Cutting Tool Selection, life,shape.pptx

Cutting Tool Selection, life, shape

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Operating Conditions
(a) high temperatures (b) high contact stresses (c) rubbing along the
tool-chip interface and along the machined surface
Desired Characteristics
Hot hardness, so that the hardness, strength, and wear resistance of
the tool are maintained at the temperatures encountered in machining
operations. No plastic deformation, retaining shape and sharpness
high speeds--- high hot hardness
Toughness and impact strength (or mechanical shock resistance),
Impact forces--interrupted cutting opera-
tions such as milling and turning
Forces due to vibration and chatter during
machining do not chip or fracture the tool.
Cutting Tool Materials

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1. High hardness
2. High hardness temperature, hot hardness
3. Resistance to abrasion, wear due to severe sliding friction
4. Chipping of the cutting edges
5. High toughness (impact strength) (refer to Figure 21-4)
6. Strength to resist bulk deformation
7. Good chemical stability (inertness or negligible affinity with the
work material
8. Adequate thermal properties
9. High elastic modulus (stiffness)
10. Correct geometry and surface finish
Cutting Tool Materials Characteristics

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Hardness at ambient temperature-Different Materials

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Cutting Tool Materials: Hot hardness Cutting sped

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Thermal shock resistance to withstand the rapid temperature cycles encountered in interrupted cutting.
Wear resistance, so that an acceptable tool life is obtained before replacement is necessary

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° Hardness and strength ==mechanical properties of the workpiece ° Impact strength ---- interrupted cuts in machining, such as
in milling.
° Melting temperature----temperatures developed in the cutting zone.
° thermal conductivity and coefficient of thermal expansion----thermal fatigue and shock.

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Failure of Tool
Fracture Failure- Temperature Failure- Gradual Wear

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where n is an exponent that depends mostly on tool material but is
affected by work material, cutting conditions, and environment and C
is a constant that depends on all the input parameters, including feed

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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.

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Machinability testing
Machining performance of material ref. to base:
(1)tool life, (2) tool wear (3) cutting force, (4) power in the operation,
(5) cutting temperature, and (6) material removal rate under standard test conditions
machinability rating (MR) = Machiniablity of a
material/machinability of standard material
MR > 1 ,
easy machining,
Difficult machinging MR <1
Example Taylor Equation = speed of cutting for standard
material/speed for target material

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work material factors .
hardness increases, abrasive wear of the tool increases-life is reduced
Strength---cutting forces---specific energy--cutting temperature
increase, making the material more difficult to machine.
Very low hardness– poor machining- problems with chip disposal
A metal’s chemistry-wear mechanism—interact with tool
C- content- high- reduce machining performance.
Chromium, molybdenum, tungsten
Carbides- increase the wear of tool
MACHINABILITY

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lead, sulfur, and phosphorus-reduce µ - and improve machineability.
A free machining steel.
A metal’s chemistry
MACHINABILITY

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2. Direct instruments mesurement

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TOLERANCES AND SURFACE FINISH

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Texture after machining operation.
(1) geometric factors, (2) work material factors, and (3) vibration and machine tool
factors.
Geometric Factors:
(1) Type of machining operation (2) cutting tool geometry, most importantly nose
radius; and (3) feed.
The surface geometry that would result from these factors is referred to as the ‘‘ideal’’
or ‘‘theoretical’’ surface roughness, which is the finish that would be obtained in the
absence of work material. vibration, and machine tool factors.
Type of operation refers to the machining process used to generate the surface. For
example, peripheral milling, facing milling, and shaping all produce a flat surface;
however, the surface geometry is different for each operation because of differences in
tool shape and the way the tool interacts with the surface. Possible lays of surface for
different tools.
SURFACE FINISH IN MACHINING

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Possible lays of a surface.

Geometric Factors
• Machining parameters that determine surface geometry:
– Type of machining operation, e.g., milling vs. turning
– Cutting tool geometry, especially nose radius
– Feed
• The surface geometry that would result from only these factors
= "ideal" or "theoretical" surface roughness

Effect of End
Cutting Edge
Angle

shape of tool point.
a larger nose radius---
feed marks to be less pronounced
Same nose radius
larger feed increases the separation
between feed marks, leading to an
increase in the value of ideal surface
roughness
ECEA
For high enough feed rate –end
Cutting edge creates new surface
a higher ECEA ---higher surface
roughness value
A zero ECA—perfect surface
Tool Geometry and Feed

Ideal Surface Roughness
where Ri = theoretical arithmetic average surface
roughness; f = feed; and NR = nose radius
NR
f
Ri 32
2


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Work Material Factors
Achieving the ideal surface finish is not possible in most
machining operations because of factors related to the work
material and its interaction
with the tool.
1 built-up edge effects—as the BUE cyclically forms and
breaks away, particles are deposited on the newly created work
surface, causing it to have a rough ‘‘sandpaper’’ texture.
2 damage to the surface caused by the chip curling back into the
work
3 Tearing of the work surface during chip formation when
machining ductile materials.
4 cracks in the surface caused by discontinuous chip formation
when machining brittle materials.

