I decided to make the whole thing out of wood and to use the laser cutter at uni to cut out all the pieces. There’s something about this construction technique that I really like.

Everything came together in my head. I need a row of eight switches and a “go” button. I need two hinged doors – one at the top for the robot to come out and another on the front for the locked compartment. I also need some kind of linear actuator to move the arm left and right, and a motor to make the arm poke out.

I decided to use a trolley riding on a threaded rod for the linear actuator. This is the easiest way as far as I can tell and is also the same way that the CNC I built worked.

Trolley riding on threaded rod. The servo will screw onto the top.

Then, I need a way to spin the threaded rod. Stepper motors give a nice controlled motion, but they are all a bit big. A servo motor also gives controlled motion, but might not be fast enough and most of them won’t spin continuously. A DC motor is simple and can be fast and small, but there’s really no way to know how far it has spun and hence how far the trolley has moved.  This can be fixed by placing eight micro-switches along the threaded rod so I know when the trolley has moved to the next switch for the robot to flick – that’s all the feedback  I need.

Ok, but mounting the motor on the end of the threaded rod wastes a load of space, I’d rather have the motor in the box somewhere connected by a series of gears and/or belts. A belt design sounds like it would be quietest and less prone to jamming – but rubber belts seem expensive and hard to find in the right size. Gears are also difficult to find in exactly the right size, but I can also cut them on the laser cutter to any specification – they won’t be accurate and quiet but I don’t need much accuracy and hearing the mechanical noise might be quite cool.

I designed the gears using Matthias Wandle’s web tool, traced them on Creo and laser cut them.

I still haven’t figured out how the locking mechanism is going to work, but I think I’ll find it easier to visualise once I have everything else built.

I modelled everything in Creo and cut the main parts on the laser cutter. Here are my designs and the build so far.

Wireframe drawing in Creo
Solid rendering of mechanism
The build so far