Inductive vs capacitive proximity sensors?

Group,

I know this subject is on the fringe of our group topic. Does anybody have
any experience with proximity sensors? I have a part I want to sense in the
collet before the cnc closes the collet. I don't want to machine air or snap
the collet. The part in question is a hand inserted but later possibly robot
inserted aluminum blank. I understand ferrous parts are easy with inductive
sensors. Do aluminum parts trigger inductive sensors? Any body play with
capacitive sensors? Are they as a group sensitive to soluble oil? mineral oil
based coolant?

Any comments are appreciated. I've not played with these things before.

Peter
THRD, Inc.