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How to select theRight Grinding Wheel

FACTORS AFFECTING GRINDINGWHEEL SELECTION

As the grinding wheel is a form of cutting tool,the abrasive, grit-size, grade and bond typeshould be correctly selected to fit the particularjob. There are 6 factors which affect the choiceof a grinding wheel specification.

1. Material to be ground and its hardness.2. Amount of stock removal and finish required.3. Whether the grinding is done wet or dry.4. Wheel speed.5. Area of grinding contact.6. Severity of the grinding operation.

MATERIAL TO BE GROUND AND ITSHARDNESS

Let us first consider the material to be ground,and its hardness which affects the choice ofabrasive, grit-size and grade.

Abrasive : Generally, Aluminium Oxideabrasives are well suited for steels and ferrousmetals, while Silicon Carbide abrasives are idealfor grinding cast iron, non-ferrous metals andnon-metallic materials.

Grit Size : A relatively fine grit-size works beston hard and brittle material. A coarser gritcapable of taking heavier cuts can be usedadvantageously on soft and ductile materials

that are readily penetratable. The figures givenbelow illustrate this.

To explain, on hard materials the increasednumber of cutting points, on the face of amoderately fine grit wheel (Fig. 1) will removestock faster than the fewer cutting pointspresented by a coarser wheel (Fig. 2). The largerabrasive grains in a coarser grit wheel can notpenetrate as deeply into the hard work-piecewithout burning it.

HARD MATERIAL HARD MATERIAL SOFT MATERIALFig. 1 Fig. 2 Fig.3

On soft ductile materials, however, the largergrains penetrate easily and provide thenecessary chip clearance to minimise wheelloading (Fig. 3) and heat generation.

Grade : The hardness of the material to beground also affects the choice of the wheelgrade or hardness. A harder grade can be usedon soft, easily penetrated materials than on hardmaterials which naturally tend to dull thewheel faster. However, the softer grade wheelreleases the dulled grains more readily, enablingthe new, sharp grains lying under it to do thework.

How to select theRight Grinding Wheel

AMOUNT OF STOCK REMOVAL AND THEFINISH REQUIRED

The second factor in selecting the correct wheelis the amount of stock to be removed and thefinish required. These affect the choice of grit-size and bond.

Grit-size : As a rule, coarser grit is selected forfast-cutting action and fine grit where a highfinish is required.

Bond : Vitrified bonded wheels are generallyused for fast-cutting action and commercialfinish. Resinoid, Rubber and Shellac bondedwheels produce the highest finish.

IS THE OPERATION WET OR DRY?

Generally for precision grinding, coolant isnecessary. However, in some cases (e.g. ToolRegrinding) the process may be dry, in which

case a softer grade wheel may be necessary.Whereas for wet grinding, a one grade harderwheel can be used as the coolant reduces theheat generated in grinding.

WHEEL SPEED

The speed at which the grinding wheel is to beoperated often dictates the type of bond. VitrifiedBonded wheels should not be used at peripheralspeeds over 33 meters per second except forspecially designed wheels.

Standard organic bonded wheels (resinoid,rubber and shellac) are used in mostapplications of over 35 meters upto 45 metersper second, and specially designed wheels forspeeds upto 80 meters per second.

The speed at which a grinding wheel revolvesis important. Too slow a speed means wastageof abrasive without much useful work achieved,whereas an excessive speed may result in a hardgrinding action and may introduce the dangerof breakage. Hence the safe operating speedmarked on the wheel or blotter, in revolutionsper minute must never be exceeded.

As a general rule, it is best to operate a grindingwheel at somewhere near the speedrecommended for a certain grinding operation.The wheel design is based on the assumptionthat approximately the recommended speedswill be employed.

Some of the more common operating speedguidelines for particular types of grinding wheelsare listed below, in surface meters per second.

A mild abrasive such as 86A, 32A or 38A is best

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FACTORS AFFECTING SELECTION OF GRINDING WHEELS

Type of Operation CommonOper. Speedin mtrs/sec

Cut-off (Rubber, Shellac and 45 � 80Resinoid)

Crankshaft Grinding 36 � 60

Cylindrical Grinding 25 � 45

Disc Grinding 20 � 45

Internal Grinding 20 � 80

Mounted Points(Speed varies with shape,mandrel diameter andoverhang) 45 � 48

Snagging � Resinoid 45 � 48

Snagging � Vitrified 28 � 33

Surface Grinding 20 � 33

Tool Grinding 25 � 33

Cutting off(Below 400 mm diameter) 48 � 80

Thread Grinding 33 � 80

Track Grinding 33 � 125

Camshaft Grinding 33 � 80

Centreless Grinding 33 � 60

Note: Higher speeds are recommended onlywhere the bearings, protection devices andmachine rigidity are adequate.

The speed table given on Page 47 will be ofassistance to you in determining therecommended revolutions per minute forvarious diameters of grinding wheels.

As a general rule, wheels are expected to beoperated at or near the recommended optimumspeed for the specific application. This is to beborne in mind when selecting the specification.If, however, the operating speed is different fromthe one recommended, then at least a changein grade would be necessary.

AREA OF GRINDING CONTACT

The area of grinding contact between the wheeland the work affects the choice of grit-size andgrade.

Grit-size : A coarser grit is required when thearea is relatively large, as in surface grinding, toprovide adequate chip clearance between theabrasive grains. As the area of grinding contactbecomes smaller and the unit pressure tendingto break down the wheel-face becomes greater,finer grit wheels should be used.

Grade : On large areas of contact, a soft gradewheel provides normal breakdown of the grit,ensuring a continuous free-cutting action.On the other hand, a harder grade is needed towithstand the increasingly higher unit pressure,as the area of contact becomes smaller.

SEVERITY OF THE GRINDING OPERATION

This affects the abrasive and grade.

Abrasive : A tougher abrasive like A isrequired for a severe grinding operation likesnagging.

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FACTORS AFFECTING SELECTION OF GRINDING WHEELS

for light grinding operations. An intermediateabrasive like 68A or 19A is used for grindingjobs of average severity.

Grade : The severity of the grinding operationalso influences the �grade�. Hard grades providedurable whels for rough grinding such assnagging, while medium and soft grade wheelsare generally used for less severe precisiongrinding operations.

CONCLUSION

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Selecting the right grinding wheel for a specificapplication, if properly understood, is notdifficult. In actual practice, where rate ofproduction as well as accuracy and surfacefinish are important, the grinding wheel must beselected with the utmost care.

Sometimes, the first selection of the wheel maynot turn out appropriate and modifications ingrade or grit-size, or both, may be necessary.In such cases, a correct specification could beestablished after conducting trials in two or threedifferent grits and arriving at the optimum grade.

The skill of the operator also plays an importantrole in the correct use of a grinding wheel. Askilled operator can make a wheel act one gradesofter or harder by varying the other parameterslike work speed, traverse, feed, etc.

FACTORS AFFECTING SELECTION OF GRINDING WHEELS