Impact riveting, where a solid or hollow rivet is formed with a single blow, is one of the oldest imaginable joining methods and, therefore, is often considered to be ‘low technology’ and only suitable for low-value, high volume assemblies or those where tolerances are not particularly tight.
While this is partly true, impact riveting remains the optimum fastening method for many applications, such as leatherwork, tools, toys, kitchen utensils or general hardware, where the speed of the process is attractive and there is no justification for a high degree of control over the process or a need for particularly tight tolerances.
Because of the nature of the process, it is fair to say that there has been relatively little research and development in the field of impact riveting over the past 40 years. However, there are some related processes, namely spin riveting, orbital riveting and orbital forming, that are experiencing a resurgence in interest, partly as a result of recent developments in real-time process control technology.
Spin riveting is, in comparison with impact riveting, a closely controlled process whereby two or more components are joined using one or more separate hollow or solid rivets. Instead of the single axial stroke that is used in impact riveting, the forming tool (also referred to as a peen) contacts the rivet head at an angle of three to six degrees from the vertical. The tool then rotates around the vertical axis, while also applying an axial load. This gradually forms the rivet head into the desired shape, the actual geometry of which depends on the rivet, peen and process parameters. For example, heads can be flat, conical, crowned, shouldered or flared into countersunk holes.
Importantly, modern real-time process controllers enable the forming to be closely controlled so that, for example, the rivet head can be formed to a desired height, or the rivet can exert a specified load on the finished joint. For this reason, spin riveting is particularly useful for assemblies where a pivot is required, such as scissors, pliers or hinges. Compared with impact riveting, there is no risk of unwanted damage to the parent material, and the gradual forming process does not crack or otherwise damage the rivet. If the rivet passes through, say, a polymer or soft metallic component, a load-spreading washer can be used beneath the head of the rivet – which might also be shouldered.
One of the advantages of spin riveting – and orbital riveting and orbital forming – is that the resultant joint is far more resistant to vibration than if threaded fasteners are used. Consequently such processes are popular for safety-critical assemblies and those where vibration or thermal cycling might otherwise cause problems (Fig.1).
Another factor that should be taken into account is the noise; spin riveting is significantly quieter than impact riveting. EU Directive 2003/10/EC on the minimum health and safety requirements regarding exposure of workers to the risks arising from physical agents (noise), which will repeal Directive 86/188/EEC, was adopted on 9 December 2002 and came into force on 15th February 2003. The Directive tightens the legal requirements in relation to noise by lowering the exposure action values to 80 and 85dB(A). EU Member States have until 6 April 2006 to transpose the Directive into local Regulations, after which employers will be obliged to meet the tighter requirements. For that reason, quieter joining processes are more attractive than ever before.
Despite the relatively tight tolerances and excellent consistency that can be achieved on the height and joint load, spin riveting remains very economical, even for small volumes. Part of the reason for this is that the process is performed cold, which keeps the equipment comparatively simple, and the tooling is low-cost and long-lasting. Spin riveting is commonly used in applications as diverse as low-volume aerospace assemblies and high-volume automotive components.
Of course, spin riveting has its drawbacks compared with impact riveting. However, these are primarily limited to the cost of the equipment and the cycle time (typically 2 to 15 seconds).
In some applications it is desirable to avoid using a separate rivet – perhaps in order to keep the parts count down, or to avoid the risk of intermetallic corrosion. Moreover, where volumes are high enough, it is likely to be more cost-effective to design-in a joining element on one of the components in the assembly, rather than using a discrete rivet. In such applications, the term orbital riveting is often used instead of spin riveting.
Essentially the same process, orbital riveting also uses an offset peen to gradually form the material into a head that holds the assembly together. However, without the need for a rivet, the scope of the process is dramatically increased. One of the interesting characteristics of the process is that it can be used with most metals – including heat-treated steels, case hardened materials and high-alloy steel up to Rockwell54C – as well as some polymers (with a lower equipment cost than for ultrasonic staking). Unlike welding, it is straightforward to join dissimilar materials, plus there is no heat affected zone, no weld spatter and the finished surface is smooth, with no sharp edges.
In terms of the production engineering, orbital riveting is quiet, creates no hazardous fumes, and does not heat the component and make it difficult for operators to handle safely. Furthermore, because the axial load applied is often as much as 80percent lower than for impact riveting, the equipment is comparatively lightweight, which reduces its footprint, and the fixturing can be relatively simple (Fig.2). Indeed, orbital riveting and orbital forming heads can be made remarkably compact, which enables these processes to be used where other joining methods would be difficult to execute.
Orbital forming takes the concept of orbital riveting a stage further. If the diameter of the ‘rivet’ form is increased, a larger peen can be used to form a ring of material, perhaps to leave a clear diameter through which another component can pass. In some cases orbital forming can be used to create a coined form and eliminate the need for snap rings or other retaining fasteners.
Alternatively, the technique can be used to crimp an external component so as to retain another inside, or as a swaging or drawing process. Almost any diameter can be formed, from about 2.5mm to 40mm.
Normally the peen will have a smooth surface in order to produce a near-polished finish on the component.
However, because the peen always retains the same rotational relationship with the material being worked, any form engraved or etched on the peen will be imprinted on the material. It is therefore possible to indelibly mark a part number, logo, symbol or date code on the assembly.
The relatively low axial forces involved mean that tooling is long-lasting, even with peens used to apply markings.Another application for the versatile orbital forming technology is to create flared or closed forms, typically on tubular parts, which would be costly to produce using alternative methods or that might be prone to cracking if other techniques were used.
Orbital forming is a remarkably versatile process if clear access is available at the face to be processed. However, there is another related process that can be used around the periphery of an assembly with, say, a thin metallic sleeve that needs to be formed to retain an end cap. Roller forming is available from a company called Orbitform and the forming heads usually have three identical rollers mounted in a circular pattern within a housing (Fig. 3). The rollers have a profile that rolls the material to gradually create the desired form as the head advances along the axis of the assembly.
Note that roller forming can also be used in conjunction with orbital forming to create two joints simultaneously, or an inner and outer set of rollers can be used. In all cases of roller forming described so far, the rollers are mounted in fixed positions. However, if the rollers are mounted such that they can be moved inwards, it is possible to crimp a component almost anywhere along its axis. Again, the profile of the roller will determine the formed profile.
Perhaps the biggest single reason for spin riveting, orbital riveting, orbital forming and roller forming not being used more widely is that design engineers are simply unaware of these processes and their capabilities today.
Thanks to modern real-time process control, and a better understanding of material behaviour, the processes are highly controllable and repeatable.
Furthermore, data can be collected from the equipment and used for statistical process control and quality assurance.
As with so many other technologies with which designers are unfamiliar, the best advice is to discuss the project with experts.
Companies such as DMG Engineering, of Pershore, UK, can assist with component and assembly design. It can also undertake to supply a turnkey machine to perform the joining operation.
The author wishes to thank Jon Isaacs, managing director of DMG Engineering, for his assistance with this article. "