vertical movement for robotic arm

You might try setting/mounting the base for your robot on top of an old
carjack and run the screwdrive for raising/lowing the jack from a stepper
motor.  Just a thought...

    --That would work but it wouldn't be real precise; i.e. I think it
would wobble. To do it accurately you want the devce to be mounted on two
vertical rails. Rails are usually precision ground steel rods, but crs would
probably do fine. In industry where you'd want the thing to survive several
million cycles the "car" that rides on the rails would have
linear bearings, but bronze bushings or even nylon ones would be adequate.

--
        "Steamboat Ed" Haas         :  I still miss          
        Hacking the Trailing Edge!  :  rock and roll...
                          www.nmpproducts.com
                   ---Decks a-wash in a sea of words---

You might want to look at linear actuators like below.

http://www.firgelliauto.com/show_info.php?page_id5