> > or should I select something slightly larger? The question is: will
> > the screw slide through if I use a drill bit exactly 3 mm in diameter?

> You need what is called a "clearance" hole, which for M3 is 3.2mm.

> In practice a 3mm hole will likely work since a 3mm drill bit guided by
> hand will most always cut slightly larger than 3mm, but you need to
> consider the tolerance stack up of the holes in all the parts you are
> screwing together.

The following is a "very naughty" thing to do and of course real
people [tm] would never do such things.
Can be useful when you don't have the right drill though.

As Michael notes, a hand held drill bit will make a somewhat larger
hole than its nominal diameter.
If you drill the hole and then tilt the drill in the hole at an angle
it will drill a slightly oval hole.
Doing this again at 90 degrees to the first tilt, or at several angles
or rotating work and drill bit conically will produce a more evenly
enlarged hole.

A 4 degree tilt (hard to avoid by hand) will produce a "crater" which
is 3.2mm diameter in 3mm thick material. The centre will nominally
still be almost only 3mm (oval projection of drill is slightly larger)
but in practice it works if "enough" angle is used. Real machinists
and people with a full set of drills are now rising up in fury :-)..

The hole so formed has a conical "crater" and is all in all a nasty
piece of work. BUT it will turn a close interference hole into a
clearance hole with the same drill, and this may sometimes be worth
the nastiness - depending on how "proper" and how desperate you are.

The above only works 'well' in relatively thin material. Say material
thickness is ideally not more than about the same thickness as the
drill diameter and the thinner the better. The truly desperate can
make it work for much thicker material. Somewhere along the line you
are using the drill as a low grade mill and it's something that should
be avopided.
..
Final thought - doing the above sort of thing may tempt people to
hand hold  work and use  a drill press. This is a technical no-no as,
apart from the lack of precision, it encourages the jamming and
throwing of the work. Bad injuries can occur. Death if you are clever
enough, but you'd have to try very hard to do that with a 3mm drill
and a PCB drilling machine. Better, invest in a few appropriate
drills.



 R
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