Converting Tapered Threads to Straight Threads

I have a large quantity job that we are ramping up to do for one of our customers. The general idea is an adjustable pipe support.

In order to accomplish this, my customer wants to re-tap the threads on the small side of a bell reducer from tapered NPT to straight. This will allow us to screw the bell reducer onto the stand and then thread a pipe through the small end for the adjustable portion.

My first thought was to chase the threads with single point. However, we are not sure how to correctly locate the threads without compromising them.

Another suggestion was to run a straight tap through the threads, the only concern there being that we are going to be tapping 3"+ Stainless Steel reducers and the cost of the tap is not negligible. My biggest concern being that the tap will wear out quickly or break.

Any suggestions are greatly appreciated.

Tap it. The straight tap will pick up the existing thread. It is easy to resharpen a straight tap. There are tapping fluids that will extend the life of the tap. Running slow will extend the life of the tap. Price the job accordingly.

Larry

The reducers are probably tapped initially with a collapsible tapping head. Think OD threading head turned inside out. Because they use replaceable chasers, the cost per thread is far less than with a solid tap. The collapsing feature is nice in that you don't have the time or wear associated with unscrewing the tap from the finished part, but its not an absolute necessity. I've never priced one, but I'd imagine the collapsing mechanism makes it a pretty expensive thing. A fixed carrier for the same inserts would do the same job at some sacrifice of speed, and is something that could be shop made if not commercially available.

In a cnc lathe it would be possible to load parts by putting them on a threaded arbor mounted in one of the tool positions on the turret and then having the turret move into position and place the part in the chuck. This would give you a repeatable orientation of the thread such that you could single point it. The drawback is that the existing thread would have to be concentric with the OD of the reducer, and that's generally not the case with most pipe fittings.

3" pipe thread is rather large and a tap would be expensive. Sooo, buy a short 3" pipe nipple and chuck it up so the threads run true. Part it off and weld it to a plate with a stub shaft welded on concentric with the axis. Put the fitting adapter in the tailstock chuck and then screw the fitting on the fitting adapter Now chuck the end of the fitting in the lathe chuck and back the fitting off the adapter in the tailstock.

The fitting threads should be running true if you did good work and you are now ready to single point the threads in the fitting.

Have fun.

Carl, that still gets you back to requiring that the thread is concentric with whatever part of the fitting you're chucking on. In most cases, the threads themselves aren't even concentric with each other, much less with the OD of the fitting.

I'd imagine a large quantity of any big screwed stainless reducer would be made to order and not laying in stock in any manufacturer's warehouse. If so, they could likely order them with the 3" end left blank so the entire thread could be single pointed. Given the inconsistencies in screwed fittings, that'd likely be a lot simpler than any sort of modification.

If they plan to start with fittings sourced from China or some other part of Asia, all I can say is good luck. On half or more of that junk its a major accomplishment to just get the fitting to screw onto a piece of pipe and end up headed in the right general direction. Any attempt to pick up an existing thread would be a nightmare since they tend to be both non-concentric and laying on centerlines that have no relation to the part itself or to the other thread.

Metlmunchr calls out some serious "issues" with this job on a lathe,
Concentricity (lack of) will make holding size a real bear.

IF you do end up with this job as "single point" work, consider using the cutting edges from an EXTERNAL pipe threading tool. Readily available (cheap) and easily sharpened. Right now I'm thinking of a floating holder that would hold two opposing dies like a spade drilling bit. hmmmm

Starting from blanks is looking pretty good just now.

CalG

Not so metalmunchr, if the thread fixture in the tailstock is running true and you screw the fitting on it and then with a 4 jaw chuck up on it with care you can get the thread very close, close enough for what he is doing. He made no statement as to the concentricity between the two threads. While they are not always perfect they are fairly close and for an adjustable stand leg it would work.

Of course there is still the issue of setting up on the taper thread which will not be easy. I have done a job similar to that and it was a not something I would like to do again.

He may even have to single point a thread on the fixture to get it concentric but it will work.