Cogging is a method of metalworking used to change the grain of a piece of metal. It is a hot forging technique involving two anvils with shaped surfaces. This kind of shaped anvil is called a "die." The term "cogging" may come from the fact that the metal is meant to be pressed into the shape of the die in the same way two cogs will fit into each other, though the shape of the die is not always rectangular.
Cogging is often an intermediate stage in the metalworking process. It is used to change the internal structure of ingots. An ingot is formed after raw metals have first been melted, then poured into molds. The metal is then easier to handle, but will need to undergo further forging and tempering before it can be used. After cogging, the finished ingots, now called billets, are refined pieces of metal which can then be forged or cast into more complex components.
The cogging process begins with heating metal ingots until they are malleable, but not molten. The ingots are placed between two shaped dies. The surface of the dies are molded so as to encourage the metal to form a certain shape. The hot ingot will then be rotated and struck repeatedly between the two dies. Depending on the shape of the anvils used, the finished ingot, now called a billet, can take on many different shapes. It can be rounded, square, or even hexagonal.
During the cogging process, the ingot becomes a little longer with each blow. As the ingot lengthens, the metal grain changes and becomes longer and more homogenized. The rotation of the ingot between strikes encourages even shaping and lengthening. This effect, coupled with the compression of the center of the ingot as it is repeatedly struck, further strengthens the metal. In a variation on this process, the metal is sometimes shaped by the pressure of rollers into the shape of the die, and then rotated until the final form is achieved.
Cogging metal is a long process that requires many adjustments. The metal will need to be reheated between strikes so that it remains malleable. The ingot may need hundreds of blows along its length before it reaches the desired shape and strength. Some forging machines have been developed to make this process easier. The number of strikes and the amount of force used can be programmed in so that the ingot remains uniform.