From Scratch??

I am flirting with the idea of building a Raprep from scratch. I have messed with 2d cnc home builds so I am certain I could do it. My question is: I have noticed that most machines the bed moves on the Y axis and the head moves on the X and Z axis. Would there be any advantage or disadvantage to making the either the head or the bed move on all 3 axis?? Also instead of using belts would there be any drawback to using screw movement on all 3 axis?

Separating X and Y movement is mostly to minimize mechanical complexity/# of parts, and to have a larger build area in a smaller space. As far as print quality, no I don't think there is any disadvantage.

The drawback to using screw on X and Y axis is speed... It would be horribly slow, and there would not be drastic accuracy improvement.

-Nick

I come from a background in cnc machining and cnc maintenance and i had the same questions as you before jumping in. Heres what i quickly found out after having a Prusa. As stated above, using a bolt or even a ball screw on X and Y would be very very slow and the accuracy would not be any better than the belt setup. Amazingly to me the stepper motors and belt setup are very accurate, fast and repeatable. The way the standard Prusa is designed is very simplistic and can be tweaked to be very accurate and fast. Simpler is always better. The more complex you make it the more parts it takes and the more hassle, potential for failure etc etc. There may be a better way to build a reprap but i cant see a way thats a lot easier and cheaper.

The other reason for separating the movement of the axes is to reduce inertia. Acceleration and speed are more important with FDM than with conventional machining.

I have been pleasantly surprised at the accuracy of the belt approach (I also have a background in CNC and precision machining).

It is important to remember that there is virtually no force generated on the part during the build so pretty much the only forces the axes have to contend with are acceleration of the mechanism and part (which is generally quite light).

You also have to accept that FDM will never give you the sort of accuracy and surface finish you can get from subtractive machining but it will give quite good results and is capable of making shapes that would be impossible with subtractive machining.

Another reason for separating axes is accuracy. If you mount one axis on top of another, this other axis will inherit the inaccuracy of the underlying one. So ideally, you'd mount all three axes directly to the frame, which isn't possible, though. But mounting X and Y directly and only Z on top of another (or X on top of Z) is doable.

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