Help identifying and duplicating this thread.

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Bill Shields
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Re: Help identifying and duplicating this thread.

Post by Bill Shields »

Making a tap out of a lead screw is a last resort measure if you cannot get the nut.
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Torch
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Re: Help identifying and duplicating this thread.

Post by Torch »

My long term plan is to make one out of bearing bronze instead of brass.
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Bill Shields
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Re: Help identifying and duplicating this thread.

Post by Bill Shields »

Ah now there is a plan. Wonder how long the brass nut will last....

I have used brass nuts in CNC machinery over the years...not a lathe but a welding machine...and they have lasted for...well...coming up on 20 years now...but I am willing to bet that nobody has checked for wear in that application... probably will not be checked until the axis stoops moving ... :mrgreen:
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Torch
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Re: Help identifying and duplicating this thread.

Post by Torch »

The first one lasted 2 or 3 years, IIRC. When I changed it, I realized that the slot milled lengthwise down the shaft across the threads left sharp edges that wasn't doing the brass nut any favours, so I dressed the corner on each edge of each thread on the steel shaft with a file to dull the edges. The second nut is now well worn again but it took around 10 years so that certainly helped.

Hey, maybe I'll get really lucky and the updated nut is already bearing bronze.
Torch
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Re: Help identifying and duplicating this thread.

Post by Torch »

Got the parts today and the shaft was indeed included with the superseded nut. So if you need a new shaft, buy the assembly -- it's cheaper!

At first glance, unmagnified, the thread looks square a the root. I'm at work but will have a look on the OC tomorrow.
Torch
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Re: Help identifying and duplicating this thread.

Post by Torch »

New shaft is definitely a different thread form. Straighter flanks, flat root and slightly rounded at the corners. Still 30°included angle, 8TPI and 20mm OD though.
New:
Image
Old:
Image

New:
Image
Old:
Image
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neanderman
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Re: Help identifying and duplicating this thread.

Post by neanderman »

Wow!
Ed

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Torch
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Re: Help identifying and duplicating this thread.

Post by Torch »

I finally got around to installing the new pair today. The new nut does not thread onto the old leadscrew and the old nut does not thread on the new leadscrew. The new leadscrew fit the machine perfectly, although the end of the new leadscrew was tapped 5mm instead of 4mm, so I had to dig out a new machine screw to secure the hand crank.

Backlash is now 0.006". I can live with that -- better than 0.040" anyway.

And now I have a spare nut and leadscrew that I can use in the crafting of a bearing bronze nut for the future. It's still an oddball thread -- metric diameter, 30" included angle, 8tpi pitch, but the straight sides should be easier to grind than the earlier mystery thread.

Now, should I make a tap out of some O1, or attempt an internal thread?
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Harold_V
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Re: Help identifying and duplicating this thread.

Post by Harold_V »

Torch wrote: Wed Mar 23, 2022 8:00 pm Now, should I make a tap out of some O1, or attempt an internal thread?
A tap would be a poor choice, at least in my opinion. It may or may not cut size and may or may not cut straight and on location.

I'd suggest a tap only if the length of the nut prohibited single pointing the thread, where you'd have complete control over the outcome.

Keep in mind, if you use sharp tools, the nut should cut beautifully. It's not like machining steel.

H
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Torch
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Re: Help identifying and duplicating this thread.

Post by Torch »

Harold_V wrote: Thu Mar 24, 2022 12:27 am A tap would be a poor choice, at least in my opinion. It may or may not cut size and may or may not cut straight and on location.

I'd suggest a tap only if the length of the nut prohibited single pointing the thread, where you'd have complete control over the outcome.

Keep in mind, if you use sharp tools, the nut should cut beautifully. It's not like machining steel.
I don't think length will be a problem -- without measuring, it's a little over an inch long, maybe 1-1/4". It's more the diameter -- specifically the root diameter. I have come across the formula Drill diameter = Acme OD - (1/ pitch). That would work out to 0.662", and jives with an online calculator I found.

But while this bastard of an unholy union between metric and inch is very similar to an Acme thread, it's not an Acme thread. The measured depth (using the 20x OC image) of the external thread is 0.070", not 0.0625" and the angle is 30° not 29°. So I'm thinking the bore size should be .001875" smaller -- call it 0.002" since I"m not that good and the OC is not that precise, so 0.660"?

Tap vs single point. I can address the straight and location concern by including a pilot on the end, similar to an Irwin hand tap, to hold it straight in the bore. And give it a nice looong taper to help keep it there. I'm not finding any commercially available threading inserts in this form factor, internal or external. Accurately grinding an external form tool is much easier than an internal one at my skill level and an external tool is inherently more rigid, being close to the tool holder. Examining and measuring the root diameter of an external thread is child's play compared to an internal thread.

But, it's a one-shot. Goof up the tapping and it's start over from scratch. Bearing bronze has a reputation for yielding then closing back in on the tap unless it is very sharp. In theory, I could first make a go/nogo gauge and then creep up on the final dimension with a single point tool (Oh great, now there's two tools I have to grind accurately! lol) compensating for any error caused by flex of the tool. Unless of course it yields at a couple of light passes then suddenly grabs as I try to take a little bit more. I have made some oddball nuts out of steel (12L14) this way, although they were standard 60° V threads. I've never attempted a trapezoidal thread (internal or external) before, so maybe I'm missing something here.

Decisions, decisions. Good thing this is a hobby and the part is not urgently needed.
f350ca
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Re: Help identifying and duplicating this thread.

Post by f350ca »

30 degrees is the standard for metric trapezoidal threads.
Ran into the same thing on my Polish made Summit lathe. Metric OD with 30 degree angle but imperial pitch for the North American market. All the other threads on the lead screw for bearing mounts were metric.

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Bill Shields
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Re: Help identifying and duplicating this thread.

Post by Bill Shields »

Sure . Why not?

Same tooling, bar stock and machines.

Just program in a different pitch.

The rest of the world has been doing this for decades to support the imperial thinking dial world.
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