I need to drill two 8mm and one 7mm holes in the sides of 25 Hammond 1550Q boxes.

To save changing bits, I've been using a stepped cone cutter, but I've not found a way to prevent slag building up on the trailing edge of the cutter.

I've tried the fastest speeds, the slowest speeds, slow plunging, rapid plunging, adding cutting fluid to the box and/or the cutter, but to no avail.

Is this slag simply due to the type of aluminium from which these boxes have been made, or am I still not cutting/plunging at the correct speed ?

Worst case I could drill the holes with individual HSS bits, but even with them I've found these boxes clag up the flutes.

Adrian.