Here?s how to join piece parts mechanically with or without loose rivets.

Assemblers generally want to eliminate loose rivets from an assembly operation. Whenever possible, they prefer to use studs or bosses that are cast or formed in one of the parts. As a result, the use of orbital and radial forming has spread beyond traditional formed metal parts. Forming methods are also widely used on die-cast aluminum, thermoplastic and fine-blanked parts. Riveting, in the common sense of using loose rivets, accounts for less than 20 percent of the applications for these joining methods.

Orbital and radial forming are nonimpact cold-forming processes. Cold-forming means that the workpiece is not heated before forming. Instead, it is formed at room temperature. Both processes can be used on any material that can be cold-worked.

Many believe that either orbital or radial forming yields superior results to the other in specific applications. Other experts believe that there is no practical difference between the two processes. Here is a close-up look both methods.

Orbital Forming

Orbital forming produces heads on rivets, pins and posts. The process is quiet, consumes little energy, and produces high-quality joints and heads, at high cycle speeds. The smallest orbital forming machine produces up to 400 pounds of force. The largest model can generate as much as 36,000 pounds of downward force. Orbital tooling is generally set up to produce extremely tight assemblies where no movement is acceptable. Precision machines operate at 1,700 rpm. Medium- and heavy-duty machines run at 900 rpm and 1,200 rpm.

Orbital forming combines pressure and orbital motion to form a head to fasten parts. A tool, called a peen, is mounted on a rotating spindle with the axis of the peen fixed at an angle with the spindle axis that varies between 3 to 6 degrees. As the spindle rotates, the peen orbits the spindle axis. The peen does not rotate on its own axis. The peen axis intersects the spindle axis at the working end of the peen. The peen presses on the rivet shank along a radial line that begins from the shank center. As the peen moves in an orbital path, a minute quantity of material is displaced with each rotation of the forming head until forming is complete. Because the peen is nutating, but not rotating, it orbits around the part with minimal friction between the workpiece and peen. Unless a large mass of material is being formed, the workpiece is normally cool to the touch.

The peen is generally made of M2 drill-rod steel and heat-treated to a Rockwell C hardness of 60 to 64. It is held in place by a setscrew, so it can be changed quickly. Changing the shape of the peen changes the shape of the finished rivet head. Conical, flat, eyelet and crown are some common head shapes.

More than one point on a part can be headed at the same time. This can be accomplished with a multipoint head?an orbital head with two or more peens. A multipoint head can be used if all of the points to be formed are on the same plane and if the center lines are between 0.3125 and 6.25 inches apart. A multipoint head uses an orbiting thrust-plate to transfer the motion to multiple rotating peens. A single-point machine can be retrofitted with multipoint head. There is no specific limit on how many points can be formed simultaneously on one part, but the center lines of the points must be a minimum of 0.3125 inch apart.

Spring-loaded parts or loose assemblies are no problem for orbital forming. They can be held in place with spring-loaded devices. These devices, called pressure pads, fit around the spindle and extend below the peen. Because it extends below the peen, it contacts the workpiece first, holding it in place.

A warm orbital forming process also exists. With warm orbital forming, the part to be formed is heated in an induction heater prior to orbital forming. Warm orbital forming can be used to form a mild-steel shank with a diameter larger than 1.25 inch, form extremely hard materials, form some plastics that do not form well cold, and form certain shapes or materials that may be unable to withstand the pressure of forming.

Because orbital forming creates a moving line of pressure, it will not create a galled or hammered finish. A solid rivet will have a smooth finish, whether it has a crowned, conical or flat head. Semitubular rivets will not split. Some plated surfaces will also remain intact, depending on the amount and quality of the plating material.

Radial Forming

Radial forming was developed in Europe in the 1960s and is often confused with orbital forming. The two processes do share similarities. Like orbital forming, the radial process produces heads on rivets, pins and posts. The operation is quiet and produces joints and heads of high quality and slightly polished in appearance. The tools can be adapted to work in close quarter. Radial forming can be adjusted to produce a tight or loose fastener.

But there are differences between the two processes. One difference is that the control of a joint can be held within limits that are slightly better than orbital forming.

However, the major difference lies with the process, itself. With radial forming, the peen axis is not held at a fixed angle with the spindle axis. The working end of the peen passes over the end of the rivet shank on a path that is an 11-loop rosette. The angle between the peen axis and spindle axis varies continuously between 0 to 6 degrees. The angle between the two is 0 degrees when the axes are aligned.

The peen rotates as the tool is orbiting the rivet. Forming pressure is applied following the rosette pattern, so the pressure line on the peen moves repeatedly through the common centers of the peen and the rivet shank. The process of the peen pressing against the rivet shank spreads the rivet material radially outward, radially inward and tangentially overlapping.

Pros and Cons

Orbital forming can form all types of malleable material, with no damage to fragile or delicate material. In fact, orbital forming can handle metals with a hardness of up to Rockwell C hardness of 35. This includes virtually all grades of mild steel, most alloys and nonferrous metals, such as aluminum, brass and copper. Polycarbonate, ABS, Noryl and glass-filled nylon can be formed orbitally. Most thermoplastics lend themselves well to the process. However, for tight assemblies, pure nylon and other unstable polymers are not suitable.

Orbital forming also offers some benefits for plastic assemblies. Plastic can form strong, consistent heads. Because orbital forming is heatless, the material?s properties are not changed. The basic integrity of the material is maintained after assembly. In fact, fiber-filled plastics become work-hardened with orbital forming and actually increase in strength after forming.

On the downside, it is claimed that the grain structure of orbital formed heads is not of the quality obtained with twin-spin rolls and radial forming. The rivet or pin being worked orbitally cannot be headed without deformation of the shank under the formed head.

In many cases, orbital forming can replace compression rivets. However, both sides of the assembly must be accessible. Blind riveting cannot be accomplished with orbital forming. Also with the right pressure, orbital forming equipment can be used for staking.

The main advantage of radial forming is that is does not deform the molecular structure of the metal as much as other riveting processes. It can also form nonround applications, such as a shaft with a single or double D shape, which is often required for high-torque applications.

Because radial riveting is said to improve the conductivity of metal parts, this method is often used to form heads that are electrical contacts. It is often preferred for delicate work in small diameters, and is used to engrave, mark, coin and burnish surfaces.

Radial motion generation is complex and is someArial considered a disadvantage. Other problems encountered include poor powertrain wear factors under heavy forming conditions, forming tools cost more to manufacture than orbital tool inserts, and the length of the forming pass increases cycle Arial.

However, radial forming can form ferrous and nonferrous metals, die cast materials and case-hardened fasteners. And because the machines can be set to precise pressures, it has been used successfully on assemblies containing Bakelite, ceramics, glass and other brittle materials. Radial forming facilitates riveting of precision parts, precious metals and various thermoplastics.

Who Uses Orbital and Radial Forming?

Orbital and radial forming has been used extensively in the automotive industry for years. The processes are typically used to assemble automotive locks, hinges, latches, seats, window cranks, wheel hubs, air bags, and engine and transmission components.

Other industries have discovered the benefits of orbital and radial forming, too. This assembly process is used to assemble appliances, electronics, audio equipment, toys and medical devices.