High speed sheet joining - by mechanical fastening

TWI Bulletin, January - February 1996

Steve Westgate joined TWI in 1972, he is Technical Specialist, Resistance Welding. His work includes resistance welding processes and mechanical fastening, particularly related to sheet joining.

Self-piercing riveting and clinching are relatively new mechanical fastening techniques. Steve Westgate is currently studying these two methods as part of TWI's sheet joining activity.

Mechanical fastening using rivets is probably one of the oldest joining techniques. For centuries it has provided a reliable means of joining metal sheet and plate, from suits of armour to engineering structures. Conventional rivets have largely been replaced by other fastening techniques and welding; but, in the high speed assembly of sheet materials, self-piercing riveting and clinching are finding increasing application.

Background to mechanical fastening

A fastener may be defined as 'a device which will position and hold two or more members of an assembly in a desired relationship to each other'. There is a wide range of fasteners available which can be used successfully for joining different types of material. Standard fasteners and methods are well documented in a number of handbooks and engineering guides. ^[1-5]

Most fasteners can be classified within the broad groups of permanent or non-permanent systems. Rivets are most typical of the permanent group, which also includes mechanical clinch systems which do not require a separate fastening consumable. Screws and threaded fasteners are usually identified as non-permanent because of their capability of disassembly. Both groups span a broad spectrum of assembly requirements ranging from high strength, heavy structural applications to light duty, high volume assembly. However, the types of systems within each group vary widely and reflect the primary demands of the different application areas.

The demand for structural strength of fasteners in sheet metal assembly is limited by the relatively low strength of the sheet material, compared to the assembly of machined parts from plate, castings or forgings. Assembly cost is usually, therefore, a more important parameter. In the assembly of products by mechanical fastening, the fastener cost is often small compared with the labour cost of hole preparation and fastener placement. It is more appropriate to analyse fastener economics in terms of total 'in-place' costs. It is evident that the greatest improvement in productivity is achievable by reducing labour costs. Development of fastening systems has therefore been governed by requirements of ease and speed of placement.

Many of the methods are also compatible with adhesives and, unlike weldbonding (spot welding through adhesives), it is feasible to make mechanical joints through film adhesives as well as the paste types. The advantages of adhesive bonding can therefore be combined with the automated point joining capability of fasteners. As with weldbonding, curing of the adhesive would follow the joining operation.

Apart from machine threaded fasteners, there are few standards relating to screw, rivet or clinch fastening methods. Most of the systems have been developed to meet specific requirements and a wide range of proprietary systems are available to the user.

The main types of mechanical fastening systems are described with particular emphasis on the self-piercing riveting and clinching techniques for sheet materials. A broad comparison of the attributes of the various fastening systems together with resistance spot welding is presented in the Table.

Table: Comparison of features for joining systems

Outline of fastener systems

Conventional rivets

The traditional solid rivet requires location through aligned holes, and the shank is clinched by hammer action or rotating tooling. Tubular or semi-tubular rivets are similar but may be set in a single press action, see Fig.1. Although automatic feed of rivets is possible, the need for the pre-drilled hole is a disadvantage.

Hot riveting provides a compressive load between the head and set shank of the rivet due to shrinkage on cooling. This creates a frictional force between the parts joined which contributes to the shear strength of the joint.

Cold riveted joints rely solely on the performance of the rivet. In addition, control of the setting action allows such joints to be used for hinge pin applications. Spin, orbital or radial riveting methods incrementally set the rivet by line contact with an orbiting tool (Fig.2). This requires lower forces, and allows close tolerances to be achieved.

a) solid type;
b) semi-tubular type

Blind rivets

Blind rivets form a family of rivets which are used particularly when access is only available to one side of the component. Figure 3 illustrates the operation. The rivet is normally of a tubular form with a headed mandrel which is placed in a pre-drilled hole and the mandrel pulled back. The head expands the rivet against the reverse side of the sheets and, as the rivet is set, the load in the mandrel increases, breaking it at a notch behind the head within the rivet body. Manual or pneumatic tools are available and automatic feed of the rivets can be incorporated.

A wide range of fastener types may be inserted in this way. Rivets range from low duty to high performance types, including self sealing ones. There are also many inserts, studs and threaded fasteners which use this principle.

