NPT threads

[qaqc1]

We need to have 2 1/2" (8 pitch) NPT female threads in one of our components. The tapping is in a partially blind hole. (at the bottom of the thread there is a 1/2" opening)
1) What is the right procedure for drilling & tapping. Should we taper machine the hole before we machine. How to decide dimensions for this taper.
2) How many threads should be available in the component so that when a pipe nipple is installed it will seal effectively.

 

[deanc]

ASME SecI says 7 threads and min. 1" plate thickness at 500psi. ASME SecVIII says 8 threads. Your drill size is 2 5/8s",The tap is tapered.

 

[mrainey]

This free program might help you with threading questions:

http://bellsouthpwp.net/r/s/rsnmar/METhreads.zip

 

[qaqc1]

Our component is such that if you provide seven threads as per Section I then the pipe will bottom out. What I mean in that the pipe nipple will not pass beyond the tapped hole. (as the tapped hole is semi-blind that is it has a smaller hole at the bottom.)
On the tap you are saying we need not mchine any taper but just drill. Is it adequate?

 

[mrainey]

see this discussion:

http://groups.google.com/groups?dq=...ups?hl=en&lr=&ie=UTF-8&group=alt.machines.cnc

 

[deanc]

A standard pipe tap should work fine in the correct hole.No reason for a tapered hole.

Why dont you try cutting a little off the end of your pipe. Since the threads are tapered it will take up faster. May be some trial and error.

 

[Ralph2]

Some of these guys obviously have never tapped a 2.5" pipe tap by hand. If you have the capability to taper the hole by all means do it. You may need to have 2 taps. One to start the hole and tap as deep as possible then because the tap can not go in far enough you need to cut some of the tap off at the bottom and then re tap. As far as I know all pipe tapping stays consistant regardless of the size and thus you need to end up with ~7 treads still showing on the tap when it is done.
Good luck

 

[deanc]

Actually Ralph2,I have. Sometimes all that matters is how bad you need it. If you cut a tapered hole you can intoduce new problems. Why not apply a little extra effort up front and allow the tool to work as designed. A good pipe tap should give you what you need in one shot as long as the pipe is properly threaded. As I said earlier you may need to compensate alittle on the thread length of the pipe. If the pressures needed are high I may agree with you,but then a welded socket joint may work better anyway.

 

[pennpoint]

qaqc1
Why not bore the 2-5/8" hole through as required for the pipe tap. Tap the hole as required.
Then weld in the disk with the 1/2" hole after all is done.
Or weld the disk into the pipe that going to be screwed into this tapped hole. The result would be the same.

pennpoint

 

[TBP]

Does it matter where the 1/2" hole is? Can you have a full 2-1/2" thread engagement if you use a 2-1/2" X 1/2" bushing? Or just drill and tap a 2-1/2" cap? You could have the 1/2" hole at the other end.

Something else to consider is - what are the service conditions? Any piping code considerations?

 

[TBP]

I said "cap", I meant "plug".