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GRINDINGAlso called as abrasive machining

Material removal process, in which abrasive particles are contained in bonded grinding wheel, that operates at very high surface speed.

Material is removed in the form of fine chips, almost as dust particles by the abrasive action of the wheel.

The grinding wheel is usually in disk shaped and is precisely balanced for high rotational speeds.

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The feeds and depths of cut in grinding are small, while the cutting speed is high.

It is especially employed for grinding the cutting tools & sharpening cutting edges.

Thickness of the material chips removed by grinding ranges from 0.25 to 0.5mm in finishing & general grinding operations

Grinding may be classified as non-precision or precision, according to purpose and procedure.

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Non-precision grinding

Done primarily to remove stock that can not be taken off as conveniently by other methods.

The work is pressed hard against the wheel or vice versa.

The accuracy and surface finish are of secondary importance.

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Precision grinding Precision grinding is concerned with producing good surface finishes and accurate dimensions.

3 types of precision grinding exists

–External cylindrical grinding

–Internal cylindrical grinding

–Surface grinding

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Abrasive materials

Abrasives are the mineral material employed for sharpening, grinding and polishing operations.

They are hard substances used in various forms as tools for grinding and other surface finishing operations.

They are also able to cut materials which are too hard for other tools and give better finishes and hold closer tolerances.

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Types of abrasive materials

1. Natural abrasives: Eg: Emery, corundum, quartz, sandstone, diamond etc.

2. Artificial abrasives:

Silicon carbide abrasives Aluminous abrasives

Eg: Carborundum Eg: Alundum(Al2O3)Made from pure glass Made from bauxiteor silica & carbon

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Properties of abrasives

Hardness

Toughness

Uniformity in grain sizes

Sharpness

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Common abrasive materials & applications

1. Aluminum Oxide (Al2O3): Known as Alundum or Aloxide. Used to grind carbon steel, alloy steel, HSS,

wrought iron, annealed malleable iron etc. Because of its high MP (>2000°C), it is used as a

refractory in the linings of furnaces.

2. Corundum: Natural mineral, consists of Al2O3

Hardest natural substance after diamond Used in shaping, finishing & polishing tool edges

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3. Cubic Boron Nitride (CBN): Boron nitride with a cubic crystalline structure CBN together with diamond – Super abrasives Used for hard materials such as hardened tool steels

and aerospace alloys.

4. Emery: Natural abrasive containing Al2O3 & of iron oxide Low hardness – can’t be used in industries Used in the sharpening & polishing of tool edges.

5. Zirconia Alumina: Mixture of Zirconium oxide & Aluminium oxide Used as an abrasive in casting & foundry industries

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6. Diamond: Hardest substance, a pure form of carbon, both natural and artificial (synthetic). Used on hard materials such as ceramics, cemented carbides and glass.

7. Silicon Carbide (SiC): Known in trade as Carborundum and Crystalon. Harder than Al2O3 but not as tough. Used to grind: aluminum, brass, stainless steel, cast irons, certain brittle ceramics, surfaces of marble, stones & granite etc.

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Grinding wheelA grinding wheel is made of abrasive grains held together by a bond.

These grains cut like teeth when the wheel is revolved at high speed and is brought to bear against a work piece.

Principle dimensions that designate the size of a wheel are the outside diameter, width and hole diameter.

Two types of abrasive wheels:1.Coated abrasive wheels2.Mounted abrasive wheels

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Coated abrasive wheels

A coated abrasive is applied to the surface of a wheel uniformly & held together by a suitable bonding material.

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Mounted abrasive wheels Small bonded abrasive wheels that are mounted on mandrels of various shapes such as a cone, cylinder, etc.

They are hand guided & driven by a flexible shaft

Used in internal grinding & deburring processes

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Abrasive Wheel Shapes

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Bonding materials & Processes To get wide range of properties needed in grinding wheels, abrasive materials bonded by using organic or inorganic materials.

Important bonding processes are:

1.Vitrified bonding process

2.Silicate bonding process

3.Elastic bonding process

4.Rubber or vulcanite bonding process

5.Resinoid bonding process

6.oxychloride bonding process

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1.Vitrified process: Abrasive is mixed with feldspar & clay, heated to high temp in kilns until clay fuses & bonds the ingradietns.It can be made strong and rigid for heavy grinding and not effected by water, oil, acids. Most (80%) grinding wheels have vitrified bonds.

