Philip over here at FK Instruments, running into a issue on a part I just programmed & set-up, was hoping to get some help or guidance on how to attack this!

Material: 15-5 CG .375

Turning tool: DCMT-3-1-4 IC8150

Part length: 3.460"

Machine: Swiss Citizen L32

I'm currently running a part with a +/-.001 true position call out on an OD. I'm not sure what is going on with my part, as there's no consistency or repetition from part to part. I don't know if it's tool deflection, material whip/bend, or bad collets. I am maxed out on my guide bushing pressure, & both my main & sub spindle. The whole 3.460" part is turned on the front side, as there's a 2.00" tapered feature that needs to be done in one pass. I can't bring in the sub for support, I don't have tool clearance. My first turn pass, about .350" "Z" move, is holding a .0002-.0004" run out & that has repeated consistently. The next turn pass goes into the taper & this is where my issue begins every time. The taper is about 2.10" long gradually getting bigger, from .204 to .275. I have an "M1" after that turn to stop & check the run out. My part is about 2.500" out of the guide bushing now. Closest to the guide bushing, the part is holding true, only running .0002" as it should. When I put my indicator to the front & indicate close to the face of the part, it's .002-.010" never repeating & that's where my +/-.001 true position call out is at. This has to be tool deflection right? Or is it the material stress relieving? I'm stumped & need help!