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Identifying an odd thread.

MrWhoopee said:
Perhaps it was your measurment rather than your math, .787 is almost exactly 20mm.
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It's a very difficult measurement to do. It's a threaded shaft down inside a casting hole. I can't get a mic or a caliper in there. I had to make a gauge that just fit and then measure that.

Regardless, you were correct - it's an M20-1.25 thread.

Lots of great ideas here. When the dust settles, I'll write this all up and post it separately in tips and tricks with attributes to those who contributed.
 
An employer of mine had a nice dial type bore gauge. I needed a longer extension rod for it to measure the cylinder bore taper in an engine block. It had an odd thread it appeared to be a 1mm thread pitch but cut on a .250" diameter rod as far as I could tell.
Our lathe could not cut a metric thread so I took it to a local machine shop and they made me the parts I needed. And yes it was a 1mm thread pitch cut on a .250" shaft.
 
here' what I'd do ' find a friend with a disassembled Hartford mill of similar vitage, and get a second set of measurements... :rolleyes:
 
Dabbler said:
here' what I'd do ' find a friend with a disassembled Hartford mill of similar vitage, and get a second set of measurements... :rolleyes:
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It's an M20 1.25 for sure.

You mean to say that Hartford of yours is still sitting in pieces in your shop!!!

I love what priorities do to us! Pretty sure I have shared that old saying of mine with you before. "Don't tell me that you don't have time. That has nothing to do with it. It's just not a priority for you."

Sometimes I think you spend 10x as much time helping other folks as you do working on your own stuff!
 
Susquatch said:
Sometimes I think you spend 10x as much time helping other folks than you do working on your own stuff!
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What’s so bad about that? The last 4+ months I’ve been spending almost all of my available (i.e., not sleeping, eating, exercising/PT) time “helping” (entertaining?) folks on four forums.
 
ChazzC said:
What’s so bad about that? The last 4+ months I’ve been spending almost all of my available (i.e., not sleeping, eating, exercising/PT) time “helping” (entertaining?) folks on four forums.
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Sometimes my back handed humour doesn't come through. It was intended to be a huge compliment to @Dabbler.
 
Dabbler said:
Thanks for the compliments. It boils down to Your projects are shinier and more interesting than mine!
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Dabbler certainty helped me to spend money to buy a hole into which I could pour more money.
Thanks I am having a great time learning.
Gary
 
Susquatch said:
Seems a repeat problem for me is properly identifying an odd thread.

Is it metric or imperial?

Thread gauge can be difficult to use.

Custom threads make everything difficult.

My standard process, is to make an educated guess, then use thread checkers and fastener gauges. But if that doesn't work, I mic the OD or ID, use threading charts to identify possibilities, and test fit if possible.

But quite often it doesn't seem to work very well.

What process do you use?
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I find it pretty easy to identify odd threads.

If within the first page or so a thread drifts into a discussion on plucking chickens, I immediately think, "This is an odd thread"
 
Update

The M20-1.25 tap arrived. Made a test bushing. Jambs up after about 1/2 an inch or so. Feels very much like thread pitch is off a smidge.

So I cut another test bushing with a 13/16-20 thread instead. Goes on easily with no binding. Too easily! It's loose as a fat goose. Not right either.

I hate odd threads.

I need to sleep on this, but I'm thinking I'll single point thread a custom tighter 13/16-20 bushing and try that.
 
A little math:

- 1.25 mm = 0.0492"
- 20 tpi = 0.0500
- Delta is 0.0008 per thread.
- 1 full cycle (crest to crest) is 62.5 threads
- Max interference occurs at 31.25 threads
- 50% interference is 15 to 16 threads

That is also approximately where the interference feel made the fit too difficult to continue without fear of damage.

I think my next step should be to make an M20 test bushing and single point thread it to 20tpi.
 
The adapter turned out perfect!

Well maybe not perfect. It threads on a wee bit tight, prolly cuz I threaded it just 5 thou deep at a time using a plain 90° plunge cut. Which is fine for a small thread with a HSS threading tool. Of course, that also meant picking up the thread each time. Not so easy with a fine inside thread for a guy with just one good eye. But, I reasoned that it was better to be too tight than not tight enough and I figured a little bit of axial play resulting from picking up the thread wouldn't hurt.

20250228_121823.jpg

So it's done. Unless someone has some really good arguments to the contrary, it is indeed a 20mm 20tpi thread. Just goes to show that you can't always trust measurements or even standards to be correct.

Time to proceed with the rest of the project.
 
In case anyone is interested, here is the HSS threading tool I ground for this job. It had to cut much deeper (about 2") than the normal threading operation hence the long slender grind. I could have left the tip sharp but chose to put a very slight radius on the Tip to improve the strength of the resulting threads. There is no top relief. I didn't think it would help so I just didn't do it. In hind sight, the chips all curled at the very tip, so I believe that was the right choice.

20250228_123856.jpg

20250228_123912.jpg
 
PeterT said:
Now we just need to get you set up on an insert style threading bar so you don't have to 'dedicate' your HSS blank.
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I actually have an insert style internal threading tool Peter. But for stuff like this, I prefer HSS. Even if I did have the ability to sharpen Carbide (which I don't) I prolly couldn't get it sharp enough to reliably shave a thousandths at a time. I can (and did) do that with HSS. It was actually quite a lot of fun doing it. Of course, I had already practiced with bushings so the pucker factor was gone.
 
Susquatch said:
The adapter turned out perfect!


View attachment 60676
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Nice results

Susquatch said:
Unless someone has some really good arguments to the contrary, it is indeed a 20mm 20tpi thread. Just goes to show that you can't always trust measurements or even standards to be correct.
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Makes perfect sense to me: I have an inch lathe, and even with a “metric threading gear” I can only get close, so I can turn a 20mm shaft, but can’t make an M20-1.25; however, I can make a perfect M20-20 tpi!
 
I am also a fan of HSS, especially for fine thread. 90' plunge ( thumbs up) After reading this technic on this forum, I will never go back to 29.5 unless its 4 TPI.
It's hard enough to identify odd threads, even harder when they are mix standards. Good for you for not giving up and
Kudos for being able to pick up the thread on the i.d and end up with a good part. I dread the day I will need to pick up an internal thread.
 
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