continuous chips

by virtue of a splitting action ahead of the tool.

observed in metal cutting. These phenomena

can be demonstrated with a wood plane and

8.19 Describe the advantages of oblique cutting.

Which machining proceses involve oblique cut-

ting? Explain.

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Fig. 8.21a on p. 441. Note that at any loca-

tion on a particular curve, the product of cut-

ting speed (ft/min) and tool life (min) is the

distance (ft) the tool travels before it reached

the end of its life (a specified wear land). The

(a) The economics of the machining process

is that it determines an effective rake angle for

8.22 How would you go about measuring the hot

hardness of cutting tools? Explain any diffi-

Hot hardness refers to the hardness of the ma-

terial at the elevated temperatures typical of

the particular cutting operation (see Fig. 8.30

on p. 453). Once the temperature is known

(which can be measured with thermocouples or

can be estimated), the hardness of the material

can be evaluated at this temperature. A simple

method of doing so is by heating the tool mate-

rial, then subjecting it to a hardness test while

it is still hot.

with multiphase coatings of different materials.

Describe the properties that the substrate for

multiphase cutting tools should have for effec-

tive machining.

8.24 Explain the advantages and any limitations of

inserts. Why were they developed?

8.25 Make a list of alloying elements in high-speed-

steel cutting tools. Explain why they are used.

and wear resistance at elevated temperatures.

8.26 What are the purposes of chamfers on cutting

tools? Explain.

Chamfers serve to increase the strength of in-

serts by effectively increasing the included angle

of the insert. This trend is shown in Fig. 8.34

on p. 458. The tendency of edge chipping is

thus reduced.

8.27 Why does temperature have such an important

effect on cutting-tool performance?

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8.28 Ceramic and cermet cutting tools have certain

advantages over carbide tools. Why, then, are

carbide tools not replaced to a greater extent?

Ceramics are preferable to carbides in that they

have a lower tendency to adhere to metals be-

ing cut, and have very high abrasion resistance

and hot hardness. However, ceramics are sen-

sitive to defects and are generally brittle, and

thus can fail prematurely. Carbides are much

tougher than ceramics, and are therefore much

more likely to perform as expected even when

8.29 Why are chemical stability and inertness im-

portant in cutting tools?

8.30 What precautions would you take in machining

with brittle tool materials, especially ceramics?

vent chipping, such as by using negative rake

penetrate the asperities; therefore, it will be less

effective at higher speeds. Furthermore, cut-

ting fluids whose effectiveness depends on their

chemical reactivity with surfaces, will have less

time to react and to develop low-shear-strength

films. At higher cutting speeds, temperatures

increase significantly and hence cutting fluids

should have a cooling capacity as a major at-

tribute.

8.32 Which of the two materials, diamond or cubic

boron nitride, is more suitable for machining

steels? Why?

This is because cBN, unlike diamond, is chem-

ically inert to iron at high temperatures, thus

rials, since many contain expensive materials

of strategic importance such as tungsten and

ature in machining, and explain why and how

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8.35 List and explain the factors that contribute to

poor surface finish in machining operations.

By the student. Recall, for example, in turning

8.36 Explain the functions of the different angles on

a single-point lathe cutting tool. How does the

chip thickness vary as the side cutting-edge an-

gle is increased? Explain.

These are described in Section 8.8.1 and can

also be found in various handbooks on machin-

ing. As the side cutting-edge angle is increased,

8.37 It will be noted that the helix angle for drills is

different for different groups of workpiece ma-

terials. Why?

8.38 A turning operation is being carried out on a

long, round bar at a constant depth of cut. Ex-

8.39 Describe the relative characteristics of climb

milling and up milling and their importance in

machining operations.

nent of the cutting force holds the workpiece

in place. Note, however, that workpiece surface

conditions can affect tool wear.

8.40 In Fig. 8.64a, high-speed-steel cutting teeth are

welded to a steel blade. Would you recommend

that the whole blade be made of high-speed

steel? Explain your reasons.

cutting surface and a tough, thermally conduc-

tive material in the bulk of the blade. This is an

economical method of producing high-quality

central region of Fig. 8.72 on p. 501.

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8.42 Make a list of components of machine tools that

could be made of ceramics, and explain why ce-

ramics would be a suitable material for these

be members that reciprocate at high speeds

or members that move at high speeds and are

8.43 In Fig. 8.12, why do the thrust forces start at a

finite value when the feed is zero? Explain.

8.44 Is the temperature rise in cutting related to the

hardness of the workpiece material? Explain.

Because hardness and strength are related (see

Section 2.6.8), the hardness of the workpiece

8.45 Describe the effects of tool wear on the work-

piece and on the overall machining operation.

By the student. Tool wear can adversely affect

cutting speed, with rapidly decreasing tool life.

8.47 Are there any machining operations that can-

not be performed on (a) machining centers and

(b) turning centers? Explain.

