Sanity Check - Tuning LR3 SKR Pro/Marlin Acceleration and Stepper Current to overcome skipped steps

LowRider CNC Troubleshooting
1

Background info:

  • LR3 using SKR Pro v1…2/Marlin and 24V power Supply
  • V1E Dual LR firmware, lightly modified (increased bed size, swapped X and Y orientation from portrait to landscape)
  • Fairly heavy gantry (1545mm x 3mm stainless steel X rails, 9mm Alu YZ plates, 6mm Alu XZ plates, 6mm Alu struts)
  • 1435 x 2710 mm cutting area
  • Machine has previously cut 3/8" aluminum using Trochoidal at 600 mm/min and full DOC without skipping steps

Scenario:

  • Max feedrate set to 133 mm/s (8000mm/min) using M203
  • Surfacing plywood table top using 1/2" two flute flat mill. 7000 mm/min feedrate (based on mill manufacturer recommended chip load), 75% stepover, 0.5mm DOC per pass.
  • Using Linear surfacing strategy, parallel to long (Y) axis
  • V1E default acceleration, stepper current settings

Issue:

At the end of the surfacing pass, I notice that the Y2 (Rail side) motor has skipped several steps (about 50-60mm)

  • Possibly due to acceleration too high for the increased feedrate and fairly heavy gantry,
  • or possibly due to motor slipping when router is biting into the stock at a pretty high feedrate)

Proposed settings changes:

  • Decrease X/Y (P and T) acceleration (M201) and X/Y starting acceleration (M204) to 150 mm/s/s from default 180 mm/s/s
  • Increase X/Y stepper current (M906) to 1000 mA from default 900 mA (monitor motor temps to stay < 50C)
  • Save into EEPROM with M500

Good plan? Bad plan? Am I off track here?

Comments welcome.

2

Did you mean 1000mA and 900mA driver current? Amazing if it moves at all at 90mA.

In any case, sounds reasonable to me. Although if it previously worked ok, what changed? Binding somewhere? Debris? Bearings?

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LOL, yes, default is 900, thinking to bump it to 1000. OP edited.

Previously was running single flute 1/8" mill, at a top feedrate of 600 mm/min (Trochoidal Alu) and 1100 mm/min (5mm DOC MDF). Now attempting plywood with 1/2" two flute mill at 7000 mm/min. The inertia/momentum on the sudden reversals of direction at the ends of the surfacing cut may be too much, or the speed might be too much for the stepper current to cut without skipping.

4

For acceleration… on other equipment I often run things back and forth up and down the table (in free air) at high speeds at increasing accelerations until I find the stall accelerations and consistent instances of lost steps; and then back off the settings considerably for use.

Good to change and test one thing at a time if you’re interested in what the cause/fix was, rather than throwing everything at it at once. If you’re not, your proposed changes look reasonable.

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No insights on your mods, but wanted to mention that, if you happen to be running at 12V, moving to 24V will give you more torque at the kinds of feedrates you list. In addition, I assume you’ve modified the maximum feedrate (M203) so that you really are getting the feedrates you list.

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Yes, the PS is 24V (added to OP for clarity)

Yes (including M500)