OK! I'm happy with the design now.
This is very straight forward machining, for those of you that wish to give it a try.
The goal is to have a point to make a solder bridge from the outer trace of the board to the sink, and keep everything close to flush. Anything sticking up will prevent the battery from making contact, or worse, it might make contact with something it shouldn't. Any lumps i.e. solder or a slightly raised screw head on the perimeter, and the sink will not sit square in the head. This is the reason for the flat head screw
My first step was to face a piece of raw stock on both ends.
Next I moved to the milling machine, estimated the location for the 2-56tpi brass screw (this can be eyeballed), the subsequent lathe cuts will remove the excess brass regardless of location (within reason). Spot drill and countersink with a small center drill, drill through with #50 drill and tap for 2-56. I think #50 drill is correct?
Now screw a 2-56 brass flat head screw in that hole, nice and tight, try for the edge of too tight (without breaking it). These little buggers are pretty strong.
Back to the lathe to finish turning. If you click on the photo to enlarge it, you will see my screw has been trimmed on the I.D. and the O.D. As long as the cutting bit doesn't loosen the screw, the screw will blend in very nicely with the aluminum sink.
Back to the mill for the two slots, once again eyeball and magnifying glass are good enough. I used a good quality eighth inch, four flute end mill. If you want the slot to machine clean, pilot drill through first, then drop the end mill all the way through and walk toward the center, full depth, one cut there's very little material left for the cutter, but it will wander anyway. If you use this method, it will look clean when finished.
TB