Actual Chip Formation
 The formation of the chip depends on the type of material being
machined and the cutting conditions of the operation.

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Vibration and Machine Tool Factors
These factors are related to the machine tool, tooling, and setup in
the operation.
chatter or vibration
deflections in the fixturing, often resulting in vibration
and backlash in the feed. Coz is old apparatus.
If these causes are removed then roughness wil be only because of
material and geometrical factors
Reducing chatter involves
•Adding stiffness or damping.
• operating at speeds that do not cause cyclical forces whose
frequency approaches the natural frequency of the machine tool
system,
• reducing feeds and depths to reduce forces in cutting
• changing the cutter design to reduce forces.

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Vibration and Machine Tool Factors
Workpiece geometry can sometimes play a role in chatter.
Thin cross sections tend to increase the likelihood of chatter, requiring
additional supports to alleviate the condition.

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Work material factors cause actual surface finish worse than ideal.

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(Groover)

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SELECTION OF CUTTING CONDITIONS
Practical problems in machining is selecting the proper cutting conditions for a given
operation.
SELECTING FEED AND DEPTH OF CUT
Cutting conditions are, speed, feed, depth of cut, and cutting fluid
Depth of cut is often predetermined by workpiece geometry and operation
Sequence.
Machining Operation--- roughing(big cut)—final finishing(dimensions).
(Feed and Speed)
Feed Rate:
Tooling:
HSS can tolerate higher feeds because of its greater toughness.
Ceramics and carbides are susceptible to fracture and have low FR.
Roughing or Finishing: typically 0.5 to 1.25 mm/rev.
For turning, 0.020 to 0.050 in/rev finishing 0.125 to 0.4 mm/rev ,,,,,
Constraints: Cutting forces, rigidity and sometimes horsepower.

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SELECTING FEED AND DEPTH OF CUT
Surface finish requirements in finishing : Feed and feed rate is an important
Parameter.
OPTIMIZING CUTTING SPEED
high metal removal rate yet suitably long tool life.
Knowing the cost and time components ---calculation of cutting speed
Cutting speed – balance b/w material removal rate and tool life.
maximum production rate
Minimum unit cost
Both are linked to material removal rate and Tool life.
Feed, depth of cut and work material are already set,

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OPTIMIZING CUTTING SPEED for Maximum Production Rate.
Time elements involved in production
Part handling time Th: time to load + time to unload after machining
Any additional time required to reposition the tool for the start of the next
cycle should also be included here.
Machining time Tm. This is the time the tool is actually engaged in
machining during the cycle.
Tool change time Tt: This apportioned over the number of parts cut
the tool change time per part = Tt/np.

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Total time ,
cycle time Tc is a function
of cutting speed, as speed
Increases, Tm decreases
and Tt/np
Th is unaffected.

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The number of pieces per tool np is also a
function of speed. It can be shown that
where T = tool life, min/tool; and
Tm = machining time per part, min/pc. Both T and Tm
are functions of speed; hence, the ratio is a function of speed.

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where vmax is expressed in
m/min (ft/min).
The corresponding tool life for
maximum production rate is

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Minimizing Cost per Unit : For minimum cost per unit, the speed that
minimizes production cost per piece for the operation is etermined.
four cost components that determine total cost of producing one
part during a turning operation.
1. Cost of the part handling: cost rate of man + machine=Co (€/min)
Thus the cost of part handling time = CoTh.
2. Cost of machining time: This is 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:
This cost is the cost per cutting edge Ct, divided by the
number of pieces machined with that cutting edge np.Thus, tool cost
perworkpiece is given by Ct/np.

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OPTIMIZING CUTTING SPEED; minimizing cost per unit
Tooling Cost For disposable inserts
Tooling cost requires an explanation, because it is affected by different
tooling situations. For disposable inserts (e.g., cemented carbide inserts),
tool cost is determined as
where Ct = cost per cutting edge, $/tool life; Pt= price of the
insert,$/insert; and ne =number of cutting edges per insert.
ne depends on the insert type; for example, triangular inserts that can be
used only one side (positive rake tooling) have three edges/insert; if both
sides of the insert can be used (negative rake tooling), there are six
edges/insert; and so forth.

