Help with thread finish on lathe, nothing I have ever seen before

Is there a HSS insert under the carbide, if so he should check it for cracks.
To thread 4130 with carbide at lower speeds it would help if it was prehard.
The carbide insert could also be sharpened. I've been sharpening some of my china carbide with good luck.
Have not done so with a threading tool though, some of the thread form may be lost if not done right.
 
Hey Matt,

You don't say if the cross-slide or compound-slide is being used to feed in for each cut.
I have seen some awful threads as the result of feeding with the compound set at the wrong angle.

I cannot quite see the details in the picture you posted, but here's what this condition looks like:
https://www.hobby-machinist.com/threads/threading-help.49488/post-416851

Some lathe compounds are marked with zero degrees perpendicular to the lathe access and some have it parallel to the lathe axis.
Which type you have controls what angle you need to set the compound to.
Therefore this is one thing very easy to get wrong when you're trying to do it "by the book".

Post #5 at the link above has some great diagrams showing the issue.
One face of the thread will be jagged and at the wrong angle.

-brino
 
Hey guys, thank you so much for the replies.

I do agree with what most say, it just plain looks like bad chatter to me, but he says that he is doing the exact same thing that he has always done, was never a problem before but its there now. Obviously there is some variable, but we can not find what it is.

It is doing similar in Aluminum, although not as bad, but still a similar pattern.

Checked all gibs, all are tight.

It does do it in a different TPI setting too.

The compound angle is definitely right, at first it was not and it was making a strange angle on the threads (I see that a lot since they are marked different than we are used to) but once that was corrected, thread angle correct, but still has that torn finish.

Good point on the thread dial, but it does it even if you do not disengage the half nut, like you are threading Metric.

I also do not think it is in the machine, we literally have covered every single thing I can possibly think of, but still no good. I will double check the cutter with him, I believe he also tried a HSS cutter, I will have to go back and check the emails, we have like 50 to 100 emails back and forth.

What it looked like to me was something in the tool post / cutter too, same as many have suggested, but he says he has checked that all many times. What I have seen before is someone machine the T nut for the compound slide to mount a quick change tool post, the T nut is a bit thick, so the tool post seems tight, but its actually floating. Checked that too.

I am not checking any of this myself, he is checking it all, but there is nothing left I can come up with.

Once we figure it out, I will definitely post the fix, and I really appreciate all of the help so far!
 
Where is he located? Perhaps a second set of eyes would help to confirm all the findings since you're not there to check.
 
He is In New Hampshire, just above the border with Massachusetts, I believe Nashua is the town
 
He may have gotten annealed instead of normalized bar, that will make a difference in the feeds and speeds. Also has he used the same cutter? I have seen where someone gets what they think is the same one and somewhere it was swapped in the supply system for another brand/quality.
 
I will check with him, he said he tried aluminum, and also a different cutter which was HSS, same result. I did not see the HSS or any other cutter, I will double check that. In my thinking, it almost seems like the insert is loose or something, but he said no way.

If anyone is near that location and would not mind stopping by just for a second set of eyes, Ill make it worth your while. He is a nice guy, I have met him in person before, just trying to get this figured out to help him out.
 
I suspect the lathe has something loose allowing it to chatter.
 
qualitymachinetools said:
hat I have seen before is someone machine the T nut for the compound slide to mount a quick change tool post, the T nut is a bit thick, so the tool post seems tight, but its actually floating. Checked that too.
Click to expand...
Agree! I would have him double check this. And also -- tool height.
 
Since it sounds like you all have exhausted all of the usual suspects, I have a suggestion as that looks like my threads when I'm going way too slow...

1. Choke way up on the tool, like as far as possible without eating the chuck.

2. Assuming he's sticking with the insert tool, have him thread a left hand thread at higher surface speeds. That way he can run off the end without worrying about crashing in to the shoulder. Dont even worry about matching a known thread - just run it faster.

With a TIN coated insert on 4140 shoot for 200-250SFM. I don't know if that lathe can handle threading that fast, but have a go at it within the limits of the machine.
 
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