Module 5 Design for Reliability and Quality. IIT, Bombay

Transcription

1 Module 5 Design for Reliability and Quality

2 Lecture 2 Design for Quality

3 Instructional Objectives By the end of this lecture, the students are expected to learn how to define quality, the importance of design for quality, and various methods that are followed to achieve the same. Defining Quality According to Joseph Juran, the term quality of a part (or product or component) should refer to the product features that meet customers needs and satisfaction, and to avoidance from deficiencies that would minimize the chance of failure of the part. David Garvin in 1987 also defined quality in eight basic dimensions for a manufactured part which is outlined in Table [2]. Table Severity and corresponding ranks of failures Dimensions Performance Features Reliability Durability Conformance Serviceability Aesthetics Perceived Quality Description Does the product perform to its standards? Does the product provide the intended service? What additional benefits may be added to the product? Will there be any tangible or non-tangible benefit? Is the product consistent? Will it perform well over its lifetime and perform consistently? How durable is the product? Will it last with daily use? Does your product meet with any agreed internal and national specifications? Is the product easy to service? Is the product appealing to the eye? What sort of quality perception does the marketing team want to convey in the marketing message? Will price charged reflect the quality of the product?

4 Importance of Design for Quality Design is more responsible for the quality than anything else. The designers determine the number of component in a specific part/product, decide which are to be procured externally, design the rest of the components and specify indirectly how they can be manufactured, determine how the parts must be assembled, and specify the overall function of the components in the final assembled part. In other words, the designers largely influence the entire procurement, manufacturing and assembly cycles of any small part or large component. Although manufacturing processes are often linked to the final quality of a part, both design and manufacturing are responsible for the final quality inherited by a part or component. If the quality is envisaged appropriately in the design procedure, the quality in manufacturing can also be ensured at lesser expenses and the cost of inspection reduces significantly. This leads to the concept of Design for Quality. Benefits of Design for Quality (DFQ) (1) The DFQ process allows the engineer to identify, plan for and manage factors that impact the robustness and reliability of the products in the design process. (2) DFQ reduces or eliminates the cost of quality that can be envisaged as the cost incurred in the inspection and rework, in the procurement of replacement materials. Appropriate DFQ procedure can also avoid defects and errors, scrap, degradation of factory/machine capacity, re-qualifications/re-certifications expenses, and overhead demands (3) Improved and consistent quality of parts provide better appeal to the customers that obviously lead to greater stability of the manufacturing shops and can create greater amount of opportunities. Figure schematically outlines the Deming s Chain Reaction depicting salient features of design for quality. In particular, the various factors that affect and in turn, get influenced by design for quality are clearly indicated in Figure

5 Strategy to implement Design for Quality Following are a few frequently used techniques that are used to ensure design for quality. Understand past quality issues The root causes of any past quality issue should be realized and resolved thoroughly with the help of a multi-functional team having representatives from all the departments. Such a team would brainstorm solutions not only to resolve the previous quality issues but would also come up with new possible design ideas that might improve the quality further. Designing the product rightly in the first time It is always suggested to take utmost care so as to design the product right from the first time. Further, standard manufacturing techniques must be followed so that the quality in manufacturing can also be obtained from the first time itself. If quality is not assured by the initial design, then expensive changes would be required in later stages of the product cycle wasting valuable engineering resources and time. Figure Schematic outline of Deming s chain reaction with respect to design for quality

6 Simplify the Design Simplify the design such that the product can be built from the smallest number of parts. Minimize Cumulative effect of Part Quality and Quantity The quality of a product or component can be approximated by the average quality level of the parts following the expression as: Q n+ 1 p (Qa ) = (1) where Q p refers to the quality level of the product, Q a is the average quality level of parts and n is the number of parts in the product. Equation (1) states that the quality of the product (the firstpass accept rate) will be equal to the quality level of the parts to the exponent of the number of parts assuming perfect manufacturing processing. Therefore, high-quality parts and simplified design which give fewer parts would help to attain higher quality product. This is also known as minimizing the exponential cumulative effect of part quality and quantity. For example, a product consisting of 17 parts with an average quality level (Q a ) of 98% would lead to a product quality level (Q p ) of (0.98) In other words, only 70% of the products will be good with an average quality level of the parts as 98%. This assumes perfect factory quality. Other unforeseen factory quality problems will lower the level of product quality even further. Select Parts for Quality Too often parts are selected for functionality and cost. However to ensure quality by design, parts must also be selected for quality. Optimize processing Be sure that the manufacturing process selected is robust enough and can produce high quality products in production quantities. Also automating the process can be a good option. Automated production lines often help to produce better and more consistent quality parts / products than manual production. Reuse Proven Design and Part Use proven standard parts and design features that have been used successfully before and would be most likely to minimize risk and assure quality. A key goal for the designer should be to use proven design, parts and modules.

