Thread Class Compatibility for 3/4" UNF

[johnchrc]

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.

- CJ

 

[Tmoose]

So individual nozzles resist being installed ?

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/images/products/2014/06/pitch-diameter-allowances.jpg

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?

 

[johnchrc]

It was the coating combined with 3A class thread. Remove phosphate and nozzle threads on just fine. We have years worth of nozzles so I may have them coat the housings then tap the threads and apply kendex or some rust inhibitor that can be easily removed with solvent until we make new nozzles with class 2A fit. Thanks.

- CJ

 

[tbuelna]

One issue I had with manganese phosphate coating of screw threads was the process spec required a grit blast of the surfaces prior to phosphating, and a salt spray test after phosphating. This seemed to leave some residue on the thread surfaces that created a bit of interference. But after giving the threads a thorough scrubbing with a brush and solvent, and applying some oil, they fit together very nicely.

 

[Gilmiril]

With the UN thread series, the thread allowance is applied to external threads only, so an internal thread (regardless of class) has no allowance. For a 2A/2B assembly, if only one of the mating threads is coated, you can use the 2A thread allowance as a coating allowance because so long as 4*Tmax is less than the allowance, you shouldn't be tighter than a Class 3 fit (i.e. worst case is size-on-size). Note that the 4*Tmax is due to geometry of the 60-degree thread form; 2*Tmax/cos(60) is the maximum change in diameter on the thread flanks. At the thread crests and roots, it's only 2*Tmax, which is why if there's going to be a problem, it will be at the pitch diameter (and not the major or minor diameters).

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...