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When the teeth of wheels are to be cut in a gear-cutting machine the accurate spacing of the teeth is determined by the index plate and gearing of the machine itself; but when the teeth are to be cast upon the wheel and a pattern is to be made, wherefrom to cast the wheel the points of division denoting the thickness of the teeth and the width of the spaces are usually marked by hand.

A face view of index plate i is shown in Fig.

An index plate secured to the outer end of the bush, and an adjustable arm and index pin attached to the projecting end of the worm, complete the dividing mechanism.

The proper change gears to use for cutting a given number of threads to the inch is ordinarily determined by referring to a table or “index plate” I which shows what the size of gears a and c should be, or the number of teeth each should have, for cutting any given number of threads per inch.

Calculating Change Gears for Thread Cutting.—As previously mentioned, the change gears for cutting threads of various pitches are shown by a table or “index plate” attached to the lathe.