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Dimensions Don’t Have to Come From Datums, They Just Have to Get You Back There
(In accordance with Y14.5M-1994 standard)

PDF of this Tip

Here is a Tip that can really pay off. Production was troubled because datum feature C had a very small surface roughness requirement. Feature C is a thrust surface and functionally should be the secondary datum feature. Production did not feel they could keep setting up on feature C to establish a measurement plane for axial measurements because continually touching datum feature C could damage the surface. I suggested that if they held the surface shown here as zero to a very tight location, they could measure from it rather than feature C. The only error would be the amount they allowed that surface to deviate from C. Just because a feature has a 0.2 mm tolerance doesn’t mean production has to use it all. I also suggested that design dimension from the surface production would use. It doesn’t matter how a part is dimensioned as long as the basic dimensions get you back to the datums. Problem solved.

Back to Tips Tip added May 2005