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X_Twist_Compensation.md

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X Twist Compensation

This document describes the [x_twist_compensation] module.

Printers with a probe that have a probe y_offset may give inaccurate probe results due to a twist in the probe's X axis. This is common in printers with designs like the Prusa MK3, Sovol 06 etc and is further described under probe location bias. It may result in probe operations such as Bed Mesh, Screws Tilt Adjust, Z Tilt Adjust etc returning inaccurate representations of the bed.

This module uses manual measurements by the user to correct the probe's results. Note that if your axis is significantly twisted it is strongly recommended to first use mechanical means to fix it prior to applying software corrections.

Overview of compensation usage

Tip: A feeler gauge is recommended in place of paper for more accurate measurements

Tip: Perform all levelling and calibration steps at your printing bed temperature

  1. Manually tram the bed to the gantry as much as possible (eg. using paper at all bed screws)
  2. After setting up the [x_twist_compensation] module, perform X_TWIST_CALIBRATE
  • The calibration wizard will prompt you to measure the probe z-offset at a few points along the bed
  • The calibration wizard recommends a probe z_offset
  1. Load the X twist calibration using X_TWIST_PROFILE_LOAD NAME=<profile_name>
  2. Verify that the calibration is active using X_TWIST_COMPENSATE_STATUS
  3. Perform automatic/probe-based bed tramming operations, such as Screws Tilt Adjust, Z Tilt Adjust etc
  4. Home all axis, then perform a Bed Mesh if required
  5. Perform a test print, followed by any fine-tuning as desired

Tip: Disable the X twist compensation using X_TWIST_PROFILE_CLEAR and perform Screws Tilt Adjust/Z Tilt Adjust/Bed Mesh to compare the difference

[x_twist_compensation] setup and commands

Configuration options for [x_twist_compensation] can be found in the Config Reference.

Commands for [x_twist_compensation] can be found in the G-Codes Reference

Compensation profiles

The modules stores saved compensation profiles at the bottom of the printer.cfg file. An example is as such:

#*# [x_twist_compensation default]
#*# z_compensations = 0.116667, -0.053333, -0.063333
#*# recommended_z_offset = 2.516667
  • z_compensations lists the z compensations calculated at the points between start_x and end_x
  • recommended_z_offset is the recommended probe z_offset note that this is just a recommendation and should be fine tuned as required for desired squish

Fine-tuning

If required, the user may fine tune the z_compensations values.

For example, the value of 0.116667 represents the z_compensation applied when the printer is probing at start_x. This means that if the non-compensated z-height measured at start_x was 2.0, the compensated z-height is now 2.0 + 0.116667 = 2.11667, resulting in a higher point on the recorded mesh.

Should the squish on the side of the bed represented by start_x be insufficient, the user may specify a lower z_compensation value in place of 0.116667, such as 0.016667. This would result in the mesh being lowered by 0.116667 - 0.016667 = 0.01mm at points along start_x.

Reference

The calculations for this module are largely based off those found in Marlin PR #23238.

A visualization of the issue can be found in this discussion thread.