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In various CNC systems the MCU, servo controllers and sensors all need to be accurately synchronized for time stamping of data and motion control. This paper presents a time synchronization method for CNC systems based on switched Ehternet in which message delay variation through switches must be minimized or compensated to obtain accurate time synchronization. The method uses the switch as the time reference instead of the usual using the clock of MCU. Also 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 frame can be transmitted immediately when the actual time is equal to that. Finally, experimental results are shown and discussed.