Weekly Assignments Final Project About me

10. Mechanical Design

Group Work Blender Fusion 360 KiCAD Printing Reinforcement Design Files
Neil's assignment for this week: 
  group assignment
      - design a machine that includes mechanism+actuation+automation+application
      - build the mechanical parts and operate it manually
      - document the group project and your individual contribution

Group work

-->Link to Group Project Page<--

Concept: Dinosaur Dentist Game

Crocodile Dentist game

We were kicking around ideas for this week's project, and someone mentioned the "alligator game", and my partners (who are all younger than me) all nodded in understanding, but I was clueless: "What's the alligator game?" Turns out it's called "Crocodile Dentist", and it's a neat mechanical toy where you open the jaws of a toy plastic crocodile and then push on his teeth. One tooth is "sore", though, and will make the crocodile snap his jaws shut on your finger. The cool thing is it's a different tooth every time you reset the jaws.

So we decided to do an electromechanical version of this game. We also decided to do it as a dinosaur head rather than a crocodile. Each tooth will have a momentary contact switch and an LED underneath it. The jaws will be opened by a servo. The game starts with all the teeth glowing, but when you push one, the tooth goes dark, the servo advances and the jaw opens a little more, until the servo goes too far, and the jaw falls down on your finger. If possible, we want to have the LEDs glow with different brightnesses, giving players a general idea of how much that button will open the jaw. Players will take turns pushing buttons, and the one who gets "bitten" loses.

Our project has only one degree of freedom of motion, so it's not as fancy as a stepper-operated gantry robot, but it does satisfy the requirements of machine week: mechanism, actuation, automation, and application.

Our group documentation and video is here (no link yet).

Our tasks were split up as follows:

In addition, I did a lot of work giving advice and suggestions to the rest of my group, since I'm more familiar with mechanical design and Arduino programming than the others.

Individual Work: Blender

I've never worked with Blender before, and I'm a terrible artist, so I signed up to create the overall design of our dinosaur head. I was really relieved when Zach suggested we go for a "low-poly" aesthetic, that sounds a lot easier than realism. This side of the project really needs Blender. Fusion 360 is a terrible tool for working with free-form organic meshes, even low-poly ones.

Blender took some getting used to, but here's my process:

My goal wih the Blender stage was just to create a skin surface. I deliberately didn't make the design a 3-dimensional watertight solid ready for the printer: there's a lot more work to be done, and it's easiest to do that in Fusion 360. I found it very difficult to make Blender care about whether the design was physically possible.

Individual Work: Fusion 360

I worked in Fusion 360's mesh environment briefly to import the Blender file. I used "Insert Mesh" to bring the STL files into Fusion, and "Convert Mesh" to change them into a Fusion "surface body". In my experience this totally doesn't work if you've got a complicated mesh with thousands of faces, but for a low-poly model like this it works great. I then thickened the surface to create a hollow shell.

Once I was done, I decided the head looked a little too scary, and was also too slim to fit our servo in. So I made a "chubby" version just by scaling the whole head width-wise.

Finished dino shell Finished dino shell (chubby version)

Finally, I shared these designs with the rest of my group using the project sharing tools in Fusion 360. Zach added a hinge to allow the head to swing open, plus a clever pawl system to allow the servo to re-set the head for another round of play after the jaw has closed. Evan designed some teeth and a tongue-and-gums part that fits into the outer shell of the head. I then followed Evan's design process to add an upper palate and gums, with more teeth.

Cross section of lower jaw showing tooth spider
Cross section view of lower jaw showing tooth spider

Somewhere along the way, I bought the original "Crocodile Dentist" game so I could take it apart and learn from it. In particular, how does a purely mechanical device generate random numbers? It's a really clever design, internally it's a bit like a roulette wheel crossed with a mousetrap. Anyway, one of the ideas we got from taking it apart was to make the teeth one single connected part rather than individual tooth pieces. The teeth are part of a spider-shaped part: the central body attaches to the rest of the dinosaurs, and the arms hold the teeth in position with enough flex to allow the teeth to be pushed down. This is crucial for the lower teeth that have buttons, but we also did this for the upper jaw, so the teeth have some springiness and will hurt less if they bite you. I printed the teeth out of translucent white PETG, which is more flexible than PLA and should allow the lights to show through nicely.

I also added colors, which was totally unnecessary, and added joints so you can articulate the jaw and servo on the computer screen, which I thought would be useless but highlighted some minor design problems with the servo action. Anyway, here's a rendering of what it will hopefully look like when it's done:

Rendering of the finished result

Individual Work: PCB Design

I took responsibility for designing the circuit board that would hold the buttons and lights beneath the teeth. Originally, the plan was to mount a Xiao module in this circuit board, but we quickly realized that we needed far too many inputs and outputs for the Xiao, which has 11 GPIO pins. We need 8 pins for button inputs, 8 for LED outputs, plus 1 to control the servo.

I imported a DXF from Fusion showing the shape of the head and the position of the teeth, so I could design the board to fit in the head, with buttons and lights under the teeth.

My first design tried to use a shift register to control the LEDs. This is a sound idea in principle, but I ran into three problems. First, there are still not enough pins on the Xiao: I need 8 for the buttons, 3 to control the shift register, plus one for the servo, which is one too many. Second, I thought the difficulty of controlling a shift register would put an unfair burden on the group members responsible for the Arduino programming. Finally, our goal was to use PWM to control the brightness of each individual LED. If it's possible to do PWM through a shift register, it's certainly beyond my skillset.

First version of circuit board (schematic)
First version of circuit board (schematic). It was only when I got to this point that I realized I needed one more pin to control the servo!
First version of circuit board (PCB)
First version of circuit board (PCB)

So I started over from scratch, with a design based on an Arduino Uno. The Uno has more than 17 GPIO pins, so the shift register was no longer necessary. The only limitation is that it only has six PWM-capable pins, so some of the teeth will have to be on full brightness.

Final PCB schematic.
Final PCB board design.
I made a soldering guide to help my group mate who would actually build the board know where to put things.

Individual Work: Printing

 I printed the jaw and teeth pieces on my Prusa Mini at home, freeing up time on the lab printers for other group members to print the other parts.

Individual Work: Reinforcement Panels

 As we printed the model, it became clear that we had both design and printer problems. The "cheek panels" of the lower jaw were to thin and weak to support the head hinge properly. In addition, the printer we used was under-extruding badly for some reason, so this area was extra-flimsy. We also had trouble with the servo mount being too weak. I solved this problem by creating some reinforcement pieces that we hot-glued to the inside and outside of the cheeks to reinforce them. There's also a plastic rod that the jaw screws thread into, that runs across the width of the head.
Fusion design of reinforcing pieces I designed, colored gray in this image.
Reinforcement piece mounted on the final dino.

Individual Work: Other Tasks

 I also helped my group members with several other tasks. We worked together to assemble and test the design, and I gave advice and encouragement for the programming work, which was mainly done by Bella.

Design Files