The Cold Forming Value Chain in Micro/Miniature Component Design - Part 1


Madan Mathevan, Sussex Wire Inc. | 3/18/2016

New product design engineers rely heavily on design and manufacturing techniques learned through education, repeated application, and empirical success. Many alternate techniques are often approached with caution, particularly on mission-critical applications where a known solution is proven, and thus considered “safe.”

In spite of its relatively low visibility, there is a time-tested manufacturing technique that is not well understood, and therefore is not being fully utilized to achieve cost, quality, yield, and waste reduction objectives that manufacturers seek to improve quality and reduce cost. That technique is cold forming.

Cold forming is not a new technology for manufacturing micro-precision components, yet it is still not well known or understood among New Product Development (NPD) teams. One of the reasons for this is a widespread lack of related course work in engineering schools. Here, we will provide a fundamental description of the cold forming process, discuss its features and benefits, and review some of the advantages over alternate manufacturing methods such as machining, MIM, and stamping.


Shown is an example of a part often submitted for a roll-forming quote. The designer in this instance has not taken into account "where is the material for the annular ring coming from?" (Source: Sussex Wire)

Successful application of cold forming depends inherently on each project’s four key contributors: the component manufacturer; the materials provider; the tooling designer/manufacturer; and finally, the machinery manufacturer. Each plays an important role in determining how a part design can be executed most effectively and provides feedback to the customer regarding critical dimensions, tolerances, potential limitations, and alternatives to improve strength and reduce cost while achieving all functional objectives.

New product design engineers are well advised to explore these strengths and limitations early in the design process, and to engage in a consultative relationship with the component design and manufacturing firm to collaborate on modifications that may well improve the component’s strength and facilitate its manufacturability, while at the same time, providing the lowest cost option available.

All too often, by the time a micro/miniature part geometry is fixed by the NPD engineer, the opportunity for making improvements has passed. Even though these parts might play a small role in the greater project, their designs can be streamlined to meet all the designer’s objectives if known ahead. In other words, trade-offs can sometimes be made on non-critical dimensions & tolerances in order to meet functional specifications, especially if the designer and component manufacturer are working together before the final part design is approved. In fact, this collaborative approach works for any manufacturing technique, not just cold heading or roll forming.

What Is Cold Forming?

Cold forming is the application of force with a punch to a metal blank staged in a die. The force exceeds the alloy’s elastic limit, causing plastic flow until the metal blank assumes the shape bound by the punch and the die. As the name implies, this method of forming is achieved by force alone, forgoing the application of additional heat or cutting and shearing. Consequently, cold forming does not re-anneal or mechanically damage the material’s original metallurgical grain structure like other processes can.

There are four principal advantages to cold forming:

Reduced scrap/cost

Cold forming is a net shape solution. During the process, wire is transformed by a sequence of die blows into a specific shape, with the material flowing to fill the part geometry and dimensional tolerances defined by the tooling engineer. So there is virtually no waste created. Without scrap to deal with, there is little to no recycling cost associated with the process, less lubricant to reclaim, and minimal labor to handle it all. In general, the wire feedstock is less expensive than the bar used for machining. Because sizing can be done in line with the heading operation, the tolerances on the wire feedstock do not need to be as tight as those demanded for the Escomatic Quality wire feedstock.

With all forms of screw machining, including single- and multi-spindle and Escomatic processes, scrap is not only unavoidable, it is a significant by-product of the process, often equivalent to 50% of the final part’s mass.

Throughput

With an optimized part formation progression for a complex component, cold heading delivers yields at a rate of 90-300 parts/minute (PPM), standard. Generally, yields for a similar design produced from a multi-spindle screw machine will be in the 6 – 20 PPM range, an order of magnitude faster for cold forming.

Since the cost of each part must absorb a proportionate share of manufacturing overhead, it becomes clear very quickly that a cold headed part can amortize that cost by many multiples over that of a screw machined part. At the same time, the man-to-machine attendance rate favors cold forming by a factor of two or more.

Strength

Cold forming is a process in which the native tensile strength of the material is increased through die forging and punch upsetting (or heading). Here’s how it works: For every 1% of area reduction of a part’s cross section due to cold forming, its tensile strength increases by a factor of ~0.6 - 1.5, depending on the alloy. This physical property is known as the work hardening rate of the material. The work hardening rate varies depending on starting tensile strength and material composition. Stainless steels tend to have higher work hardening rates than carbon steels. Super-alloys tend to have higher work hardening rates than stainless steels. Ferritics (body-centered crystal structure, i.e. carbon steels, annealed 410, & 430) tend to have a lower work hardening rate than austenitics (face-centered cubic crystal structure, i.e. 302HQ, 304, 316 …). Elements like nickel and copper will suppress work hardening rates while elements like carbon and nitrogen will accentuate the work hardening rate. No process that removes material from the native shape, such as screw machining, can achieve this.

Roll forming is an ideal method for creating full radii, tapers, undercuts on solid material, and tubular parts with or without a head, splines, and more -– all while strengthening the part at the same time, rather than weakening it. Roll forming is versatile, too. It can produce knurls, single or multiple cross threads for plastic inserts, barbs for one-way inserts, and clips, detents, or custom shapes for retaining two parts. Engineers can achieve stronger product assemblies and grips by employing roll forming techniques, while taking advantage of its superior speed and economy.

Eco-Friendly

Because cold forming produces virtually no metal scrap, it requires less reprocessing, along with its associated costs of transportation, fuel, and labor. Lubricant is used in the die formation process, but at a fraction of the rate for a screw machine (cutting) operation. Furthermore, cold forming lubricants don’t include metal scraps and so are cleaner and easier to recycle.

For screw machine operation, materials often contain free machining additives. These free machining additives are added directly to the molten metal before casting into ingots. These additives are not soluble in the alloy composition and form discrete particles of various shape and size in the ingot casting. As the ingot goes through a series of hot and cold work operations to achieve microstructure, size and shape, these particles known as inclusions get stretched out. During the machining operation, these inclusions are very useful because they not only act as weak spots where chips can break but also reduce the friction between the cutting tool and work piece. From a pure engineering perspective, the inclusions have no value. From a cost perspective, the addition of free machining additives are critical. They significantly increase the ability to machine parts and increase production rates.

For stainless steels, elements like sulfur and selenium are the most common additives. Grades like 303, 416, and 430F are very common in the screw machine world. In carbon steels, sulfur and phosphorus are used in 1200 series grades while 12L14 gets an extra boost from Lead (~0.15 - 0.35 wt%) additions to enhance its machinability. 360 brass contains ~1.5 - 2.0% lead, purely for machinability. Cold forming does not require these additives. From an engineering perspective, because the raw material has fewer inclusions, a cold formed part lends itself to a superior potential design.

See Part 2

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