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Methodology for Process Capability Tolerancing |
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Excerpt of Process
Capable Tolerancing (PCT) article written by Don Day,
Tec-Ease Inc, USA, Martin Raines and Ken Swift,
CapraTechnology Ltd, UK
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The Process Capable Tolerancing methodology (PCT) requires that the designer or design engineer verify that each assigned tolerance be process capable. PCT is the one activity which links design, manufacturing and quality. This activity must be a part of any company's "phases and gates" product development process if they are to succeed.
For years quality and manufacturing areas have been gathering massive amounts of process data. This data is used to generate control charts but is seldom shared with design. Consequently, companies acquire vast amounts of data without ever acquiring any real knowledge.
The problem is compounded when suppliers are involved. Usually suppliers will tell you how good they are. Rarely will they tell you how good they are how often. Early in a program, suppliers will submit sample parts, but these parts are rarely representative of the parts you will receive in production. Design needs a tool that will tell them what to expect once the product is in full production, therefore tolerances must be determined based on real data. It is possible for a company to create its own database provided there is an understanding of how to model the data in a fashion usable to the designer. However, this is a very difficult, time consuming and costly task. Alternatively, there is now software available with a database of knowledge covering most processes used by most manufacturing companies. Ref. (3)
The PCT methodology is illustrated in Fig. 8. It is shown against the background of the product introduction process, illustrating phases of operation. The process involves setting process capability targets for the design characteristics in question, assessing process capability or allocating process capable tolerances based on process variation and assessing the effects of design geometry and material on tolerance capability.
The main objective of the methodology is the assignment of process capable tolerances to component design characteristics. Other objectives include assessing the acceptability of a design characteristic against its likely failure severity and specifying appropriate process capability (Cpk) targets for internal manufacture and external supply.
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Fig. 8. Process Capable Tolerancing (PCT) methodology
It has been assumed that for each process there is a fundamental level of inherent variability associated with processing the 'ideal' design under conditions of good practice
in manufacturing
operations1. Therefore, central to the determination of Cpk are the
process capability maps2 that plot achievable tolerance (for an ideal design) against characteristic dimension. There are currently around 70 maps incorporated within the
analysis covering processes from sand casting to honing. A sample map can be found in Fig. 9.
In addition, the analysis embodies the main design dependent factors that influence process variability. The factors included in the methodology relate to material and design geometry effects. (This kind of variation can be considered as special-cause). Assessing the degree to which the material under consideration is away from the 'ideal' specification is key to the analysis. (Machinability or formability ratings are examples of the data types employed in an assessment). The analysis of design geometry enables form and feature to be linked to variability during manufacture; for example, parting lines on castings and long unsupported sections. In order to assess the risk of an out of tolerance situation both a likely estimate of occurrence and a measure of failure severity are needed. Therefore, in addition to understanding the likely parts per million (ppm) defective from the Cpk estimate, the designer must consider the severity of potential failures. A technique useful in this connection is Failure Mode and Effects Analysis (FMEA). The conformability map (Fig. 10) relates FMEA Severity Rating to occurrence probability and Cpk. The map is divided into regions of acceptable design, special control and design review, and includes lines defining quality failure costs. In this way the map provides a risk assessment tool. For more information the reader is directed to references (4) and (5).
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Fig. 9 Sample Process Capability Map, Ref. (6) Fig. 10 Conformability Map.
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1This common-cause or inherent variability is due to the set of factors that are inherent in a machine/process by virtue of its design, construction and the nature of its operation; for example, positional repeatability, machine rigidity, which cannot be removed without undue expense and/or process redesign. When only common cause variability is present, the process is performing at its best possible level.
2The data given in the maps are representative of good practice in the industry concerned. It has been collected from a wide range of sources including International standards, specialist organizations, engineering texts and experimental studies. Particular maps would include as many as twenty different data sources. In all cases the data is consistent with processes that are well established and fully developed. The predicted values of Cpk have proved to have very close correlation (98%) with SPC results
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