Cold forming is a quick process that imparts a fine surface finish, meets tight tolerance requirements, and produces ready-to-ship parts without wasted material.
Clack-tick-clack-tick-clack-tick-clack-tick. That's the sound of a six-die, cold forming machine spitting out microparts at rates up to 200 pieces per minute. In addition to being quick, the process imparts a fine surface finish, meets tight tolerance requirements and produces parts that are often ready to ship. And there's no wasted material because cold
forming doesn't produce chips.
Cold-formed parts are common. A visit to a hardware store's fastener section reveals numerous examples, including screws, nuts, bolts, rivets and nails. But far more than just fasteners are cold-formed. The process is pushing the manufacturing envelope for creating parts—including many microscale ones—that are impractical or nearly impossible
to make by other methods.
Starts with round stock
Cold forming involves feeding wire or round stock, typically from a spool, into a high-speed, single- or multiplestation, automated reciprocating machine. The machine shears off a small portion of the raw material to create a blank with the same volume as the finished part.
Then, via a set of feed fingers, a pick-and-place attachment or a die, the machine feeds the blank into a series of horizontal punch and die sets, which rogressively "spank" it—at a rate of one hit per station—into the correct shape.
This multistep process can involve numerous operations, such as coining, piercing, extruding, shearing, trimming, threading and knurling. With a few exceptions, any operation that can be performed on a lathe or mill can be coldformed.
Similar to squeezing toothpaste from a tube, cold forming forces brass, copper, steel and even difficult-to-machine materials such as Inconel, tantalum and molybdenum into each uccessive die cavity. At each step, the material takes the mirror shape of the station's die before moving to the next station.
Because virtually no material is wasted, cold forming is especially beneficial when manufacturing parts from precious
metals. Your scrap man might not be happy, but because there are no chips, it's easier to find microparts on a cold
former than in a lathe's chip pan.
Another advantage is that cold-formed parts are stronger and more durable than machined parts, because there's no
interruption of the material grain flow as there is with traditional machining processes like milling and turning. This
makes it possible to create complex shapes while holding close tolerances. (For example, one manufacturer of coldformed parts holds tolerances of ±0.0005" on a 0.010"-dia. part.)
However, despite the many benefits, cold forming does have limitations. It's challenging, for example, to cold-form
parts with long length-to-diameter ratios, undercuts or "choke" diameters, and complex parts with multiple features.
Want to cold-form?
If you're looking to purchase a cold-forming machine, your options range from breaking open your kid's piggy bank to
buy an old, used machine for as little as $500 to groveling for a bank loan to buy a new machine with all the bells
and whistles, which can cost upwards of $500,000.
The reality for most shops is probably somewhere in between. Cold-forming machine builders offer various versions. For instance, Tiffin, Ohio-based National Machinery LLC's offerings range from its new Microformer machine with a 3mm-dia.-wire capacity to a six-die monster capable of processing 34mm-dia. stock with forming pressures up to 600 metric tons. National equips its machines with features such as quick-change tooling, CNCs, zero-clearance slide mechanisms and linear feeds.
Needless to say, effective cold forming requires more than just buying a machine. Cold forming is an art, especially when making microparts. Be prepared to develop your own tooling, processes and possibly your own equipment.
That's what leading medical-component manufacturer Deringer-Ney Inc. does. The 200-year-old company, headquartered in Vernon Hills., Ill., performs stamping, machining and insert molding, and develops custom preciousmetal alloys.
Microforming, however, is the company's sweet spot. "A 5mm part is huge for us," said Tom Schieber, product development engineer. " 'Micro' to us means anything under 0.5mm."
At its Marshall, N.C., plant, Deringer-Ney specializes in forming microparts, including antenna leads; actuator pins; medical-grade, implantable, radio-opaque markers; and miniature screws for medical and electronic devices. It forms standard and high-value work materials, including gold, platinum and tantalum.
