"DoN. Nichols" wrote in message
...
On 2017-08-22, Przemek Klosowski wrote:

[ ... ]

(*) check out the plot of drill diameter vs. drill index number: I
would
like to know how THAT came about, as the line is all over the
place.

Well ... start with counting the passes of drawing wire through
reduction dies. (They *are* "wire sized drills" after all.

Then, start moving some sizes a bit to avoid near fractional
sizes. (I think that the only counter-example is #7 = 1/4"). This
helps
make a number of sizes between the two series. The next thing is
when
metric sizes came into the game so move things a little more to make
the
combination of wire sizes, fractional sizes, and metric sizes fill
in
most of the gaps in the individual sets. Note that when using roll
taps
in imperial sizes, the recommended tap drill is often a metric size.

Enjoy,
DoN.

http://www.harveytool.com/secure/Con...rill_Chart.pdf
-jsw