8.11. In theory, every cutting operation can be

performed on a machining center, if we consider

machining?

ics, as described in detail in Section 8.2.

8.49 Emulsion cutting fluids typically consist of 95%

water and 5% soluble oil and chemical addi-

tives. Why is the ratio so unbalanced? Is the

machining fluids have, as their primary pur-

pose, the cooling of the cutting zone (water be-

ing an excellent coolant). However, the oil is

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8.51 Assume that you are asked to estimate the cut-

ting force in slab milling with a straight-tooth

better when it is moved back and forth. Con-

sider factors such as the material being cut, in-

By the student. One obvious effect is that the

longitudinal movement of the knife reduces the

vertical component of the friction force vec-

tor, thus the material being cut is not dragged

downward. (Consider, for example, cutting a

couraged to inspect the cutting edge of knives,

especially sharp ones, under a microscope and

8.53 What are the effects of lowering the friction

at the tool-chip interface (say with an effective

cutting fluid) on the mechanics of cutting oper-

ations? Explain, giving several examples.

8.54 Why is it not always advisable to increase cut-

8.55 It has been observed that the shear-strain rate

in metal cutting is high even though the cutting

speed may be relatively low. Why?

tion 8.2, and is associated with Eqs. (8.6) and

(8.7) on p. 421.

8.56 We note from the exponents in Eq. (8.30) that

the cutting speed has a greater influence on

temperature than does the feed. Why?

8.57 What are the consequences of exceeding the al-

lowable wear land (see Table 8.5) for cutting

tools? Explain.

(a) As the wear land increases, the wear flat

will rub against the machined surface and

thus temperature will increase due to fric-

tion.

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8.58 Comment on and explain your observations re-

garding Figs. 8.34, 8.38, and 8.43.

By the student. For example, from Fig. 8.34

on p. 458 it is clear that edge strength can be

8.59 It will noted that the tool-life curve for ceramic

cutting tools in Fig. 8.22a is to the right of those

for other tools. Why?

8.60 In Fig. 8.18, it can be seen that the percentage

of the energy carried away by the chip increases

with cutting speed. Why?

lubricity, specific heat, and chemical reactions

(see Chapter 4 for details). The students are

encouraged to develop their own ideas for such

tests.

8.62 Describe the conditions that are critical in ben-

and vibrations (see Section 8.12). Tool geom-

etry and setting is also important to minimize

stresses and possible chipping. The workpiece

Explain.

Thermal conductivity is important because

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8.64 It will be noted in Fig. 8.30 that the tool mate-

rials, especially carbides, have a wide range of

hardness at a particular temperature. Why?

By the student. There are various reasons for

sten carbide, showing a fine-grained (left)

and coarse-grained (right) tungsten car-

bide. (Source: Trent, E.M., and Wright,

the carbon content can be different, as can

the level of case hardening of the tool.

about recycling used cutting tools. Comment

on any difficulties involved, as well as on eco-

nomic considerations.

By the student. Recycling is a complicated

subject and involves economic as well as envi-

ronmental considerations (see also pp. 12-15).

Fortunately, cutting-tool materials are gener-

ally non-toxic (with the exception of cobalt in

carbide tools), and thus they can be disposed

of safely. The main consideration is economics:

Is recycling of the tool material cost effective?

8.66 As you can see, there is a wide range of tool

materials available and used successfully today,

yet much research and development continues

to be carried out on these materials. Why?

8.67 Drilling, boring, and reaming of large holes is

generally more accurate than just drilling and

reaming. Why?

a boring tool requires an initial hole, so the

drilling step cannot be eliminated. Reaming

is a generally slow process and produces good

surface finish on a precisely produced hole.

8.68 A highly oxidized and uneven round bar is be-

ing turned on a lathe. Would you recommend a

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wear rapidly, and thus it is highly desirable to

cut right through the oxide layer during the first

the hole depth increases? Explain.

the change is more pronounced on the torque.

Because of elastic recovery along the cylindri-

8.70 Explain the advantages and limitations of pro-

By the student. Thread rolling is described in

Section 6.3.5. The main advantages of thread

difficult operation to perform.

8.71 Describe your observations regarding the con-

tents of Tables 8.8, 8.10, and 8.11.

8.72 The footnote to Table 8.10 states that as the

depth of the hole increases, speeds and feeds

Reduction in feeds and speeds can compensate

8.73 List and explain the factors that contribute to

poor surface finish in machining operations.

or tool-edge chipping (see Fig. 8.28), or vibra-

8.74 Make a list of the machining operations de-

scribed in this chapter, according to the diffi-

culty of the operation and the desired effective-

8.75 Are the feed marks left on the workpiece by

a face-milling cutter segments of a true circle?

Explain with appropriate sketches.

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8.76 What determines the selection of the number

of teeth on a milling cutter? (See, for example,

Figs. 8.53 and 8.55.)

