Transcript Slide 1

Machining
Cutting action involves shear deformation of
work material to form a chip
 As chip is removed, new surface is exposed
Figure 21.2 (a) A cross-sectional view of the machining process, (b)
tool with negative rake angle; compare with positive rake angle in (a).
Orthogonal Cutting Model
Simplified 2-D model of machining that
describes the mechanics of machining
fairly accurately
Figure 21.6 Orthogonal cutting: (a) as a three-dimensional process.
to
r 
tc
where r = chip thickness ratio; to =
thickness of the chip prior to chip
formation; and tc = chip thickness
after separation

Chip thickness after cut always
greater than before, so chip
ratio always less than 1.0

Based on the geometric parameters of
the orthogonal model, the shear plane
angle  can be determined as:
r cos 
tan  
1  r sin
where r = chip ratio, and  = rake angle
Shear Strain in Chip Formation
Figure 21.7 Shear strain during chip formation: (a) chip formation
depicted as a series of parallel plates sliding relative to each other,
(b) one of the plates isolated to show shear strain, and (c) shear
strain triangle used to derive strain equation.
Shear strain in machining can be
computed from the following
equation, based on the preceding
parallel plate model:
 = tan( - ) + cot 
where  = shear strain,  = shear
plane angle, and  = rake angle of
cutting tool
Chip Formation
Figure 21.8 More realistic view of chip formation, showing shear
zone rather than shear plane. Also shown is the secondary shear
zone resulting from tool-chip friction.
1.
2.
3.
4.
Discontinuous chip
Continuous chip
Continuous chip with Built-up Edge (BUE)
Serrated chip
Discontinuous Chip
Brittle work
materials
Low cutting
speeds
Large feed and
depth of cut
High tool-chip
friction




Figure 21.9 Four types of chip
formation in metal cutting:
(a) discontinuous
Continuous Chip





Ductile work
materials
High cutting
speeds
Small feeds and
depths
Sharp cutting
edge
Low tool-chip
friction
Figure 21.9 (b) continuous
Continuous with BUE




Ductile materials
Low-to-medium
cutting speeds
Tool-chip friction
causes portions of
chip to adhere to
rake face
BUE forms, then
breaks off, cyclically
Figure 21.9 (c) continuous
with built-up edge
Serrated Chip



Semicontinuous saw-tooth
appearance
Cyclical chip
forms with
alternating high
shear strain then
low shear strain
Associated with
difficult-tomachine metals
at high cutting
speeds
Figure 21.9 (d) serrated.
Forces Acting on Chip


Friction force F and Normal force to friction N
Shear force Fs and Normal force to shear Fn
Figure 21.10 Forces in
metal cutting: (a) forces
acting on the chip in
orthogonal cutting
Vector addition of F and N = resultant R
 Vector addition of Fs and Fn = resultant R'
 Forces acting on the chip must be in
balance:

› R' must be equal in magnitude to R
› R’ must be opposite in direction to R
› R’ must be collinear with R
Coefficient of friction between tool and chip:
F

N
Friction angle related to coefficient of friction as follows:
  tan 
Shear stress acting along the shear plane:
Fs
S
As
where As = area of the shear plane
t ow
As 
sin 
Shear stress = shear strength of work material during cutting
Cutting Force and Thrust Force


F, N, Fs, and Fn cannot be directly measured
Forces acting on the tool that can be
measured:
› Cutting force Fc and Thrust force Ft
Figure 21.10 Forces
in metal cutting: (b)
forces acting on the
tool that can be
measured

Equations can be derived to relate the
forces that cannot be measured to the
forces that can be measured:
F = Fc sin + Ft cos
N = Fc cos - Ft sin
Fs = Fc cos - Ft sin
Fn = Fc sin + Ft cos

Based on these calculated force, shear
stress and coefficient of friction can be
determined

Of all the possible angles at which
shear deformation can occur, the
work material will select a shear
plane angle  that minimizes
energy, given
 by

