Grinding wheels consist of abrasive particles bonded by some other substance, which may be anything from rubber to a kind of glass. The bond is meant to fail. As a particle becomes worn, the drag on it increases and friction makes it hotter, just as a plane with a dull blade requires more force than one with a sharp blade. The increased drag either fractures the particle, exposing a new cutting edge, or tears the worn particle out of the wheel, exposing fresh abrasive grains with unused cutting edges. The bonding should be such that, under the conditions in which the wheel is meant to be used, worn particles come out and unworn ones don't. The grade of a grinding wheel (“hard,” “soft”) refers to the tenacity with which the particles of grit are held, not to the hardness of the abrasive particles themselves.
Grinding wheels are marked with a specification that consists of alternating letters and numbers separated by dashes. Large wheels carry at least four and as many as six terms, but small wheels are marked only with grain size and grade – the other information is on the packaging. In order, the terms refer to:
1. Abrasive type (a letter). A is aluminum oxide; C is silicon carbide.
2. Grain size (a number). This corresponds to the grades of sandpaper.
3. Grade (a letter). On a scale from A to Z, with A being softest.
4. Structure (a number). This term is optional. Structure refers to the spacing between the grains of abrasive. The higher the number, the wider the spacing; ordinarily between 0 and 16.
5. Bond type (a letter). Describes what holds the grains together.
|E||shellac or elastic|
|B||resinoid (synthetic resins)|
6. (optional) Manufacturers may use this position for lot numbers or other private information.
Sizes typically used on home bench grinders are (first dimension is the diameter of the wheel, the second the wheel's thickness): 5″× ½″; 6″ × ¾″; 8″ × 1". The special wheels used for side grinding are thicker, e.g., 6″ × 1". Aluminum oxide wheels cannot be used to sharpen carbide bits and blades.
Generally speaking, grinding wheels should not be used with soft metals like aluminum and lead. They load up too easily and cease cutting. Use a moving abrasive belt instead.
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An excellent, comprehensive lists of do's and don't's in using grinding wheels:
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Last revised: 20 April 2015.