On Feb 20, 12:26*am, croy wrote:
On Sat, 19 Feb 2011 12:58:57 -0800 (PST), 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.

Assuming that all discussed here are "cutting" taps, and not
"forming" taps....

A lot of drill charts will show a tap-drill size that will
theoretically provide for 75% [?] of full thread profile.
You can use a smaller drill to get closer to 100% thread
profile, but it's likely to cause more swearing due to
broken taps. *For shade-tree machinists, it's best to use
the largest recommended tap-drill, unless you've got good
reason to worry about the strenth of the join--and then it
means you're working on critical aerospace parts, or the
design is off.

--
croy

All of that makes sense, but I still find it curious that Hanson
changed the recommended bit size for their tap. They went from a
larger one a few years back (G) to a smaller one today (F).

Perhaps, as I said in another post, the lack of a pronounced taper, in
addition to the smaller bit, helps them fulfill their claim of the tap
being "self aligning" as noted on the package.