Spring 2016 EML4321 Manufacturing Engineering HW-8
(Due on April 18, 2016)
Q1. Explain why is it not always advisable to increase cutting speed in order to increase
production rate.
Answer:
From the Taylor tool-life equation, VTn = C, it can be seen that tool wear increases rapidly with
increasing speed. When a tool wears excessively, it causes poor surface finish and higher
temperatures. With continual tool replacement, more time is spent indexing or changing tools
than is gained through faster cutting. Thus, higher speeds can lead to lower production rates.
Q2. Answer the following questions.
(1) True or false? Knurling is performed on a lathe, but it is a metal forming operation rather
than a metal removal operation.
Answer: True
(2) Which one of the following cutting tools cannot be used on a lathe?
(a) single-point-turning tool
(b) cut-off tool
(c) drill bit
(d) broach
(e) threading tool
Answer: (d)
Q3. List four basic characteristics required for cutting tool materials.
Answer:
(1) hardness (hot hardness)
(2) toughness
(3) wear resistance
(4) chemical stability
Q4. Explain the consequences of a cutting tool coating with a coefficient of thermal expansion
that is different than that of the substrate.
Answer:
Consider the situation where a cutting tool and the coating are stress-free at room temperature
when the tool is inserted. Then consider the situation when the tool is used in cutting and the
temperatures are very high. A mismatch in thermal expansion coefficients will cause high
thermal strains at the temperatures developed during machining. This can result in a separation
(delamination) of the coating from the substrate. (See also pp. 107-108.)
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Spring 2016 EML4321 Manufacturing Engineering HW-8
(Due on April 18, 2016)
Q5. Of the two materials, diamond or cubic boron nitride, which is more suitable for machining
steels? Why?
Answer:
Of the two choices, cubic boron nitride is more suitable for cutting steel than diamond tools.
This is because cBN, unlike diamond, is chemically inert to iron at high temperatures, thus tool
life is better.
Q6. Referring to Fig. 8.31, how would you explain the effect of cobalt content on the properties
of carbides?
Answer:
Tungsten-carbide tools consist of tungsten-carbide particles bonded together in a cobalt matrix
using powder-metallurgy techniques. Increasing the amount of cobalt will make the material
behave in a more ductile manner, thus adversely affecting the strength, hardness, and wear
resistance of the tungsten carbide tools. Cobalt improves toughness and transverse-rupture
strength.
Q7. A 3-in. diameter stainless steel cylindrical part is to be turned on a lathe at 500 rpm in one
pass. The depth of cut is 0.1 in. and the feed is 0.02 in./rev. What should the minimum
horsepower of the lathe be?
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Spring 2016 EML4321 Manufacturing Engineering HW-8
(Due on April 18, 2016)
Q8. Calculate the same quantities as in Example 8.4 but for high-strength cast iron and N = 500
rpm.
Answer:
Q9. Orthogonal cutting experiments using an aluminum alloy workpiece (flow stress Yf of 120
MPa, thermal diffusivity K of 97 mm2/s, and volumetric specific heat ρc of 2.6 N/mm2C)
give the following: chip thickness tc=0.23 mm, cutting force Fc =430 N, and thrust force Ft
=280 N. The cutting conditions for these experiments were depth of cut t0=0.13 mm, width
of cut w=2.5 mm, rake angle α= -5, and cutting speed V=2 m/s.
(1) Calculate the following:
(a) shear angle φ [] (use Equation 8.1)
(b) friction of coefficient at the tool-chip interface µ
(c) shear stress τ [MPa] and shear strain γ on the shear plane
(d) chip velocity Vc [m/s]
(e) shear velocity Vs [m/s]
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Spring 2016 EML4321 Manufacturing Engineering HW-8
(Due on April 18, 2016)
(f) total specific energy μt [N·m/mm3]
(g) specific energy for friction μf [N·m/mm3]
(h) specific energy for shearing μs [N·m/mm3]
(2) The equivalent form of Equation (8.29) for SI units is shown here as Equation (1).
𝑇=
3.8𝑌𝑓 3 𝑉𝑡0
√
𝜌𝑐
𝐾
[C]
(1)
where Yf is the flow stress [MPa], K is the thermal diffusivity [mm2/s], V is the cutting
speed [mm/s], t0 is the depth of cut [mm], and ρc is the volumetric specific heat
[N/mm2C].
Using the Equation (1), calculate the approximate temperature at the tool-chip interface T
[C].
Answer:
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