The low cost of riveting equipment and the flexibility of the process make it ideal for a wide range of application areas. However, the need to provide a pre-drilled hole makes the technique less attractive for higher volume work.

There are systems available which will self drill the sheets as the rivet is inserted with a special tool, this combines the advantages of blind riveting with the freedom from pre-drilled holes. An example of this type is shown in Fig.4.

Self-piercing rivets

Self-piercing rivets are of a tubular type, which pierce and clinch the material in one operation by means of special punch and die tooling. No pre-drilled hole is required, see Fig.5. This technique has been refined by a number of rivet manufacturers so that the rivet does not completely pierce both sheets during setting, but forms a button on the reverse side. Self-piercing riveting provides a high speed means of sheet joining, and is described in greater detail below.

Clinching

Almost an offshoot of self-piercing riveting, clinching or press joining provides a mechanical interlock between sheet materials without inserting a fastener. The sheets are deformed using a punch and die, as shown schematically in Fig.6 and 7. This technique has a similar speed and suitability for automation as the self-piercing technique, and again is produced using proprietary tooling developed by a number of equipment manufacturers. More details are provided below.

a) Circular tool non-piercing
b) Rectangular tool semi-piercing

Threaded fasteners

The basic engineering types of threaded fasteners are bolts or screws with machine threads, which are attached into nuts or tapped holes. Although fully standardised, these methods are of limited use for sheet material assembly because of the high cost of fastener and assembly. Self tapping systems are widely available and tend to be proprietary items with numerous patented shapes and thread forms. In most cases, pre-drilled holes are required, and on assembly, the screw forms a thread in the materials being joined. Flow drilling provides sufficient material for thread engagement in thin sheets, alternatively, an additional nut or clip is used to engage the thread. While such systems are valuable for non-permanent fastenings, they are still a relatively high cost approach in comparison to the riveting and clinching methods.

Process details

Self-piercing riveting systems

Self-piercing riveting techniques vary in detail between suppliers. The design of tools and rivets is normally covered by patents. Advantages of the self-piercing riveting systems are as follows:

• Fast automated operation
• Ability to join unweldable materials and dissimilar material combinations
• No pre-drilled holes required
• Little or no damage to pre-coated material
• No fume and low noise emission
• Long tool life, >200 000 operations before replacement
• Low energy demand
• Environmentally and user friendly

Riveting equipment is available in various forms, depending on the application requirements, from portable manually operated tools, to multi-head, automated machines and robot mounted tools. Examples are shown in Fig.8 and 9. Rivets may be delivered to the tool via a plastic tape, or air blown individually along a tube from a bowl feeder.

Disadvantages which may need to be considered are:

• Limitations on access due to required accuracy of tool alignment and relatively high setting forces.
• Lower strength than spot welds in many cases, depending on the material being joined. This also depends on the requirements and design of the joint layout.
• Unacceptable visual appearance at least on one side for some applications.

There is no specific standardisation of methods and each supplier has in-house data to allow the choice of tooling or rivet type for specific applications. Options available for rivet types include:

• Material type: steels, stainless steel, aluminium alloys, copper alloys
• Heat treatment: to control hardness, strength and formability
• Dimensions: length and diameter - depending on sheet thickness and strength required
• Coating: platings or polymer coatings, eg for corrosion protection or colour match
• Head type: typically countersunk, domed or flat head, depending on application and strength requirements

In operation, sheets to be joined are first held together by a clamping ring around the rivet. This may be spring loaded or applied by a controlled hydraulic force. When a high clamping force is used, lateral displacement of material is restricted during setting, this reduces the eventual distortion of the component.

Data are often available for joint properties, but these are usually only released by suppliers in support of their application studies. Much work has also been done on a European industrial collaborative basis between manufacturers and users.

