Chapter 4 -1STUD Abrasive Machining(GRINDING)

7/28/2019 Chapter 4 -1STUD Abrasive Machining(GRINDING)

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Manufacturing technology

Chapter 4

Abrasive Machining Processes

4/7/2013 Prof Dr Adel M. Abdelmaboud Manuf. Technology Chapt.4 Abrasive Machining Processes 1


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References

1. Manufacturing, Engineering & Technology, Fifth Edition,by Serope Kalpakjian and Steven R. Schmid.

2. Fundamentals of Modern Manufacturing , Upper Saddle

River, NJ. John Wiley & Sons, Inc. M. P. Groover,

3. NON TRADITIONAL MANUFACTURING PROCESSES, Adel M.abdelmaboud . Menoufia university I.S.B.N. 477-5800-39-0.

4. Advanced Machining Processes. H.El hofy McGraw-Hill.ISBN00714533

4/7/2013

Prof Dr Adel M. Abdelmaboud Manuf. Technology Chapt.4 Abrasive Machining Processes

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Abrasives Machining processes

Abrasives Machining Processes are a Machiningwith Small, Hard Particles with Sharp Edgesand Irregular Shapes can Remove Small

Amounts ofMaterial, Producing Tiny Chips.

Abrasive processes can produce Fine (Smooth)

Surface Finishes and Accurate DimensionalTolerances.



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Abrasive Machining and FinishingOperations

There are many situations where the processes ofmanufacturing required dimensional accuracyand/or surface finish.

Fine finishes on ball/roller bearings, pistons,

valves, gears, cams, etc. The best methods for producing such accuracy and

finishes involveAbrasive mMchining



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The Main Uses of Grindingand Abrasive Machining

1. To improve the Surface Finish of a partmanufactured by other processes.

Examples: (a)A steel injection molding die is machined by milling.

The surface finish must be improved for better plasticflow, either by manual grinding using shaped grindingtools, or by electro-grinding.

(b) The internal surface of the cylinders of a car engineare turned on a lathe. The surface is then made

smooth by grinding, followed by honing and lapping toget an extremely good, mirror-like finish.

(c) Sand-paper is used to smooth a rough cut piece ofwood.



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2. To improve the Dimensional Tolerance of apart manufactured by other processes

Examples:

(a)ball-bearings are formed into initial round shapeby a forging process; this is followed by a grindingprocess in a specially formed grinding die to getextremely good diameter control ( 15m).

(b) Knives are made from forged steel; the steel isthen hardened; finally, a grinding operation isused to give a sharp cutting edge.



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3. To remove unwanted materials of acutting process

Examples:(a)Drilling and milling often leave tiny, sharp chipsalong the outer edges of the surface created by

the tool these are called burrs.

Tapered grinding wheels are used to remove theburr(the process is called deburring).



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4. To cut hard brittle materialsExamples:

(a)Most semiconductor IC chips are made from

silicon; the starting point is a long bar of acrystal of silicon (the diameter is usually 8cm,15cm or 30cm, and length up to 200 cm).

This rod must by sliced into thin circular slices;

each slice is used to make a large number ofICs.A diamond abrasive wheel is used to cutthe rod into slices.



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Abrasive Characteristics

Characteristics of abrasive must be:1. Harderthan material being ground

2. Strong enough to withstand grinding

pressures3. Heat-resistant so that it does not becomedull at grinding temperatures

4. Friable (capable of fracturing) so when

cutting edges become dull, they will breakoff and present new sharp surfaces tomaterial being ground



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Abrasive Processes Classifications

There are THREEprinciple types of abrasive cutting toolsaccording to the degree to which abrasive grains areconstrained.

1. Bonded abrasive tools: abrasive grains are closelypacked into different shapes, the mostcommon is theabrasive wheel. Grains are held together by bondingmaterial. Abrasivemachining process that use bonded

abrasives include grinding, honing, superfinishing.

2. Coated abrasive tools: abrasive grains are glued onto aflexible cloth, paper or resinbacking. Coated abrasivesare available in sheets, rolls, endless belts. Processes

include abrasive belt grinding, abrasive wirecutting.



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Abrasive Processes Classifications

3.Free abrasives: abrasive grains are not bondedor glued. Instead, they are introducedeither inoil-based fluids (lapping, ultrasonic machining),

or in water (abrasive water jet cutting) or air(abrasive jet machining), or contained in asemisoft binder (buffing).



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Why a smooth surface?

1-Reduction in Frictiono Heat - Bearings

2-Reduction in Wearo

Bushings/Bearings

3-Appearanceo Car Body, Furniture

4-Clearanceo Disk Head

5-Sharpnesso Cutting Tools



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How do we get a smooth surface?

