Might be worth looking at a Zeobot type actuator http://www.technobots.co.uk/en-gb/dept_528.htmlhttp://www.technobots.co.uk/en-gb/dept_528.html you can either buy the kit or make your own. I made one a while back and used a threaded rod from a workmate and put a drill motor on the end.

http://www.teamblazer.50megs.com/images/flybot_arm_testing.jpghttp://www.teamblazer.50megs.com/ima...rm_testing.jpg

http://www.teamblazer.50megs.com/images/arm.jpghttp://www.teamblazer.50megs.com/images/arm.jpg