Winding machines wrap copper wire around the stack of steel plates in the armature so fast the eye barely follows. In brushless motors the windings sit on the stator and magnets ride on the rotor.

A balancing machine spins each rotor, finds its heavy side and grinds a little metal off the plates. Some lines add a dab of weight compound instead. The dished scars on a tool rotor come from this step.

The body is injection molded as two mirror-image halves, often at a separate plant. One half sits in a fixture on the line with alignment pins pressed in.

The reduction gears, the clutch that slips before a screw head strips, and the chuck that grips the bit arrive as one sealed unit built elsewhere. It sits at the front of the motor.

A worker solders the heavy power wires from the switch assembly to the motor contacts. Thinner control wires clip on with plastic connectors.

Motor, switch and gearbox go into the clamshell, the second half drops on, and drop-down pneumatic screwdrivers set to a fixed torque drive the screws in a numbered order.

A station writes the tool's settings, such as the temperature at which it shuts off, into its memory chip through the battery connector. Then the chuck is coupled to a dynamometer-style rig that loads the motor and compares it to the design limits.

Lithium-ion cells are wired in series with a control board that manages charging and discharging, then sealed in a plastic case that slides onto the handle. Packs at 18 V are the most common.

Tools, batteries and chargers go into cases that are set on a scale. If the weight is off, something like a belt clip or manual is missing.