My X2 minimill has been running like a champ for a couple of years cutting 1000s of parts for my fly fishing reel kits. A few weeks ago, I decided to to do some upgrades and repack the ball nuts with oversize bearings to remove some backlash. I had enabled backlash comp last year (.002" on X and .003" on Y) to help a bit.
Long story short, after the upgrades and bearing repack I had a problem where the Y axis would slowly skew over the course of a run. I tried several programs that I had used 100s of times and they all failed to varying degrees. I checked all the mechanicals, backlash, make sure backlash comp had been disabled, etc. For several weeks I have been out of commission. I can not hear missed steps when running. I slowed my rapids down from my normal 125IPM to 75IPM and accel from 25 to 5. Still no change. I even ordered and installed a much bigger stepper on Y and the problem persisted.
Today, I went back to basics. I first wrote a little gcode program to exercise the mill. Here it is:
M98 P01 L50 (call subroutine 1)
G0X0Y0
G4P4
M98 P02 L50 (call subroutine 2)
G0X0Y0
G4P4
M98 P03 L50 (call subroutine 3)
G0X0Y0
G4P4
M98 P04 L50 (call subroutine 4)
G0X0Y0
M30
O01 (subroutine "01")
(simple short distance movement on Y)
G0Y0
G0Y.1
M99 (return)
O02 (subroutine "02)
(small rectangle on X & Y)
G0Y0
G0X.1
G0Y-.2
G0X0
M99 (return)
O03 (subroutine "03")
(diagonal movement on X & Y)
G0Y.1X.2
G0x0y0
M99 (return)
O04 (subroutine "04")
(small triangle on X &Y)
G0Y.1
G0X.2
G0X0Y0
M99 (return)
%
When I run this, only subroutine 4 introduces the skew along Y. The others do not. In 50 calls, the Y skews about 1/8" (eyeballed). I then switched my X & Y stepper cables to see what I might learn. Now the skew shifts to the X axis and in the same magnitude. If I run 100 cycles, the skew is about 1/4", 150, 3/8" etc - so its cumulative.
Thinking about the amount of skew: my Y "steps per" are roughly 10,000. So, to skew 1/8" would require 1250 (missed) steps, which would be 25 (missed) steps per call to the triangle path! I do not hear anything indicating missed steps. And, the diagonal and back-forth movements on Y do not result in this skew. I re-ran at 7.5 IPM and the same amount of skew occurs.
Then I realized that my previous .003" backlash comp on Y * 50 cycles = .150" - in the ball park of the eyeballed 1/8" skew. I double checked backlash comp and although the checkbox was unchecked to disable it, the X and Y values were present. So I zeroed them and restarted Mach (actually rebooted the PC too). The skew still occurs.
Then I remembered that I have a saved profile from last year before I enabled backlash comp. I loaded it and ran. No skew! It ran flawlessly. I even ran it 10000 cycles and saw no skew! I then restarted Mach and loaded my current profile and again the skew appeared. Restart with the old profile, no skew.
I suppose the easiest thing will be to rebuild a new profile from scratch but this is nagging at me! Anyone have any ideas or configuration parameters that I can check?
cheers,
Michael