Oddball Acme Thread

[recoilless]

This must be a strange thread day. I'm restoring a 1917A1 tripod part of which requires me to fab some threaded studs that help lock the three legs in place. I've got one stud and all three locking nuts. Got around to studying the threads which are a double start acme (no big deal), but 7/8" x 7 tpi (wtf!!).

I can't readily turn 3-1/2 tpi on my Logan or Clausing. Also checked various mfr's, no luck on even a single start 7/8" 7 tpi, which wouldn't help anyway.

I suppose I could manufacture 8tpi double start and have it be "unique" or pay to have it custom made(takes all the fun out of it).

Has anyone run across this thread before or (prayers) have some of this laying around...SCIH.

[SGW]

You should be able to do it with an extra gear. On my South Bend 10K, there's normally a 20T stud gear on the spindle and a 56T screw gear on the gearbox. To get the top row of threads on the chart, you replace the 20T gear with a 40T gear. I think Logan does something similar. That's how you get 4, 4 1/2, ... 7 TPI.
Now, I think if I replaced the 56T screw gear with a 28T gear, I'd also divide the TPI settings by 2, so the 7 TPI would become 3 1/2 TPI.
I may have something backwards, and Logan probably uses different tooth counts for its set of gears, but I think that's how I'd approach the problem.

[recoilless]

SGW: Thanks, I thought there might be a way. I'll do a little investigating tonight and peruse some of my older texts that cover gearing, etc.

Edit: Came to me as I walked away from my desk. Logan swaps the 24 tooth gear out for the 48 tooth for the QC settings called out on 4, 4.5 up tp 7. Yes.