I needed to cut and draw some big dang circles!
This model is currently designed to work with two 6mm rods that I find are a typical size of steel rods in printers. Hopefully you have some that fit. There are currently two attachments, one that holds a utility knife blade and a normal sized sharpie.
There are two pivot or pin mounts, one that is fully printable and another that uses a 3mm pin which I sanded to a point using a Dremel tool.
There are markings on the attachment and pivot mounts where you can align a ruler to mark how large you want your circle to be.
Both the attachment and pivot mounts use 4mm bolts and nuts to allow for tightening to the main rods. The utility knife blade holder uses a 3mm bolt which fits through one of the two grooves at the top of the blade.
I will upload another set soon that fits common 5/16" or 8mm wooden rods you can buy at hobby stores and 4mm wooden rods for the pivot pin mount.