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GD&T Straightness
Straightness control in GD&T can be a surface Straightness of a feature or an axis Straightness of a cylinder. If the axis straightness is called out, it is usually called out with an (MMC) maximum material condition.

 

Straightness of a surface

Straightness can be used on a feature to control the flatness without refer to a datum. Straightness only measures the variance on a straight line. In the following example, straightness is only called out on the surface of the feature, the height differences are measured to see how flat or straight the line is along the indicated surface.

surface straightness GT&T

 

Straightness along axis

Straightness along axis in GD&T, the maximum material condition ( MMC ) is commonly called out together to ensure both the diameter and it axial straightness are controlled.

Example : Cylinder of OD 10.0 +/- 0.1 with Straightness Tolerance of 0.05 under Maximum material condition ( MMC )

straightness MMC GD&T

The straightness controlled at 0.05 if cylinder OD measured at 10.1 ( MMC ). Extract bonus can be applied if the cylinder produced to a smaller diameter size.
Diameter 9.9 for instance, the allowable straightness can be (10.1-9.9 )+0.05 = 0.25

 

Click on the following links to get more information on GD&T concept.

 

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