desired grinding time t from a thumbwheel switch or potentiometer 84 the divider 82 output proportional to the quotient of the input of the square root generator 80 and potentiometer 84 is then applied to a servo input for control of the slide 34 the output of the square root generator 80 is also applied to a summation amplifier 86 where its value is added to that of a voltage source 88 the voltage source 88 is proportional to the value a that is the distance between cycle commencment and grinding wheel contact positions 12a and 12b fig7 and 8 schematically illustrate alternate digital schemes to implement the control schemes of fig3 and 4 respectively in fig7 an analogue signal representing the grinding path length c is generated and converted to a digital signal in block 90 the output of block 90 which should also be an absolute encoder is applied to an arithmetic unit 92 a second input to the arithmetic unit 92 is applied from a digital thumbwheel switch 94 the arithmetic unit 92 provides a digital output signal proportional to the quotient of the inputs from unit 90 and thumbwheel switch 94 which is then applied to a feed mechanism either directly as a digital signal.