To maintain horizontal straightness accuracy of right/left movement of a main spindle head by fitting a servo control means which compares a corrected NC command signal with an actual position and controls the rotating position of a ball screw in respective cross rail drive devices to a command position.
When correction values ΔL, ΔR are given at the position (x) of a ram 10 by a load correction function, a ram position correction signal generator 27 outputs a correction signal having the number of pulses and polarity corresponding to the correction value. The correction signal is superimposed with an NC spindle command signal by an adder 28, and the corrected NC spindle command signal is inputted in a servo control circuit 25. As a result, the rotating positions of ball screws 12a, 12b are controlled at a initial designated position in an absolute coordinate on a control axis regardless of expansion/ contraction of the ball screws 12a, 12b.
YOSHIDA HIROMICHI
KUROBE MASANORI