  45 


2

2
Derived by Eugene Merchant
Based on orthogonal cutting, but
validity extends to 3-D machining
  45 


2


2
To increase shear plane angle
› Increase the rake angle
› Reduce the friction angle (or
coefficient of friction)
Force relationships Merchant circle

Ski
edge angle

shear angle
Snow
p
Fs
Fc
Fn
F

R
N
--
Ft

-
Forces
Fc = centrifugal
(cutting)
Ft = thrust
Fs = shear
Fn = normal to
shear plane
F = friction on ski
N = normal to ski

The most important geometry’s to
consider on a cutting tool are
› Back Rake Angles
› End Relief Angles
› Side Relief Angles

Small to medium rake angles cause:
› high compression
› high tool forces
› high friction
› result = Thick—highly deformed—hot chips

Larger positive rake
angles
› Reduce compression
and less chance of a
discontinuous chip
› Reduce forces
› Reduce friction
› Result = A thinner,
less deformed, and
cooler chip.

Problems….as we increase the angle:
› Reduce strength of tool
› Reduce the capacity of the tool to conduct
heat away from the cutting edge.
› To increase the strength of the tool and
allow it to conduct heat better, in some
tools, zero to negative rake angles are used.

Typical tool materials which utilize
negative rakes are:
 Carbide
 Diamonds
 Ceramics

These materials tend to be much more
brittle than HSS but they hold superior
hardness at high temperatures. The
negative rake angles transfer the cutting
forces to the tool which help to provide
added support to the cutting edge.

Positive rake angles
› Reduced cutting forces
› Smaller deflection of work, tool holder, and
›
›
›
›
machine
Considered by some to be the most efficient
way to cut metal
Creates large shear angle, reduced friction and
heat
Allows chip to move freely up the chip-tool zone
Generally used for continuous cuts on ductile
materials which are not to hard or brittle

Negative rake angles
› Initial shock of work to tool is on the face of
the tool and not on the point or edge. This
prolongs the life of the tool.
› Higher cutting speeds/feeds can be
employed

Factors to consider for tool angles
› The hardness of the metal
› Type of cutting operation
› Material and shape of the cutting tool
› The strength of the cutting edge
HIGH STRESSES & TEMPERATURES
 GRADUAL WEAR
 MANY VARIABLES
 MATERIAL
 CUTTING FLUIDS
 TOOL SHAPE
 SPEEDS & FEED RATE
 CHIPPING

Good cooling
capacity
2. Good lubricating
qualities
3. Resistance to
rancidity
4. Relatively low
viscosity
5. Stability (long life)
1.
6.
7.
8.
9.
Rust resistance
Nontoxic
Transparent
Nonflammable
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
Most commonly used cutting fluids
› Either aqueous based solutions or
cutting oils

Fall into three categories
› Cutting oils
› Emulsifiable oils
› Chemical (synthetic) cutting fluids
35

Two classifications
› Active
› Inactive

Terms relate to oil's chemical activity or
ability to react with metal surface
› Elevated temperatures
› Improve cutting action
› Protect surface
36
Those that will darken copper strip
immersed for 3 hours at temperature
of 212ºF
 Dark or transparent
 Better for heavy-duty jobs
 Three categories

› Sulfurized mineral oils
› Sulfochlorinated mineral oils
› Sulfochlorinated fatty oil blends
37
Oils will not darken copper strip
immersed in them for 3 hours at 212ºF
 Contained sulfur is natural

› Termed inactive because sulfur so firmly
attached to oil – very little released

Four general categories
› Straight mineral oils, fatty oils, fatty and
mineral oil blends, sulfurized fattymineral oil blend
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Mineral oils containing soaplike material
that makes them soluble in water and
causes them to adhere to workpiece
 Emulsifiers break oil into minute particles
and keep them separated in water

› Supplied in concentrated form (1-5 /100
water)
Good cooling and lubricating qualities
 Used at high cutting speeds, low cutting
pressures

