Alternative to Linear Actuators or Linear Actuators with small load limit and 70cm movement
Assuming you only need positioning accuracy to ~0.1 mm, you can use standard threaded rod driving a carriage with a single matching nut. My local hardware store has 3-foot sections of 8-32 threaded rod for less than $10. Any sort of small DC motor with a gearhead will do to drive it. Making an encoder for the rotation of the rod is harder, but you only need to sense rotation to 45 degrees to get 0.1 mm resolution.