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Positive rake position= 3 edges/insert
-ve rake position= 6 edges/insert
Tool cost + cost of regrinding
For regrindable tooling (e.g., high-speed steel solid shank tools,
brazed carbide tools), the tool cost includes purchase price plus cost
to regrind:

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Minimizing Cost per Unit : For minimum cost per unit, the speed that
minimizes production cost per piece for the operation is etermined.
four cost components that determine total cost of producing one
part during a turning operation.
1. Cost of the part handling: cost rate of man + machine=Co (€/min)
Thus the cost of part handling time = CoTh.
2. Cost of machining time: This is 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:
This cost is the cost per cutting edge Ct, divided by the
number of pieces machined with that cutting edge np.Thus, tool cost
perworkpiece is given by Ct/np.

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Tooling Cost For disposable inserts
Tooling cost requires an explanation, because it is affected by different
tooling situations. For disposable inserts (e.g., cemented carbide inserts),
tool cost is determined as
where Ct = cost per cutting edge, $/tool life; Pt= price of the
insert,$/insert; and ne =number of cutting edges per insert.
ne depends on the insert type; for example, triangular inserts that can be
used only one side (positive rake tooling) have three edges/insert; if both
sides of the insert can be used (negative rake tooling), there are six
edges/insert; and so forth.

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Positive rake position= 3 edges/insert
-ve rake position= 6 edges/insert
Tool cost + cost of regrinding
For regrindable tooling (e.g., high-speed steel solid shank tools,
brazed carbide tools), the tool cost includes purchase price plus cost
to regrind:

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Where Ct=cost per tool life, $/tool life
Pt=purchase price of the solid shank tool or brazed insert, $/tool
Ng=number of tool lives per tool,which is the number of times the tool can be
ground before it can no longer be used (5 to 10 times for roughing tools and 10
to 20 times.for finishing tools);
Tg = time to grind or regrind the tool, min/tool life; and
Cg = grinder’s rate, $/min.
Cost of grinding

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Cost of part handling time.
Cost of machining time.
Cost of tool change time.
Tooling cost.
Cost of Cutting Process.
Cc = CoTh + CoTm + CoTt/np + Ct/np
Cost in terms of speed

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Derivative assuiming, cost zero= Vmin

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Example Problems
vTn = C, T. Equation

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Example Problem
Tt= (C/V)n
Rate=T/Tm
average production cycle time for the operation is=
Hourly production
Cost per piece Cc = CoTh + CoTm + CoTt/np + Ct/np

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I Abrasive Machining and Finishing
Operations:
 Introduction.
 Abrasives and Bonded Abrasives
 The Grinding Process, Grinding Operations and Machines,
Design Considerations for Grinding, Ultrasonic Machining
 Finishing Operations.
 Deburring Operations.
 Economics of Abrasive Machining and Finishing Operatio
ns
 Kalpakjian

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Abrasive Machining and Finishing
Operations:
Grinding is a material removal process accomplished by abrasive particles that are contained in a bonded
grinding wheel rotating at very high surface speeds.
It has special import as it imparts high dimensional accuracy and surface finish.
Coated or Bonded Abrasive:
polishing, buffing, honing, and sanding.
Loose Abrasive:
ultrasonic machining, lapping, abrasive flow machining, and electrochemical machining
and grinding
Application
Any part requiring high dimensional accuracy and surface finish.

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Introduction
Abrasive Machining and Finishing
Operations:
An abrasive is a small, hard particle having sharp edges and an
irregular shape, unlike the cutting tools described earlier.
hone, lap, buff, and polish
Friability
Bonding

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Possible Geometeries
Abrasive Grinding
a wide variety of workpiece geometries,

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Abrasive Grinding
Tolerances
dimensional tolerances can
be less than 1 micron, and
surface roughnesses can be
as fine as 0.025 micron.

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Abrasive Grinding
They are used for hardened metals and other hard components in
service
(a) finishing of ceramics and glasses,
(b) cutting off lengths of bars, structural shapes, masonry, and
concrete,
(c) removing unwanted weld beads and spatter, and
(d) cleaning surfaces with jets of air or water containing abrasive
particles.

