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Reorder variation

When designing and ordering custom manufactured parts do not rely on the outcome of prior orders. Each order must stand alone and specify all details independently of other orders. 

Just because a prior order met your expectation or functioned correctly does not mean a reorder will do the same. Tolerance controls conformance, not prior orders. There will always be some variation from one order to the next due to machinist preferences, process changes, wear of cutting tools, variations in methods, etc. The purpose of tolerances is to specify an allowable range of variation. This applies to all the specifications in your order.

For example, suppose you place an order for a part with a 1" hole into which you plan to insert a shaft. And suppose you set the general tolerance to +/- .005. On a first order all the holes might, by chance, range from 1.001 to 1.005 and the shaft fits through. On the next order some or all of the holes fall into the range .995 to .999 and the shaft does not fit. The parts are in spec but non-functional. It is your responsibility to specify tolerances so that your part will function throughout the tolerance range. In this example you might have needed to specify a hole size of 1.006 +/- .005.

As another example, suppose your first order has parts with a surface texture or appearance that you like. Then you follow up with an order that has a different surface texture or appearance. Both orders will be considered in spec unless you specified a specific texture or appearance that was not met. 


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