On Oct 9, 9:46 am, "Joe AutoDrill" wrote:
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?

The number and letter sizes, just like wire gauges, represent a more
logarithmic distribution of sizes than you would get just going up by
say 0.1mm or say 128ths each and every step.
You need a finer step in your scale at the small sizes and a larger
step at the larger sizes. So at the #60 drill range the step size is
just one or two thousandths at each step, but by the time you get to
#10 you're stepping by two or three or four thousandths, and by the
time you're up at X or Y or Z you're stepping by 7 or 8 or 9
thousandths.

That said, there are some oddball spacings in the letter drill
sizes... e.g. M to N is a step of 7 thousandths, but N to O is a step
of 14 thousandths, so they aren't really logarithmically spaced
either. I'm guessing at some point in the distant past there was a
good reason to make M be 0.295 inches, N be 0.302 inches, and O be
0.316 inches, and there was no need for a drill in the 0.309 inch
step, but I can't explain the reasoning there!

Tim.