bad twice. While this approach to operating inspection increases the number of defective parts shipped, it reduces the internal reject rate. Te question becomes: “Is the added cost of inspection and greater defectives reaching the customer less than that saved by having fewer internal rejects? To answer that question, the
Fig. 4. The total cost of quality is shown for one inspection under varying levels of inspection effectiveness. The p value has been fixed at p = 0.10. The cost case is 1 per Table 2. Similar plots (though at different y-intercepts) can be created from the other cost cases. The point at $700 is generated from perfect inspection, the lowest cost (and highest outgoing quality condition).
manufacturing operations. It is not unusual to find inspection operations performing well below 80% overall effectiveness. Figure 4 highlights the impact of
inspection improvement on total cost of inspection, using cost case 1 where N = 1,000 and the incoming actual defective parts is 10% (p = 0.10). Inspection effectiveness (in terms of
miss rate and false alarm rate) was allowed to vary, indicating the total cost. A single point on the plot rep- resents “perfect” inspection with zero misses or false alarms. It is of interest to evaluate whether the total cost of quality is reduced if the producer subjects those initially rejected to a second inspection, then rejects only those that were deemed
case where N = 1,000, m = 0.15, f = 0.05 and p = 0.10 was considered, assuming the second inspection was equivalent in effectiveness to the first inspection. In this scenario, the top half of Table 3 provides the costs associated with the initial, single inspection for the four cost cases. Tis cost picture can be compared to that with reinspection of the rejected parts from the initial inspection. It is clearly evident that, in every case, the cost of internal rejects is reduced, but the cost of the added inspection plus the cost of the added external defects overwhelms that savings. Te total cost for such a procedure is always
50 | MODERN CASTING November 2012
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