Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How to machine a NPT internal thread?

  1. Nov 23, 2009 #1
    I want to tap a NPT (NPT-3/4) thread hole on a plate. And I found some related standards, these standards told me the detailed dimensions of the thread. My question is how to machine the NPT thread with a tap? How to control the parameter mentioned in the standards?
    thanks in advance!
  2. jcsd
  3. Nov 23, 2009 #2
    Last edited by a moderator: Apr 24, 2017
  4. Nov 23, 2009 #3
    Make sure you drill the hole to the appropiate size before using that tap. I think it is 59/64 but you better double check me.
  5. Nov 23, 2009 #4
    Last edited by a moderator: Apr 24, 2017
  6. Nov 23, 2009 #5
    thanks.what kind of hole does it need? Do I need to drill a taper hole?
  7. Nov 24, 2009 #6
    Just a regular round hole,just try to make sure you drill perfectly perpendicular(a drill press or mill will work best), the 59/64 measurement should be right about the size of the minimum diameter of the tap, as you screw the tap in it will cut the threads and the taper all at once. A t-handle makes it easiest, but a crescent wrench will work though. Make sure to use plenty of oil, they make some exclusively for cutting threads but I usually just use WD-40. Make sure the tap is absolutely perpendicular to the plate in the hole and twist. As you are cutting you should cut(clockwise) for a turn or so then back it off(counter-clockwise) to break the cut material loose. Take your time and you will have a perfectly tapped 3/4" npt hole when done. If the plate is mild steel it will be the easiest, aluminum is softer so it will cut like butter but it will also be easier to rip out the cut threads ruining the piece, and stainless will work harden very easily if it get hot at all making it a chore. Good luck.
  8. Nov 24, 2009 #7


    User Avatar
    Science Advisor

    Depending on how close you want to hold to those dimensions, you will have to look into standard gauge rings/plugs to make sure you are to spec.
  9. Nov 24, 2009 #8
    I'm not sure what parameter you mean.
    NPT is a tapered pipe thread.
    http://en.wikipedia.org/wiki/National_pipe_thread" [Broken]
    That means it's made to guarantee interference at some depth.
    Usually the pipe will be wrapped with teflon tape to seal it as well.

    You can use a drill press and rotate the chuck manually to get it perpendicular but that's seldom critical with piping.

    You tap the proper sized hole (get the tapping chart somewhere) to a certain depth and try the pipe. If it doesn't penetrate far enough, tap some more until it does. It will stop hard when the tapers jam.

    Did that help any?
    Last edited by a moderator: May 4, 2017
  10. Nov 24, 2009 #9
    Jasongreat,thanks very much.
    one question is what the screw depth should be appropriate for the given NPT thread? you know, the NPT thread is a taper thread, if the screw depth is different, the major diameter of the end of thread is not the same.
  11. Nov 24, 2009 #10


    User Avatar
    Science Advisor

    What table did you get your "dimension" from? There are plenty of pipe tap charts out there that list length to hand tight and wrench take up lengths. I have posted them a couple of times on here. Do a search and you'll find them.
  12. Nov 24, 2009 #11
    I care about the NPT dimensions(major/pitch diameter of the end of thread). in fact, I need to control the srewed depth of the NPT fitting. So the parameter of NPT thread should be controled as precise as we can. So I wonder how the NPT thread is machined and how to control the quality.
  13. Nov 24, 2009 #12
    Benchtop, thanks for your help.
    how to determine the certain depth? is there any calculation method?
  14. Nov 24, 2009 #13
    FredGarvin, thanks.
    my question is: how to control the parameters of NPT thread during the machining process?

