Subject: DES: Hole Sizes on PCBs for tight tolerance non-plated holes

       2.  I have a standard table of drill bit sizes.  Does the PCB
     industry have custom drill bit sizes or are these all in the
     standards?  If there are custom sizes are these published anywhere?

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 >>>>>>>>>>>>THAD, I don't know about the US of A, but in Europe, with the
exception of 1/8" drills, most drill sizes are metric and are in steps of
0.05mm ie. they start at 0.15mm, and continue 0.20, 0.25 etc all the way
up to 6.35mm. Other intermediate sizes based on mils will be more difficult
to obtain, and cost more as a result. If we are asked for hole sizes in mil,
we round up to the nearest 0.05mm size, and this may be where you have to
lose some of your tolerance on finished size, especially for non plated
holes.

Dougal Stewart
Exacta Circuits
Scotland

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