Bending titanium rods is hard

How to build an electric mbira from titanium bicycle spokes and radio controlled boat parts

It started with an Instagram post from Meng Qi, a fantastic musical instrument designer from Beijing.

It showed a handful of brightly-coloured titanium rods; “Titanium kalimba blades with single piece bridges. From Yasushi of Mbiraski, true high end.”

That was how I discovered Mbiraski, which sells beautiful one-off thumb pianos , made in Japan, with built-in preamps and headphone amplifiers for £350-£400.

Mbiraski instruments. Note how much better they look than mine.

Inspired, I set out to make my own. This is how I did it.

Parts

The Tines

• The tines are Titanium bicycle spokes, 14g grade, which are 2mm in diameter.
• The rainbow-anodised spokes that Mbiraski uses can cost £90+ for a pack of 40, but I found an eBay store selling individual similar ones in raw (grey) titanium for £1.59 each and bought six. The matt texture of the titanium is nice.
• Detailed googling about Young’s Modulus and Density has left me pretty sure that steel spokes will work just as well — they’re a lot cheaper. I don’t think spokes with a plastic coating (like these ones) would be very good.

The Mounts

• I spent a lot of time trying to work out how to fix the spokes.
• The ‘saddles’ in Meng Qi’s picture look very elegant, and I have no idea what they are.
• These linkage stoppers are used in remote controlled planes and boats to connect push rods. They’re tiny, designed for 2mm rods. £2.87 for 20.
• They come in lots of different designs. I wanted some that looked sturdy, and where the bolts were threaded all the way, so they’d do up tight.

The Rest

• I used a Hammond 1590B cast aluminium box, £7 from Bitsbox, but you can work with any size/shape.
• To amplify it, I attached two Piezo disks, (similar to these from Maplin) to the bottom with quick drying epoxy glue, wired up to two 3.5mm sockets.

Building it

Once you have all the parts, construction is really simple and fast. Map out where you want your ‘bridges’ (it’s worth thinking about this more than I did), drill 2mm holes. Drill holes for the sockets, then bolt everything loosely into place.

Cutting the tines — theory

To work out how long to make the tines, you need to use a version of the Twelfth root of two, the formula calculated in 1636 by the French mathematician Marin Mersenne, which is the basis of Equal Temperament.

I cut the first tine at random — it was the top straight tine with the curled up end — attached it and plucked it. It was roughly A, so I tuned it to exactly A (by moving it back and forward on the bridge using the grub screw).

Then I knew what length titanium bicycle spoke makes an A. It was 7.2cm (plus the 1cm on the other side of the bridge).

Using Exmis Free Bar Length Calculator, I was able to enter 7.2 and calculate a whole range of lengths up and down the scale. For the five straight tines I chose a minor pentatonic scale — A, C, D, E, G.

Here’s another version of the calculator as a Google Sheet, which I found easier to work with.

The longer, tines are folded to take up less space, but as you can see, this is a skill that needs a bit of practice.

Cutting the tines — practice

The spokes have a screw thread on one end, which is slightly larger than the rest of the rod, so needs to be removed before they’ll fit into the stoppers.

I cut the tines with a Dremel, which worked fine and was pleasingly spectacular. Remember to smooth off the ends, because you’ll be playing them.

Epoxy glue to attach the piezo disks.

Wiring and electrics

I used two piezo disks. They’re more ‘directional’ than I expected, so it works well. I use Mikrophonie modules to amplify the disks through my modular (if you find the Mikrophonie module distorts — the mbira can put out big bassy sine waves — consider changing R10 from 56k to 10k to reduce gain). Google ‘piezo preamp’ and you’ll find lots of projects.

Mbiraski uses piezo strips rather than disks, which should give a much more even response.

Anodizing the tines

Once the box is assembled, carefully hold the tines over a gas ring, and they will turn amazing blue/black colours.

Improvements for next time

• Plan the ergonomics — how will it fit into your hand and how will you play it? Mine doesn’t do either very well.
• Plan the notes; work out a sensible range. Nearby or harmonically related notes will resonate with each other.
• Buy more bits than you need — you might want to build a second one as soon as you’ve finished the first.