Chapter 1P3 -Cutting Tool.pdf

Manufacturing Engineering II
Mechanical Engineering 4th year
Chapter 1
11/15/2022
TRADITIONAL MACHINING PROCESSES
BDU-BiT-FMIE Manuf. Eng. I By Gessessew L & Yibeltal W. 1

Outline:
• Introduction
• Turning, Milling
and Drilling
processes
Operations
Tool Motions
Time calculation
Material removal
rate
Cutting Tools
• Tool Life and Tool
Wear,
• Tool geometry
• Surface finish
• Machinability
• Productivity and
• optimization
11/15/2022 BDU-BiT-FMIE Manuf. Eng. I By Gessessew L & Yibeltal W.
2
Mechanics of machining
Chip Formation
Forces and Power
Merchant Circle
Cutting Temp.
Cutting Fluid

11/15/2022 BDU-BiT-FMIE Manuf. Eng. I By Gessessew L & Yibeltal W. 3
• Machining operations are accomplished using cutting tools.
• The high forces and temperatures during machining create a very harsh
environment for the tool.
• If cutting force becomes too high, the tool fractures. If cutting temperature
becomes too high, the tool material softens and fails. If neither of these
conditions causes the tool to fail, continual wear of the cutting edge
ultimately leads to failure.
Cutting tool technology has two principal aspects:
1. Tool materials: develops material that withstands the force,
temperature and the wearing action
2. Tool geometry: that optimizes the geometry of the cutting tools for
the tool materials and for a given operation
Cutting Tool

There are three possible mode of cutting tool failure in machining:
1. Fracture Failure: mode of failure occurs when cutting force at
the tool point becomes excessive, causing it to failure suddenly
by brittle fracture.
2. Temperature failure: failure occurs when the cutting
temperature is too high for the tool material, causing the
material at the tool point to soften, which leads to plastic
deformation and loss of the sharp edge.
3. Gradual wear: Gradual wearing of the cutting edge causes loss
of tool shape, reduction in cutting efficiency, an acceleration of
wearing as the tool becomes heavily worn, and finally tool
failure in a manner similar to a temperature failure.
Tool life and Tool Wear

• Fracture and temperature
failures result in
premature loss of the
cutting tool. These two
modes of failure are
therefore undesirable.
• Gradual wear is preferred
because it leads to the
longest possible use of the
tool, with the associated
economic advantage of
that longer use.
Tool life and Tool Wear

Tool Wear
• Gradual wear occurs at two principal
locations on a cutting tool: the top rake
face and the flank.
• Two main types of tool wear can be
distinguished: crater wear and flank
wear,
• Crater wear, consists of a cavity in the
rake face of the tool that forms and
grows from the action of the chip sliding
against the surface. High stresses and
temperatures characterize the tool–chip
contact interface, contributing to the
wearing action. The crater can be
measured either by its depth or its
area.

• Flank wear, occurs on the flank, or
relief face, of the tool. It results from
rubbing between the newly
generated work surface and the flank
face adjacent to the cutting edge.
• Flank wear is measured by the width
of the wear band, FW. This wear
band is sometimes called the flank
wear land.
Tool Wear

The mechanisms of cutting tool wear (wear at the tool–chip
and tool–work interfaces) /(Gradual wear):
1. Abrasion: This is a mechanical wearing action caused by hard
particles in the work material gouging and removing small portions of
the tool. This abrasive action occurs in both flank wear and crater
wear; it is a significant cause of flank wear.
2. Adhesion: When two metals are forced into contact under high
pressure and temperature, adhesion or welding occur between them.
These conditions are present between the chip and the rake face of
the tool. As the chip flows across the tool, small particles of the tool
are broken away from the surface, resulting in attrition of the
surface.
Tool Wear

3. Diffusion: This is a process in which an exchange of atoms takes place
across a close contact boundary between two materials. In the case of tool
wear, diffusion occurs at the tool–chip boundary, causing the tool surface
to become depleted of the atoms responsible for its hardness. As this
process continues, the tool surface becomes more susceptible to abrasion
and adhesion. Diffusion is believed to be a principal mechanism of crater
wear.
4. Chemical reactions: The high temperatures and clean surfaces at the
tool–chip interface in machining at high speeds can result in chemical
reactions, in particular, oxidation, on the rake face of the tool. The
oxidized layer, being softer than the parent tool material, is sheared
away, exposing new material to sustain the reaction process.
Tool Wear

5. Plastic deformation: It is a plastic deformation of the cutting edge. The
cutting forces acting on the cutting edge at high temperature cause the
edge to deform plastically, making it more vulnerable to abrasion of the
tool surface. Plastic deformation contributes mainly to flank wear.
Tool Wear

TOOL LIFE AND THE TAYLOR TOOL LIFE EQUATION
• As cutting proceeds, the
various wear mechanisms
result in increasing levels
of wear on the cutting
tool. (flank and crater
wear)
• The general relationship
of tool wear versus cutting
time is shown in the fug.