39
Also called synthetic fluids
 Introduced about 1945
 Stable, preformed emulsions

› Contain very little oil and mix easily with water

Extreme-pressure (EP) lubricants added
› React with freshly machined metal under
heat and pressure of a cut to form solid
lubricant

Reduce heat of friction and heat caused
by plastic deformation of metal
40
Good rust control
2. Resistance to rancidity for long
periods of time
3. Reduction of amount of heat
generated during cutting
4. Excellent cooling qualities
1.
41
Longer durability than cutting or soluble
oils
6. Nonflammable - nonsmoking
7. Nontoxic??????
8. Easy separation from work and chips
9. Quick settling of grit and fine chips so
they are not recirculated in cooling
system
10. No clogging of machine cooling system
due to detergent action of fluid
11. Can leave a residue on parts and tools
5.
42
Chemical cutting fluids widely accepted
and generally used on ferrous metals. They
are not recommended for use on alloys of
magnesium, zinc, cadmium, or lead. They
can mar machine's appearance and dissolve
paint on the surface.
43

Prime functions
› Provide cooling
› Provide lubrication

Other functions
› Prolong cutting-tool life
› Provide rust control
› Resist rancidity
44

Heat has definite bearing on cutting-tool
wear
› Small reduction will greatly extend tool life

Two sources of heat during cutting action
› Plastic deformation of metal
 Occurs immediately ahead of cutting tool
 Accounts for 2/3 to 3/4 of heat
› Friction from chip sliding along cutting-tool
face
45
Water most effective for reducing heat
by will promote oxidation (rust)
 Decrease the temperature at the chiptool interface by 50 degrees F, and it
will increase tool life by up to 5 times.

46

Reduces friction between chip and
tool face
› Shear plane becomes shorter
› Area where plastic deformation occurs
correspondingly smaller
Extreme-pressure lubricants reduce
amount of heat-producing friction
 EP chemicals of synthetic fluids
combine chemically with sheared
metal of chip to form solid compounds
(allow chip to slide)

47
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
48
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Heat and friction prime causes of
cutting-tool breakdown
 Reduce temperature by as little as 50ºF,
life of cutting tool increases fivefold
 Built-up edge

› Pieces of metal weld themselves to tool
face
› Becomes large and flat along tool face,
effective rake angle of cutting tool
decreased
50
Built-up edge keeps
breaking off and
re-forming
Result is poor
surface finish,
excessive flank
wear, and cratering
of tool face
51
1.
2.
3.
4.
5.
Lowers heat created by plastic
deformation of metal
Friction at chip-tool interface
decreased
Less power is required for machining
because of reduced friction
Prevents built-up edge from forming
Surface finish of work greatly improved
52

Water best and most economical
coolant
› Causes parts to rust
Rust is oxidized iron
 Chemical cutting fluids contain rust
inhibitors

› Polar film
› Passivating film
53
Rancidity caused by bacteria and
other microscopic organisms,
growing and eventually causing bad
odors to form
 Most cutting fluids contain
bactericides that control growth of
bacteria and make fluids more
resistant to rancidity

54
Cutting-tool life and machining
operations influenced by way
cutting fluid applied
 Copious stream under low pressure
so work and tool well covered

› Inside diameter of supply nozzle ¾ width
of cutting tool
› Applied to where chip being formed
55
Another way to cool chip-tool interface
 Effective, inexpensive and readily
available
 Used where dry machining is necessary
 Uses compressed air that enters vortex
generation chamber

› Cooled 100ºF below incoming air

Air directed to interface and blow chips
away
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Hardness (Elevated temperatures)
 Toughness (Impact forces on tool in
interrupted operations)
 Wear resistance (tool life to be
considered)
 Chemical stability or inertness (to avoid
adverse reactions)

Carbon & medium alloy steels
High speed steels
Cast-cobalt alloys
Carbides
 Coated tools
 Alumina-based ceramics
 Cubic boron nitride
 Silicon-nitride-base ceramics
 Diamond
 Whisker-reinforced materials




