A few months ago, a former colleague lead a project where students designed, prototyped, and pitched a model of an urban watershed for use as a teaching tool. They had already interviewed local water department staff and watershed educators. Now the instructors contacted me for ideas about supplies and activities for prototyping. Materials selection and Ashby charts are rarely covered in depth for most STEM students or teachers, so I wanted to give them strong guidance. Here's my advice:

1. Find a maker or engineer to visit or consult. I put a call out to my network and got a response from an exhibit designer at the city’s science museum.

2. Drawings are the quickest way to explore design ideas, so don’t jump immediately to the laser cutter! In addition to 2D paper sketches, consider Sketchup or another modeling program to build 3D visualization skills, especially for younger students.

3. Essential questions: What is the timeline? budget? What concepts or lessons is this model trying to teach? What goals does it serve? What functions or properties does it need to accomplish that?

   Ideally students would come up with these questions, and some of their answers, during the project introduction. One way to get started is to ask students to think about the 5 Ws: Who will use this? When? Where? To do What? Why?

   Or, have them fill in the blank: This model needs to be __

   (cheap, durable, easy to build, easy to fix, fixable without special tools, expandable/upgradable, waterproof, easy to clean, not too heavy, collapsible, creative)

   Write down these answers and post them on the wall for easy reference.

4. Grab some boxes and take an hour after school gathering up prototyping materials: plywood, lumber, acrylic sheet, felt/fabric, plastic bottles/containers, metal bars, tile, XPS foam, glass pebbles, PVC pipes, polycarbonate sheet, landscaping/craft foam, aluminum foil, plastic wrap... Also gather scissors, glue guns, box cutters, Gorilla glue, tape, glue sticks, and other tabletop cutting and fastening supplies. Maybe also markers, graph paper, and rulers. Use the boxes to organize everything per classroom and per group. Anything you can’t find or don’t have enough of, put in an order now!

5. For pointers on how to break down the materials selection process, I recommend MIT D-Lab’s excellent Materials Selection poster. (They also have resources on Adhesives and Fasteners.)

   Ask students to look at the table at the top to get acquainted with the key questions and comparisons between wood, metal, and plastic. Which parts of this chart are relevant to this project? What would you add? Is color or paintability important, or not? Having samples of materials on hand can help demonstrate differences in weight, strength, absorption, etc. and get ideas flowing.

6. Next, to ground it in the logistics of the fabrication shop, ask students to split up and make a list of machines and the materials each one can cut or join. Add notes on how much time/effort is involved in each method; for example, the entire 10th grade probably can’t design and laser cut multiple iterations in a week. Students may realize they need to improve their CAD skills or get trained on a machine - now is the time to schedule some tutorials!

7. Step back and check your list of which properties are most important for the prototype and the final product (remember, they may not be the same!) A model you hope to manufacture in bulk as a teaching aid might need to be light, strong, cheap, and waterproof. What combination of materials and manufacturing techniques can accomplish that? You may need to try something out, like comparing the behavior of plywood vs. lumber in water, or do more research, like finding out exactly which plastics can be cut safely on the laser cutter.

   By the time their prototypes are built, students should be able to explain why they chose the materials and manufacturing methods they did. Their analysis can also inform recommendations on how the final product should be produced, although design for manufacturing is a whole topic on its own.

Often, teachers design activities to use what's on hand, or what can be acquired cheaply and in bulk. But students need to learn how different materials behave, just like they need to learn how different tools work. Materials selection is part of the engineering design process, so I hope I've given you some ideas on how to include it in your next maker project!