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Female NPT reaming/tapping tips

Clodbuster

Aluminum
Joined
Jan 29, 2012
Location
Tri-Cities, WA, USA
I'm looking for tooling & technique suggestions for making female NPT threads. Sometimes in a one-off part on the mill, sometimes in the field. In mild steel, aluminum and polypropylene. 1/8-2" NPT.

I'm thinking taps but don't know anything on what kinds work best vs. cost. Do I need taper reamers to get these to work right?

Are you all using reamers? What's your process?
 
You haven't given enough information (as usual) but what you are talking about is very generic tapping that is done with taps all the time. Unless you have a specific tolerance you are trying to hold (that you haven't told us about) or some other out of the ordinary requirement...you are ordinary.

Drill the hole per a tapping chart, chamfer it, run the oily tap in straight to the proper depth, remove tap, clean out chips and oil.
 
When I use NPT taps to make female threads they bottom out too early. Comparison with CNC made parts confirm this. The purchased parts are
machined so that the threads extend further into the hole. So in some cases I use a tap that has the tip ground down. Then I use that tap
to chase the thread further down into the hole. Another thing I do is taper the the rough hole about 2-3 degrees before I start the tap.
Helps reduce the amount of force and wear on the tap.
 
Recipe for 2" NPT thread.

1) Drill hole to 2.1875".
2) Chamfer.
3) Start tap.
4) Use pipe wrench and cheater bar to turn tap to depth.
5) OPT- use fork-truck on cheater bar.

:leaving:
 
I found for soft materials a spiral tap works best and dont be afraid to drive it in fairly fast, spin to slow and they tend to tear.
I marked the depth on the tap and drove it in one shot with the appropriate lube.
 
When I use NPT taps to make female threads they bottom out too early. Comparison with CNC made parts confirm this. The purchased parts are
machined so that the threads extend further into the hole. So in some cases I use a tap that has the tip ground down. Then I use that tap
to chase the thread further down into the hole. Another thing I do is taper the the rough hole about 2-3 degrees before I start the tap.
Helps reduce the amount of force and wear on the tap.

Yes...about 90% of everything I buy with an NPT thread is improperly threaded. Most are threaded way too shallow....the first thread hardly starts. I wonder about the average Joe buying this stuff, knowing he won't have any taps on hand or the knowledge to use them. China strikes again.....
 
The OP's question is legitimate. For many years I just drilled the hole straight to the minimum diameter and then tapped the hole to my desired depth. As many have stated that depth may be all over the place. I said to MY required depth for the fit I want. To day I own a taper reamer for every NPT or BSPT thread I cut. The taper required is 3/4" per foot. I made the investment in reamers and I don't regret it. It removes all that excessive torque required when the hole being tapped is straight and it greatly improves the quality of the thread produced.
 
I use to use tapered reamers when I tapped NPT's. Since I only thread mill them now I have not used a reamer. Is there much improvement to be made when reaming before thread milling?
 
NPT reamers are good is you're hand tapping.
Also the interrupted tooth taps can give better results.
Other than that, I think the people above have it covered.
 
Yes...about 90% of everything I buy with an NPT thread is improperly threaded. Most are threaded way too shallow....the first thread hardly starts. I wonder about the average Joe buying this stuff, knowing he won't have any taps on hand or the knowledge to use them. China strikes again.....

Would like to ding China on this. In my experience I have been using OSG and other quality taps. They all bottom out too early. But it probably
has to be that way for the tap to engage a non-tapered hole. So for some cases a shorter tap (ground down a little) is needed to finish out the
thread at the bottom. I actually don't like shortening a high quality tap.
 
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Best thing I ever found is the 'skip tooth' pipe tap. A good one with the proper coating greatly reduces required torque, makes good threads and lasts a long time (machine tapping on a turret lathe).
 
You haven't given enough information (as usual) but what you are talking about is very generic tapping that is done with taps all the time. Unless you have a specific tolerance you are trying to hold (that you haven't told us about) or some other out of the ordinary requirement...you are ordinary.

Drill the hole per a tapping chart, chamfer it, run the oily tap in straight to the proper depth, remove tap, clean out chips and oil.

Not sure what else I can give for info? What I am doing is pretty ordinary, but the way I've been trying to do it hasn't worked well. I've been tapping straight drilled holes, and it's not giving me threads that make up tight like a factory female pipe fitting does. That's what I'm trying to correct.

This isn't a tight tolerance thing really, I just need to make female threads that snug up before I run out of male thread. And I would like a tool and process that will work for steel, aluminum and polypropylene, the three materials I work with most. I'm guessing that last one will need different treatment.
 
For the guys who mentioned measuring the depth you tap to for NPT, how and where are you getting that from?

The taps I've used don't give any indication of that other than a drill size to use, and that seems like it would definitely affect the final size and fit.
 








 
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