loosing steps in one direction

[mick]

hello to all,
              i have a small 4 axis mill x@y recentley fitted with 16mm *5mm ball screws and angular contact bearings ,driven by 320oz steppers through a cnc4pc g11 bob and arcuro drivers,after cycling each axis 50 times 0 to 127 to 0 the error is about .150" dti.
no combination of acc @ vel seems to improve on this, gen/config/step pulse(5),dir (5).
drives have been swopped with other motors,tried  sherline mode  but the problem is still there.
                                       thankfull for any suggestions,    mick.

[jimpinder]

I am trying to envisage what a "small" mill is that needs 320 oz motor to drive it. You do not say what your maximum travel is, therefore I cannot work out the accuracy. There would appear to be no benefit in moving the axis 50 times, because that confuses the issue.

If it is a problem with backlash, depending on where you start, (say the centre) then loving left, moving right and back to centre should get rid of backlash and be accurate. Doing it 50 times would make no difference. So you can eliminate backlash (and do not need to compensate for it)  by doing this. DO NOT start at one end and go up and down, because you only get one set of backlash.

You seem to be saying that you inaccuracy is 0.150 inches which seems to be a fair bit if you have ball screws etc. but test for back lash  - move in one direction then the other an equal distance and see if you have any back lash. Only do it once.

If you have rid of back lash then the other problem is you travel is not correct. Test your travel (in one direction only) and see if your table moves say 1 inch at a time, using your digital calipers. You may have to adjust your number of pulses per inch on your motor configuration to get this accurate, particularly if you axis thread is metric and you are working in inches or vica versa. Having said that, if it is wrong in both directions, it cancels itself out when you do it twice, let alone fifty times.

There really is not other reason I can think of, other than then, you are missing steps driving your stepper motors, but if you are saying this 0.150 is common to all axis, it is difficult to envisage each axis missing a similar number of steps and all in one direction (because if they missed steps in both directions you could not see anything wrong.

I have a feeling it is backlash - but it seems rather a large figure to me.

[mick]

JIMPINDER thanks for replying,

motors are 220ncm ? 1.8deg, drivers set 1600 steps per rev,320 per mm
 
the backlash is all but zero.

the reason i ran so many times is, after 10 cycles from 0-127-0 mm i measured a -.0011" error and wanted to know if it was
accumulating, to me it says that steps are lost in one half of the cycle,

is this correct?

[Graham Waterworth]

Hi Mick,

if you reset your drivers to 400 steps/rev and retune mach3 to match, then run your tests again are the errors 4 times greater?

Graham.

[mick]

thanks Graham,

will try that tomorrow .
                                    mick.

[comet]

hi,
 is it 100% repeatable?
if your steps per inch were no where near correct it should still
zero on return.
  if your not missing steps that is...
  tony

[mick]

good morning Graham,
                               have tested as you suggested,and yes the error is 4 times as big.

also ran driver test:
                         cpu 1597
                         pulse per sec 24.136
                         apic  4142
                         shortest  39.5
                         longest   46.6
mach ver 2.2

        regards mick. 

[mick]

Tony,
      thanks for your input,
                                    mick.

[Ian Ralston]

It might be worth trying a simple, straight through BOB just to eliminate it as the problem. I had a CNC4PC C11 board that did not like my driver cards and had symptoms similar to yours. Ended up using the CNC4PC C11G card, which has worked perfectly. Let us know how you solve this one.

Ian

[mick]

IAN,
     thanks for the info, i have been thinking along those lines.

ordered a set of new chips for this board this morning , but by the sounds of things maybe its money wasted.

                                    regards mick.