Technical Oddball NF(?) Thread pitch nut

If you know a machinist or have a good machine shop nearby they may have one of these, it will eliminate all guess work.
http://www.starrett.com/metrology/p...ools/fixed-gage-standards/Screw-Pitch-Gages/6

 

Experience has me going with bobs396 on this one. 8-36. Check McMaster-Carr.

 

Just my .2 have you looked into what gun smiths use. They have some different sizes .

 

Thanks Fordors! I will slide by one of our local shops and see! Regards, Dave.

 

Thanks kindly Dean! I will check their website! Regards, Dave.

 

We have one in town and I'll put them on my list of places try tomorrow - Thanks for the idea! Regards, Dave.

 

UPDATE: Went to one of our oldest machine shops and talked to one of their oldest guys (looked a little younger than me, but then I'm older than dirt!). He hauled out his vintage-looking set of thread pitch gauges. "Yeah, looks like an "8"......34?......Nope!.........36?.........Nope!.........38?...........Nope!........40?.........YUP!!
Cool! Thanks! Got any? Searched a large pullout drawer in an old filing cabinet filled with nuts but no go! Went to electrical supply outlets, gun shops, and bike shops. Nope! Ran out of time to check the Harley dealership in the next town. No sweat! At least I know what it is now! May order one online or a tap and build my own nut! Thanks again to everyone for your help! Regards, Dave.

 

Once you get a thread count and a tap...I've so far never found an oddball thread that I could not get tap or die for from MSC industrial...the nut becomes easy. You just need to find a nut of any sort that has slightly smaller hole and drill/tap as needed. If really troublesome getting large enough or whatever nut as a basis, Just use something round, drill and tap, and tighten the thing either by putting on a simple pair of flats with a grinder or just grabbing it with vise grips.

 

or square stock

 

Thanks Bruce! Yes, I was thinking along those lines too. I think it would be better if the nut was wider to offer as much locking area as possible so just find a wide one with a smaller hole and do as you suggest. Thanks for your other options as well! Regards, Dave.

 

There's another good idea! Thanks tb33anda3rd! Regards, Dave.

 

"Save the parts. Grind the gauge. Change the prints."

 

Sorry for the delay! Update time on this!: Got an 8-40 tap and checked it with the thread in the "dimple wheel". It ran the full length of the tap threads smooth as glass -Good! Got a nut with an undersized hole and the correct tap drill ( a #28 according to several charts) and drilled it out. Tapped it, cleaned it and tried it. Couldn't start it on the armature's threads - WTF?! Tried doing another undersized nut - Nope! To quote Mr. Spock, "This is highly illogical Captain!". What am I missing here guys?! Re-checked that I'd used the correct drill and the end of the armature's threads look fine plus I re-checked both nuts threads with the tap and they're good and the "wheel" spins up the armature threads just fine! I guess the next step is an 8-40 die nut on the armature threads??? Crazy!! (Yes, maybe I am or the Alezheimer's is starting to kick in faster than I thought!) Regards, Dave.

 

Any chance they are left hand threads?

 

Nope - I checked that possibility when I examined the armature's first few threads, but thank you for the observation! Regards, Dave.

 

In Mac's Auto Parts 2017 catalog ( Model A ) they have an adjustment screw isentified as SPECIAL but not giving the thread pitch. It sells for 1.95 Part # A13804. You can see it on Mac's Autoparts.com Hope that helps

 

Could the nut have stretched while you were tapping it, then sprung back to undersized when the tap was removed?



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Electrical threads are pretty standard anymore. The oddball stuff disappeared in the 50s. The one oddball you'll still occasionally run into is a 12-24.

Did you try running the tap fully into the nut? More than once? Be aware that there are over-and-under sized taps and dies but they're not commonly used.

 

Actually ran into something similar recently with a light fixture. Had to use a longer fastener, but it would not screw into the mounting bracket. It was 8-32, but the screws were made undersize so you couldn't just use a regular 8-32. I fixed that with a tap.

I've also worked at places where they would come up with stupid stuff like that to ensure that the customer could only purchase replacement parts from us. On the other hand, we also spent a lot of time reverse-engineering products we bought from vendors so we wouldn't have to purchase replacement parts from them...

 

Thanks! Will check it out! Regards, Dave.

 

I actually did think of that possibility Dave after the first one didn't work as I had the nut in a vise to hold it although I didn't think it took more than the normal amount of pressure to run the tap in. I did the second nut with less pressure on the vise, but no go. Thanks for your input! Regards, Dave.

 

Thanks for your observations Steve! Yes, ran the tap the full length of it's cutting portion twice on the first nut and three times on the second nut, cleaning the flutes out each time. The dimple wheel spins onto the tap as if the tap were just a bolt so I don't think I have any reason to suspect that the tap is wrong. Regards, Dave.

 

Thanks for your thoughts cfmvw! As the horn is from the late 20's/early 30"s, I would think that everything would be straight up but hey, you never know! I will see if I can locate an 8-40 die nut so I can rule out the starting threads on the armature shaft. Regards, Dave.

 

10-40 maybe?

 

Thanks for your response Pat! Actually, I got a PM from one of our members about it possibly being a 9-40 which is VERY obscure but I was able to find one supplier for the tap. As that is just a red one larger, it may be the ticket. Regards, Dave.

 

Perhaps use a larger tap drill?

 

Dave, take a micrometer and mike the OD of the tap and the OD of the threads. Providing the pitch is correct, if the tap is smaller than the thread OD by any amount, you will not get the threads to assemble. Some times the male thread can be too large on the OD. Simply file some off of the OD of the male threads if that is the case. Also, any good machinist can check the pitch diameter of the male thread with a set of thread wires and determine exactly what you have. Machinery handbook will have the dimensions for what the min/max pitch diameter is you can compare to. The pitch diameter is what is important in determining if the parts will assemble providing the OD's and pitch are correct. Most tap drill sizes give you around 75% thread engagement so I would not think that is your problem unless you used a smaller drill. I worked in the fastener mfg business for 40 years. Let me know if you have any questions.

 

There is this "one size fits all" device called a welder. LOL I thought I had it bad a couple years ago when I ran across a 4.5mm screw in an off topic carb. I had lost the screw on the road and couldn't find one locally or McMaster-Carr, so it got re-drilled and tapped 10-32

 

This is all good info!
I wonder if in this case, the thread is slightly oversize for self locking purposes. Normally the nut is deformed, but engineers sometimes do odd things.

 

Did you try going up a drill size (to a #27) when making a nut?
There's not a lot of meat on the bone, that small, and you can lose % of thread really quick, but if you don't need a lot of force...