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Tensegrity table

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Wend

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I've seen a number of tensegrity projects recently, and thought it would be fun to have a go at making a side table. My plan was a wooden frame with 5 chains, one at each corner and one in the centre, holding it up, but I was having trouble visualising how many chain links would fit in - too few and I don't think it would look right. Normally I just leap in, start building, and hope for the best, but for this I needed to actually do some design work first.

I discovered openscad, which made this reasonably simple. Measuring the side tables we have, something like 35x55x52 (width x length x height) seemed about right. Lots of searching the 'net for chain sizes later, and a bit of fiddling with openscad, I had this:
tensegrity.png

with the L's being 3/4 the height, and the chain being short link DIN766.

Not sure yet if I was working on the prototype or the final thing, I reduced some of the cheapest softwood money can buy to 42mm squarish pieces and got about chopping mortices and tenons.I decided to put wedges in the tenons, partly because I'd never tried it before, partly because it ought to be stronger, and partly because it seemed the easiest way to get something gap-free. I'm not sure exactly what the right process was, but I drilled some 3.5mm holes as close to the edges as I dared about 1cm from the end, and cut down to them.
1000.jpg

I discovered a beech offcut I have was just a shade thicker than the mortices, so I used that to make the wedges. I wasn't sure how wedged to make them, so I just winged it. They aren't particularly uniform, but as I wasn't sure what size to make them, and my gaps weren't equal anyway, that didn't seem too important. I also made them longer than necessary, so if they turned out not to be wedgey enough then I could try cutting the thinner end off.
1002.jpg

I used a simple L-shaped block to make the ends pointy. Here working on a softwood one, in case the beech ones didn't work for some reason.
1003.jpg

At this point I'd run out of excuses, and it was time to start gluing up.
1004.jpg

To minimise the chances of getting something half-way in when the glue grabbed, I glued it up one side at a time. Here's the first glue-up, where I forget to add clamps clamping the cheeks together, so you can see how comically over-sized the wedges are. These were actually so oversize that I cut them off, re-sharpened the tip and used the other half for another joint.
1005.jpg

The second glue-up looked like it was meant to. But what you can't see is that I somehow managed to forget to apply glue to the wedges this time. I think enough glue probably seeped in from elsewhere, though; at least, the wedges didn't want to come out easily.
1006.jpg

All thing considered, I'm pretty happy with how the joints turned out!
1007.jpg

The two halves of the frame are now complete. They are (theoretically) identical, i.e. either could be the bottom or the top.
1008.jpg

Back into the workshop, and I've used offcuts to clamp them roughly where they want to be to check how many chain links I want. Amazingly, the answer is exactly what the CAD model predicted!
1009.jpg

When cutting the stainless steel chain to length, I tried oil, but found that simple water seemed to be the best coolant.
1010.jpg

Finally, a hint of the final form, as I work out how far along the arms I want to make the holes for the central chain. The legs are now cut to their final length and, as the finest furniture deserves the finest treatment, I've sanded everything all the way up to 80 grit.
1011.jpg

Central chain attached and tensioned, and it works! I'm not really too sure where the weakest link is, but it's stayed together for a couple of days already, so things are looking promising.
1012.jpg


I have originally planned to take the central chain off in order to stain and varnish it, but having put it together I'm not sure I want to put it through the stress of being taken apart and reassembled, so I'll probably just do it as-is,

I'm not sure whether I'll add adjustable feet to it. How "level" the bottom is depends on which bit of our floor you put it on, so depending where it ends up living it may or may not need them. Unfortunately, the feet probably couldn't go right at the ends, as that's where the chains attach. And I'm not sure I'd want to put them where the M&T is. So they'd end up a fair way towards the middle.

The other thing I haven't worked out yet is what sort of table top to use. One option would be a sheet of plywood, simply glued on, but I'm not sure how I feel about the juxtaposition of plywood edges and M&T joinery. Making a top out of pine is more tempting, but I'm not sure how best to attach it. I think breadboard ends, and gluing the ends to the top would be OK expansionwise, but there wouldn't be much glue surface. Making the top without breadboard ends would be easier, but then I'm not sure how to best attach it. Perhaps I could glue some 42mm blocks to the top's underside, and then screw piece of overhanging wood to those blocks, clamping the frame between the top and the overhanging pieces?

Anyway, I'll take a break for a bit and mull it over.


Take care,
Wend
 

Fitzroy

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Cool! I’d attach atop with traditional buttons.
Fitz.
 

MikeG.

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Excellent. Chunky, though. As it's a gimmick, , you could put a sheet of glass on top for the table top, perhaps.
 

Duncan A

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Clever, although I have to confess I'd be tempted to cheat by welding up the corner chains, rather than getting the lengths and tensions spot on!
I like Mike's idea of a glass top - just adds to the novelty of it.
Duncan
 

Droogs

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Ah, so all the weight of the top is just taken by the little chain in the middle? The corners are just to stop it canting (no compresive forces), they don't keep it up in any way?
 

Wend

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Excellent. Chunky, though. As it's a gimmick, , you could put a sheet of glass on top for the table top, perhaps.
Yeah, I went for chunky as I wasn't sure how well thinner arms and M&Ts would stand up to the forces they are under - especially in softwood.
 

Wend

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Do they wobble?
Yes, although mine doesn't as much as most I've seen online. I think that's because I've used threaded bolts to attach the central chain to the arms, allowing me to put that chain under more tension than most people have.
 

Droogs

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Do the 2 frames have to be symmetrical? Can they be asymmetrical as long a they are hung at eaches own centre of balance?
 

Wend

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If you imagine it with only the central chain, then the top piece is hanging from the arm of the bottom piece. But it is very unstable: If you push one corner down, then the opposite corner will rise. The corner chains stop the opposite corners from rising, and hence stop the original corners from falling.

As long as the hanging point is within the polygon described by the other chains (when you look down on it), each piece could be any shape you like.
 

John Brown

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I made one out of bamboo skewers, blind cord and a rubber band. It wasn't pretty, but demonstrated the principle. I might make another one day, but have too much to do right now.
I think yours looks great, by the way.
 

AndyT

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Great stuff. I saw one on YouTube as well and spent a morning hot glueing bits of scrap together and tying knots in thin thread, just so I could play with one and understand how it worked.

I predict that anyone seeing your rather nice table will feel the same way, if they have any curiosity at all.

And I agree that a glass top would be cool.
 

TheTiddles

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Maybe it’s my occupation, but I’ve not managed to understand the fascination or lack of understanding with this design, isn’t it obvious? Says Aidan who finds so very many other things confusing...

yes the chain is in tension, but only because there’s some wood in compression/bending to keep it in tension.

they use this method to make expanding structures for spacecraft on a much more elaborate scale

nice smart M&Ts you have there

Aidan
 
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