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by EFFALO, published

domekit-prototype by EFFALO Mar 13, 2011

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domekit-prototype by EFFALO is licensed under the Creative Commons - Attribution license.

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this is a prototype connector design for assembling geodesic domes and spheres. rather than using preset strut angles, it uses a flexible ball joint strut connection that allows the structure to adapt to a variety of configurations. therefore, all that's required to build a dome of any scale or frequency is the proper strut length calculations, available from: http://simplydifferently.org or http://desertdomes.com

the design is based off of william adams' yazzo disc joint openscad script, modified to include a shape language of hexagons and pentagons for the six- and five-way connector types. the strut caps used in this prototype should fit 3/8" diameter dowels (but they might not)

update 3/16 — added strutcap.scad. other code forthcoming.


depending on your application, print out the proper number of hexagonal and pentagonal connectors. for each strut required, you'll need two strut caps that will attach to either end of the dowel. then, simply snap the ball-end of the strut cap into the receptacles on the connector to build your desired space frame.

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Do you have more photos of that assembled dome? That's really great. How did it hold up? No buckling of the joints under the stresses?

I've been thinking of modifying the balls so they have dimples, and the socket so it has a nipple. that way they can "click" into some positions. That plus a dab of glue would make the joint way strong once you had it set at the proper angles.

After considering this design somewhat, and thinking about why the balls would not fit, I have the following observation. A fundamental feature of the original design was in the flexibility of the material between the balls. There was enough give such that the first balls goes in fairly easily, and by the last ball, it's extremely hard to get the ball into the slot.

This is achieved by finding the right diameter for the disc, such that you get enough rigidity, as well as flexibility. Looking at your pentago, I think it might be too stiff, which means there's not much flex, and your balls simply have to fit perfectly on their own, without much benefit of bein
g squeezed as further balls are inserted into their sockets (thus causing you to make your balls smaller to fit initially).

So, you might try using a smaller diameter on the pentagon so the inter-joint material is less, and a bit more flexible. You might also find you get better grip as it will ti
ghten up as you put more balls into their sockets.

agreed, flexibility is tops when you're dealing with so many joints. getting the tolerances correct was difficult; especially across different 'bots and plastics. i wonder about adding slits between the holes on the disc or cutting a + shape into the ball itself would help. need to try that out!

— i really dig your use of ratios for widths and heights of things; that should probably be taken into consideration with these discs! the sharp edge needs some tuning as well.

I think I'm at my limit with OpenScad on making the edges of things smooth. I recently purchased Alibre, so I might revisit those designs with that tool. I suppose you could use something like MeshLab though to do some general smoothing.

Trying to calculate the proper ratio for the disc radius, based on the number of vertices was a challenge I left undone with the original design. I thought about it, but didn't tackle it. I need to take a look at some flowers, or spider webs, or something to get some inspiration.

Your idea of a
cross in the middle might be a good one. Or even a slit per vertex.

I think it's a good basic design, and I'm sure someone will be inspired to come up with something even better at some point.

I've printed off three of them so far...they all are 9/32 according to my drill set and the dowel rod I bought for a sanity check...

they're supposed to be 3/8" but it's hard to get it exactly. the popular opinion has been to thicken the wall and then ream out with a drill to whatever size you need.

*of the strut caps.

Just saw one of these in person -- this design rocks!

I guess after printing the sharp edges are less dangerous then they look in the rendering. ;)

I recommend to round or at least tapper any edges humans come in contact with. Always.

It's just good practice and makes thing much more comfortable to handle.

the edges really aren't an issue once the balls are in the joints. you'll also notice this is work-in-progress, too.

Marcus is really good at finding the positive aspect of anything and not commenting on it. Aren't you Marcus?

I struggled with the sharp points when I did the original design. I wasn't sure how to round them using OpenScad. Perhaps someone else can take up a design that rounds things out and makes them more friendly. This would be more critical if it were rendered in metal I think, but I get the aesthetic point.

Of course I think this is really cool!

I was recently contemplating using this combination as the vertices for my PolyBot. One of the things I would change about the strut cap is to allow the strut to go into the center of the ball. That will make it easier to measure strut lengths, and possibly give it more stability from shearing.

Maybe you've already done this.

Also, do you find you need to print the balls lying on their sides to prevent delamination under stress? Or does it work out find for your purposes?

Lastly, I've taken to putting a hole atthe center of the disc so that you can possibly attach things there, like bu
ngee cord or something.

This is one of the intended uses I had for the disc, so it's great to see someone take it further.

first and foremost, thanks for the awesome openscad code!

we've been printing strut caps with strut opening against the build platform and the ball facing up (like it's shown in the preview), haven't run into any overhang or shearing issues. we modified the strut to go a bit deeper into the ball and put a hole in the middle of each connector, too.

trying to make a huge 3V 5/9 icosahedron and need about 420 pieces printed (55 hexas, 6 pentas, and 360 strut caps) too bad our extruder is down!