Abstract: A motorised measurement arm apparatus for a machine tool includes a base for attachment to the machine tool and an arm member extending from the base for holding one or more sensors. The arm member is moveable relative to the base between a stowed position and an operative position, the operative position being defined by engagement of a mechanical stop arrangement. The apparatus also has a motor for moving the arm member relative to the base and a motor controller for energising the motor to move the arm member relative to the base. The motor controller is configured to energise the motor when the arm member is in the operative position to maintain engagement of the mechanical stop arrangement. An operative position having improved repeatability is thus obtained.