DerbyDad03 wrote:
The decimal equivalents for F & G size drill bits are as follows

F 0.2570
G 0.2610

I've got an old Hanson 5/16-18NC Tap that has "G drill" printed on it.
When I used it, I used a G sized bit.

I need to do a bunch of tapping and decided to start with a fresh tap,
so I picked up a 5/16-18NC Tap and Bit combination by Irwin. It came
with an F size bit which is also what it says on the Tap.

BTW...the F size is what the charts say for a 5/16-18NC Tap.

Other than calling for different bit sizes, there are 2 differences
between the Taps:

Minor: The old one says "Hanson", the new one says "Irwin Hanson"

Major: The package for the new one states that it "Starts Straight"
and there is a noticeable difference in the taper of the shaft. The
newer one does not taper as sharply as the old one therefore it fits
more firmly in the hole. (No, it's not a "bottom tap" in case anyone
was thinking that).

So I wonder...why did the old Hanson call for a G drill, which is
larger, and have a deeper taper, thus making it sloppier when starting
in the hole?

The new one is a breeze to work with but I changed 3 things
(sharpness, hole size, taper) and that's a lot a variables to
consider.

From my understanding, there is some wiggle room in tap drill sizing
depending upon the material you are tapping into, and what percentage of the
threads you want to penetrate. Some tap drill charts will actually indicate
this disparity, and give you a choice as to what tap drill you prefer to
use.

In all honestly, my life was a lot easier when I didn't have to make this
determination.

Here is one such chart, from which you can get the pleasure of having to
make the decision for yourself:

http://www.physics.ncsu.edu/pearl/Tap_Drill_Chart.html

Jon