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.

[etfrench]

Will a 3d printed coupling withstand the water pressure?

[jmbones]

I am not sure but I do not think there is much pressure, it is just a drain cap at the bottom of the filter housing. The filter itself usually has about 18psi normally.

[etfrench]

The easy way to create each of the threads is to find a model of each online, then combine those with your model.  If this doesn't work for you, then there is a method to create custom threads (helixes) on any arbitrary shape.

[jmbones]

Would it need to be some type of 3D or CAD model? I found an image of the thread:

[etfrench]

Do these fire hose connectors have your threads?

[jmbones]

I don't think so, NPT is not NPSM.

[jmbones]

I can't seem to find any models of it but I did find some measurements, however I am not skilled enough yet to create the coils in Fusion 360. The existing threaded cap looks to be 1.5" inside the threads. So on the below table I'm guessing it would be 1 1/2" x 11 1/2". Is anyone able to draw a small pipe with those threads on the inside using the measurements I provided? It would need to be about 2" tall.

[jmbones]

I am still working on this. I have found that by simply editing an existing or creating a new thread.xml file in the Fusion360 file structure, I can add any custom thread to the "Thread" menu in F360 as long as the variables are known. However since I am not an engineer, I do not know if I am inputting the correct variables for the thread into the XML file. Anyone care to give it a go?

Straight pipe thread variables are here: https://www.engineersedge.com/fluid_flow/straight_pipe_threads_13376.htm

I am now able to create a pipe with the selected internal straight pipe thread size (1.5") but it does not look like the thread on the cap that I have. Wondering if I am not putting the correct values into the XML file. Attached is a picture of the thread I am trying to duplicate.

My XML code:

<?xml version="1.0" encoding="UTF-8"?>
<ThreadType>
  <Name>NPSM Pipe Threads</Name>
  <CustomName>NPSM Pipe Threads</CustomName>
  <Unit>in</Unit>
  <Angle>60</Angle>
  <SortOrder>9</SortOrder>
  <ThreadSize>
    <Size>1</Size>
    <Designation>
      <ThreadDesignation>1x11.5</ThreadDesignation>
      <CTD>1x11.5 </CTD>
      <TPI>11.5</TPI>
      <Thread>
        <Gender>external</Gender>
        <Class>A</Class>
        <MajorDia>1.293</MajorDia>
        <PitchDia>1.2311</PitchDia>
        <MinorDia>1.281</MinorDia>
      </Thread>
      <Thread>
        <Gender>internal</Gender>
        <Class>A</Class>
       <MajorDia>1.211</MajorDia>
        <PitchDia>1.2462</PitchDia>
        <MinorDia>1.201</MinorDia>
        <TapDrill>1.210</TapDrill>
      </Thread>
    </Designation>
  </ThreadSize>
<ThreadSize>
    <Size>1.5</Size>
    <Designation>
      <ThreadDesignation>1.5x11.5</ThreadDesignation>
      <CTD>1.5x11.5 </CTD>
      <TPI>11.5</TPI>
      <Thread>
        <Gender>external</Gender>
        <Class>A</Class>
        <MajorDia>1.877</MajorDia>
        <PitchDia>1.8144</PitchDia>
        <MinorDia>1.865</MinorDia>
      </Thread>
      <Thread>
        <Gender>internal</Gender>
        <Class>A</Class>
        <MajorDia>1.794</MajorDia>
        <PitchDia>1.8302</PitchDia>
        <MinorDia>1.785</MinorDia>
        <TapDrill>1.794</TapDrill>
      </Thread>
    </Designation>
  </ThreadSize>
</ThreadType>

[MRWakefield]

I think your thread is a 1.25" NPSM as this seems to match up with the 1.5" Minor dia you've measured on your cap.

I've taken the mean averages of the Max and Min values for Major dia (for external threads), Minor dia (for internal threads) and Pitch dias from Machinery's Handbook (which seems to be where your Engineers Edge data came from) and added a bit of clearance to get the values for External Minor and Internal Major. I don't know whether I should've taken the mean or gone with the max or min to be honest. Anyway, they might not be strictly correct but they should be close enough. I've changed some of your original values as they weren't quite right (like a minor dia value larger than the pitch dia on an external thread - I know, it can be very confusing!).

