SA Pin - 2.4- 20 vs.16 - Y can thd 16 but not 20 TPI...Insert?

[johnchrc]

I made a mistake by specifying too fine a pitch thread for a 2.48-20 SA. It can be made but was not practical as well as stupid... Overthinking...

Anyhow, Machine Shop called and asked if I could change 20 TPI to 16 TPI, which , after quick calc check, I agreed.

Machinist tried to explain they carried no insert with width for 20 tpi... down to 16 tpi. Make scence?

He could have made with another insert but would have had to shipt over after successive passed to get correct form. Sounds like trouble.

Was trying to keep pin thread and thread shear as strong as possible but effect is negligible and the parts are a pitch to make -up, clean, protect and prevent from damaging.

Can anyone explain why the available inserts for this thread is 16 tpi or larger for this thread?.

What exactly does the machinist have to do different? Oh, material was Inconel 718 120 ksi MYS. Thread will be peened.

I would love a simple response or diagram/pic/photo. THANKS.

 

[Cockroach]

Okay, difficult to follow. You have a2.480-20 Stub Acme-A thread specified on a piece that cannot be made due to lack of a commercially available thread insert. You find out 16 to 20 TPI is problematic and there is no commercially available insert in this size. Your question is why.

I would think that 16-20 TPI is extremely fine and the width od the root is too narrow to have an insert of sufficient strength. The tool would chip or produce threads of poor quality.

But I think you could cut a multiple-start thread leading to 20 TPI. Why not cut a four or five start thread, assuming you could space it into a 2 1/2 inch nominal piece? For example, 2.480-5 Stub Acme-A-4 Start? Usually larger sized threads are coarse, 8 TPI and lower. This provides for a stronger insert and one commercially available.

I've seen 8 TPI in these sized threads. It is uncommon to go beyond a 3 start thread. Would 2.480-8 Stub Acme-A 3 Start work for you? That would give you an equivalent 24 TPI thread. The 8 TPI insert is strong and have a descent depth thread. Looks like you have sufficient geometry.

 

[johnchrc]

I used a fine pitch shallow thread form to maximize the tensile area across the root. I ended up running thread out so there was no thread relief to reduce tensile area. The Thread shear is > than the tensile through body.

I usually use a 6 tpi SA for this size of part. To be conservative, I use the thread root for the tensile calculation but this isn't correct. How do you actually determine the load required to part the joint in tension? As the thread is a helix, I could cut a plane through the part and look at area through any section. Seems unnecessary.

This question is really about splitting hairs but I would like to know the answer. Could I use a larger pitch thread, with no relief, and achieve the same strength (neglecting thd shear)?

 

[cloa]

If you want some more answers, I suggest making a link thread in the most appropriate subforum- Machine and Machining.

 

[vpl]

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[johnchrc]

cloa,

Probably so,and I appreciate the help, but the question has both design and manufacturing elements. I found the answer to my "manufacturing question" in this forum. The second part is purely a mechanical design question regarding everyday thread strength calculations. With no relief, is the tensile area calculated at the thread root, thd OD or something in between.

 

[Cockroach]

This is indeed the proper forum. There have been a multitude of thread questions asked here, so that is not the issue. It just so happens that knowledge in the machining trade often makes for better designs and understanding of the function. So keep on posting.

I will do a complete analysis on this thread calculation for you. The tensile area is regarded over the full depth of threading, but is only one calculation in the analysis. You also need to compute shear for Box and Pin. Since you cannot make a 16 TPI thread, I will go with 2.480-8 Stub Acme-A-3 Start, something close to your needs.

I note you can put in a thread relief to ease machining at the thread run-out. I also suggest clipping the start(s), this improves assembly. You can model this extremely easily in SolidWorks, this would show you make-ups.

 

[johnchrc]

Cockroach,

I have attached the values I calculated for a 2.48-16 SA 2G Connection with 2" Thread Engagement. Box is 3-1/2 OD and Pin has .875 dia hole thru. I calculated root at last pin thread but think this is conservative because a thread relief isn't used and the helical form means more material actually exists at any give x-section.

See attachment. This is single start thread... Weak point is thread shear for this connection. I'm interested to see what values you have. I do have the part modeled in Pro/E so I can look at sections and model multiple starts.