The grinding wheel is one component in an engineered system consisting of wheel, machine tool, work material and operational factors. Each factor affects all the others. Accordingly, the shop that wants to optimise grinding performance chooses the grinding wheel that is best suited to all other components of the process.
In a grinding wheel, the abrasive performs the same function as the teeth in a saw. But unlike a saw, which has teeth only on its edge, the grinding wheel has abrasive grains distributed throughout the wheel. Thousands of these hard, tough grains move against the workpiece to cut away tiny chips of the material. Abrasive suppliers offer a wide array of products for various grinding applications in metalworking. Choosing the wrong product can cost the shop, time and money.
Factors affecting the selection of a grinding wheel
A number of factors must be considered in order to select the best grinding wheel for the job at hand. The abrasive, grit size, grade and bond type should be correctly selected to fit a particular job. There are six factors for selection:
· Material to be ground and its hardness
· Amount of stock removal and finish required
· Whether the grinding is done wet or dry
· Wheel speed
· Area of grinding contact
· Severity of the grinding operation
First, the material to be ground and its hardness is taken into consideration, which in turn affects the choice of abrasive, grit size and grade.
Choice of abrasive
Aluminium oxide abrasives are well suited for steels and ferrous metals. Silicon carbide abrasives are ideal for grinding cast iron, non-ferrous metals and non- metallic materials.
Fine grit size works best in hard brittle material. Coarser grit capable of taking heavier cuts can be used advantageously on soft and ductile material. For example, on hard materials, the increased number of cutting points on the face of a moderately fine grit wheel will remove stock faster than the fewer cutting points presented by a coarser wheel. Besides, the larger abrasive grains in a coarser grit wheel cannot penetrate as deeply into the hard workpiece without burning it.
On soft ductile materials, however, the larger grains penetrate easily and provide the necessary chip clearance to minimise wheel loading and heat generation.
The hardness of the material to be ground also affects the choice of the wheel grade or hardness. A harder grade can be used on soft, easily penetrable materials than on hard materials that naturally tend to dull the wheel faster. However, the softer grade wheel releases the dulled grains more readily, enabling the new, sharp grains lying under it to do the work.
Amount of stock removal and finish required
The second factor in selecting the correct wheel is the amount of stock to be removed and the finish required. These affect the choice of grit size and bond.
As a rule, coarser grit is selected for fast-cutting action and fine grit where a high finish is required.
Vitrified bonded wheels are generally used for fast-cutting action and commercial finish. Resinoid, rubber and shellac-bonded wheels produce the highest finish.
Is the operation wet or dry?
Generally for precision grinding, coolant is necessary. However, in some cases (eg, tool regrinding) the process may be dry, in which case a softer grade wheel may be necessary. For wet grinding, however, a one grade harder wheel can be used as the coolant reduces the heat generated in grinding.
Grinding wheels are generally labeled with a maximum safe operating speed. Do not exceed this speed limit. The safest course is not even to mount a given wheel on any grinder fast enough to exceed this limit.
The speed at which the grinding wheel is to be operated often dictates the type of bond. Vitrified bonded wheels should not be used at peripheral speeds over 33 m/sec except for specially designed wheels.
Standard organic bonded wheels (resinoid, rubber and shellac) are used in most applications for over 35 m/sec up to 45 m/sec, and specially designed wheels for speeds up to 80 m/sec.
The speed at which a grinding wheel revolves is important. Too slow a speed means wastage of abrasive without much useful work achieved, whereas an excessive speed may result in a hard grinding action and may introduce the danger of breakage. Hence, the safe operating speed marked on the wheel or blotter, in revolutions per minute must never be exceeded.
Area of grinding contact
The area of grinding contact between the wheel and the work affects the choice of grit size and grade.
A coarser grit is required when the area is relatively large, to provide adequate chip clearance between the abrasive grains. In case the area of grinding contact becomes smaller and the unit pressure tending to break down the wheel- face becomes greater, fine grit wheels should be used.
On large areas of contact, soft grade wheel provides normal breakdown of the grit, ensuring a continuous free-cutting action. On the other hand, a harder grade is needed to withstand the increasingly higher unit pressure, as the area of contact become smaller.
Severity of the grinding wheel
The severity of the grinding wheel affects the abrasive and grade as well.
A tougher abrasive is required for grinding operation like snagging. For light grinding operations, intermediate abrasive is used for grinding jobs of average severity.
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.
Dos and donts
Grinding wheels must be handled, mounted and used with the right amount of precaution and protection.
The wheels should always be stored in order, to protect these from banging-and gouging. The storage room should not be subjected to extreme variations in temperature and humidity as these can damage the bonds in some wheels.
Immediately after unpacking, all new wheels should be closely inspected to ascertain that these have not been damaged in transit. All used wheels that are returned to the storage room should also be inspected.
Wheels should be handled carefully to avoid dropping and bumping, as it may lead to damage or cracks. Wheels should be carried to the job, not rolled. If the wheel is too heavy to be carried safely by hand, use a hand truck, wagon or forklift truck with cushioning provided to avoid damage.
Selecting the right grinding wheel for a specific application, if properly understood, is not difficult. In actual practice, where the rates of production, accuracy and surface finish are important, the grinding wheels must he selected with utmost care.
Sometimes, the first selection of the wheel may not turn out to be appropriate and modifications in grade or grit size, or both, 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 skills of the operator also play an important role in the correct use of a grinding wheel. A skilled operator can make a wheel act one grade softer or harder by varying the other parameters like in-work speed, traverse, feed, etc.