Turned a 3/4-14 BSPP thread and measured with wires but it didn't fit.

[Edster]

I made a few fittings with a 3/4-14 BSPP thread. I measured them with wires but the thread still didn't screw into the manifold. I'm not real familiar with BSPP threads, and I'm not sure where I went wrong.

I used Thread Pal to come up with the major (1.0410") and minor (.9496") diameters, entered my thread wire size (.040") and hit the number for measurement over wires (1.0533") within a thou. It was kind strange that there was no minimum or maximum on the Major Diameter or Minor Diameter. Just a field for Gage Plane and Pipe Face but both of those numbers were the same. There was also no "mean" number for the measurement over wires just one number. I checked the machinery's handbook but didn't find much on OD BSPP Threads.

I used a laydown threading insert that was 55 deg. It wasn't a full form and I wonder if that might have been the problem. There is a root and crest radii (.0098"). I don't know what the radius on the tip of the insert is but there has to be one. Not using a full form tool so I don't have any radius on the crest. Could this have been the problem or is there something else I'm overlooking?

Thanks in advance!

 

[sfriedberg]

Yes, that certainly could have been your problem. If the pitch diameter, as indicated over wires, is correct, then either the major diameter is too large, the minor diameter is too large (or both) or the pitch or thread form is wrong. Since you're using a pre-formed insert, we'll presume the pitch, thread form and minor diameter are correct. That leaves your major diameter. Have you established what the actual major diameter is, or did you just cut to the specified pitch diameter?

 

[gregormarwick]

Max. Major 1.0410
Min. Major 1.0298
Max. Minor 0.9495
Min. Minor -
Max PD 0.9952
Min PD 0.9896

Max over 0.040" wires: 1.0533
Mean over 0.040" wires: 1.05049
Min over 0.040" wires: 1.0477

You're on top limit for your sizes, coupled with no crest radii and you may have interference.

 

[Peter from Holland]

Did you enter the 55deg in your Tread pal ??

peter from holland

 

[Edster]

Did you enter the 55deg in your Tread pal ??

peter from holland

Yes it is automatically entered when you select BSPP.

 

[Edster]

Max. Major 1.0410
Min. Major 1.0298
Max. Minor 0.9495
Min. Minor -
Max PD 0.9952
Min PD 0.9896

Max over 0.040" wires: 1.0533
Mean over 0.040" wires: 1.05049
Min over 0.040" wires: 1.0477

You're on top limit for your sizes, coupled with no crest radii and you may have interference.

Can I ask where you found your numbers. Thread Pal only gave me one set and I didn't find anything helpful in machinery's handbook. I'd like to not be bitten in the @ss in the future.

 

[Edster]

Yes, that certainly could have been your problem. If the pitch diameter, as indicated over wires, is correct, then either the major diameter is too large, the minor diameter is too large (or both) or the pitch or thread form is wrong. Since you're using a pre-formed insert, we'll presume the pitch, thread form and minor diameter are correct. That leaves your major diameter. Have you established what the actual major diameter is, or did you just cut to the specified pitch diameter?

I turned the OD to the major diameter. My numbers were the max sizes. I'm going to order a full form insert and cut to the mean numbers now that I know what they are.

 

[gregormarwick]

can i ask where you found your numbers. Thread pal only gave me one set and i didn't find anything helpful in machinery's handbook. I'd like to not be bitten in the @ss in the future.

iso 228-1:2000

 

[Edster]

Thank you.