Material Selection Cheat Sheet

Designers work in a void.  One of the voids to be filled is selecting appropriate materials.  I have my favorites.  The selection is usually based on performance characteristics such as yield strength and ability to transfer heat.  However, the feel of a material lies beyond conductivity numbers and surface hardness.  Many elements of touch points need to be approached qualitatively.

Material selection is a balance of money, performance and manufacturability. Parts that are not subject to extreme temperatures can usually be made out of plastic, with polyethylene and other commodity plastics at the low end to the ubiquitous ABS on up to high strength, high temperature superstars.  Parts made out of A36 hot rolled steel or gray cast iron are cheap and strong yet we love titanium and gold at the other extreme.

There are natural divisions of material selection based on strength, weight, temperature, thermal and electrical conductivity, production volume and of course cost.  However, at the end of the day you have to pick something so here is a summary of my favorite material types.  This is just a starting point and is largely directed at metals.

Steel

• A36
• 1018
• 1040 (high strength)

Chromium (Stainless) Steel

• 304 (low Chromium)
• 316 (high Chromium, highly corrosion resistant)

Aluminum

• 5052 (good choice for flat work, not machining)
• 2011 (free machining alloy)
• 2024 (high strength, heat treatable)
• 6061 (high strength, structural alloy)

Brass

• C36000, Free Machining (machines like butter)

Polymers

• ABS (common injection moldable polymer)
• Low Friction: Delrin (lots of great surface treatments or impregnations are available with many polymers)
• Epoxies are used in carbon and aramid fiber composites.

Machined Prototype Materials

• Delrin, Nylon 6/6, brass and aluminum. 
• Additive manufacturing has changed prototyping approaches and material selection!

Favorite specialty materials

• Cu-Ni Alloy (Monel, cupro-nickel) and Torlon.

Designers worry

 

Designers worry about everything.  They worry about a product's function, appearance, sound, touch, and feel.  It is amazing how much we care about the little things. The passion for excellence coupled with a myriad of small decisions produces an odd inner stress that leaves you vulnerable to criticism of your design — it is your baby and no one can say anything bad about it.

We try to think about how a design is going to be used and misused.  We try to blend evidence and creative expression in producing an excellent design.  Every dimension is thought out, every material pondered, every movement studied, every sound analyzed.  In addition to function, we think of things such as color, material, finish, tactile response, weight, center of gravity, heat transfer and sound.

After over 30 years of designing things, I’m still impressed at how all in the design community consider all the small things.

There is a giant difference between a good idea and a commercialized product.  Good ideas are cheap and easy – commercialized products take a lot of work and skill.

Reflecting on life, cogitation on inaction

I have written almost a quarter million words in my journal.  Describing my life, reflecting on experiences in prose and poem.  While no one should care about my life, with the possible exception of my children, I write because I must.  Although I write to the wind, it gives me a chance to reflect on things.  We are fortunate if even two generations of our progeny care about us.  After that, we are just names and dates. 

However, too much time on introspection: can be a problem.  Did Emily Dickinson live life robustly or did she just document a provincial dwelling robustly?  Sometimes we need to reflect, sometimes we need to act.  While General Ian Hamilton’s troops were being slaughtered at Gallipoli during World War I, he thought broadly and deeply about life.  He shared these thoughts in long diary entries.

“Each evening their commander in chief before retiring in his bunk on the battleship, wrote five thousand word entries in his diary, reflecting upon the mysteries and ironies of life.”   (The Last Lion, William Manchester, Little Brown and Co., 1983, p 551.)

I would probably do the same thing. Maybe people of action don’t leave a long paper or video trail but they live life robustly.

A preachy reflection on faculty commentary

     

        “I always remember my professor telling me…”

      "My professor used to say …."

These are the words that educators wish to hear. But is that what we want?  No!  Better memories for students are:

      “In that class I learned that I’m really good at …."

      “In that class I realized that …."

Because faculty usually don’t have tangible, market rooted abilities, they often try to "produce" students who share their views.  The goal can be to have students think like us.  This may seem reasonable if we ‘profess’ expertise in a discipline.  However, our expertise is a datum and the world is much more complex than we can capture.  

Why should a student care about our opinion on anything?  We should care about students’ opinions or opinions of those that don’t hold a power position in the classroom. 

Subjective assessment of student work is required for many of us. We like to call it "trained subjectivity" as we work through rubrics and past experience to critique work.  However, in trained subjectivity we must present our positionality with respect to our data.  For me as a Christian, the only sayings worth quoting are from the Bible, which presents what I believe to be the only eternal truths.  To quote the aphorisms of others is pointless.  

Of course, we can learn from great minds and those who have dedicated themselves tirelessly to our disciplines.  However, throwing out quotes from "experts" is weak exposition.  Where expert commentary is offered from the lectern, it should be two opposing opinions thereby allowing students to struggle with what they believe.  Our individual commentary is always connected with some intellectual ecology and worldview.  When we contend with various viewpoints, our inclination is to side with the greater expert, which is lazy epistemology. 

Don’t have your students quote you, don’t have them remember your opinions.  Let your role be such that they only remember their own intellectual walk.  Your vanity shouldn’t be satiated by young minds but rather confidence in the excellence of your teaching. Now, I just have to get over my own vanity and condescension…