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Conventional abrasives
° Aluminum oxide (Al2O3)
° Silicon carbide (SiC)
Superabrasives
° Cubic boron nitride (cBN)
° Diamond
Desired characteristics
1. Hardness
2. Friability: Ability to break and expose new sharp surface.
e.g SiC> Alumina
shape and size govern friability—easy breakable, large and flate.
Abrasives and Bonded Abrasives

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Natural Abrasives—emery, corrundum– impurities –inhomogenieties
Synthetic
Aluminum oxide :
Produced by fusing bauxite, iron filings, and coke. Fused aluminum oxides
are categorized as dark (less friable),
white (very friable).
and single crystal.
Abrasive Types.

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° Seeded gel :purest form of unfused aluminum oxide. It also is known as
ceramic aluminum oxide. It has a grain size on the order of 0.2micron, which
is much smaller than other types of commonly used abrassive grains.
These grains are sintered to form larger sizes
Friable and hard than fused alumina
used especially for difficult-to-grind materials.
Synthetic Abrasive Types.

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Silicon carbide
made with silica sand and petroleum coke. Silicon carbides are
divided into black (less friable) and green (more friable) and generally
have higher friability than aluminum oxides.
Hence, they have a greater tendency to fracture and remain sharp.

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cubic boron nitride (CBN)= Borazon
cubic boron nitride is made by bonding a 0.5 -to-1-mm layer of polycrystalline
cubic boron nitride to a carbide substrate
by sintering under high pressure and high temperature
Functioning
Carbide base( tough)– Shock resistance:
cBN layer -----very high wear resistance and cutting-edge strength
Charaterisitcs
Inert to iron and nickel at elevated T.
Its resistance to oxidation is high
Applied To
Hardened ferrous and high temperature alloys and at high speed machining oper.
Used as an brasive
Their brittleness demands workplace free of vibration and chatter.
Phases of BN and maximum hardness of each phase??

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Diamond
the hardest substance is diamond-- As a cutting tool, it has highly
desirable properties, such as low friction--high wear resistance, and
the ability to maintain a sharp cutting edge
Diamond is used when a good surface finish and dimensional
accuracy are required, particularly with soft nonferrous alloys and
abrasive nonmetallic and metallic materials (especially some
aluminum-silicon alloys).
Synthetic or industrial diamonds are widely used because natural
diamond has flaws and its performance can be unpredictable,
as is the case with abrasives used in grinding wheels.
Single Crystal diamonds Applications=??
PCD- mounted on carbide substrate, another example is Die for wire
drawing

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tool shape--- and sharpness are important.
Low rake angles generally are used to provide a strong cutting edge
(because of the larger included angles).
Proper mounting and crystal orientation in order to obtain optimum tool life.
Wear may occur through microchipping (caused by thermal stresses and
oxidation) and through transformation to carbon (caused by the heat
generated during cutting).
Diamond tools can be used
satisfactorily at almost any speed, but are most suitable for light,
uninterrupted finishing cuts.
In order to minimize tool fracture, the single-crystal diamond must be
resharpened as soon as it becomes dull.
Because of its strong chemical affinity at elevated temperatures (resulting
in diffusion).
Diamond

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Abrasive Grain Size.
The size of an abrasive grain is identified by a grit number, which is a
function of sieve size
a grit number 10 is typically regarded as very coarse,
100 as fine, and 500 as very fine. Sandpaper
and emery cloth also are identified in this manner.
Abrasive-workpiece-material Compatibility.
Aluminum oxide: Carbon steels, ferrous alloys, and alloy steels.
° Silicon carbide: Nonferrous metals, cast irons, carbides, ceramics,
glass, and
marble.
° Cubic boron nitride: Steels and cast irons above 50 HRC hardness
and high temperature alloys.
° Diamond: Ceramics, cemented carbides, and some hardened
steels.

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Grinding Wheels- wear and abrasion mechanism

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http://www.arceurotrade.co.uk/Catalogue/Diamond-Tools/Diamond-Grinding-Wheels
Grinding Wheels

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Superabrasive Wheel configu

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marking system for aluminum-oxide and silicon-carbide

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Standard marking system for cubic boron nitride and diamond bonded
abrasives.
abrasives, $30 to $100 for diamond, and $50 to $300 for
cubic boron nitride
wheels

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Bond Types
Vitrified.
Glass- Ceramic bonding
Feldspar + Clay
mixed with
Abrasive
And fired to sintered body-1250C
Poor shock resistance is biggest disadvantage
Steel backing is an improvisation

