Did 5/16 Taps Shrink?

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.
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DerbyDad03 wrote:

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
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re: "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.
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Hmmm...senior moment here. There is a regular tap with the long taper, the bottoming tap with almost no taper and one in between but I can't recall the name of that one. From your descritption it does sound like the middle type tap vice a regular one.
Harry K
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This is pretty confusing.
If I look here, it looks like my new tap is a "intermediate tap" except that the picture of what they call the plug tap looks more like a bottoming tap, so I don't trust their descriptions. I though a plug tap came to a distinct point.
http://www-mdp.eng.cam.ac.uk/nagty/resources/enginfo/cookbook/manufacturing/tapping/tapping.html
However, if I look here it looks like my new tap is a taper tap. Key point: The Plug tap has much more of a taper than a Taper tap. Go figure.
http://upload.wikimedia.org/wikipedia/commons/thumb/f/fb/TapTypes.gif/220px-TapTypes.gif
The packaging for the new tab simply says "Self Aligning Tap" with no mention of Taper, Plug or Bottoming.
So I'm going to combine the two sites and assume that both of my taps are Tapering taps because neither one comes to a point. I'll further assume one of 2 things:
Either the new one is an "intermediate taper" tap or Hanson has modified the basic shape of a taper tap to hopefully make the tap more "self aligning" as claimed.
Of course, none of this really addresses the issue as to why the drill size changed from a G to an F. I'll guess that Hanson decided that a smaller hole would make for better alignment. As long as the user is a bit more cautious when tapping, the result will simply be deeper threads.
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On Sat, 19 Feb 2011 12:58:57 -0800 (PST), DerbyDad03

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

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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.
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On 2/19/2011 3:58 PM, DerbyDad03 wrote:

I don't know the numbers anymore but I've had problems with holes too large and the threads stripping. I now often use the next size smaller drill then spend 10 times longer than normal turning the tap in and out, cleaning and lubricating the tap and hole each time. Results are great but like I said, it takes a lot longer. Go easy and you won't break a tap.
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