Taps and dies are tools used to create new threads, which is called threading. Dies are used to cut or form the male portion, where the threads are on the outside of the surface. R.S. Hughes carries both round dies and hex dies.
Round dies are threading tools used for producing external threads - creating the female portion of the mating pair. Round dies are round, commonly have three or more cutting edges, and designed to fit in a T-handle tool. Round dies are usually used for creating new threads in the workpiece.
Hex dies are threading tools that are shaped like a hexagon, and commonly used with hand tools to help create or repair threads. The die material should be harder than the workpiece material. Hex dies are usually used in applications where threads require repair - known as "chasing", and they are used to create the external or female threads for the workpiece.