Form tolerances are applicable to single (individual) features or elements of single features; therefore, form tolerances are not related to datums. The following subparagraphs cover the particulars of the form tolerances - straightness, flatness, circularity, and cylindricity.
Straightness is a condition where an element of a surface, or an axis, is a straight line.
18.104.22.168 Straightness Tolerance
A straightness tolerance specifies a tolerance zone within which the considered element or derived median line must lie. A straightness tolerance is applied in the view where the elements to be controlled are represented by a straight line.
This on the drawing:
The derived median line of the feature actual local sizes must lie within a cylindrical tolerance zone of 0.04 diameter at MMC. As each actual local size departs from MMC, an increase in the local diameter of the tolerance cylinder is allowed which is equal to the amount of such departure. Each circular element of the surface must be within the specified limit of size.
- The maximum diameter of the pin with perfect form is shown in a gage with a 16.04 diameter hole;
- with the pin at maximum diameter (16.00), the gage will accept the pin with up to 0.04 variation in straightness;
- with the pin at minimum diameter (15.89), the gage will accept the pin with up to 0.15 variation in straightness.