Remove Material1. Abrasive Machining

Flatten1. Burnishing

Fill in Voids1. Add material

2. Paint3. Finish

4. Wax



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Bonded Abrasive1-Grinding

The principal differences between the cuttingaction of grinding and of milling lies in thefollowing:

1-Grinding grits are sufficiently hard to cut fullyhardened steels of the order of 850HV.

2-The cutting angles of the grits have a randomgeometry.

3-The pitch of the grit cutting edges is muchsmaller than the pitch of milling cutter teeth.4-The size of the chips cut is very small for

grinding as compared with milling.



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1- Grinding

GRINDING WHEEL COMPOSITION

1-Abrasives Materials

2-Bonding Materials



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Abrasives Materials

A-TypesI-NaturalII-Artificial

I-Natural abrasives

They include sand stone, diamond, Corundum andemery.

II-Artificial abrasives1-Aluminium OxideAl2O3,, denoted by letterA

Most important abrasive Make up 75% of grinding wheels Used for high-tensile-strength materials



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Abrasives Materials

A-Types2-Silicon Carbide, SicDenoted by the letterS1. Suited for grinding materials that have low

tensile strength and high density2. Harder and tougher than aluminum oxide3. Color varies from green to black4. Green used mainly for grinding cemented

carbides and other hard materials5. Black used for grinding cast iron and soft

nonferrous metals (also ceramics)



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Abrasives Materials

B- Grit (Grain) SizeThe number of Meshes per inch of the screen through which

the grains of crushed abrasive are passedfor grinding. The

coarser grit will remove the stock at a faster rate and finer finish willrequire a finer grit.

The standard range of the grit size is as follows

1. Coarse 10-24

2. Medium 30-60

3. Fine 80-1804. Very fine 220-600


8-grain 24-grain 60-grain


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Factors Affecting Selectionof Grain Sizes

1. Type of finish desire

2. Type of material being ground

3. Amount of material to be removed

4. Area of contact between wheel and workpiece



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Bonding Materials

A-Types1. Bonding materials are used as binders to hold the abrasive particles

in place.

2. The bonding material determines whether the wheel is Rigid orFlexible.

3. It also determinesthe force that is required to dislodge an abrasiveparticle from the wheel which plays a major role in the cutting action.

Type of bond Symbol

1. Vitrified bond V

2. Resinoid bond B3. Shellac bond E

4. Rubber bond R

5. Silicate bond S



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Bonding Materials

B-Bond Strength (Grade)Grade indicates the strength with which the bondingmaterials holds abrasive grains in the grinding wheel.

Hard GradeWhen bond posts very strong (retain abrasive grains during

grinding operation) Soft Grade

Grains released rapidly during grinding operationThe degrees of Hardness (Grade)are specified by the use of

the letter of the alphabet as follows

1. Soft A to H2. Medium I to P3. Hard Q to Z



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Wheel Structure


It indicates the Spacing Between theAbrasive Grains

1 2 3 4 5 6 7 8. Close (dense) ........9 10 11 12 13 or more Open

Open Close

Dense structure has close grain spacingOpen structure has relatively wide spacing


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Grinding Wheel Manufacture..


Most grinding wheels used for machine shop operations aremanufactured with vitrified bonds.

Main operations in manufacture of vitrified grinding wheels are:

1. Mixing

2. Molding

3. Shaving

4. Firing (Burning)

5. Truing

6. Bushing

7. Balancing

8. Speed Testing


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MARKING SYSTEM OF GRINDING WHEELS



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Specifications of GrindingWheels Example

Xxx A 60 K 5 V xxx The letter Adenotes that the type of abrasive isAluminum

oxide. In case of silicon carbide the letter C is used. The number 60 specifies the average grit size in inch

mesh. For a very large size grit this number may be as small as 6where as for a very fine grit the designated number may be ashigh as 600.

The letter K denotes the hardness of the wheel, whichmeans the amount of force required to pull out a single bondedabrasive grit by bond fracture.

The letter symbol can range between A and Z,A denoting the

softest grade and Zdenoting the hardest one. The number 5 denotes the structure or porosity of the

wheel. This number can assume any value between 1 to 20, 1indicating high porosity and 20 indicating low porosity



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GRINDING WHEEL SHAPES

1-Straight wheel, for surface, cylindrical, internal and centerless grinding.

2-Beveled face, or tapered wheels for grinding threads, gear teeth, and the like.

3- Straight recessed wheels for cylindrical grinding and facing.

