Reference for screw threads with tolerances?

[David_M]

In a world where PTC refuses to provide us with an integrated screw thread library, I need a decent reference for standard internal and external screw threads, with tolerances. I expect there are some standard rules for the tolerances, but all of the resources I've found so far omit this information entirely. Right now I'm interested in a 1/2" UNC thread, but I'm looking for a resource that covers the most common thread types, so that I don't have to find another one in the future.

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[esorensen]

Machinery's Handbook

Threads and Threading Section

[BenLoosli]

Go to the source: ASME B1.1 and use the latest revision.

[BillChapman]

Both source mentioned will have this information, but basically there are 3 standard tolerance classes 1,2,3.  There are several dimensions that are controlled to obtain accurate threads. 

Class one is the loosest tolerance and class 3 is the most precise.  If you do not have access to the above sources the manufactures of GO / NO GO gages to inspect the three above mentioned thread classes will show most of these specs.

[Chris3]

"Common threads" varies from industry to industry. That being said though I am willing to bet the most common US fastener is a 1/4-20 Class 2 thread.

Some MIL specs standardize on UNC for #8 and below and UNF for #10 and higher. As a result folks that use these MIL specs have adopted those standards. It is possible to have different class threads too. You can have say a class 3 female and a class 2 male. There are also UNJ threads which are used aerospace.

Any of these factors will change the fit of your joint.