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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.
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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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