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There are three basic designs for abrasive belt machines: slack-of-belt, platen and contact wheel.
Slack-of-belt is limited to low-pressure finishing of contoured or cylindrical objects. Platen machines have a flat plate backing up the abrasive belt, making these machines suitable for grinding and finishing flat surfaces. However, contact-wheel machine designs are the most common and versatile types for abrasive applications.
In fact, after selecting the proper abrasive belt, the contact wheel is the most important factor in how a belt will ultimately perform. Contact wheels serve are the back-up to the belt in most abrasive machine designs from the simple "backstand" to sophisticated automatic or robotic polishers.
The contact wheel determines how effectively the belt will cut, and what kind of finish will result. It also allows proper cooling and a more efficient abrasive grain breakdown (this factor is important on hard steels where glazing can be a problem).
There are just a few basic concepts to consider when selecting a contact wheel:
- Construction: Contact wheels for polishing use rubber-covered aluminum or fabric materials such as felt, cotton cloth, or laminated canvas plies as the surface material. Also available are pleated rubber wheels.
- Hardness: The basic rule of thumb is the softer the wheel, the softer the scratch, and the better the conformability to the workpiece. Rubber contact wheels are easily measured in "durometer" hardness values (20 being the softer end of the scale and 90 at the harder end). Cotton wheels are given hardness values related to the number of plies per inch of the face width.
- Serration: On rubber contact wheels, serrations are grooves cut into the face of the wheel. This action allows more pressure on the belt supported on the "lands" between the grooves (compared to a smooth-faced wheel). The pattern of "grooves and lands" allows better breakdown of the abrasive grains and cooler cutting with less working pressure by the machine operator. Contact wheels can be ordered with various groove-to-land ratios, groove depths and shapes, and angles of serration. A typical contact wheel for general-purpose grinding or finishing would be 14" in diameter, 50-60 durometer hardness, 3/8" to 3/8" groove-to-land ratio, 3/8" groove depth, and have a 45-degree serration angle.
General Guidelines for Contact Wheels
- Cloth contact wheels allow more conformability than rubber wheels.
- Faster speeds cause contact wheels to act harder.
- The thickness of the rubber covering plays an important part in how soft/hard the wheel acts. Worn wheels should be resurfaced or replaced.
- Hard wheels cut faster, but give coarser finishes.
- Small contact wheels are more aggressive than large-diameter contact wheels.
- Large groove-to-land ratios are more aggressive in cut and finish.
- The angle of serration affects the rate of the cut and the belt life. An angle of 45 degrees is most common. Lower angles (minimum of 7 degrees from the side) are less aggressive and high angles (toward 90 degrees) are more aggressive.