How to drill Diecast Aluminium Boxes- Hammond 1591

[vinMax]

I have 32 Hammond 1591LSBK boxes that I want to drill one 9 mm hole and and two 10 mm holes on the sides. I have tried drilling a few using a stepped cone cutter. My aim of using the stepped cone cutter was to save the changing bits. However, the process has been derailed by the accumulation of slag on the trailing edge of the cutter.
To try to safe the situation, I tried low speed and high speed but none of the options helped. I also tried slow plunging and later the rapid plunging but still there was no improvement.
What could be the problem? or am I missing something?

[Ulfreds]

Try putting the cutting fluid both on the box sides and the cutter.

[Alex Plainlater]

The Hammond must have been made from pure aluminium. As Ulfreds says, try using some lubricants such as WD40. if that doesn't work, find an oil based fluid.