One kind of die is a tool used for cutting a thread on a rod. It usually takes the form of a disc of tool steel, with a center hole and two or more radially disposed holes which break into the central hole to form cutting edges. The central hole is first tapped with the required screw thread. The outer edge of the disc has flats or shallow holes which allow it to be gripped in a die holder or die stock when in use. The finished die is hardened and tempered, and some dies have a slit from one edge to the center hole to enable this cutting hole to be opened or closed slightly when fixed into the stock. Split dies are made in two halves which are put together in the screw stock; they normally have one size of thread only, but are sometimes made with a number of differently sized holes in the split. A die nut is similar to an ordinary die but of hexagonal or square outer shape for cleaning up threads and useful for threads which cannot be reached with dies held in stocks. Another, entirely different tool called a die is a shaped metal block used to produce a form or shape by metal stamping or pressing; these are called 'metal-forming' or 'punching' dies. Die casting is a process in which molten metal is forced under pressure into a die or mould to produce a casting.