After having revisited this model recently I noticed some improvements that could make it easier to clean up and improve its reliability.

This is version 2 of a multipart accessory to any standard servo that converts it output into a linear motion. Designed, tested and 3D printed on makerbot style (FDM) printers this should work for you and your next robotics project.

You should sand and lubricate the tracks that the rack gears sit in well.