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Twisted X Extrusion Diagnostics

If a first layer is constantly high on one or both sides and is low on the other side or in the middle, the printer may have a twisted X extrusion. than can happen from the factory or be a result of a head crash.

The sensor is correctly offset from the bed based on the Bed_Tilt or Quad_Gantry_Level, but the nozzle is ~25mm in front of the probe. As the head moves along the twisted X extrusion the toolhead is rotating about the X axis swinging the nozzle in an arc. Moving along Y should not change this rotation.

This process helps to identify a twisted X extrusion without tearing down the printer.

Note: The values used in this exmaple are for a printer with a 300x300 bed. If the printer being tested is larger or smaller, compensate accordingly.

Prep

Perform the following homing procedure(s):

V1 

BED_MESH_CLEAR
G28
BED_TILT
G28

V2 

BED_MESH_CLEAR
G28
QUAD_GANTRY_LEVEL
G28

Measure

Measure Middle Center

  • Move to center of bed
G90
G0 X150 Y150 Z5
  • Find bed location by moving bed down 0.1mm at a time until it touches a piece of paper.
G91
G0 Z-0.1
  • Retrieve current location
M114
  • Record position

Measure Remaining Points

  • Middle Right (X250, Y150)
  • Middle Left (X50, Y150)
  • Front Center (X150, Y50)
  • Front Right (X250, Y50)
  • Front Left (X50, Y50)
  • Rear Center (X150, Y250)
  • Rear Right (X250, Y250)
  • Rear Left (X50, Y250)

Calculations

For each Y position (Middle, Front, Rear), calculate the Z offset of each side with the following calculation:

Z Offset = (Recorded Z Value - Center Z Value)

Compute the 6 resulting values for (Middle, Front, Rear) and (Left, Right).

Comparisons

If the Z offsets match for the same three X positions along the different Y values there is likely a twisted X extrusion - (or the bed is a perfect taco)