Overconstrained Axes

I was looking at the mendel apollo thread and musing on the problems of rigidity and overconstrained axes and wondered: has anyone thought of hanging the extruder on a gantry supported by 3 threaded rods that can be pulled/pushed in a controlled manned from the points where they are hung (much like the z axis is driven).

This would totally solve the problem of overconstrained axes at the cost of more complex software to control the extruder head movement (and possibly lower head speed?)

I have attached a picture to help you visualise what I mean.

The square would be the extruder gantry. The triangle at the top would be the support structure (the rest of the structure to hold this in place is omitted).

Hi, and welcome to the forums!

Interesting idea and construction. Great picture as well! This would combine the x and z axis, right?

But (and there always have to be one, sorry), wouldn't the rods that hold the extruder have to be rather long if you want a big build area like in Apollo? I'm no engineer, but say you want to move the extruder 20cm on the x-axis from the current position in the drawing, then the rod(s) opposite the movement direction would have to extend quite a bit.

And if you want huge build height as well, the rods would stick out quite a bit when the extruder is in the top position.

Or am I totally misunderstanding everything now?


About the speed, perhaps it would be possible to use some miniature hydraulics instead of threaded rod?

No, I think you understand perfectly regarding the length of the rods. When you were nearing the maximum build height, they would stick out a long way.

It would combine the X, Y and Z axis. The axes wouldn't really be x,y and z anymore. You would need to translate your xyz position to determine the length you needed to push each rod.

I remember someone mentioning that hydraulics would be good for a large-heated-compartment granule extruder (as opposed to using a pressurised gas chamber) because with pressurised gas, you dont get direct control - the gas is springy). Hydraulics would also simplify coupling of the movement to the rods. Mounting steppers as I was thinking might be tricky, because their orientation would have to move with respect to the frame. Do you think you could get good enough absolute positional control with hydraulics?

Another thing this would allow you to do is have some control over the orientation of the print head. Im not sure if that is useful.

Avoiding head crashes might also be interesting with this setup.

Edited 1 time(s). Last edit at 05/30/2010 04:11AM by Greg Frost.

You've just described a Delta Robot for which we all ready have a working group..

Delta Robots and Stewart platorm

I would suggest heading over there if you want to help them with development etc