Summary Don't worry about the red and black... I was just running out of red so I pressed on pause changed for black and resumed. 2 functions for that compass... first: Make circles, like any compass. second: Reproduce irregular shapes like a rock or in this video a shower. If you need to fit a piece of wood tight close to it. If it is horizontal or vertical, you can even help the process with a torpille bubble level. Look at 2 images and the video.

Instructions You will also need a ~1 1/2 inch wood screw, and a set of bolt and nut 1/4 -20 x 1" It works better with those carpenter flat pencil but it works with any pen pencil marker or chalk. I designed it for my work (carpenter), but every kid that come home go with my compass and I have to print myself a new one! If you need bigger circles. You can print only the slider, and use a piece of wood (10mm x10mm) instead of the main piece.

Update april 2017

I designed a second shaft that uses a washer as a wheel to help slide on something ruff like a rock.