huangyhg

how to call out a npt thread
how do you call out a npt thread length on a drawing?
i have a part that is 17.75 mm thick and i called out a 1-11.5 internal npt pipe thread. the tap has a tapper on it, s if i don't tell where to the machinist to stop in depth with the tap, the hole will be over sized, so the ntp fitting will not thread in tight. do you understand?
so how do i call out on a drawing, how much of the tep in length to us?
frank
i would dimension the max dia of the hole allowable.
tapered pipe thread is an abomination, and should not be used in the first place.
17.75 mm is only slightly longer than the minimum recommended length of engagement for 1 inch npt, so it is very likely that you will never get a good seal on this part.
so in order to get a a good seal i should increase the thickness of the parts?
can you change the npt fitting to one which uses a parallel thread?
don't know if this helps.
what cbl said.
metric: iso/din 228/1 (bsp, jis b0202) parallel threads with appropriate o-ring or gasket.
inch: sae or ms 16142
i rather not change to parallel fittings, because i've all ready speced out all the fittings
frank
you can use low-profile (short?) npt plugs, which reduces the thickness requirements, or are you going to live witht he plugs sitting above the workpiece surface?
to specify a shallower tap, just dimension it on the drawing, but make sure you talk to the machinist if you can to confirm that he notices it. i know, the drawing should speak for itself, but this is reality.
jim sykes, p.eng, gdtp-s
profile services
cad-documentation-gd&t-product development
a less than perfect but usually workable alternative is to use a npsf female thread.
whichever way you go, with npt fittings it is advisable to use teflon tape or other sealing substance to help achieve a seal.
...let me guess most you guys are limeys ...huh!!? *l* when you use tapered pipe thread in the varying sizes they all have a normal thread engagement dept (good machine shops "should" know this) ...and if i did the math correctly 17.75mm is 3/8" pipe ...normal thread engagement is 19/64" by my chart. also it is not called out as "internal pipe thread", its called fnpt (female national pipe thread) and mnpt (male national pipe thread)...do i need to explain the difference between female & male part? *g*
and as mr. corblimeylimey pointed out they do make a straight nation pipe thread (nps)...this is the stuff you'll find on electrical conduit and conduit fitting. this is not the normal threading for pipe that you don't want to leak.
...and mr. mintjulep my friend, you would not do well here in u.s.a. oil field environment! *l* …Good luck all! …Mark

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