ASME Y14.5 talks about five categories of geometric dimensioning and Tolerance (GD&T) controls. Form is one among them. Form itself has four sub-types of GD&T control: Straightness, Flatness, Circularity, and Cylindricity.
This GD&T control is used for line elements (straight edge of the surface). It restricts the line elements from distorting.
The Straightness of 0.004 of the above picture signifies that, the actual line should lie between a set of parallel lines 0.004 apart.
The concept of flatness is similar to the straightness except that it is applied for a plane instead of a line. Or, in other word the straightness is for 2D and the flatness is for 3D. The flatness is represented by a parallelogram.
The circular shape of a cross section of a shat or cylinder is controlled by the Circularity GD&T control.
The above picture explains the significance of circularity of 0.006 as the circular cross section should lie in between the two 0.006 apart concentric circle.
Instead of across section, the cylindricity applies for the whole cylinder. And it says that the cylindrical surface should lie in between two concentric cylinder of specified distance.
General Rules for Using the Form Controls
- The form types of GD&T controls don’t require a datum.
- The ASME Y 14.5 Rule#1 is applicable to these types of GD&T controls.
- These types of GD&T controls are always attached with a leader and never associated with a dimension.
- Material modifiers (MMC, LMC or RFS) are never used with these kinds of GD&T controls.So, the concept of the bonus tolerance is not applicable.
The Form types of geometric dimensioning and Tolerancing controls are used to control the form of geometry. There are four sub-types under the form types of GD&T. These GD&T symbols are used 2D (lines, circles) as well as 3D (surface, cylinder) geometric features.
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