When most people think of riveting, they think of the classic blind rivet-the two-piece fastener consisting of a headed, hollow rivet body and a solid mandrel.
However, riveting actually encompasses a wide range of technologies, including impact rivets, self-piercing rivets, self-clinching fasteners, rivet-nuts, and orbital and radial forming. Each has its own niche, its own advantages and limitations. Which to choose depends on the application.
Self-Clinching FastenersOne of the newest developments in riveting is the SpotFast self-clinching fastener from PennEngineering (Danboro, PA). The fastener permanently joins two sheets of metal. It installs smooth with the top sheet, and flush or sub-flush with the bottom sheet. The sheets can be different materials and different thicknesses.
Standard fasteners can be installed in sheets as thin as 0.03 inch thick, explains Jay McKenna, product manager with PennEngineering. The sheets can be aluminum, brass or steel-any metal with a hardness of 80 or less on the Rockwell B scale. (Fasteners for harder metals, such as stainless steel, will be introduced later this year.) Made of low carbon steel, the fasteners are available in four standard lengths, ranging from 0.059 to 0.123 inch. Standard head diameters are 0.139, 0.148 and 0.219 inch.
“The thinnest panel defines what length of fastener to use,” says McKenna. “Above a certain minimum, the thickness of the bottom panel doesn’t matter. But, the thickness of the top panel is important because the fastener has to be long enough to reach into the bottom panel.”
The fasteners are installed with a press. First, a hole is drilled or punched into each sheet. The hole in the bottom sheet should be slightly smaller than the hole in the top sheet, and secondary operations, such as deburring, should not be performed. Next, the bottom panel is placed on the anvil, the top panel is placed over it, and their two holes are aligned. The fastener is then inserted in the hole, with the narrower end going in first. The head of the fastener will sit slightly above the top panel at this point. Finally, the press squeezes the fastener and the two panels together, causing panel material to cold-flow into the fastener’s two separate clinch profiles.
“You could also clinch the fastener into the top panel first, perform some operation on it, and then clinch the bottom panel. It could be a two-step process,” says McKenna.
A unique aspect of the SpotFast technique is that a single installed fastener can act as a hardened pivot point. The top sheet is effectively captured between the head of the fastener and the lower, nonrotating sheet. The top sheet is then able to rotate freely. A nylon washer or a wave spring can be inserted between the two parts to control the amount of friction between the two panels and thus determine how easily the top sheet can rotate.
A similar fastener is the Fas-Ner from AKH Inc. (Indianapolis). Cylindrical with concave sides, the fastener resembles an hourglass. Made of hardened aluminum or high carbon steel, the fastener acts as its own punch.
Like the SpotFast, the fastener is installed with a press equipped with special tooling. When the insertion tool contacts the parts, the fastener pierces the materials. The lower die shears the material and allows the slug to pass through. The fastener, which is the same length as the total thickness of the materials being joined, takes the place of the pierced section. The upper and lower dies exert pressure around the fastener, forcing the sides of the pierced hole to flow into and form around the concave section of the fastener. The joint is locked tight, and the fastener is flush with both surfaces of the assembly.
The fastener can join multiple layers of metal. The metal can be plated or coated, and the layers can be different materials and different thicknesses.
Orbital and Radial FormingDon’t ask Michael J. Shirkey, president of Orbitform (Jackson, MI), about orbital riveting. Not unless you want an earful about how the technology is more correctly called orbital forming and how riveting is just a small part of what this versatile technology can do.
In truth, however, orbital forming, and its cousin, radial forming, are mostly (perhaps 60 percent of the time) used to form heads on loose rivets or, more commonly, studs.
“In the application of riveting, orbital forming is best used to upset a rivet or stud when either the assembly is delicate or the components are required to rotate once they’re assembled,” explains Shirkey. “Both of those are hard to accomplish with impact riveting or staking.”
Orbital forming is a cold-forming process in which an orbiting tool held at a fixed angle (3 to 6 degrees) is used to progressively move malleable material into a predetermined shape. The technique heads the rivet without disrupting the grain structure of the material. “Orbital forming can be used to form a rivet out or a shoulder in,” says Shirkey.
