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).

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.

r
 t o t c
 where r = chip thickness ratio; t o
= thickness of the chip prior to chip formation; and t after separation c
= chip thickness
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: tan
 
1 r
 cos
 r sin
 where r = chip ratio, and

= rake angle

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.

equation, based on the preceding parallel plate model:
  = tan(   ) + cot 
 = shear strain,  = shear

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





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






Ductile work materials
High cutting speeds
Small feeds and depths
Sharp cutting edge
Low tool-chip friction
Figure 21.9 (b) continuous





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




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.



Friction force F and Normal force to friction N
Shear force F s and Normal force to shear F n
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 F s and F n
= 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:
S

F s
A s where A s
= area of the shear plane
A s
 t o w sin

Shear stress = shear strength of work material during cutting



F, N, F s
, and F n cannot be directly measured
Forces acting on the tool that can be measured:
› Cutting force F c and Thrust force F t
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 = F c
N = F c
F s
F n
= F
= F c c sin  cos cos sin 


+ F
- F
- F
+ F t t t t cos  sin  sin  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)

 edge angle
Ski
 shear angle
Snow p
Fs
Fc

F
N
R   

Fn
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

1.
2.
3.
4.
5.
Good cooling capacity
Good lubricating qualities
Resistance to rancidity
Relatively low viscosity
Stability (long life)
8.
9.
6.
7.
Rust resistance
Nontoxic
Transparent
Nonflammable
34



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
38





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

1.
2.
3.
4.
Good rust control
Resistance to rancidity for long periods of time
Reduction of amount of heat generated during cutting
Excellent cooling qualities
41

6.
7.
8.
9.
5.
10.
11.
Longer durability than cutting or soluble oils
Nonflammable - nonsmoking
Nontoxic??????
Easy separation from work and chips
Quick settling of grit and fine chips so they are not recirculated in cooling system
No clogging of machine cooling system due to detergent action of fluid
Can leave a residue on parts and tools
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

49




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

4.
5.
1.
2.
3.
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
56





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
(Powder metallurgy)

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








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 (Al
2
600Mu.in)
O
3
)thickness range – 2-15 µm (80-
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