Community Tip - You can Bookmark boards, posts or articles that you'd like to access again easily! X
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.
Machinery's Handbook
Threads and Threading Section
Go to the source: ASME B1.1 and use the latest revision.
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.
"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.