[mclausen]

I have a Bridgeport Series II R2E3 Boss 8 mill which I have been cutting a large 7075 Aluminum mold. The mold has a lot of contours, and I have using cutters from 2" down to 0.04".
Everything looks beautiful on the first two plates. While completing the roughing of the last plate, the machine quit gave an axis limit notice, and would not retract the quill. I turned the machine off, and because the cutter was down in the mold cavity, I retracted the Z by removing the cover and turning the belt. The belt was very hard to turn. It did get easier, and I turned the machine on. Everything seemed to be working, except there was now about 0.010 backlash in the Z which was never there before.

I should note that I had just done a lot of roughing with a 2" cutter that chattered on plunge. (100 LBS of metal removal). Right before the machine quit I had been having to use air to clear the chips out of the cavity as it cut, and chips were flying everywhere. A chip could have got into the head.

Due to ceiling height restrictions, I am unable to remove the head of the machine, but I lifted it enough to be able to flush off the ball screw with solvent. I also replaced the lube metering units in the head. Everything now moves easily up and down, so I adjusted the belt tension and put it all back together.
There is still about .008 of backlash in Z.

Is the ball screw damaged? It happened too fast to be wear.
Could the pulley be loose on the ball screw?

Any help would be appreciated. I need to finish this mold!

[mclausen]

I might add that this machine has software backlash compensation for X and Y, but not Z.

[polaraligned]

That is a lot of backlash. It may be your ball screw needs re-balling or your ballscrew bearings are shot, or lost their preload.

[mclausen]

Yes, that is a lot of backlash. Something is wrong, but I do not think it can be just wear because it happened all at once. There was not backlash when I set the tool length offset on the previous program the day before.

I suppose I could pit a dial indicator on it and push up and down without turning the motor which would narrow it down to ballscrew or bearings and eliminate pulleys and belt.

I may need to cut a hole in the ceiling to remove the head.

[mclausen]

Well, this evening I opened up the front cover, and put a dial indicator on the ball screw arm. I chucked up a rod, and pried up and down on it with no measurable ball screw movement.
I moved the belt by hand, and there is a dead zone of belt movement without any ball screw movement. I can see what I think is the ball nut moving with the belt.

I would think this rules out thrust bearings, and there is no vertical slop in the ball screw from prying, but I still don't know what is wrong. I don't see how the ball screw could have rotary slop without also having vertical slop.

[mclausen]

I spoke with a ball screw rebuilding company, and they think from the symptoms that some balls have gotten out of track, or a return is not working. It looks like I will need to remove the ball screw assembly and send it off for repair.

[scottly]

I had the nut at the bottom of the quill come loose on my series 1 due to heavy chattering once. You might want to check that?