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Chapter 9
Material-Removal Processes:
Abrasive, Chemical, Electrical, and
High-Energy Beams
Questions
9.1 Why are grinding operations necessary for parts
that have been machined by other processes?
9.2 Explain why there are so many different types
wheels because of the different types of opera-
9.3 Why are there large differences between the
and in machining (Table 8.3)? Explain.
also been discussed as a contributing fac-
9.4 Describe the advantages of superabrasives over
conventional abrasives.
9.5 Give examples of applications for the grinding
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monly used for surface grinding with hand-held
9.6 Explain why the same grinding wheel may act
soft or hard.
and wheel diameter increase. Equation (9.6)
9.7 Describe your understanding of the role of fri-
ability of abrasive grains on the performance of
grinding wheels.
9.8 Explain the factors involved in selecting the ap-
propriate type of abrasive for a particular grind-
ing operation, as this will reduce the effective-
ness of the abrasive. The abrasives should be
9.9 What are the effects of wear flat on the grind-
ing operation? Are there similarities with the
through friction. Wear flats are undesirable be-
cause they provide no useful work (they play
9.10 It was stated that the grinding ratio, G, de-
why.
The grinding ratio, G, decreases as the grain
in opposite ways; higher wheel speed re-
duces the force on the grains, which re-
9.11 List and explain the precautions you would take
when grinding with high precision. Comment
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dressed often.
9.12 Describe the methods you would use to deter-
mine the number of active cutting points per
unit surface area on the periphery of a straight
(Type 1; see Fig. 9.2a) grinding wheel. What
is the significance of this number?
and use Eq. (9.5) on p. 532 to determine C.
Another method involves rolling the grinding
and expressed as a number per unit area. Note,
9.13 Describe and explain the difficulties involved in
grinding parts made of (a) thermoplastics, (b)
(a) Thermoplastics have a low melting point
p. 702).
able for soft and ductile metals.
In ultrasonic machining, the stresses developed
from particle impact should be sufficiently high
to cause spalling of the workpiece. This in-
9.15 It is generally recommended that a soft-grade
wheel be used for grinding hardened steels. Ex-
9.3.2) will tend to cause wear and dulling of
soft-grade wheel (see Figs. 9.4 and 9.5) means
9.16 Explain the reasons that the processes de-
scribed in this chapter may adversely affect the
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also the role of residual stresses, particularly the
9.17 Describe the factors that may cause chatter in
to chatter in machining, hence many of the
factors discussed in Section 8.12 apply here as
9.18 Outline the methods that are generally avail-
able for deburring manufactured parts. Discuss
ter are the physical properties of the workpiece
material important? Explain.
9.20 Give all possible technical and economic reasons
that the material removal processes described in
listed in Section 9.1. Students are encouraged
9.21 What processes would you recommend for die
sinking in a die block, such as that used for
forging? Explain. (See also Section 6.7.)
9.22 The proper grinding surfaces for each type of
wheel are shown in Fig. 9.2. Explain why grind-
ing on other surfaces of the wheel is improper
wheels are made stiffer in the directions in
which they are intended to be used. There are
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9.23 Note that wheel (b) in Fig. 9.3 has serrations
The basic advantages of this design are the fol-
lowing:
grooves.
9.24 In Fig. 9.10, it will be noted that wheel speed
and grinding fluids can have a major effect on
sipated in plowing and sliding abrasive grains
over the workpiece surface without chip gener-
piece temperature to rise excessively in grinding
(a) If excessive, it can cause metallurgical
burn and heat checking.
will contract and the dimensional toler-
9.26 Comment on any observations you have regard-
ing the contents of Table 9.4.
tant manufacturing process? Explain.
Recall that the advantages of creep-feed grind-
ing (see Section 9.6.6) is the ability for high
9.28 There has been a trend in manufacturing indus-
tries to increase the spindle speed of grinding
tures [see Eq. (9.9) on p. 535] and associated
9.29 Why is preshaping or premachining of parts
generally desirable in the advanced machining
processes described in this chapter? Explain.
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9.30 Why are finishing operations sometimes neces-
sary? How could they be minimized to reduce
product costs? Explain, with examples.
9.31 Why has the wire-EDM process become so
widely used in industry, especially in tool and
die manufacturing? Explain.
eral reasons (see also Section 9.13.2). The pro-
cess is relatively easy to automate, and numer-
9.32 Make a list of the material removal processes
described in this chapter that may be suitable
for the following workpiece materials: (1) ce-
ramics, (2) cast iron, (3) thermoplastics, (4)
(b) Cast iron: chemical machining, elec-
trochemical machining, electrochemical
(f) Annealed copper: Chemical and electro-
chemical processes, EDM, and laser-beam
9.33 Explain why producing sharp corners and pro-
files using some of the processes described in
this chapter can be difficult.
9.34 How do you think specific energy, u, varies with
respect to wheel depth of cut and hardness of
the workpiece material? Explain.
