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.
Perform the following homing procedure(s):
BED_MESH_CLEAR G28 BED_TILT G28
BED_MESH_CLEAR G28 QUAD_GANTRY_LEVEL G28
- 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
- Record position
- 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)
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).
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)