• The slope of the tool wear curve in the
steady-state region is affected by work
material and cutting conditions.
• Harder work materials cause the wear rate
(slope of the tool wear curve) to increase.
• Increased speed, feed, and depth of cut have
a similar effect, with speed being the most
important of the three. If the tool wear curves
are plotted for several different cutting
speeds, the results appear as in the Figure.
As cutting speed is increased, wear rate
increases so the same level of wear is reached
in less time.

Tool life is defined as the length of
cutting time that the tool can be used.
Taylor tool Life Equation:
C= 𝑉𝑇𝑛
Where:
V: Cutting Speed, m/min
T: Tool life, min
• C and n are parameters
• Value of n constant for the
specific tool material;
• Value of C depends on tool
material, work material, and
cutting conditions

Tool Life Criteria in Production:
Although flank wear is the tool life criterion in the Taylor equation, this
criterion is not very practical in a factory environment because of the
difficulties and time required to measure flank wear. Following are nine
alternative tool life criteria that are more convenient to use in a production
machining operation, some of which are admittedly subjective:
1. Complete failure of the cutting edge (fracture failure, temperature failure, or
wearing
until complete breakdown of the tool has occurred). This criterion has
disadvantages.
2. Visual inspection of flank wear (or crater wear) by the machine operator
(without a
toolmaker’s microscope). This criterion is limited by the operator’s judgment
and ability to observe tool wear with the naked eye.

3. Fingernail test across the cutting edge by the operator to test for
irregularities.
4. Changes in the sound emitting from the operation, as judged by the
operator.
5. Chips become ribbony, stringy, and difficult to dispose of.
6. Degradation of the surface finish on the work.
7. Increased power consumption in the operation, as measured by a
wattmeter connected to the machine tool.
8. Workpiece count: The operator is instructed to change the tool
after a certain specified number of parts have been machined.

9. Cumulative cutting time. This is similar to the previous
workpiece count, except that the length of time the tool has
been cutting is monitored. This is possible on machine tools
controlled by computer; the computer is programmed to keep
data on the total cutting time for each tool.

Exercises
1. Turning tests have resulted in 1-min tool life at a cutting speed = 4.0 m/s and a 20-
min tool life at a speed = 2.0 m/s. (a) Find the n and C values in the Taylor tool life
equation. (b) Project how long the tool would last at a speed of 1.0 m/s.
2. In a production turning operation, the work part is 125 mm in diameter and 300
mm long. A feed of 0.225 mm/rev is used in the operation. If cutting speed = 3.0 m/s,
the tool must be changed every five work parts; but if cutting speed = 2.0 m/s, the
tool can be used to produce 25 pieces between tool changes. Determine the Taylor
tool life equation for this job.

Tool Material
1. Toughness: To avoid fracture failure, the tool material must possess high
toughness. Toughness is the capacity of a material to absorb energy
without failing.
2. Hot hardness: the ability of a material to retain its hardness at high
temperatures. This is required because of the high-temperature
environment in which the tool operates.
3. Wear resistance: Hardness is the single most important property needed
to resist abrasive wear. All cutting-tool materials must be hard. However,
wear resistance in metal cutting depends on more than just tool hardness,
because of the other tool-wear mechanisms.
The three important properties required in a tool materials:

Tool Material

Tool Geometry
Many of the principles that apply to
single-point tools also apply to the other
cutting-tool types, simply because the
mechanism of chip formation is basically
the same for all machining operations.
There seven elements of tool geometry;
(Called tool signature)
Back Rake, ( b)
Side Rake, ( s)
End Relief (ERA)
Side Relief, (SRA)
End Cutting Edge, (ECEA)
Side Cutting Edge, (SCEA)
Nose Rides, (NR)

In a single-point tool, the
orientation of the rake face is
defined by two angles, back rake
angle ( b) and side rake angle ( s).
• These two angles are influential
in determining the direction of
chip flow across the rake face.
The flank surface of the tool is
defined by the end relief angle
(ERA) and side relief angle (SRA).
• These two angles determine the
amount of clearance between the
tool and the freshly cut work
surface.
Tool Geometry

