Andreas,

I do covers for microwave housings all the time on vacuum. I run down to .002 or less from the fixture so as to not break the seal, (alum parts, alum fixture), then just tear them apart after I take them off the machine. It's like alum foil.

The fixture has to be real flat, with a fairly accurate machine. Also, I run strictly HAAS', and thermal expansion must be monitored throughout the day: as the spindle/ambiant temp goes up, the tool/spindle/head assembly expands, moving the tool down and will (at least on my machines) move as much as .002 or more depending on the temp swing..