The sheet metal assembly process is a complex series of operations that differs from the traditional rigid body assembly processes. The compliant nature of the components leads to assembly process interactions influencing the final assembly output. These interactions make the assembly process difficult to model, sensitive to process variables and problems, and make faults more difficult to diagnose. Given these complexities, a wide variety of modelling and analysis approaches have been used to understand the sheet metal assembly process. Previous research has applied control theory, pattern recognition and artificial intelligence algorithms to investigate these problems, and to attempt to reduce variations in production.
An automobile body, otherwise known as a Body In White (BIW), is constructed from hundreds of individual components that are assembled together. To achieve the end result, a large number of assembly operations are performed that build smaller assemblies, or sub-assemblies, before they are combined to form larger assemblies, and eventually the complete BIW. This process may be repeated many times, where smaller sub-assemblies have other components added on; building on the assembly.
The most basic element of this larger chain of processes is a single assembly process. Chang and Gossard (1997) represented the assembly of sheet metal components by a four stage process, the Place, Clamp, Fasten andRelease (PCFR) cycle (Figure 2.2). Components are placed on an assembly station, aligned using control pins and rests on control surfaces. The clamps are then applied, pushing the
§2.2 The Sheet Metal Assembly Process 15
components onto the control surfaces. The fastening procedure is then performed; this is commonly performed by spot welding, although self piercing rivets are also used. Finally, the components are un-clamped from the assembly station.
Figure 2.2: The Place, Clamp, Fasten and Release cycle (Chang and Gossard, 1997)
To locate a component onto the assembly station, control strategies are used to restrain the component’s available degrees of freedom. Translations and rotations about each axis for a rigid component are restricted by a 3-2-1 locating system (Figure 2.3). The locating strategy defines the number of points of contact in each plane, where three points of contact are used in the primary datum plane, followed by two in the secondary and one in the tertiary (Krulikowski, 2007). For compliant bodies, Caiet al.(1996) showed that a N-2-1 locating strategy is effective in reducing dimensional variability from external loading during the assembly of compliant bodies. In this strategy, additional locating surfaces are used in the primary plane to constrain the flexibility of the component and bring it to the desired position. The secondary and tertiary planes are typically constrained using a locating pin with a hole and a slot respectively.
Figure 2.3: The 3-2-1 locating strategy. Clamps and pins are indicated byCi and Pi
respectively. In this example, the pins also coincide with the location of two of the locating surfaces. (Ceglarek and Shi, 1999)
The combination of control surfaces and pin tooling elements used to support a workpiece are assembled into a supporting framework. These supporting frameworks will be referred to as assembly fixtures. These assembly fixtures form part of the assembly station, which also consists of the joining equipment and appropriate safety equipment. The clamps that are used to hold the components onto the control surfaces are most commonly pneumatically operated to aid the automation of the assembly process.
The joining operation is then performed. The most common type of join is a Resistive Spot Weld (RSW); however, self-piercing rivets are growing in use, particularly when joining materials which are dissimilar or hard to join. For resistive spot welding the methods of tip actuation and control can also vary. The most common types are the positional and compensating tip control methods, although servomechanism controlled tips are becoming increasingly popular. For positional tip control, one weld tip is moved into position before the other is clamped down onto it. Compensating weld tips close simultaneously and utilise pneumatics as a force balance. Servomechanism controlled tips utilise force sensors that can control the force exerted by the tips to control the position and clamping force. The primary benefit of a servomechanism is the improved control of the force exerted during welding, which is critical to the integrity of a RSW (Zhang et al.,
§2.2 The Sheet Metal Assembly Process 17
2000).
Measurement of the assembly can be described by a similar technique, known as the Place, Clamp, Measure andRelease (PCMR) (Ceglarek and Shi, 1999) cycle. In this cycle the components undergo a similar process, but are measured rather than joined. In both the PCFR and PCMR process cycles, significant interactions and deformations can occur to the component.
Due to the complexity of the sheet metal assembly process, any deviations of the component from their target, or nominal, values become difficult to incorporate into a tolerance analysis method. This affects the entire design-to-manufacture process, and, as such, identifying the source of any variability within these deviations is a critical concern. This can be visualised graphically by considering the potential sources of variation using a fish-bone diagram, as shown in Figure 2.4. Due to the need to understand the causes of variations in the assembly process, early work in the area of sheet metal assembly focused on root cause diagnosis.
Figure 2.4: Fish-bone diagram illustrating potential root causes of variation that feed into the assembly process (Huet al., 2001)1
1Note that PCWR is used here to represent the PCFR cycle, where W refers to weld instead of F for fasten.