Abstract: A device for indicating the touching of a grinding wheel and a workpiece. On a shaft connected to a spindle of a grinding wheel there is fixed a radially slotted or gear wheel. On the shaft there is also rotatably mounted another identical slotted wheel, which is connected by means of a spring to the shaft, and a stable supporting plate provided with magnetic scanning heads situated over the respective gear wheels. The thus mounted slotted wheels, which are made of magnetic metal, such as steel, the scanning heads, and the phase meter function to detect changes in speed of the grinding wheel. The outputs of the scanning heads are connected to a phase meter, the output of which, after being amplified, controls the in-feed speed of the grinding wheel.