Pure coincidence, but a couple years ago I made a couple dozen parts with a 4"-8 internal thread. I first made a gauge on a manual lathe for checking the female threads. Cut that to the middle of the tolerance measured with wires.
I used a bar with top notch inserts for the internal thread, cut to the upper end of the tolerance since the end use of the part would work best with fairly loose threads. I used an insert whose largest internal pitch was 8 tpi. Threaded the first hole and my gauge would only go in a couple turns and lock up. Material was 1117 and the thread was slick as could be. Checked my numbers and re-ran the thread cycle. Final pass took no metal but skimmed off most of the Dykem I'd sprayed in the thread.
Finally. for no reason other than not being able to think of anything else to try, I switched to an insert that was good down to 4 or 5 tpi. Futzed around with offsets till I was about cross-eyed as I didn't want to scrap an $80 chunk of stock. Ran the cycle, and the gauge threaded in perfectly with the little bit of slop I'd expected. Rest of the parts came out on the money with no additional spring passes, etc.
About a year later I was looking for some top notch grooving inserts and happened to notice the manufacturer had changed the max internal pitch on the insert I'd initially used to 9 tpi rather than the original 8 tpi. Nice to know, as I'd wasted about 3 hours to figure out they didn't work for 8.
I am getting lost in this story....
You said that you threaded the internal threads to the upper limit, but you was using your previous part as the gauge.
Since the gauge wasn't fitting, how did you determine that you were at "the upper end of the limit"?
I'm guessing simply by X position - determined by touch-off?
When you went with the bigger insert, you had to touch that one off as well? *
And then you ran it to the same final X position in your code - and all was well?
Am I following?
Ifso - the difference between the inserts is the nose rad.
That smaller insert was good "up to" 8 TPI, but it was good "down to" what ??? 18, maybe 24 TPI?
It would have been rated down to the finest pitch that the tip rad is rated for.
Max is mere overall width.
The difference is that the bigger insert would have had a bigger tip rad, and was only rated "down to" 13 ??? TPI?
So when your cheat sheet is telling you that your thread depth is "x" for this thread, it is also ass_u_ming a certain tip rad, and if you have one much sharper, you would need to go deeper - which is what you did with the bigger insert. You may have only went "x" deep from your touch-off ref point, but to theoretical sharp - you were much deeper.
Just guessing if this is the case, and if not - I would like to understand the issue.
I just cut an 8.628 - 8 internal (AN-44 bearing nut) "class III" a cpl weeks ago and was like "How in the world doo I qualify THAT! without $1600 and 4 weeks for plug gauges?" (Or maybe a "GageMaker" setup)
So - like you - I made a plug to about means of spec per wires. (I HATE thread wires!) Then made my part - sneaking up on what it takes to let my plug spin in with a slight wiggle. But I had to go .01 if not .02 oversized to get to that point, and I concluded that it was my NT3R insert that was the problem.
I would have liked to have used a much larger tool, but didn't realize until it was time to make the part that it even had a thread in it, and with all the tools around, I couldn't really find a bigger bar/insert to make the part on hand, so I run-what-I-brung. ... and again - I think that an insert made specifically for 8TPI would have had a much larger tip rad. Prolly enough to blame .01 on diameter to anyhow.
Now, what's more interesting is - I wonder how they used that part in application? I've never seen a "bearing nut" thread actually "IN" a part before. Seems like an odd application.
???
* Getting a bigger threading insert offset close enough to re-run would be a major chore, and would almost certainly widen the thread a bit jist from slight mis-alignment. So that would have helped as well.
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Think Snow Eh!
Ox
