VMC 800 "Z" growing

[Lesmark]

Recently replaced Z axis thrust bearing...went to check Z travel looks good...ran a program and looped it about 30 times....checked Z and it grew .006 . Shut down and restarted and Z went back to zero...ran a loop and z grew again....????? any clues
Thanks
Mark

[KTD1]

Originally Posted by Lesmark Recently replaced Z axis thrust bearing...went to check Z travel looks good...ran a program and looped it about 30 times....checked Z and it grew .006 . Shut down and restarted and Z went back to zero...ran a loop and z grew again....????? any clues Thanks Mark

Wow, 0.006” is quite a bit. My 760 will cut about 0.003” deeper after running @ 6000 rpm for a all day.

Is the spindle getting hot? I would think you couldn’t keep your hand on it to get that much. It might “break in” after a while but that would concern me. I replaced spindle bearings in an old surface grinder that ran hot for a bout the first 2 - 3 hours. I wrote it off as wasted money on bearings but it does just fine now.

Is it possible that you are accumulating some encoder error on Z axis? I’m not sure which controller you have but I’ve seen some CNC controllers do it when using a lot of incremental moves. Especially when running inch mode with metric screws.

If your loop was rapid z moves (like peck drilling) maybe your Z screw is the culprit and it’s getting heated.

Any chance the coupling between servo and screw is slipping?

The one spot in the screw that should always remain the same is the home position. I would try measuring Z position, run program with slow spindle speed to prevent spindle heating and then measure it again. The encoder has only one home position so it should be exactly the same spot on the screw and the same measurement.

If it doesn’t measure the same I would say the problem is somewhere in the encoder, coupling, or screw.