MANUFACTURING_TECHNOLOGY_I

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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.

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

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.

equation, based on the preceding parallel plate model:

  = tan(   ) + cot 

= shear strain,  = shear

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.

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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 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

Cutting Force and Thrust Force

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)

Force relationships Merchant circle

 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

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Good cooling capacity

Good lubricating qualities

Resistance to rancidity

Relatively low viscosity

Stability (long life)

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7.

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

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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

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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

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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

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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

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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

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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

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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.

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Prime functions

Provide cooling

Provide lubrication

Other functions

Prolong cutting-tool life

Provide rust control

Resist rancidity

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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

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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.

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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)

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Copyright © The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.

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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

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Built-up edge keeps breaking off and re-forming

Result is poor surface finish, excessive flank wear, and cratering of tool face

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4.

5.

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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

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Water best and most economical coolant

› Causes parts to rust

Rust is oxidized iron

Chemical cutting fluids contain rust inhibitors

Polar film

Passivating film

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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

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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

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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

(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

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