An unconventional fastening technique cuts the cost of many material forming and assembly applications.
Kevin L. Haueter
Most engineers tend to rely on traditional fastening methods to join parts, but a lesser-known technique, orbital forming, often provides better results at lower costs. Orbital forming, as the name implies, is a cold-forming process that uses an "orbiting" tool. Sometimes called spinning, radial riveting, or spin riveting, orbital forming is most often used to head, swage, crown, flare, or form a column or projection of material in fastening and assembly operations.
The process is somewhat similar to impact and compression forming, where the tool applies a compressive axial load to plastically deform the part. The difference is that in orbital forming, the tool rotates at a fixed angle typically 3 to 6° and applies both axial and radial forces to progressively move malleable material into a desired, predetermined shape. Unlike impact or compression forming, where the process is complete in a single pass, orbital forming requires several tool revolutions and typically takes 1.5 to 3 sec to complete. Most of the work during orbital forming is focused at the tool's line of contact, not along the entire tool surface. This reduces axial loads by as much as 80%, which has several advantages.
- Fasteners and mating parts see less stress.
- Orbital forming produces a smooth surface finish and, in some applications, eliminates cracks caused by impact riveting.
- Since the effective forming load typically does not exceed the fastener's column strength, cold-head forming is possible without bending or swelling the fastener shank.
- Because less axial load is required for forming, a smaller press can be used, which reduces equipment tonnage, floor space, and costs typically associated with producing large parts.
- Due to lower forming forces, less rigid fixturing is required, and tools last longer.
- Orbital forming is quieter than other cold-forming processes such as impact forming or peening.
The process is suited for many materials, from metals to plastics, including virtually all grades of mild steel, most stainless steels and heat-treated alloy steels, case-hardened materials, and nonferrous metals such as aluminum, brass, and copper. Even high-alloy steel hardened to Rockwell 54C can be orbitally formed. Standard machines are available that can handle up to 1.5-in. diameter parts in solid mild steel, and larger diameters in softer materials. Still, the ultimate physical size limit of orbital-forming applications is constantly being expanded. New customer applications are resulting in larger orbital forming machines.
Although riveting is by far the most common use for orbital forming, any application with a symmetrical or near symmetrical structure that requires material forming is a candidate.
In fact, the process has been successfully used in many nontypical applications such as crimping and sealing end caps in air-bag gas generators, roller retention in cam followers and rocker arms, and sealing and retaining the assembly of tie-rod ends, ball joints, and idler arms. It has also been used to seal antilock brake manifolds, join tubes to mating end plates, attach gears to shafts, seal welch plugs in driveshaft yokes, and retain nuts in reinforcing plates.
Orbital forming can replace fasteners such as screws, snap rings, threaded caps, and rivets. For instance, orbitally forming studs cast in steel, aluminum, and zinc, or molded into plastic, can eliminate rivets. Using the process to form over semishears and extrusions retains the mating part without riveting.
In one recent application, a production lot number was etched into the forming tool and then transferred to the workpiece during the forming process, marking the part.
Equipment suppliers should be contacted early in product development to enhance any orbital-forming application.