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Parts Do Not Have Centerlines—Features Have Axes
(In accordance with Y14.5M-1994 standard)

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I continue to see datum feature symbols placed on center lines. The Standard allows this only if datum targets are used to indicate the datum feature. That was not the case for a company designing lens systems for use in space. Here is a simplified drawing of one of their lens barrels. They did not associate the datum feature A symbol with the Ø.8423. Instead they placed it on the center line which violates the Standard. The supplier used a convenient feature to establish the axis to check the total runout. In this case probably the largest outside diameter. The parts passed. The customer knew that the lens barrel would mount on the smallest outside diameter so they checked it by holding the part there. The parts failed and the customer refused to pay for the parts. As a result there was a lawsuit brought against the customer. The drawing had a loophole and was open to interpretation. The customer lost millions in the lawsuit all because they did not make it clear which feature to use when checking the total runout.

This revised drawing clearly indicates which feature to use to establish an axis of rotation when checking the total runout. Center lines are a drafting convenience. Associate the datum feature symbol with datum features!

Back to Tips Tip added Sep 2007