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Optimizing Defect Levels and Losses from Gage Errors

Guard Banding

The risk of passing reject units, Figurę 3, is of great concem in finał test applications where all of the product that is passed by the gage is shipped to the customer or sent on to another process step that will add substantial cost to the product. Because of this, many manufacturers impose guard bands on these gages to help minimize the level of defects that are passed on as good product. Figurę 7 shows the improved effect of adding a guard band. There would obviously be a reduction in the defect level; however, the price paid is the potential increase in gage losses.

How are guard bands for test system selected? Several methods are utilized in industry to determine guard bands such as the "best guess" system. The "best guess" system is an iterative approach to guard banding. As long as there are no known defects sent to the customer, the guard band remains the same. However, when the customer retums a defective unit, the guard bands are tightened. This often works fairly well. However, there are three disadvantages to this method. One disadvantage is that it may take a very long

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