Z axis mill - reverse axis overshoot

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30 Oct 2023 07:29 #284103 by msurmei
I am building a mini mill and all is good, except the following.
After I have fully loaded the Z axis ( around 100kg, no counterweight ) I have noticed an overshoot at changing direction:
1) moving the axis for 1mm for several times in the same direction gives proper figures on the dial indicator (around 1mm)
2) changing  direction will give me 1.05 at first move in a consistent way, then it keeps 1mm for the following moves.
3) it does not matter if direction is up or down

It was ok before loading.
I am using open loop stepper NEMA42 22Nm (6A) and DM860A (set on 5.7A peak/4A rms), with ballscrew ( previous measurement showed 0 backlash).
If I add BACKLASH=-0.05 on the Z joint the final reading is 1mm but it goes to 1.05 and back ... kind of "negative" backlash :-)

Any suggestion?
 

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02 Nov 2023 23:04 #284393 by tommylight
Does the error occur only when Z is going down?
Almost always that is mechanical issues, but with steppers loaded to much, also happens.
Adding a counterweight is very advisable, all that weight will have to be handled by the ballscrew.
You can also try lowering the Z axis acceleration to at least half and do some tets.
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03 Nov 2023 17:34 #284487 by msurmei
I am ashamed ... it seem the gibs were loose. I have tightened them and now is ok.
Funny but I realized that only after I've cranked up the motor current and everything was vibrating.
Yes, I'm planning some counterweight setup.
Thanks for your help!
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03 Nov 2023 17:53 #284489 by tommylight
You are welcomed, always.

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