Lessons learned today: Do not believe tap drill charts that say 2.5mm pilot hole for an M3 tap in aluminium.
Just failed miserably by following that advise - screw stripped the thread straight out. (Lucky it was a test piece.) Just to add insult to injury the nearest drill down I had was 2mm and I couldn't even get a taper tap started in that. Had to 'peck' a 2.5mm hole at the start of it and could finally tap the hole.
TEA value? I'm doing a little practical pre-design for making some instrument cases (cf previous discussions, esp. with bd139) and was seeing if I could get away with 3mm of thread in Al to hold covers down. Conclusion, yes, just about, but it would be chancing it. Better to go with steel or more thread.
2.5mm into aluminum CASTING for m3 COARSE thread; 2.3mm for m3 FINE. As with steel, go a little smaller OR make the hole with a prickpunch/die for sheetmetal. 
Also, make sure you are in fact using a m3 tap not a m2.5, m2.6 or m2.8...
mnem
been dere, dunnat. 
The evil chart in question claimed 2.5mm for 1050 Al plate and rod, which is exactly what I was tapping. And it was M3x0.5 coarse (or just M3, what kind of weirdo uses M3x0.35 fine). Theory says that 3mm - 2 x (0.5mm thread x 0.614 thread factor) => 2.386mm thread root (or top depending on what way you're looking at it) for a 100% thread. The chart claimed to be for a 70% thread (sounds reasonable in Al), which calculation says would want a 2.57 mm hole and with plastic deformation as well as cutting you'd get a little bit more than 70%. So, in theory, what the chart says is reasonable, in practice in something as soft as 1050 it clearly is not. The point of using a chart is that it
ought to temper theory with practice, not just save you from doing the arithmetic (which is not exactly onerous) which the chart I'd foolishly saved away didn't - bastards.
You're talking to a man who meters out diodes before soldering them, of course I read the engraving on the tap even though it came out of the little box with all 3 types of M3 tap in, that says 'M3' on the front. What do you take me for, some normal person without an obsessive distrust of what Murphy's going to do to me?
Anyway, I think I'd have got off better with a random M2.8x0.5, had I owned such an aberrant non-standard beastie, and left the steel screw to finish the actual thread forming as even the finished thread drilled at 2mm feels slack. These are supposed to be decent taps, but I think a new set of something reassuringly expensive is in order; I've never been particularly pleased with the results from these ones in anything apart from Perspex or similar hard plastics - where they give a lovely result. There's something deeply satisfying about a well tapped hole, satisfaction that I'm not going to be permitted to have today. At least I was just mucking about.
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