Mech5656

Refer ASME B1.1-Para 5.8. Since your thread size is special one, I think, you can use ASME B1.1 formula to calculate the tolerance.

Thanks. Is there any chart online or some book may have these min and max values?

This is the best I could find online but is not what I need.

https://www.ebay.com/itm/184224844111

I will look into your provided source now. Thanks

Here is that formula:

P=1/12=0.083

Major diameter external tolerance: 0.06 x(0.083x0.083)^1/3

=0.0114”

So this tolerance is plus what and minus what?

So will -+0.005 work? Any feedback?

Quote (AMSE B1.1)

5.7 Disposition of Allowance and Tolerance
The disposition of allowance, tolerance, and crest clearances for the various thread classes is shown in Figures 5 through 8 and 10 and 11.

Quote (Mech5656)

So will -+0.005 work? Any feedback?

I am not an expert in GD&T, but 0.005" looks tight tolerance instead of calculated 0.0114". It will be difficult to achieve, I guess. May be better you can consult with GD&T guy.

The typical outer diameter maximum for any thread is the nominal diameter. Tolerance is how much smaller that diameter can be.

In certain classes of thread there is also a required diameter reduction - class 1 threads for example, start off smaller than the nominal and then there is a tolerance on how much smaller yet is allowed.

I think in class 2 and class 3 threads there is no required offset - the max diameter is the nominal diameter.

NRP99:
Thanks for your response. I meant to ask you about the calculated number for tolerance. To me, tolerance is the total allowable change in a size.

Should it be +0.000/-0.011, +0.011/-0.000 or half and half on each side? Most probably +0.000/-0.011, but curious to know what the standard says. And yes, the calculated number looks higher than what I thought.

By the way, machining and holding tolerance of +0.005/-0.005 is not hard where I work.

Anyway, I ended up putting major diameter (in this case 2.687) +0.000/-0.002 tolerance on my drawing. I believe it is a safe number.

There is a class 4 thread where there is a plus tolerance. It can be anything you like if you specify "MODIFIED" and state the modification. Otherwise thread Major diameter tolerances are 0 or negative.

Mech5656,

If there is not a Machinery's Handbook sitting on your bookshelf, there should be.

It is The Bible.

It will answer your question.

--
JHG

Mech5656
As 3DDave suggested and looking at figures in ASME B1.1 the basic tolerance is +0 to -tolerance value. So basically the nominal size is tolerated to be decreased from nominal size towards decreased diameter. So +0.000/-0.002 is good if achievable, I think. But why do you want to use these values instead of following standard? Did you ensure that the mating internal threads will be having same tolerance?



Looking at the formula you used, you are considering class 2A/3A threads, I think.

And +1 to drawoh. Machinery's Handbook is must for any kind of help in day to day life of Mechanical engineer.

Thank you all for replying to this post. I have to admit that this is turning into a very interesting discussion. This is all fascinating stuff.

drawoh:
The whole point of this post is that this is a rare thread size and is not mentioned in Machinery's Handbook. I am sorry if I was not clear in my original post. That is why I calculated the tolerance of major diameter shown above. Please check your Machinery's Handbook and reply back if you find this thread size. Thanks!


NRP99:
Thank you for clarifying the tolerance limit. The machinist has the locknut (with female threads) that goes against these threads of shaft. He can match the threads so he did not need the class of threads. Usually, we use 2A/2B class of threads. Thanks again!

You can specify that tolerance if you want.. but 2 thou on a thread that large, and asking the machinist to 'match the threads' is making your machinist's life more difficult, and your product cost higher, than it needs to be.

If this is a one-off, maybe no problem.. but from the standpoint of taking the right approach, I wouldn't suggest this process as standard practice. Thread standards exist for a reason, and when followed they are unquestioningly reliable.

Mech5656,

Your thread size is not on the Macinery's Handbook's tables. The Unified National thread specification is described in sufficient detail that you can work out the tolerances.

--
JHG

Oh boy, making threads by fitting to a nut.

How long is the nut?
Who made the nut?

I'd require first piece and job lot inspection of the bolt threads to include a pitch diameter as determined by "over wire" measurements, and pitch and thread form proven with a name brand thread pitch gage with good light.

• https://files.engineering.com/getfile.aspx?folder=02a557b3-0295-4c15-a772-f

Tmoose,

I have called up weird, custom threads, and the machine shops have asked for the mating part so that they could inspect theirs. In productioh, they can always make an inspection gauge.

Of course, people who call up 1.782-31UN threads, do not deserve to go into production.

--
JHG

Make that LH and that thread is perfect.

3DDave,

Thanks. I don't know how I missed that.

--
JHG

Drawoh:

You are correct. After searching in Machinery’s handbook 29th edition, on page 1869, I found the above formula used to calculate tolerance of major diameter. So yes, it was there.



Tmoose:
The nut is 6 inches long and the internal threads are 1.75” deep from one side. It also has set screw holes to go against shaft OD. We made the nut in our shop. They were both (shaft and nut) made together. Thanks for replying.

drawoh - I forgot to add in the multi-start thread option. Considering I have a Monty Python record with three sides, it seems like there must be some similar option - like a single start RH thread and double start LH thread. That way the user has more choice of mating nut.

3DDave,

I have that LP too. I wonder how the CD works.

--
JHG

SwinnyGG:

I agree with you. On my print, I did specify tighter tolerance (+0/-0.002) which was mainly because of the time constraint. I had to make a decision within a day (notice the dates above).

Yes, It was a one time thing and it is lesson learned. Thanks again!

Thanks to all who replied. I learned how to calculate tolerance of major diameter of unified threads. What an amazing platform this is for learning. The threaded nuts were machined and installed on shaft. It is used to lock impeller in place.