7 Document thoroughly and completely In the rush to develop products, many designers fail to document every aspect of the design thoroughly. Drawings, manufacturing instructions, and bills-of-material sent to the manufacturing or vendors need to convey the design unambiguously for manufacture, tooling, and inspection. Imprecise drawings invite misunderstandings and interpretation, which add cost, waste time, and may compromise quality. Centralize the most current data with good product data management. Implement incentives that reward quality Many a times there are a lot of incentives for achieving the production deadline. Similar incentives should be defined to meet quality standards as well. Utilize Quality Function Deployment Quality function deployment (QFD) can be used define products to capture the voice of the customer the first time without the cost and risk of changing the design. QFD is a tool for systematically translating the requirements of the customer into product design specifications and resource prioritization. Its strength is to translate the objective and the subjective wants of the customer into objective specifications that engineers can use to design products. The basic structure is a table with "Whats" as the labels on the left and "Hows" across the top. The roof of the table is a diagonal matrix of "Hows vs. Hows" and the body of the table is a matrix of "Whats vs. Hows". Both of these matrices are filled with indicators of whether the interaction of the specific item is a strong positive, a strong negative, or somewhere in between. Additional annexes on the right side and bottom hold the "Whys" (market research, etc.) and the "How Much". The rankings based on the Whys and the correlations are used to calculate priorities for the Hows. Figure depicts a typical structure of a QFD table.

8 Hows vs. Hows Hows Whats vs. Hows Whats Why How much Figure Schematic presentation of quality function deployment (QFD) house / table Figure provides a physical insight how the QFD table / house can be prepared for a new part. Figure depicts a complete QFD table / house for a new part.

9 Figure Basis of the development of quality function deployment (QFD) house / table

10 Figure Typical quality function deployment (QFD) house / table for a part

11 Utilize Poka-Yoke Poka-yoke is a Japanese term that means mistake-proofing. The Poka-Yoke principles to product design are meant to prevent mistakes by design in addition to the traditional manufacturing techniques or to prevent incorrect assembly or fabrication. The Poka-yoke principles ensure that proper conditions exist before actually executing a process step, preventing defects from occurring in the first place. It refers to techniques that can identify and keep away defects out of products and processes and, substantially improve quality and reliability. It can be thought of as an extension of FMEA. The step-by-step process in applying poka-yoke can be envisaged as Understand the design Analyze and understand the ways a product can fail. Decide the right poka-yoke approach, such as using a o shut out type (preventing an error being made by modifying the design), or o an attention type (highlighting that an error has been made by adding more features to the design) Do appropriate modifications in the design to incorporate the above approach Trial the method and see if it works Finalize the design and proceed ahead Figure depicts a typical example of applying poka-yoke principle. Figure Application of Poka-Yoke principles

12 Proactively minimizing all types of risk Proactively minimize all types of risk, not just functionality. Use Failure Modes Effects Analysis (FMEA), which is one of the techniques used to identify and analyze failure and actions that need to be taken to reduce their occurrence. Optimize tolerances Optimize tolerances for a robust design using Taguchi Methods to ensure the high quality by design. This is a systematic way to optimize tolerances to achieve high quality at low cost, which is often achieved by using the principles of Design of Experiments to analyze the effect of all tolerances on functionality, quality, and manufacturability. The procedure can identify critical dimensions that need tight tolerances and precision parts, which can then be taken care of appropriately. The unique strength of this approach is that it can minimize cost while assuring high quality by identifying low demand dimensions that can have looser tolerances and cheaper parts. Such a design would be considered robust so that it could be manufactured predictably with consistently high quality and perform adequately in all anticipated usage environments. Without a methodical way to determine tolerances, the alternatives would be either to make all tolerances tight which is expensive or inadvertently (or deliberately) make tolerances too loose, leading to manufacturability and quality problems.

13 Exercise Develop a QFD table for a pen. Reference [1] David M. Anderson and David M. Anderson, Design for Manufacturability and Concurrent Engineering, CIM Press, [2] G Dieter, Engineering Design - A Materials and Processing Approach, McGraw Hill, NY, [3] [4] [5] [6]