ST-40 Inconsitent diameters when roughing.

I am currently running parts on a less than 4 year old ST-40L machine and i am using a high feed roughing insert (Sandvik DNMM 433-QR). We have been using these inserts for a couple of months now with a depth of cut up to 0.250 with a feed of 0.024"/rev with great results one of which was nearly a 50% decrease in production time (had a lot of material to remove). I have recently tried to use them on another set up and things worked well for the first week or so, but then i started to notice that the predictability of the insert was starting to wander. I.E. where there once was 0.02" for finishing the part is now undersized 0.005-0.01" after roughing. I even backed off the depth to 0.150" and the results are the same.

I have asked our service company about the problem and they basically told me they didn't know what the problem is. He said the ball screw should be fine and that it might be the part of the machine that accounts for the coefficient of thermal expansion. I am beginning to think that the machine is most likely the cause of my problems as we have had consistent results with a deeper cut previously.



Other details:
- I am using a soft jaw holding 3.5" of material on a 10" part.
- We just had the turret aligned
- There have been no major components changed i.e. ball screws since the machine arrived brand new in our current shop.
- If we switch back to a CNMG and cut at 0.150 depth and a 0.014 feed it works (just adds three minutes to the parts time)
-The part does have 3 different diameters to rough to so the canned cycle does cut at different depths (I also think this may be part of the problem)
- I am still fairly green as a machinist so any advice would be greatly appreciated

That being said my questions are:
- Has any one had any similar problems and what were the solutions if any
- Am I pushing the machine too hard?
- Is it simply a rigidity issue?

Please advise,

Addam

Could you part be shifting in the jaws?

Have you checked to see that the tool and/or insert are not shifting?

Can you stop on one of your last roughing cuts to verify position ie Control shows cut dia of 6.000. Stop midway on cut and measure, verify size.

I have had thermal comp issues, but I found my numbers slowly creeped away as machine warmed up over an hour or so. Pushing machine...maybe is generating lots of heat...that is a factory question. I called factory and a tech worked with me to tweak the comp parameters.
let us know what you find.

I'm with SIM on this one. I think the cutting pressure on the part is overcoming the clamping force of your jaws and the slug is pulling into your tool.

I'm all about reducing cycle time, but I'm not keen on ripping a slug that size out of the jaws.
I'd go back to what you know runs reliably and go to town.