Drilling Sheet Metal
The British Horological Institute has archived and edited the following from e-mails sent to the Clock/Clockers mailing lists on the Internet. The information here does not necessarily indicate a method approved by the BHI, we are only publishing this digest so that others can decide for themselves whether the methods listed below will suit them.
From:
Ron Schorr, Lee Werling, Murray R. Falk, W. R. Smith. Also a letter from C.J.baldwin in the Feb 1987 HJ
Drilling the winding arbors in dials. The aluminium plate seemed easy to cut but raised some nasty ridges around the edges of each hole.
Method 1.
First of all cover the approximate area where the holes are to be, with masking tape, at least an inch all around the new hole position. Overlay the old scratched dial and gently scribe the two holes. Use the centring tool in the bushing machine to make a tiny dimple in the centre of each circle aligning it by eye. Drill a small starter hole large enough to insert the tip of your smallest reamer. Progressively worked my way up through 5 sizes and used the chamfering tip to clean the hole edges after each pass, both on top and bottom. Next, use a large "T"-handle reamer and gently open each hole to the correct size using my scribe marks. You can also insert the reamer in the original hole and make a pen mark on the large reamer. This mark will let you know when you are getting close. Use a drill bit to clean the final reaming rough edges, then insert a grommet in each hole and the job is done.
Method 2.
One method is to use a sheet metal drill. They require only the slightest centre mark, do not walk, and instead of drilling a three sided hole in thin sheet metal like a normal twist drill, they drill a perfect circle. Just pick a twist drill of the size required and grind it to shape using a standard bench grinder. See Chapter 9 in Bill Smith's book, Clockmaking & Modelmaking Tools And Techniques. Sheet metal drills will drill circular holes in decreasing metal thickness' until it is becomes so thin it doesn't have enough to have the body to resist the drilling force- maybe 0.005" thick.
Method 3.
Another method described in the HJ in Feb 1987, by C.J.baldwin is to simply use a small square of fine emery paper under the cutting point of the drill. This will enable the drill to cut sheet metal with no fuss and will give a good round hole. Obviously the only time you cannot do this is if there is surface finish that cannot be protected. However, most times there is one side that will not be seen (the back of the dial for example where the rubbing marks of the emery will not matter.
Method 4.
If your not fortunate enough to have access to a hole puncher that makes good round clean holes. A possible method is put a piece of wood under the dial, align the hole under the drill in the drill press, cover the dial with a piece of masonite, clamp everything in place, and drill through the masonite and dial into the wood.
