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Form Types of GD&T - Know Where to Use and Where Not to Use

written by: Suvo • edited by: Lamar Stonecypher • updated: 12/16/2009

Form is one the five types of GD&T control. This geometric dimension and Tolerancing tutorial will be concentrating on the form types of GD&T controls.

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    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.

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    This GD&T control is used for line elements (straight edge of the surface). It restricts the line elements from distorting.

    Gd T Straightness  

    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.

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    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.

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    The circular shape of a cross section of a shat or cylinder is controlled by the Circularity GD&T control.

    GD T Circularity  

    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.

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    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.

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    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.
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    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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    Related Reading

    How to Perform Assembly Tolerance Stack Up Analysis by Worst Case Method:This GD&T tutorial will talk about assembly tolerance chain stack up analysis and will describe the procedure of calculating the tolerance chain stack up analysis in a practical manner.

    GD&T Tutorial – Learn How to Identify the Feature of Size Dimension: This GD&T tutorial will talk about the concept of feature, feature of size, non-feature of size and also about the application and identification of the geometric dimensioning and Tolerancing concept in engineering drawing.