TOPIC:

Just out of the blue I started wondering about something here.....

Let's say I wanted to tap/thread holes in some aluminum, to be used with M3 (3mm) screws. Should I then drill the hole with a 3mm drill bit, or should I use a slightly smaller bit?

Here, this should explain it:





The "M3" is the Dmaj dimension, i.e. the outer diameter. for a tapped hole you need to drill with something smaller. Dmaj - 5/8H, I guess ?

en.wikipedia.or...rew_thread

It's a good question, and I'm quite certain the hole you drill has to be smaller than 3mm.
I found this site, whether it's correct or not I don't know. It suggests 2,5mm drill bit.

www.shender4.co..._chart.htm

Thanks Lars.
That link is very useful!

drill 2.5mm for an M3 thread.
Use WD40 or paraffin or spit(!) as a lubricant for aluminium.
Use a proper taper-tap (First or second) for the first cut, to help keep it straight. Don't try to do the job with the cheap pointy parallel taps because you'll just monkey it up.
"Spiral point" taps are awesome if you have access to them.