Skewcal

Test plate

Thin-rim Standard Cube: XY, XZ, YZ

Slicing rules

• Don't rotate or mirror it in the slicer. The three small holes mark the origin and the X and Y directions. The origin hole must end up at the front left of the bed.
• Use your normal print settings, without a brim. A brim makes the plate edge hard to find.
• Dark filament is easiest. Light filament works if something dark is behind the holes when you scan.
• To check a correction, print the plate again with the correction turned on and measure it the same way.

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Reference sheet phone photos only

Print at 100% / Actual size. Use the dark-window version for light-coloured plates. After printing, measure the two arrowed spans for exact scale.

Flatbed scanner

Most accurate

1. Lay the plate on the glass near the middle, either side down.
2. Scan at 600 dpi, greyscale or colour. Save as TIFF or PNG so the resolution is stored in the file.
3. Turn the plate 90° on the glass and scan again. Don't rotate the image afterwards; the plate itself has to move.
4. Optional: turn it twice more (180°, 270°) for four scans. The scatter between them then shows the real uncertainty.
5. Open the Measure tab, choose Scanner, and add all the scans.

JPEG scans work too if the file keeps its dpi. Otherwise enter the dpi by hand.

Phone camera

Take four photos

1. Show the reference on a tablet (below), or print the paper sheet at 100% and tape it to a flat, hard table.
2. Lay the plate inside the window, first-layer side down.
3. Shoot from 40–50 cm with the 2× or 3× lens, the whole sheet in frame. The 1× lens up close distorts too much for accurate scale.
4. Use soft, even light and no flash. Check that no lamp reflects off the plate as bright bands.
5. Turn the plate 90° on the sheet and take another photo. Repeat until you have four: 0°, 90°, 180°, 270°. Turning the phone doesn't count.
6. Use the original files. Photos sent through chat apps are shrunk and lose the focal length.

With paper, absolute scale depends on the sheet printing at exactly 100%. Enter your measured spans for exact scale. A tablet screen needs no measuring.

Cube walls

X–Z and Y–Z

Snap the four walls off the cube at the corner tabs. Each wall is then measured exactly like the flat plate, with the scanner, the tablet or the paper sheet, lying flat.

• Every piece is labelled on its outer face: FRONT XZ, BACK XZ, LEFT YZ, RIGHT YZ and FLOOR XY. The arrow points to the top as printed. Front and back measure X–Z, left and right measure Y–Z, the floor measures X–Y.
• Lay every piece letters-up, on the scanner, the tablet or the sheet. The raised letters must not touch the glass, and letters-up also means you never flip a wall by mistake.
• Two images per wall, turned 90° between them. Turn it, don't flip it over: flipping and turning together doesn't cancel the scanner's own error.
• Measure both walls of a pair together (four images), and choose Plate: Cube wall with the right plane. The two walls cross-check each other.

Tablet as the reference

Exact scale, perfectly flat

Open this page on an iPad, lay it flat, and show the marker ring full screen. A screen's pixels are spaced exactly, so scale needs no ruler check, and glass is flatter than paper. The plate lies on the screen; its holes glow from the backlight.

Turn brightness up and auto-lock off, and don't zoom the page. Wipe the screen first: dust under the plate lifts it. A 100 mm bar is drawn beside the ring. For exact scale, measure it with calipers and enter the length when you measure. Models marked "calibrated" have been checked this way already.

PrusaSlicer post-processing

Set once

Every file you slice is corrected automatically. Needs Python 3, which macOS and Linux already have.

1. Save skew_gcode.py and unzip it somewhere permanent.
2. In PrusaSlicer open Print Settings → Output options → Post-processing scripts.
3. Paste this line, with the real path to the file:

Turn off Printer Settings → General → Supports binary G-code. The script can only edit text G-code.

Correct one file here

For other slicers, or to try it first. Pick a plain-text .gcode file. It's corrected in your browser and saved as a .zip.

Add a G-code file .gcode, not binary .bgcode