3D-Printable Reusable Tensegrity Modules

I'm giving a talk at the Joint Math Meetings next month on my tensegrity activity for math circles. There's not really time to run it as a hands-on activity in the 30-minute slot and still give a sense of the full arc, since all of the time would be spent making struts with little time left to explore. What I'm instead planning to do is introduce things by modeling the strut construction from dowels and rubber bands and switch over to using a toy I've developed that will save the audience the work of making struts. I've brought the toy to a variety of SUMM events and people always ask where they can buy it. SUMM will be piloting selling these in small kits soon, but if you have access to a 3D printer and want to print a set for personal or educational use, I'm making the 3D print files available here.

Reusable Tensegrity Modules

There are two pieces you'll need: the printable strut and the band.

Component: Struts

There is only one print file you'll need, but I'm providing three versions of it. Two feature the text "SUMM" along a flat side or two. If you want to support the Seattle Universal Math Museum, who makes this work possible, consider including some with SUMM on them for a conversation starter. They do take longer to print and use more plastic, though.

I tend to print 20-25 at a time and have found printing text-side-down works better than text-side-up.

Component: Band

I've been using these hair bands, which come in packs of 250 for about $10.

They work well enough, but there's a point of failure where the band is glued. I've only seen them break when kids are super rough with them, but you might want to keep some spare bands.

The printable struts were designed to work with the dimensions of this band (2 in. loop diameter, 1/8" cross-section diameter), so if you deviate from this band, pick something with very similar dimensions.

Assembly

At each end of the strut, the space between the prongs connects to an inner hole via a slit. The band should be passed through the slit until it rests in the inner hole. (There'll be some resistance, so I do this part against a pillow in my lap!)

After you do this on one end, pass the band through the slit on the other end.

Since the glued bit of the band doesn't play nicely with the prongs, I try to keep it about a quarter down the band length so it is unlikely to get grabbed by the prong of another module.

Try to get about the same amount of slack on both sides. You can test this by grabbing each length of band in the middle and stretching it open:

Example Models

To give you a sense of what builds look like with these, here are some models from my other post built with these reusable units instead of dowels and rubber bands. All of them can be made with 30 or fewer modules. I try to use colors to highlight features of the polyhedra I model these on and have found that six colors suffice for most things. A nice starter kit could be 30 struts, 6 colors, 5 of each.

Octahedron (6 modules)

Pentagonal Antiprism (10 modules)

Elongated Square Bipyramid (10 modules)

Cuboctahedron (12 modules)

Rhombicuboctahedron (24 modules)

Icosidodecahedron (30 modules)

Since these modules are more robust than the dowel and rubber band modules, there are some new techniques involving doubling up bands that become available. The next four models are a sampling of what this allows you to build.