Lecture 14

Production Engineering
(MET 351)
B.Tech. (6th Sem) Spring 2024
Department of Mechanical Engineering
NIT Srinagar
Course In-charge
Dr. Noor Zaman Khan
Course Outcomes
At the end of the course, a student should be able to:
• CO1: Determine the shear angle and cutting force in machining and understand the basics
of metal cutting.
• CO2: Estimate tool life and explain the tool wear mechanisms and abrasive machining
process.
• CO3: Analyze the forming process behavior for conventional and advanced metal forming
processes.
• CO4: Understand the basics of limits, fits and tolerances in manufacturing.
UNIT II
• Mechanisms of tool wear; Types of tool wear, Tool life: Variables
affecting tool life-Cutting conditions; Tool angles specification
systems; Tool materials; Desirable Properties of Cutting Tool;
Determination of tool life; Machinability, Economics of machining.
• Abrasive Machining Process: Introduction; Grinding: Characteristics

of a grinding wheel; Specification of grinding wheels; Mechanics of
grinding process; Grinding operations; Wheel wear; Surface Finish;
Selection of grinding wheels.
PCD
• Diamond is the hardest known (Knoop hardness ~ 8000 kg/𝑚𝑚2 )
material that can be used as a cutting tool.
• It has most of the desirable properties such as high hardness, good
thermal conductivity, low friction coefficient, and good wear
resistance.
• It is used when good surface finish and dimensional accuracy are
required.
• Factor against PCD: high cost, possibility of oxidation in air, allotropic transformation to graphite above
temperatures of 700°C, very high brittleness and difficulties associated in shaping it to suitable tool form.
• However, natural diamond is unreliable in performance because of the impurities present.
• Artificial diamonds are basically polycrystalline (PCD) in nature. Polycrystalline diamond tools are
metallurgically bonded to a tungsten carbide substrate and cut into small bits.
• Strong chemical affinity at high temp. PCD not recommended for Ti, Carbon steel, Ni based alloys, Co based
alloys.
Cubic Boron Nitride (CBN)
• Introduced in 1962. CBN is next in hardness only to diamond (Knoop hardness ~ 4700 kg/mm2).
• It is not a natural material but produced in the laboratory using a high-temperature/high-pressure process
similar to the making of artificial diamond.
• CBN is less reactive with materials like hardened steels, nickel base and cobalt-based super alloys, and
hence is used effectively for machining these alloys.
• These are more expensive than cemented carbides but in view of the higher accuracy and productivity
possible for difficult to machine materials, they are used in special applications as mentioned above.
• However many of the cutting-tool manufacturers such as Sandvik, Widia provide detailed literature to
help in the choice of cutting tools.
Max possible Cutting Speed for various tools to machine mild steel
Machinability
Machinability
• The term "machinability", which loosely means the ease of machining, is used quite common.
• However, the criteria for judging this ease may be different
• For example, machining a lead piece with an HSS tool may be quite easy so far as the force requirement is
concerned, but a good surface is extremely difficult to achieve.
• Thus, for a given operation, the machinability may be considered to be good or bad depending on the criterion.
• The major criteria for judging machinability are
(i) Machining forces and power consumption: A machining requiring a large force indicates low machinability
and vice versa. When the strength of the tool is a matter to worry about, this is the criterion to be considered.
(in) Surface finish: In some situations, quality of finish become the major concern and, depending on the severity
of this problem, the machinability may be low or high.
(iii) Tool life: The length of the period for which a tool can be used is defined as the tool life. This criterion is also
linked up with the productivity and economics and can be a very good index for an overall judgement of a
machining operation.
Limits of tool wear
• If sufficient power is available and good surface finish with precise machining is not required the tool
life may be considered the index for judging the machinability.
• In cases where the major objective is to remove a large amount of material quickly and cheaply, the tool
life is used as a direct measure of machinability.
• A precise definition of tool life is also not a simple problem. As a machining operation progresses, the
wear on the flank and rake surfaces keeps on increasing.
• So, limits on these wears have to be chosen for defining the

tool life.
• The commonly recommended criteria for HSS and carbide
tools are given in Table.
• In a very large number of cases, the criterion of the average
flank wear ℎ𝑓 serves well.
Tool life: Growth of flank wear for different speeds
• Using ℎ𝑓 = 0.3 mm as the tool life criterion, we note that the tool lives are
𝑇1 , 𝑇2 , and 𝑇3 .
• Also, it is obvious that a higher speed of cutting leads to a lower tool life.
• It has been experimentally established that the tool life equation is
• 𝑣𝑇 𝑛 = 𝐶
• Where, C and n are constants depending on the tool and work material,
tool geometry, and cutting conditions (except speed).
• Though cutting speed is the most dominant variable, the other cutting parameters, e.g., the uncut
thickness and width of cut, also affect the tool life.
• When machining is through the production of continuous chips without a built-up edge, the
generalized Taylor equation can be written as
• Where, 𝐶 ′ , n, p, q are constants. The units of v, T, t, and w are m/min, min, mm, and mm, respectively.
Cutting speed-tool life relation for different cutting tool materials
• Typical variation of tool life with speed for

HSS, WC, and ceramic tools, keeping the
other conditions the same.
• It is clear that the tool life for a given speed is
normally much higher with WC than that with
HSS.
• A ceramic tool performs better at a high
cutting speed.

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Production Engineering

• Abrasive Machining Process: Introduction; Grinding: Characteristics

• So, limits on these wears have to be chosen for defining the

• Typical variation of tool life with speed for

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