A very old CNC machining center with Fanuc 6ME CNC controller, there is vibration and abnormal noise while moving the Z axis under Jog mode, and the CRT shows ALM431.

Alarm 431 means the Z axis error is too big, check DGN802(Position following error diagnose parameter) then found the error was exceed the tolerance.

As the CNC machining center is using semi-close-loop servo system, to identify the defected part we can disconnect the servo motor and the ball screw, then run the machine and the servo system is running well, so it must be mechanical parts’ problem.

Turn the Z axis mechanical parts by hand, it is not smooth and the bearings of the ball screw are heating, check carefully then found out the Z axis guide way has no lubrication any more, which caused the Z axis friction force was too big.

Fix the Z axis lubrication system the machine resume to normal.