Basically this is just a simple linear actuator. I've used lot of them to move a paper surface fixed in the top of the shaft.
The linear actuator is moved by a 9gr servo, gear and rack are intended to be printed in plastic material (i.e. ABS), the supporting structure has been cut in MDF wood.
It can be easily modified without the need to model a new one.
It works ! Servos run without stopping for three days.
During the Maker Faire 2014 a friend suggest me to share it, maybe it can be useful for different purposes, in example to move vertically the pen in writing robots.
Use it and let me know !
Fab Lab Olbia

It's very easy, just print gear and rack (every 3d printer) and (laser) cut the wood. Parts can be distingued by colors. Servo is black. Two screw are needed to fix the shaft.Enjoy.
Fab Lab Olbia