There are obviously a lot more sizes that aren't in this file but perhaps someone who actually uses these threads might like to fill out the remaining sizes and maybe correct what's already there if it's not right.

I've attached the xml file but I haven't tested it in Fusion. You'll probably get to test it before I do so let us know how it goes.

[jmbones]

Thank you MRWakefield! I will give it a shot. The filter manufacturer actually gave me the thread as 1" x 11 1/2 NPSM with higbee start. But me not being an engineer kept thinking how could it be 1" when nothing about the existing drain cap or male bung on my filter is 1".

[jmbones]

So I quickly drew up a pipe using your internal 1.25" NPSM thread option. Just from looking at the threads that were modeled it looks pretty darn close, at least for the thread shape and style. So I printed it out. It is just slightly too small to thread onto the existing male bung on the filter. The finished size ended up being ~38.90mm while the existing cap is about ~40.50mm. Could it be that the printing process of my printer isn't exact or is it actually 1.5" ? Did you change the specs in the XML file for the 1" and 1.5" as well? I will try printing the 1.5" and see how that measures up.

[MRWakefield]

Yes sorry, I had a thought after I'd posted that if you're 3D printing this you might want to increase the Major, Pitch and Minor dias as I thought it might come out a bit tight. Obviously I don't know how tight but you could try adding about 8 to 10 thou.  Rather than changing the values you could add another size designation especially for 3D printing.

I'm certain 1.25" is the size you want as the 1.5" thread diameter is nearly 1/4" bigger and you could probably throw it on from 100yds away! (well, maybe not 100yds 😉 )

Yes, I think I changed all of the values, I certainly changed some. I knocked up a spreadsheet to calculate all of the values from 1/8" to 6" but I'm a bit busy with other stuff to be entering all that data as XML (especially if it isn't quite right!). I thought about writing an application to spit out a F360 compatible XML file but again I'm a bit busy to be doing that and it might never get used again.

I've attached the Excel spreadsheet in case someone fancies checking/amending the values and adding the remaining sizes to the XML.

[jmbones]

Thanks again! I will fiddle with it a bit until it fits as a printed piece.

[jmbones]

Something has gone wrong with my Fusion360 app. I have been adjusting the diameters in the XML file as you described and slowly working my way larger. All of a sudden when I choose the NPSM option from the F360 thread options box, it puts the thread on the outside as the attached picture shows. I reverted back to the original XML file you sent which worked with no issues and it still does it. It does not do it with any other thread selection.

Any ideas what's going on? Did I mistakenly tick a checkbox somewhere that I am missing? 

[MRWakefield]

My first guess is that the Minor and/or Pitch dia values are bigger than the Major dia value. Make sure that the Minor dia. is smaller than the Pitch dia. which should in turn be smaller than the Major dia. When tweaking the values make sure you add the same amount to each (Minor, Pitch and Major) so that the geometry of the thread remains constant. This is all I can suggest without seeing your current values.

I'm sure we'll get there in the end 😉

[jmbones]

Thanks for the quick response, I really appreciate the assistance. I deleted the NPSM XML file that I had been editing and replaced it with the original unedited (by me) version that you supplied which had been working with no issues and it still produced the same issue. Does F360 cache some of the XML settings somewhere? I restarted the app and the PC but get the same results.

[MRWakefield]

I wouldn't have thought that Fusion caches it anywhere but who knows! I have no experience with this apart from hopefully helping you out with your predicament Perhaps someone who has more experience with custom threads might be able to help. If I get time I'll give it a go but you said it has worked just fine (until it didn't!).

[MRWakefield]

Here's a new XML file with an extra 'Class' I've called 3DP on the 1.25" size. This has a 0.01" larger female thread and 0.01" smaller male thread. I've tested it and it looks ok to me but you'll need to test an actual print.

[jmbones]

Thanks, I'll give it a shot. I have found a work around, it seems if I open one of the files that I have already created, it will allow me to adjust the existing thread without the issue. Then I go back and edit the XML file with another size bigger, and change it again in F360. Still haven't got the right size, I am up to:

Major: 1.676

Pitch: 1.6273

Minor: 1.5905

(I didn't physically test this one yet and it just finished).

Incidentally, the threads in the cap I have seem to look "larger", maybe more pronounced vs. the threads being printed so far.