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Using Profile of a Surface Tolerance on Size Controls the Form Better... Part ll
(In accordance with Y14.5M-1994 standard)

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I received several requests to further explain how profile of a surface, when used to control size, also controls the form of the feature better than when a size dimension and tolerance are used.

This drawing uses direct tolerancing to control the size of the pin. According to the Y14.5 standard, this pin must pass through a 30.1 ring gage or equivalent and every cross section must be larger than 29.9. This cross section is described in the Y14.5.1 math standard, in theory, as a sphere that might be dragged through the pin without breaking out of the surface. In practical terms, any two-point, micrometer type measurement cannot be less than 29.9. There is no requirement that the 29.9 measurements be centered on the 30.1 ring. Therefore, the form (straightness, circularity and cylindricity) of the pin can be as great as 0.2, the size tolerance. The graph shows that as the feature’s size departs from MMC, the allowable form error increases.

On the second drawing, size is being controlled with a basic size dimension and a profile of a surface tolerance. The profile tolerance is on the true or basic surface which is a radius of 15. The tolerance is implied centered. The tolerance zone is two concentric circles with a radial separation of 0.1. As the size of the pin approaches 29.9 or 30.1, its form must improve. The greatest form error may occur if the pin is made at 30.

Back to Tips Tip added Dec 2005