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March 8th, 2009 - Doug Ayen's Blacksmithing Blog — LiveJournal [entries|archive|friends|userinfo]
Doug Ayen

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March 8th, 2009

tensegrity experement [Mar. 8th, 2009|11:01 pm]
Doug Ayen
A couple of weekends ago, I went for a visit to the Smithsonian. After once again noting forlornly that the old Arts and Industries building is still closed for renovations, I went wandering through Freer and Sackler, looking for inspiration.

Sadly, enough of the museums were under renovation that there was little to find. Having abandoned the arts of the East and the ancient past, I wandered up Constitution to the Hirshhorn.

The middle third of the place was being set up for a new exhibit, but on the top floor I found myself washed up by the crowd of mostly children and bored parents watching a video loop of a number of improbable actions resulting in a simple objective to an area overlooking the tensegrity tower in the courtyard. Needle Tower by Kenneth Sneson.

Now, years ago I bought myself a toy, one that allowed you to easily build structures using tensegrity. It's called Tensegritoy, It came with enough tension and compression bits (dowels with elastic cords, basically) to make a few simple polyhedra, but nothing complex. I dragged it out after my visit to the museum, and made a tetrahedron and an octahedron, but then ran out of sticks. It only comes with 30, and over the years I've lost 3. So, I needed moor.

I checked online, but the manufacturer has gone out of business, and they're basically not available except in occasional partial-kits on ebay. No worries, they're simple enough, just need some dowels, something for end caps, and some elastic cords.

Then I got to thinking. This is usually a sign of immanent danger. PVC pipe is damn cheap, endcaps are easy to come by one aisle over, bungees and cord are cheap and easy to get in bulk.

So, today, I went and got 4 ten foot pvc pipes, cheapest they had for the prototyping, 24 end caps, twelve sixteen inch bungee cords, and a tool.

I've wanted this tool since I learned they existed. I mean, who wouldn't want a laser-guided miter saw? I've been using a hand saw and miter block for ages, and it just doesn't scale for larger projects, and is a pain to get everything clamped down and so on. And, well, it was on sale.

Once I got home, I set up the saw, cut each pipe into 3 3'4" lengths, then cut a slit at each end about one inch deep. Using some spare cord I had lying around, I made 24 bits of string with a bowline on one end and a barrel knot in the other, with an overall length of about 6".

I put on each of the twelve pipes first one of the strings, set with the barrel knot caught in the slot, then to the string I attached a bungee cord, then another string, pulling tension onto the line to streach it so the other knot would catch in the slot.

This is enough to make an octahedron, so I started assembly and ran into a couple of hitches. First, the slots were too shallow, so there wasn't enough room for all the cords and still have enough room for the caps. Secondly, on those couple of rods I had experementally cut deeper it was clear that the caps wouldn't stay on anyway, not enough friction.

So, I took everything apart and set up a jig to cut a slot 2" deep. I have also put a coat of contact cement on the inside of each endcap -- once thoroughly dry, they shouldn't be sticky or tacky, but should offer enough friction to keep the end caps on.

If I can make an octahedron work then there's some more complex polyhedrons I'd like to try.
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