Materials #

When you’re in school, you have relatively limited resources. Here are a few families of materials and fabrication methods that you may be able to use.

  1. cardboard and foamcore + X-Acto knife and hot glue
  2. 3D-printed PLA or TPU, possibly with heat-set inserts
  3. laser-cut acrylic, plywood, MDF or chipboard
  4. construction lumber + miter saw, band saw, and table saw
  5. 80/20 structural aluminum framing + miter saw and drill press
  6. sheet metal + tin snips, jump shear, and box brake
  7. Aluminum or steel + lathe and mill

Once you get to industry, you have more choices– LOTS MORE.

Aluminum #

There are at least 530 different aluminum alloys. For only $150, you can buy the ANSI H35.1 standard and read all 12 pages (includes appendix).

Alternatively, here are a few guidelines that will help you get your bearings. (That’s almost a pun, but bearings are made of 440C steel, not aluminum.)

Alloy Composition Most common example
1xxx pure (>99%) aluminum 1100
2xxx alloyed with copper 2024
3xxx alloyed with manganese 3003
4xxx alloyed with silicon ???
5xxx alloyed with magnesium 5052
6xxx alloyed with Mg2Si 6061
7xxx alloyed with zinc 7075
  • 1100: easy to form, cheap

  • 3003: easy to form, cheap, but a little stronger

  • 5052: strong, great corrosion resistance, not heat-treatable

  • 6061: strong, good corrosion resistance, heat-treatable, machines well

  • 2024: very strong, still formable

  • 7075: even stronger, less formable, better corrosion resistance

  • 7075-T6: the most bad-ass aluminum

Reminder about fatigue: aluminum fatigues; steel doesn’t.

Steel #

Steel = iron + carbon (0.04% - 1.5%)

You can read about different steel alloys in the SAE A29 standard, only $56 for 17 pages (almost 4x cheaper than the H35.1 aluminum standard!).

Generally, the last two digits of the alloy designation are the carbon concentration by mass in hundredths of a percent.

Alloy Composition Example
1xxx plain carbon steels 1010, 1018
2xxx nickel
3xxx nickel-chromium
4xxx molybdenum 4130 (gears), 4140 (stiffer, harder)
5xxx chromium 5160
6xxx chromium-vanadium
7xxx tungsten
8xxx nickel-chromium-molybdenum
9xxx silicon-manganese

Tool steel #

Designator Description
A2 Air hardening
M2 Machine tool cutters
O1 Oil hardening
S2 Shock resistant

Here’s what these materials look like.

Yield strength tradeoffs #

Notes on material usage, based on notes taken by Ananya Ram (thanks!) #

Plastics: #

  • LDPE → milk jug material, cheap
  • HDPE → stronger, terrible for the environment, can’t really be machined
  • MDPE → recycled material, still terrible for the environment
  • Polycarb sheet → like acrylic, but can absorb a lot of energy (high energy toughness)
  • Acrylic → can’t machine because it shatters~~
  • Delrin → optimized for machining
  • Nylon → super strong plastic, some are stronger than steel

Composites #

  • Fiberglass → boats, super stiff
  • Carbon fiber → also super strong, $$,

Woods: good for lightweight structures #

  • Pine → cheapest, lightest, crappiest material
  • Cedar → even lighter than pine, eh about everything else. Smells good briefly…
  • Fir → Pine 2.0, stiff, tables and long-term stuff
  • Mahogany → looks nice, fancy furniture
  • Maple → hard, for cutting board
  • Red Oak → heavier means stronger, and it’s very strong, good for floors
  • Baltic Birch Plywood → plywood = alternating grain
  • Purple Heart → one of the densest woods that you can just buy

Metals, usually alloys: #

  • Copper → thermally conductive, expensive, not an alloy
  • 1018 Steel → crappy steel, still expensive (nice but heavy)
  • Stainless Steel 304 → same as mild steel, non-magnetic, doesn’t rust (nice but heavy)
  • A2 Tool Steel → air-hardened steel, very uniform, used for knives (nice but heavy)
  • 6061 Aluminum → cheapest aluminum engineers use
  • 5052 Aluminum → easily formed or welded
  • 2024 Aluminum → used for airplanes etc. extra stiff allow
  • Brass → copper + zinc(?); mostly used for aesthetic
  • Titanium → $84, stiffest material