A tool (or spindle) represents a physical device that tightens a bolt. It can for example be held by a human being, attached to a robot, or be part of a multi-tool fixture.

A tool has usually a drive to perform the tightening operation. There may also be some kind of sensors involved to get the process status.

Note: Spindle is a common term used in Fixtured system to represent a specific type of Tool.