to permit machining of a workpiece into a highly accurate tapered configuration , a wire - cutting edm including a numerical controller that reads programmed shape data is utilized . to precisely taper the workpiece , it is necessary to move the upper and lower guides of the wire so that they start and stop moving simultaneously . from the programmed shape data , the respective speeds of the upper and lower guides are computed . the upper guid is interpolated by the upper guide speed , and the lower guide is interpolated by the lower guide speed so that the guides can start and stop moving simultaneously .