to close its contacts by the operation of a limit switch 44 at a suitable time in the cycle the digital setting from a standard length digital switch 45 to produce a difference reading 34 d 34 at a read out 46 the magnitude of the signal emitted from the read out 46 is divided in two by the element 47 this signal is fed through a line 48 to a line 49 which receives a signal from a standard feed digital switch 51 the two signals on the lines 48 and 49 are combined in the element 52 and serve in the conventional way to control the number of pulses being fed to the feed motor 19 the operation of the present invention will now be readily understood in view of the above description particularly in connection with fig6 11 with the grinding machine 10 operating in the conventional way the workhead motor 18 operates through the workhead 12 and the platen 39 to rotate the workpiece 26 about the axis of the cylindrical surface which is to be generated referring to fig6 the workpiece 26 is shown as the outer race of a roller bearing as such it has a first end radial surface 53 which is engaged by the platen 39 and a right hand end surface 54 which is engaged on occasion by the gage 21 the workpiece 26 has an outer cylindrical surface 55 which rests in the shoes 41 the workpiece also has an inner groove 56 whose surface is to be finished this groove having a cylindrical bottom surface 57 the left hand radial end surface 58 and a right hand radial end surface 59 finally the workpiece has a lip 61 defined by the two radial surfaces 53 and.