Automatic measurement/compensation apparatus for tool diameter

An automatic measurement/compensation apparatus for the tool diameter of a machine tool which machines a workpiece by rotating a tool mounted on a main spindle, comprising a first non-contact detector provided on a fitting table of the machine tool which detects the displacement in one direction from the center of rotation of the rotating tool, a memory which stores the initial value X.sub.0 measured by said detector of a reference tool when it is mounted on the main spindle and rotated and the initial value x.sub.0 measured by said detector of an object tool before working when it is mounted and rotated, a first subtractor which calculates the thermal displacement amount AX.sub.i of the difference between the measured value X.sub.i of the object tool after the predetermined number of workpieces by said detector and the initial value X.sub.0 stored in said memory, a first register which stores the subtraction value AX.sub.i from said first subtractor, a second subtractor which calculates the variation in tool diameter of the difference between the measured value x.sub.i of the object tool after the predetermined number of workpieces detected by said detector and the initial value x.sub.0 stored in said memory, a third subtractor which calculates the tool compensation value AT.sub.i of the difference between the tool variation Ax.sub.i from said second subtractor and the thermal displacement amount AX.sub.i stored in said first register, a second register which stores the tool compensation amount AT.sub.i from said third subtractor, an accumulator which accumulates the values sequentially stored in said second register, a comparator which compares the accumulated value AS.sub.i of said accumulator with a tool life criterion .alpha. and outputs an alarm signal if the relation AS.sub.i .gtoreq..alpha. holds, and outputs a tool compensation signal, if the relation AS.sub.i <.alpha. holds, and a first adder which adds the tool compensation value AT.sub.i of said second register to the initial tool compensation value T.sub.1 for generating a tool compensation signal CS, and for applying said CS signal to an NC system.