"Joe AutoDrill" wrote in message
news:fDLOi.1693$d2.1294@trnddc08...
Okay... I understand why fractional drill sizes and metric drill sizes
exist... They are common sense to me.

But... Why do number and letter drill sizes exist? Do they correspond to
a wire size or some other standard?

Regards,
Joe Agro, Jr.

The number sizes correspond to the "Stubs steel wire gage" a more or less
obsolete system of measurement.

However, it exactly matches the size and number of the holes in a recently
manufactured drawplate I have in the jewelry shop.

It may be a legacy of draw plate manufacture, but I suspect that the numbers
are based more on the amount you can reduce a wire in one pass, than on any
other number.(weight per ft, decimal steps)

I imagine that wire manufacturing started first, creating sizes that were
convenient to their process.

Later the drill sizes were created to match. This could be for 2 reasons. #1
to make holes that fit the wire, and #2 because the drill bits were made out
of wire, which was supplied in wire gage sizes.

I don't know about the letter series. I suspect that, as Ed says, they were
created to fill in awkward gaps in the decimal set.

Paul K. Dickman