Original poster: Ed Phillips <evp-at-pacbell-dot-net> "Outside dia. of ball cutter matches that of the OD of the copper tubing. I use a 1/2 inch ball cutter with 3/8 ID (1/2 inch OD) copper tubing. Ball cutter sizes usually are 1/4, 5/16, 3/8, 1/2, 5/8, and 3/4 most popular sizes from J&L Industrial. Dr. Resonance" How deep is the cut? Ed