# How to mill NPT thread using a threadmill



## koenbro (Mar 3, 2022)

I am making a manifold for compressed air, and need advice on how to thread mill NPT threads, specifically 1/4" - 18. I looked up the thread and the small diameter is 0.4774", the large dia is 0.4916", and the length of useful thread is 0.4018". A drill size of 7/16" is recommended.

This is what the manifold will look like:







The hole on the front left will punch through and there will be one inlet on top, and three outlets on the front, right, and left walls.  These are the 4 holes that I want to tap NPT 1/4" - 18 to attach air fittings. The two tabs will be used to attach to 8020 1010 profile.

I have done thread milling for 1/4" - 20 holes and liked it a lot more than tapping. I have a Tormach #34694 threadmill (for 1/4" and M6 threads) and the Tormach #34696 (for 3/8, 7/16, 1/2, M10, M12, M14 threads), which is what I plan to use.

How do I drill the tapered starter hole?  Should I use a boring bar and drill a hole exactly to teh small diamater?  Does anyone have any experience to share on how to do this type of thread with a threadmill?


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## davidpbest (Mar 4, 2022)

Fusion 360 has support for NPT thread milling tool paths now.


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## mmcmdl (Mar 4, 2022)

Pipe tap reamer .


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## mmcmdl (Mar 4, 2022)

koenbro said:


> How do I drill the tapered starter hole? Should I use a boring bar and drill a hole exactly to teh small diamater? Does anyone have any experience to share on how to do this type of thread with a threadmill?


For a 1/4" NPT I would just drill and tap it with the machine and not thread mill it . We used thread mills when the machines didn't have the torque to drive the larger taps .


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## Jason812 (Mar 4, 2022)

Drill, ream, tap or get a NPT thread mill.   The NPT thread mills have the taper and you program them the same as a normal treadmill.  2 passes split 75%/25%.


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## koenbro (Mar 4, 2022)

What size (diameter) drill should I use?


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## Jason812 (Mar 4, 2022)

Every thread mill I ever used I would use the recommended drill off a drill/tap chart.  I was fortunate to work for a large manufacturer and I didn't have to use a single pointed thread mill.

These were all on large CNC machines.  The only NPT I ever pre reamed the taper for was a 1/16.  It was for an orifice in the bottom of a hole and I had to use a long tap.  I'm not sure how a smaller "hobby" machine would handle cutting the taper.

What material are you using?  If aluminum, personally, I would just use a tap in a non production environment.


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## mksj (Mar 4, 2022)

I made an air manifold for my mill coolant system out of aluminum, drilled the internal connection holes 3/8", used a straight drill and then 1/4" NPT tap. Chart says to use a 7/16" drill if not using a taper reamer. Done similar tapping in steel, brass and PVC.





						Tap Chart NPT National Pipe Taper Threads
					

Tap Chart NPT National Pipe Taper Threads



					www.carbidedepot.com


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## mmcmdl (Mar 4, 2022)

I've made quite a few of these manifolds in the past for the work benches . Nothing special , but it's nice having the extra outlets when needed .


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## koenbro (Mar 5, 2022)

This is what the finished item looks like. Very pleased with the dimensional accuracy. Have used the mill for various small jobs like drilling, squaring ends, and chamfering but  this is my first CNC project.





The mill is, for now, trammed quite well. This photo shows (barely) the fly cutter patterns crossing. I know it will get out of tram from usage, but I lucked out for now.














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## koenbro (Mar 5, 2022)

I don't have a 7/16" drill or a NPT tap. So my workaorund was to spot drill, then 1/4" endmill, then 7/16" endmill, and then a boring head/ boring bar. I targeted 0.4774" but oveshot by 10 thou as this was my time using the boring head, and wasn't sure how to adjust the wormgear. Then threadmilled.  


















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## Firebrick43 (Mar 5, 2022)

Something is wrong with those, I "assume" the hole is drilled oversize.  NPT depends on thread crest to seal and those have quite the "flat top" on them which will be a leak path.


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## koenbro (Mar 5, 2022)

Yeah, I haven't tested them yet, but have no experience with thread milling and there are a few adjustments that look critical. Will let you know how thhey work.


