Using a step drill bit

[whaler23]

Looking for some pointers on using a step bit to drill into my aluminum hardtop supports to begin antennae and radar cable runs. Looks like I will need from 1/4 up to slightly over 1/2 inch holes and not quite sure what material the bit should be made of and the proper technique. Thanks.

[Boss Lady]

Drill a pilot hole with something small, and then put a couple of layers of masking tape or similiar around the area. This is just in case the step drill jumps and wont mar the finish, it also keeps the shoulder of the step drill from marring the area outside the hole. You should seal the aluminum where you drill it since the outter surface is probably anodized, and exposing unprotected aluminum will allow corrosion to start. Use a good clear paint on the exposed aluminum. I like to split some rubber vacumm line and fit around the inside of the new hole, it looks professional and will keep the wires from chaffing. Anybody else have a different solution?

[mike]

Grow a native oxide on the surface of the Al by cleaning the drilled surfaces using alcohol and a scotch brite. Rinse with any debatured alcohol and keep, water off the surface for 2 or 3 days while leaving the surface exposed to the natural atmosphere. The net result will be a 10 to 20 ang. thick Al oxide that will protect the surface. It is not excessively robust if scratched, however for an application such as a pass through it should do the trick. The oxide will prevent surface corrossion just like a more traditional andoization.

[Nonchalant1]

Any good hardware store will have many sizes of rubber grommets that should fit instead of a split vacuum line.

[rmaher]

I saw an interesting suggestion for drilling holes in aluminum tubing from an Atlantic Towers installation video. They took a scrap piece of angle iron, drilled a small hole at the bend, flipped the angle iron so the hole and apex of the bend faced out away from the tube (picture it like this: drill bit < O) Insert bit in hole and drill away. The bit is much less likely to slip!

Hope this makes sense - Rob

[Boss Lady]

That technique works with normal drills, not a step drill.

[luckydave215]

Go to where you buy your hardware and look for a "Uni Bit". It looks like a full circle instead of a twist drill, with a flute cut out of one side for 45 degrees or so. they are usuallly of a "stepped" design, come in several sizes, and are made to cut round holes in sheet metal. (round instead of ragged triangular holes, like twist drills cut in thin stock)
Because most of the diameter of the tool is in contact with the work at all times, they don't tend to jump around when cutting, especially when using a drill motor and the Armstrong method.
WD-40 (kerosene) makes a good cutting fluid for aluminum, use plenty.