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Bond Types
Resinoid.
Thermosetting resins are available in a wide range of compositions and
properties. Because the bond is an organic compound, wheels are called
organic Wheels.
Manufacturing
(a) mixing the abrasive with liquid or powdered phenolic resins and additives,
(b) Pressing/injection molding the mixture into the shape of a grinding wheel,
and
(c) curing it at temperatures
of about 175°C.
Problem– flexibilty::: Polyimide is an improvisation

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Bond Types
Reinforced Wheels.
layers of fiberglass mats of various mesh sizes---laminate structure provides
reinforcement in resinoid wheels by way of retarding the disintegration of the
wheel should it break for some reason during use, rather than improving its
strength.
Large-diameter resinoid wheels can be supported additionally with one or more
internal rings made of round steel bars inserted during the molding of the wheel.
Thermoplastic.
ln addition to thermsetting resins, thermoplastic bonds are used in
grinding wheels. Wheels are available with sol-gel abrasives bonded with
thermoplastics.

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Rubber
(a) mixing crude rubber, sulfur, and the abrasive grains
together,
(b) rolling the mixture into sheets
(c) cutting out disks of various diameters, and
(d) heating the disks under pressure to vulcanize the rubber. Thin wheels
can be made in this manner and are used like saws for cutting-off
operations (cutoff blades).
Bond Types

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Metal.
abrasive grains (usually diamond or cubic boron nitride)
are bonded
to the periphery of a metal wheel to depths of 6 mm or less
Metal bonding is carried out under high pressure
and temperature. The wheel itself (the core) may be made of aluminum,
bronze, steel, ceramics, or composite materials--depending on
requirements such as strength, stiffness, and dimensional stability.
Super abrasive wheels may be layered so that a single
abrasive layer is plated or brazed to a metal wheel with a particular desired
shape.
Layered wheels are lower in cost and are used for small production
quantities.
Bond Types
Shellac Bond- resin secreted by bugs and bond is used for good surface finish grinding
wheels

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Wheel Grade and Structure
Grade is a measure of its bond strength- type and the amount of bond in the
wheel grade is also referred to as the hardness of a bonded abrasive.
Thus, a hard wheel has a stronger bond and/or a larger amount of bonding
material between the grains than a soft wheel.

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The Grinding Process
The individual abrasive grains have irregular shapes and are spaced
randomly along the periphery of the wheel
The average rake angle of the grains is highly negative, typically -60° or
even less. Consequently, grinding chips undergo much larger plastic
deformation than they do in other machining processes.
The radial positions of the grains over the peripheral surface of a wheel
vary; thus, not all grains are active during grinding.
Surface speeds (i.e., cutting speeds) in grinding are very high, typically
20 to 30 m/s, and may be as high as 150 m/s in high-speed grinding
using specially designed and manufactured wheels.

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Undeformed chip length, l
and chip thickness, t
Nc=vwC
where v ¼ wheel speed, mm/min (in/min); w ¼ crossfeed, mm (in)
and C= grits per area on the grinding wheel surface, grits/mm2
(grits/in2).

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As an example, I and t can be calculated for the following process parameters: Let D = 200
mm, d= 0.05 mm, v= 30 m/min, and V = 1800 m/min. Using the C as the number cutting
points per unit area= 2/mm2 – r is the ratio of chip width to average undeformed chip
thickness and has an estimated value typically between 10 and 20, use the average value of r
Please calculate, I=
And T=?
Grinding Forces.
A knowledge of grinding forces is essential for
• Estimating power requirements.
• Designing grinding machines and work-holding fixtures and devices.
• Determining the deflections that the work piece, as well as the grinding machine itself, may undergo.
Note that, unless accounted for, deflections adversely affect dimensional accuracy and are especially
critical in precision and ultraprecision grinding.

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Grinding Forces.
Specific-energy requirements in grinding are defined as the energy per unit
volume of material ground from the workpiece surface
° Chip formation
° Plowing, as shown by the ridges formed in Fig.
' Friction, caused by rubbing of the grain along the workpiece surface.
Energies consumed in the grinding process are higher than the ones
consumed in the machining process.

