Realistic tolerances set by expert

New software aims to provide a straightforward, rapid and consistent way of allocating process capable limits to design dimensions. Tom Shelley reports

The main objective of 'The Tolerance Capability Expert' 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.

The software has been developed by Capratechnology, based in Yorkshire. An analysis begins with the selection of a manufacturing process and the determination of an ideal or best achievable value of Cpk for a given tolerance. The user is guided through a series of questions to enable an evaluation of the effect of geometry and material.

When studying a particular component, the user can select processes to be investigated. Tabs are provided to move from one process group to another. Clicking on a process displays the appropriate capability map. There are currently around 70 such maps covering processes from sand casting to honing. Double clicking the process brings up the process setup screen where the characteristic dimension and its tolerance can be entered and assessed. Selecting the 'Summary' button on the 'Process setup' screen gives access to the results obtained, and enables a user to derive an appropriate Cpk target. The failure severity rating can be entered and an appropriate process capability target obtained from a Conformability Map. The map relates Failure Mode and Effects Analysis Severity Rating to occurrence probability and Cpk. If the engineering requirement is not met, the software can be used to explore the effect of design changes. Changes in geometry, materials, processes and tolerance can be rapidly investigated.

Martin Raines, director of Capra Technology, claims that the software can reduce quality failure costs by as much as 75%. Dyson has recently purchased the technology after comparing values predicted by the software against those being achieved in production. Extensive trials and product studies have also been undertaken in TRW over a number of years during the development phase. A single copy of the software is priced at £950. A site license for 10 users costs in the region of £2,500. The figure does not include maintenance or training.


* The software allocates process capable limits to design dimensions

* Tolerance capability can be predicted for different manufacturing routes whose relative effectiveness may thus be compared

Capra Technology
email Capra Technology

Tom Shelley

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