Addison Pemberton, of Pemberton and Sons Aviation in Spokane, WA, sent this tip:
This is a way to make a really big holes in sheet metal with a hole sow or fly cutter and get a really round on size hole. This idea came from Nick Bishop at West Coast wings and it worked great on our Goose Center Section to add additional 4 3/4" inspection holes. The holes came out on size with in .015 and round and beautiful in .063 2024-T panels.
- Drill 1/4" pilot hole
- Open up pilot hole to 3/4 with a Unibit
- Install a 1/4 Bellcrank flange bearing with 2 ea #6 screws and nuts
- Let the pilot drill ride in the ID of the bell crank bearing
- Use a slow tuning drill motor and use WAD or oil lube unit you break through.
Thanks Nick for the great help this worked perfectly.