Thursday, August 17, 2006
Dual Disk Sketch
Okay. Here is a sketch of a dual disk concept. I've left a lot of detail out (for example, how the extruder head might work -- it would need to be attached to something above the work space, or to a part that straddles the base with enough clearance to allow the inner disk to rotate the ~45 degree span.)
I'm not sure if it is an improvement or worse than a threaded model; but I thought I'd throw it out there anyway. Some of the problems that it might have:
- friction between the plates. What is the coefficient of friction for two plates of plexiglass split by a layer of vaseline?
- mounting the motor to the plates. I've sort of made this abstract, but I think it could be done.
- donut print area ; As you get closer to the centers, it becomes a lot harder to control (one motor has to be driven uber fast, the other, uber slow; it seems rather than trying to fix the singularity, just don't go there). I figure the ring form would allow a reasonably wide variety parts to be made.
- available resolution varies : the resolution at a point will be different for different angles. As you get closer to the center of the top disk, resolution in the direction of it's rotation will be higher
- It is a reasonably small footprint for the size of pieces it can make.
- It would be relatively easy to add a 2D motion sensor to monitor the top disk motion (Upside down optical mouse mounted beyond the radius of the inner disk.)
- It should be pretty easy to build the XY axis. The Z axis would probably still be a little more difficult.
Here are a two wireframe top down images that show the range of motion of the center disk (The top disk is allowed to rotate 360 fully.)
The extruder nozzle is the square/triangle pair near the top.
An outer disk with 30cm diameter would fit on a frame of about 40cm square, and could print solid cubes with a base about 10cm x 10cm, long rectangular pieces of about 2cm x 25cm, and of course, larger hollow pieces of up to about 30cm in diameter.
Oh, and here is how you would add a second print head (click for a larger image). This would not allow you print two parts at the same time (you'd need a second top disk for that), but could be used to fill with a second material
**** HOW IT WORKS ****
There was some confusion as to how this XY axis would work. In the drawing here, I hope you can see how you could move any point in the ring under the extruder head. Look at just the green lines for a moment. If motor A were the only motor turning, you would form the arc of a circle between the two darker green lines radiating out of the center of that circle. If motor B were the only motor turning, you would get an arc of a circle between the two darker green lines radiating from that circle. If both motors were running, you would get some curve (or line, if you controlled the motor speeds accurately) that represents the sum of two circular arc vectors.
The same would happen when moving along the blue line, and the red line. I think it might be easy to visualize if you turn the two disks into lines, put the extruder head out on the tip of the line, and pretend that this is above a working surface --> You get a robot arm. If you reverse the concept, and put a working surface on the robot arm instead of putting the extruder on it, this is what you get.
I was stumped for a while too, but I think I've got it! The partial disk is anchored to the square base, and, it -supports- the full disk by its center, which can let it spin any way it wants.
Very novel, where did you get such an idea Beagle?!
BTW, I think you could reduce your friction between the two disks to near zero, just by feeding a stream of preasurized air between the two surfaces. Kinda like a hovercraft. I like hovercrafts. :)
Hopefully, it is enough to demonstrate the action.
As far as where I got the idea, Vic's idea of a two motor reprap machine was my inspiration. He had one linear axis, and a combination linear/circular axis. I just took it to the next level by wondering what you might get if you just used two circular beds for the XY motion instead of the linear axis.
That might work.. Another solution might be to just make a smaller yet sturdy inner hub/bearing for the surface to ride on (maybe 1/2 to 1/3 the diameter of the work circle.) This would also give a place for the wires to get into and out of the motor on the second stage. The way it is with the current sketch, you either have to string them down from above and figure out a way to unstring large donut shape objects you've made, ; or bring the wires up thru the bottom, required a long slot in the bottom surface.
Okay, now the nitty gritty question. What's the advantage in doing it this way?
I see two advantages to this design, one is that motors could be embeded in the perimeter of the disks. Just a ring of electromagnets and you have a reprappable motor! Also, if the two disks are smooth and rest on each other, there is next too no structural flexing going on. Good for softer more flexible reprapped plastic I suspect.
Ah! The fog begins to clear!
Just a ring of electromagnets and you have a reprappable motor!
Interesting. I hadn't thought of that idea.
I was thinking a chunkier pedastal to mount each platform would be the reprappable platform; you could add embed roller bearings, use radially aligned motors driving pinion gears that mesh with a round rack, etc. The 'hole' in the middle is smaller than the largest solid piece, so you just need two printed parts to get a solid disk of diameter up to the work diameter.
Making the motors part of the platform sounds better than my idea! :)
Okay, now the nitty gritty question. What's the advantage in doing it this way?
I'm not sure. There are speculations that I have, that might be arguably advantageous:
1. You can add one extra head without reducing the print area, or doing complicated head swapping actions.
2. The machine requires ~2x footprint area of the largest part it can make. A threaded X/Y axis requires area ~4x the largest part.
3. It would be ideal for making wheels and large round parts. The hole in the middle can be made as small as you want, if you solve the singularity at the center where one motor strains to spin 180 degrees as the other motor strains to move .04 degrees to achieve a .1 mm resolution of a 15cm radius work surface.)
4. Sort of an extension of 3 above, it could be adapted to make a specialized high speed high resolution gear maker - spin the top disk at a good clip, and twitch the lower disk back and forth to create each tooth, use spiral infills that change direction each layer.
The motor idea becomes less an advantage when you realize that you can do this for linear style XY axis too -- just use your rack and pinion, and build a big gear with embedded rare earth magnets in it to drive it back and forth.
I beg to differ in the ~4x area for the threaded X/Y axis... That's assuming your axis are moving the platform (instead of the extruder), and that there is little/no overhang beyond the end of the axis...
By mounting the extruder on the axis, you can do away with the '2x' space requirements, and achieve the 'margin' requirements that printers/plotters have. (consider the differences between the old style typerwriters, where the paper moved but the 'keys' stayed in one spot, to the new style typewriters/printers, where the paper stays in one spot and the 'print head' moves back and forth...)
I'm also not a fan of spiral infills... Even moving in opposite directions, the layered 'strands' would still create weak spots that could form into cracks/etc. At the very least, some form of radial infil every other (or every third) layer would be required for good stability...but you'd probably have a much easier time just doing basic cross hatching (than trying to get the radial fill to work right)
As far as spiral infills, I wasn't proposing a single solid infill, but rather, 50 or 60 radial lines. The cross angle between alternating layers would probably be between 30-90 degrees. I don't think a linear cross hatch would be any stronger/weaker.
In any case, I'm also somewhat skeptical whether this would have straight advantages over a straight threaded rod solution (Except for the specialized gear/wheel reprap - simply because it's natural motion flows more closely to the shapes being built.)
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