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## aliva (Mar 5, 2022)

As Firebrick43 said those threads are flat, that's not likely  seal. Pipe threads rely on a good crest on the threads to tighten into the taper .  The taper creates the seal.


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## koenbro (Mar 5, 2022)

Yes the threads look funny and there is indeed an airleak. I think I programmed the threadmilling cycle wrong. Will try again and leave the hole at 7/16"  or 0.4375" (instead of enlarging it to 0.4774). What PDO (pitch diameter offset do you recommend)?


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## bretthl (Mar 6, 2022)

NPT threads also rely on pipe dope to seal properly.  The only threaded connections that seal without pipe dope are metal to metal connections such as PH-6, Vam Top, etc.  API LTC and STC are similar to NPT (tapered V thread) and rely on pipe dope to seal.


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## WobblyHand (Mar 6, 2022)

bretthl said:


> NPT threads also rely on pipe dope to seal properly.  The only threaded connections that seal without pipe dope are metal to metal connections such as PH-6, Vam Top, etc.  API LTC and STC are similar to NPT (tapered V thread) and rely on pipe dope to seal.


What you say is true.  However, no amount of pipe dope will make the OP's threads seal.  They are way off.

I had an HF pipe threader, whose dies were wrong, they cut too much metal.  No matter what I did, teflon, pipe dope, those threads wouldn't hold 100 psi, nevermind 200.  Yes, they threaded in easy, too easy.  My friends at HF didn't execute the NPT taper correctly.  Replaced the dies with a Ridgid, and only a tiny amount of pipe dope was required to seal.  The thread dimensions matter a lot for NPT.


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## Jason812 (Mar 6, 2022)

bretthl said:


> NPT threads also rely on pipe dope to seal properly.  The only threaded connections that seal without pipe dope are metal to metal connections such as PH-6, Vam Top, etc.  API LTC and STC are similar to NPT (tapered V thread) and rely on pipe dope to seal.


NPTF will seal without dope or tape.


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## ropedrag (Mar 6, 2022)

NYC CNC has great youtube content for thread milling and yes NPT as well. Watched their video and went to work, nailed 10 1/4 NPT in one
part no issues. It's black arts!


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## koenbro (Apr 14, 2022)

Thank you for the recommendation to the videos by NYC CNC. I followed their method of making a conical bore and then tapping it and it works great. Still used Teflon tape and have no perceptible air leak. 












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## Winegrower (Apr 14, 2022)

Just FYI, I have been making some 1/4” npt brass parts for a high vacuum system…the female threads are very easy to make with a tap, I use a YG1 nptf tap and it’s very impressive.   For the male fitting threads I use the Geometric Die Head with 1/4” npt chasers.   I leave a  stub about 0.7“ long and 0.54” diameter and the Geometric chasers cut a beautiful male thread.  The tap does a similar job on the female.


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## FOMOGO (Apr 14, 2022)

Pipe dope really acts more like a lubricant on pipe threads to allow proper tightening. It does have some sealing component, but pipe threads are designed as a metal to metal seal. Have run thousands of black pipe connections. Mike


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## Firebrick43 (Apr 14, 2022)

FOMOGO said:


> Pipe dope really acts more like a lubricant on pipe threads to allow proper tightening. It does have some sealing component, but pipe threads are designed as a metal to metal seal. Have run thousands of black pipe connections. Mike


I concur.  Teflon tape is the same, more of a lubricant to prevent galling and get the thread to tighten up enough.  Only a very small amount is used to block the leak path.  See a rehash on the old hemp string/paste idea with locktite 55


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## AlanB (Apr 14, 2022)

There are many sources of engineering data on tapered threads that indicate NPT threads do not inherently seal while NPTF threads do seal mechanically, the design is slightly different. NPT threads require sealant to avoid leakage while NPTF do not. I know I've had cases where making things tighter did not stop leaks while reassembly with sealant did.


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## Jackle1312 (Apr 14, 2022)

You can also wrap a string around the threads then apply some sealant. The string compresses as the fitting is installed and helps seal.


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