Skew

What is skew?

Skew is when the axes of your printer are not perfectly orthogonal, i.e. the angle between them is not 90 degrees. If this happens and you print a square object, it will come out as a rhombus.

Effects of a skewed printer

If the skew angle of your printer is denoted by $\theta$, then over a printbed of size $L$ there is an offset of $L\sin\theta$ across the length of the bed. Thus, for example, a skew angle as small as 0.1 degrees across a 235mm bed results in a 0.4 mm offset. FOr a large-form printer such as a 350mm Voron 2.4 or Voron Trident, this grows to 0.6 mm. This doesn't sound like much, but that much change could cause functional prints to not line up properly

Fixing skew

If you measure the diagonals as well as the x- and y-axis of Calistar, the spreadsheet will calculate your skew angle and estimate the error in the calculation. Unlike dimensionality, it is not possible to iteratively tune skew, though this is planned for the future.

Once you complete the spreadsheet, skew is fixed in your printer's firmware. Common firmware are discussed below.

Some special cases

The Prusa i3 MK3S+ and MK4 will measure and correct skew automatically. They do this by measuring the location of magnets embedded in the print bed. Thus, on these printers, there is no need to use Calistar to measure skew, unless you want to crosscheck your Prusa's results.

Some printers such as the Prusa Mini and Bambulab printers cannot correct skew. In these cases, you can measure it with Calistar, but not do anything about it.

Klipper

The way that you fix skew in Klipper is described here in the Klipper documentation. This is done in the skew_correction section of your printer.cfg. The spreadsheet automatically provides the SET_SKEW XY command that you need to fix your skew. Ensure you save your skew settings with SKEW_PROFILE SAVE=my_skew_profile, and load it in your print_start macro with SKEW_PROFILE LOAD=my_skew_profile.

Marlin

Marlin skew correction is planned, but currently not implemented.

Reprap

Reprap skew correction is planned, but currently not implemented.

Iterative skew correction

This feature is planned, but is currently not implemented.

Next: Dimensionality (if you haven't already done it).