1. Machining
Cutting action involves shear deformation of work material to form a chip
As chip is removed, new surface is exposed
Figure (a) A cross‑sectional view of the machining process, (b) tool with negative rake angle; compare with positive rake angle in (a).

2. Orthogonal Cutting Model
Simplified 2-D model of machining that describes the mechanics of machining fairly accurately
Figure Orthogonal cutting: (a) as a three‑dimensional process.

3. Chip Thickness Ratio
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

4. Determining Shear Plane Angle
Based on the geometric parameters of the orthogonal model, the shear plane angle  can be determined as:
where r = chip ratio, and  = rake angle

5. Shear Strain in Chip Formation
Figure 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.

6. Shear Strain
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

7. Chip Formation
Figure 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.

8. Four Basic Types of Chip in Machining
Discontinuous chip
Continuous chip
Continuous chip with Built-up Edge (BUE)
Serrated chip

9. Brittle work materials
Discontinuous Chip
Brittle work materials
Low cutting speeds
Large feed and depth of cut
High tool‑chip friction
Figure Four types of chip formation in metal cutting: (a) discontinuous

10. Continuous Chip Ductile work materials High cutting speeds
Small feeds and depths
Sharp cutting edge
Low tool‑chip friction
Figure 21.9 (b) continuous

11. 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

12. Serrated Chip Semicontinuous - saw-tooth appearance
Cyclical chip forms with alternating high shear strain then low shear strain
Associated with difficult-to-machine metals at high cutting speeds
Figure 21.9 (d) serrated.

13. Forces Acting on Chip Friction force F and Normal force to friction N
Shear force Fs and Normal force to shear Fn
Figure Forces in metal cutting: (a) forces acting on the chip in orthogonal cutting

14. Resultant Forces 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

15. Coefficient of Friction
Coefficient of friction between tool and chip:
Friction angle related to coefficient of friction as follows:

16. Shear Stress Shear stress acting along the shear plane:
where As = area of the shear plane
Shear stress = shear strength of work material during cutting

17. 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 Forces in metal cutting: (b) forces acting on the tool that can be measured

18. Forces in Metal Cutting
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

19. The Merchant Equation
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
Derived by Eugene Merchant
Based on orthogonal cutting, but validity extends to 3-D machining

20. What the Merchant Equation Tells Us
To increase shear plane angle
Increase the rake angle
Reduce the friction angle (or coefficient of friction)

21. Force relationships Merchant circleFc Fn Ft R N F 
--
-   p Fs
Snow
Ski
edge angle
shear angle
Forces
Fc = centrifugal
(cutting)
Ft = thrust
Fs = shear
Fn = normal to
shear plane
F = friction on ski
N = normal to ski

22. Cutting tool geometry

23. Tool Geometry
The most important geometry’s to consider on a cutting tool are
Back Rake Angles
End Relief Angles
Side Relief Angles

24. Rake Angles Small to medium rake angles cause: high compression
high tool forces
high friction
result = Thick—highly deformed—hot chips

25. Rake Angles 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.

26. Rake Angles 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.

27. Negative Rake Tools
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.

28. Negative Rake Tools

29. Summary Positive vs. Negative Rake Angles
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

30. Summary Positive vs. 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

31. Tool Angle Application
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

32. Tool Life HIGH STRESSES & TEMPERATURES GRADUAL WEAR MANY VARIABLES
MATERIAL
CUTTING FLUIDS
TOOL SHAPE
SPEEDS & FEED RATE
CHIPPING

33. Tool Wear

34. Characteristics of a Good Cutting Fluid
Good cooling capacity
Good lubricating qualities
Resistance to rancidity
Relatively low viscosity
Stability (long life)
Rust resistance
Nontoxic
Transparent
Nonflammable

35. Types of Cutting Fluids
Most commonly used cutting fluids
Either aqueous based solutions or cutting oils
Fall into three categories
Cutting oils
Emulsifiable oils
Chemical (synthetic) cutting fluids

36. Cutting Oils 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

37. Active Cutting Oils
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

38. Inactive Cutting Oils
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 fatty-mineral oil blend

39. Emulsifiable (Water Soluble) Oils
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

40. Chemical Cutting Fluids
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

41. Advantages of Synthetic Fluids
Good rust control
Resistance to rancidity for long periods of time
Reduction of amount of heat generated during cutting
Excellent cooling qualities

42. 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

43. Caution
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.

44. Functions of a Cutting Fluid
Prime functions
Provide cooling
Provide lubrication
Other functions
Prolong cutting-tool life
Provide rust control
Resist rancidity

45. Functions of a Cutting Fluid: Cooling
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

46. Functions of a Cutting Fluid: Cooling
Water most effective for reducing heat by will promote oxidation (rust)
Decrease the temperature at the chip-tool interface by 50 degrees F, and it will increase tool life by up to 5 times.

