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A Datum Feature Is Not a Datum
PDF of This Tip
Note: This Tip is in accordance with the ASME Y14.5M-1994 standard.
 First, it is not considered good practice to have such a large flatness tolerance on the datum feature and smaller tolerances on the other considered features. However, it is technically correct and this approach was taken on this drawing to make a point. The parallelism is relative to the datum plane established by datum feature A not to datum feature A.
The question I get is “How can the datum be bowed and there be a tight parallelism tolerance relative to the datum?” Well, the answer is, “It Can’t!” The problem is that people are constantly confusing the datum with the datum feature. On this part the bottom surface is the datum feature. No matter how “bowed” it is, it is used to establish a datum plane which in theory is flat. According to the ASME Y14.5M-1994 standard in Section 4.2.1, since measurements cannot be made from theoretical planes, the processing equipment serves as a simulated datum plane for inspection purposes. One way to inspect this parallelism is to place the datum feature on a surface plate. The surfa ce plate is used as the simulated datum. The travel of the indicator measuring the parallelism error is parallel to the surface plate—not the datum feature.
So, as is shown in the last figure, the datum feature may be out of flat as much as 0.5 but the top surface might be parallel (which also controls flatness and straightness) within 0.1. You might ask what you should do if datum feature A is bowed the other way. That will be explained next month and will be indexed on the website under Datum, Candidate Datum Set.
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