I think this is a genuinely useful tool.

I don't know if you've ever tried to cut a piece of wood to fit an irregular wall, but it's hard to gauge how much to cut out of it to match the wall.

With this tool, you can run the wheel up the wall while the pencil marks a precise line on the wood (or wallpaper, or cupboard, or whatever else you're trying to fix up).

1. Load up both parts (they're in a single STL file). You may need to rotate them 90° so they sit flat on your bed.

2. Print them out. I used 30% fill, and that worked fine.

3. Snap the wheel onto the axle. Push it right down before trying to turn it, so it clicks over the enlarged cap that will then hold it in place.

4. Stick in a pencil. The hole is chamfered, so it should hold several different sizes of pencil.

5. Scribe away!