What Abrasive Does

What Abrasive Does. If a grain of carborundum is placed between two disks of glass and one disk is slid over the other, a rolling action is imparted to the" grain. As its sharp corners impact the glass surfaces, fragments are chipped out, leaving pits.

The carbo grain is itself broken down in size until effective grinding ceases. Increasing the rolling speed beyond a certain point merely increases the rate at which the grain is broken down; in fact, the chipping action on the glass is reduced.

By increasing the pressure, however, the force of impact is increased, and larger chips are broken out of the glass. When grinding a telescope mirror, if the carbo is too thickly applied, the separate grains are crowded together and are not free to roll. Instead, they are dragged en masse around the work, scratching and scoring as they go, until most of them have been pushed over the sides of the tool, and the rest worn down to a size where much of their efficiency is destroyed.

It is a needless waste to apply the abrasive thickly, and the amount of grinding obtained will be small in proportion to the effort expended. Apply it thinly, and use plenty of pressure, with emphasis on pressure. Speed of stroke is also an important factor. Too rapid a stroke retards the hollowing-out process, and will flatten the edge zones of the mirror. Not more than 60 to 80 strokes per minute should be taken, calling a stroke, in this instance, the combined back-and-forth motion.

Working in this manner, glass can be removed rapidly, and with a minimum expenditure of elbow grease and carborundum. Just enough water should be used to keep the work wet. It may be necessary to add water from time to time as the abrasive breaks down, or as evaporation takes place. In rough grinding to curve, it is a waste of time to break each charge down fine.

The purpose here is to remove glass, and when the sound of the coarse grinding
ceases, a fresh charge should be added. The accumulation of "mud," consisting of crushed abrasive, glass, and water, should be flushed from the surfaces occasionally; otherwise it may cushion the action of the carbo and slow up the grinding. This is important when working with the finer abrasives.

How the curve is obtained