A wide variety of wheel-cutting engines have been made over the past three centuries, but all follow more or less the same general principle. It is generally agreed that the wheel-cutting engine evolved from manual methods of dividing a wheel in the 17th century, when revolving cutters to cut the separate wheel teeth spaces replaced hand filing, and when the blank to be cut was divided mechanically into the required number of teeth. For this purpose a dividing plate was used, on the arbor of which the blank wheel to be cut was mounted. The dividing plate was supported in a frame of various designs according to country and date, and a detent on the frame allowed the dividing plate, and thus the work, to be rotated at the chosen number of intervals required for cutting the chosen number of wheel teeth, according to the divisions of the circles on the dividing plate. As the blank was indexed round, therefore, a tooth space was cut by the rotating cutter, which was driven by an endless cord from a foot wheel or other power source. In some cases the cutter was advanced into and retracted from the edge of the blank, while in others the blank was advanced to the cutter. Early engines produced work which still required the teeth to be formed to their correct shape later with a rounding-up tool, but later machines were more advanced and had cutters which were shaped to produce the final tooth form, sometimes using three or more cutters mounted in a multiple cutting head for progressive work. Modern wheel cutting is normally done on a lathe with specially designed dividing heads, while in clock factories a cutter traverses a large number of wheel blanks mounted on a common arbor, cutting wheels in stacks.