Build Complete; Painting ...

 Last week, as expected, I finished building the last of the pieces of the giant Soma cube. (I'd predicted a Thursday finish, but it turned out to be Friday. Not too bad, though.)

Piece Y was fun to build because, in spite of being made of square shapes, it has rotational symmetry of order three around a diagonal axis. For the same reason, it has a good strong corner shape from which to reckon right-angles. I took a little longer over it than I had planned because I wanted to do it justice, and it did come out very nice and square.

So this is what all the sides of all the pieces look like. There are ... quite a lot of them!

On Sunday I bought a handful of paints to experiment with colours and textures. I confirmed that undercoat/primer is definitely going to be worthwhile, as is a quick sanding once that's applied.

Yesterday I sorted the convex sides into the ten cube faces to which they belong, ready for painting. I also bought enough undercoat for the entire job. Mike very kindly did the first layer of undercoat, and took only 90 minutes, which was encouraging.

The last thing I did on Monday was to cobble together a crude frame for holding a projector vertically at ceiling height. The shelves have holes in them, that were originally going to accommodate rods supporting the lowest shelves. Those rods turned out not to be necessary, but the holes have found a new purpose: the frame is held on by just two dowel pins that fit into those holes.

With the projector pointing downwards from a height of over 2.5 metres, I can lay out each cube face on the floor and project its design onto it, which should make it easy to produce large-scale imagery. The frame took under an hour to build, and should save many times that. 

Today all the convex faces got a gentle sanding and then their second undercoat. Here they are, drying. I realised that if I put a screw part-way into a carefully chosen screw-hole on each face then that could be used to prop the faces a good distance apart.

Also I finished rigging the projector with power and signal. Amazingly, from ceiling height it produces an image on the floor that's 1.22m tall – exactly the size it needs to be. So that's one advantage of living in a place with high ceilings.

Still Building

A week after my last post, I've finished four out of seven pieces of giant Soma cube, and am half-way through the fifth. The first four were all comparatively simple: because they are essentially flat shapes, I could attach most of the frame to the two large sides, then join it all together with simple rectangles. The fifth (piece P) is more complicated. There's no obvious order in which to put it together, and it needs screws at some less accessible angles.

 To cap it all, this piece is more likely than others to suffer odd tension forces, so I decided to put a long coach bolt through the central joint. Working out how to dril the hole for that accurately enough was a novel task.

Once I'd finished assembling piece P, I put cube X on it to get an idea of how the pieces will look when fitted together. I'm happy with the result. As you can see, the pieces are built slightly smaller than the ideal mathematical cubes that they represent, leaving a gap that is just large enough for the feet that will eventually be on every side.

After I've finished gluing piece P together, the next piece will be Q which is the mirror image of P. And then after that, only Y will be left. I've been timing my work with a stopwatch app, and I reckon there are about 20 hours of construction work left. So the basic construction should be complete by Thursday.

Under Construction

Piece R in progress

I've started building proper cube pieces. It's taking a lot of measuring, double-checking, and Pythagoras' theorem to make sure the pieces don't end up too warped. I suppose the process would be less laborious if I had been able to cut the faces and frames to higher precision.

Roughly speaking, the steps to build each piece are:

• Collect all the parts required.
• For each length of frame:
• Clamp it onto the side to which it will be glued.
• Drill pilot holes for all the screws it will need.
• Secure it temporarily with just two screws.
• Fit the faces together, and add more pilot holes and again secure with two more screws on each edge. This is the step where the alignment of the holes can be fine tuned, to help keep the faces flat.
• Number the faces on the inside, and also mark which other faces they are adjacent to, for easy reassembly on site.
• Drill pilot holes for all the feet.
• Remove the second set of temporary screws.
• For each side:
• Label the corners with letters on the inside, and label the ends of the frame with the same corner letters.
• Remove the first set of temporary screws, to release all the lengths of frame.
• Countersink all the pilot holes.
• For each length of frame:
• Glue it onto the side, and secure it permanently with all the screws it will need.

Preliminary construction of piece S

I weighed piece S after assembling it. It is 16.7kg, which is about what I was expecting. One person can lift it, but it would definitely be more fun for two people.

Gluing the frame onto one side

Meanwhile, I did a bit more work on the maths. I found that, by adding variables that model how much of each solution face appears on the outside of each solution, I can use the same integer constraint solver as before to discover a set of ten solution faces where solving for one face doesn't nearly reconstruct any of the other faces. This required 360,000 constraints and I wasn't sure the solver would be able to deal with that. But it turned up an answer in only 30 seconds ... in spite of being single-threaded.

Back in the physical world, I still need to work out how to varnish and paint the thing. The wood is very absorbent, so it could probably use a coat of varnish before painting with acrylic or similar.

If you've got any recommendations, let me know.

Ready for decoration