GD&T Straightness

What is the meaning of the form control Straightness? When called out how to interpret it?

Straightness is a form control in GD&T . When applied to a surface , straightness basically controls the variation of each of the lines with respect to their peaks and valleys .

Here is an example of a block on which straightness is applied.

The line segments are inspected for variation from highest points to lowest points.

From the crest to the trough .

Tolerance zone is a 2 D rectangle with a width of 0.02 .

Each cross section of the surface is checked for straightness.

Inspecting planar surface straightness involves running probes or dial gauge indictors across the line segments of the surface as shown . The total indicator reading is taken m if it exceeds 0.02 for any instance then the surface is not acceptable in straightness.

When Straightness applied on a cylindrical surface the meaning remains the same but instead of a flat surface the surface is cylindrical.

Shape of the tolerance zone is still the same a 2D rectangle with width of 0.5 mm.

The inspection using dial gauge indicator :

Axis straightness

Axis straightness is a special type of straightness control . When straightness feature control frame is attached to the size dimension , it is said that axis straightness is being controlled.

Here is an example :

Tolerance zone definition is different in this case .

It’s a cylinder with a dia of 0.5

The fact that it’s a cylinder is indicated by the dia symbol .

Straightness in two directions is also a way of controlling straightness in perpendicular directions as shown in the views below :

The tolerance values can change if straightness in one direction is to be controlled more tightly as compared to the other.

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Geometric Dimensioning and Tolerancing: Basics

Categories: : GD & Tolerancing