run out issues

[ark2115]

Hello. I'm trying to make a 5.3 long part on a sr-20 screw machine. The part has a .160 diameter 2.5 long. .There are two 7/16 diameter,one at the end of the .160 diameter and one at the end of the part and the middle is .215 diameter. The Stem is allowed a little bit of run out but the body has to run true within .001 to each other.. I'm getting about .0025-.0035 run out on datum A which is the two 7/16th diameters.
I'm not sure what to do here. The guide bushing is set right,no out of roundness at all. My cutting tool is a .004 nose radius and on center. Any help would be greatly appreciated. Thanks

[CitizenBrian]

I would start by tightening up the guide bushing little bit's at a time. Even though it may feel right there could be crap in it giving a false feel. Next would be your turning insert, I think the 4r is good but have you tried different geometry's? A little more info would help too, rpm, material, process.

[ark2115]

Thanks for your reply. The material is 3/16 ss and is precision ground half inch diameter. The insert I'm using is a high rake Kyocera.

[JMS4287]

Is it A runout issue, or is the part bending, are you turning everything with the same tool? Are you turning in one pass, if not, make sure your not doing any Z moves that could be pulling the stock out of the Guide Bushing.