# how to spec threads?



## wyoguy (Nov 24, 2008)

Hi,
Long time lurker - first time poster...
I'd like to have a local machine shop make me some custom bodies and I have a question about how to spec the threads - hopefully someone here can help so I don't look like an idiot when I walk into the shop.

Where the body and the tailcap thread together, lets say the threads on the body want to be 0.92"-20 (these are internal)

Then the tailcap threads that mate to the body (external) probably need to be slightly smaller, but how much? Or do those typically get spec'd as exactly the same 0.92"-20?

Is there anything else I will want to tell the machinist?

Thanks for any advice.


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## precisionworks (Nov 24, 2008)

In North America, threads are called out by fractional size + threads per inch ... like 1/4 x 20. The following chart shows both UNC (Coarse) & UNF (Fine) threads:







Both external & internal threads use the same size designation.

Metric threads are called out by MM + TPI, similar to the table above. There are also classes of thread fit, depending on how closely you want the male & female threads to fit.


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## gadget_lover (Nov 24, 2008)

The depth of the threads is a determined by a geometric function based on the number of threads per inch. You don't have to specify much more than that as far as the threads go.

More important to you might be the clearances you need for whatever goes into the tube. 

You could specify the minor diameter (the crest of the internal threads) as .92 and the thread as 20 tpi. The machinist will then calculate the clearance and sizes for the mating male thread.

Daniel


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## Anglepoise (Nov 25, 2008)

When a factory is turning out thousands of threaded parts, they check the threads with go/no go gauges. These are simply a master thread in male or female configuration. If possible always give the machine shop an example of the item that will be 'screwed' to the part they are making. This then becomes their gauge. If strictly working off of plans, you now have a tolerance factor that might or might not cause fit problems later.


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