The cutting edge of a single point tool
is divided into two sections, side
cutting edge and end cutting edge,
• These two sections are separated by
the tool point, which has a certain
radius, called the nose radius.
• The side cutting edge angle (SCEA)
determines the entry of the tool into
the work and can be used to reduce
the sudden force the tool experiences
as it enters a work part.
• End cutting edge angle (ECEA)
provides a clearance between the
trailing edge of the tool and the
newly generated work surface, thus
reducing rubbing and friction against
the surface.
Tool Geometry

Machinability
Machinability denotes the relative ease with which a
material (usually a metal) can be machined using appropriate
tooling and cutting conditions.
There are various criteria used to evaluate machinability, the
most important of which are:
1. Tool life,
2. Forces and power,
3. Surface finish, and
4. Ease of chip disposal

Productivity and Optimization
• Selection of cutting speed is based on making the best use of the cutting
tool, which normally means choosing a speed that provides a high metal
removal rate yet suitably long tool life.
• Mathematical formulas have been derived to determine optimal
cutting speed for a machining operation, given that the various time and
cost components of the operation are known.

• The formulas allow the optimal cutting speed to be calculated for
either of two objectives:
1. Maximum production rate, or
2. Minimum unit cost.
• The formulas are based on a known Taylor tool life
equation for the tool used in the operation.
• Accordingly, feed, depth of cut, and work material
have already been set.
• The derivation will be illustrated for a turning operation.

Maximizing Production Rate
• For maximum production rate, the speed that
minimizes machining time per workpiece is determined.
• Minimizing cutting time per unit is equivalent to
maximizing production rate.
• This objective is important in cases when the
production order must be completed as quickly as
possible.
• In turning, there are three time elements that contribute
to the total production cycle time for one part:

Three time elements:
1. Part handling time Th: the time the operator spends loading the
part into the machine tool at the beginning of the production cycle and
unloading the part after machining is completed. Any additional time
required to reposition the tool for the start of the next cycle should also be
included here.
2. Machining time Tm: the time the tool is actually engaged in
machining during the cycle.
3. Tool change time Tt:At the end of the tool life, the
tool must be changed, which takes time. This time must be divided over
the number of parts cut during the tool life. Let np=the number of pieces cut
in one tool life (the number of pieces cut with one cutting edge until the tool
is changed); thus, the tool change time per part=Tt/np

The sum of these three time elements gives the total time per unit
product for the operation cycle
𝑇𝑐 = 𝑇ℎ + 𝑇𝑚 +
𝑇𝑡
𝑛𝑝
Where: Tc-Total
cycle time per piece
The cycle time Tc is a function of cutting
speed. As cutting speed is increased, Tm
decreases and Tt/np increases; Th is
unaffected by speed. These relationships
are shown in Figure.

𝑇𝑚 =
𝜋𝐷𝐿
𝑉𝑓
Where:
Tm=|Machine Time, min
D=Work diameter, mm
L=Work length, mm
V=Cutting velocity,
mm/min
F=Feed, mm/rev
The number of pieces per tool
np is also a function of speed.
It can be shown that
𝑛𝑝 =
𝑇
𝑇𝑚
Where:
np=No. of pieces machined
Tm=Machine Time, min
T=Tool Life, min

𝑛𝑝 =
(𝐶
𝑉)1/𝑛
(𝜋𝐷𝐿
𝑓𝑉)
Type equation
here.Taylor Tool
Life Equation;
𝑉𝑇𝑛
= 𝐶
𝑛𝑝 =
𝑓𝐶1/𝑛
𝜋𝐷𝐿𝑉
1
𝑛
−1
𝑇𝑐 = 𝑇ℎ +
𝜋𝐷𝐿
𝑓𝑉
+
𝑇𝑡(𝜋𝐷𝐿𝑉
1
𝑛
−1
)
𝑓𝐶1/𝑛
Differentiating with time and equating it to zero, i.e.
𝑑𝑇𝑐
𝑑𝑉
=0

Differentiating with time and
equating it to zero, i.e.
𝑑𝑇𝑐
𝑑𝑉
=0
Finally Solving this equation yields the cutting
speed for maximum production rate in the
operation
𝑉
𝑚𝑎𝑥 =
𝐶
1
𝑛
− 1 𝑇𝑡
𝑛
Where:
Vmax =the velocity for
maximum production
𝑇𝑚𝑎𝑥 =
1
𝑛
− 1 𝑇𝑡
Where:
Tmax =the time for maximum
production

