On Oct 11, 10:30 am, "Paul K. Dickman"
wrote:


Some more research indicates that the "Stubs steel wire gage" extended into
letters that exactly match the letter drills. They indeed extend fairly
neatly from the number series. The step to number 1 from A is pretty darn
close to the step from number 2 to number 1.

While Stubs wire gage was undoubtedly the source, my
American Machinists' Handbook says of twist drill gage sizes
"rod sizes by this gage should not be confused with
Stubs' Steel Wire Gage sizes. The difference betwen the sizes
of corresponding numbers in the two gates ranges from about
..0005 to .004 inch, the Stubs sizes being the smaller..."

#30 drill is 0.1285, and #30 Stubs' steel wire is 0.127, for example.

I suspect the Stubs' gages were measured as an afterthought from
some salesman's sample kit, but the number drill sizes were
designated in absolute measure (thousandths of an inch)
for an industry that could handle precision measurement.