Tapered thread

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Jan 25, 2019
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New York
Hi all...... Question about cutting a tapered thread. I was threading a piece of 5/16 hex brass that had turned down to to 1/4" using 1/4-40 tapered die from PM research and noticed that when i was finished and backed off the die that the 1st 4 or 5 threads looked like crap. I tried another piece with the same result. I then looked at the threads with a magnifying glass and it looked like the thread was "split" ...like a knife had been run down the peak of the thread. I've attached a photo, I'm not sure if it's going to show what I'm talking about that well.
I've never cut a tapered thread before so I don't know if this is normal (doesn't look normal) or if there might be something with the way I did it or if the die itself has a defect. Any thoughts or opinions would be greatly appreciated. Thanks.


Did you make sure that the taper was the correct angle and the start and end diameters were correct for the thread form?

If it is a split die you could try altering the diameter of the die and see if that helps.

I would also contact PM Research to check the required diameters.

It looks as if the die is dull. Perhaps some cutting fluid will help. I use Chroma Tap but I'm sure that guys on this forum have their favourite cutting fluid for brass.

When wrapped in teflon tape I'm sure that your thread will hold and not leak.
... I was threading a piece of 5/16 hex brass that had turned down to to 1/4" using 1/4-40 tapered die ...

You could try putting a taper on the material before you thread it with the die, instead of just making it 1/4" all along.
Just the sake of "transparency"....I'm currently working on the boiler feed pump from PM research....I bought the 1/4-40 mpt tap and die. I tapped the inlet and outlet on the pump and was looking to thread the fitting that I'd make to fit the tapped openings.
After trying the one from the photo I posted I tried another using "tap magic" and got the same result....I then tried another, this time I tapered the material and used the tapping fluid and got the same result. Oh, I also have PM's 1/4-40 UST die...I tried that one as well and got the same result so it must be something I'm doing. I doubt that 2 separate dies are the problem....both dies were brand new, 1st time use. So now I'm not sure what to try. I may give PM a call tomorrow and get their opinion.
What does the thread look like peeking down inside the die before you back it off the part? Doing that, maybe you can tell whether the thread is getting torn up while unscrewing the die, in which case something may be preventing free axial movement of the die as it unwinds, or you are somehow dragging it hard over the outer few threads. Are you free-handing it, or using a guide in the lathe? If the former, be sure not to get the die cockeyed on the way back off. If using a guide, is it free-moving and does it have enough travel?
one thing that might clean up your threading with a die is to stop before you go full depth. Remove die and clean all chips out and lube and cut to full depth and back out. When chips get in the threads blocking the way out they will tear the threads up.
Describe your setup, please. How are you holding the work? How are you holding the die? Is the die being guided?
With a taper thread you have to remove the die at the pitch rate. When removing the die and it is part way off the formed thread the crests can be cut by the following threads if the die is not moved at the correct pitch rate. It is not like a parallel die which will follow the thread all the way off.
Hi all....

To answer John's question:
The workpiece is held in the lathe chuck
The die is held in the die holder which is held against the workpiece and guided by the tailstock quill. As far as I can tell the die is going on square to the work.

TonyM......how do I go about making sure I'm removing the die at the proper pitch rate?

I have spoken with Jim at PM research and he recommended running the die on piece of steel which should help "clean up " the die. I tried that and it didn't seem to help which is why what TonyM said is intriguing to me.

I thank everyone for their opinions.
I wonder about some of the suggestions.

If there is no pressure on the die as you remove it, it will take the path of least resistance. So I can not agree with that.

I have never used my dies or taps on both brass and steel. They need to be kept separate. Steel will do more damage to a tap or die than brass.
When you start a thread with the die holder against the tailstock quill it is possible to let the die get ahead of the quill and get a cockeyed start. It is also possible to overfeed the tailstock quill and mess up the threads. I assume you are backing off the tailstock before you back off the die so that would eliminate backing off as a cause of your problem.

I have a die holder that floats on a dowel pin held in a chuck. It works well assuring a straight start every time. It also eliminates any axial load on the thread after the initial push to start the die. It works similar to the tool shown here:

After reading "john_reese's" post where he mentioned overfeeding the tailstock, I got to thinking that this is what I may have been doing. I tried again and made sure not overfeed the tailstock an ended up with a nice looking thread. So, thank you "john_reese" for pointing that out....I greatly appreciate your input as well as the others that have offered their ideas.
Looks like you found the issue.

I might mention that we ran into a bad batch of standard (non tapered) dies at work. They came in an imported set and would tear up threads (in aluminum) pretty bad. For a while I thought it was me but buying a good quality die proved that wrong. Just wanted to point out that sometimes it is the tool.
I might agree if it had a given resistance but when backing of the die it should not have enough pressure to cut more metal like that. By the way this is a non-electric power cut, all by hand. If you feel that kind of resistance there is something wrong. It is the interference that create more of a cutting action.
Hi Nelson. Whilst you are right if it was done by hand, it is a small cut on the crests of a couple of brass threads so not a lot of resistance required to do that.
However the OP says it was done in the lathe so the probability of backing off incorrectly was high.
To answer John's question:
The workpiece is held in the lathe chuck
The die is held in the die holder which is held against the workpiece and guided by the tailstock quill.