The number of teeth will affect the surface fin-

finer the teeth, the greater the tendency for chip

to clog. At many facilities, the choice of a cut-

8.77 Explain the technical requirements that led to

the development of machining and turning cen-

ters. Why do their spindle speeds vary over a

wide range?

parts in small lots.

8.79 Why is thermal expansion of machine-tool com-

ponents important? Explain, with examples.

When high precision is required, thermal distor-

tion is very important and must be eliminated

8.80 Would using the machining processes described

in this chapter be difficult on nonmetallic or

rubber like materials? Explain your thoughts,

ometries, and the fixturing required.

By the student. Rubber like materials are dif-

that is to be machined from a rectangular

blank. Suggest the type of operations required

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8.82 Select a specific cutting-tool material and esti-

(b)

(c)

Pitch: 12.7 mm

75 mm

50 mm

By the student. Students should address the

8.84 What are the advantages and disadvantages of

dry machining?

8.85 Can high-speed machining be performed with-

out the use of cutting fluids? Explain.

8.86 If the rake angle is 0◦, then the frictional force

is perpendicular to the cutting direction and,

therefore, does not contribute to machining

power requirements. Why, then, is there an in-

crease in the power dissipated when machining

with a rake angle of, say, 20◦?

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p. 428) will now affect the position of the re-

sultant force, R, and thus have a component

8.87 Would you recommend broaching a keyway on

a gear blank before or after the teeth are ma-

chined? Explain.

8.88 Given your understanding of the basic metal-

toughness, thermal conductivity, and thermal

for ceramic, diamond, and cubic boron nitride

tools. Why?

8.90 If a drill bit is intended for woodworking ap-

plications, what material is it most likely to be

made from? (Hint: Temperatures rarely rise to

400◦C in woodworking.) Are there any reasons

why such a drill bit cannot be used to drill a

few holes in a piece of metal? Explain.

that carbon steels maintain a reasonably high

hardness for temperatures less than 400oF. For

8.91 What are the consequences of a coating on

a cutting tool that has a different coefficient

of thermal expansion than does the substrate?

thermal strains at the temperatures developed

8.92 Discuss the relative advantages and limitations

or treat before its disposal. This has other im-

plications in that the workpiece doesn’t have to

controlled machine tools, which types of metal

chips are undesirable and why?

By the student. Continuous chips are not desir-

able because (a) the machines are now mostly

untended and operate at high speeds, thus chip

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produced using chip-breaker features on tools,

8.94 Explain why hacksaws are not as productive as

8.95 Describe workpieces and conditions under

which broaching would be the preferred method

of machining.

8.96 With appropriate sketches, explain the differ-

ences between and similarities among the fol-

8.97 Why is it difficult to use friction sawing on non-

ferrous metals? Explain.

8.98 Review Fig. 8.68 on modular machining cen-

ters, and explain workpieces and operations

sizes can be accommodated and the tool sup-

By the student. There are some workpieces that

cannot be produced on machining centers, as by

8.100 Give examples of (a) forced vibration and (b)

self-excited vibration in general engineering

also engineering texts on vibration.)

8.101 Tool temperatures are low at low cutting speeds

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Fig. 8.18 on p. 439). At higher speeds, conduc-

tion cannot take place rapidly enough. At even

8.102 Explain the technical innovations that have

technical advances that have made high-speed

machining possible include the availability of

mainly because of the absence or reduction of

8.103 Assume that in orthogonal cutting the rake an-

crease in chip thickness when friction is dou-

bled.

β= 11.3◦, and hence

φ1= 45◦+15◦

or to/tc= 1.16. Therefore, the chip thickness

8.104 Prove Eq. (8.1).

Refer to the shear-plane length as land note

from Fig. 8.2a on p. 419 that the depth of cut,

validity of the statement in the last paragraph

in Example 8.2.

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8.106 Using Eq. (8.3), make a plot of the shear strain,

α, as a parameter. Describe your observations.

The plot is as follows:

20

Friction coefficient, µ

0 0.2 0.4 0.6 0.8 1.0

.

(

8

.

2

1

)

The cutting ratio is given by Eq. (8.1) on p. 420

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nal cutting experiments. In both cases, depth of

cut (feed) to= 0.13 mm, width of cut w= 2.5

mm, rake angle α=−5◦, and cutting speed

to= 0.13 mm, w= 2.5 mm, α=−5◦and

V= 2 m/s. From Eq. (8.1) on p. 420 ,

tc

0.23 = 0.565

or

µ=Ft+Fctan α

Fc−Fttan α=280 + 430 tan(−5◦)

430 −280 tan(−5◦)

= 359 MPa

From Eq. (8.3) the shear strain is given by

ut=Fc

wto

=430

(2.5)(0.13) = 1323 MN-m/m3

wto

us=ut−uf= 1323 −419 = 903 MN-m/m3

rc= 0.224 φ= 12.3◦

Problem 8.110 for the following workpiece prop-

erties:

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