On Feb 19, 4:15*pm, Smitty Two wrote:
In article
,



*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.

.004 diameter is .002 on a side, so that's not a lot of difference
between the F and G. Machinists use different tap drill sizes for
different materials, and depending on the "% thread" or "thread class"
callout if there is one noted on the print.

Number of threads of taper on a tap also vary, as you note, between
taper, plug, and bottoming taps.

Obviously, starting the tap straight is critical, and various methods
are used to assist with that, the simplest being a block of steel with
appropriate holes for various tap major diameters. The "starts straight"
label on the package sounds like advertising hype to me.

"The "starts straight" label on the package sounds like
advertising hype to me."

I'd tend to agree, but I will admit that the new tap - with the lesser
taper - works pretty darn well.

It almost looks like it's a bottoming tap with the first 3 or 4
cutting threads ground almost down to the flutes, then a couple of
cutting threads that "taper up" towards the full sized ones.

On the older one the flutes themselves are tapered.

I just pulled out the mics...the new one has a tip diameter of .245".
The old one measures .215".

So it looks like they are using the wider shaft to help keep the tap
straighter in the hole.