WaterJet/ShopBot

For this assignment, we had stock of 4'x4'x.55" plywood, and some 1/16" and 1/32" aluminum sheets smaller than 2'x2'.  After mulling over some outlandish ideas, I decided to try to do two things. 

The first was to try and make as many of the parts for the RepRap open source 3D printer as I could make.  The print bed was a particularly good use of the plywood, and I thought I could try and make a jib to cut some of the parts with 3D topologies (but which nonetheless only required full through cuts on a plane) out of 4"x4" lumber stock.  The necessity of this arises from the RepRap printing many of its own parts.  You either have to have an existing printer make these parts, create your own bootstrapped version out of materials which require no printed material, or create the printed parts out of something else.  They are detailed enough that it would be impossible to do by hand, and voluminous enough that you'd want to do them out of something cheap like wood.

The second was to cut out an interrupted map view of the world, which I have liked since I was a kid, out of the thin aluminum stock, and to bend it around to see what sort of a globe I could make.  I found a jpeg of a modified Mollweide projection.  Interrupted maps introduce cut views to achieve more proper proportion and area.  They are obviously not perfect 2D representations of spherical topology, but they are closer to that ideal.  I used an online program to convert image files to dxf (img2dxf), which did a really poor job and would only do monochrome line drawings.  Here's what I got before editing:



So after cleaning that up and importing it into a dxf in Omax Layout, here's what I have:



After cleaning that up again, generating lead-ins and lead-outs, generating a toolpath, and exporting to a .ord file for the waterjet, I was ready to cut.  I tried it first on a thicker sheet of 1/16" aluminum (which we had only at 1' to the short side), because I thought I'd want to hammer at it to bend it into a spherical shape, and the 1/32" bent too much.  However, I should have seen that I had woefully little material along some primary bend axes, and the thing wasn't going to bend at all without snapping.  Here's a picture of that first attempt:



So I recut with the larger 2'x2'x1/32" sheet, lopping off Alaska, Kamchatka, the British Isles, and part of South America for good measure.  Still, woefully little bend material.  I began to realize that even with all land masses gone, I probably wouldn't be able to bend this thing into anything resembling a sphere before it snapped.  So I did just that, and I was right.  It's really hard to make a sphere out of an interrupted projection at the scale I was endeavoring to do so.  The problem is, you really only have one good axis to bend around. With a spherical surface, you need two to get the correct curvature, so you really end up with disjointed edges with no semblance of tangential continuity.  Here's what I got:





Even if the world didn't work for me, I thought making some RepRap parts wouldn't be as bad.  To an extent, this was correct.  The print bed was quite easy to do on the waterjet cutter out of the .55" plywood, which was only a mm or two thicker than the plans call for.  Here's what the physical assembly of the RepRap looks like, and here's a picture of the print bed I made:





One note of advice for using wood on the waterjet cutter is to make sure you elongate your lead-ins and lead-outs, as otherwise you get splintered junctions.

Now, I looked through the other .stls, the parts which can be seen in white, and tried to see how many could be made with clean through cuts, even if they had to be rotated one or two times to complete the part.  Even though the waterjet cutter is a little overqualified for wood, I thought I could create a jib and automate the process of creating parts such as the corner mounting bracket.  Once you've created the jib(s) and got your drawing views set, you could in one batch create a large number of parts.   Each different part family would require its own jib(s), and it's a pretty labor-intensive fixed cost of time upfront, but afterwards, once everything's set, it's not a bad way to make the parts.  Unfortunately, I got stuck on back-solving for parametric drawings from .stl files.  All the tools SolidWorks has for converting to part geometry from .stl preserve the polygons in any resulting drawings derived from the parts.  I ended up either having to hand delete lines (which was way bad for things like fillets and some fractured faces) or to just use some of the nodes of the .stl as references for overlay vector drawings, which was a bit faster but still quite annoying and not at all extensible.  Ideally you would have the vector file formats of the parts as they were drafted, but often these are just not to be found, especially for CNC or 3D printing applications where they are irrelevant.  Here's an example of an .stl file imported into SolidWorks using the stl import plugin:



Those isosceles triangles leading to the corners would have to be hand selected and deleted, likewise for the fillet, unless you wanted to redraw.