Numerical control system having synchronous control function between units

A numerical control system comprising:

• a master unit controlling a master axis of a machine tool;
• and
• a slave unit controlling a slave axis of the machine tool, the slave unit connected to the master unit by a communication path, wherein, for synchronizing movement of the machine tool on the master axis with movement of the machine tool on the slave axis, the master unit periodically transfers synchronization information including position of the machine tool on the master axis to the slave unit, and the slave unit performs the synchronous control based on the synchronization information transferred from the master unit,
• wherein the slave unit is configured to:
• acquire a reception time of the synchronization information received from the master unit, the reception time is a time interval between output of a timing signal generated in the slave unit at a predetermined period and reception of the synchronization information,
• record a history of the reception time of the synchronization information, as reception time history data,
• detect retransmission of transfer of the synchronization information,
• correct the reception time of the synchronization information with a correction amount based on the reception time history data when the slave unit detects the retransmission of the transfer of the synchronization information, the correction amount corresponding to a delay due to the retransmission of the synchronization information, and
• compute an estimated position of the machine tool on the master axis based on the synchronization information and the reception time corrected by the slave unit, and control a position of the machine tool on the slave axis based on the estimated position.