Written Muqi Wulan.
The material provides further information on design for assembly (DFA) at Level 2, and the contents focus on design for two assembly methods other than manual assembly.
• Design for high-speed automatic assembly.
• Design for robot assembly.
Design for assembly (DFA) means the design of the product for the ease of assembly (Boothroyd, Dewhurst & Knight 2011: 1). As described before, the implementation of DFA depends on the selection of assembly methods. Then for each assembly method, design guidelines are developed in the form of simple design rules for designers to follow and consolidate manufacturing knowledge into a design. Based on the design guidelines for manual assembly, the guidelines for the other two assembly methods will be presented as follows.
Design guidelines for high-speed automatic assembly
Compared to manual assembly, the automatic assembly requires more careful considerations on product structure and component part design. The simple example shown in Figure 1 illustrates significant problems in automatically assembling a slightly asymmetrical threaded stud since it is much harder for automatic handling to recognise its orientation. The savings resulting from product redesign often outweigh those from automation. On the other hand, the operation of orientation, handling and feeding of parts on assembly machines has a considerable effect on the assembly cost (Boothroyd, Dewhurst & Knight 2011: 185) (El Wakil 1998: 438).
Figure 1 – Design change to simplify automatic feeding and orienting
Table 1 and Table 2 summarise the design rules for high-speed automatic assembly in product design and part design.
Table 1 – Design rules of product design for high-speed automatic assembly
Table 2 – Design rules of part design for high-speed automatic assembly
Design guidelines for robot assembly
In robot assembly, the production volume is higher than that of a manual assembly system but lower than that of an automatic assembly system that incorporates special-purpose machines. Therefore it fills a gap in production volume between the other two assembly systems (El Wakil 1998: 434). Three representative types of robot assembly can be chosen (Boothroyd, Dewhurst & Knight 2011: 206):
• Single-station with one robot arm.
• Single-station with two robot arms.
• Multistation with robots, special-purpose workheads, and manual assembly stations appropriate.
Many rules of product design for manual assembly and high-speed automatic assembly can apply to product design for robot assembly. However there are two crucial considerations that have to be taken into account when designing a product or parts for robot assembly (El Wakil 1998: 445).
• Design a part so that it can be grasped, oriented, and inserted by the robot’s end gripper. Failure to do it will result in the need for an additional robot and high assembly cost.
• Design a part so that it can be presented to the robot’s arm in an orientation appropriate for grasping. Also, eliminate the need for reorienting assemblies (or subassemblies) during the assembly operation.
Summary
Design for assembly (DFA) is a method on improving the product design for the easy and low-cost assembly. It is an assembly-conscious design approach. The first step toward a rational design for assembly is the selection of the most appropriate method for assembling the product under consideration. Although there are a lot of common rules shared with design for manual assembly, the design guidelines for high-speed automatic assembly and robot assembly have their own characteristic considerations in product design.
Selected References
Boothroyd, G. (1996) ‘Design for manufacture and assembly: the Boothroyd-Dewhurst experience’. in Design for X: concurrent engineering imperatives. ed. by Huang, G. Q. London: Chapman & Hall
Boothroyd, G., Dewhurst, P., Knight, W. A. (2011) Product design for manufacture and assembly. 3rd edn. Boca Raton: CRC Press, Taylor & Francis Group
El Wakil, S. D. (1998) Processes and design for manufacturing. 2nd edn. Boston: PWS Publishing Company
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