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The ball screw that moves the spindle head on a large vertical machining center locked up last month so we replaced the unit, now the new ball screw has seized up too. The new ballscrew cost over $6000, could it have been a faulty part or is there another problem?
It is unlikely to have been a faulty ballscrew.
The chances are that it was not the old ballscrew at fault either. If you had not worked on the spindle unit or carriage and nothing was done to cause the problem, then it is most likely associated with the counter balance system. The spindle carriage on a large machining center can weigh a ton or more, so to allow for efficient operation, some sort of balancing system must be employed or the ball screw and servo drive will be required to lift a dead weight. Some machines utilize a mechanical balancing system, others use a hydraulic cylinder. I will repeat the standard warning - working on a counter balance system can be extremely dangerous, (really dangerous) so should only be attempted by personnel fully conversant with safety procedures.