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Thread cutting dies?
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Posted by: bart_graco

01/02/2007, 07:13:34

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Because we want to erpair damaged threads, we bought a standard set of NPT thread cutting dies and threading taps. Now we start using these dies and we got an unexpected surprise. When we cut the tread as far as we could with our die; we didn't reach the end of the die. So the thread is still not repaired. Tisn't cut deep enough

Can you help us to find out what we did wrong? Are there different thread cutting dies for NPT threads, do they have different widths?








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Re: Thread cutting dies?
Re: Thread cutting dies? -- bart_graco Post Reply Top of thread Forum
Posted by: randykimball
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01/02/2007, 21:48:43

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I'm confused.

Did your die thread until the end of the pipe was flush with the other side of the die or did it go farther and still not cut any fresh threaded material? To my experienced mind your question has conflicting information.





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Modified by randykimball at Tue, Jan 02, 2007, 22:00:30


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Re: Re: Thread cutting dies?
Re: Re: Thread cutting dies? -- randykimball Post Reply Top of thread Forum
Posted by: bart_graco

01/03/2007, 03:26:04

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I added a picture to clarify it.

die.jpg (30.1 KB)  






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Re: Re: Re: Thread cutting dies?
Re: Re: Re: Thread cutting dies? -- bart_graco Post Reply Top of thread Forum
Posted by: Kelly Bramble

01/03/2007, 09:13:58

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Sounds like what is being said here is that the standard die cannot reach and re-cut the last several threads that are already in place on the shaft feature because of a obstruction in the path of the thread die.

Three things come to mind:

1. Thread dies and taps will never get closer than 1.5 x pitch relative to an intersecting wall. This is because there are lead in cutting threads on the die or tap.

2. When the threaded feature was originally designed, the engineer or designer did not consider or understand thread manufacturability issues, or had no choice in the how close the thread feature was to the obstruction or intersection wall. Good design for manufacturing practice is to allow at least 1.5 x pitch and prefferably more space between the last required thread and any threading tool obstruction.

3. The original threads where cut using a different process than a standard thread cutting die. They could have been cut using a lathe or a modified thread die.


I'm not a manufacturing professional, however this is the kind of design that realy %^$&@*$ them off in high production senarios. Not to mention drives up cost and reduces maintainability.







Modified by Kelly Bramble at Wed, Jan 03, 2007, 13:15:30


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Re: Re: Re: Re: Thread cutting dies?
Re: Re: Re: Re: Thread cutting dies? -- Kelly Bramble Post Reply Top of thread Forum
Posted by: randykimball
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01/03/2007, 23:14:02

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OK, ...
Here is what you may be able to do, depending on your situation. You may be able to saw off the first couple of threads, then thread higher up the shank. I can't tell how large the shoulder is where the threads stop, you may need to remove some of the shoulder material. If you have several of these to repair, I suggest you place them in a lathe jig and re-machine the parts to remove a couple of threads and enough shoulder material to allow for the die to chase up the threads. I am quite sure these parts were machined on a lathe fixture or a CNC mill with single tool path actions to where the threads could be cut to any size they desired reguardless to that shoulder. ... but.. Now you need to chase the threads with a die.




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Re: Re: Re: Re: Re: Thread cutting dies?
Re: Re: Re: Re: Re: Thread cutting dies? -- randykimball Post Reply Top of thread Forum
Posted by: Kelly Bramble

01/04/2007, 08:13:49

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Another consideration is that the threads which you cannot cleanup may not matter. If you can assemble the part, look at the actual thread engagment required. If the bolt or whatever never has to thread onto the area of concern, I would call it a success.







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