Abstract: A metal can top rests on a tool seat for the purpose of riveting a pull-up ring to the metal can top. Riveting takes place with the aid of a dolly which moves back and forth relative to the tool seat. Transmitters are arranged in the tool seat to check that no foreign objects are present in it. Transmitters sense the movement of the dolly past a reference level as it moves towards the tool seat. In the riveting position the dolly presses the metal can top and associated pull-up ring against the tool seat, whereby the pull-up ring actuates a sensing pin and moves it to an indicating position which is sensed by a transmitter. The signals from the respective transmitters are received by a delay and control device.