Had a job to do on the mill and part of it was doing some scribing. Started off ok, going about .005" deep with a single flute scribe. It was a pretty slow process since I can only run at 1600 rpm for now. There was, I think, 8 different parts to scribe with 6 different scribe areas per part. So the mill head was up and down while moving through the parts all in a row.
I have no way to measure the depth of each scribe area, but they got progressively deeper as the program wore on toward the end. This was in .031 O1 steel, not heat treated. I hazard a guess that the better half of it was .020" deep. But my dro showed a constant Z-.005". Parts are probably scrap, but the fellow they belong to hasn't seen them yet.
I suspected something going on with the head, but didn't really see anything of notice during the run of that program. I had to then flip the parts and scribe them on the opposite side. Same thing, started out good, then got deep as the mill moved through the program. Since they were now individual parts at that point I was able to keep reseting Z to account for whatever was happening. I was suspecting cutter slip in my ER32 collet at that point.
I decided to abandon the single flute scribe at one point and go with my spring loaded scribe to finish them up. So I got through them and may have to get some more material and do them over. Today, on another project, just finishing one cut to bring it within tolerance and Z starts to get wacky again. This time it's in a pretty good tool holder so I'm not thinking tool slippage. I started watching the pulley and belt and noticed it cogging down. Maybe a thou or two at a time. Again with no indication from the dro that the Z was moving. It read .0000" the whole time. I decided I could live with some reduction in the Z value, so I kept going to see where it would end. Dropped a total of .030".
Last night I was changing parts after a program ended, or was reaching for my wrench to do that. I turned back to the mill to find the head slowly lowering itself. I was using a spoil board to hold my parts, so the cutter had a semi soft place to stick and it went about 1/2" into it. Then the drive faulted. It was just like the power had been shut off to the motor and was free wheeling down the screw. I reset the drive and things were good again. Never happened again, but it shouldn't have happened even once.
I'm thinking two separate issues here. I have problems trouble shooting things unless I can duplicate them and test different fixes. I'm sure the creeping is fixable, once I find the cause. Not sure I can duplicate the other issue though. So, going through the creeping Z in my head trying to think of the possible causes. I'm not really coming up with anything concrete to even look for right now. I did try to retune the drive the night before last and was getting some strange oscillations from the motor. I never felt I had it tuned as good as I had in the past, but going beyond were it is now just seemed to make the thing skittish. Could the gain and damp of the drive cause the head to creep down? And to think, I was getting ready to start milling on my belt drive conversion. That's now on hold until this thing is fixed and thoroughly tested.
Bob