Spanning a void

s0lstice requested that the next time HydraRaptor was extruding into fresh air I should post a video, so here is a video of a part with a large covered void being made: -


Spanning a void from Nop Head on Vimeo.

It is a bracket to mount a tiny stepper onto a Darwin corner bracket.



As you can see, the unsupported layer is very untidy, but the next layer sorts itself out. The reason I had to make this side the top is that there are projections on the bottom to mate with the corner bracket: -



Here is what it looks like installed. I need to sort out a shaft coupling. I have no idea if the motors will be powerful enough though.



I also made some feet for my Darwin to stop it scratching the table: -



I stuck felt pads, that I happened to have, on them, but rubber would probably be better for non slip.

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.