machining theory 4

8/7/2019 machining theory 4

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Prepared by dr. Mohamed AhmedPrepared by dr. Mohamed AhmedAwadAwad

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Machining FundamentalsMachining Fundamentals


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AdvantagesAdvantages

More dimensional accuracy mayMore dimensional accuracy may

Parts may have external and internalParts may have external and internal

profilesprofiles

Special surfaces or textures may beSpecial surfaces or textures may be

needed.needed.


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DisadvantagesDisadvantages

Removal processes waste material.Removal processes waste material.

Require more energy and labor thanRequire more energy and labor than

forming and shaping operations.forming and shaping operations.

Removing a volume of material isRemoving a volume of material is

time consuming.time consuming.


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Variables in the Cutting ProcessVariables in the Cutting Process

The quality resulted depends on:The quality resulted depends on:-- Tool materialTool material

Tool shape, surface finish, and sharpness.Tool shape, surface finish, and sharpness. Work piece material, condition, and temperature.Work piece material, condition, and temperature.

Cutting conditions (such as speed, feed and depth ofCutting conditions (such as speed, feed and depth ofcut).cut).

Type of the cutting fluid.Type of the cutting fluid. Machine tool characteristics (such as stiffness andMachine tool characteristics (such as stiffness and

damping).damping).


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Cutting ToolsCutting Tools

A singleA single--point tool cuts parts using only onepoint tool cuts parts using only one

edge of the tool.edge of the tool.

AmultiAmulti--point cutting tool cuts parts usingpoint cutting tool cuts parts usingtwo or more edges of the tool.two or more edges of the tool.


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Cutting AnglesCutting Angles


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ChipsChips

Metal cutting is essentially a chip formationMetal cutting is essentially a chip formation

process.process.

Metal chips are the unwanted bits of materialMetal chips are the unwanted bits of materialthat are removed from the workpiece duringthat are removed from the workpiece during

the cutting processthe cutting process ..


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Chip FormationChip Formation


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The point contacting the metal makes the cut.The point contacting the metal makes the cut.

Depending on the operation and the machine, the workpiece may beDepending on the operation and the machine, the workpiece may bemoving against the cutting point or vice versa.moving against the cutting point or vice versa.

As the workpiece is moved past the cutting tool, the workpiece metal isAs the workpiece is moved past the cutting tool, the workpiece metal isforced along at great pressure.forced along at great pressure.

Just ahead of itself, the cutting tool deforms a bit of the metal,Just ahead of itself, the cutting tool deforms a bit of the metal, This elongates its crystalline structure, somewhat like the stretching of aThis elongates its crystalline structure, somewhat like the stretching of a

plastic bag.plastic bag.

A boundary line between the undeformed metal and the deformed metalA boundary line between the undeformed metal and the deformed metalemerges.emerges.

At this boundary line, the cutting action takes place.At this boundary line, the cutting action takes place.

The force of cutting deforms metal and introduces shear forces at theThe force of cutting deforms metal and introduces shear forces at theboundary. The shearing creates a chip. Eventually, the tool separates theboundary. The shearing creates a chip. Eventually, the tool separates thechip from the partchip from the part ..


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Chip TypesChip Types

There are three common types of metal chipsThere are three common types of metal chips

created by cutting tools:created by cutting tools:

continuous chips,continuous chips,

discontinuous chips,discontinuous chips,

continuous chips with a builtcontinuous chips with a built--up edgeup edge..


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Continuous chipsContinuous chips

This type is generally formed when machiningThis type is generally formed when machining

ductile metals.ductile metals.

Ductile metals include mild steel, copper, andDuctile metals include mild steel, copper, andaluminum.aluminum.

The resulting surface finish is generally good,The resulting surface finish is generally good,


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Discontinuous chipsDiscontinuous chips

Discontinuous chips consist of distinct chip segmentsDiscontinuous chips consist of distinct chip segmentsthat come off the workpiece in small chunks orthat come off the workpiece in small chunks orparticles.particles.

Discontinuous chips often form with hard or brittleDiscontinuous chips often form with hard or brittleworkpiece material that cannotworkpiece material that cannot

withstand the highwithstand the highshear stresses involved.shear stresses involved.

These continuous chips are long, tightly curled stringsThese continuous chips are long, tightly curled stringsof cutting, such as cast iron or hard bronze.of cutting, such as cast iron or hard bronze.

Since the cutting edge generally smoothes over theSince the cutting edge generally smoothes over thesurface after thesurface after the chips break off, a reasonably goodchips break off, a reasonably goodsurface finish remains after cuttingsurface finish remains after cutting..


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BuiltBuilt--up Edgeup Edge

Continuous chip with a builtContinuous chip with a built--up edge (BUE).up edge (BUE).

The builtThe built--up edge often forms when cuttingup edge often forms when cutting

soft, ductile metalssoft, ductile metals

Small workpiece particles adhere to the toolSmall workpiece particles adhere to the tool

due to high pressure and heat.due to high pressure and heat.

By increasing speeds and cutting angle, youBy increasing speeds and cutting angle, you

can reduce the occurrence of the builtcan reduce the occurrence of the built--up edge.up edge.


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Built Up EdgeBuilt Up Edge


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Cutting Tool MaterialsCutting Tool Materials

Hardness (particularly at elevatedHardness (particularly at elevated

temperatures).temperatures).

