“Cold heading is basically putting material in a die and hammering on it” according to one industry expert. Unlike hot forging, where the metal is heated before forming, or casting, where the metal is melted and solidified, cold heading and other methods of cold forming cause the metal to deform at room temperature. Though there was a U.S. patent issued in 1794 for a “cold header” (really a rivet machine), cold forming became a practical fabrication technology after World War II.

The typical one-die, two-punch method is common in producing headed parts, especially fasteners. The first blow combines partial head upset (coning) with shank extrusion. Then the second blow finishes the head shape. (Illustrations courtesy of Capenter Technology Corporation.)

Die and Punch

Modern cold-forming machines, also called headers or parts formers, provide from one to seven die stations, opposite a number of punches mounted on a slide, which usually moves horizontally. A common type used for making bolts or screws is the one-die, two-blow header, similar to the example shown. This has one die, which is the diameter of the shank of the bolt or screw being formed. The material in the die is struck with two different punches, one after the other.

There is a limit to how much you can deform the material with a single blow, so two hits are often necessary to create the correct geometry of the head, said Steve Copeland, vice president of sales and marketing, Reed Machinery, Inc., Worcester, Mass. The first blow makes a shape like a tulip. Then a shifting mechanism moves the first punch over and places the second one in position. The second punch comes in to produce the finished shape of the head. After the second blow, a knockout pin pushes the piece out of the die. On a multiple-die header, there is a transfer rack with fingers that grab the part and move it from one die station to the next. If needed, the transfer mechanism can turn the part around 180 degrees so the next punch hits the other end of the part.

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.

Example of a part converted from screw machining to cold forming. Originally, the flat on the right side of the shank was machined and the hole was drilled. On the cold-formed part, the flat is formed and the hole is created by backward extrusion. A multi-die parts former produces these at 140 pieces per minute. (Photo courtesy of Reed & Prince Manufacturing Corporation.)

In addition to fasteners, many kinds of parts 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.

All this goodness does not come for free, however. Tooling costs for cold forming can be significant, perhaps $5,000 to $25,000 for a tool set, depending on how complex the part’s geometry is. Lead time for design and setup to run cold-formed parts is often measured in weeks, much longer than it would take to program and set up the same part on a CNC lathe or machining center. For small quantities, cold forming may not be cost effective.