Materials

Most of your robot will be aluminum. Polycarbonate shows up for guards, panels, and light brackets. Steel shows up in shafts and certain COTS components. That covers 95% of what you'll work with.

Aluminum

Aluminum is the default material for almost everything structural and mechanical on the robot. It's light, strong enough for nearly all FRC applications, and easy to cut, drill, and tap.

Alloy	When to use	Notes
6061-T6	Everything unless you have a specific reason not to. Frame rails, gussets, mounting plates, bellypan, brackets.	The standard. Cheap, widely available, easy to work with.
7075-T6	High-load parts where you want to use thinner material to save weight. Gearbox plates, arm pivot plates, heavily loaded brackets.	About 50% stronger than 6061 but more expensive and more brittle (cracks at sharp corners rather than bending).

If you're not sure whether you need 7075, you probably don't. Default to 6061 and only use 7075 when 6061 isn't strong enough for the application.

Common forms in FRC:

Form	Typical sizes	Common uses
Box tube	1"x1", 1"x2", 2"x2" (1/16" or 1/8" wall)	Frame rails, superstructure, elevator stages
Sheet / plate	1/8" and 1/4" thick	Mounting plates, gussets, bellypan, gearbox plates
Hex shaft	1/2" and 3/8"	Shafts (usually buy ThunderHex from REV instead of raw hex bar)

Polycarbonate (Lexan)

Polycarbonate is a clear plastic that's nearly impossible to shatter. Unlike acrylic (which looks similar but shatters into sharp pieces), polycarbonate flexes on impact instead of cracking. This makes it safe and useful in a lot of places on the robot.

Common uses: electronics guards, intake guides, shooter hoods, sensor mounts, light panels, anywhere you want a lightweight cover that can take a hit.

Thicknesses: 1/16" for light guards and covers, 1/8" for anything that needs more rigidity.

Working with it: Easy to cut on a bandsaw or with a jigsaw. Can be bent with a heat gun or strip heater to form shapes. Scratches easily, so keep the protective film on as long as possible during fabrication.

Don't confuse polycarbonate with acrylic. Acrylic is the clear plastic that shatters into sharp pieces when it breaks. It should not be used on an FRC robot. When buying clear sheet plastic, make sure it says polycarbonate.

Steel

Steel is about 3x heavier than aluminum, so you would never use it for structure or plates. However, it's harder and stronger, which makes it the right material for a few specific things.

On most FRC robots, steel shows up in parts you buy rather than parts you make:

Part	Why it's steel
Hex shafts (ThunderHex, etc.)	Shafts under heavy load need the hardness that aluminum can't provide
Gears and sprockets	Surface hardness resists wear from repeated tooth contact
Shoulder bolts and dowel pins	Precision diameter and hardness for use as pivots
Bearings (internal components)	The balls and races inside bearings are steel

You generally don't need to buy raw steel stock or fabricate steel parts. If something on your robot needs to be steel, there's almost always a COTS part that already exists for it.