47. Functions of a Cutting Fluid: Lubrication
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)

48. Cutting fluid reduces friction and produces a shorter shear plane.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

49. Cutting fluid reduces friction and produces a shorter shear plane.

50. Cutting-Tool Life
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

51. Built-up Edge Built-up edge keeps breaking off and re-forming
Result is poor surface finish, excessive flank
wear, and cratering of tool face

52. Cutting Fluid's Effect on Cutting Tool Action
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

53. Rust Control Water best and most economical coolant
Causes parts to rust
Rust is oxidized iron
Chemical cutting fluids contain rust inhibitors
Polar film
Passivating film

54. Rancidity Control
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
Rancidity Control
When lard oil was the only cutting fluid used, after a few days, it would start to spoil and give off an offensive odor. This rancidity is caused by bacteria and other microscopic organisms, growing and multiplying almost anywhere and eventually causing bad odors to form. Today, any cutting fluid that has an offensive smell is termed rancid.

55. Application of Cutting Fluids
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

56. Refrigerated Air System
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

57. Characteristics of cutting tool
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)

58. Cutting tool materials
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

59. Carbon and Medium alloy steels
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

60. High speed steels Hardened to various depths Good wear resistance
Relatively
Suitable for high positive rake angle tools

61. Two basic types of HSS
Molybdenum ( M-series)
Tungsten ( T-series)

62. M-series Contains 10% molybdenum, chromium, vanadium, tungsten, cobalt
Higher, abrasion resistance
H.S.S. are majorly made of M-series 

63. T-series 12 % - 18 % tungsten, chromium, vanadium & cobalt
undergoes less distortion during heat treating

64. H.S.S. available in wrought ,cast & sintered (Powder metallurgy)
Coated for better performance
Subjected to surface treatments such as case-hardening for improved hardness and wear resistance or steam treatment at elevated temperatures
High speed steels account for largest tonnage

65. Cast-Cobalt alloys 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

66. Carbides : 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

67. Tungsten 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

68. Titanium carbide
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

69. Inserts

70. Inserts 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

71. Insert Attachment
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.

72. Edge Strength
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.

73. Chip breakers: 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

74. Coated tools :
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

75. Coating materials Titanium nitride (TiN) Titanium carbide (Tic)
Titanium Carbonitride (TicN)
Aluminum oxide (Al2O3)thickness range – 2-15 µm (80-600Mu.in)
Techniques used :
Chemical –vapor deposition (CVD)
Plasma assisted CVD
Physical-vapor deposition(PVD)
Medium –temperature chemical- vapor deposition(MTCVD)

76. Properties for Group of Materials
Fig : Ranges of properties for various groups of tool materials.

77. Cutting tool Characteristics for coating :
High hardness
Chemical stability
Low thermal conductivity
Good bonding
Little or no Porosity
Titanium nitride (TiN) coating :
Low friction coefficients
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.

78. Ceramics : 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.

79. Multiphase Coatings
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.

80. Diamond Coated tools : 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

81. New Coating 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

82. Alumina-Based ceramics:
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.

83. Cubic boron Nitride ( CBN ) :
Made by bonding ( mm ( 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).

84. Silicon-Nitride based ceramics (SiN)
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

85. Diamond : 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

86. Polycrystalline-Diamond ( PCD ) Tools:
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

87. Whisker –reinforced & Nanocrystalline tool materials
New tool materials with enhanced properties :
High fracture toughness
Resistance to thermal shock
Cutting –edge strength
Hot hardness

88. Whiskers used as reinforcing fibers :
Examples: Silicon-nitride base tools reinforced with silicon-carbide( Sic)
Aluminum oxide based tools reinforced with silicon-carbide with ferrous metals makes Sic-reinforced tools
Progress in nanomaterial has lead to the development of cutting tools
Made of fine grained structures as (micro grain) carbides