Problem- CNC threading problem

[3bmachine]

Hello,
I am having a problem on threading with our Hwacheon CNC turning center. This is the first time I have experienced this. I am attempting to OD thread a 5/8-11 thread on a 3/4 diameter x 3.75 long piece of 303 stainless steel. There is a shoulder turned back .875 from the face of the part. This is what is being threaded. Using a tailstock. The "go" thread gage will thread on all the way til the last 2 threads or so - then it just tightens up - can't go any more. Have tried both top notch and lay down threading inserts - no difference. Using a G76 threading cycle. taking many light passes. there is a thread relief machined up against the shoulder. the only way i can get the thread gage to go on all the way is to program a .002-.003 taper into the threading cycle - kinda opposite of what you would do for a NPT thread. I have threaded thousand of parts on this machine over the last 8 years or so - never had this problem. The last thing i did tonight before calling it quits was to check alignment of the turret - while it is not perfect (.002 when running an indicator in the x axis in a tool pocket), I am of the opinion that this would not cause this problem. As I mentioned, I can get it to work with a programmed taper, but this is obviously not the correct answer. Considering I have to have 500 pcs delivered to a new customer on tuesday, I am in a little bit of a panic. Any input would be GREATLY appreciated.
Thanks,
SR

[Ricky Bobby]

Are you the only one operating the lathe? Is it possible someone changed the pullout parameters for the threading cycle. You probably want a 90 Deg pullout for this with the shoulder so close.

Just a couple thoughts hope you figure out your problem.

[Nakauc]

If your tailstock is set to hard it will cause a taper on the material. If this is the case i would just check your tolerances with parrallels or on a shadow graph an run the job programming out the taper especially if you need them by tuesday.

[g-codeguy]

All of the above are possibilities. Make sure someone didn't change the retract to something other than 00. I often have to program tapers when using a tailstock regardless of the pressure being used. Too hard to keep them perfectly centered, and they will pull the material one way or another. Also make sure the threading insert is close to being on center.

I assume your RPM is correct since you have made plenty of them before. RPM hasn't been increased has it? Slower RPM will keep the tool from pulling out too quickly. How wide is the under-cut?

[pixburghenat]

I too was going to say the pull out, it might be in the gcode too.

I also wondered if you measured the diameter at the tight spot, it may be bigger there.

pix

[rapidtraverse]

I totally understand your frustration. I've experienced it my self. What blows my mind is when you get a good part and suddenly this conditions appears. What is even worse is when you are late to discover this cutting condition and you have a bunch of parts you have to chase or worse scrap.

There is nothing wrong with including an "R" in your G76 to take the taper out. Afterall you are removing material.

In addition, it probably wouldn't hurt to replace inserts. By the way what type of inserts are you usingn?

I have experienced simular phenomenon as the cutting conditions seems to change as the bar cycles it's way through the operation.

It's baffleing I know, and damn hard to accept however, chalk it up to material conditions. You know...stress releif, the quality of the material, or even bar orientation from the bar feeder.

Of course, you will have to keep an eye on it as conditions will continue to change.

With that, it is possible this condition will not repeat on the next job.