Why do repraps have a bed that moved in the Y Axis?

I'm just curious why most repraps seem to have a bed that moves in Y (or doesnt move at all), where as non-repraps e.g. Makerbot seem to have bed that moves in Z.

Is this a function of print bed size and hence weight of print bed versus the weight of the X axis carriage (and extruder etc)?

Thanks

Edited 1 time(s). Last edit at 10/17/2013 04:29AM by rogerclark.

it is more expensive to build a device that supports the x/y axis also supporting the extruder. the mendel design constrains motion to 1 axis to allow easier alignment and less play and slop. it is a matter of costs because if extruder is allowed both x/y motion then design needs to be very precise and parts need to be relatively strong to work with minimum distortion.


Also a different constraining system is needed on z axis for makerbots and ultimakers. you need stainless steel shafts, with high quality bushings that can handle a lot of load. so parts need to be very precise for z to not have much wobble/ or distortion, or even slight slant in angle.


it really is nothing new to do such a design. the original darwin printer from reprap had extruder move in both the x and y axis.
so basically if your table surface, or extruder moves and it moves in more than 1 axis, it is really hard to constrain with adjustments, and requires very precise, very expensive bushings, very precise and accurate stainless steel rods.

it is a matter of cheaper costs for reprap machines.

There is no hidden mystery. All likely options have been explored. It depends on what the designer is familiar with or what they think will fill the goals for the minimum effort or cost.

The basic choice is should the extruder shake, should the model shake or should they both shake. If building large items then having only the extruder shake is a good plan especially with a Bowden extruder. The Z axis usually moves slowly so it is less of a constraint. With Delta Bots it is possible to have the model fixed and only the head moves.

To get an idea of the variety you can look at those designs that are documented in the RepRap wiki that have been categorised in terms of the head motion.

[reprap.org]

---

Kalle
--
Lahti, Finland
The only stable form of government is Open Source Government. - Kalle Pihlajasaari 2013