Precision metal stamping forms tight-tolerance parts in high volumes and with a process that lends itself to automated assembly operations
Edited by David S. Hotter
Engineering Manager, Stamping Div.
Weiss-Aug. Co. Inc.
East Hanover, N.J.
When it comes to forming intricate metal parts, engineers have a tough time finding a process that can handle the job as well as precision metal stamping. In much the same way that injection molding works for making plastic components, precision metal stamping forms tight tolerances at production rates that keep it cost competitive with other forming processes. Precision stamping competes with screw-machining operations, cold heading, and powder-metallurgy techniques, to name a few.
In addition, precision stamping easily adapts to automated downstream processing and assembly operations. Processing ease and competitive costs represent only a sampling of advantages. Further advantages include the ability to form varying thicknesses on the same part, work with a broad selection of materials, and make use of grain orientation to strengthen components.
Precision metal stamping is used to describe the process of stamping components that are smaller than 1⁄2 in. in any direction (although lengths sometimes exceed this), less than 0.010 in. thick, and have dimensional tolerances from 0.001 to 0.002 in. on lengths and 0.0005 to 0.001 in. on roundness.
The process starts with metal in strip form, typically specialty alloys and often precious metals rather than standard cold-rolled grades. This gives engineers a broad range of materials to choose from because most metals come in strip form. Strip metals let engineers maximize strength-to-weight ratios because of the high mechanical properties inherent to strips. Mechanical properties also improve with heat treating and work hardening.
The grain orientation in strip materials is also referred to as its texture. Stamping is one of the few metal-fabrication methods that takes advantage of texture. When a material has a specific grain orientation, it is able to withstand flexing across the grain more than parallel to grains.
Other metal-forming processes such as die casting, forging, screw machining, cold heading, and pressed powder metals don’t impart a grain orientation, which is often used to improve qualities such as wear resistance. For example, by switching from screw machining to precision stamping, one manufacturer was able to increase the life of a retainer contact tenfold. Besides boosting life, stamping cut the weight of the retainer by 30% and reduced overall costs by a remarkable 80%.
Because stamping gives engineers a choice of grain orientation, parts can be designed for different qualities in separate areas. Indicating the orientation of a part on the strip of stock material maximizes qualities such as strength. For instance, a component will be stronger if it bends across the grains. For parts that will flex in several directions during use, the components should be oriented at 45° to the grain direction.
Tooling can be made after determining the orientation. While tight tolerances are inherent to precision stamping, consistency from part to part is also critical for such small designs. Fabricating stamping dies from high strength tool steels gives them enough wear resistance to maintain 0.001-in. tolerances for production rates that exceed millions before they need sharpening. In contrast, machining tools typically require sharpening after producing as few as a thousand parts.
Because even small part-to-part variations can halt an entire automated assembly line, the consistency of miniature stampings also helps lower assembly times and costs. In some cases, holding tight tolerances is more critical to downstream assembly operations than it is to end-use performance.
Small parts can be fabricated several ways and with many materials. The following guidelines can help with the selection of a manufacturing method.
Varying thicknesses: One advantage of precision metal stamping is that it allows engineers to form components with multiple thicknesses. It can also replace designs using strips of different metals welded together to form varying thicknesses.
Small changes in part thickness can be produced by adding a coining operation to the stamping-forming process, as part of the sequence of progressive dies. Such methods make it easy to produce part-thickness reduction ratios up to 2.5:1 with most materials. When engineers require greater reduction ratios, strip stock can be machined before putting it through the stamping die. Coining is the preferred method, however, because it work hardens and strengthens metals while forming them. It also produces a better grain flow or orientation than machining, which helps eliminate stress concentrations that weaken components.
Doubling back: When designing one end of a component thicker than the other to boost strength, manufacturers can take advantage of doubling material back on itself. For cylindrically shaped parts, this is called collaring when the material is doubled back inside itself, and cuffing when folded outward. Besides boosting strength, doubling material over also helps parts assemble more easily and lets them dissipate heat more efficiently.
Applying precious metals: Precious metals are often added to stamped electrical components such as connectors and lead frames to increase conductivity and reliability of contact points. To aid the process, parts should be designed so they are easily used in selective cladding or plated before stamping.
The choice between the two methods is determined by the required thickness of the coating layer. A rule of thumb is that for a thickness less than 0.00003 in., plating is more economical; for thicker coatings, cladding or inlaying material works better. Cladding also offers a wider choice of contact materials, including various low-cost precious metals. In contrast, plating is usually restricted to pure precious metals, although advances in plating technology are helping change this limitation.
In the past, concerns for porosity in plating and poor mechanical properties have led engineers to avoid plating. However, the latest developments in plating technology eliminate such concerns and provide coatings with the same integrity produced by cladding.
Specifying radii: When forming bends in stamped components, radii should be specified no smaller than three times the material thickness. Smaller radii may weaken parts and increase tooling costs, making it impossible to form sharp corners.
All radii on material blanks should be at least as large as the strip thickness. Smaller radii lead to premature punch and die breakage, causing burrs and stress concentrations.
Besides forming tight tolerances on small components, precision metal stamping also makes it easier for manufacturers to assemble parts. This is particularly critical for tiny components. One can easily imagine the formidable task of separating a handful of nested fishhooks in a bag, for example. By forming parts on carrier strips, stamped shapes remain separated and organized on a metal coil. Looking beyond the forming process, engineers must consider the importance of feeding stamped components into automated-assembly and packaging equipment. Some points to remember include the:
Strip camber: If stamped strips bow or twist, they will jam assembly equipment. To avoid this, designers should use an adequate amount of material for the width of the carrier strip, which may require additional width than the part alone requires. Ladder rungs connecting carrier strips to parts can also be used to boost stability. As component designs become more delicate or include severe forming, support becomes crucial.
Another method for avoiding camber is by dimpling the carrier strip, particularly for long and thin stampings. Dimpling stretches or lengthens the carrier strip to provide more support. Usually, adjustments such as those to the carrier strip can only be made after initial assembly trials, as a retrofit correction.
Index pin breakage: Assembly machines rely on indexing pins to properly align parts while building components. To prevent indexing pins from breaking in equipment, specify large indexing holes — at least 0.078 in. in diameter — in the carrier strip.
While it may seem that designing more complex carrier strips and larger indexing holes increases tooling and material costs, the slight increases are easily offset by the costs eliminated from lower scrap and greater machine uptime.