Tapping Pipe Threads

[sansbury]

I am trying to make a manifold for my air compressor and am having the time of my life trying to tap the holes.

Material is 6061, using hardware store taps for 1/4 and 1/8 NPT. I'm using the recommended size drills and thread cutting oil, and I am having an awful time getting enough threads to make any kind of seal. Is there any sort of trick for this that no one ever told me about?

In case anyone is wondering, it's a 100PSI compressor, little tiny thing, and I'm using 1" bar so I think the hose would blow up before the manifold....

[SGW]

Yes -- the trick is not to use taps from a hardware store. Most of them are terrible, fit only for cleaning up existing threads. Buy a good (expensive, but worth it) name-brand tap from a place like MSC www.mscdirect.com or Travers www.travers.com or any of the other good industrial suppliers. You will be amazed at the difference.

[Black_Moons]

Use teflon tape and lots of it? NPT are not designed to seal directly.

[camdigger]

Use teflon tape and lots of it? NPT are not designed to seal directly.

??? what's the taper for?

Having said that, they do work better with some thread compound or teflon tape to lube the contact faces but I have always been told that NPT and assorted other threads seal on the flanks of the tapered thread. As opposed to rotary shouldered connections that seal on the radially flat shoulder or ORB threads that seal on the Oring at the base of the male pin.

[Black_Moons]

The taper is so it bottoms out and goes/stays tight. Its not for sealing. Iv never seen ANY of my store bought NPT connections seal without teflon tape or some other sealant compound.

There ARE npt style connections that 'seal' without compound/tape, but they are some other standard.

[Mcruff]

Get a decent HSS tap from MSC or such, if you have a lot to do get the tapered reamer (yes they make a taper reamer that looks just like a tap for them) that goes with it. As far as NPT they make both wet and dry seal. I always tape them or use teflon pipe dope on them regardless. 2nd, Carbon steel taps are horrible in aluminum. I have seen days at work where I tapped literally hundreds of NPT holes, the reamer makes it much easier.

[sansbury]

Yeah, I misspoke--I always use pipe tape for the final seal.

I was having a hard time getting a deep enough cut to seat the fitting firmly, let alone provide an airtight seal. At 100PSI I think there's a good chance they'd blow out (ducks)...

[Black_Moons]

If you mean it just took too much torque, Yea thats normal

Consider a normal tap just taps with the tip, and each tooth just takes a small portion of the thread, and only the last few threads do any cutting anyway.

a NPT tap, every tooth cuts with the full face of the tooth. And the deeper you go, the more teeth that are cutting.

I had to use a 18" long wrench and secure the part in my bench vise and put praticaly all my body weight into it to tap a 3/8" NPT tap in aluminum deep enough.

Use lots of cutting oil, reverse every 1/2 turn to break the chips, and basicly just use insane amounts of torque untill the job is done, those NPT taps should be tough as nails so don't worry much about snaping em.

Almost makes me consider using an impact gun but somehow I suspect that would be the one way to quickly break a tap.

Doing NPT taps requires so much force/power, I actualy heated the aluminum block up considerabley just taping it once by hand!

[camdigger]

Yeah, I misspoke--I always use pipe tape for the final seal.

I was having a hard time getting a deep enough cut to seat the fitting firmly, let alone provide an airtight seal. At 100PSI I think there's a good chance they'd blow out (ducks)...

3 - 3 1/2 threads to hand tight is what I've been told.
Interrupted tooth taps will decrease the amount of torque needed.
Have you tried A9 cutting fluid? Seems better than some.
One more insultingly obvious thing... did you double check the tap drill size? (been stung on that one more than I care to admit)
Misalignment will increase the torque very quickly... Are the taps square to the work?

[Black_Moons]

Interrupted tooth taps will decrease the amount of torque needed.

I looked at these and considered buying them and thought it might reduce torque due to number of teeth engaged, but then I realised each tooth would have to remove 2x the amount of material anyway, you'd only get a small advantage in not having so much cutting surface engaged.

If you have actualy used them and compaired them, id love to hear your approximation on how much less torque they require.