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OA47

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Joined: 11/04/2012
Location: Australia
Posts: 981

Posted: 04:14am 21 Apr 2023

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I built a benchtop CNC machine and recently after upgrading the motor to an 800W air cooled unit (I wanted software speed control as well) I also upgraded the Z axis bearings to linear units. The problem is that I now have slippage of the Z axis. As far as I am aware the stepper units are locked electrically during other operations but gravity takes effect when I am lowering the tool.

Is there a mechanical or electrical fix to this problem?

0A47

phil99

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Joined: 11/02/2018
Location: Australia
Posts: 2407

Posted: 08:02am 21 Apr 2023

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One possible mechanical solution could be a counterweight for the motor. It would be a bit clumsy though. The middle of the counterweight arm would require a support pillar attached to the Z-axis support beam (X or Y axis) and a linkage from the end of the arm down to the spindle motor body.

One possible electrical solution could be increasing the width of the stepper pulses. The increased overlap between phases might reduce the tendency to overshoot after a step. Combined with slowing down the decent rate to reduce momentum might be just enough.

Edit
A electromechanical solution might be a counter-force motor. A small DC motor with a bobbin on the shaft, attached to the Z-axis support. Thread from the bobbin is attached to the to the spindle motor body. A resistor in series with the motor limits its torque to just counteract the mass of the spindle motor.

Edit 2
Or use a bigger Z-axis stepper motor. Hopefully it will have greater holding torque.

Another mechanical method would be reduction gears between the Z-axis and it's stepper motor. This would reduce the load on the motor at the expense of more steps per mm.
Edited 2023-04-22 09:08 by phil99

OA47

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Posted: 01:46am 23 Apr 2023

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Thanks Phil for the suggestions.
I will try the simpler ones first. Changing the width of the stepper pulses wiil not be a problem and I do have a bigger stepper motor on hand but that will require a bit of wiring change.

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OA47

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Posted: 02:31am 23 Apr 2023

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Here's another issue. I thought that if I write a piece of G-code to raise and lower the X-axis 80mm, 100 times at 1200 feed rate to show the issue I could then determine that I have fixed it or not. Problem is after 100 raises and lowers there is no error showing in the z-axis height.

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Bryan1

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Joined: 22/02/2006
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Posted: 09:11am 06 May 2023

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Before we start to solve this problem we do need to know the diameter of the ball screw thread and the weight of the 800 watt unit along with the stepper motor used.

It does sound like the stepper is too small and the forward weight of the unit is over powering the holding torque of the stepper.

But mate a picture says a 1,000 words so put one up so we can have a look and we can go from there.

Cheers Bryan

OA47

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Joined: 11/04/2012
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Posts: 981

Posted: 02:02am 09 May 2023

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Bryan, I have tracked down the issue and implemented some workarounds. The Z axis stepper does seem to be locking during process as it should but the issue is apparent when jogging the z-axis small increments upward. On close inspection the spindle does slide back down sometimes particularly closer to Z0. This situation occurs as the stepper is not instructed to hold when jogging and the issue is exasperated as the software indicates that the spindle has risen the required amount.
I have run several programs with this in mind and have not had an incorrect cut depth. I will investigate further a mechanical counterweight to fully overcome this issue.

Regards
0A47
Edited 2023-05-10 08:00 by OA47

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