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).

Machining

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.

Orthogonal Cutting Model

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

c

o

tt

r

Based on the geometric parameters of the orthogonal model, the shear plane angle can be determined as:

where r = chip ratio, and = rake angle

sincos

tanr

r

1

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 Chip Formation

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

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.

Chip Formation

1. Discontinuous chip2. Continuous chip3. Continuous chip with Built-up Edge

(BUE)4. 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

Discontinuous Chip

Ductile work materials

High cutting speeds

Small feeds and depths

Sharp cutting edge

Low tool‑chip friction

Figure 21.9 (b) continuous

Continuous Chip

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

Continuous with BUE

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

Serrated Chip

Figure 21.9 (d) serrated.

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

Forces Acting on Chip

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:

Friction angle related to coefficient of friction as follows:

NF

tan

Shear stress acting along the shear plane:

sinwt

A os

where As = area of the shear plane

Shear stress = shear strength of work material during cutting

s

s

AF

S

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

Cutting Force and Thrust Force

Equations can be derived to relate the forces that cannot be measured to the forces that can be measured:

F = Fc sin + Ft cosN = Fc cos ‑ Ft sin

Fs = Fc cos ‑ Ft sinFn = 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

Derived by Eugene Merchant Based on orthogonal cutting, but

validity extends to 3-D machining

2245

To increase shear plane angle › Increase the rake angle › Reduce the friction angle (or

coefficient of friction)

2245

Fc

Fn

FtR

N

F

--

-

pFs

SnowSki

Force relationships Merchant circle

ForcesFc = centrifugal

(cutting)Ft = thrustFs = shearFn = normal to

shear planeF = friction on skiN = normal to ski

shear angleedge angle

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. Good cooling capacity

2. Good lubricating qualities

3. Resistance to rancidity

4. Relatively low viscosity

5. Stability (long life)

6. Rust resistance

7. Nontoxic8. Transparent9. Nonflammabl

e

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 fatty-mineral 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

pressures39

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. Good rust control2. Resistance to rancidity for

long periods of time3. Reduction of amount of heat

generated during cutting4. Excellent cooling qualities

41

5. Longer durability than cutting or soluble oils

6. Nonflammable - nonsmoking7. Nontoxic??????8. Easy separation from work and chips9. 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

42

43

Chemical cutting fluids widely acceptedand generally used on ferrous metals. Theyare not recommended for use on alloys ofmagnesium, zinc, cadmium, or lead. Theycan mar machine's appearance and dissolvepaint on the surface.

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 chip-tool 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

48Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

51

Built-up edge keepsbreaking off and

re-forming

Result is poor surface finish, excessive flank

wear, and cratering of tool face

1. Lowers heat created by plastic deformation of metal

2. Friction at chip-tool interface decreased

3. Less power is required for machining because of reduced friction

4. Prevents built-up edge from forming5. 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 case-hardening 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% chromium10%-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 (80-

600Mu.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 Sic-reinforced tools

Progress in nanomaterial has lead to the development of cutting tools

Made of fine grained structures as (micro grain) carbides