Hi,
I Want to use my cnc machines 4 th axis as turning spindle i have ac servo motor on 4th axis. I am using smoothstepper plugin control card. Is it possible to use A axis  as turning spindle as well as indexing axis for rotary milling.

Do a search and you will find a lot of info on what you want to do. The user "Simpson" has posted a lot about developing a 4th axis for both indexing and using it as a spindle. I will remark that a servo should be used and not a stepper. Stepper is ok for indexing but not to drive it as a spindle.
RICH