Cogging is a hot forging technique that changes the grain of metal using two shaped anvils called “nuts”. It is an intermediate step in metalworking, used to change the internal structure of ingots. The process involves heating the metal ingots until they are malleable, then rotating and tapping them repeatedly between two shaped molds to stretch and homogenize the metal. The finished ingot, now called a billet, can take on many different shapes. Cogging is a long process that requires a lot of adjustments, but some forging machines have been developed to facilitate this process.
Cogging is a metalworking method used to change the grain of a piece of metal. It is a hot forging technique involving two anvils with shaped surfaces. This type of shaped anvil is called a “nut”. The term “cogging” may come from the fact that the metal is intended to be pressed into the shape of the die in the same way that two gears fit into each other, although the shape of the die is not always rectangular.
Cogging is often an intermediate step in the metalworking process. It is used to change the internal structure of ingots. An ingot is formed after the raw metals are first melted down, then poured into moulds. 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 that can then be forged or cast into more complex components.
The cogging process begins with heating the metal ingots until they are malleable, but not molten. The ingots are placed between two shaped molds. The surface of the dies is shaped in a way that encourages the metal to form a certain shape. The hot ingot will then be rotated and tapped 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 bar gets a little longer with each stroke. As the ingot stretches, the grain of the metal changes and it stretches and homogenizes. Rotation of the ingot between strikes promotes uniform shaping and stretching. This effect, coupled with the compression of the center of the ingot when repeatedly struck, further strengthens the metal. In a variation of this process, the metal is sometimes shaped by the pressure of the rollers into the shape of the die and then rotated into the final shape.
Cogging metal is a long process that requires a lot of adjustments. The metal will need to be heated between shots so 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 facilitate this process. The number of strikes and the amount of force used can be programmed so that the ingot remains uniform.
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