Acceptable Quality Levels (AQL)
It should be recognized that components produced in large volume are not exactly identical even though produced on only one machine. Inherent machine repeatability variations are the cause. The components produced are usually within the tolerance limits but a small percentage may not be within the tolerance. To allow for this variation, the military developed MIL-STD-105E (or latest revision), Sampling Procedures and Tables for Inspection by Attributes. This specification allows a small percent of imperfect components in any given lot. The percent of production allowed to exceed the tolerance limits is the Acceptable Quality Level (AQL).
Note: Military specifications and standards are distributed by the Defense Technical Information Center, which is a branch of the Defense Logistics Agency. Click here for more information.
Informing the producer of the quality level required is essential to obtaining usable parts. The producer of your parts or components should be able to determine the AQL the very first time he sees the print.
In addition, he can determine from the print whether or not special gages or inspecting procedures will be necessary and then be able to coordinate his final inspection with that used by the receiving inspector.
It is recommended the inspection information be shown to the producer either on a separate form accompanying the print or in the form of a note on the drawing itself. When practical, the latter method is preferred.
If you have a formal AQL system it is a relatively simple matter to incorporate the basic information on the drawing; see the example in Drawing #5 .
A simple coding can be used. For critical dimensions use , for majors use , and for minors use , or any method you desire, with a note to show the values for each of the three symbols. A chart explaining your sampling procedure should answer any additional questions.
If you do not now have a formal AQL system and plan to adopt one in the future, it is recommended that you adopt MIL-STD-105E, Level II – Normal Inspection, and with it, the coding system suggested above.
Specifying the quality actually required assures that you receive full value for the money spent. On the other hand, over-specifying AQL will lead to waste. An unnecessarily restrictive dimension or surface finish can prove to be a substantial, and avoidable, expense.
Every consideration should be given to using as open an AQL as is consistent with function. Where function requires close limits, this should be explained clearly in the information sent to the part producer.