somewhat OT: Why do pipe taps have extra threads?

[kwfiggatt]

This may be a somewhat simple question, but I've been taught that when tapping a hole with an NPT pipe tap, leave 6~8 threads of the pipe tap visible above surface of the part (I shoot for 7), and generally this works well.

Why are NPT pipe taps made with the extra threads? If you sink the tap down into the part until the top threads are flush with the part, then the hole is WAY oversized. I understand that all pipe threads aren't made to exactly the same size - especially field threading of pipe - but 7 extra threads?

Anybody know the history / answer?

Thanks,

Kevin

 

[Mike1974]

WAG, but I would say for manufacturing purposes? Imagine if pipe taps were made so the top thread was exactly the perfect height to give you a good thread by "bottoming" it out. Probably significantly more expensive to produce and QC and such.

Also, even though there is a gage to check a pipe thread, by their nature of being tapered, you can compensate some of that by going an extra (or minus) turn or so and still make a "good" (usable) thread... if that makes sense.

 

[Myrmidon]

Mike Production pipe taps are made like that, buried is flush on the gauge.

 

[Mike1974]

Mike Production pipe taps are made like that, buried is flush on the gauge.

Where do you get those? I've never seen one and have run production before using pipe taps..? Are they new, or a specialty item?

 

[lazz]

Pipe threads are tapered.
As you tighten the pipe the thread fills the hole to make a seal.

Electrician conduit threads are usually the same as a pipe thread without the taper.

 

[Milland]

Imagine tapping a thread at an angle into a casting - you'd need some extra cutting threads to ensure full "access" to the mating part. Yes, some other interference is to be expected with most fittings, but not (for instance) a socket plug.

In other special circumstances you could repair a damaged pipe thread by going oversize, then making a new fitting to take advantage of the extra diameter.

 

[DMF_TomB]

This may be a somewhat simple question, but I've been taught that when tapping a hole with an NPT pipe tap, leave 6~8 threads of the pipe tap visible above surface of the part (I shoot for 7), and generally this works well.

Why are NPT pipe taps made with the extra threads? If you sink the tap down into the part until the top threads are flush with the part, then the hole is WAY oversized. I understand that all pipe threads aren't made to exactly the same size - especially field threading of pipe - but 7 extra threads?

Anybody know the history / answer?

Thanks,

Kevin

.
i check all npt threads with a npt thread plug gage thats been checked by calibration
.
more likely get fired for eyeballing threads of tap visible above hole as only way of checking threads

 

[kwfiggatt]

Imagine tapping a thread at an angle into a casting - you'd need some extra cutting threads to ensure full "access" to the mating part. Yes, some other interference is to be expected with most fittings, but not (for instance) a socket plug.

In other special circumstances you could repair a damaged pipe thread by going oversize, then making a new fitting to take advantage of the extra diameter.

The angled hole idea makes sense. Thanks everyone else for your ideas - lots of good thinking.

And regarding the calibrated gage to check the hole after tapping - I definitely agree with this point. However, most of my stuff are not production parts, but one-off prototype stuff and we don't have check gauges in the shop...so I count tap threads... 8-)

Kevin

 

[johnfalco]

WAG but... so the same taps can be used on oversized plugs?

 

[Froneck]

I too have the gauges for tapping pipe threads when doing work for others from a print they provided. Using a rule of thumb leaving 6-8 threads exposed might get the work rejected. On the other hand I have tapped beyond the 6-8 thread rule of thumb when doing work for myself especially when I want a pipe plug to go in further. Also when doing work on castings that had opening to remove core sand then plugged. Especially in one job that had 3 holes that were plugged with 1" pipe plugs then machines smooth and after machining 2 of the holes were tapped 3/8" pipe and the 1 plugged hole untouched. Tapping the casting with more threads allowed the plug to go deeper so that after machining there was enough material for the 3/8" pipe tapped holes. I would assume that the additional threads on a typical pipe plugs is to give the user the option of using the additional threads.

 

[4GSR]

Our predecessors who wrote the standard for pipe taps determine that taps needed the extra threads on them. Apparently, it has never been contested otherwise. I would think the most threads needed would be for the deepest location of the gage. Don't remember if that was L2 or L3 depth.
Will say, there is one pipe thread series that was developed by A.P.I. back at the turn of the century of 1900 that threads were made to about three threads more engagement. This is the "Line Pipe" series, basically the same thread as NPT, but with a counter bore in front of the thread. Gaging is a bit different because of the counterbore, but the thread engagements were different on some of the sizes.
I've never used any of the smaller line pipe threads, so I can't say for sure if they have the same engagement as NPT threads have. Do know the 2" Line pipe thread had a different engagement length by a increase of about three threads at one time, and still used on certain products built today. Ken

 

[rons]

Agree with Froneck. I have reduced the length of NPT taps to enlarge a non-through hole.

 

[Doozer]

I like to grind back ONE flute on a pipe tap
to where I know the gauge line is. That way
you just keep tapping until you are deep enough
where the one flute has been ground back.
I does not really hurt anything, it just tells
you where the gauge depth is. You can still go
deeper if you want, just the cut-back flute
will be along for the ride, so to speak.
Kind of an easy modification that speeds up
tapping pipe thread holes repeatedly.

--Doozer

 

[99Panhard]

More often than not when I'm using an NPT tap it is to plug a hole where the male piece is never going to be removed. I usually run them down to the end to take advantage of the full depth of the plug.

 

[Cole2534]

Our predecessors who wrote the standard for pipe taps determine that taps needed the extra threads on them. Apparently, it has never been contested otherwise. I would think the most threads needed would be for the deepest location of the gage. Don't remember if that was L2 or L3 depth.
Will say, there is one pipe thread series that was developed by A.P.I. back at the turn of the century of 1900 that threads were made to about three threads more engagement. This is the "Line Pipe" series, basically the same thread as NPT, but with a counter bore in front of the thread. Gaging is a bit different because of the counterbore, but the thread engagements were different on some of the sizes.
I've never used any of the smaller line pipe threads, so I can't say for sure if they have the same engagement as NPT threads have. Do know the 2" Line pipe thread had a different engagement length by a increase of about three threads at one time, and still used on certain products built today. Ken

Oil and gas threads could have a library written about them.

Sent from my SM-G973U using Tapatalk

 

[kwfiggatt]

Lots of neat info - thanks everyone for posting!

Kevin