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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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Summary of design analysis
- Clearly the tolerance stack is not capable as only one characteristic is acceptable. The failure rate will be extremely high given the low values of Cpk (the associated failure cost was calculated at over $6 million per annum against product sales revenues of around $16 million). Problems with the stack resulted from confusion with the suppliers about the capability of the impact extruded Body and Magnetic Pole, and with that of the plastic molded Bobbin. In connection with the impact extrusions radial capabilities had been assumed on axial dimensions! (The software has separate process capability maps for each forming direction).
A redesign was undertaken which required focusing on improving capability and/or eliminating problematical parts from the stack.
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Table 1 TCE results produced for the original design
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Summary of Smart Solenoid Redesign - Based on the analysis results redesign solutions were generated by the design team. (See Fig. 14 for one of the solutions). The redesign shown has eliminated the Bobbin component from the stack. The Magnetic Pole (molded
into the Bobbin) has been positively located against a machined step in the Body component.
(The body was already machined at one end to provide location on the Fuel Port Block).
The TCE analysis results are tabulated in Table 2. All components are now process capable. As can be seen from Table 2 the tolerance stack is now capable and the design can be produced with low levels of failure cost. This redesign alternative resulted in a calculated failure cost of only $6 thousand per annum. (The additional machining of the Body was not significant in the calculations.)
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Table
2 TCE results for Smart Solenoid redesign
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Concluding Remarks
The importance of the process capability index Cpk in the design of process capable products has been illustrated and a methodology has been introduced for enabling process capable tolerancing. The approach provides a means of predicting process capability in the early stages of the design process, eliminating problems before they occur. Software developed to support the application of the technology and its role in the process of creating capable products has been overviewed. Further developments with the software will extend the coverage of geometric tolerances and include the effects of assembly/fastening processes on process capability.
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