Seperate Z-Axis Drive

I'm only in the initial planning of building a RepRap, but one thing that strikes me is that the z-axis doesn't need to be as fast as the x- and y-axis. Lurking around the forums, it looks like there are a lot of ideas are out there for replacing the expensive stepper motors, so I thought I'd throw anther one out: use a simple DC motor with a screw drive on the z-axis. Then at least you only need two steppers instead of three.

Precision needs to be maintained. Without a stepper you would need a shaft encouder so that you would know where you were. Without it, you are driving blind. I agree, however, that the z-axis is a good place for cost reduction. Use a smaller stepper motor if nothing else. Good idea.

Demented

I think you will find that stepper motors now aren't nearly as expensive as they were in the recent past.

I've been through the stepper versus constant dc motor debate before and ultimately the constant dc motor with optical encoder wound up being only a few pennies cheaper (including the drive electronics and programming time). Which is great if you are mass producing a product, but for limited builds, it may not be worth the effort.
But if you feel more comfortable using a dc motor with encoder then there is no reason that it cannot be done. You will wind up with the same resolution and
probably the same speed.

Personally I hate mixing steppers and constant dc motors in the same application. Having both means that you have to have 2 different types of drive components which means more thought has to go into the boards.

I spent the better part of nine months hammering dc motor/encoder systems. It ain't worth the bother.