    I have got the table of the NPT thread. I need to tap a NPT thread on a through hole and then screw a NPT fitting into it. if the major diameter of the end of the NPT thread hole is not stable, NPT fitting's engagment is varible. for example, if the end of the NPT thread hole's major diameter is larger than the parameters given in the standards, the engagement of NPT fittings should be bigger than normal.
  15. Nov 24, 2009 #14
    I'm not real clear on what you're trying to do, but the basic control in pipe threading is length of engagement, and there are several ways to arrive at that. Are you trying to control depth of engagement in some precise way?
  16. Nov 24, 2009 #15

    yes. I am trying to control the depth of the engagement precisely. could you please give me some suggestion? I have no idea about that.
  17. Nov 24, 2009 #16


    User Avatar
    Science Advisor

    What you need are pipe tap plug gages. You tap the hole to a certain depth, remove the tap, clean out the hole and then run the go-no go gage into the hole. If the gage says you are good then you are done. If not, you either tap the hole farther or scrap the part because you went too deep.



  18. Nov 24, 2009 #17
    thanks, FredGarvin
    I think I need the gage to control that NPT thread.
  19. Nov 24, 2009 #18


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Start tapping threads in scrap, learn how to get what you need by trying.
  20. Nov 24, 2009 #19
    According to the pipefitters handbook, which can be found at google books, in section 4 it starts explaining threaded fittings. Some pages are missing so I hope the info you need isnt on one of those pages, my copy is in storage so I cant reference it. In section 4 part 10 it states that the depth of the threads will be .55 inches, engagement of threads when tightened by hand is .34 inches and when fully engaged will be 9/16ths of an inch. There is all the info you will ever need to fit a pipe properly contained in the book you may want to check it out. There is also a plumbers pipe fitters handbook there.
  21. Nov 25, 2009 #20
    thanks very much. I will check it out soon.
  22. Nov 25, 2009 #21
    The question is what you mean by precisely and whether you're doing a "one-off" or production. Pipe threads are interference fit (when done right and cold weld when not) and cannot be extremely precise. If you're doing one only, follow FredGarvin's gaging solution and make sure you have really sharp dies and taps. Clean the threads well. You should be able to get about +/-0.05" tolerance for a 0.50" pipe in ordinary steel or brass.

    If you're doing production, pipe threads are the wrong choice. Don't use over-tightening or extra dope/tape to try to compensate.
  23. Nov 25, 2009 #22


    User Avatar
    Science Advisor

    Personally I have never had to hold pipe threads "that" tight. I simply use the tables and tell the machinist how far he needs to plunge the tap. I'd say 1 in 10 times I have to take it back to run the tap in farther. I have never had an issue not using gauges.
  24. Nov 25, 2009 #23
    Oh, I didn't mean to suggest that was a good idea. I certainly would never try for anything tighter than +/- 0.25" if I could help it. I only know about the (roughly) 0.05" becuase I got assigned a job like that a few years back. My supervisor insisted he wanted an adapter put in with precision, and I did, albeit very, very slowly and with a couple dollars labor each. :yuck::cry::cry:
  25. Aug 18, 2010 #24
    This poor guy is trying to find out how far to turn the tap into the hole to ensure the NPT Thread is conforming. He would like to do it without checking the hole with the appropriate NPT Thread gage after every 1/4 turn of his tap. Isn't the tap itself marked somehow to show how deep it needs to go to make a conforming NPT threaded hole? This is a tapered thread and the specs say that it the finished threads must be wider at the hole entrance than in the bottom of the hole. Isn't there a simple, fool-proof way, when using an NPT tap, to know when you've gone deep enough? Also, if your L1 NPT Thread gage stops two threads before the flat spot on the gage, do you just insert the tap again (by hand until it stops) and then crank it one or two more turns to make the threads OK?
  26. Aug 19, 2010 #25
    This is something that just comes with experience, but it's very rare you need a thread to a specific (tight tolerance) depth.

    So say you needed a thread of 12 TPI but wanted to go in 0.5 inch, you know that you want to go in about 6 threads. Or you can do it the other way round, you know that your tap is about an inch long, so you do in about half way. Or if it can be no longer than an 0.5 inch, you take it to just under by eye, then start using the gauge.

    Most female threads I use are undercut, I can't remembe off hand if NPT can be or not. Meaning you can tap it all the way, and the thread depth is controlled by the previous machining process.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook