“Optimus Prime cake!” my 5 year old son exclaimed. His 6th birthday was approaching and like previous years my wife was planning to make him a themed cake of his choice. But this year was going to be different because a little idea got stuck in my head… what if the cake actually transformed?
I set to work sketching, designing, modeling, 3d printing, motorizing, testing, failing, and eventually succeeding to build an articulated multi-tiered cake platform (you can read details about this specific build on my blog). Eight months of effort paid off as my son had a fun birthday and was captivated by his one-of-a-kind cake. Here’s what I learned along the way.
Whether you buy it or make it, the type of cake you get will influence what icing you can use and vice versa. If you are icing a themed cake to look like fur, snow, grass, water etc you might use a nice light, fluffy buttercream and because of that you could use an equally light sponge cake.
For the transforming cake my wife chose fondant because it’s flat, ridged-looking surface gave the robot a strong structural look. However, fondant is very dense and you need a dense cake like a mud cake to support it. We tried rolling the fondant really thin, like 1-2mm but it begins to fall apart when it’s any thinner.
My point here is that the weight of your cake will have an enormous impact on your design and the grade of components you’ll need to use. Plan ahead. Mud cake is about 0.6g/cm3 and sponge cake about 0.3g/cm3 (or 50% less). So whatever your goal, its best to make sure your structure and motors can handle the weight.
Have you ever watched a movie fight scene without sound? It removes all the drama and leaves the scene looking empty and fake. Sound sells the story and is equally important as the picture.
Optimus Prime is famous for his commanding voice and it wouldn’t be right if I left my robot cake voiceless. I used an Arduino with a MP3 player module to play voice and sound effects in time with the cake’s movements.
As a shortcut, try just hiding a Bluetooth speaker under or near your next cake and using your phone to play sound effects or theme music. Imagine a Finding Nemo cake with beach sounds or a Thomas the Tank Engine cake with railways sounds!
The platform for the cake is made from 8mm MDF. I chose 8mm so that I had plenty of wood to screw into from underneath. Plus it was nice and heavy because I feared Optimus would topple over when he stood up!
More than just decoration, the gaps along the edges play a few roles: it lets the sound out from the hidden speaker, gives finger room to pick up the heavy cake securely, and the removed material from the side helps make the box appear thinner and empty which in turn helps sell the illusion that it is just a cake box not an experiment in electromechanical engineering.
The box was finished with a sheet of sticky back plastic sheet (like you might use to cover school books) not only was the metallic look a nice finishing visual touch but it keeps the cake away from the porous MDF and is easily wiped clean.
To lift the cake I used a stepper motor (and a few springs and elastic bands for luck). Stepper motors are commonly found in 3D printers and are awesome if you need fine control of the range and speed of motion. When I shopped for related parts — like belts, pulleys, bearings, and mounts — I found better prices by buying the standard parts used in 3D printers like the RepRap.
To control the stepper motor, you will need to be familiar with a microcontroller like Arduino. There are libraries written for Arduino that enable acceleration and deceleration of stepper motors so you can move your cake creation smoothly without jerky or unnatural starts and stops.
The main downside for their use in moving cakes is that the stepper motor sound is quite loud and they sound like a flatbed scanner when working, which is cool when your cake is shaped like a robot, but less cool if it’s a fairy cake.
Metal fishing wire, as it turns out, is great for animating cakes. It’s skinny, super strong, and the stainless steel is both rust proof and food safe. Best of all, it will conduct electricity for hidden lights or motors.
I originally tried to use brushes (like on the bottom of a R/C track car) to pass electricity to the wire but the wire had a habit of catching and bending the brush. For better results I turned to metal guide bearings, which conduct electricity through the wire by connecting power to the bearing’s bolt.
Whatever your crafty brain dreams up remember the end result should be like a magic trick. Keep all the machinery hidden and wow your guests with a delicious illusion.
If you do make an animated cake please let me know on Twitter, I would love to see it!