At 09:36 AM 06/08/2010, you wrote:
> > > 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.

Anything of reasonable size can be hand-held with a 3mm/0.125" drill
bit. You have enough leverage to easily break the drill, and I can
state this from personal experience. ;-)

Also, twist drills ALWAYS tend to make holes a few thou larger than
their nominal size (maybe 0.1-0.15mm larger). If you want an accurate
hole size, you drill perhaps 0.01" undersize (maybe using a center
drill or spot drill to locate the hole) and then use a reamer.

For a typical clearance hole for M3 and small production, none of this
is very important, just get the hole location reasonably close and use
an appropriate hole size for standard fit (not too tight or too loose)

For real production work these days you'd likely want to specify the hole
size and location using Geometric Dimensioning and Tolerancing (GD&T)
so that each instance of every part ALWAYS will fit together with its
mate.

For example: http://home.eol.ca/~hgibson/tolerances/node6.html

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m



--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.