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

Might be worth posting a new thread with the title 'Need assistance near Nashua, NH or Massachusetts - reward offered' :) If it was SoCal I'd be more than happy to help out.
 
Everyone has covered most all the bases. I struggled threading and finally worked all the issues out. I use HSS, cut away from the chuck, plunge straight in, tool upside down, compound locked. It allows me to have higher surface speeds. None of this was my invention, credit Joe Pieczynski. PM1440HD.
 
Starting with the basics:

1. Did you use a thread fish (center gauge) to be sure the cutting edge of the insert was square with the work piece?

2. Are the gibs on either the carriage, cross slide, or the compound gibs loose enough to cause the tool to dive when making contact with the work?

3. Is the tool post holding the tool holder square to the work piece, and tight enough to prevent it from diving when contacting the work?

4. Are the carriage half nuts worn enough that the tool can vibrate when engaged in the work piece?

5. Is there any play in the lead screw mounts that allow the lead screw to move causing the tool to engage in a slightly different location each time the half nuts are engaged?

6. Have you checked the profile of the insert against a new one to be sure it's sharp, has a true 60* angle, and is properly relieved so the lower edge isn't rubbing.
8. Is there any runout in the chuck or work piece causing the tool to loose it's top edge?

9. Is there any debris between the tool post and the compound, or between the block holding the tool post to the compound?

10. Are the nuts locating the compound to the cross slide tightened sufficiently to keep the compound properly located?

11. Is the lead screw or nut on the compound worn enough that the tool does not advance the amount shown on the dial?

12. Are you cutting the proper amount with each pass? The recommended amount is usually around.005 on the first couple passes, then .002 until the desired depth. Remember some lathes read the depth of cut while others read the amount being removed from the diameter of the work piece.
 
Matt: one very remote possibility; some kind of stiction/friction problem with the carriage? But I really doubt it
 
Thank you for that list, and yes, from what he says, we have gone over every single one of those things. The #12 I am not sure about since I can not be there, it does look like the passes are deeper than .005 but Ill have him double check. I also gave him the link here and told him to sign up so he can also post, so I am not being the middle man.

The machine is only about 1.5 years old, one man shop, so definitely not worn. As far as the friction, it does turn just fine, the problem is only when threading. I am going to post some more pictures of the threads.
 
Strange that the angle would be initially set wrong this time since he threaded with it a year ago with no problem. But, aside from that, did he just not thread anything with that machine for a year or has it sat idle for that long? I see in the pic that he turned the work piece before he threaded it, a good thing to do, but did he perchance check runout on the turned work piece before threading it? I agree with others that the tool looks to be sticking quite a way out of the tool post. Quite the mystery.

Tom
 
Upon rereading the original post and the fact that it turns fine but threads poorly is saying to me that it's either the angle or a tool issue or the gear-train for threading has an issue.
 
Looking at the picture it doesn't really look all that bad- It would help to have a closer-up view
Choice of material has a big effect on the smoothness of finish when threading
 
spumco said:
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.
Click to expand...

Hmm. This is really good troubleshooting advice. I will have to save this thread so I remember to do this when I encounter a problem of similar nature. It kind of covers a fundamental troubleshooting principle:

- try something you aren't comfortable with but in a safe way. The factory motto we use on this is "fail fast and fail cheap". You will learn something quickly at a very low cost and start to isolate all of the variables. In this case, you can feed on a left hand thread at a higher speed, different depth of cut, different TPI, different tools, different inserts, locked carriage, unlocked carriage, etc., without fear of buggering something up or crashing into the chuck.

Good advice. Thanks for sharing.
 
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