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DIAMETER/DIMENSKJN (nun) Figure 2.11 Employment of the tolerance data in the generation of a process capability maps relationship, therefore (1.7)2 = 3 and (3)2 = 9. It follows then that risk indices of 'A' greater than 1.7 would not be process capable. In essence, the spacing of the lines A = 1 to A = 9 represent decreasing percentages of the tolerance band at any given dimension as the value of 'A' increases. However, the log-log axis as used on the maps show the difference as a linear step. A further development of the use of the maps is that the 'A' values can be interpolated between A = 1 and A = 9 values bounded on the map. This ultimately improves the accuracy in determining the risk value. Therefore, to determine the tolerance risk value 'A', look along the horizontal axis until the characteristic dimension is found, and locate the adjusted tolerance on the vertical axis. Read off the 'A' value in the zone at which these lines intersect on the map by interpolating as required between the zone bands, A = 1 to A = 9.

The knowledge contained in the maps is also useful in determining the tolerance requirement at an early stage in the detailed design process. In this capacity, the region of process capable tolerance is bounded by two bold lines at A = 1 and A = 1.7 on the maps. Of course, this does not take into consideration the material and geometry effects initially, for example parting line allowances. Reference to Swift and Booker (1997) can be made for approximate parting line allowances. In most cases, guidance is also given on the maps for the need of a secondary process if the dimension/tolerance combination defined gives a risk index greater than 3 (which is considered to be out of manufacturing control).

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