Oldest of tool materials
 Used for drills taps,broaches,reamers
 Inexpensive ,easily shaped,sharpened
 No sufficient hardness and wear resistance
 Limited to low cutting speed operation

Hardened to various depths
 Good wear resistance
 Relatively
 Suitable for high positive rake angle tools

Molybdenum ( M-series)
 Tungsten ( T-series)

Contains 10% molybdenum, chromium,
vanadium, tungsten, cobalt
 Higher, abrasion resistance
 H.S.S. are majorly made of M-series

12 % - 18 % tungsten, chromium,
vanadium & cobalt
 undergoes less distortion during heat
treating


H.S.S. available in wrought ,cast & sintered

Coated for better performance

Subjected to surface treatments such as casehardening for improved hardness and wear
resistance or steam treatment at elevated
temperatures

High speed steels account for largest tonnage
(Powder metallurgy)







Commonly known as stellite tools
Composition ranges – 38% - 53 % cobalt
30%- 33% chromium
10%-20%tungsten
Good wear resistance ( higher hardness)
Less tough than high-speed steels and sensitive to
impact forces
Less suitable than high-speed steels for interrupted
cutting operations
Continuous roughing cuts – relatively high g=feeds &
speeds
Finishing cuts are at lower feed and depth of cut
3-groups of materials






Alloy steels
High speed steels
Cast alloys
These carbides are also known as cemented or
sintered carbides
High elastic modulus,thermal conductivity
Low thermal expansion
2-groups of carbides used for machining operations


tungsten carbide
titanium carbide

Composite material consisting of tungsten-carbide particles
bonded together

Alternate name is cemented carbides

Manufactured with powder metallurgy techniques

Particles 1-5 Mum in size are pressed & sintered to desired shape

Amount of cobalt present affects properties of carbide tools

As cobalt content increases – strength hardness & wear
resistance increases

Titanium carbide has higher wear resistance
than tungsten carbide

Nickel-Molybdenum alloy as matrix – Tic
suitable for machining hard materials

Steels & cast irons

Speeds higher than those for tungsten
carbide







Individual cutting tool with severed cutting points
Clamped on tool shanks with locking mechanisms
Inserts also brazed to the tools
Clamping is preferred method for securing an insert
Carbide Inserts available in various shapes-Square,
Triangle, Diamond and round
Strength depends on the shape
Inserts honed, chamfered or produced with negative
land to improve edge strength
Fig : Methods of
attaching inserts
to toolholders : (a)
Clamping and (b)
Wing lockpins. (c)
Examples of
inserts attached to
toolholders with
threadless
lockpins, which
are secured with
side screws.
Fig : Relative edge
strength and
tendency for
chipping and
breaking of inserts
with various
shapes. Strength
refers to the
cutting edge
shown by the
included angles.
Fig : edge preparation of
inserts to improve edge
strength.
Purpose :
 Eliminating long chips
 Controlling chip flow during
machining
 Reducing vibration & heat generated
 Selection depends on feed and depth
of cut
 Work piece material,type of chip
produced during cutting
-
High strength and toughness but generally
abrasive and chemically reactive with tool
materials
Unique Properties :
 Lower Friction
 High resistance to cracks and wear
 High Cutting speeds and low time & costs
 Longer tool life
Titanium nitride (TiN)
 Titanium carbide (Tic)
 Titanium Carbonitride (TicN)
 Aluminum oxide (Al2O3)thickness range – 2-15 µm (80600Mu.in)

Techniques used :
 Chemical –vapor deposition (CVD)
Plasma assisted CVD
 Physical-vapor deposition(PVD)
 Medium –temperature chemical- vapor
deposition(MTCVD)
Fig : Ranges of properties
for various groups of
tool materials.





High hardness
Chemical stability
Low thermal conductivity
Good bonding
Little or no Porosity
Titanium nitride (TiN) coating :





Low friction coefficients
High hardness
Resistance to high temperatures
Good adhesion to substrate
High life of high speed-steel tools
Titanium carbide (TiC) coating:

Titanium carbide coatings on tungsten-carbide inserts have high flank
wear resistance.




