COLD FORMING: WASTE NOT, WANT NOT!
As an engineering technique that has been with us for many years, precision cold forming (or cold heading) can be a key method of reducing waste metals in manufacturing and engineering.
None of us like waste. Not in our personal life, at home, and probably least of all in our workplace. So why is it that many of us continue to use practices, systems and equipment that are inherently wasteful?
In the engineering workshop, for example, we use modern CNC machine tools that are highly sophisticated, extremely accurate and are capable of producing precision components consistently and in large volumes. Yet, regardless of the skill machine operator, conventional machining – cutting, milling or grinding parts from a solid block of metal – creates considerable waste.
For many components this waste (swarf, chips and offcuts) can represent more than 80 percent of the total volume of the original block of metal. The scrap has to be recycled, often at a cost to the company and the environment, while the need to purchase more metal than is required for the finished component leads to higher part costs to the end user, or lower margins for the producer.
Although there are many parts where conventional machining is the only solution, there are far more instances where the use of precision cold forming, combined with CNC machining, can reduce the size of the initial blank of metal by 75 percent and scrap volumes by up to 90 percent.
Cold forming, or cold heading, is an engineering technique that has been with us for many years. It is a proven process that is straightforward and reliable, and can be used to manufacture large volumes of high-quality components quickly and consistently.
Cold forming is perhaps best known for the manufacture of low-cost components such as rivets, bolts and fixings. What is perhaps less well known, however, is that the technique is also capable of producing parts that can be both accurate to within a few microns and extremely complex, for use in applications in aerospace, automotive, power generation and medical.
Copper, brass, aluminium, steel and even stainless steel can all be cold formed. Parts are normally produced at ambient temperatures, so no heating or cooling is required.
A simple blank, sawn or cut from a round bar or wire, is placed into the cold forming press, where die and a punch tooling is used to extrude the metal under extreme pressure, typically between 100 tons and 200 tons. But this pressure can be far greater in some systems, up to 2,000 tons.
The metal is stretched beyond its yield strength, so that it takes on and retains the exact shape of the mold without adversely affecting its tensile strength. Unlike conventional machining, where the material removal processes cut across the grain structure of each part, cold forming allows the grain structure of the material to follow the contours of the die or mold. As a result, the strength of the part is maximized along its length, while internal surfaces take on a highly polished finish.
Parts effectively undergo work hardening during the extrusion process. This improves their machinability and durability still further, as it aligns the molecular structure of the metal in such a way that it prevents subsequent rearrangement. This increase in strength is comparable to that achieved by heat treatment, making it more cost effective to cold form a less costly and weaker metal than it is to hot work a more expensive material, especially where a precision finish is required.
The ability to extrude metal from a small blank means that there is almost no waste. This only occurs if subsequent machining is required but is typically far lower – typically up to 90 percent less, with a similar saving on the cost of raw materials.
Precision cold forming also offers other advantages. For example, in the production of copper parts it is normal to use oxygen-free copper wire, which is more readily available at lower cost that the Tellurium copper required for machining.
In addition, precision cold forming using oxygen-free copper produces significantly better results than machining in many applications. This is due to its superior electrical and thermal properties, which result in components with optimized mechanical and surface characteristics.
Parts can have highly complex geometries; indeed, complex internal shapes that would be almost impossible to achieve using traditional machining can easily be produced. Other important characteristics of copper, such as its antimicrobial properties and the ease with which it can be recycled, remain unaffected by cold forming.
These benefits are consistent with all types of metal, not just copper. Recent developments have even enabled stainless steel to be cold formed – something that has traditionally been extremely difficult to achieve commercially.
It’s important to recognize that precision cold forming is not the holy grail of engineering. It is an extremely cost effective manufacturing technique, which produces exceptional results. In common with all engineering processes, however, it has its strengths and weaknesses – it’s unsuitable, for example, for extremely low volumes due to the cost of tooling – but used in conjunction with other techniques it can offer engineers a valuable option with exciting technical and commercial advantages, especially if the goal is to cut the cost of waste materials.
About the Author: Mark Jennings is the technical director at Dawson Shanahan Ltd., Cranbourne Industrial Estate, Potters Bar, Hertfordshire EN6 3JN, United Kingdom, 01 707 602000, Fax: 01 707 602049, www.dawson-shanahan.co.uk.