Thread Class Compatibility for 3/4" UNF
Thread Class Compatibility for 3/4" UNF
(OP)
We have a cylindrical housing with 16 3/4"-16 UNF 2B threaded holes. The housing and port are coated with manganese phosphate. A 3/4"-16 UNF 3A nozzle (pin) thread made with D2 Tool Steel and no coating is called out to make up into the hole. Our Supply Chain Manager says he is having hell trying to make up this threaded connection. I looked into this and believe that there are two problems.
1) the phosphate thickness in the holes and ability to control inside the threaded ports
and
2) the thread class 3A external thread should not make up to a 2B internal thread.
I propose changing the external 3/4-16 UNF Nozzle to a 2A class and leaving the threaded ports in the housing class 2B. Is this the proper solution? Can someone enlighten me about how to check thread class make-up. Do you compare PD, and Major diameters to verify there is no interference? Thanks.
1) the phosphate thickness in the holes and ability to control inside the threaded ports
and
2) the thread class 3A external thread should not make up to a 2B internal thread.
I propose changing the external 3/4-16 UNF Nozzle to a 2A class and leaving the threaded ports in the housing class 2B. Is this the proper solution? Can someone enlighten me about how to check thread class make-up. Do you compare PD, and Major diameters to verify there is no interference? Thanks.
- CJ





RE: Thread Class Compatibility for 3/4" UNF
Do you gage the housing threads after coating?
Do you gage the nozzle external threads to see they are not oversize PD, or some other feature?
"manganese phosphate coatings are typically between .0002 and .0004 inches thick."
http://www.actonmetal.com/phosphate_coatings.html
Sounds like the coating thickness shouldn't be a big deal.
The pitch diameter is the only possible zone of interference in properly made threads.
general relationship of PD tolerances here -
https://www.fastenal.com/content/merch_rules/image...
As you suspect, a 3A external thread //could// end up a line-to-line fit with any internal thread.
What feature is expected to seal the nozzle to the housing?
RE: Thread Class Compatibility for 3/4" UNF
- CJ
RE: Thread Class Compatibility for 3/4" UNF
RE: Thread Class Compatibility for 3/4" UNF
Switching to a 3A nozzle thread means that there is no longer any thread allowance that can be used for coating, so the thicker the coating, the less likelihood of getting parts to fit properly. One alternative solution if you want to stick with the 3A nozzles is to get the holes made oversize with a coating allowance so that they'll be back to standard after coating. But that requires special tooling...