> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of V G
> Sent: 05 August 2010 19:46
> To: PICLIST
> Subject: [OT] Selecting a drill bit size for 3 mm screws
>=20
> Hi all,
>=20
> I'm going to be using hex standoffs to stack PCBs on top of each
> other. These hex standoffs use M3 screws which I have found to use
> screws 3 mm in diameter. For drilling holes appropriate for these
> screws, I have chosen to use tungsten carbide drill bits. Should I
> select a drill bit exactly 3 mm in diameter (such as this one:
> http://cgi.ebay.ca/Tungsten-Carbide-Multipurpose-Drill-Bit-3mm-
>
/160439853750?cmd=3DViewItem&pt=3DUK_Home_Garden_PowerTools_SM&hash=3Ditem2=
55a
f5
> e2b6#ht_673wt_913)
> 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. =20

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.

Mike

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