Lesson 3.1: Selecting Materials for FTC Structures
Technical Context
Material selection on an FTC robot is a trade among four things: stiffness, weight, workability, and cost. There is no best material, only a best material for a specific part with specific loads.
The mistake that costs teams the most is using one material for everything because it is what the shop stocks.
The Materials FTC Teams Actually Use
| Material | Stiffness | Density | Workability | Typical use |
|---|---|---|---|---|
| 6061 aluminum | High | Moderate | Good, cuts and taps cleanly | Frames, extrusion, brackets, shafts |
| Mild steel | Very high | High | Harder to cut, heavy | Rarely used, occasional shafts and gears |
| Polycarbonate | Low | Low | Excellent, but notch sensitive | Guards, funnels, non-structural plates |
| Printed PLA | Low, brittle | Very low | Prints easily, poor impact | Prototypes and jigs only |
| Printed PETG or ABS | Low | Very low | Good, tougher than PLA | Light brackets, spacers, guides |
| Printed nylon or CF nylon | Moderate | Low | Harder to print, expensive | Load bearing printed parts, gears |
| Delrin / acetal | Moderate | Low | Machines beautifully | Bushings, low friction guides |
Stiffness Is Not Strength
These get used interchangeably and they are different properties.
Strength is the load at which the part breaks or permanently deforms.
Stiffness is how much the part deflects under a load below that point.
For FTC robots, stiffness usually matters more. A frame rail that bends 0.2 in under a defensive hit does not break, but it moves your intake 0.2 in out of position, and the mechanism stops working. Nothing failed, yet the robot stopped scoring.
The material property that governs stiffness is Young's modulus:
6061 aluminum about 69 GPa
Mild steel about 200 GPa
Polycarbonate about 2.3 GPa
Printed PETG about 2.0 GPa
Steel is roughly three times stiffer than aluminum and roughly three times denser, so for the same shape it is stiffer and heavier in about the same proportion. That is why the shape of the section matters more than the material choice, which is Lesson 3.3.
Aluminum has about a third the stiffness of steel and about a third the density, but it cuts, drills, and taps easily with the tools a school shop has. The combination of adequate stiffness and easy fabrication is why nearly every FTC frame is aluminum.
Where Polycarbonate Belongs
Polycarbonate is excellent at absorbing impact and terrible at resisting deflection. That combination points at specific uses:
- Good: guards, funnels, ball deflectors, protective panels over electronics, intake side walls
- Bad: anything that must hold a mechanism in position, any beam carrying load over a span, anything a bearing presses into
The failure mode teams hit is using a polycarbonate plate as a structural side panel because it is easy to cut. The panel survives every hit and flexes enough under load that the two shafts it supports stop being parallel.
Under sustained load, polycarbonate slowly deforms and does not fully return. A bolt torqued hard through a polycarbonate plate will be loose weeks later. Use large washers, do not overtighten, and do not rely on a polycarbonate joint to stay tight over a season.
Printed Parts
3D printing is transformative for prototyping and useful for production parts within limits.
Printed parts are anisotropic: much weaker along the layer lines than across them. A printed bracket loaded so that the load tries to peel layers apart will fail at a small fraction of its apparent strength. Orient the print so the load runs across layers, not along them.
Practical rules:
- PLA for jigs, fixtures, and prototypes. It is stiff, prints easily, and fails brittle under impact.
- PETG or ABS for parts that see moderate load and some impact.
- Nylon or carbon fiber filled nylon for parts that must survive real robot loads.
- Never press a bearing into a printed part and expect it to stay tight, since the plastic creeps.
- Add a fillet at every internal corner. Sharp corners in printed plastic are where cracks start.
Cost and Availability Are Real Criteria
A material you cannot get in time is not an option. A material your team cannot cut with the tools available is not an option either. Both belong in the decision matrix as explicit criteria rather than as an unspoken reason a good design gets rejected later.
- Confirm material legality for the current season in the game manual: FIRST Tech Challenge Game and Season Info
Fill-in-the-Blank Practice
- How much a part deflects under load without breaking is described by its
__________, not its strength. - The material property that governs stiffness is called
__________modulus. - Printed parts are weaker along their
__________lines, so orientation must be chosen with the load direction in mind.
Show answers
- stiffness
- Young's
- layer
Exercise
Pick three parts on your robot and, for each one, write the material, the load it carries, and whether stiffness or strength is the governing requirement. Then ask whether the material was chosen for that reason or because it was the material on the shelf.
Ready to move on?
Sign in with Google to save your progress with Telemark, or continue without saving.
Stuck on this lesson?
Ask about anything on this page. It can see which lesson you have open and which part you are reading.