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sult in higher forces on the grains, as seen in
9.35 It is stated in Example 9.2 that the thrust force
in grinding is about 30% higher than the cut-
ting force. Why is it higher?
We note in Fig. 9.7 that abrasive grains typi-
cally have very high negative rake angles. Let’s
now compare the force differences in grinding
of the thrust force in grinding? Explain.
9.37 Why is the material removal rate in electrical-
discharge machining a function of the melting
point of the workpiece material? Explain.
sequently, the higher the melting point, the
higher the energy required.
9.38 Inspect Table 9.4 and, for each process, list and
describe the role of various mechanical, physi-
9.39 Which of the processes listed in Table 9.4 would
not be applicable to nonmetallic materials? Ex-
By the student. The following are generally
not applicable to nonmetallic materials: elec-
trochemical machining, electrochemical grind-
ing, EDM, and wire EDM.
9.40 Why does the machining cost increase rapidly
as surface finish requirements become finer?
(a) ceramics, (b) thermoplastics, and (c) ther-
mosets? Explain.
(b) thermoplastics: chemical machining, high-
energy-beam machining, water-jet and
9.42 Other than cost, is there a reason that a grind-
ing wheel intended for a hard workpiece cannot
be used for a softer workpiece? Explain.
By the student. Recall that a soft workpiece
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9.43 How would you grind the facets on a diamond,
such as for a ring, since diamond is the hardest
material known?
at the asperity scale it is possible for abrasion
to occur on diamond.
9.44 Define dressing and truing, and describe the dif-
ference between them.
tion 9.5.1, dressing is the process of condition-
significance? Does heat checking occur in other
manufacturing processes? Explain.
9.46 Explain why parts with irregular shapes, sharp
corners, deep recesses, and sharp projections
can be difficult to polish.
9.47 Explain the reasons why so many different de-
burring operations have been developed over
the years.
9.48 Note from Eq. (9.9) that the grinding tem-
perature decreases with increasing work speed.
Does this mean that for a work speed of zero,
the temperature is infinite? Explain.
Consider the heat flow in grinding: The heat
lowing mechanisms:
heat being physically moved with the ma-
terial.
action of metalworking fluids in machining vs.
grinding operations.
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properties such as viscosity, wetting, surface-
ing, polishing, and buffing? Explain.
9.51 Is the grinding ratio an important factor in eval-
uating the economics of a grinding operation?
Explain.
9.52 Although grinding can produce a very fine sur-
face finish on a workpiece, is this necessarily an
indication of the quality of a part? Explain.
9.53 If not performed properly, honing can pro-
duce holes that are bellmouthed, wavy, barrel-
center and the axis of the tool is aligned with
9.54 Which of the advanced machining processes de-
scribed in this chapter causes thermal damage
to workpieces? List and explain the possible
including polymers and ceramics, can be laser-
beam machined using different types of lasers.
The presence of a concentrated heat source and
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It is presently impossible to produce graphite
wires, although significant effort has been di-
9.57 What is the purpose of the abrasives in electro-
chemical grinding? Explain.
9.58 In a surface-grinding operation, calculate the
chip dimensions for the following process vari-
ables: D= 8 in., d= 0.001 in., v= 30 ft/min,
V= 5000 ft/min, C= 500 per in2, and r= 20.
The approximate chip length, l, is given by
Eq. (9.1) on p. 530 as
l=√Dd =p(8)(0.001) = 0.0894 in.
The undeformed chip thickness, t, is given by
creased by 50%, what should be the percentage
decreases in the wheel depth of cut, d, in or-
der to maintain the same grain force, all other
variables being the same?
9.60 Taking a thin, Type 1 grinding wheel, as an
example, and referring to texts on stresses in
rotating bodies, plot the tangential stress, σt,
and radial stress, σr, as a function of radial dis-
tance (from the hole to the periphery of the
wheel). Note that because the wheel is thin,
this situation can be regarded as a plane-stress
problem. How would you determine the max-
imum combined stress and its location in the
wheel? Explain.
and
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ro
Normal
stress,
θ
9.61 Derive a formula for the material removal rate
in surface grinding in terms of process parame-
ters. Use the same terminology as in the text.
tion 8.10.1). Therefore,
ing carried out under the following conditions:
v= 0.3 m/s, and V=25 m/s. What is the dif-
ference in the temperature rise from the initial
condition?
p. 535. Note that the value of Cis not known,
tween the two cases, so it can be ignored in this
∆T∝D1/4d3/4V
v0.5
For the second case, we have
∆T∝D1/4d3/4V
v0.5
9.63 For a surface-grinding operation, derive an ex-
pression for the power dissipated in imparting
kinetic energy to the chips. Comment on the
P=1
4(V C) = vd
The same expression can be derived by noting
Work = mV 2
P=dW
dL
9.64 The shaft of a Type 1 grinding wheel is attached
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