A vertical machining center with Fanuc 6M CNC control system, ALM403 and ALM441 alarm shows on CRT display during AUTO machining.
According to the alarm codes, the 4th axis driver is not ready, the OVC light of the 4th axis speed control unit is on, it means the speed control unit is overloaded.
Turn the rotary table and it can be loosed as normal, try to run it without workpiece and it is fine, so the rotary table has no problem.
Then check carefully and find there are hydraulic clamp devices on both tailstock and rotary table side of the A axis, to rotate the A axis need to release these two simultaneously. The hydraulic clamp device of the tailstock could not be loosed properly caused the alarms, fix it then the machine back to normal. | 
