A keychain yarn cutter in the shape of a pie! This will work for yarn up to about worsted weight, maybe aran if you're lucky. I made these as swag for a Pi Day knitting retreat. Both single color and painted profiles are available.

Materials required:

28mm rotary blade (available at craft and quilting stores)

M3 x 8 flat (countersunk) screw

After printing, place the blade on the bottom piece and carefully align the pie top so the notches line up. Drive the screw in from the bottom. For light use the printed plastic should be enough to hold it securely together, but if you're concerned about it coming loose you can either put a dab of glue inside the hole on the top piece, or insert a heat set screw thread into the hole.