Grinding machine

A grinding machine comprising:

• (a) a first rotational drive source for rotating a grinding tool;
• (b) a second rotational drive source for rotating a workpiece to be ground by said grinding tool;
• (c) means for displacing said grinding tool and said workpiece with respect to each other;
• and
• (d) a control unit controlling said displacing means according to change of a load imposed on said first rotational drive source wherein said control unit includes a power detector connected to said first rotational drive source for detecting the load on said first rotational drive source, a comparator connected to an output terminal of said power detector for comparing an output load signal from said power detector and a normal load setting signal, and an adder for adding an output signal from said comparator and a command signal for displacing speed of said displacing means and an output terminal of said comparator is connected to a proportional-plus-derivative control circuit, a zero crossing detector and an analog switch, said zero crossing detector selectively opening and closing said analog switch in response to the output signal from said proportional-plus-derivative control circuit to said adder for controlling said second rotational drive source.