In article
,
Jim Wilkins wrote:

On Nov 25, 9:04*am, Joseph Gwinn wrote:
In article ,
*"Michael Koblic" wrote:
"Carla Fong" wrote in message
...
This brings up my question: Is there a way to calculate the decimal inch
sizes of letter- and number- denominated drill bits, or are they ad hoc?

There probably is a simple mathematical rule used to generate the drill
diameter tables. *I don't know the rule, but I would guess that it is
documented in the standard ANSI/ASME B94.11M-1993 (or earlier issue,
which may be free), which is cited in Machinery's Handbook as the source
of the tables.
Joe Gwinn

The wobbly metric conversion graph here suggests that the sizes were
chosen arbitrarily to fill in the gaps in a fractional set:
http://en.wikipedia.org/wiki/Drill_bit_sizes
It didn't mention who decided on the number sizes. Stephen A. Morse
invented the twist drill and Morse tapers.

I see a five-segment piecewise-linear function, slightly smeared by
roundoff errors.

The sizes will not be arbitrarily chosen, as there is money at stake.
The problem being solved is how to choose diameters such that the fewest
number of sizes will cover the needed range. Drill gauges were modeled
on wire gauges, and wire gauges most definitely follow a rule.

From the look of the wiki plot, I would guess that if one plots log of
diameter against drill size (number or letter), one will get a roughly
straight line, although the segments will remain visible.

Anyway, I imagine that ANSI/ASME B94.11M has the story, or at least
cites the story.

Joe Gwinn