Bits and Pieces
Builders' Tip - Step Drills
By Ian Brown, Editor - Bits and Pieces, EAA 657159
You may have come across these drills already, and if so you probably love them like most people. If not, get one or two. Aircraft suppliers such as Van's and Aircraft Spruce & Specialty sell them, but I've also seen them at Canadian Tire.
These drills are ideally suited to thin sheets of metal or Plexiglas.
For aircraft sheet aluminum, they produce a very clean hole of whatever odd size you require. They stay centred on your pilot hole and cut really cleanly, with no burring. Since they are essentially shaving the side of the hole a bit at a time, they put minimal stress on Plexiglas, so there is much less risk of cracking that precious, expensive canopy; but remember to drill it when the canopy is very warm and go slowly.
Use a bit of masking tape to indicate the depth, i.e. hole size, you require. You will just need a pilot hole at least as big as the smallest diameter at the tip of the drill, or if you need two, as in the photo, you can start with the smaller one at a 1/8-inch hole and enlarge to 1/4 inch before "stepping up" to the second one.
Step drills: 1/8 inch to 1/2 inch and 1/4 inch to 3/4 inch