I have long dreamt building my own 3D printer on the cheap, but the idea of using linear actuators always scared me of doing so. First of all they are expensive, and secondly great care needs to be taken during construction to have good accuracy in the end due to the open loop nature of traditional implementation.
I eventually realized that moving a plane is not that hard without using any linear actuators. You can basically take an holonomic drive system from a robot to do so. In sum, is like having an robot turned upside down and fixed to the ground, and having a work plane above it. Since the robot is fixed, when the wheels turn the work plane moves instead.
The main problem with this approach is the accuracy of moving the work plane since my idea is to use either servos or DC motors. To compensate for that the plan is to use optical mice (more than one for accuracy) to correct the movement of the work plane. The Z axis should use a similar approach, but with a simple wheel.