2. Silicate process:Abrasive is mixed with silicate of soda; mixture is subjected to low baking temp around 260°C (20-80 hours).It holds grains more loosely than a vitrified bond and give closer cut. Employed for smooth grinding processes –tool grinding requiring a sharp edge

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3. Elastic process: Abrasive is usually bonded with shellac; the mixture being baked at a low temp (around 150°C) for few hours.Very thin wheels can be produced.Employed for cutting metal tubes, wires, etc.Extremely high finish for the wheels.

4. Rubber or vulcanite process:Abrasive is mixed with pure rubber & sulphur, then heated under pressure to a temp sufficient to vulcanize the rubber.Bond is strong & tough enough to make very thin wheelsWheels are more resilient, less heat resistantEmployed for the cutting of narrow slots & grooves

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5. Resinoid processes: abrasive grains are mixed with synthetic resins (bakelite) & other compounds, heated in moulds at about 200°C. The resin sets to hold the abrasive grains.Used for purposes which require a strong, free, high speed wheel.They can remove the stock very rapidly.

6. Oxychloride process:Abrasive grains are mixed with oxide & chloride of magnesium. Mixing of bond & abrasive is performed in the same way as for vitrified process.Bond ensures cool cutting action.Employed in making wheels & wheel segments for use in disc grinding operations.

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Grinding wheel terminology

1. Grit

2. Grade

3. Wheel structure

4. Bond type

5. Abrasive

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Grinding wheel terminology

1. Grit:

Represents grain size of the abrasive grain Important parameter in determining surface finish and

material removal rate. Small grit sizes produce better finishes, larger grit sizes

permit larger material removal rates. Harder materials need smaller grain sizes to cut

effectively, while softer materials require larger grit size.

Represented as a number. Coarse grain size 10-24 & Medium grain size 30-60 Fine grain size 60-180 & Very fine is 220-600

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Grinding wheel terminology

2. Grade:

The grade of a grinding wheel is a measure of how strongly the grains are held by the bond

Designated by alphabets

A- very soft material Z – very hard grade

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3. Wheel Structure:

Typical structure of a grinding wheel contains abrasive grains, bond material and pores (air gaps)

Volumetric proportions can be expressed as Vg+Vb+Vp= 1

If Vp relatively large and Vg relatively small, it is called open structure.

If Vg relatively large and Vp relatively small, it is called dense structure.

Grinding wheel terminology

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Wheel structure (continued….)

Open structures are recommended in situations where the clearance for chips must be provided.

Dense structures are used to obtain better surface finish and dimensional control.

Hard wheels used to achive high stock removal rates and for grinding of relatively soft work materials

Soft wheels generally used for applications requiring low material removal rates and grinding of hard work materials.

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Grinding wheel terminology

4. Bond type:

Denoted by alphabets V- vitrified bond B – resinoid bond R- rubber bond E- shellac bond s- silicate bond O-oxychloride bond

5. Abrasive:

Denoted by alphabets A-aluminium oxide C-silicon carbide etc

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Abrasive Industries Association’s marking system for standard wheels

• Prefix-Abbrasive type-Abrasive grain size-Grade-Structure-Bond type-Manufacturer’s wheel type symbol

• Example: S - C - 51 – L – 7 – V -16

Prefix is:Manufacturer symbol indicating exact kind of abrasive

Abrassive type: A:Aluminum oxide C:Silicon Carbide

B: Boron Nitride D:Diamond Grain size: coarse:8-24, medium:30-60, fine:70-180,

very fine: >220

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Grade: Ranges from A-Z, where A represents soft, Z represents hard wheel grade.

Structure: Scale isnumerical. 1: very dense and 15:very open.

Bond type: B : Resinoid E : Shellac

R : Rubber S : Silicate

V : Vitrified O : Oxychloride

Manufacturer’s wheel type symbol: Manufacturer’s

private marking to idetify the wheel.

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Methods of Grinding & Types of Grinding Machines

1. Surface Grinding

2. Cylindrical Grinding

3. Centreless Grinding

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Surface Grinding Machine

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Surface Grinding Operations

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Surface Grinding Operations

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Cylindrical Grinding Machine

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Cylindrical Grinding

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Cylindrical Grinding Operations

Examples of various cylindrical grinding operations. (a) Traverse grinding, (b) plunge grinding, and (c) profile grinding.

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Centerless Grinding Machine

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Centre less Grinding Operatioin

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Centerless Grinding

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Centerless Grinding