Plates — Flat pieces of material (usually sheet or plate stock) used for mounting, brackets, and structural panels. Plates are easy to cut, drill, and machine, and they provide broad support and attachment points.
Shafts — Used to transmit rotation and support bearings, gears, and pulleys. Shafts are chosen for stiffness and precision; they locate rotating parts and carry torque.
Tubes — Hollow profile stock (square, or rectangular) used where strength-to-weight matters. Tubes save weight compared to solid shafts or plates and are common for frames, spacers, and lightweight structural members.