Saturday, June 14, 2008



It is surprising what can be built without support material. Kyle Corbitt has designed a RepRapable solar collector described here.

The structure is made up from a triangular lattice like this : -

The risers only overhang 30°C, so they are no problem but the horizontal beam looks like it should need support material. Kyle asked me to try building it without, so I gave it a go. Here is what it looked like after it was made: -

Very hairy but basically sound. This is it after being cleaned up with a scalpel: -

It took about 45 minutes to make and used only 7g of ABS, not including the raft. Head travel while not extruding was about 42% of the filament length but as I move twice as fast as I extrude that was only 21% of the time.

Despite the risers only being about 3.7mm thick it is very strong and rigid. I loaded the centre of the beam to 1.5Kg and it showed no sign of breaking. I also loaded one end to 6Kg with no sign of movement, so the beam could easily support 10Kg and possibly a lot more.

At the top of the base beams the triangular section goes down to zero width. The top four layers are only one filament wide so are very fragile. I don't think they add much to the strength so it would be better to truncate the top of the triangle. Interesting though because it is the first time I tried to make something this thin (0.6mm) in ABS.

Enrique added an option to make the infill go along the length of bridges but it is not actually needed for this shape. The top beam has an inverted triangular section so the first layer of it is just two parallel outlines which span the gap. The rest of the beam builds out from this at 30° so it does not matter which way the infill goes. The first few layers did sag a bit but the top of the beam is flat. An inter layer pause may have reduced the sagging.

So this looks like a good way to make large structures that are light and quick to build, but still strong.

Another example of getting away without support here.

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Hey if you ca print that why not try printing a Sierpinski Pyramind. It's a 3d fractal, which means it's infinitely expandable.

DXF files here
The overhang issue has never created undue concern for me. If I want to make a bell I wouldn't extrude the clapper in place, I'd make it separately and install it post build. This points out clearly how good engineering gets around such problems.

Has anyone designed an interlocking block like Legos for extruding by reprap?
Very nice. I am planning my repstrap (up at 3am working on ideas for an extruder drive - watch this (tired) space)

Support structures allow full build without human assembly ops required, but I believe a multi-talanted robot like the hexapod with grippers etc would be able to assemble items.

btw, there is no S on Lego.Ever. but I love Lego and always wanted to make my own special parts (when teen) and my nephiew is going to attempt a mindstorms chocolate extruder. yum.

Hopefully, the project will not become stuck in a Sierpinski tar pit. I guess the minimum detail should have some reasonable limit, say 3x line width...
What a blast from the past! When I was a wee graduate student in the early 1970's, I was very interested in this kind of solar concentrator. For my money the guy who had thought most deeply about it was a Frenchman by the name of G. Francia. He eventually built that huge solar concentrator using heliostats in the French side of the Pyrenees.

In my opinion, though, he built a much more interesting concentrator in Italy near Genoa on the coast. It fit into a small vinyard and generated about 150 kg/hr of superheated steam at 600C. While his boiler design was brilliant, his way of focussing the mirrors was much less well-described but even more brilliant, in my opinion. If you can get a copy of the article that he published in the International Solar Energy Society Journal (ISES Journal) which would be Vol. 12, Issue 1, September 1968, pp 51-58, you will get some good, if a little opaque description of the mechanics and some awe-inspiring pictures. You should be able to get it through interlibrary borrowing or you could buy it direct from Elsevier (Science Direct)

for about $31. Interlibrary borrowing should be cheaper.

The trick to his focussing array is that it ran off of a single pendulum clock and depended on a very clever kinematic mechanism with, iirc, chain drives to make it all work. It would be a perfect reprap project.

Thank you for publishing this. I had a very nice few hours last evening remembering how all that worked.
Could you not rotate the design so that both horizontal beams are lying on the raft?

Vik :v)
The problem (in the absence of a filler solution) is not that you can't figure out a way to build most things - but that it takes significant skill to do so. If RepRaps are to become household appliances up there with the Microwave and the inkjet printer then these things have to "just work" regardless of poor attention to overhangs and how the object is placed on the raft. Digital photography driving colour printers is a case in point. If you had to drag your photo into an art tool and fiddle with the controls to turn it into a Cyan/Magenta/Yellow/Black image manually - and if you sometimes had to print the photo upside-down to get the colour to come out right then those would be specialised tools that only professional printers could use.

RepRap needs to "just work" no matter what 3D shape you toss at it - and for that, we ultimately need a decent filler solution.

(And as for the solar collector - it's a neat project - but wouldn't a sheet of aluminized mylar sucked into a large bucket-like object with a partial vacuum provide an almost perfect mirror without all of that complex structure and fiddly mirror adjustment? I'm trying to recall what shape a sucked mylar mirror forms - but I believe it would be at least as good at focussing light onto a reasonably large collector such as the one envisaged here.)
Steve, RepRap will "just work" one day, but at the moment we're still at the stage of it only just working - gotta start somewhere. However, the stage we've just reached is the one that allows proliferation and hence evolution, at least among the technically astute.

You see, although no overhangs would be a problem for mass adoption, it is one that doesn't actually stop replication - so at present it's relatively low on the priority list. Things like build quality and metal injection spring to mind.

Feel free to build one and join in the fun! Maybe you could be the support material hero?

Vik :v)
Very cool and impressive. Tools shape the way we think and things like this show how the limitations of RepRap can be overcome with a little creativity and luck.

Regarding Steve's point, the issue is not in RepRap, but the software that generates the models. Right now the focus is mostly on the hardware/firmware since that needs to be made to work well first. As that begins to mature (arguably Nophead's stuff is there) then the time will be right to start thinking about how to optimize models for fabrication.
Cool, great stuff (sorry I'm late to the party, I've been out of country for three weeks)!

Thanks for testing this. I'm trying to think of ways to systematically determine the average RepRap's ability to print overhangs...
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I'm just a that bit in my thesis where I say: it's impossible to print a horizontal overhang without support material.

Unless you're nophead
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