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You Can Still Use Your Calipers, If You Really Want To
(In accordance with Y14.5M-1994 standard)

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I constantly hear that where profile of a surface is used, you have to use a coordinate measuring machine or some other sort of high-tech inspection equipment. The assumption is that if the part had been toleranced with the old plus/minus approach, rather than geometric tolerancing, the part would be much easier to inspect with simple equipment such as calipers. Of course this is nonsense. A caliper measurement is a one-dimensional measurement of a three-dimensional feature whether it is toleranced with GD&T or not. Calipers may be used on this part, for instance, to do a quick check of features with a profile tolerance applied. Since the profile of a surface tolerance is relative to the datum reference frame established by A, B and C, an accurate, 3-D inspection of these features would require establishing the datum reference frame and measuring from the datums. Here are a couple of quick caliper type measurements that could be made.

Imagine trying to inspect the R8 dimension if it had been toleranced without GD&T as is shown here. Try to determine if the center of the 4X R8±0.1 are on the Ø36±0.1 and at the implied 90°±0°30’. By the way, how centered does the Ø36 have to be on the Ø28. There are many ways to "interpret" this drawing. However, the drawing with GD&T has "one clear meaning." Bottom line, if you want to do an inadequate job of inspecting a part using calipers-go ahead, regardless of how the part is toleranced.

Back to Tips Tip added Dec 2006