Mechanical

Our mechanical design for the Ultimate Etch-a-Sketch was fairly straightforward. It consisted of a sheet metal body to hold the etch-a-sketch, motors, battery, and handyboard; traction drive wheels using o-rings as the friction surfaces; and a spring-loaded release mechanism to allow easy removal of the etch-a-sketch while providing adequate compression for the traction drives when engaged.

The drive wheels are designed to provide enough traction to turn the etch-a-sketch shafts while at the same time allowing the easy removal of the toy from the body for erasing purposes. We chose a friction drive because it would have less backlash than gears, and the wheels would be easier to separate than belt-driven wheels. With friction drive wheels, we would also have to be less precise in our alignment of the wheels as long as there was enough compression to prevent slippage.

Since we were less constrained with alignment than we would have been using belts or gears, we were able to design our drive wheels based on other constraints. We wanted to ensure that our stepper motors were strong enough to overcome the friction of the etch-a-sketch's shafts without slipping. We also wanted high resolution and were less concerned with the motors' top speed. This led us to decide not to make wheels that were a 1:1 gear ratio. Instead we wanted a gear ratio greater than one to give us the added torque and precision that our project asked for.

We decided to use o-rings as our friction surfaces because of their rubbery surface and their availability in many different sizes in the shop. For the driving wheel we chose a 1.75 x 1.5 x 1/8 o-ring, and for the driven wheel we chose a 2 x 1.625 x 3/16 o-ring. Our wheels we manufactured out of aluminum stock by turning using the dimensions of our chosen o-rings for the right fit. We removed the original knobs from the etch-a-sketch and filed flats on the etch-a-sketch shafts. We then added set screws to our drive wheels.

When testing we found that o-rings were not the best at transferring torque without slipping. We thought again about using gears or pulleys, but instead found an easy solution that gave the o-rings more traction. We used spray-on belt dressing, a rubber-cement-like compound used to increase traction on belts, and found that our o-rings were subsequently able to grip much better, solving our problem.

The body is built as two levels. The first level is designed to hold and protect the motors, battery, and handyboard while allowing an easy interface to the etch-a sketch, which is housed on the second level for everyone to see. In the accompanying images, the dashed lines indicate bends that we made on the original patterns.

On this second level the etch-a-sketch is held tightly to the drive wheels by its spring-loaded release mechanism. On it we punched holes for attaching the motors. Two bolts that connect the two levels function as a hinge, allowing easy access to the first level.

The spring-loaded release mechanism is also attached to this hinge connection. A spring-loaded bolt holds a hinge bracket connected to two more spring-loaded bolts. This bracket holds the top edge of the etch-a-sketch to the "floor" of the second level while the two springs compress the etch-a-sketch to the drive wheels. The third spring simply holds the bracket in place. To remove the etch-a-sketch, this third spring is released, allowing the bracket to swing out of the etch-a-sketch's way.

We added smaller additional brackets for the battery, the front of the etch-a-sketch, but these did not follow any pattern, but were very simply-made bent pieces of sheet metal that bolted or screwed onto the body. We also cut small access holes on the sides of the body to enable us to grip the etch-a-sketch for removal.