NPT threads

[reinux]

Any chance we can get NPT threads? Thanks!

[NulAsh]

I'm working on it:

use <thread_profile.scad>
eps=0.01;
$fn=360;
NPT2_profile=[
    [-0.0115996249670774927, -1.0430672674082538],
    [-0.077989141850213598, 1.0813972728521016],
    [1.67577597265425, 0.06886051189757472],
    [1.68000348111203267, -0.06641975875147245]
];
intersection() {
    cylinder(h=100, d=60.325);
    union() {
        cylinder(h=120, d1=55.8673, d2=55.8673+120*0.0625);
        translate([0,0,-2.208695652173913]) tapered_thread(section_profile = NPT2_profile, pitch = 2.208695652173913, turns = 40, r=55.8673/2-2.208695652173913*0.03125, higbee_arc=0, fn=360, left=false, taper=0.0625);
    }
}

This code uses thread_profile.scad from my fork of threadlib. This profile and pitch is for 1" to 2" NPT (11.5 TPI). Soon I will calculate for other sizes.

[NulAsh]

The following is all for external threads.
For TPI=27:

P=0.94074074074074074;
profile=[
    [-0.004940581004495969, -0.44426939167388586],
    [-0.03321759745472061, 0.46059513473330253],
    [0.7137564327971805, 0.02932947728970775],
    [0.71555703825142132, -0.028289897245997524]
];

For TPI=18:

P=1.4111111111111111;
profile=[
    [-0.0074108715067439537, -0.6664040875108288],
    [-0.04982639618208091, 0.6908927020999538],
    [1.0706346491957708, 0.043994215934561628],
    [1.073335557377132, -0.042434845868996286]
];

For TPI=14:

P=1.8142857142857143;
profile=[
    [-0.0095282633658136547, -0.8568052553710656],
    [-0.06406250937696117, 0.8882906169856549],
    [1.3765302632517053, 0.05656399191586495],
    [1.38000285948488398, -0.05455908754585237]
];

For TPI=11.5:

P=2.208695652173913;
profile=[
    [-0.0115996249670774927, -1.0430672674082538],
    [-0.077989141850213598, 1.0813972728521016],
    [1.67577597265425, 0.06886051189757472],
    [1.68000348111203267, -0.06641975875147245]
];

For TPI=8:

P=3.175;
profile=[
    [-0.016674460890173896, -1.4994091968993648],
    [-0.112109391409682047, 1.554508579724896],
    [2.4089279606904843, 0.09898698585276366],
    [2.415005004098547, -0.09547840320524164]
];

Nominal pipe size inches	Outer diameter of the pipe mm	TPI	K₀ mm
¹⁄₁₆	7.9375	27	6.1341
⅛	10.287	27	8.47852
¼	13.716	18	10.99566
⅜	17.145	18	14.4145
½	21.336	14	17.81556
¾	26.67	14	23.12924
1	33.401	11.5	29.06014
1¼	42.164	11.5	37.78504
1½	48.26	11.5	43.85564
2	60.325	11.5	55.8673
2½	73.025	8	66.5353
3	88.9	8	82.31124
3½	101.6	8	94.92996
4	114.3	8	107.55122
5	141.3002	8	134.38378
6	168.275	8	161.1884
8	219.075	8	211.6709
10	273.05	8	265.31062
12	323.85	8	315.79312
14	355.6	8	347.34246
16	406.4	8	397.82496
18	457.2	8	448.30746
20	508.0	8	498.78996
24	609.6	8	599.75496

As for the internal threads, I'll work on them later.

[NulAsh]

Looks like I slightly f'd up the profiles - on the drawing in the standard the narrow end of the pipe is upside, so I calculated all the points with this assumption, but I'm starting with the narrow end downside. So the profile for TPI=11.5 should probably look like that:

NPT2_profile=[
    [-0.07798914185021359, -1.0813972728521016],
    [-0.0115996249670774927, 1.0430672674082538],
    [1.6800034811120326, 0.06641975875147245],
    [1.67577597265425, -0.06886051189757472]
];

[NulAsh]

Yes, just turning it upside down should work. Here's the updated profiles:
For TPI=27:

profile=[
    [-0.03321759745472061, -0.46059513473330253],
    [-0.004940581004495969, 0.44426939167388586],
    [0.71555703825142132, 0.028289897245997524],
    [0.7137564327971805, -0.02932947728970775]
];

For TPI=18:

profile=[
    [-0.04982639618208091, -0.6908927020999538],
    [-0.0074108715067439537, 0.6664040875108288],
    [1.073335557377132, 0.042434845868996286],
    [1.0706346491957708, -0.043994215934561628]
];

For TPI=14:

profile=[
    [-0.06406250937696117, -0.8882906169856549],
    [-0.0095282633658136547, 0.8568052553710656],
    [1.38000285948488398, 0.05455908754585237],
    [1.3765302632517053, -0.05656399191586495]
];

For TPI=11.5:

profile=[
    [-0.077989141850213598, -1.0813972728521016],
    [-0.0115996249670774927, 1.0430672674082538],
    [1.68000348111203267, 0.06641975875147245],
    [1.67577597265425, -0.06886051189757472]
];

For TPI=8:

profile=[
    [-0.112109391409682047, -1.554508579724896],
    [-0.016674460890173896, 1.4994091968993648],
    [2.415005004098547, 0.09547840320524164],
    [2.4089279606904843, -0.09898698585276366]
];

Everything else should be the same.

[myearwood1]

@NulAsh, I'm not clear what you're doing here. Am I to add these bits of code (profiles) somewhere? Don't I just download your fork and done?

[adrianschlatter]

According to Wikipedia, National pipe threads are conic. Supporting these will need much more than just a new profile definition.

[myearwood1]

Hmm. I was not aware of that. An extra flag for type might work?

…

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[NulAsh]

@adrianschlatter , have you checked my code? I made a fork of threadlib, added thread_profile.scad to it with my modification to allow it to make conic threads.

[adrianschlatter]

@NulAsh, I missed that. I see you found a quite elegant solution to add the conical tapering. I have no idea how conical threads are specified. Do you have links to (excerpts of) norms so I can have a look at that? I see you have already found a table with thread specs. Do you also have the pictures and explanations related to that?

[NulAsh]

@adrianschlatter https://files.catbox.moe/0z456a.pdf I took this and did some trigonometry.

[adrianschlatter]

@NulAsh: That is a fantastic source :-) I guess the way to extend threadlib by conic threads, NPT in particular, would be approximately like this:

• obtain a license from @MisterHW to include thread_profile.scad in threadlib directly. Preferably he would commit it personally so that he visibly keeps the credit for his own work.
• include your tapered_thread() module (or merge it into the straight one with an added argument)
• go the standard route of creating thread specs for threadlib (tabulate specs, write convertor to OpenSCAd, add unittest)