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Learn the ins and outs of centerless grinding

Anyone who earns a living at a machine shop probably has heard about centerless grinding, but it is an obscure process often only familiar in name.

The three basic types of centerless grinding are through-feed, in-feed and end-feed. In each case, the fundamental configuration of a machine is identical. The three primary components of a centerless grinder are the grinding wheel, regulating wheel and work rest, or blade.

Components of Centerless Grinding

Grinding wheels do the work as they would on other grinders. Abrasive materials are the same as with other grinding wheels, but the size and shape of centerless wheels differ from other kinds of grinding wheels. Centerless wheels most frequently are bonded with resin materials, unlike other sorts of wheels, which are vitrified. Resin bonding enhances the efficiency of centerless grinding and produces a more resilient wheel.

Like a grinding wheel, a regulating wheel is made of an abrasive material but usually is bonded with rubber or some other similar substance. As the name implies, a regulating wheel regulates the speed of a part as it is rotated against a grinding wheel. A regulating wheel may be more critical to the process than a grinding wheel because a regulating wheel controls the material removal rate, surface finish and geometry.

Like a regulating wheel, a work rest might be more important than a grinding wheel. A work rest supports a part during grinding. It is easy to see in the diagram on Page 56 how the whole process would be impossible without a work rest. It most often is made of a hard material that resists the tendency of a part to pick up material from a rest. A rest commonly is capped with a carbide strip. The geometry of a rest is crucial as uneven surfaces can allow a part to flex, resulting in chatter, poor surface finish and incorrect geometry. A work rest generally has an angled working surface, which plays a vital role in the efficiency of the process. As the angle becomes steeper, the rounding action of the process is enhanced.

These three components can be configured in several ways to allow a part to sit on the centerline of the wheels, above center or below center. Most setups place a part above center of the wheels. The angle of the work rest surface height above center and the tangency of the regulating wheel and grinding wheel to a workpiece create a unique geometric arrangement that allows the centerless process to efficiently generate round parts.

Types of Centerless Grinding

Through-feed grinding is the most popular form of centerless grinding. Through-feed grinding is performed by traversing a part from one side of the machine to the other, between the grinding wheel and regulating wheel, without stopping. Axial feed is created by dressing a regulating wheel and tipping it at an angle relative to the blade and grinding wheel, normally about 3°. This combination of factors pulls a workpiece across the work rest and between the grinding wheel and regulating wheel. Through-feed grinding is very productive. At a previous employer where we made power steering gears, we would grind 0.305 mm (0.012″) from a 31.75 mm-dia. (1.25″) steel bar that was 914 mm (36″) long. The full process took 33 seconds, and we held the diameter to a 0.0127 mm (0.0005″) tolerance and produced roundness within 0.0051 mm (0.0002″).

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