rivet, headed metal pin or bolt whose shaft is passed through holes in two or more pieces of metal, wood, plastic, or other material in order to unite them by forming the plain end into a second head. The button-head rivet has a hemispherical head; the countersunk-head rivet has a flat head made to fit a countersunk hole. A large rivet for building construction is first heated so that the pneumatic hammer used to set it can more easily squash the plain end into a head. When the hot rivet cools, it shrinks and pulls the parts tightly together. For critical work, holes are drilled and reamed to exact size; for most other work they are punched. Full tubular and split rivets can be driven through soft materials without the necessity of first making a hole; after passing through the materials, the rivets' plain ends spread out to form heads as they strike a hard substance. More complicated blind rivets are used when only one side of the work is accessible. The mandrel type is a tube in which a rod with an enlarged end is inserted. After the rivet is pushed into the hole, the rod is pulled back through, crushing the end of the rivet into a head and forcing the sides of the tube against the walls of the hole. The drive-pin type is a tube with an opening at the headless end smaller than at the head end. As the pin is driven through from the head end, it spreads the tube out over the edge of the hole. The explosive blind rivet is filled with an explosive; when the head is heated with an iron, the explosive ignites and expands the headless end over the edge of the hole. Rivets are made of steel, aluminum, copper, and many other metals, and of plastics.