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It requires accurate synchronization among the MCU, servo controllers and sensors in CNC systems for time stamping of data and motion control. This paper presents the design and implementation of time synchronization for CNC systems which employ switched Ethernet as its communication module. The method adopts the clock of switches as the master clock instead of that of MCU which usual is. To obtain accurate time synchronization, it uses a low-level time-stamping scheme and blocks the time synchronization frames until the actual time is equal to the time in the payload of the frames. A preemptive mechanism is introduced to make sure the time synchronization frames can be transmitted immediately when the actual time is equal to that. Finally, experimental results are shown and discussed.