A CNC milling machine with SIEMENS 802S CNC controller and Stepper motors, move  axes under JOG mode, the coordinate on the controller screen are moving, but the machine no response, no alarms on the CNC controller.

The Z axis stepper motor is heating and exciting, disconnect the stepper motor and the ball screw, then run Z axis again, the motor runs. So the problem should be from mechanical parts.

Finally we checked the guard rail stuck which caused the Z axis could not moved and the stepper motor lose its step.

Fix the guard rail the machine resumed.