Die and Punch

The form of metal stock most often used in cold forming is “wire,” which may be a half-inch or more in diameter. It comes in large coils, convenient for feeding into machines making many parts per minute. When metal is compressed within a die, it is important to introduce just the right amount of material into the die, often within plus or minus one percent, or even less. Too little material won’t fill the die, making a bad part. Too much material can result in a malformed part, or produce flash that needs to be removed. The excess material can cause the die to split when the punch hits. Cold-forming machines are designed to cut off a precise length of wire, but the diameter of the wire must also be precise. To provide this, many shops will do a final draw of three to 10 percent on the wire before it is cut and formed to get a nice, tight diameter tolerance, said Kevin Hughes, specialist in wire products, Carpenter Technology Corporation, Reading, Pa.

Not Just Fasteners Anymore

Originally, cold heading was used to create heads for fasteners. A piece of wire was held in place and an impact to one end of it caused some of the material to spread or “mushroom,” creating a head for the fastener without having to machine away a lot of material to form the shaft. Once the head was formed, then the threads could be either cut or rolled. This process not only formed the fastener quickly, but it made efficient use of the material, producing little or no scrap.

with complex shapes can be made by cold forming. The machines may be called “headers,” but they do much more than heading.

Tooling can be designed to extrude the material. When the punch hits at each station, it presses the material forward into the die to create a narrowed section, taper or shank. Or the punch can press on the material within the confines of the die so the material extrudes backward over the punch, creating the walls of a hole. In addition, shapes and contours can be built into the tooling to create splines, gear teeth and other features.

Upsetting, another term for heading includes forming a bulge in a cylindrical part, as well as forming a head on one end.

Using sophisticated tooling producing combinations of upsetting and extrusion when the punch hits each station, a cold forming machine can produce complex parts. Dimensional tolerances and surface finishes can rival those achieved with machining, depending on the shop and precision of the tooling.

So cold forming can produce parts at near-net-shape, using the minimum of material, to close tolerances, and at the rate of dozens or hundreds per minute. In addition, cold-formed parts exhibit excellent strength, as the material flows into its final form, rather than being cut, as in machining.