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Grinding Forces.
wear flat, high negative rake angles of the grains (which require more
energy), and a possible contribution of the size effect (the smaller the
chip, the higher the energy required to produce it).
Specific energy: Cutting vs Grinding
The grinding force and the thrust force in grinding can be calculated from
the specific-energy data.
Example 26.1 A surface-grinding operation is being performed on low
carbon steel with a wheel of diameter, D = 250 mm that is rotating at
N = 4000 rpm, and a width of cut of w = 25 mm. The depth of cut is
d=.05mm and the feed rate of the workpiece, v, is 1.5m/min. Calculate the
cutting force(the force tangential to the wheel), Fv, and the thrust force
(the force normal to the workpiece surface), Fn

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MRR = dwv , u=specific energy=40Ws/mm3
Power= (u)MRR
Power=Tω and T=FcD/2== Fc=24N and Fn is 30% higher
than Fc.
Grinding Forces.

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Temperature Rise in Grinding.
Grinding action can lead to increase in temperature
Upto 1600 C -----Still grinding?
Overtemperature can hamper surface properties of the workpiece,
including metallurgical changes.
Temperature rise can cause residual stresses on the workpiece.
Temperature gradients in the workpiece cause distortions due to
thermal expansion and contraction of the workpiece surface, thus
making it difficult to control dimensional accuracy.

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.
Sparks. chips-glow
color, intensity, and shape of the sparks depend on the composition of the metal
being ground
For high heat , chips can melt, acquire a spherical shape (because of surface tension),
and solidify as metal particles.
Tempering
An excessive temperature rise in grinding can cause tempering and softening
of the workpiece surface. Process variables must be selected carefully in
order to avoid excessive temperature rise. The use of grinding fluids is an
effective means of controlling temperature.
Burning.
Excessive temperature- burning---A burn is characterized by a bluish color.
It can be detected by etching and metallurgical techniques.
A burn may not be objectionable in itself, unless phase transformations.
For example, martensite forming in higher carbon steels from rapid cooling is
called a metallurgical burn. Ductility and toughness is hampered.

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Heat Checking. High temperatures in grinding may cause the workpiece
surface to develop cracks; this condition is known as heat checking. The
cracks usually are perpendicular to the grinding direction. Under severe
conditions, however, parallel cracks also may appear.
such a surface lacks toughness and has low fatigue and corrosion
resistance.
Residual Stresses:
Temperature gradients within the work piece --- residual stresses.
Residual stresses usually can be reduced by lowering wheel speed and
increasing workpiece speed (called low-stress grinding or gentle grinding).
Softer grade wheels (known as free-cutting
wheels) also may be used.

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. Grinding-wheel wear is caused by three different mechanisms:
1. attritious grain wear,
2. grain fracture,
3. and bond fracture.
Attritious Grain Wear.
cutting edges of an originally sharp grain become dull and
develop a wear flat.
Wear involves--physical and chemical reactions.
Diffusion,
Chemical degradation or decomposition
fracture at a microscopic scale,
plastic deformation, and
Melting
The selection of the type of abrasive for low attritious wear is based
on the reactivity of the grain with the workpiece and on their relative
mechanical properties, such as hardness and toughness.
Grinding-wheel Wear

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Grinding-wheel Wear
Grain Fracture
Ideally, the grain should fracture or fragment at a moderate rate, so that
new sharp cutting edges are produced continuously during grinding
Bond Fracture.
Bond strength should be adequate to facilitate the grains dislodging.
Softer bonds are recommended for harder materials and for reducing residual stresses and thermal
damage to the workpiece.
Hard-grade wheels are for removing large amounts of material at high rates.

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Grinding Ratio
Grinding-wheel wear is generally correlated with
the amount of workpiece material ground by a
parameter called the grinding ratio, G, defined as
2~200
It is a relative term, i.e
Grinder may act soft or hard, grinding ratio may
be improved by the application of lubricants.

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EXAMPLE 26.2 Action of a Grinding Wheel
A surface-grinding operation is being carried out with the wheel running at a
constant spindle speed. Will the wheel act soft or hard as the wheel wears down
over time? Assume that the depth of cut, d, remains constant and the wheel is
dressed periodically (see Section 26.3.3).
Wear and Grinding Force

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Dressing, Truing, and Shaping of Grinding Wheels
Dressing is the process of
° Conditioning worn grains on the surface of a grinding wheel by
producing sharp new edges on grains so that they cut more effectively.
Dressing is necessary when
Grains are dull
Wheel is clogged with chips
Truing, which is producing a true circle on a wheel that has become out
of round.

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Dressing Techniques
A specially shaped diamond-point tool or diamond cluster is moved across the
width of the grinding face of a rotating wheel
A set of star-shaped steel disks is pressed manually against
the wheel. Material is removed from the wheel surface by
crushing the grains.
method produces a coarse surface
used only for rough grinding operations on bench or
pedestal grinders.

