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Process Capable Tolerancing® (PCT)

Don Day, Tec-Ease, Inc, USA

Martin Raines and Ken Swift CapraTechnology Ltd, UK

Quality costs often consume some 25% of total revenues in manufacturing business. Even the quality leaders face intimidating quality losses. The vast majority of quality costs are failure costs that include rework, scrap, warranty, product liability claims and recall costs. In general, the cost of failure is the difference between the actual production costs and what it would cost if there were no failures. This represents lost profit. Engineering change to address such issues is invariably difficult, protracted and costly. Ref. (1) and (2)

Just look at any engineering drawing revision column and it will become clear that most designs are not right the first time. Each revision is a rework of the design and many times the words REDRAWN AND REVISED appear at the top of the column. This means the previous drawing revision column had been filled and in order to provide room for subsequent revisions a new drawing was issued. There is tremendous pressure to get the drawings released, and worry about fixing them later. Most companies wait until they find a problem and then fix it. This is the "if it ain't broke-don't fix it" attitude. Later in the development program when fixes are expensive, we troubleshoot the problems and are then proud of our troubleshooting skills. Clearly, a better approach is to avoid the problems in the first place. Problems must be avoided by recognizing issues early in the design phase. Many tools are available to the designer - DFA, DFM, DOE, etc. but these tools are often too cumbersome or time consuming to use. Several attempts have been made through the Six Sigma effort to compare tolerances to process variation based on historical data. There are many tolerance analysis packages that facilitate this activity. The problem is that these packages often are not user friendly and require an internal expert or outside consultant. What is needed is a fast, simple way for designers and engineers to determine whether or not the tolerances they apply are reasonable and producible.

The answer is found in Process Capable Tolerancing (PCT) - a methodology for designing components and products that are robust to process variation. The methodology involves setting process capability targets, predicting process capability levels and minimizing quality failure costs. A key element in all this is the process capability index - Cpk.

In most organizations designers and design engineers have not been involved with process control activities. As a result, the Cp and Cpk indices are not well understood in the 'design engineering' arena. Certainly, designers do not need to have as thorough an understanding of these indices as quality and process engineers. At the risk of offending the statisticians who might read this article, the following is a simplified explanation of Cp and Cpk aimed at illustrating the way an understanding of these terms is crucial to process capable tolerancing.

Predicting Cpk when tolerances are allocated allows the designer to greatly reduce:

The tolerancing of parts is typically a two-step process. First, tolerances are allocated to the features on the parts. Second, when there is time, these tolerances are analyzed to assure that design requirements are being met. Typically, the allocation of tolerances is based upon:

Sadly, none of these methods relates to the quality initiatives being driven in world-class quality companies today. All too often tight tolerances have caused problems in manufacturing that design is unaware of. Tolerances found in handbooks are frequently based on 3s tolerancing and have not been revised to reflect today's quality demands.

The predicted cost of a product is determined, to a large degree, by the processes and materials used to produce the parts. These materials and processes should determine the tolerances allocated. Designers need a way to instantly know if the tolerances they assign are reasonable for the processes and materials to be used.

Due to the length of the article we have created individual pages for each section. Please follow the links below to these sections or, click here for a PDF of the entire article.

References

  1. Dale, B. G. (1994) (Editor), Managing Quality, 2nd Edition, Prentice Hall, NY.
  2. Leaney, P. G. (1996) Design for Dimensional Control. Published in Design for X - Concurrent Engineering Imperatives. Huang, G. Q., (Editor) Chapman Hall, London.
  3. CapraTechnology Limited, Birch Lea, Walkington, East Yorks, UK.
  4. Batchelor, R. & Swift, K. G. (1996) Conformability Analysis in Support of Design for Quality, Proc. Instn. Mech. Eng., Part B, 210, 37-47.
  5. Booker, J. D. Raines, M. & Swift, K. G. (2001) Designing Capable and Reliable Products, Butterworth Heinemann, London.
  6. Batchelor, R., Raines, M. & Swift, K. G. Tolerance Capability Analysis, 4th Int. Conf. On Quality, Reliability and Maintenance, QRM 2002

(Edited by G. J. McNulty), Professional Engineering Publishing, 307 - 310.