4Abrasive disks for cut off and slotting operation



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GRINDING WHEEL SHAPES



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GRINDING OPERATIONS


TWO Main Types:1-Cylindrical grinding

2-Surface grinding


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1-Cylindrical Grinding

Three Main TypesA. Traverse grinding

B. Plunge grinding

C. Centerless grinding



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A-Traverse grinding

In the transverse method, both the grinding wheel and the workpiece

rotate, and longitudinal linear feed is applied so that theentire length can be ground



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B-Plunge grinding


In the plunge grinding, grinding is achieved through the cross feedof the grinding wheel, and No Axial Feed isapplied.


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Cylindrical Grinding


Combined traverse and plunge


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C-Centerless grinding

Centerless grinding involves passing a cylindrical workpiece, which is supported

by a Rest Blade, between two wheels (i.e., the grinding wheel and theregulatedor feed wheel) .The grinding wheel does the actual grinding, whilethe regulating wheel is responsible for rotating the workpiece as well asgenerating the longitudinal feed. Centerless grinding can be classified as

Internal and external grinding.

Thee Main Types of centerless grinding.

1-Through feed

2-End feed

3-In feed



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Main Types of centerless grinding

1-Through feed 2-End feed 3-In feed



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Advantages of centerless grinding

1. Work chucking and its centering is not required.

2. The workpiece is supported rigidly during theoperation and therefore no deflection takes placeduring machining

3. Very rapid process

4. Very little skill is required of the operator.

5. A fairly wide range of components can be ground

6. Large grinding wheels can be used so that wheel.7. Excellent accuracy and fine surface finish can be

achieved wear is minimized.



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Disadvantages of Centerless Grinding

The major disadvantages are as follows:1. Special machines are required that can do no other type

of work.

2. The work must be roundno flats, such as keyways, canbe present.

3. Its use on work having more than one diameter or oncurved parts is limited.

4. In grinding tubes, there is no guarantee that the OD andInternal Diameter (ID) are concentric.



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2-Surface Grinding



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Surface Grinding Types



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Internal grinding



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Inspection of Wheels

Inspect wheels after they have been received Damage might occur during transit

Suspend and tap lightly with screwdriver handle for

small wheels or with wooden mallet for larger wheels Vitrified or silicate wheels give clear, metallic ring when sound

Organic-bonded wheels give duller ring

Cracked wheels do not produce ring


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Grinding Wheel Truing and Dressing

TruingWhen the grinding wheel is mounted to thegrinding wheel spindle, the run-out on wheeloperating surface isremoved, the wheel duringcontour grinding is trued or worn grinding wheel

is corrected. Dressing

When the sharpness of grinding wheel becomesdull because of glazing and loading, dulled grainsand chips are removed(crushed or fallen) with a

proper dressing tool to make sharp cutting edgesand simultaneously, make recesses for chips byproperly extruding to grain cutting edges. Thus,these operations are for the dressing.



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Truing of Grinding Wheel


Truing is the act of regenerating the requiredgeometry on the grinding wheel, whether thegeometry is a special form or flat profile.Truing produces the macro-geometry of the grindingwheel.Truing is also required on a new conventional wheel toensure concentricity with specific mounting system.In practice the effective macro-geometry of a grinding

wheel is of vital importance and accuracy of thefinished workpiece is directly related toeffective wheel geometry.


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Truing of Grinding Wheel



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Truing Methods



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Dressing of Grinding Wheel

Dressing is used to remove the lodged metal chips.

Dressing is the conditioning of the wheel surface whichensures that grit cutting edges are exposed from thebond and thus able to penetrate into the workpiecematerial.

Also, in dressing attempts are made to splinter theabrasive grains to make them sharp and free cutting andalso to remove any residue left by material being ground.

Dressing therefore produces micro-geometry. The

structure of micro-geometry of grinding wheel determineits cutting ability with a wheel of given composition.

Dressing can substantially influence the condition of thegrinding tool.



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Dressing Methods

4/7/2013

Prof Dr Adel M. Abdelmaboud Manuf. Technology Chapt.4 Abrasive Machining

Processes 49


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Dressing Methods



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Grinding Wheel Truing and Dressing

Grinding wheels, like other cutting tools,require frequent reconditioning of cuttingsurfaces to perform efficiently. Dressingis theterm used to describe the process of cleaning

the periphery of grinding wheels. This cleaningbreaks away dull abrasive grains andsmoothes the surface so that there are nogrooves. Truing is the term used to describe

the removal of material from the cutting faceof the wheel so that the resultant surfaceruns absolutely true to some other surfacesuch as the grinding wheel shaft.



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Difference betweentruing and dressing of grinding wheel?

Truing= process of making a wheel round

and concentric producing the form or

shape of the wheel.

Dressing= operation of removing the dull

grains and metal particles.



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Finally

Chapter 4 -1STUD Abrasive Machining(GRINDING)

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