Alternative alignment

The Darwin build instructions recommend squaring up the frames by adjusting the corner blocks to get the correct length stubs sticking, out as the excerpt below shows: -

This assumes that the rods are exactly the same length. I think what is actually important is that the gap between the rods is exactly 400mm. To achieve that I made a temporary jig from a couple of diagonal tie brackets and a piece of the 8mm rod and adjusted it so that the outside of the brackets was 400mm. I then used that to space all the rods of the lower frame. I also set the short stubs to 20mm using the first method above. Any variation in rod length then ends up on the the 28mm stub.

Knobs

I wanted some wingnuts to let me quickly clamp the bed material to the table and release it again. The XY table came from the US so it has 2BA, rather than metric threads, in it. That means I can't get them locally and would have to order them. The cost would be about $6 for 10 including postage, but I only wanted 4.

Then I remembered I have a machine than can make things so I made some knobs with captive nuts: -



Easy to design, but the hexagonal cavity is a pain to model in ArtOfIllusion. You have to start with a six sided polygon. You then convert it to a triangle mesh and then extrude it to make a hexagonal prism to subtract from the cylindrical shaft.

The three types of solid primitive: cube, cylinder and sphere all have editable dimensions but for some reason polygon primitives don't show any dimensions. To get round this you have to set the grid spacing to the dimensions you want and snap the polygon's bounding box to the grid.



I intended the nuts to be a push fit but they were too tight so I pushed them in with a hot soldering iron. The small M4 one on the left was a test to see if the design scales. I think the nut cavity needs to be a bit shorter for metric nuts.

These cost less than 6 cents each in plastic so that saved me $5.76. A good example of the economics of RepRap. Although it is no doubt cheaper to make wingnuts by traditional means in large numbers, the cheapest way for an individual to obtain them in small quantities is to RepRap them. Of course I needed some plain nuts, but they are a lot cheaper and easier to obtain.

Unsupported!

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.