Lecture 8 TOOL MATERIALS AND FLUIDS Modified

8/3/2019 Lecture 8 TOOL MATERIALS AND FLUIDS Modified

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CUTTING TOOLLIFE, MATERIALS

& CUTTINGFLUIDS


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Taylor Tool Life Equation

where V = cutting speed; T = tool life;and n and C are parameters that

depend on feed, depth of cut, workpiece material, tooling material, and thetool life criterion used


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Characteristics of cutting tool

Hardness (Elevated temperatures)

Toughness (Impact forces on tool ininterrupted operations)

Wear resistance (tool life to be

considered) Chemical stability or inertness (to

avoid adverse reactions)


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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


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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

High speed steels (HSS) Hardened to various depths

Good wear resistance

Suitable for high positive rake angle tools


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Two basic types ofHSS

Molybdenum ( M-series)

Tungsten ( T-series)

M-series - Contains 10% molybdenum, chromium, vanadium,tungsten, cobalt

Higher, abrasion resistance

H.S.S. are majorly made of M-series

T-series - 12 % - 18 % tungsten, chromium, vanadium &cobalt

undergoes less distortion during heat treating


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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 wearresistance or steam treatment at elevatedtemperatures

High speed steels account for largest tonnage


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Cast-Cobalt alloys

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 impactforces

Less suitable than high-speed steels for interrupted cuttingoperations

Finishing cuts are at lower feed and depth of cut


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Carbides :

3-groups of materials Alloy steels

High speed steels Cast alloys

These carbides are also known as cemented orsintered carbides

High elastic modulus,thermal conductivity

Low thermal expansion

2-groups of carbides used for machining operations

tungsten carbide

titanium carbide


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Tungsten Carbide

Composite material consisting of tungsten-carbide particlesbonded together

Alternate name is cemented carbides

Manufactured with powder metallurgy techniques

Particles 1-5 Mum in size are pressed & sintered to desiredshape

Amount of cobalt present affects properties of carbide tools

As cobalt content increases strength hardness & wear

resistance increases


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Titanium carbide

Titanium carbide has higher wear

resistance than tungsten carbide

Nickel-Molybdenum alloy as matrix Tic suitable for machining hard

materials

Speeds higher than those for tungstencarbide


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Inserts


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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


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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.


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Edge StrengthFig : Relative edge

strength andtendency for chippingand breaking ofinserts with various

shapes. Strengthrefers to the cuttingedge shown by theincluded angles.

Fig : edge preparation ofinserts to improve edgestrength.


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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 ofchip produced during cutting


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Coated tools :

- High strength and toughness but generallyabrasive and chemically reactive with toolmaterials

Unique Properties :

Lower Friction High resistance to cracks and wear

High Cutting speeds and low time & costs

Longer tool life


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Coating materials Titanium nitride (TiN)

Titanium carbide (Tic)

Titanium Carbonitride (TicN)

Aluminum oxide (Al2O3)thickness range 2-15 m (80-600Mu.in)

Techniques used :

Chemical vapor deposition (CVD)

Plasma assisted CVD

Physical-vapor deposition(PVD)

Medium temperature chemical- vapordeposition(MTCVD)


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Properties for Group of Materials

Fig : Ranges of properties

for various groups of

tool materials.


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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

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.


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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 thermalconductivity

Intermediate layer Bond well & compatible with bothlayers

Coatings of alternating multipurpose layers are also

formed.


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Multiphase Coatings

Fig : Multiphase coatings on atungsten-carbidesubstrate. Threealternating layers ofaluminum oxide areseparated by very thinlayers of titanium nitride.Inserts with as many asthirteen layers ofcoatings have beenmade. Coating thicknesses are typically inthe range of 2 to 10 m.


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Diamond Coated tools :

Use of Polycrystalline diamond as a coating

Difficult to adhere diamond film to substrate

Thin-film diamond coated inserts nowcommercially available

Thin films deposited on substrate with PVD & CVDtechniques

Thick films obtained by growing large sheet ofpure diamond

Diamond coated tools particularly effective inmachining non-ferrous and abrasive materials


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Diamond :

Hardest known substance

Low friction, high wear resistance Ability to maintain sharp cutting edge

Single crystal diamond of various carats usedfor special applications

Machining copperfront precision opticalmirrors for ( SDI)

Diamond is brittle , tool shape & sharpened isimportant

Low rake angle used for string cutting edge


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Whisker reinforced & Nanocrystalline

tool materials

New tool materials with enhanced properties :

High fracture toughness

Resistance to thermal shock

Cutting edge strength Hot hardness


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Cutting-Tool Reconditioning

When tools get worned, they are reconditioned for further use

Reconditioning also involves recoating used tools with

titanium nitride

Cutting Fluids: (Lubricants + Coolants)Used in machining as well as abrasive machining processes

Reduces friction wear

Reduce forces and energy consumption

Cools the cutting zone

Wash away the chips

Protect Machined surfaces from environmental corrosion


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Application of Cutting Fluids

Fig : Schematic illustration of

proper methods ofapplying cutting fluids invarious machiningoperations: (a)turning,(b)milling, (c)threadgrinding, and (d)drilling

Lecture 8 TOOL MATERIALS AND FLUIDS Modified

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Transcript of Lecture 8 TOOL MATERIALS AND FLUIDS Modified