Saturday, July 26, 2008


Every little helps

The Darwin design has 10 diagonal tie bars across the corners of all but the top face of the cube, making it very rigid. These are attached by 20 diagonal tie brackets: -

The brackets are held onto the protruding 8mm stubs by M5 set screws through a captive nut. The diagonal bars are then held in place by M8 nuts either side of the bracket.

When fitting them I noticed that the set screws and nuts are not necessary. All the holes I make come out a little undersized and stringy so I clean these out with an 8mm drill. This makes them an interference fit onto the M8 rods. The force exerted by the M8 nuts is enough to squeeze the bracket to make it a tight fit. This is the case when they are made from ABS with 25% fill. Other plastics may be too strong or brittle.

This shortcut saves 20 grub screws and nuts and the time to fit them (inserting the nut can be quite fiddly). Not only that, the bracket can be simplified and made smaller because it does not need space for the nut and grub screw. This optimisation is well worth doing because, although these brackets are quite small, there are 20 of them so they are a significant part of the time taken to replicate.

Here is my smaller design which uses 21% less plastic and reduces the time to make 20 from 11.5 hours to 9 hours on my machine :-

I also used a truncated teardrop for the lateral hole. This relies on the fact that filament can span gaps as well as being able to build out at 45°. The drawing below illustrates that, even for an 8mm hole, the difference between a proper circle, which would require support material, and this truncated shape is very little. It also shows where the full teardrop would extend to.

Here is a picture of it installed alongside the old design: -

I think this is a beneficial mutation that will slightly increase the rate at which Darwins reproduce in the wild. The new DNA can be found here.


Yes! This way the amount of parts (and assembly time) could be reduced and thus automatically the proportion of replicable parts increases. Nice.

Perhaps one day we will print the diagonals as well out of a couple of parts. They mostly need to resist a pulling force and I think it could be made strong enough. It will also reduce the weight of the already pretty heavy Darwin. You need to have the two plastic parts lock onto itself, perhaps with an adjustable screw in the middle that will tighten it. That way you can have the parts that go in to the corner bracket printed as well (or just use a screw there.

I've posted sketches in the forum
Nophead, what about cutting a slice across the smooth rod hole, such that tightening the threaded rod clamps the smooth rod directly, like the PCB clamps?

I ask because I broke one of my cast tie brackets by over tightening; I didn't use grub screw either. I couldn't get it to grip the smooth rod.

I'd try this myself, but I still haven't got a usable version of the software running on my Darwin.
I considered a slot but it uses a bit more plastic (more wall), takes a bit longer to print, and is weaker in the axis where you want it to be solid.

The cast blocks are solid PU so less flexible and possibly weren't a tight fit to start with. I imagine this design will work with PCL and HDPE as well. Possibly PLA would need a slot but the movement required is minute as long as you drill with the same drill as your rod diameter.

The grub screws I got from BFB grip the rods very well. They have a sharp ring on the end that cuts into the rod.
Great work as usual, Nop. I wait with bated breath to see what the controller system for Darwin you talked about designing looks like.
That looks beautifully clean, as always. Brilliant!
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