hello, i have a mazak 8g cnc lathe, and i cant get the z axis to move, when it is powered up it comes on as normal with no alarms, when put into a handle mode you can turn the wheel and the numbers on the position screem move as normal but the table(gang tool lathe) wont move, i replaced the limit switch and it does work, but if it jogs at any speed it will alarm out with a z axis servo overload. i called mazak and they dont really know what to say. this machine is late 80's. anybody have suggestions

What limit switch did you replace? Did you test for binding in the Z?
With the power off, how easy is it to turn the axis by hand?
Al.

Is the Z-axis a vertical axis?? If it is it could be the brake not pulling in. This is often a common problem on vertical axises when theres a servo load issue. Also here is my website link that could help you out troubleshooting a bind or servo alarm. Machine Tool Help (http://www.machinetoolhelp.com/Troubleshooting/Servo%20%26%20Drive%20alarms%20Tr.html)

the z axis is free, this machine ran parts until a week ago, but i can turn the ballscrew when the power is on and it is free, this is a horizontal lathe also
thanks

That vintage a machine, I would suspect it has DC motors? Did you check the brushes and also inspect the commutator. It is normal to see the display run even when the motor is not turning for some reason, and then it will reach a following error and then display fault message.
If it was a mill, I would say maybe the Z axis counterbalance is not working but on a lathe Z is usually a Horizontal axis.
Al.

Sorry didn't see it was a lathe in your first post. I think Al may have hit the nail on the head.

I'm going to vote for a blown driver board, if you can turn the screw with the power on, then there isn't any juice going to the motor. The machine usually doesn't get mad until you move the axis and it doesn't get any feedback. If you can switch the X and Z board, that should tell you pretty quick. On some machines you just need to move over the outputs, if they will reach. Also I would check all the breakers and fuses before investing any money in new parts. One to watch out for though I doubt it is common is on an old bandit controller, once in a while my Y axis would run away, then it was fine, turns out that where the filament was spot welded to the end of the fuse had broken, so at times it would be in the right position and work and some vibrations would throw it off and the Y would go for a random walk.