Odd tapping day - educate me if you dare ;)

[vectorwarbirds]

Drilled and tapped eight 1/4-28 holes using the tap you see in the picture. Given to me by an aerospace guy, brand new, US made, three flute, marked as follows:
VT FT
1/4-28 NF
HS GL15
D0999

Used #3 cobalt drill, drilled and tapped in the mill. Not one 1/4-28 bolt would start into any hole. Not cheapo or NAS aircraft bolt would go. Ran a new everyday Greenlee through the holes and everything worked as usual. Could feel it barely making a cut. Do they make undersized taps? Never seen this before and wonder if yall ever have? Searched online but no joy there.

[old mart]

It looks a coarser pitch than 28 but that might be an optical illusion. Try a known one or a thread gauge on it.
I have had to hide some of the taps at the museum just in case somebody taps a thread with an oversize one +0.002" and +0.005" are common pre plating or anodising taps. On the continent (Europe rather than the UK), sets of three taps of the same form but increasing sizes are common, I had only heard of and used sets of three taper, second and bottoming or plug taps. Maybe yours is one of the incremental type.

[oxford]

It looks a coarser pitch than 28 but that might be an optical illusion. Try a known one or a thread gauge on it.

I thought the same thing when I looked at the picture. I think he would have noticed it when he ran a different tap through if it was.

[MattiJ]

I'm not willing to dig into details but I remember that aerospace threadform is slightly different from UN thread.
Look up for UNJF.

[tom_d]

The tap is designed to cut undersize. The designation code letter G is for Ground thread, the letter L is for the pitch diameter to be below basic size, and the 1 should be below basic by .0005"
Typically we see taps designated GH, where the H is the thread above basic pitch diameter with a number designating how much oversize.

[BCRider]

.....Do they make undersized taps? ......[/ATTACH]

Yes, they do. I've got a 1/4-20 here which produces an undersized thread just like you found. I can screw in cheap grossly undersized carriage bolts but all other options are from quite snug to needing a good wrench right from the first thread entering the hole.

No idea where I got it from but it might have been the old Boeing Surplus Center many years ago. It's also pretty clearly very new and very lightly used. It cuts very easily so it's not just dull and cutting undersize as a result.

[old mart]

It only has the very old NF on it , which is strange as it has a tin coating. Most of the Unified taps I used on aircraft parts were UNJEF which has a slightly different root or crest radius for better fatigue resistance, but screwing in ordinary commercial fasteners in would not be a problem as they are looser fitting.
I still think it is the first size of a set, as I mentioned earlier.

[reggie_obe]

This may be totally wrong.... https://www.bola.de/en/technical-inf...14-28g-thread/

[Tungsten dipper]

Tom D is correct it has to do with class of fit. Has NOTHING to do with thread/inch.

[vectorwarbirds]

Thanks Tom D! The pitch is dead on 28 tpi. So I am guessing this is destined for the trash, nothing I will ever need it for, and don't want to end up using it again.

[danlb]

Rather than trashing it, you might want to hold onto it for working in extra tough materials, and use it as step 1 of 2.

Dan

[vectorwarbirds]

Rather than trashing it, you might want to hold onto it for working in extra tough materials, and use it as step 1 of 2.

Dan

Thanks Dan that's an excellent idea and why I didn't think of it....

[Richard P Wilson]

A dab of paint on the shank will remind you that its the undersize one.

[Black Forest]

A long time ago I bought a set of taps and dies in a very nice case. There were three of each size tap. Wow that is quite nice I thought. I had lots of trouble using those taps. Then a member pointed out to me that what I had were hand taps and they were intended to be used in a series leading up to the finished size. The rings on the tap shank designated which to use first and so on. I didn't know such a thing existed. Live and learn. I know this is probably not the case in your situation as there is no ring on the shank to designate it to be a starter tap. I threw out the hand taps and replaced them with really good quality German made taps. Did I mention that they came in a really nice case?

[vectorwarbirds]

A long time ago I bought a set of taps and dies in a very nice case. There were three of each size tap. Wow that is quite nice I thought. I had lots of trouble using those taps. Then a member pointed out to me that what I had were hand taps and they were intended to be used in a series leading up to the finished size. The rings on the tap shank designated which to use first and so on. I didn't know such a thing existed. Live and learn. I know this is probably not the case in your situation as there is no ring on the shank to designate it to be a starter tap. I threw out the hand taps and replaced them with really good quality German made taps. Did I mention that they came in a really nice case?

Somebody taught me that too, my high school shop teacher, Mr. McPherson