Tuesday, January 12, 2010

 

Meet Ola, the RepolaRap (At least, her first parts)

Having acquired all the non RP parts to build a Mendal, I started down the path of building the RP parts out of MDF and angle aluminum. The STL model files are not very suitable to be built directly out of wood, at least, with my own woodworking skills; I decided to alter things around a little, and after 3 versions of the X-Axis chassis and bearings, I was still unhappy with the state of things and the progress.

I figured there must be an easier way to DIY, and decided to try an idea I posted on this blog about 3 years ago. I discarded other ideas because, well, frankly, I think a polar bot would look really cool, and has some intriguing possibilities -- being able to print an 28cm / 11" diameter timing belt gear pulley to replace the MDF, for example, assuming I can avoid warp, so I can give friends mostly non-MDF Repolarap parts of their very own, maybe even more quickly than I could print out the parts for a mendel for them (maybe? Okay, maybe not.. that would be a really big part and very liable to have accidents partway thru; and thats not even thinking about the 42cm / 16" gear needed by the radial arm.)

So, I created a design in blender, tweaked it a bit (Okay, a lot), and posted in the forums to see if anyone had tried anything similar or had any suggestions or comments. After becoming more confident that something like this might work after creating a bearingless "mockup" in MDF (the wheels two wheels and an unusable upper radial arm that I did not take a picture of), I decided to take it further and ordered some 5mm steel balls to use in the bearing races, timing belts, and pulleys for the steppers sitting idly in the "here are my Mendel parts" bin. Unfortunately, I separated the orders by a few days -- I bought the steel balls thinking I wouldn't be ready to attach motors for another few weeks. Looks like I underestimated my woodworking skills, or overestimated the difficulty in building the XY stage prototype, or both. Anyway, the steel balls arrived today.





The build platform (medium circle), bearing races (the two smallel dark circles, steel balls already inserted), upper radial arm platform (the funny shape one with one of the bearing races on it), lower radial arm (BIG circle), and the base (the square with a bearing race on it). You probably don't need to, but can refer to the draft WIKI page on this design for a render I put up while working thru the design in blender.






I anchored the build platform to the upper radial arm platform, with intervening bearings to make it turn easily and smoothly. I'll still need to figure out how to put something on top of this to get plastic to adhere. The 4 machine screws visible are just there because I was too lazy to take them out when I disassembled the structure to take pictures.






Then, I anchored the lower radial arm anchored to the base, with intervening bearings to make it turn easily and smoothly. You have to proceed in this order because the nut (or bolt head) to lock down the pivot points become entirely inaccessible between the upper and lower platform. The hole you see here provides access for the bottom bit on the build platform pivot. Theoretically, I could drill out windows to get access to these, and still might because I may like to adjust platform bearing compression without having to take everything apart.








Finally, I screwed down the 4 machine screws into captured nuts I had already installed on the lower radial arm platform to firmly attach the radial arm parts together.

If I were to do it over again, I'd change the machine screws with inverted bolts up, glued or epoxied to prevent them from rotating, and ensuring enough recess space to allow the lower platform to rotate.




It'll be a few more days before I can actually see if the motors are going to move this. The wheels spin freely, have very little play in any except their intended degrees of freedom, but are a bit heavy. It might take a while to visuallize the motions. Think sort of like "Spirograph (tm) on steroids" and you get a feeling for how it works -- Radial motion on the build platform makes concentric circles around the build platform pivot. Radial motion on the radial arm makes circles half the size and touching the rim of the build platform. If you move each axis at a constant angular velocity, it draws big Rose Curves. Oh, the extruder stays stationary relative to the two platforms -- it will move only up or down relative to someone watching the machine.

Anyway, I plan to build motor mounts with a simple box in the corner on the upper bearing arm platform, and on the base. A 10 tooth pulley will drive a belt that wraps entirely around the appropriate circular platform. At 200 steps or 400 half-steps per rev, that should give near about .1mm accuracy everywhere the toolhead can reach. The belt doesn't seem to move up and down vertically when interfaced to the rim on the MDF, but I may need to put some guides nearby to keep it on the straight and narrow.

The wheels are a little wobbly (1-3mm wobble or so). I made the mistake of cutting out the circles prior to drilling out the center. Oops. If I were to do it over, I'd trace the template on a square with the hole drilled in, and be a bit more careful when cutting that out. I can always remake these, in any case; with a scroll saw, it takes about 10 minutes.


Comments:
Its great to see new designs. It'll be interesting to see how the drive software turns out.

Sort of begs for a polar to/from cartesian look up table. Wonder how much space that would take up. Fortunately you could do it with a table stored on an SD card
 
Hey thats looking realy good very compact in fact I imagined it would look a lot larger.

Will it need a much larger base to support the extruder gantry. Also for moving the print table down from the extruder. Or are you going to move the extruder up from the print table base?
 
>> Sort of begs for a polar to/from cartesian look up table. Wonder how much space that would take up. Fortunately you could do it with a table stored on an SD card

My plan is to compute all transidental functions on the host, which will convert them to cubic splines. The firmware will be updated to process cubic spline paths. See http://beaglefury.blogspot.com/2009/01/spline-path-computation-using-integer.html for the approach, and calculations I intend on using in firmware.

>> Will it need a much larger base to support the extruder gantry. Also for moving the print table down from the extruder. Or are you going to move the extruder up from the print table base?

It should be possibly to use any Z axis design, as long as it has the clearance needed (Either a unsupported 15cm+ overhang, or ~32cm+ across dual support arch (set diagonally at one of the base corners.)

My vision is to put the Z axis as a module that attaches to the base. I'm toying with the idea of creating a single sturdy drawer slide Z axis about ~15cm wide by ~54cm tall, with unsupported overhang of ~15cm. Assuming I can reprap most parts for a new Z gantry, this would allow me to build multiple Z axies and have a two or three headed RepolaRap for the price of another extruder or two, a stepper per added axis, and a few hardware parts.
 
I like it, its something Ive been thinking of for a long time too. The issues you will encounter revolves around knowing where you are. Because the result of the motors are not linear, you will have to compensate in software (easy) the table pulses per rotation (easy) and where you are to start with accurately (not so easy).

You can produce an auto calibration method. Every time you switch the machine on, you place a set square on the table (cheap available and accurate enough) next, eater with a pin (electrical conductivity test) on the extrude head or a webcam (optical setup) you locate 3 points on each of the 2 axes, from there the software can calculate the radius of each arc and the relative position of the 2 centers. P.S. I found you can get lazy Suzan bearings at cabinet shops. they eliminate a lot of play while being available and relatively cheap. P.S. on the webcams, by opening the case you can usually focus them to less than an inch, but they do need a good light source.
 
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