I have drilled and machined thousands of soft copper contact blocks and brazed on silver contacts. You DO NOT want to break the edge of the cutter. It needs to be as sharp as possible with a large rake. The problem is not cutting the metal, which is easy, but getting the chip to slide over the cutter. Large rake and clearance and a cutting oil with the best lubricating qualities. For a lot of them we use Mobilgear 636, which is a mildly sulfurized gear lube. Normally you do not want a cutting fluid with high film strength because it makes it harder for the tool to penetrate, but that is not a problem with copper.
We also fly cut silver surfaces. If the cutter edge is the least bit rounded, it will drag on the machined surface and ball it up. We grind the cutter on a wheel to make it concave, then finish it with a fine stone held against the edge and heel, in the same manner as sharpening a hollow ground razor.
Most buss bar is cold rolled so it is not dead soft. We drill, tap and mill slots with standard cutters. If you are getting good chip clearance and still have a grabbing problem, it is probably lack of rigidity in your setup.
Bill
