SCT thread specs

[nikolav]

Hey there

For various DIY projects I have in mind I want to design/create my own SCT adapters and etc.

To start with I have very very limited experience in CAD software, but I'm trying to learn...

So far I was able learn how to do it in Fusion 360, but what I miss are the specs of the thread (at least). All I could find online is that it is 2", 24 TPI which tells me it is 2 inches wide and has 24 threads per inch. Nothing about the internal or external diameter.

I can tinker around the following variables below. These values are mine and nowhere near the real life sizes. The only correct one I believe is the TPI value. And I guess they should be different for male and female threads.

    <Size>2</Size>
    <Designation>
      <ThreadDesignation>1-24</ThreadDesignation>
      <CTD>1-24</CTD>
      <TPI>24.0</TPI>
      <Thread>
        <Gender>internal</Gender>
        <Class>2B</Class>
        <MajorDia>2.072</MajorDia>
        <PitchDia>2.0598</PitchDia>
        <MinorDia>2.0475</MinorDia>

Can someone knowledgeable shed some light here and help me fill the gaps?

[BlondeBimbo]

I understand that it's a standard uns thread form, so 2" would be the major dia, and 24 the tpi. (So the inverse is the pitch.) H can be calculated from the pitch as sqrt(3)/2 x P (0.866P roughly) the radius of the thread trough is not shown on the diagram attached but is 0.1443P

Although if you are generating it in fusion 360 you can just insert the thread from the nominal and the tpi, and fusion will generate the correct form on either external or internal threads.

You can also change the tolerance - being a uns type thread it ought to use the A/B tolerance scheme, so likely class 3A/B, ( you set it as 2) although more popular now is using the L/G/H system - typically the external thread is size (G) and the internal is oversize say 4H

 

Your diameters are way out btw

Edited September 5, 2019 by BlondeBimbo
Error

[BlondeBimbo]

Oops,  just note the H for the thread depth in the diagram ( which you can calculate) is not the same H as the tolerance H - which is just the reference indication that the 4 applies to the hole - internal thread tolerance making the hole 0.0015"-0.0020" larger than size.

(4L would apply the same 0.0015"-0.0020" smaller than size on an external thread)

 

Only noticed the confusion once I posted!

 

Hence Blondebimbo! 

Edited September 5, 2019 by BlondeBimbo
Spelling!

[nikolav]

Thanks mate!

I'll have a play with it as well.

Before I read this I took more scientific approach.  I used this calculator click. As I wasn't very sure how exactly this works I got an SCT design in STL format that was confirmed working - link. "Sliced" in Tinkercad and measured everything manually. After lots of trial and errors tinkering the xml numbers I got it visually pretty close to my "template"

I ended up with these numbers from the calculator:

  <ThreadSize>
    <Size>2</Size>
    <Designation>
      <ThreadDesignation>1-24</ThreadDesignation>
      <CTD>1-24</CTD>
      <TPI>24.0</TPI>
      <Thread>
        <Gender>internal</Gender>
        <Class>3a</Class>
        <MajorDia>2.0105</MajorDia>
        <PitchDia>1.9774</PitchDia>
        <MinorDia>1.9619</MinorDia>
      </Thread>
    </Designation>
  </ThreadSize>
  <ThreadSize>
    <Size>1.99</Size>
    <Designation>
      <ThreadDesignation>1-24</ThreadDesignation>
      <CTD>1-24</CTD>
      <TPI>24.0</TPI>
      <Thread>
        <Gender>external</Gender>
        <Class>3b</Class>
        <MajorDia>2.0000</MajorDia>
        <PitchDia>1.9729</PitchDia>
        <MinorDia>1.9504</MinorDia>
      </Thread>
    </Designation>
  </ThreadSize>

I'll have a test 3D print tonight to check how that has worked.

Cheers!

