m3 hotbed screws won’t come out.  

Best Answer by Netpackrat:

Drilling out steel or stainless screws surrounded by softer material (aluminum, or composite as in the case of the heat bed) is always perilous because as soon as the bit gets off center a little, and starts to bite into the softer material, that's the direction it will want to go.  What I would do is get a 3mm drill bit, and the key here is to only drill down far enough to take the head off the countersunk screw.  Don't try to drill any deeper than that, and don't try to drill down the center with a smaller bit like you would if using a screw extractor (there's no point in doing that).  Once you get the heads off any stubborn screws, the heat bed should come off, then with the bed and spacers out of the way, you can get on the remnants of the screws with vise grip pliers.

If you can remove the bed and carriage from the printer, and take everything else off the carriage, and then do the work on a drill press, that would be best.  The countersunk screws should guide the bit to some extent, but the more square to the work the bit can be held, the better.

For a 3mm HSS drill in stainless, a good target RPM will be about 1600.  People run into problems with small bits and difficult materials when they try to go too slow, more than going too fast.  Use a new bit (buy spares and change out if one starts to dull).