Thursday, August 17, 2006


Here you go...

Lord Cat said... I've always had the idea of making a 'robotic arm' to hold the extruder. Something similar to this (but a little more complex) to reach the "entire work surface". I was figuring either mounting it to the 'wall' with a vertical axis to it, or mounting it to the 'floor' with the first 'segment' being long enough to ensure that the arm doesn't violate the 'extruded space' as it moves up the layers.

Attach your xy threaded rod positioning stage to the rightmost red knobbed node, put in a longer slide threaded vertical axis and put a Mk II in place of the leftmost red knobbed node and you should be good to go. :-D

Cool picture. :)

It still seems to me though that the armature structure would need to be made out of metal. Would plastic parts be able to take the stresses that are going to be torquing the the hinges, and supporting an extruder head? If you could keep the extruder head light, it would be less of an issue. (How much does the MkII extruder weigh in production mode?)

I'll try to create a 3D mockup of my imagined disk model when I get home. I think it has the same amount of motive freedom, but can ride on top of structural supports.
Maybe 2 oz. max.
Most of the reprap designs I've been seeing use some sort of structural metal (bolt/rod/tubing/etc), I don't see why you couldn't just use 'steel rods' for 'bones' inside the armature, and cap/sleeve them in extruded plastic.

What actually sparked my idea was one of those desk lamps that are hinged on the end of an 'arm', with a pivot at the base where it attaches to the desk. Replace the springs with threaded rods and and some structural stability, and it'll be able to achieve all three axis at once. (You'd probably need a 3rd segment to it so that you're not violating the extruded space).

My biggest 'sell' about the robotic arm is the footprint of it. Pretty much every other machine being designed is as big as, or bigger, than the objects it can create. A good robotic arm would have a very small footprint (especialy folded up against the wall or ceiling), would easily be capable of creating every piece that makes it up (instead of having to break up your moving platform into 4 pieces so that it'll fit on itself while being prototyped/etc).
Beagle, I was just going to ask you guys if some of these ideas could be drawn out! Even the roughest sketch helps us slower ones follow all these ideas.

A bit of math, and we should be able to see how much stress there would be on those hinges. Plastic can do a lot of stuff, it's like aluminium, you just need to put more... ;)

Lord Cat, cartesian coordinate arms are rather different from the one you are discribing, but they also have many of the advantages you mention, plus they are much easier to programme. Also, they might be much easier to build with low level RepRap -ish tech, not so dependant on very accurate/powerfull encoders/motors and such.
With an arm that long, I'd want to make sure it's sturdily built out of metal. The Z-Axis looks like it would have a tendancy to wobble, and not support a heavy extruder arm.

That being said, it's definitely a neat solution to the speed issues for the motors. If the potential kinks were worked out, I bet it'd look pretty neat.
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