The technique is similar to impact riveting or forming, except that the axial load imparted to the rivet is much less, due to the mechanical advantage of the angular orbiting tool and progressive forming action. In fact, orbital forming requires 80 percent less axial force to form a rivet head than would be needed in a comparable impact riveting application.
The technology can be used to rivet mild steel studs as small as 0.02 inch in diameter and as large as 1.5 inches in diameter. Any malleable material is a candidate for orbital forming, including steel, stainless steel, aluminum, brass and copper. Even high alloy steel with a hardness of 54 on the Rockwell C scale can be orbitally formed. “We’ve formed a wide array of materials,” says Shirkey. “The only materials we haven’t formed are things like glass or ceramic.”
It typically takes 1.5 to 3 seconds to advance the tool, form the stud, and retract the tool. The exact cycle time is determined by the type of material, the diameter of the stud, the desired head style, and the stroke.
Impact RivetingImpact riveting is a cold-forming process that uses short, metal rivets to permanently join parts. The rivet is cylindrical with a head on one end. After the shank is passed through holes in the parts, compressive axial force is applied to flatten the opposite end of the rivet and form a new head. Access to both sides of the assembly is required.
The rivet setting machine can be electromechanical, pneumatic or hydraulic. The process is typically semiautomatic, but the equipment can also be integrated with a rotary indexing table or other automated assembly system. The parts are placed in a fixture, the rivet is automatically inserted into the hole, and the head is formed an instant later. The rivets are fed from a hopper or a vibratory bowl, down a track, to the rivet setter’s jaws.
Impact riveting is inexpensive, quick and reliable. “Impact riveting is best when you’re trying to securely fasten two or more components together and you don’t necessarily want relative motion between them,” says Shirkey, whose company also supplies impact riveters. “It lends itself very well to simultaneous insertion and heading of the rivet, which is a little more challenging to do with orbital riveting.
“When delicacy is not required, and you’re just putting parts together, impact riveting is usually the most cost-effective method, because it’s a little quicker.”
There are two types of impact riveting: solid and semitubular, says Shirkey. In solid impact riveting, compressive force is applied to the end of the rivet shank. This causes the rivet shank to shorten and swell. Once the shank fills the hole, the unrestricted end of the rivet expands, forming a head that holds the parts together. In semitubular riveting, the head is formed differently. The end of the rivet is hollowed out. The forming tool is shaped to flare out this hollow portion, rolling it down until in touches and traps the top part. The force required to form a semitubular rivet is typically 40 percent less than that required for a comparably sized solid rivet.
Solid and semitubular rivets are typically steel, but they can also be made of aluminum, brass and stainless steel. A variety of head styles and finishes are available. The body diameter of semitubular rivets ranges from 0.06 to 0.3 inch. The body diameter of semitubular rivets ranges from 0.06 to 0.3 inch. Solid rivets can be as wide as 1 inch in diameter.
Blind RivetsThe ability to set blind rivets without accessing the opposite side of the assembly makes their use mandatory in many instances. However, there are many other reasons to recommend these versatile fasteners. Blind rivets can be installed quickly and economically. The installation tools are inexpensive, lightweight and easily portable, and the installation process won’t mar the surface of the assembly. Joints fastened with blind rivets are strong, tamper-proof and vibration-proof.
A blind rivet consists of a headed, hollow rivet body and a headed, solid mandrel. To install a blind rivet, the fastener is manually or automatically inserted into the jaws of the riveter. The rivet body is then inserted in a hole in the materials to be joined. When the tool is actuated, the jaws grip the mandrel and pull the mandrel head into the rivet body. The rivet body expands and forces the materials together, forming a strong, tight, reliable joint. At a predetermined setting force, the mandrel breaks and falls away.
The rivet and mandrel can be made of aluminum, steel, stainless steel, Monel, copper, plastic or a combination of materials. For example, an aluminum rivet body can be paired with a mandrel made of aluminum, steel or stainless steel. The rivets are available in a wide variety of finishes, lengths, diameters, head styles and body styles.
Three body styles are available: filled, semifilled and hollow. With a filled rivet, the break point of the mandrel is approximately flush with the rivet head. These rivets provide high shear strength. A semifilled rivet retains a shorter section of the mandrel. A hollow rivet does not contain any part of the mandrel. This style is advantageous when light weight is important. It’s also useful in electronics applications, because it eliminates the possibility of loose mandrels disrupting circuits, and it leaves a grommeted hole for ventilation or wires.
