NPT27

[Mike BrettParticipant @mikebrett89695]

How to cut thread

[Mike BrettParticipant @mikebrett89695]

Hi all

I am trying to cut a NPT 27 tapered thread on the end of a piece of brass bar to make a removable plug for a hole on my piston cylinder . I started the thread ok but only got a few turns before it jammed. Do I need to taper the end of the brass bar first.

Mike

[old martParticipant @oldmart]

The starting diameter should be 0.405" (10.23mm). The thread, being tapered will get progressively harder to tap as you get further on. The angle of taper if you intend to use a lathe, is about 1.5 degrees.

[Clive FosterParticipant @clivefoster55965]

Proper taper would be best but, in brass, steps will work well enough with care.

Make sure you have the thread length correct too.

Clive

[Anonymous]

I've never cut an external NPT thread but have tapped them. As you'd expect the tap quickly gets more difficult, followed by impossible, to turn. The tapping drill tends to be slightly larger than one might expect. I expect a die would be similar. I'm not sure if a taper on an external item would work. The die will go on further before it cuts, but I suspect it'll come to a halt more quickly. So possibly not much gain.

This is how I cut external tapered threads:

I was screwcutting BSPT threads but I think they're the same taper as NPT. I thought NPT threads were specified by a nominal imperial dimension representing the pipe size rather than tpi?

Andrew

[roy entwistleParticipant @royentwistle24699]

Andrew wrote above

I've never cut an external NPT thread but have tapped them.

How do you tap an external thread ?

[old martParticipant @oldmart]

Now now, Roy, you could get a tap on the head for that (and water on the brain).

[Mike BrettParticipant @mikebrett89695]

Hi all

Thanks for your replies. I managed to cut a reasonable tapered thread after three goes. Only goes in half way , but makes a good seal.

Many thanks Mike.

[Martin ConnellyParticipant @martinconnelly55370]

The plug gauges for pipe taper threads have a flat below the rear surface and the rear surface to act as limits for threading. It is surprising how few threads are enough to be classed as acceptable.

Picture of a taper thread gauge

Martin C

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