A robot assembly can, instead of a welding function, also have an integrated nutrunner. E.g. before tightening a cap onto a barrel, the robot goes picking the cap in a cap feeding system. A camera checks in the meanwhile in which position the barrel hole is located where the cap needs to be assembled. Ones the robot has picked the cap he goes to the correct position of the hole an the nutrunner tightens the cap on the correct torque value. The reaction forces are absorbed by a manipulating arm which is linked onto the robot.