Problems:
1. A cemented carbide tool is used to turn a part with a length of 400 mm in and
diameter = 100 mm. The parameters in the Taylor equation are: n=0.25 and C=305
m/min. The rate for the operator and machine tool =45 Birr/hr, and the tooling cost
per cutting edge = 2.50 Birr. It takes 2.5 min to load and unload the work part and
1.50 min to change tools. The feed = 0.4 mm/rev. Determine A. Cutting speed for
maximum production rate, B. Tool life for max prod, and C. Cycle time.
2. A high-speed steel tool is used to turn a steel work part that is 300mm long and
80mm in diameter. The parameters in the Taylor equation are: n=0.13 and C=75
(m/min) for a feed of 0.4 mm/rev. The operator and machine tool rate=30 Birr/hr,
and the tooling cost per cutting edge=Birr 4. It takes 2.0 min to load and unload
the workpart and 3.50 min to change tools. Determine (a) cutting speed for
maximum production rate, (b) tool life in min of cutting, and (c) cycle time and cost
per unit of product.
Note: Your Calculations should be in two decimal places.

Minimizing Cost Per Unit
• For minimum cost per unit, the speed that minimizes
production cost per piece for the operation is determined.
• To derive the equations for this case, we begin with the four
cost components that determine total cost of producing one
part during a turning operation:
Four four cost component:-

Productivity and Optimization, Minimizing Cost Per Piece
2. Cost of machining time: the cost of the time the tool is engaged in
machining. Using Co again to represent the cost per minute of the
operator and machine tool, the cutting time cost = CoTm.
3. Cost of tool change time: The cost of tool change time = CoTt/np.
4. Tooling cost: In addition to the tool change time, the tool itself has a cost
that must be added to the total operation cost. This cost is the cost per
cutting edge Ct, divided by the number of pieces machined with that
cutting edge np. Thus, tool cost per work piece is given by Ct/np.
1. Cost of part handling time: This is the cost of the time the operator
spends loading and unloading the part. Let Co= the cost rate (e.g.,
Birr/min) for the operator and machine. Thus the cost of part handling
time = CoTh.

Total Cost per unit product, Cc
𝐶𝑐 = 𝐶𝑜𝑇ℎ + 𝐶𝑜𝑇𝑚 +
𝐶𝑜𝑇𝑡
𝑛𝑝
+
𝐶𝑡
𝑛𝑝
Cc is function of cutting speed V
𝐶𝑐 = 𝐶𝑜𝑇ℎ + 𝐶𝑜
𝜋𝐷𝐿
𝑓𝑣
+
(𝐶𝑜𝑇𝑡 + 𝐶𝑡)(𝜋𝐷𝐿𝑉
1
𝑛
−1
𝑓𝐶1/𝑛

Productivity and Optimization; Minimizing Cost Per Unit
The cutting speed that obtains minimum cost per piece for the operation
can be determined by taking the derivative of Cc with respect to V , setting
it to zero, and solving for Vmin
𝑑𝐶𝑐
𝑑𝑉
= 0 𝑉𝑚𝑖𝑛 = 𝐶(
𝑛
1 − 𝑛
∗
𝐶𝑜
𝐶𝑜𝑇𝑡 + 𝐶𝑡
)𝑛
Vmin= Velocity for minimum cost per piece
𝑇𝑚𝑖𝑛 = (
1
𝑛
− 1)(
𝐶𝑜𝑇𝑡 + 𝐶𝑡
𝐶𝑜
)

1. A high-speed steel tool is used to turn a steel work part that is
300mmlong and 80mm in diameter. The parameters in the Taylor
equation are: n = 0.13 and C =75 (m/min) for a feed of 0.4 mm/rev.
The operator and machine tool rate = 50 Birr/hr, and the tooling
cost per cutting edge=Birr 25. It takes 2.0 min to load and unload
the work part and 3.50 min to change tools. Determine: A. Cutting
speed for minimum cost, B. Tool life for minimum cost, and C. Unit
cost per piece.
2. The tool change time = 3.0 min. The tooling cost is paid at a rate of
20 Birr/hr. Machine and operator cost of the lathe=24 Birr /hr. The
feed = 0.30 mm/rev. The parameters in the Taylor equation for this
grade are: n=0.25 and C=300 (m/min). Determine: A. Cutting for
minimum cost per piece, B. Tool life for minimum cost, C. Cost per
piece, D. Cutting speed for maximum production.
Note: Your Calculations should be in two decimal places.

Chapter 1P3 -Cutting Tool.pdf

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