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Abrasive sticks may be used to dress grinding wheels, particularly softer
wheels. --------------not appropriate for precision grinding operations.
Electrical-discharge and electrochemical machining for metal-bonded diamond
wheels involve the use of-----
,
crush dressing or crush forming ---- for form grinding
Hardened steel, carbide or nitride tool
Computer Aided Dressing and other auxiliaries.
Dressing Resolutions
For alumina grinders 5 to 15micron
for a CBN wheel, it would be 2 to 10micron.
modern dressing systems 0.25 to 1micron
Dressing Techniques

110
Dressing Techniques

111
Grindability of Materials and Wheel Selection
How easy it is to grind a material , it includes;
the quality of the surface produced
surface finish,
surface integrity,
wheel wear,
cycle time, and overall economics of the operation.
Grindability of a material can be enhanced greatly by:
proper selection of process parameters
grinding wheels,
and grinding fluids,
as well as by using the appropriate machine characteristics,
fixturing methods, and work-holding devices.

112
Ductile-regime Grinding. ???

113
Grinding Operations and Machines
The selection of a grinding process and machine depends on:
workpiece shape and features,
size,
ease of fixturing,
and production rate required

114
Modern grinding machines are computer controlled with features:
automatic workpiece loading and unloading,
part clamping,
dressing, and wheel shaping.
Modern machines additionally may contain gadgets and sensors.
Please enlist the sensors applied in the automated, semi-automated
grinding machines and explain their working.?
Grinding Types:
Surface grinding is one of the most commonly applied.

115
a) Traverse grinding
Surface grinding

116
Surface grinding – Blanchard type machine

117
Cylindrical Grinding.
In cylindrical grinding ~ center-type grinding
external cylindrical surfaces and shoulders of workpieces
crankshaft bearings, spindles,pins, and bearing rings are ground.
(a) traverse grinding
plunge grinding
profile grinding.

118
The workpiece in cylindrical grinding is held between centers or
in a chuck, or it is mounted on a faceplate in the headstock of the
grinder.
For straight cylindrical surfaces, the axes of rotation
of the wheel and workpiece are parallel.
The wheel and workpiece are each driven by separate motors and at
different speeds.
Long workpieces with two or more diameters can be ground on cylindrical
grinders. As form grinding and plunge grinding, cylindrical grinding also
can produce shapes in which the wheel is dressed to the
workpiece form to be ground

119
universal grinders, both
the workpiece and the wheel axes can be
moved
cams grinding on a rotating workpiece.

120
Thread grinding

121
Internal Grinding.
In internal grinding a small wheel is used to
grind the inside diameter of the part like:
bushings and bearing races.
Internal profiles also can be ground
with profile-dressed wheels that
move radially into the workpiece.
The headstock of internal grinders can be
swiveled on a horizontal plane to grind
tapered holes

122
Centerless Grinding.
Continuously grinding cylindrical surfaces in which the workpiece is not
supported by chucks or magnetic plateforms.
Large wheel
Small Wheel
Other types are - infeed/plunge grinding internal grinding
Applications of centerless grinding are
Roller bearings, piston pins, engine valves, camshaftscomponents.
Parts with diameters as small as 0.1 mm can be ground

123
Creep-feed Grinding.
Grinding for large-scale metal-removal operations :similar to milling,
broaching, and planing.
depth of cut, d > 6 mm
speed is low
The wheels are softer grade resin bonded
and have an open structure.
Special power features, up to 225 kW.
High stiffness
high damping capacity,
variable spindle and worktable speeds,
and ample capacity for grinding fluids.
equipped with dressing facility, using a diamond roll .
Applications
grinding shaped punches, key seats,
twist-drill flutes, the roots of turbine blades.

124
Creep-feed Grinding.

125
Heavy Stock Removal by Grinding.
heavy stock removal- by increasing grinding process parameters.
Competitive to cutting and other machining processes
Surface finish is secondary requirement
Dimensional tolerances~ as obtained in other machining processes
It is performed on welds, castings, and forgings to smoothen weld beads and
remove flash.

