Z axis print bed

I am surprised that more people/companies don't use a z axis print bed and have the y axis and the x axis be at the top but fixed in the z axis.

I know makerbot does this and so does type A machines, but not a lot of reprappers do it.

Any reasons why?

Are there mechanical downsides to this?

No mechanical downside for the size of most RepRap printers. It doesn't scale up too much without losing some performance to the mass of hte print bed. Incidentally, the first RepRap printer (Darwin) was configured as you describe. The Mendel design was developed in part to address some of the difficulties of that implementation of that configuration. Primarily, Mendel is a simplification. Myself, I prefer the Ultimaker and Tantillus configuration. Someday I'll build something similar to that design.

Edited 1 time(s). Last edit at 04/06/2013 03:24PM by Dale Dunn.

as far as why the repraps use seperate x and y axis's is because it uses less smooth rods and bushings, and it requires a less rigid frame, which is good because it uses less parts to make, but its less rigid, and therefore sucky. now combining the x and y for the gantry also has the advantage of using significantly less space than using a moving bed, about 1/2 if the bed is moving in one axis and 1/4 if the bed moves in two axis. of course if you have a small enough printer, where the extruder becomes heavier than the bed, and the mechanisms for moving the extruder take more space than it would take to move the bed, then it become advantageous to move the bed, but in most cases this is not true. for example my new printer has a 500x500mm bed, moving the bed would require a substantial amount of space, a minimum of 1000x1000mm, also the bed weighs quite a bit with the whole heating mechanism, glass, and insulation, not to mention the additional weight of the rails needed to move it, my bed does move in the z axis, by using 3 m6 threaded rods(i chose m6 because they are 1mm per turn and they can flex so if they get bent it wont affect the movement of the bed) and the bed is on a set of rails that keep it true to the z axis, even if the threaded rods are bent. however something like the cupcake cnc has a small 100x100mm bed that is quite light and the frame for the printer couldnt be that much smaller anyways so it makes sense to move the bed instead of the hot end, also its not using a bowden tube so that also comes into play.

when i was designing my new printer i considered many options for x y and z movement, i finally decided on the h bot configuration which uses two rails for the y axis with one rail inbetween them for the x axis. the bed moves up and down. what i like about this configuration is that all of the motors remain stationary and in reality i only need one motor per axis, but because i want to do some cnc milling of aluminum on my machine, it uses a total of 4 motors on the x/y axis, and hbot configuration is basically the same as corexy, except it uses less pulleys. so if you want to learn about it just google corexy. now the advantage of the hbot over the ultimaker is that it only has to move one rail back and forth instead of two, so its lighter for the x axis, and the same on the y axis, also it doesnt have to rotate the smooth rods, so that cuts down on inertia as well.

I am building my machine with a Z bed. It's a large machine, and there's no other way to do it. Mine will have a welded 1 piece steel frame with adjustable Y & Z rods, and multiple extruders with bowden tubes. I agree the design of the printer is all a matter of what works at a given size. As you get past a cubic foot build size, you have to begin considering the weight of the print as well as other things. I prefer to put all the weight in the fixed bits and make the moving parts as light as possible.

Im starting my first 3d printer design so I am by no means an expert on the subject....

I went with the z axis bed design mainly because I felt that my current printer which has a fixed Y axis works great but the z axis is tall and when the weight of the extruders is sliding around high off the bed the machine looses stability.

I found a nice solid linear setup surplus and decided that by mounting this assembly firmly to the chassis the whole enclosure would be even more solid the effects of the heavy extruders should be more uniform irregardless of how high they x and Y axis are off the ground..

Its all guess work for me but in my head it makes sense. I would just suggest that you make sure that the z axis assembly adds to the stability of the unit (im building a heated enclosure box structure) if you go with a frame design that is not solid when th weight of the bed moves up it might give the unit more wobble,,,

Wont really know till I start the build next week....