Cost calculation of defects in relation to the design

The design evaluation tool uses process capability data to establish the robustness of the design, using process capability indices, discussed previously. It has been discussed that there is a need for manufacturing engineers to have more tangible data to measure the manufacturability to the customer requirements. With this in mind the design evaluation tool develops by introducing costs, translating the complex CP & CPK values into more tangible supportive data to aid the direction of the design ensuring the customer requirements and predetermined costs can be met prior to manufacture and detail design stages commencing.

The design evaluation tool looks at two main areas of cost; first the concession and non conformance and secondly the repair cost to bring the product back to within the designed or concession design standard.

Concession cost is the related engineering cost in bringing the defected product back to an acceptable standard that can ensure its use. In many manufacturing environments such as automobile manufacture most of the defective products will be scrapped as the engineering costs to rework the product would exceed the cost of the product itself, even with larger products such as cylinder heads etc. However, in airframe manufacture, many components have a high monetary value e.g. the main structural frames in an aircraft structure may cost in excess of £100,000. It is not only the physical costs of products but the traceability paperwork also has a large contribution to the overall cost. Major products on an aircraft have to maintain traceability, where all the data such as inspection, material batch numbers, protective treatments etc. have to be tagged to the product and maintained for the life of the aircraft.

Deciding how to repair a defective product involves various engineering disciplines. Design engineering need to model the defective component, looking how it will affect the design and the overall assembly. Stress engineers need to recalculate and assess the loads that will be affected by the defects on the product. Assembly manufacturing engineers need to assess the potential issues which may occur on the assembly build, due to the defective products. There are other engineering disciplines that may need to be involved in the assessment. On completion of the concession, the quality team need to establish corrective action plans to ensure that the defect does not reoccur.

The design evaluation tool estimates the potential of a concession, therefore allocating this to the overall defects cost. As highlighted previously, concession costs can be a large part of the overall defect cost. Defects per unit (DPU) are used to calculate the probability of a concession, as illustrated in figure 5.17. A sensitivity marker is set to control the probability of the concession:

The DPU of the surface profile for metallic ribs is 0.618, there will always be a DPU value for the key features. It is therefore up to the organisation to set the level of the sensitivity value, e.g. a sensitivity value of 0.5 would include the concession cost in the overall defect cost; however a sensitivity of 0.7 would not include this in the overall defect cost. An organisation can establish this value by reviewing the past concession rates in their business.

The second source of defect cost is the cost to rework the product to ensure the incorrect feature is acceptable to the design requirements enabling the product to be used on the aircraft. To calculate these costs the design evaluation tool calculates the probability of the defect and whether the probability of the defect exceeds the USL or the LSL, allocating a cost to the appropriate defect area. To explain this area more clearly an example of hole diameter can be used:

The design specification for a hole in a component is a diameter tolerance of 6mm + 0.1mm / - 0.000mm. In terms of a defect the hole could be under the design specification; say 5.8mm, or over the design specification; say 6.2mm. These two potential scenarios would create very different issues to the performance of the product. The under size hole would produce an issue on assembly that the fastener could not be installed. If the hole were to be oversized then there would be a loose fit fastener in the structure, this could reduce the amount of load that would be carried through the joint, e.g. there would have to be some deformation before the fastener

would carry load and fretting of the fastener would also be a potential issue. Taking these into consideration the hole would have to be reworked back to the designed tolerance and there would be different rework processes for the two scenarios.

The undersize hole rework would be to open up the hole with a tool to achieve the design standard, this would introduce little cost to the product. If the hole was oversize the hole may have to be opened up to a larger size and a bush or sleeve fitted to ensure the designed tolerance is achieved, this would substantially increase the cost to the product than the previous rework process.

Taking this into consideration it would not be acceptable for the design evaluation tool to associate a single cost to a defect. This would give a misrepresentation of the actual rework costs. With this in mind, the design evaluation tool calculates the highest probability of the defect, whether it is exceeding the USL or the LSL. Figure 5.18 illustrates this;

Figure 5.18 – Selecting defect costs for a hole diameter

Using the Pz values from key feature hole diameter for metallic ribs, as illustrated in figure 5.18, the Pz values indicate that there is a higher probability that

µ 6.1mm 6mm Designed Upper spec Limit Designed Lower spec Limit Defects below 6mm Pz lower = 25.93% Defects above 6.1mm Pz upper = 18.04%

manufacturing a hole with the current process capability in place the hole with be undersize. Therefore, the design evaluation tool will allocate a cost to open the hole diameter to within design limits.

The cost is calculated by the amount of time it takes to rework the feature. In the above example the time to repair for over sized hole would be the time it takes to open up the hole (0.75 hrs), Counter bore for bush (0.250 hrs), fit the bush (0.250hrs), there will be an additional time of (0.5hrs) for manufacture of bush therefore the total time being 1.5 hours. For an undersize hole, the hole would require opening up to within the design limits, the time allocated to this is 0.333 hrs, and these hours are to be multiplied by the appropriate hourly rate in pounds for the specific process. Figure 5.18 shows the Pz value for an undersize hole (25.93%) and an oversize hole (18.04%). Therefore to ensure that the design evaluation tool predicts an accurate rework cost it has been programmed to select the highest Pz value to apply the rework cost, in the above example it would be 0.333 x £50 = £16.65. Each key feature has a time associated to the repair of a defect outside the USL and LSL. This is shown in appendix 2.

The author has identified a “grey area” as to whether a cost should be allocated in the design evaluation tool. This area has been addressed by the introduction sensitivity makers to control the probability of concessions, rework costs and main contributors; these have been highlighted in figures 5.8 and 5.9. The author has discussed this in more detail in Section 5.5.5.

5.6 Conclusion

In summary of the present chapter the author has presented a number of solutions to the gaps identified in the current academic work, identified in chapter 4. The author

discusses the importance of the selection of the environment of which the design life- cycle operates in. A novel selection tool is discussed, after analysis from senior industrial experts from the aircraft industry it was concluded that for high complexity low volume products, such as aircraft, a concurrent environment would be the most effective.

The author introduces a novel design life-cycle that enables a structured approach to the design of high complex products such as aircraft. This structured approach helps control the large amount of technical data that is required in this environment. The core of the process flow is based on seven phases; requirements definition, conceptual design, embodiment design, detail design, production planning, manufacturing, and maintenance and support. These distinct steps help structure the maturing design. To control the complexity of the data mass, three main drivers are introduced; requirements definition, review gates and a decision maker. After these phases and drivers had been developed by the author, it became apparent that is was difficult for manufacturing engineers to introduce tangible data at the review gates. This derived the main area of the research by presenting a solution to this problem.

The literature survey highlighted the need for manufacturing engineering to have the ability to assess the design for manufacturability. The author introduces a solution to this gap in the existing academic work by the introduction of a design evaluation model. The key characteristics are selected from the design and using manufacturing process capability data, they are mathematically analysed to predict the additional costs that will be incurred by the business if the design moves into manufacture. Using the proposed design life-cycle, each discipline has to pass a review gate to enable a reduction of risk through the design life-cycle. Engineering disciplines like weight engineering and structure analysis engineering can offer this

information, however manufacturing engineering has not been able to introduce tangible data in the same way as our disciplines. This chapter offers a solution to this by using the model to develop a design evaluation tool measuring the manufacturability of the design during the design phase. The outputs from the tool aid the decision maker to decide whether to allow the design to pass through the review gates by validating the products manufacturability to the customer requirements. This tool introduced by the author will also enable a comparison of the design from various engineers, highlighting how well the designer has thought about the manufacturability and cost of the product. The tool will assist in establishing best practice through the design life-cycle.