Newbie help with NPSM threaded adapter design

[jmbones]

Hi, newbie here with zero CAD or 3D design experience, just got Fusion 360. Recently picked up a 3D printer and want to make a threaded adapter piece for my pool filter. Basically custom female NPSM thread (National Standard Free-Fitting Straight Mechanical Pipe Thread) with higbee start on one end and 2" male pipe thread on the other end for connection to sch40 PVC. The thread specs are from the manufacturer of the pool filter and there is currently a female drain cap in place with the following: 1" x 11 1/2 NPSM. I want to replace that with the adapter I 3D print. The finished product would be similar to the attachment. Any and all help is appreciated.

[MRWakefield]

Hmmm, that's quite a lot oversize (0.04"). I'm surprised you need it that much bigger. I have some experience with a Formlabs Form 2 SLA printer but I'm guessing you're printing this on an FDM which I have no experience with but I'm guessing needs more 'adjustment' in the sizes.

[jmbones]

Yes it's a Flashforge Finder unit. I printed out 5 different sizes and will check them tomorrow to see which is the closest. And that is just for diameter wise, the thread shape and size does not look similar to the cap.

[jmbones]

So I tested a few prints, it looks like the closest one to the real cap via inside measurement is with these thread specs:

Maj 1.716

Pitch 1.6673

Min 1.6305

That one measures approx. 40.8 inside thread and the real cap measures around 40.5 however it does not thread on. I'm beginning to think that the thread specs the manufacturer gave me are not correct because the threads still do not appear to match the inside of the cap. I have attached a pic of both. Don't mind the pink print, my kids like pink and I'm too lazy to change the filament.

[MRWakefield]

Yes, after your post last night I started to think that it might not be an NPSM thread after all. Looking at your photo of the two caps I'd say that the pitch of the original is much coarser than the NPSM thread. Are you able to measure the pitch of the original? Even if it's just rough we might be able to identify what it is.

Actually can you measure the following:

• OD of male thread
• ID of female thread
• Pitch of thread

That should give us the best chance of identifying what it is.

[jmbones]

Here's what I came up with:

OD Male: ~44.0mm

ID Female: ~40.6mm

Pitch: ~3.25mm (assuming this is measuring the distance between two peaks of the male thread)

[MRWakefield]

Ok. Yes, that's the correct way to measure thread pitch (as long as it's a single-start thread, which I'm pretty sure it will be ;-)). 3.25mm equates to 7.8 TPI. If this is an inch based thread then it would probably be 8 TPI. If it's a metric thread then it could be 3mm or 3.5mm (I guess it could be 3.25 but it's not as likely).

44mm is a bit less than 0.5mm (20 thou) under 1-3/4" so my guess is that your thread is a 1-3/4" x 8 UN. This is a standard thread that you'll find already included with Fusion. Give this a go and let us know how you get on. Again, you may find it's a bit tight and you might need to increase the Minor dia, Pitch dia and Major dia just a little to make it fit nicely.

Anyway, let us know how it goes.

[jmbones]

I changed the thread to use the default value in F360 for 1.75" x 8 UN, exported as an STL file and imported directly into my 3D printer software to print out. The printed piece threaded right on to the male nipple of the pool filter housing with no issue. So it looks like we got the correct size now. Now I need to design an adapter to go from that thread to 1.5" solvent/glue joint for PVC valve, which shouldn't be too hard.

Thank you very much for your assistance!

[MRWakefield]

That's great. Thanks for letting us know.

You're welcome.

[markaudacity]

I found and am attaching a PDF of ASME B1.20.1, the standards document for NPT/NPS threads.

MRWakefield, my numbers don't agree with yours on the external minor and internal major dimensions, calculating from the ASME thread form dimensions. Your external major/internal minor and external/internal pitch diameter are correct, though.
I'll be uploading a complete XML file with all the NPSM sizes from 1/8" to 6", but I'm in the middle of modeling a part I need to make 600 of by next week so I'm not sure when I'll get to it. 😋

[markaudacity]

I have attached a PDF scan of ASME B1.20.1, the standards document that defines NPT and NPS threads.

MRWakefield, some of your numbers don't agree with the ones I calculated based on the ASME thread profile dimensions. Your external major/internal minor and pitch diameters are correct, but the external minor/internal major numbers are slightly off (at least for a 2A/2B fit).

I'll knock up an XML with the correct dimensions for all the NPS threads from 1/8" to 6", but I'm in the middle of modeling a part that I need to make 600 in the next couple weeks so I can't promise when I'll get to it. 😋

[MRWakefield]

I'm sure you're correct. IIRC all I did was take an existing thread in the Fusion360 XML file and adjusted the values accordingly to get something that worked for the OP. I'm sure yours will be more correct. I've not looked into it extensively but I'm sure I remember noticing that some of the thread data supplied with Fusion360 doesn't agree with the published standards. That might be where the discrepancy has come from.

[Anonymous]

Did you ever create the XML file?

[Anonymous]

Did you ever create the xml file? I am looking for the 1/2" size

[MRWakefield]

No, I haven't done any more work on this since my last post.

[kevin.pintoDK62F]

Another vote for the NPSM file.  I also need the 1/2” size.  Thanks so much in advance!