Guidelines for Selecting Grinding Wheels

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Selecting right grinding wheel for a specific application, if properly understood, is not difficult. Some times, the first selection of the wheel may not turn out appropriate and modifications in specification may be necessary.

Factors Affect Choice of Right Grinding Wheel

There are 6 factors which affect choice of a grinding wheel specification.

1. Material to be ground and it’s hardness
2. Required finish and amount of stock removal
3. wet or dry grinding
4. Area of contact
5. Wheel speed
6. Severity of the operation

1) Material to be ground and its hardness

The first thing to consider when selecting a grinding wheel specification is what are we grinding? What is material, and how hard it is?

By reviewing their elements, we can select the correct abrasive type, grit size, and grade. Abrasive: Generally, Aluminium oxide grains are well suited for steels and ferrous metals, while silicon carbide grains are ideal for grinding cast iron, non ferrous metals and non metallic materials.

Grit size

For easy to grind materials, we would want a coarser grit. This is because the grain can easily penetrate the material, make and remove chips. Using a coarser grit maximize the stock removal, reducing cycle time. A coarser grit can be used advantageously on  soft and ductile materials. A relatively fine grit size works best on hard and brittle materials.

Grade

The hardness of material to be ground also affects the choice of the wheel grade or hardness. A harder grade can be used on soft, easily penetrated materials than on a hard material which naturally tend to dull the wheel faster. However, the softer grade wheel releases the dulled grains move readily, enabling the new, sharp grains lying under it to do the work.

2) Required finish and amount of stock removal

The second factor in selecting the correct wheel is the amount of stock removal and the finish required. These affect the choice of grit size and bond.

Grit size

As a rule coarser grit is selected for fast cutting action and fine grit where à high finish is required.

Bond

Vitrified bonds are generally used for fast cutting action and commercial finish. Resinoid, rubber and Shellac bonded wheels produce the highest finish.

3) Wet or dry grinding

The third factor we consider is the coolant use. If coolant used:- vitrified bonded wheels will act the softer because the lubricity of coolant helps to reduce the friction/energy in grinding zone, which can help keep the grain from dulling.

This keep it sharper and freer cutting. Generally for precision grinding, coolant is necessary. Flow over, in some case(eg tool regrinding) the process may be dry in which case softer grade wheel may be necessary.

4) Area of contact

The area of grinding contact between wheel and work affects the choice of grit size and grade.

Grit size

A coarser grit is required when the area is relatively large, as in surface grinding, to provide adequate chip clearance between abrasive grains. As the area of contact becomes smaller and the unit pressure tending to break down the wheel-face becomes greater, finer grit wheels should be used.

Grade

On large area of contact, a soft grade wheel provides normal breakdown of the grit, ensuring a continuous free cutting action. On the other hand, a harder grade is required to withstand the increasingly higher unit pressure, as the area of contact becomes smaller.

5) Wheel speed

Wheel speed determines what bond type is most suited for the required speed or if a special high speed bond might be required.

As a general rule:- Vitrified bonded wheels should not be used at peripheral speeds over 33 meters per second except for specially designed wheels.- Standard organic bonded wheels ( resinoid, rubber and Shellac) are used in most applications of over 35 meters per second up to 45 meters per second, and especially designed wheels for speed up to 80 meters per second. The speed at which a  grinding wheel operates is important.

Wheels will act differently based on their speed. An excessive speed may result in a hard grinding action and may introduce the danger of breakage. Hence safe operating speed marked on wheel or blotter, in revolutions per minute must not be exceeded.

Bonus Content : Grinding Wheel Ring Test Procedure

6) Severity of the operation

Here we consider how much or how heavy the grinding pressure will be in grinding zone. The higher the grinding pressure or force per grain, the more severe the operation. This factor affects the abrasive and grain.

Abrasive

A tougher abrasive like BA is required for severe grinding operation like snagging, because this type of grain can tolerate the large amount of pressure generated during the operation. Tough abrasives are better able to withstand the pressure and not break down too soon.

Severity of operation also helps  determine the grit size. For operations that are more severe, we want to use a coarser grit so that the grain will hold up to the grinding pressure. For light grinding operations, an intermediate abrasive like MA is used for grinding jobs of average severity.

Bonus Content : Why Use Vitrified Bond for Grinding Wheels

Grade

The severity of the grinding operation also influences the ‘grade’. Hard grades provide durable wheels for rough grinding such as snagging, while medium and soft grade wheels are generally used for less severe precision grinding operations.

Selecting right grinding wheel for a specific application, if properly understood, is not difficult. Some times, the first selection of the wheel may not turn out appropriate and modifications in specification may be necessary.

In such cases a correct specification could be established after conducting trials in two or three different grits and arriving at the optimum grade.

The skill of operator also plays an important role in the correct use of grinding wheel. A skill operator can make a wheel act slightly softer or harder by varying the other parameters like work speed, traverse, feed, etc.