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. 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.

https://www.youtube.com/watch?v=JSm0pXgfxlo

I added to the design brims at the ends of the main piece because it was shrinking a lot on my printer. Don't worry, it is easy to remove with a knife.

You will also need a ~1 1/2 inch wood screw, and a set of bolt and nut 8-32 (1 1/4)

You may have to enlarge the hole to fit the wood screw with a drill.

You may have to use a knife to remove a very little bit of plastic on the smallest side of the pencil pusher to fit it really tight. once in its place it will not fall again.

You can glue the bolt in it's place or you can heatup the bolt and make it stick in the molten plastic.

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 instead of the main piece.