STRAIGHTNESS

GD&T Symbol:                 

Relative to Datum:              NO

MMC or LMC applicable: YES - FOR AXIS STRAIGHTNESS

Straightness is a Form type of Tolerance.

It ensures the entity or elements should be in one line to which tolerance is applied. 

Straightness serve two roles; one is surface straightness and second is Axis Straightness.

Surface straightness controls the form of line on the surface.

Axis Straightness controls axis of the feature. This can be called out with MMC (Maximum Material Condition).

The Straightness Tolerance value must be less than size tolerance value.

Drawing Callout:

Surface Straightness:

Straightness symbol is called out on the surface of the part using the leader. 

Surface Straightness ensures that the part is uniform across a surface or feature. 

Straightness can apply to a flat feature or to the surface of a cylinder along the axial direction.

In Above Image you can see that feature control frame with straightness symbol has arrow head pointing at the surface of cylindrical part.

Lets take a look at the difference between the circularity and straightness. In both articles same part drawing has been used only difference is symbol.

So, if the Straightness is mentioned to cylindrical part, the drawing call-out indicate to measure tolerance along the axis.

and, If Circularity is mentioned, the drawing call-out indicate to measure the tolerance perpendicular to axis of cylinder.

Axis Straightness with MMC: 

In the above drawing you can see the method to show Axis Tolerance, As axis tolerance can come with MMC, so MMC symbol is also mentioned in feature control frame.

Straightness symbol comes with the size dimension of the cylindrical part.

The form tolerance, Straightness that controls the curve of the central axis of a part is called as Axial Straightness.

This tolerance call-out tells about- how straight the axis of a part (usually a cylinder) is?

It is a 3D tolerance that constrains the center axis of the feature from twisting or bending

MMC applied to dimension signifies the additional control on the allowable bend and twist of axis.

GD&T Tolerance Zone:

Surface Tolerance:

Above image give the glance of pictorial meaning of surface straightness tolerance.

It is a 2D tolerance, so the line (red) along the axis of cylindrical part can be allow to deviate withing the blue tolerance zone of 0.05 mm. the surface deviation within this blue zone is acceptable.

Axis Tolerance:

Above image give the glance of pictorial meaning of Axis straightness tolerance.

The allowable deviation of axis of the cylindrical part is the, cylindrical boundary of diameter of give tolerance value, around the true central axis of the part.

Gauging and Measurement:

Surface Straightness:

The part is constrained in the fixture and the gauge measures the height variation on the surface along the straight line.

Axis Straightness:

To measure the Axis straightness cylindrical gauge is used, So the MMC is mentioned to make the go gauge.

Application:

Cover plated of Hydraulic components or transmissions, Transmission shaft, gear shifting linkages etc.