For Happ 3" trackballs.
Used once. I'll never need it again. The blade cost me $6 and the mount was $13. It cut a perfect hole in my Lexan -- it's as if this was made for Happ trackballs.
How about $13 and you get the blade for free. (You get to clean the bits of mdf stuck to the teeth, though.)
ScoopKW