Back for 2014... with an exoshark (and tips on how to make one)
It's been a while - in my 'other life' I'm publishing a book, so my 3D project has been running a bit slow for a few months, but now I'm back... and it's time to kick some laser-sintered mechanical shark butt.
Meet Exoshark, a 146mm long articulated shark (marine nature fans will recognise it is based on the Bull Shark, Carcharhinus leucas, my personal favourite) which is designed to move in sections, a bit like those crappy plastic snake toys we've all had, and broken, as children.
I got interested in this kind of simple mechanical design on a recent trip to Qatar, where I bought a bunch of wooden snakes and crocodile toys, made in the traditional way of sewing together slices of wood. My kids loved them so much I (foolishly) said I could 3D print a shark that would do the same kind of movement. Anyway, after months of asking me when it would be finished, I finally managed to work out the mechanism and body design that would a) print out and articulate in plastic via Shapeways and b) be tough enough to withstand a 2 year old and 6 year old road testing them for me.
It's actually more complex than you might expect. You need to consider clearances for the moving parts - and allow enough space for shrinkage / warping in the joints. Given the material specs at Shapeways (min. clearance for plastic is 0.5mm) and the shrink/warp potential of about 5% of the overall length, I've made all the clearances at least 1mm - both inside the internal link backbone, and between the backbone and the body.
I've also put a lot of thought into the strength of the connections - there are weak points. The problem is, to deal with the clearances you've got to make the link connections 2mm at their thinnest point (or the body is too big to print around the internal backbone) - but that means the tail portion is attached by a 2 x 2mm thick piece of plastic... however, there's a good degree of flexibility in the plastic, plus the clearances around other link joints in the back bone means pressure is distributed along the whole backbone (up to a point). In simple terms, you'd have to try to snap it to snap it, it shouldn't happen by accident.
At least that's the theory.
So I've ordered two models in plastic (one for each of my kids) and a third one in Alumide for me. The Alumide is brittle, so that one won't be playing shark attack in the bath.
Sculpting a shark is surprisingly easy for someone with my weak modelling skills btw - here's a quick guide:
1. draw the outline of a shark's body (no fins or tail) in illustrator - SIDE VIEW
2. draw the outline of the dorsal profile - TOP VIEW
3. draw the cross section shape - FRON END ON VIEW
3. draw the fins, tail etc - SIDE VIEW
4. save them as SVGs and bring them into Blender
5. extrude them as curves to get the proportions right (except the cross section, make that longer than you need it) then convert them all to mesh objects (ctl+c)
6. Take the long extruded cross section, place the extruded side view within it (so the height and width are about the same) and apply a boolean intersect to the cross section, applying the side view mesh as a modifier.
7. Repeat that process on the (now roughly shark shaped) results of step 6 with the top view. The result is a shark body.
8. Add the fins.
9. From there, you can hollow it out (boolean difference works well for this), then design and build a simple link system like a bike chain, and slot it into the body cavity.
10. Finally, carve out portions of the body in slices (I made mine eliptical to enable a wide degree of flexibility) so the links and articulate. You now have a wiggly shark.
From a design perspective, you can then add details, cut outs, make it look a bit cyber and funky and... hopefully, the kids will love it.
I'll post a vid of the finished item in a few weeks ;)
