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Thread: Proper depth of pipe thread

  1. #1
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    Post Proper depth of pipe thread

    How do you calculate the proper depth of NPT when you are tapping? Do you count the number of turns or go for a certain depth? I am tapping 1/8-27 NPT in 304 stainless.

    Thanks for the help.

    Joe

  2. #2
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    I just posted on your other thread Joe. Hope it helps some.

  3. #3
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    Use a male 1/8-27 NPT as a gauge?

  4. #4
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    Joe,the basic rule is 5 complete turns forms the maximum effective seal surface of any NPT thread.Thats what I was taught,I also have a hydraulics formula book and chart that gives thread engagement for various pressures.

    Most of the time I have found on standard length NPT taps(not dryseal)four threads from the top is about right.

    Hope that helps.
    I just need one more tool,just one!

  5. #5
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    Thanks for the responses so far. I have been cutting 10 to 10-1/2 turns and then I have to relieve the top thread. Like you do Joel. This seems to be the right depth but I have broken several taps when I tap by hand getting to that depth. I have tried several different taps from different manufacturers, all high quality taps not foreign made. I am going to build a jig a try using a Tapmatic and see what it does. Have some taps that are long enough to fit the Tapmatic standard taps are to short. Have been using Tapmagic as a tapping fluid and have some Castrol MoliDee, think that is the name,that I will try.

    I may be getting the stainless too hot and causing it to harden. Am going to uses a different coolent and see what happens.

    Any help or thoughts would be appreciated.


  6. #6
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    Pipe threads are tapered, and for larger sizes, it is a good idea to use a tapered reamer to size the hole before tapping. I don't know if one is available for 1/8".

    Stainless may need a size or two larger tap drill to prevent breakage.

    [This message has been edited by JCHannum (edited 07-24-2005).]
    Jim H.

  7. #7
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    Darin, when you say 4 threads from the top are you talking about the tap?

    I am tapping 1/8 NPT and using an R size drill. I have also used and 11/32" drill but it seems to be to big.

    Joe

  8. #8
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    <font face="Verdana, Arial" size="2">Originally posted by WJHartson:
    Darin, when you say 4 threads from the top are you talking about the tap?

    I am tapping 1/8 NPT and using an R size drill. I have also used and 11/32" drill but it seems to be to big.

    Joe
    </font>
    Yes,the threads on the tap.304 is very tough on taps,I use either sulpher threading oil (Texaco Sultex-D is what we use at work)or the wax sticks like "Edgelube".Might even try common candle wax.

    Like JC said a reamer will help things,also an interupted thread tap may make things easier.Don't know if they make them in NPT,but the Greenfield EM taps work miracles in SS.

    I typically use the 11/32 drill for 1/8NPT,it starts out loose,but since the thread is tapered it cuts to full profile anyway.

    I just need one more tool,just one!

  9. #9
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    Thanks to all, Darin the last tap I broke was a Greenfield EM. Haven't tried the sulfur threading oil.

    I will try the 11/32 drill and see what happens. Who carries the tapered reamers?

    Joe

  10. #10
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    Joe, I just looked in Travers, J&L and Production Tool catalogs. They all carry them down to 1/8". Look under pipe reamers.
    Jim H.

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