Hi John,
Standard machine shop drill tolerance is +/-.003, but you can
realistically get down to +/-.001 on FR-4. Actually, the tolerance is
+0,-.001 of the bit size unless the shop runs their bits so long the
diameter of the bit gets smaller, in which case, you might get -.002.
There is not an infinite gradient of bit sizes, and more importantly,
shops overseas use metric, so choose your hole size carefully. The
shops might complain, but we've been getting boards, (and measure hole
size on every shipment), using a spec of +/-.001 for 10 years or more.
Unfortunately, our tooling hole size is inch, so our metric board shops
have to order custom drill bits to match because of the tight tolerance.
(I've tried getting the board shops some more tolerance, or at least a
change to metric hole size, but SMT manufacturing did want to change
tooling pin sizes or allow extra slop between the tooling pin and hole.
:(
Regards,
Ryan
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Babcock John
Sent: Friday, December 02, 2005 2:32 PM
To: [log in to unmask]
Subject: [TN] Drill Tolerances
We've had problems with this in the past so maybe the group can help me
out.
I have a mill-max receptacle pin with a .040 diameter. The datasheet
calls for a .042 minimum dia. plated through hole.
Is it better to call out a .048 hole with a +/- .003 drill tolerance?
(because if you stack the .003 positional tolerance as well, you could
get down to the min. .042 hole).
Or is it better to call out a .047 hole with a +/- .002 drill tolerance?
(because with the .003 positional tolerance you could get down to the
min. .042 hole).
Or does this depend on which board house you go to as well?
John
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