Deringer-Ney regularly cold forms parts with dimensions down to 0.25mm—only three times the width of a human hair.
"The rules change when you move into the micro world," said Garth Boyd, the company's vice president of marketing. "For example, during a normal cold-forming operation, a stock diameter reduction of up to 75 percent might be possible. But when you're below 0.5mm, that size reduction might be limited to only 55 percent due to a number of constraints, including material ductility, tooling accuracy and workpiece grain structure."
Tolerances also shrink with part size. Boyd noted that a standard tolerance might be ±0.002" on a 0.5"-dia. part, but a 0.5mm (0.02") part might have a ±0.0002" tolerance. This tolerance reduction means tools must be more accurate and be better aligned, and surface finish and tool wear become bigger factors. Deringer-Ney monitors dimensions when microforming far more frequently than for macroscale work, and replaces microtools more often.
Equipment is another challenge Deringer-Ney faces. "Commercially available equipment frequently doesn't cut it," said Dana Dubuc, vice president of business development. As a result, Deringer-Ney sometimes modifies standard equipment to meet tighter tolerances and process requirements—in effect, turning a Chevy into a Ferrari. And if that's not possible, the company designs and builds achines to meet challenging customer requirements.
High-volume part runs are desirable from the standpoint of amortizing tooling and equipment, of course, but that doesn't preclude Deringer-Ney from scheduling short runs. "Our minimum lot size is one piece," said Schieber. The reason is that when working with exotic materials and precious metals, sometimes it's not possible to conventionally machine a micropart. "Show a Swiss-machine operator a 0.25mm part made out of tantalum and he'll just shake his head and say, 'It's not possible,' " said Scheiber. "That's because some materials, while perfect for cold forming due to their ductile nature, are just plain tough to machine. These include nickel alloys, molybdenum and other exotic materials. But with microforming, we can spank out parts like that all day long." Sometimes by the millions.
As an example, Deringer-Ney was approached by a medical device company to help design a part and produce
several million of the parts annually. Deringer-Ney's designers got involved early in the process to create part features
conducive to cold forming.
"We spent a week on the part design and then they asked us to make a dozen prototype parts from two raw materials," said Scheiber. "We designed tooling for a 1mm (maximum wire), two-die cold former. The midsection of the part was only 0.030", so the wire diameter was slightly smaller—0.0275". We built punches and dies in-house and delivered two dozen parts in 6 weeks. The customer tested the parts, settled on the material and decided to make a few minor dimensional changes. We tweaked our process and sent them another thimble-full of samples in a week. We later added a textured surface finish for cosmetic reasons." Deringer-Ney then received an order for 250,000 parts, deliverable in 2 weeks.
Keeping it in-house
Another company specializing in microforming is Bigelow Components Corp. The Springfield, N.J., company has a department dedicated to developing tooling and processes for efficient microforming. "We design, produce, heat-treat and maintain all tooling in-house," said Brett Harman, company president. "Most of the parts we produce are components that go into a bigger assembly. They may be electronic, electrical or mechanical in nature. On the heading side, we can produce parts from wire diameters down to 0.018" (0.46mm)."
Why not use a screw machine? "Cold forming is a scrapless process, can typically be run faster [than screw machines] and tooling is less expensive," Harman said. In addition, many of Bigelow's customers require products made from lead-free and sulfur-free materials. It would be difficult to process such parts on multiple-spindle equipment because the material additives that promote free machining have been eliminated.
"One of our customers was machining a lead-based material to produce an integral part that required magnetic properties for the end product to function correctly," said Harman. "The machining process they were using was slow, expensive and left a cutoff mark that caused defective components. We changed the raw material to low-carbon steel, because of its superior magnetic qualities, and produced a component that was more stable, defect-free and costeffective to produce and assemble."
MICROmanufacturing April 1st, 2011 -About the author: Kip Hanson is a manufacturing consultant and freelance writer.