Apparatus for compensating position errors of spindle head and machine tool provided with same

A machine tool controlled by a numerical control unit comprising:

• a column;
• a spindle head vertically movable while being guided by a guide on the column;
• a ram carried by the spindle head so as to be horizontally movable while being guided by a guide on the spindle head;
• a pair of ball screw mechanism for driving vertical feed motion of the spindle head, including first ball screw feed mechanisms disposed on the front side of the ram and second ball screw feed mechanisms disposed on the rear side of the ram;
• a Z-axis ball screw mechanism for driving horizontal feed motion of the ram;
• a Z-axis servomotor for driving the Z-axis ball screw mechanism;
• first Y-axis servomotor for driving the first ball screw feed mechanism;
• second Y-axis servomotor for driving the second ball screw feed mechanism;
• a compensator for compensating a vertical deviation of the ram resulting from the fluctuation of the center of gravity of the spindle head caused by the feed motion of the ram, wherein said compensator comprises:
• a Y-axis control means for controlling the first and second Y-axis servomotors on the basis of spindle head fee instructions provided from the numerical control unit;
• a Z-axis control means for controlling the Z-axis servomotors on the basis of a ram feed instruction provided from the numerical control unit;
• correction data storage means for storing correction data determining the relationship of a feed distance of the ram to a compensation value for correcting a vertical deviation of the ram;
• and
• correction means for being introduced the ram feed instruction provided from the numerical control unit, and for reading out the compensation value corresponding to the feed distance of the ram from the correction data storage means, to correct the spindle head feed instruction by adding or subtracting the compensation value to or from the spindle head feed instruction.