231 Design for assembly techniques

Early work looking at designing products for mechanized assembly started over 30 years ago (Boothroyd and Redford, 1968). Large cost savings were found to be made by careful consideration of the design of the product and its individual component parts for ease of assembly. Commercial DFA techniques are now used successfully by many companies in either workbook or software versions. The three most referred to methods are:

• Boothroyd-Dewhurst Design for Manufacture and Assembly (DFMA) (Boothroyd et al., 1994)

• Computer Sciences Corporation's (CSC) DFA/Manufacturing Analysis (MA) (CSC Manufacturing, 1995)

• Hitachi's Assembly Evaluation Method (AEM) (Shimada et al., 1992).

In fact, the use of DFA techniques is now mandatory in some companies, such as Ford and LucasVarity (now TRW) (Miles and Swift, 1998). These techniques offer the opportunity for a number of benefits, including:

• Reduced part count

• Systematic component costing and process selection

• Lower component and assembly costs

• Standardized components, assembly sequence and methods across product 'families' leading to improved reproducibility

• Faster product development and reduced time to market

• Lower level of engineering changes, modifications and concessions

• Fewer parts means: improved reliability, fewer stock costs, fewer invoices from fewer suppliers and possibly fewer quality problems.

A team-based application and systematic approach is essential as there are many subjective processes embedded, but many companies have found them to be pivotal techniques in designing cost-effective and competitive products (Miles and Swift, 1998).

An overview of each of the main commercial methods can be found in Huang (1996), but in general, a number of design for assembly guidelines can be highlighted (Leaney, 1996b):

• Reduce part count and types

• Modularize the design

• Strive to eliminate adjustments (especially blind adjustments)

• Design parts for ease of handling (from bulk)

• Design parts to be self-aligning and self-locating

• Ensure adequate access and unrestricted vision

• Design parts that cannot be installed incorrectly

• Use efficient fastening or fixing techniques

• Minimize handling and reorientations

• Maximize part symmetry

• Use good detail design for assembly

DFA minimizes part count by, for example, consolidating a number of features found on two components in a single component, but it cannot indicate the process capability for the design proposed. DFA techniques can only indicate that it may be costly to assemble the component or that the cost of the component may be relatively high, but still more cost effective than the two components it replaces. Production costs should not be the only measure of performance with which to select designs. The analysis should extend to the potential variability associated with the design when in production.

There is a strong correlation between assembly efficiency and reported defect levels for a number of Motorola products evaluated using the Boothroyd-Dewhurst DFMA technique (Branan, 1991). From the results of the study, it is claimed that DFA cuts assembly defects by 80% and, therefore, has a direct influence on manufacturing quality. This may be due to the fact that more efficient design solutions have relatively fewer component parts which naturally infers fewer quality problems. However, early life failures, which are caused by latent defects, are not necessarily highlighted by DFA techniques.

Quality or robustness is seen as a natural outcome of the product when effectively using DFA techniques, and is commonly listed as a potential benefit, but it is not evaluated explicitly by any of the commercial techniques. Too often, assembly is overlooked when assessing the robustness of a design and DFA techniques do not specifically address variability within the assembly processes. Undoubtedly, the better the assemblability, the better the product quality in terms of fewer parts and simpler assembly operations. Fewer parts lead to fewer breakdowns, fewer workstations, less time to assemble and fewer overheads. Simpler assembly operations imply that the product fits together more easily, leading to shorter lead times and less rework (Leaney, 1996b).

From the above arguments, it has been recognized that there is a need to predict variation at the assembly level. To facilitate an assembly analysis in the first instance, it is essential to understand the structure of the proposed product design and the assembly sequence diagram is useful in this respect.

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