Low thermal conductivity ,resistance ,high temperature
Resistance to flank wear and crater wear
Ceramics are suitable materials for tools
Al2O3 (most commonly used)
Multi Phase Coatings :




First layer –Should bond well with substrate
Outer layer – Resist wear and have low thermal
conductivity
Intermediate layer – Bond well & compatible with both
layers
Coatings of alternating multipurpose layers are also
formed.
Fig : Multiphase coatings on
a tungsten-carbide
substrate. Three
alternating layers of
aluminum oxide are
separated by very thin
layers of titanium
nitride. Inserts with as
many as thirteen layers
of coatings have been
made. Coating thick
nesses are typically in
the range of 2 to 10
µm.






Use of Polycrystalline diamond as a coating
Difficult to adhere diamond film to substrate
Thin-film diamond coated inserts now
commercially available
Thin films deposited on substrate with PVD & CVD
techniques
Thick films obtained by growing large sheet of
pure diamond
Diamond coated tools particularly effective in
machining non-ferrous and abrasive materials







Titanium carbo nitride (TiCN)
Titanium Aluminum Nitride(TiAlN)
Chromium Based coatings
Chromium carbide
Zirconium Nitride (ZrN)
Hafnium nitride (HfN)
Recent developments gives nano coating & composite coating
Ion Implementation :




Ions placed into the surface of cutting tool
No change in the dimensions of tool
Nitrogen-ion Implanted carbide tools used for alloy steels & stainless
steels
Xeon – ion implantation of tools as under development






Cold-Pressed Into insert shapes under high pressure and
sintered at high temperature
High Abrasion resistance and hot hardness
Chemically stable than high speed steels & carbides
So less tendency to adhere to metals
Good surface finish obtained in cutting cast iron and steels
Negative rake-angle preferred to avoid chipping due to poor
tensile strength
Cermets, Black or Hot- Pressed :
70% aluminum oxide & 30 % titanium carbide
 cermets(ceramics & metal)
 Cermets contain molybdenum carbide, niobium carbide and
tantalum carbide.





Made by bonding ( 0.5-1.0 mm ( 0.02-0.04-in)
Layer of poly crystalline cubic boron nitride to a carbide substrate by sintering
under pressure
While carbide provides shock resistance CBN layer provides high resistance
and cutting edge strength
Cubic boron nitride tools are made in small sizes without substrate
Fig : (a) Construction of a polycrystalline cubic boron nitride or a diamond layer on a tungsten-carbide insert. (b) Inserts
with polycrystalline cubic boron nitride tips (top row) and solid polycrystalline CBN inserts (bottom row).

They consists various addition of Aluminum Oxide ythrium
oxide, titanium carbide

SiN have toughness, hot hardened & good thermal – shock
resistance

SiN base material is Silicon

High thermal & shock resistance

Recommended for machining cast iron and nickel based super
alloys at intermediate cutting speeds







Hardest known substance
Low friction, high wear resistance
Ability to maintain sharp cutting edge
Single crystal diamond of various carats used
for special applications
Machining copper—front precision optical
mirrors for ( SDI)
Diamond is brittle , tool shape & sharpened is
important
Low rake angle used for string cutting edge
Used for wire drawing of fine wires
 Small synthesis crystal fused by high pressure and
temperature
 Bonded to a carbide substrate
 Diamond tools can be used fir any speed
 Suitable for light un-interrupted finishing cuts
 To avoid tool fracture single crystal diamond is to
be re-sharpened as it becomes dull
 Also used as an abrasive in grinding and polishing
operations

New tool materials with enhanced properties :




High fracture toughness
Resistance to thermal shock
Cutting –edge strength
Hot hardness

Examples: Silicon-nitride base tools reinforced
with silicon-carbide( Sic)

Aluminum oxide based tools reinforced with
silicon-carbide with ferrous metals makes Sicreinforced tools

Progress in nanomaterial has lead to the
development of cutting tools

Made of fine grained structures as (micro grain)
carbides