Toughness, so that impact forces will not chipToughness, so that impact forces will not chipthe tool.the tool.

Wear resistance for a long tool life.Wear resistance for a long tool life.


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Chip typesChip types


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Cutting Tool Material TypesCutting Tool Material Types

Carbon SteelCarbon Steel

High Speed Steel (HSS)High Speed Steel (HSS)

Tungsten CarbideTungsten Carbide CeramicsCeramics

DiamondDiamond


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Carbon SteelsCarbon Steels

It has a limited tool life.It has a limited tool life.

Therefore, not suited to mass production.Therefore, not suited to mass production.

Can be formed into complex shapes for smallCan be formed into complex shapes for small

production runs.production runs. It has Low cost.It has Low cost.

It is suited to hand tools, and wood working.It is suited to hand tools, and wood working.

Carbon content is aboutCarbon content is about 00..99 toto 11..3535% with.% with.

Maximum cutting speed is aboutMaximum cutting speed is about 77 m/min.m/min. The hot hardness value is low and this is the majorThe hot hardness value is low and this is the major

factor in the tool life.factor in the tool life.


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High Speed Steel (HSS)High Speed Steel (HSS)

It consists of alloyed steel with tungsten.It consists of alloyed steel with tungsten.

When properly heat treated the tool propertiesWhen properly heat treated the tool propertieswill be improved significantly.will be improved significantly.

It can cut materials with tensile strengths up toIt can cut materials with tensile strengths up to105105..4646 Kg /Sq.mmKg /Sq.mm

It has hot hardness value much greater thanIt has hot hardness value much greater than

Carbon Steels.Carbon Steels. It is used in all type of cutters, single/multipleIt is used in all type of cutters, single/multiple

point tools, and rotary tools.point tools, and rotary tools.


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Tungsten CarbideTungsten Carbide

It is produced by sintering grains of tungsten carbideIt is produced by sintering grains of tungsten carbidein a cobalt matrix (it provides toughness).in a cobalt matrix (it provides toughness).

Other materials are often included to increaseOther materials are often included to increase

hardness, such as titanium, chrome, molybdenum, etc.hardness, such as titanium, chrome, molybdenum, etc. Compressive strength is high compared to tensileCompressive strength is high compared to tensile

strength; therefore the bits are often brazed to steelstrength; therefore the bits are often brazed to steelshanks, or used as inserts in holders.shanks, or used as inserts in holders.

Speed up toSpeed up to 300300 rpm is commonly used on mildrpm is commonly used on mildsteels.steels.

Hot hardness properties are very good.Hot hardness properties are very good.


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CeramicsCeramics

Sintered or cemented ceramic oxides, such asSintered or cemented ceramic oxides, such asaluminum oxides.aluminum oxides.

Mild steels can be cut at speeds up toMild steels can be cut at speeds up to 15001500 rpm.rpm.

These tools are the best to be used in continuousThese tools are the best to be used in continuouscutting operations.cutting operations.

There is no occurrence of welding, or built upThere is no occurrence of welding, or built upedges.edges.

Coolants are not needed to cool the work piece.Coolants are not needed to cool the work piece. Very high hot hardness properties.Very high hot hardness properties.

Often used as inserts in special holders.Often used as inserts in special holders.


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DiamondDiamond

It is very hard material with high resistance toIt is very hard material with high resistance toabrasion.abrasion.

Very good for turning and boring, producingVery good for turning and boring, producingvery good surface finish.very good surface finish.

Operations must minimize vibration to prolongOperations must minimize vibration to prolongdiamond life. Also used as diamond dust in adiamond life. Also used as diamond dust in a

metal matrix for grinding and lapping.metal matrix for grinding and lapping. For example, this is used in the finishing ofFor example, this is used in the finishing of

tungsten carbide tools.tungsten carbide tools.


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Cutting FluidsCutting Fluids


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Purposes of cutting fluidsPurposes of cutting fluids

The purposes of using cutting fluids areThe purposes of using cutting fluids are

to cool and lubrication of the tool bit and work piece thatto cool and lubrication of the tool bit and work piece that

are being machined,are being machined,

increase the life of the cutting tool,increase the life of the cutting tool, make a smoother surface finish, deter rust, and wash awaymake a smoother surface finish, deter rust, and wash away

chips.chips.

Cutting fluids can be sprayed, dripped, wiped, or floodedCutting fluids can be sprayed, dripped, wiped, or flooded

onto the point where the cutting action is taking place.onto the point where the cutting action is taking place. Generally, cutting fluids should only be used if the speed orGenerally, cutting fluids should only be used if the speed or

cutting action requires the use of cutting fluids.cutting action requires the use of cutting fluids.


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Cutting ConditionsCutting Conditions

The main cutting motions for metal cutting processesThe main cutting motions for metal cutting processes

are:are:--

Main cutting motion (R)Main cutting motion (R)

This is represented in either Linear cutting speed inThis is represented in either Linear cutting speed in

m/min or in rotational cutting speed in rpmm/min or in rotational cutting speed in rpm

Feed motion (S)Feed motion (S)

This is represented in either mm/min or in mm/rev.This is represented in either mm/min or in mm/rev. Depth of Cut (a)Depth of Cut (a)

This is represented in mmThis is represented in mm

machining theory 4

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