At 11:31 AM 5/16/2002 -0700, Brad Velander wrote:

>         The KB 1472 is so nice, they don't even suggest that letter
>characters might solve the problem so you are left to wonder. Is there a
>limit of 16 sizes even with the letter symbols?

Actually, in my opinion, the best practice is to use the hole size itself, 
which is one of the options. It may need to be very small text, but the 
purpose of showing hole positions/sizes on a drill drawing is mostly to 
make hole size checking easy. Having the hole size in place is *much* 
easier to check, because you don't need to refer to a table.

I doubt that there is a limit of 16, per se; I would not be surprised, 
however, to find a letter limit of 26. There is no limit when the size 
itself is used.

On the other hand, good PCB design practice is to reduce the number of 
different drill sizes used. It helps manufacturing by reducing the number 
of drill changes needed. Note that fabricators will typically combine hole 
sizes to a single drill where the holes are still likely to remain within 
specification (+/- 0.003 perhaps with small holes). 16 sizes is quite a 
few; though, certainly, there might be a design that demands more than 16, 
it would be unusual.

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