We use various materials based on its application.
2 x 1 Aluminum tubing:
- Drivetrain
- Some structural components, especially those that need a bearing hole
1 x 1 Aluminum tubing:
- Assemblies that need to be strong but are not structural in nature
- Thin wall versa-frame for low load applications
Aluminum Sheet (1/8" and 1/4"):
- Gussets
- Mounting brackets
- Other applications where a box tube is overkill
Lexan:
- Usually for aesthetic uses
- Can be pretty useful if you are creative with it (bend with heat gun for various shapes)