In order to improve engineering product designs and reduce the number of problems that occur during a new product launch, firms have focused on integrating downstream product development processes into the design phases. Unfortunately, this resource integration to solve problems has been less common in the back-end of product development, particularly for complex products involving many components such as an automotive body. Here, manufacturing firms use sequential validation procedures first to approve components, then subassemblies, and finally the end product. One trend has been to tighten component tolerances in efforts to avoid or minimize downstream assembly problems. These stricter component requirements, however, often result in timing delays and cost overruns due to unnecessary rework of components. For complex-assemblies, the use of "flexible criteria" and an approach called "functional build" can significantly reduce validation time and costs yet still meet end product quality objectives. This paper examines this functional build approach to manufacturing validation and demonstrates its effectiveness with an automotive case example.
AND CONCL USIONSThe traditional, sequential process-validation approach used by manufacturers for process development has shortcomings when applied to certain complex products such the automotive body. The common sequential approach starts by validating individual components, then small subassemblies, ultimately leading up to the finished body. This approach assumes that the quality of each higher level assembly is predicated on the quality of incoming, lower-level components. Validation at each step usually is measured by quality indices such as Cp and Cpk. This sequential approach has proven non-competitive for car bodies, often resulting in missed development schedules and unnecessarily high costs for process rework. Two unique attributes of sheet metal stamping and assembly processes inhibiting the sequential approach are the inability to produce component dimensions precisely at their nominal specification, and the weak correlation in dimensions between lower-level components and their assembled counterparts. Manufacturers attribute the deviations from nominal specification to difficulties predicting metal flow during forming operations, as well as the measurement process itself. In addition, no stamping diemaker has shown an ability to significantly shift the majority of part dimensions on a complex part closer to nominal on a consistent basis, even after several die-rework iterations. Manufacturers using Cpk buyoff indices, ultimately have to expand tolerances to meet the required threshold to accommodate for these mean deviations. The lack of correlation between component dimensions and first-level sub-assemblies suggests that some of this die rework is nonvalue added. These industry-wide problems have led several manufacturers to adopt a more integrated process validation approach called functional build.The functional build approach to process development focuses on the customer perceived quality of the final car body when evaluating the need for process changes. This approach shifts the development focus from optimizing individual components to the whole car body, and integrates product, process and manufacturing. Necessary changes are identified based on lowest-cost solutions. These solutions might involve modifications to a product design, stamping die, or an assembly process. With functional build, manufacturers treat original design specifications as targets rather than absolute requirements, and they generally do not use Cpk as the primary acceptance or rejection criterion. Automobile companies implement functional build using a process called a "screw-body'' to attach mating component parts. These parts are screwed (or riveted) in order to isolate the influence of the assembly process. One drawback of functional build is the subjective nature in which decisions are made, and research is needed to help quantify decisionmaking. Since the original specifications are target dimensions, deviations from specification could be partitioned into three regions: obvious die rework changes (large deviations)...
This paper introduces three auto body build concepts: net build (NB), functional build (FB) and integrated build (IB). Most manufacturers design and validate their vehicle launches using the NB concept. This concept is shown to be expensive, time consuming and not effective value added. Functional build has found growing acceptance as being preferable over NB. Functional build has tended to find the greatest acceptance as a die buyoff procedure for individual parts based on subassembly quality. Aspects of FB are shown in a case study where the manufacturer claimed to use FB for die development, but critical elements were omitted and opportunities lost. The integrated build (IB) process is an extension of the FB approach. It is a more inclusive process that considers the entire vehicle and requires closer coordination with product design. Aspects of IB are also shown in two case studies. In the first, IB concepts were included in the design process, which enabled the company to implement an effective material introduction strategy. The second is a comparison of the product development timelines of two companies and shows how IB concepts can shorten lead and production ramp time.
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