OmniTurn G4 CNC: Size-holding Issues

Size-holding Problems

Your parts are not maintaining size; measurements are different part to part. If the size variation is wandering back and forth within a limited range, the issue is probably mechanical. If it tends to accumulate errors in one direction or the other it's probably electrical (a motor or control problem).

Using Ctrl-H to diagnose repeatability problems

Pressing Ctrl-H in Jog Mode enables display of the Encoder Count. This test will quickly determine if the problem is electrical or mechanical. It is not necessary to load any special program, or to re-home the machine.

• Run your part program until the axis gets out of position (ie the thread gets bad or the size goes out of tolerance), then go into Jog mode.
• Send the slides home (using H on the Jog menu, not 9).
• Jog both axes to a point between 0 and the first hashmark on the scale, then press Ctrl-H to bring up the encoder count display.
• Jog each axis away from home at #1 jog speed.
• When the "Position" counter passes 0.20000, the "Encoder Count Captured" number should change to 4000. A variation of ± 1 count is acceptable, but anything else indicates an encoder counting problem.

If the both axes read 4000, the problem is mechanical: loose coupler, excess backlash, entire slide loose on ways, bad spindle, tooling, etc. Go to Mechanical Tests (below) for help in determining the mechanical cause of the repeatablilty or size-holding problem.

If either axis reads other than 4000, the problem could be a bad bad motor encoder, or some trouble on the motion card. To determine which, you must load a test program that can run with the motor cables swapped.

Controls ship with a program named smaltest on the harddrive. Smaltest only moves the slide one inch from home in X & Z, so it is safe to run with tooling on the slide.

• Run five or ten cycles of smaltest, then do the Ctrl-H test again.
• Note which axis is not 4000.
• Set the E-Stop to turn off servos, and swap the motor cables at the back of the control.
• Run five or ten cycles of smaltest and do Ctrl-H test again.
• If the same axis still doesn’t read 4000, the problem is in the control and we'll have to investigate further. (Both motors count correctly on one axis but not on the other, so it's not the motor).
• If the other axis is now not reading 4000, but the first axis is now good, the problem is in the motor encoder. (One motor is good on both axes and one motor is bad on both axes).
• If the problem goes away when the motors are swapped, it was probably caused by bad connection:
  Examine the pins in the motor connector and the mating connector in the control. If any of the pins look like they're not poking out as far as the others, they must not be locked in to position. Usually this can be fixed by pulling them out with a needle-nose pliers.

Mechanical Tests

To see if it is a mechanical problem, turn servos on and position the slide close to the spindle. Mount an indicator on the tooling plate so it touches the headstock, zero out the indicator and then push and pull the slide with servos on. The indicator should show some movement as you push and pull on the slide. However it should return to zero when you let go. If the slide does not come back to zero then there is something loose, (Slide, Ballscrew nut, Ballscrew support bearing, etc).

To check repeatablilty, write a program to rapid back and forth in the axis you’re testing. Mount an indicator on the tooling plate and zero it at the program end location. Be sure to slow down the feed before the indicator hits. Run cycle repeat for 100 cycles and see if the indicator is zero at the end.

After running the program and studying the way the indicater responds, you should have an idea of what your mechanical problem could be.

If the indicator repeats, you could have a tooling issue: Maybe the part is moving, check your work holding fixture; be sure the tooling is held tightly; if you have an attachment, see if the slide might be loose on the lathe.

If the indicator doesn't repeat, email the factory and describe the type of error that you are seeing, for example: constant creeping in one direction, random movement in both directions, jumping.