The key idea is that the slug of metal is red-hot when hammered into place. This results in the slug of metal having a dumbell shape which holds the two metal sheets together. An automatic jackhammer is used to force the metal into position, so giving a bulb on the inside face away from the pounding. The process (for example, of ship building) was very noisy. As the metal cooled, the two end of the rivet were pulled together, making a tight joint.