Jj104 Workshop Technology Chapter7 Grinding Machine 6a340

GRINDING MACHINES PN NORAZLINA BINTI AHMAD SARAI Department of mechanical Politeknik ungku omar

INTRODUCTION 

Grinding is a process of metal cutting,carried out with a rotating grinding wheel made up of abrasive grains for removing very fine quantities of material from the workpiece surface.

It is used to remove material from a workpiece in an abrasive action Each grain acts as a cutting tool It is only self-sharpening cutting tool

CLASSIFICATION OF GRINDING MACHINES 1)Rough or Non Precision grinding machines  These machines are used for removing large amount of metal from workpiece. Surface finish is not important. This is used mainly for removing the projections such as projection of forgings and for sharpening the hand tools. a)Floor stand grinder b)Bench grinder

2)Precision grinding machines  Precision grinders are used for accurate (machining) finishing of parts to closer tolerances. Very little amount of material is removed while grinding. Very good surface finish is obtained  This types of grinding can be done on heat treated parts for getting required surface finish and dimensions. a)Surface grinder b)Cylindrical grinder c)Internal grinder

1)Rough or Non Precision grinding machines a)Floor Stand Grinder

b)Bench Grinder

2)Precision grinding machines a)Surface grinder Horizontal grinding

Vertical grinding

•Centered grinding

Centerless grinding

Centerless grinding

GRINDING WHEELS 

Grinding wheels are manufactured by mixing the required grain size of the abrasive particle with the required bond and pressed into shape.

Grinding wheel specifications a)Abrasive b)Size of particles c)Wheel grade d)Wheel structure e)Bonding materials

Abrasive 

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These are hard materials with sufficient toughness,which acts as cutting edges for a sufficiently long time.Small particles are known as abrasive grains.These grains will fracture into smaller pieces when the force increases,which is termed as friability. This is the property which gives the abrasive the necessary self sharpening capability. Two types of abravises 1)Natural abrasives 2)Artificial abrasives

Types of Abrasives

Abrasives

Natural abrasives

Sand stone

Corundum

Artificial abrasives

Emery

Aluminium Oxide

Silicon Carbide

Cubic Boron Nitride

Diamond

Bonding Materials 

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The Bond is an adhesive substance and keeps the abrasive grains together under the action of grinding forces. Types of bonds Vitrified bond V

Bonds

Silicate bond

S

Synthetic bond

B

Rubber bond

R

Shellac bond

E

Oxychloride bond

O

Grain Size 

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The abrasive used in the grinding wheel are small,the size of an abrasive grain is generally called a grit, and is identified by a number,which is based on the sieve size used. The grain size vary from very coarse size of 6 to 8 to a super fine size of 500 to 600.The larger the grain size,the smaller the abrasive particle size. Coarse grain wheels,removes larger material and hence used for rough grinding.Fine grain wheels remove less material and hence used for finish grinding.

Grade 

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The grade is also called the hardness of the wheel.This is the force the grains are held together. The grade is denoted by Alphabets Very soft

Soft

Medium

A BCDE

FGH

IJKLMNOP

Hard QRSTUV

Very hard WXYZ

Structure 

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The Structure of the grinding wheel refers to the microscopic spaces between the abrasive grains and the bond. If the spacing of the grains are close to each other, then the wheel is a densely structured wheel. If the structure of the spacing is relatively wide, then the wheel has an open structure. Open structure wheels have a greater ability to remove material than dense wheels.

Wheel shapes and sizes 

Grinding wheels are manufactured in difference shapes and sizes.This is necessary for using them with difference types of machines and for different types of grinding applications.

Grinding Wheel Standard Marking System

Factors to the considered in selecting a grinding wheel 1)Material to be ground 2)The amount of material to be removed 3)The area of of the workpiece with the wheel 4)The type of grinding machine and its condition 5)Wheel speed and the work speed 6)Types of bond used

Safety procedures 

MOUNTING THE WHEEL

1.Select correct wheel for your operation. "Ring" wheel and inspect for cracks. Never use cracked wheel. 2.Never exceed maximum safe speed established for wheel. Be sure machine speed is not excessive. 3.Never alter hole in wheel or force wheel on spindle. 4.Use clean, recessed, matching flanges at least 1/3 wheel diameter. 5.Use one clean, smooth blotter on each side of wheel under each flange. 6.Tighten nut only enough to hold wheel firmly.

7.Adjust wheel guard and put on safety glasses before starting wheel.