Skew & Build Envelope¶

Concept¶

Several Happy Hare macros move the toolhead to fixed places near the edge of the machine: parking positions, the Blobifier tray, brush and shaker, and a toolhead cutter's pin. Those coordinates are gcode coordinates, and Klipper transforms them before the move is checked against the axis limits (position_min/position_max):

1. The gcode offset (SET_GCODE_OFFSET X/Y, or G92) is added.
2. If [skew_correction] is configured and a skew profile is loaded, the position is skewed.
3. The result is range-checked. Anything outside the limits fails with Move out of range.

Skew correction is the one that catches people out, because the correction grows with height:

• XZ and YZ skew shift X and Y in proportion to Z. A park position right on the Y limit can be fine at the first layer and out of range 200mm higher. Even a Z-only lift near the edge moves Y.
• XY skew shifts X in proportion to Y, so it affects positions at the back of the bed even at Z=0.

The shift is small per millimeter but adds up over a tall print:

Whether a given axis shifts toward or away from its limit depends on the direction of your printer's skew. Without checking, you can't tell which edge is at risk.

How Happy Hare keeps its moves in range¶

Happy Hare's own macros make their edge-of-machine moves with _SAFE_G1, an absolute G1 that applies the gcode offset and skew exactly as Klipper does. If the target would end outside the axis limits, _SAFE_G1 moves it to just inside the limit (0.01mm). If the required correction exceeds 5 mm on any axis, it rejects the move before movement. This cap includes the boundary inset and any correction needed for MAX_Z. Moves that are already in range are sent unchanged, so nothing changes on printers without skew correction or an XY offset.

These moves go through _SAFE_G1:

• Parking and restoring: every toolchange movement move, including the z-hop lift and ramp, the move to each park position and the moves back to the print. On the way back down, the toolhead moves to the restore X,Y and the print height together. Any adjustment made at the lifted height is taken out during the descent, not as a sideways move at print height.
• Blobifier: the purge row, tray, brush and shaker moves at the edge of the machine, including the Z rises while purging.
• Toolhead cutting: the approach to the pin, both cutting strokes, the return to the pin park position, the safety margin moves and the back-off after cutting.

The first time a move is adjusted in a print, Happy Hare logs a warning that names the move:

Warning: Move to (Y356) adjusted to (Y355.056704) to stay within axis limits (Y 356.933 beyond limit 356.000).
Check your Happy Hare macros for positions or movement that exceed your build volume envelope. To allow for Klipper skew compensation (or an X,Y gcode offset) it is often necessary to move park/place coordinates slightly inside the boundary. Further automatic adjustments during this print will be logged at debug level

Later adjustments in the same print are written to mmu.log as one short line, such as _SAFE_G1 adjusted X100 Y356 -> Y355.06. An adjusted move stays within the axis limits, but does not guarantee clearance from attachments or a complete cut. The toolhead did not go exactly where it was told. If you see the warning, check the configured positions with MMU_CHECK_POSITION and move them inside the boundary.

Your own macros¶

_SAFE_G1 X= Y= Z= F= can be used in place of G1 in your own macros. It takes absolute coordinates only and leaves the current G90/G91 mode unchanged. In particular, a user_park_move_macro (see Macro: Sequence) replaces Happy Hare's own park move, so it should use _SAFE_G1 for its moves to get the same protection.

Cutter clearance and travel¶

The cutter's pin approach and cut strokes use the same bounded adjustment. Pulling an endpoint inward can shorten the cut. The 5 mm cap is a defensive movement limit, not a statement of your cutter's mechanical tolerance: verify that the pin still compresses far enough to sever filament at every print height. The cutting motor current is restored even if a movement fails.

Skew correction also changes where the toolhead physically ends up for the same coordinates as Z changes. The depth of the cut stroke can therefore vary slightly with print height. Make sure pin_loc_xy, pin_loc_compressed_xy and pin_park_dist leave enough margin at every height you print at. Check both points with MMU_CHECK_POSITION (see below).

