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Time to Stop the Nonsense
(In accordance with Y14.5M-1994 standard)

PDF of this Tip

If you don’t have opposed points (180+ degrees of arc), you cannot reproducibly find the center (axis) of a radius. Often a curved surface will be defined on a drawing by locating the center and specifying a radius. There is nothing wrong with this type of dimensioning. The problem is that traditionally these dimensions are toleranced directly or use the title block tolerance. Once the part is made, someone has to determine where the center is out in space to determine if the dimensions are in tolerance.

When you have opposed points, you can contact these points with an indicator or gage pin and calculate the location of the center. Without opposed points, you have to try to fit a radius to the surface and try to determine where the center is. If you are taking sample points with a CMM or vision system, you will get an answer; but, if the radius is not perfectly round (and it never will be), contacting different points will yield a different size radius with a different center location.

This problem can be avoided by using Profile of a Surface control. The dimensioning doesn’t change but the dimensions are made BASIC and originate at the datums. The tolerance is on the surface not on the dimensions. You might think of the dimensions as the goal. You don’t want to tolerance the goal. You do want to specify how much the actual surface of the part may deviate from the goal. Now measuring the part does not involve trying to determine where the imaginary center of the radius may be.

Back to Tips Tip added Dec 1997