The tools are shaped like boards, and so they are called flat dies or sometimes board dies or rack dies. The picture besides shows the basic principle behind the workings of flat dies. As the diagram illustrates, one die in the pair of dies is stationary. The other moves in a parallel direction and shapes the work piece. The distance between the dies determines the amount of penetration, and when the work piece reaches the end of the dies, it drops out. The distance between the dies cannot be changed during processing. The advantage is high production rate; the disadvantage is limited application. Flat dies are mostly used in mass production of multi-purpose bolts and screws.
A special application that yields superior accuracy is the dual-slide type of flat die form rolling machine. Please refer to the diagram on the left. This machine uses a pair of flat dies that, while parallel, move in opposite directions at the same speed. The work piece does not move during processing. This superior accuracy is particularly applicable to involute spline or involute serration processes.