Checking a coordinate: MMU_CHECK_POSITION¶

MMU_CHECK_POSITION reports whether an X,Y gcode coordinate stays within the axis limits at every height, from Z=0 to the Z maximum, with the current XY gcode offset and a skew profile applied. It never moves the toolhead and works whether or not the printer is homed.

MMU_CHECK_POSITION X=<x> Y=<y> [SKEW=<profile>] [ERROR=0|1]

• X/Y: the coordinate as written in your macro variables. At least one is required. If you give only one, the other axis is checked at both ends of its range, because skewed X also depends on Y.
• SKEW: a saved skew profile to check with. Klipper doesn't load a skew profile by itself at startup, so at idle there is usually no skew in effect. Name your profile here to check against it without loading it (this is also safe during a print). Leave it out to use whatever skew is currently loaded.
• ERROR=1: raise a gcode error if the position is out of range (off by default).

The first line of the report always says which skew was used: the profile you supplied, the currently loaded profile, current factors that don't come from a saved profile, or no skew profile provided or loaded. The second line shows the gcode offset. Then each axis is either OK, with the machine positions it spans across the Z range, or out of range, with the height at which it goes out and a value that is safe at every height:

MMU_CHECK_POSITION X=19 Y=355 SKEW=my_skew

Skew: profile 'my_skew' (supplied) xy=0.000000 xz=0.000000 yz=-0.018666
Gcode offset: X0.00 Y0.00
Checking X19.00 Y355.00 from Z=0.0 to Z=340.0:
  X: OK (machine 19.00..19.00, limits 0.00..350.00)
  Y: exceeds maximum 356.00 by 5.35 above Z=53.6. Use Y <= 349.64 for all heights

Here, Y=355 works for prints up to 53.6mm tall. A park position of Y=349.64 or lower works at any height.

Which coordinates to check¶

Checking at print start¶

Skew profiles are normally loaded in your start-print macro with SKEW_PROFILE LOAD=<profile>. Add MMU_CHECK_POSITION ... ERROR=1 lines straight after that. If a position would be out of range, the print stops before the first toolchange. That's particularly worth doing for a toolhead cutter's pin points. With the shipped defaults (pin_loc_xy: 14, 250, pin_park_dist: 5, pin_loc_compressed_xy: 0.5, 250, cutting along X), the lines are:

SKEW_PROFILE LOAD=my_skew
MMU_CHECK_POSITION X=19 Y=250 ERROR=1    ; Cutter pin park point
MMU_CHECK_POSITION X=0.5 Y=250 ERROR=1   ; Cutter compressed point

Without SKEW=, the check uses the factors that are currently loaded, so it must come after SKEW_PROFILE LOAD. Call your start-print macro before MMU_START_LOAD_INITIAL_TOOL (see Slicer Setup) so the check runs before the first toolchange.

Troubleshooting¶

A cutter move is rejected or needs excessive adjustment. The pin park point or compressed point may be beyond the limit at the current height. _SAFE_G1 refuses corrections over 5 mm; it cannot fix an incorrectly configured cutter position. Check both with MMU_CHECK_POSITION ... SKEW=<profile> and move pin_loc_xy / pin_loc_compressed_xy inside the boundary, or add the ERROR=1 check above to catch it at print start.

The adjustment warning appears in every print. One of the positions Happy Hare uses is beyond the limit at some height. Blobifier positions at the edge of the machine are a common cause. The move stays inside the axis limits, but MMU_CHECK_POSITION will tell you how far inside the boundary to move the position (for Blobifier, increase y_offset or brush_y_offset) so it's no longer adjusted.

MMU_CHECK_POSITION says everything is fine, but moves are still adjusted. Check the first line of the report. "No skew profile provided or loaded" means the check ran without skew. Add SKEW=<profile> with the profile your start-print macro loads.

See also¶

• Toolchange Movement - park positions and the moves around a toolchange
• Purge: Blobifier - the purge positions at the edge of the machine
• Tip Shaping: Toolhead Cutting - the pin pin positions and mechanical travel
• Command Reference: MMU_CHECK_POSITION
• Klipper skew correction - measuring skew and saving profiles