Rivet setting tools are typically handheld. They can be pneumatic, hydraulic, electric or battery-powered. They are available in pistol-grip and in-line versions, and they can be mounted to tool balancers and mechanical support arms. Some tools are equipped with sensors to monitor setting force and displacement.
Self-Piercing RivetsSelf-piercing riveting is a cold-forming process for joining two or three layers of material by driving a rivet through the top layers and upsetting the rivet in the lower layer without piercing it. Access to both sides of the assembly is required. Although the bottom layer must be metal, such as steel or aluminum, the top layers can be metal, plastic or a woven material. The rivets can be installed after the parts have been coated or painted without marring the finish. The rivets can be installed in a stack as thick as 6 millimeters for steel sheets and 10 millimeters for aluminum sheets.
Self-piercing rivets produce strong joints. For a given sheet thickness, a joint made with self-piercing rivets is 30 percent stronger than a comparable spot-welded joint. Indeed, self-piercing rivets are ideal for fastening metals, such as zinc-coated steel, that are difficult or impossible to spot-weld.
Available from several manufacturers, self-piercing rivets can be made from steel, stainless steel, copper and aluminum. Various coatings can be applied to the rivets, and the heads can be painted to match the assembly. The rivets are available with countersunk, flat, pan and dome heads. Special heads, such as threaded studs and stand-off pins, are also available. Head diameters range from 3 to 5 millimeters. Lengths range from 4 to 14 millimeters.
The setting tool is essentially a small C-frame press, and it can be hydraulic, electric or battery-powered. The tool can be equipped with various sensors to measure setting force and rivet displacement. The tool can be manually operated, or it can be mounted to a pedestal for semiautomatic operation. It can also be mounted to a six-axis robot.
The cycle time for the process ranges from 1 to 4 seconds. Rivets can be supplied to the tool on a plastic tape or through a feeder bowl and a pneumatic tube. Depending on the size of the rivet and the reel, one tape can hold 800 to 10,000 rivets.
Rivet-NutsBlind threaded inserts, also known as rivet-nuts, provide strong fastening threads in thin panels. The fasteners were originally developed decades ago by B.F. Goodrich to attach rubber flaps to airplane wings. Today, rivet-nuts are found in a variety of products. For example, the fasteners are commonly used in cars and SUVs for attaching roof racks and bumpers.
A rivet-nut is a one-piece internally threaded and counterbored tubular aluminum rivet that can be headed while working entirely from one side of a panel. As with a classic blind rivet, the upset head of the rivet-nut is formed on the blind side. This head is large enough to resist being pulled out even under conditions of eccentric load. Because rivet-nuts can be installed without accessing both sides of the panel, the fasteners are ideal for attaching parts to housings, tubes or extrusions. The fasteners can be installed into all metals, most plastics and even ceramics.
Rivet-nuts are installed in a three-step process. First, the fastener is threaded onto a mandrel at the end of a pneumatic heading tool. The fastener is then inserted into a hole in the workpiece. When the tool is actuated, the mandrel spins rapidly and powerfully, pulling the threaded portion of the rivet-nut’s shank toward the blind side of the workpiece and forming a bulge around the unthreaded portion. Once the rivet-nut is clinched securely in place, the mandrel spins in the opposite direction, unthreading itself from the nut and leaving the fastener’s internal threads intact.
The heading tool can be handheld, or it can be mounted to a linear actuator for fully or semiautomatic operation. It can also be mounted to a six-axis robot.
No additional finishing is required after setting, even with coated or painted components. Thus, rivet-nuts can be fitted at any stage in the production process. Unlike welded captive nuts, rivet-nuts do not subject the base parts to the damaging effects of high temperatures. The installer is safe, too, since the process does not produce fumes or gasses, and there is no risk of fire. In addition, the fastener’s internal threads can be safely made and remade without loss of performance.
The fasteners can be made from aluminum, steel, alloy steel, stainless steel and brass. They come in many head styles, finishes and sizes. A single fastener size can typically accommodate many grip ranges.