Clinching systems

A range of proprietary designs has been developed for tools to lock sheets together mechanically, without an additional fastener. The joint is completed in one operation using a punch and die. The die allows sheet materials to be formed while the base of the die creates an anvil to deform the material to form an interlock, see Fig.6. In many cases, the parts of the die are sprung to allow lateral movement of the shearing part so that the compression between the punch and anvil part permits lateral deformation of the button (Fig.7a). Other designs rely on the specific profile of a closed die, allowing interlocking of the deformed material within the sheet thicknesses. Some process variants, typically using rectangular tooling pierce the sheets completely along part of the tool and again compression of the material against the die completes the interlock, see Fig.7b. Advantages of the clinching methods are similar to those of the self-piercing rivets described above, also, no consumables are required. Long tool life is again claimed in comparison to spot welding (typically >200 000 operations), although the punch tool, (this penetrates the sheet), may suffer greater wear than in an equivalent riveting operation.

Broadly similar forces are required for clinching as self-piercing riveting - a rigid frame to provide tool alignment and react the force. Joint strengths, particularly in peel are lower than for riveted or spot welded joints, and again the appearance of the joint may be undesirable in some cases. Such joints have been flattened after the main clinching operation while maintaining interlock between the sheets.

Equipment ranges from manual gun types through to automated or multi-tool assemblies. Fig.10 and 11 show typical units.

As with the self-piercing riveting systems, the types of clinching vary in detail between suppliers and are based on patented tooling. The suppliers are again the main source of application data and work is also included in the European collaborative effort, access to this data is restricted.

Application areas
(self-piercing riveting and clinching)

Both self-piercing riveting and clinch joining have found particular application in the domestic appliance, heating and ventilation, automotive and general sheet metal assembly industries. Their market share has come partly from replacement of spot welding applications and partly from replacement of alternative fastener systems.

Maximum thickness capability for both processes at present is 6mm total joint thickness in steel and slightly greater for aluminium alloys. Clinching punches and rivets are generally in the range 3 to 5mm diameter, larger tools may be used for higher strength applications at the expense of higher setting forces.

The techniques can be used for pre-painted and plastic coated steels and are well suited to the assembly of domestic appliance bodies, such as washing machines. In one multihead automated cell, 58 press joints are made in a cycle time of 16 sec, (see Fig.12). In this case, both pierce-through and non pierce-through types were capable of providing the earth continuity for electrical safety. An example is aluminium to steel joined by clinching in the Land Rover Discovery rear door window aperture (Fig.13).

Joining of hot dip zinc coated steels is done with press joints or self-piercing rivets in the assembly of duct work for example, see Fig.10. Weld quality problems are avoided and there are no fume emissions.

Self-piercing rivets have taken over from blind riveting systems in the assembly of aluminium alloy road signs (sheet to extrusion). The rivet is set flush on the face side to eliminate marking through the reflective film which is applied after assembly (Fig. 8).

Numerous automotive applications have been adopted using both systems on sub-components or hang-on parts. A range of brackets and attachments are joined in this way, as are boot and bonnet reinforcements, window regulator assemblies, spoilers, seat assemblies and a variety of other non-structural parts. Coated steels, aluminium alloys and dissimilar combinations are included.

Quality Assurance of self-piercing riveted and clinch joints

The use of rivets provides a reasonable visual indication of joint quality. This can be confirmed by dimensional checks on the formed button and is sufficient for quality control in many cases. Furthermore, tight quality control during rivet manufacture is advocated to ensure a consistent product and reproducible riveting performance. Monitoring of the setting force against rivet displacement has been shown to identify correct setting of the rivet. This can identify unacceptable variation in material thickness and the data may be used for statistical process control (SPC) purposes.

The quality of clinched joints may also be confirmed by dimensional checks. In this case, the thickness of the base of the button can indicate the degree of sheet compression and hence the degree of interlock between the sheets. Alternatively, when the sprung tooling type is used, measurement of the button width or diameter indicates the lateral expansion beneath the sheets. Systems are available for in-process monitoring of force against displacement during clinching, as a means of in-process quality control. The force/displacement profile is similar to that measured in self-piercing riveting and deviations from the profile measured for good quality joints may indicate problems. Failure to meet preset limits on the profile could indicate a problem with the material or parameters, or tool damage.

Summary

Both self-piercing riveting and clinching are rapidly expanding to provide alternatives to spot welding and other fastening systems. The techniques are opening up opportunities in the high volume assembly of difficult or alternative material combinations, particularly pre-painted materials. A range of low to medium duty applications is already established and the techniques are poised to take a larger share of more demanding applications, particularly in the automotive industry.

References