Rivets are produced using automatic riveting machines. Commonly used types include R-type rivets, fan rivets, pop rivets, tree shaped rivets, semi-circular head, flat head, semi hollow rivets, solid rivets, countersunk head rivets, pop rivets, and hollow rivets, which are usually connected to the riveted parts using their own deformation. Generally, cold riveting is used for sizes smaller than 8 millimeters, while hot riveting is used for sizes larger than this.