A numerical control system diagnoses and maintains a numerical control machine tool by measuring backlash and downloading a backlash correction value to the numerical control machine tool. The system includes a host computer device that downloads a monitor management program to a numerical controller of the numerical control machine tool, collects numerical control information input/output between the numerical controller and a servomotor, analyzes the numerical control information, and diagnoses the numerical control machine tool.

Regarding a backlash amount measurement system provided in the servo control device of the drive motor of machine tools and industrial machines, a technical problem is to accurately measure the backlash amount at the time of inversion of a drive shaft by means of a backlash amount measurement system having a simpler configuration with a small number of detectors. The above technical problem can be solved by a backlash amount measuring unit equipped with an inversion detection unit which detects the start of inversion of the shaft, an arrival detection unit which detects that the backlash end has been reached from the start of inversion, and a time/distance measurement unit which measures the time from when the inversion detection unit detects the start of inversion until the arrival detection unit detects that the backlash end has been reached, or the movement distance.

A host computer device downloads a monitor management program to a numerical controller of an NC machine tool in step S10, collects numerical control information input/output between the numerical controller and a servomotor in step S30, analyzes numerical control information monitored in step S40, and diagnoses the NC machine tool in steps S50 to S110.