Banging out tiny parts

Steve Taylor
     Cold Forming Engineer
     Deringer-Ney Inc.
     Mundelein, Ill.
     www.deringerney.com

The quest for ever more compact products has designers looking for ultra-small, high-precision metal components that can be made quickly and inexpensively. Cold forming (cold heading) can produce such components in net or near-net shape at rates to 400 parts/min or faster. Parts can have multiple diameters, shoulders, and heads, even letters and logos.

Cold forming takes wire from a coil and cuts it to precise lengths. Multiple blows by a series of dies and punches then shape the wire into a finished product in one machine cycle. Parts with high levels of precision and complexity are possible. For instance, outside diameters can be made less than 0.015 ±0.0005 in. Internal diameters can also be held to within 0.0005 in. and lengths to 0.005 in.

Typical cold-forming techniques include upsetting or heading, forward and backward extrusion, piercing, and trimming. Upsetting starts with wire of a smaller diameter and forms features with a larger diameter but shorter length (conservation of volume). Headed fasteners are made using this technique. Calculating volume of the upset and dividing by the area of the starting material gives final length. Length is then divided by starting diameter, yielding the number of diameters of material in the upset. Buckling limits upsetting of typical carbon steels to about 2.25 diameters. Softer alloys such as copper and silver have a buckling limit of about 1.5 diameters. Beyond these ratios an additional controlled upset is required to collect the needed material.

There are two basic types of forward extrusions, open and trapped. An open extrusion is one in which material being extruded is not confined by tooling, hence the name. This generally works for reductions of about 30 to 35%. Beyond this point the material will upset rather than extrude. Reductions approaching 70 to 75% can be accomplished by confining or trapping material in tooling during the extrusion process. Then pressure on tooling is the limiting factor.

Such guidelines work well for typical low-carbon steels and average-sized components. However, they become less dependable for miniature parts and nonferrous materials such as silver, gold, copper, and brass. This is because nonferrous materials tend to have tensile and yield strengths that are much lower than steel. As such they flow easier but have less column strength and a greater tendency to easily upset. This lowers open-extrusion-reduction percentages and upset ratios. However, the use of unconventional methods can push the design envelope.

Case in point: Basic cold-forming guidelines say a 75% reduction in area is possible for trapped-forward extrusions. But a component made from a particular alloy required an 85% reduction with a 0.015-in.-diameter knockout pin. A conventional approach puts the extrusion orifice in the die. The blank would then load into the die and be pushed with a pin from the punch-side tooling. The blank needs to be totally enclosed in the die during extrusion and the die kick-out pin would be only 0.015-in. diameter. But such a small diameter pin would break and the 0.015-in. part diameter would upset when trying to push the larger portion of the part from the die. However, turning the process around - placing the extrusion in the tool and sliding the die to push the material from the die into the punch - solves the problem. In other words, leaving a portion of the part's larger diameter in the die and using this to pull the finished part from the tool allowed a larger kick-out pin in the die.

Cold forming versus screw machining

Cold-formed parts can be churned out up to 100X faster than screw-machined parts. Cold forming produces little or no scrap and requires fewer secondary operations. In contrast, screw-machined parts with partial or through-holes may leave up to 80% scrap. Eliminating scrap can significantly lower part cost, especially when using expensive, exotic materials.

Cold forming also produces mechanically stronger parts because the process doesn't interrupt material grain structures. Screw machining typically cuts across grain-flow lines while cold forming "forms" grain-flow lines to the part geometry, work hardening the part in the process.

Metals made for screw machining typically incorporate lead, sulfur, and other additives to improve machinability. But such additives also degrade some of the other properties. Lead, for example, promotes metal surface porosity and raises toxicity. Metals used in cold forming don't require such additives so cold-formed parts can have improved mechanical properties, especially important for ultrasmall components.

Cold-formed parts are inherently smooth and free from tool marks and burrs. Surface finishes of under 8 R_A are possible right from the die, eliminating secondary finishing processes in many cases.

Cold-formed parts tend to be more consistently sized because each part is made with the same die and tool. Tools tend to wear slowly over many thousands of parts.

But cold forming doesn't work well for some jobs such as threadforming, forming undercuts, flattening, or making full points. In such cases, parts may be transferred to another station or machine to perform these operations. Some companies may incorporate secondary operations into the original heading step to reduce part cost.