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edproject

Plastic
Hello everyone!

I've started doing a bunch of work that uses stub ACME threads, which is something I'm fairly new to. All of the parts are open to my own design, but customer specifies stub ACME threads. All of my inserts are genuine Sandvik and do a great job.

Now I use a full profile insert that has a depth of 0.056" radially. So say for example, I turn a piece to 3.002" and I want my thread to be 3.0-8TPI. That would mean a minor diameter of 3.000-0.056-0.056 = 2.888 inches, that being I tell my machine to thread down to that value, with it being full profile I should end up with a diameter of 3.000 inches. This is pretty much what I do with every thread. Now it's easy with a unified thread as I just type it into my thread calculator, it tells me my middle tolerance of the Major diameter, minor diameter and therefore I can work out my radial depth, threads always come out exactly where I need them.

As for the stub ACME I seem to get different data wherever I look. So if I look on the Sandvik website no data correlates to this depth, but if I download the model then this dimension matches. I go on the ISCAR website and no data correlates, but I download the model and it's very similar to the Sandvik, 0.053". If I look around on the internet I find the radial depth should be 0.3 x pitch, but there are other variations of 0.375 x pitch and 0.25 x pitch, which gets me 0.0375", 0.0469" or 0.0313. So none match. If I used an online calculator I get a different value of 0.042", again it doesn't match.

So where am I going wrong with stub ACME calculations?

Here are the numbers I get for a 3.000-8TPI thread
Me measuring inserts and models (tolerances based on ASME B1.8 - 1988) 5/8-8TPI
External, 3.000/2.994 major 2.888/2.882 minor, 0.056 thread height
Internal, 3.020/3.035 major, 2.908/2.923 minor, 0.056 thread height

Online Calculator
External 3.000/2.994 major, 2.865/2.886 minor, 0.042 thread height
Internal 3.020/3.041 major, 2.906/2.913 minor, 0.042 thread height

Now if I go with the middle tolerances of my values I get a thread engagement depth of 0.042" (radial) which equates to the thread depth on the above calculator.

0.0475" thread height from this source.

Then I finally find a PDF for the ASME B1.5 stub ACME tolerances,
That says for a regular ACME that the thread height is (0.5 x pitch) x thread allowance. (0.01" for under 10TPI and 0.02" for over)
Appling that to my stub ACME, (0.3 x pitch) x (0.01 x 1.5) = 0.0525
So that comes out exactly what the ISCAR model states, but not the Sandvik (0.003") difference, but still very close.

So the question is, did I just solve my rambling at the end, but also, shouldn't these inserts be the same?

4GSR

Diamond
The root width of all Acme and Stub Acme EXTERNAL threads will vary with the OD and pitch of the thread. You may or may not be able to use a standard full topping insert for external threads without either side stepping your insert/tooling or buying inserts with a specific nose/root width.
Places I've worked at, we would stock three different root width inserts for cutting 8-pitch Stub Acme external threads. These three would generally get you a thread within the proper PD tolerance. You do not have this issue with internal threads, one internal insert will do most diameter threads for a given pitch.

You must be doing parts for a oilfield equipment manufacture? We use just 8-pitch SA threads on just about everything we design. 6-pitch SA is next one we use also. Ken

edproject

Plastic
Thanks Ken!

I'm curious, why would the OD root width change with the diameter?
I am doing some oilfield stuff, 8SA seems to be fairly common and that is what I have chosen to go with, the customer is open to design. I have just finished the parts using standard SA inserts from Sandvik, and the parts fit together great. I have used these pairs of inserts now from 1 inch diameter up to 4 inch diameter, and I don't see any difference between them.

4GSR

Diamond
Thanks Ken!

I'm curious, why would the OD root width change with the diameter?
...........
All I can tell you is to study the ANSI/ASME Standard on Acme threads. The formulas they use vary the root width according to the diameter. BUT they keep the nominal depth constant. There's nothing wrong with keeping the root width the same and making the thread deeper in the external thread. It's not proper but is done by many that don't understand Acme threads.
Sounds like you are threading the external thread to .0475" dept and root width to .0476". If you are, you're doing it wrong. You also have to figure in the thread class you are using, rather it's a class 2G, which is common on oilfield parts, or one of the other two dozen tolerance classes that could be used. Actually, there are six different classes of fits for Acme and Stub Acme threads.
It's the pitch diameter that has an effect on root width based on thread class used.
There's a lot to learn about these threads and I've forgotten some of the stuff over the years. I have a Excel program that calculates out all of the thread parts that I use and put on drawings I make with my real job! Ken

Cole2534

Diamond
Why stub vs full profile acme?

Philabuster

Diamond
Why stub vs full profile acme?
Stub ACME is usually called out when the thread is on a pipe whereas the regular ACME thread root diameter will make the wall too thin for strength.

4GSR

Diamond
Many of the parts I design have very small cross sections to them not allowing full depth Acme threads be used and the parts are sometimes highly stressed. Some sections I have used 10-pitch and 12-pitch Stub Acme threads on just to get a threaded joint made. No, vee threads will not work in their place. Most of these connections have to break out and make up many times in their life. Vee threads don't have a chance. Ken

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