126
Tool-post grinders- self contained grinding units fixed on lathe machine
Other Grinding Operations.
Universal tool and cutter grinders---Grinders to sharpen cutting tools
snag grinder(swing frame grinder)-----a grinder with big disks used to
remove extra metal fromCastings and weld slag.
Portable Grinders----- drive mechanisms are different
Bench and Pedestal grinders: fixed on small bench with two wheels on sides

127
Reduces temperature rise in the workpiece.
Improves part surface finish and dimensional accuracy.
Improves the efficiency of the operation by reducing wheel wear and
loadingand by lowering power consumption.
Application------ Flooding and Mist with Nozzle
Temperature regulation---by a chiller
Grinding Fluids

128
Grinding Chatter.
Cause of chatter can be understood by observing the surface(chatter marks).
(a) the bearings and spindles of the grinding machine.
(b) nonuniformities in the grinding wheel (as manufactured).
(c) uneven wheel wear.
(d) poor dressing techniques.
(e) using grinding wheels that are not balanced properly,
(f) external sources (such as nearby machinery).
Controlling Chatter
(Guidelines to reduce Chatter)
(a) using soft-grade wheels
(b)dressing the wheel frequently
(c) changing dressing techniques.
(d) reducing the material-removal rate, and
(e) supporting the workpiece
rigidly.

129
Safety in Grinding Operations.
To Avoid Fatal Accidents
follow procedures, instructions and Warnings printed on wheel label.
stored properly and protect from environmental extremes,
Visual inspection.
Inspection by ringing
Be aware of bursting speed--- expressed in rpm
Safety in Grinding Operations

130
Ultrasonic Machining
material is removed from a surface by microchipping and erosion with loose,
fine abrasive grains in a Water slurry.
amplitude of 0.0125 to 0.075 mm.
Particle-surface contact time 10-100S
frequency of 20 kHz
A special tool is required for each shape to be produced that is called a form tool.

131
Rotary Ultrasonic Machining.
No slurry
Vibration + rotaion
Applications
Deep holes and high metal
Removal from ceramics.

132
Design Considerations for Ultrasonic Machining.
Avoid sharp profiles, corners, and radii
Realize that holes produced will have some taper.
should have a backup plate.

133
Finishing Operations
Coated Abrasives.
Abrasives: Alumina, silicon carbide and zirconia alumina
On
Flexible backing material; paper, cotton, rayon polyester, polynylon.
adhered with
Matrix: resins, phenolic resin
Applications: finish flat or curved surfaces of metallic and nonmetallic parts, metallographic
specimens, and in Woodworking

134
Belt Grinding.
Coated Abrasives.
Belts with grit numbers ranging from 16 to 1500.
Speeds 700 to 1,800 m/min.
surgical implants, golf clubs, firearms, turbine blades, and
medical and dental instruments.

135
Wire Brushing.
Wire brushing is used to produce a fine
or controlled surface texture and may be
used for cleaning and small material
removal.

136
Honing
to improve the surface finish of holes
made by other process,……………..
Fluid is generally applied.
Require great skills otherwise holes may
be deshaped.

137
Honing
Superfinishing
Process is performed with very light motion of
the honing stone has a short stroke
Fluid?

138
Lapping.
Lap: soft and porous of cast
iron, copper, leather, or cloth
Abrasive: particles either embedded in the ---
or may be carried in a slurry.
Wokpiece
Superfinishing
Dimensional tolerances ±.0004 mm
Surface finish: .025~.1 micron

139
Superfinishing
Polishing.
Softening, smearing and very little metal removing
done with disks or belts made of fabric,
leather, or felt that are
typically coated with fine powders of
aluminum oxide or diamond

140
Polishing.
Chemical-mechanical Polishing.
combined abrasion and corrosion effects.
Electropolishing
Polishing in Magnetic Fields.

141
Polishing in Magnetic Fields.
Magnetic-field-assisted polishing
In the magnetic-float polishing

142
Buffing.
Buffing is similar to polishing
Finer surfacen finish than polishing
Very fine abrasive on cloth or hide

143
Deburring Operations
Pros and Cons of Burs
a) jamming and misalignment.
b) short circuits.
c) safety hazard to personnel.
d) fatigue life.
e) lower bendability
f) holding torque of screws

144
Vibratory and Barrel Finishing.
abrasive pellets, metallic or non-metalic, vibration/tumbling
Fluids to impart erosive or corrosive action, similar to electro-mechanical polishing.
Shot Blasting.
Abrasive particles(sand) + high velocity jet of air
Matte finish

145
Abrasive-flow Machining.
abrasive grains, such
as silicon carbide or diamond,
that are mixed in a putty-like
matrix and then forced back
and forth through the openings
and passageways in the
workpiece

146
Thermal Energy Deburring.
Mixture of NG + air -----heat
Robotic Deburring.
Fast
Low labor cost
repeatability
Demerits:

147
Economics of Abrasive Machining
and Finishing Operations

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