[BlondeBimbo]

Those figures look better, I did a quick manual calculation

P = 1/24 =0.041666666";

H = 0.866P = 0.03608333"

From the diagram,  the pitch diameter reduction from major is H3/8 = 0.01353125" (each side) so twice this for diameter 0.0270625" so if we subtract this from the major dia of 2" we will get 1.9729375" - this would be the nominal or size of pitch diameter - for both ext and int threads, to add tolerance since you went for 3a (0.00436") this would give a range of pitch dia (ext) of 1.97294" - 1.96858" - appears to line up with your figures now.

BTW external thread should be 3a, internal will be 3b - you have them the wrong way round 

 

With a 3a thread, it's dimensions should be within the range 1.97294" to 1.96858" - that is anywhere from nominal size down to 0.00436" below it.

With a 3b thread, it's dimensions should be within the range 1.97294" to 1.97861" - that is anywhere from nominal size up to 0.00567" above it.

So for your modelling,  pick suitable figures within these ranges - as you will be specifying just the one set of dimensions rather than a range.

One thing that appears to be missing in your thread specification is the thread angle, with UN threads it should be 60 degree, unfortunately there are others ( 55 degree,  47.5 degree and so on) so I am not sure which your software would pick by default. 

Oh, and I don't want to appear nitpicking but the thread designation 1-24 you have stated means a 1" diameter thread of 24tpi  the standard designation for the thread you are specifying is 2-24 (3b)

Edited September 6, 2019 by BlondeBimbo
Noticed 1-24

[BlondeBimbo]

If you don't mind me asking btw, what 3d printer are you using?

 

I have a 10s, and with pla, pteg, or any other filament,  I cannot produce a thread this fine. I assume you are using something other than fdm. 

[nikolav]

I have a pretty standard and pure stock FDM printer - Ender 3, which I believe is just a smaller version of your 10s. I guess every modern FDM printer is very capable of printing even finer threads. Here on Thingiverse - Thingiverse link you can see the SCT coupler I printed with PLA+. Probably the key is the layer height, the nozzle size is irrelevant. The one above is done with 0.16 mm layer height. Yesterday I realized I have a clog, so most probably this one was printed with a slightly clogged nozzle.

I still haven't tried printing with PETG, but I reckon this will be the filament of choice for printing adapters and couplers. For say planetary camera or just for a visual use the PLA should be fine. The DSLR on the picture was just to prove the concept, i'm not trusting the PLA that much

And yes, the thread angle angle is 60 degrees, it is defined in the start of the file for all threads. About the thread designation - I had no idea what this meant so I just left the value from the example. I'll change it and I'll switch the As and Bs as well.

The guy on thingiverse - Link who originally created the thread I used came back to me with the following settings last night:

    <Designation>
      <ThreadDesignation>2-24 UN</ThreadDesignation>
      <CTD>2-24 UN</CTD>
      <TPI>24.0</TPI>
      <Thread>
        <Gender>external</Gender>
        <Class>1A</Class>
        <MajorDia>1.9928</MajorDia>
        <PitchDia>1.9669</PitchDia>
        <MinorDia>1.9410</MinorDia>
      </Thread>
      <Thread>
        <Gender>internal</Gender>
        <Class>1B</Class>
        <MajorDia>2.0087</MajorDia>
        <PitchDia>1.9786</PitchDia>
        <MinorDia>1.9598</MinorDia>
        <TapDrill>1.9598</TapDrill>
      </Thread>
      <Thread>
        <Gender>external</Gender>
        <Class>2A</Class>
        <MajorDia>1.9947</MajorDia>
        <PitchDia>1.9683</PitchDia>
        <MinorDia>1.9428</MinorDia>
      </Thread>
      <Thread>
        <Gender>internal</Gender>
        <Class>2B</Class>
        <MajorDia>2.0068</MajorDia>
        <PitchDia>1.9767</PitchDia>
        <MinorDia>1.9598</MinorDia>
        <TapDrill>1.9598</TapDrill>
      </Thread>
     </Designation>

Now I have long enough list of settings to play with