Gear pitch error correcting system for numerical control apparatus

A gear pitch error correcting system for a numerical control apparatus for a machine tool including at least two gears provided in a transmission line transmitting a rotating force of a servomotor to a driven member, comprising:

• a nonvolatile memory storing pitch error correction data related to the respective at least two gears, the pitch error correction data for each of the respective gears including gear pitch error correction values, each of the gear pitch error correction values corresponding to a predetermined gear angle and collectively representing one rotation of a corresponding gear;
• a current position register, operatively connected to said nonvolatile memory, for storing data representing a current position of the driven member;
• pitch error computing means for reading the data stored in said current position register and superimposing the gear pitch error correction values of respective gears, corresponding to rotational angles of the respective gears, determined in dependence upon the read data representing a current position of the driven member, to obtain a superimposed pitch error correction value for the current position of the driven member;
• interpolating means for outputting an interpolation pulse for deriving a corrected position of the driven member;
• and
• an adder, operatively connected to said pitch error computing means and said interpolating means, for adding the superimposed pitch error correction value to the interpolation pulse to derive the corrected position of the driven member.