GD&T symbols

What are the symbols in GD&T which symbolize geometric controls and what do they mean?

There are 5 fundamental domains of geometric control in GD&T

Form
Orientation of the feature
Location of feature
Runout of rotating features
Profile of a surface

Form control is basically the control of inherent shape of the feature not with respect to any reference but with respect to itself.

Straightness is the most fundamental form control which is used to control the surface variations from the highest peaks to lowest troughs for each line segment on the surface .

While straightness controls form of line segments, flatness is used to control the form of flat surfaces.

Flatness is a more stringent form control of flat surfaces as compared to straightness.

While flatness can only be used for flat surfaces straightness can also be used cylindrical surfaces.

Form of a cylindrical surface is dependent on how close the cross section is to a true circle. This is called roundness or circularity which is shown here :

A more tighter control of form is cylindricity which instead of controlling form of individual circles at each cross section , controls the cylindrical form as a whole.

Orientation geometric control is applied when the angle between a reference face and another face needs to be controlled.

Angularity is commonly applied to do this as shown where the targeted surface is being controlled in terms of its angle with respect to a datum A

Generally associated with a basic angle as shown as 35 in the example.

If the angle is 90 degrees then angularity is replace by perpendicularity

Here is an example .

Perpendicularity controls how two faces are orthogonal to each other.

If the angle is 0 degrees that is both surfaces are parallel to each other or if one axis is parallel to a surface then parallelism control is used ,

There is always a datum referred to an orientation control. Minimum one datum is mandatory , there can be more than one .

One of the most commonly used symbols and geometric control is the true position control . an example is shown here.

This geometric control makes sure the location of a feature of size like a hole , pin or a width is within specific limits with respect to a datum system generally specified by a primary , secondary and tertiary datum.

There should be at least one datum specified when using position control on features.

Position cannot be used on plane surfaces , only on features of size.

Runout :

It is a geometric control primarily for rotating surfaces.

Any surface which is rotating about an axis uniformly can be measured in terms of its runout.

Runout not only controls form but also orientation and position of the features.

Circular runout like circularity controls each cross section or circular element and total runout controls the complete surface whether cylindrical or flat .

Profile:

Profile control in GD&T is a highly versatile and multipurpose geometric control which can be used on any type of surface.

Profile can be used to control size, form, orientation and even position of surfaces with respect to established datum features .

Here is an example.

Like straightness vs flatness there is profile of a line and profile of a surface

Most commonly profile of a surface are used for non uniform and non cylindrical surfaces which are not features of size .

To learn more about GD&T have a look at this course

Geometric Dimensioning and Tolerancing: Basics

Categories: GD & Tolerancing