The Body and Frame

You cannot build a robot without giving it a physical form. The frame is the skeleton of the robot: it holds every other subsystem in its correct position, bears the mechanical forces the robot experiences, and determines the robot's fundamental shape. Sensors must point in the right direction. Motors must be aligned with the wheels or joints they drive. The controller must be mounted securely where it will not shake loose or overheat. All of this depends on the frame. And unlike software, you cannot patch a broken frame with a software update.

Form Follows Function

The single most important principle in robot body design is that form follows function: the shape of the robot should be determined by what it needs to do. A robotic vacuum cleaner is round and flat because it needs to slide under furniture and turn in place without getting stuck in corners. A robotic arm on an assembly line is tall and slender with multiple joints because it needs to reach into tight spaces and move precisely along complex paths. A legged search-and-rescue robot is roughly dog-shaped because it needs to traverse uneven rubble that wheels cannot handle. Every geometric decision — the length of a limb, the width of a base, the placement of the center of mass — is an engineering decision driven by the robot's intended environment and task.

Materials

The choice of material shapes the robot's weight, strength, flexibility, and cost. Each material comes with tradeoffs. Aluminum is the most common structural material in mid-size robots. It is strong, lightweight, easy to machine, and does not rust. A typical robot chassis built from 3mm aluminum sheet is rigid enough to carry motors and electronics while remaining light enough for the motors to move efficiently. Carbon fiber is stronger and lighter than aluminum by weight, making it preferred for racing drones and high-performance arms where every gram matters. The tradeoff is cost: carbon fiber parts can be ten times more expensive than equivalent aluminum parts. Acrylonitrile butadiene styrene (ABS) plastic and polylactic acid (PLA) are the materials most commonly used in 3D-printed robot parts. They are cheap, easy to prototype, and good enough for many light-duty applications, but they lack the strength and heat resistance of metals. Steel is used where extreme strength and rigidity matter more than weight — large industrial robot arms that must be stiff to thousandths-of-an-inch tolerances, for example. Steel is heavy, which is acceptable when the robot is bolted to the floor.

Geometry and Stability

The geometry of a robot's base determines its stability. A wheeled robot with a wide base and low center of mass is very stable — hard to tip. A narrow-based robot with a high center of mass tips easily. This is why heavy components like batteries are typically mounted low in a robot's chassis, not on top. The number of contact points with the ground matters too. A tripod (three-point support) is always stable — three points always define a plane, so a three-wheeled robot never wobbles on uneven ground. Four wheels require a suspension system or flexible chassis to keep all wheels in contact on uneven surfaces. Six-legged robots can always have three legs on the ground at once, forming a stable tripod while the other three move — a key reason insects can walk on rough terrain so reliably.