17.5 Six Sigma Concept

3.Gaugeless Tooling: This is achieved using a theodolite system linked through a central processor. Coordinated geometry, obtained directly from CAD, is used to establish the “hard points” to meet the build, interface, and interchangeability requirements. Gaugeless tooling is required for the manufacture and periodic inspection of the assembly process.

4.Inline Assembly: This provides a progressive and balanced assembly build sequence, utilizing the maximum number of subassemblies in a cellular-type environment, which improves interchangeability.

5.Automatic Riveting: The assembly is first slave-riveted on the fixture and then moved to the automatic machine. This improves productivity and accuracy; hence, the quality impacts from human error are minimized. The manpower engaged is also reduced.

6.Tolerance Relaxation at the Wetted Surface [2]: Aircraft surface-smoothness requirements are aerodynamically driven with a stricter manufacturing tolerance to minimize drag – that is, the tighter the tolerance, the higher is the assembly cost. Trade-off studies between surface tolerance and aerodynamic drag rise can reduce manufacturing costs (see Section 17.6).

7.Six Sigma and Supporting Methodologies: An important framework in which DFM/A techniques should be conducted is that of concurrent engineering (i.e., IPPD) focusing on improvement of the product-development process by concentrating on the design stage for the entire life cycle of a product. Management strategies such as DFSS and LAM as well as effective personnel management also must be considered if improvements are to be made in assembly-system profitability. DFM/A should strengthen the team activity in all phases of the design process, thereby ensuring that the technical expertise of the participants is successfully utilized; this is a management tool.

Decisions made during product design have a major impact on cost, defects, and cycle time. In fact, about 70% of production cost is locked in during the design process. DFM/A helps reduce product complexity through minimization of parts and fastener counts, assembly and manufacturing time, and material costs. Additionally, DFM/A application reduces the potential for defects. Robust design, statistical tolerancing, and geometric dimensioning and tolerancing actually help reduce defects. A better understanding of DFM/A in reducing the cost of production requires detailed studies in material selection and different process capabilities, which are beyond the scope of this book. The DFM/A concept assists the Six Sigma management strategy.

17.5 Six Sigma Concept

The objective of robust design is to achieve product designs with few defects during manufacture and very few latent defects after a product is delivered to a customer. It focuses on identifying the characteristics of the product that are critical to meeting the product requirements and then seeking the DFSS process capability.

The DFSS is an integrated approach to design with the key issue of reducing the scope for mistakes and inefficiencies – that is, make a product right the first time to prevent the waste of company resources [2]. DFSS is a collection of product tools

and topics used to assist the design of products for manufacture by processes operating at the Six Sigma capability. It is a management-driven task to facilitate the improvement of labor efficiencies from employees and find new ways to improve on any routine approach so that the product can be manufactured at the highest quality and lowest cost, thereby satisfying all of a customer’s requirements. Six Sigma helps expose the “hidden factory” of waste that robs organizations of profits by using a routine approach to issues with the product and manufacturing process. The vision of Six Sigma is as follows:

reduce costs and improve margins in a context of declining prices

surpass customer expectations by a margin few competitors can match

improve at a faster rate than the competition

grow a new generation of leaders

Six Sigma is a systematic methodology for eliminating defects in products, services, and processes while also yielding cost and cycle-time reductions. By significantly improving process capability, it can achieve operational excellence in delivering almost defect-free products and services, at the lowest possible cost, and on time.

For manufactured products, the Six Sigma methodology makes use of a variety of managerial, technological, and statistical techniques to change the manufacturing processes, the product, or both in order to achieve the Six Sigma process capability. DFSS is the collection of tools and topics used during the design phase to achieve a Six Sigma product.

One measure of process capability is in the sigma, σ , a statistical measure. For example, when a process is operating at Six Sigma capability, the long-term yield is 99.99966%, corresponding to 3.4 defects per million opportunities. The demand for Six Sigma is high, thereby guaranteeing a robust design. Table 17.1 lists the sigma distribution in a statistical histogram of defect levels relative to process capability.

To gain a competitive advantage through customers’ satisfaction, their needs must be understood. One way to capture customer requirements is by using selected quality function deployment (QFD), which consists of a series of interlocking matrixes that are used to translate customer requirements into product functional requirements and process characteristics.

However, development and implementation of DFSS is difficult. It requires employee behavior characteristics such as leadership, commitment, professionalism, and perseverance to overcome the attitudes heard in phrases such as “no time,” “not invented here,” “doesn’t apply to us,” “we’ve been doing it for years,” “I prefer