20T pulley SKR Pro settings?

LowRider CNC Advice
1

Hello,

It seems that I never get the exact dimensions in my cut, as defined in Estlcam.

Reading in on the forum I suspect part of my problem might be caused by the 20T pulleys. I got these by sourcing them locally, for some reason 16T was difficult to get.

Now I’ve read that I could change the steps per mm down from 200 to 160.
Since I am not yet ready to mess with the FW I want to set it manually to eeprom. But I can not seem to find where to modify this as I don’t have any setting on 200 steps.

Does anyone have a lead for me?

And while we’re at it, do I need to modify anything for a 8x8 leadscrew?
Thanks.

2

SKR Pro will have X/Y steps/mm set to 100. Change to 80 for 20T pulleys.

As in a 4 start 2mm pitch 8mm lead screw? Those are what’s already expected for Z (400 steps/mm)

All of this assumes a 1.8° (200 steps/rotation) stepper motor. If you have 0.9° motors (400 steps/rotation) then the numbers need to double.

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3

Thanks Dan 100 seems like a number I can recall, going to check this ASAP.

I’ve got the recommended 1.8 stepper, so that’s good.
However for the leadscrew I only got this information:

Technical specification:
Diameter: 8mm
Thread pitch: 8mm / revolution
Screw length: 400mm

4

200 steps/revolution / 8mm/revolution * 16X microstepping = 400 microsteps/mm

5

Do you have any link, explaining the logic behind this?
Just so I can totally understand it?

6

Other than reprap.org?

It’s not too difficult. Mostly you look at the units.

steps/revolution divided by mm/revolution. Revolutions cancel each other out and you end up with steps/mm.

(Mathematical principle is that division is multiply by the inverse. Therefore:

  • steps/revolution / mm/revolution =
  • steps/revolution * revolution/mm =
  • steps/mm * revolution/revolution =
  • steps/mm * 1 =
  • steps/mm)

Now because steps are pulsed as microsteps, se multiply by the microstepping factor, which for the SKR Pro firmware is 16.

This works for leadscrews (8mm/revolution pitch) or pulleys (16T * 2mm for GT2 belts = 32mm/revolution, or 20T * 2mm =40mm/revolution)

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7

Thanks, something to read during the holidays :smiley:

8

The command to change them is M92 X80 Y80. That changes it until you reset. If you want them to be saved to eeprom for the next reset, you would send M500.

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9

Great, that worked perfect !
As far as I can see it runs the length it should be :slight_smile:

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