14 GD&T Symbols Explained: A Complete Guide
What are the 14 symbols in GD&T? The traditional 14 symbols are used to define the constraints for form,
orientation, location, profile, and runout on a drawing. We will be discussing
the specifics and applications of each geometric symbol in this guide.
What is GD&T?
Geometric Dimensioning and
Tolerancing (GD&T) is a system that helps to define the variations from a
design specification that are allowed. Rather than defining the tolerance with
a +/– constraint, GD&T controls define how the feature performs, where it
is located or oriented, and more.
GD&T symbols are applied on drawings to define the intent of the design for a specific feature of a part. The symbols can define a large range of constraints that otherwise would require a much larger description.
For instance, the GD&T symbols can define that a surface has to be flat or a feature has to be located somewhere within a tolerance zone. With the right datum references, GD&T controls define the orientation, location, and runout of features.
What are the 14 symbols in GD&T?
What are the 14 symbols in GD&T? The 14 traditional symbols are
outlined below:
1. Straightness
2. Flatness
3. Circularity
4. Cylindricity
5. Parallelism
6. Perpendicularity
7. Angularity
8. Position
9. Concentricity
10. Symmetry
11. Circular runout
12. Total runout
13. Profile of a line
14. Profile of a surface
The symbols can be broken down
into 5 categories of tolerances – form, orientation, location, profile, and
runout.
Note that modern GD&T
standards and references might contain further modifiers or distinctions.
Consult the relevant standard for complete information.
1. Straightness
Straightness controls the amount
of straightness allowed in a line element or derived median line. For a
surface, straightness tolerances apply to line elements, whereas straightness
applied to an axis controls the derived median line.
As straightness is a form
tolerance, it does not require a datum reference.
2. Flatness
Flatness controls the amount of
deviation from a theoretically perfect plane that a surface can exhibit. If you
were to imagine a theoretically perfect plane lying on top of the surface in
question, the flatness tolerance would define the space between two parallel
planes that contain the actual surface.
As flatness is a form tolerance,
it also does not require a datum reference.
3. Circularity
Circularity (or roundness)
controls the amount of out-of-roundness a cylindrical or spherical part can
exhibit. This control is a form tolerance, so it does not define an axis
dimension or require a datum reference.
4. Cylindricity
Cylindricity is very similar to
circularity, but applied to the cylindrical surface of a part. This control
defines the condition of a cylindrical surface that is equidistant from a
central axis. In other words, the control describes the whole surface of a
cylinder.
Cylindricity is another form
tolerance that does not require a datum.
5. Parallelism
Parallelism is one of the three
tolerances for orientation in GD&T. The control specifies that the
orientation of a feature has to conform to a specified direction relative to a
datum reference.
Parallelism can define the
orientation of a surface or axis. It constrains the surface or axis to be
parallel within a tolerance zone.
6. Perpendicularity
Perpendicularity is another one
of the three tolerances for orientation in GD&T. The control specifies that
the orientation of a feature has to conform to a 90-degree direction relative
to a datum reference.
If there is a requirement for a
surface or axis to be perpendicular to a specific plane, this control should be
used.
7. Angularity
Angularity is the 3rd tolerance
for orientation in GD&T. The control specifies that the orientation of a
feature has to conform to an angular direction (apart from 0 or 90 degrees)
relative to a datum reference.
This tolerance is required
whenever the feature has to be constrained at a certain angle.
8. Position
One of the most common controls,
position defines a tolerance for the location of a feature. This tolerance
specifies that the location of a feature has to conform to a theoretically exact
location by the dimensions and datum reference frame.
Position can define tolerances
for the location of any feature of size. Holes, pins, slots, and other features
employ this control. It is one of the most important controls in GD&T.
9. Concentricity
Concentricity controls the
location of median points of a feature relative to a datum axis. While it seems
that it would define the same thing as two circles having the same center,
concentricity is not as precise.
Concentricity should be avoided
as much as possible in favour of other controls that are able to define the
required tolerance better.
10. Symmetry
Symmetry is a tolerance that
controls the location of median points of a feature relative to a datum plane.
It is a location tolerance, much like concentricity. Due to being a location
tolerance, it is often best to use other controls to define the required
specifications.
Once again, modern GD&T
practices dictate using other controls to define symmetry rather than this.
11. Circular Runout
Circular runout is one of the two
tolerances for runout in GD&T. This control defines a circular tolerance
zone around a datum axis. As the part rotates around the axis, circular runout
specifies that the surface has to be within the defined tolerance zone at all
points.
Circular runout can define radial
variation, as well as axial variation depending on the part’s orientation.
12. Total Runout
Total runout is very similar to
circular runout, but the difference is that total runout controls the whole
surface of the part as it rotates around the datum axis. In effect, total
runout is a more comprehensive version of circular runout.
13. Profile of a Line
Profile of a line is a tolerance
that controls a line element of a feature. The flexibility of this control
makes it valuable, as it is able to define size, form, orientation, and
location in the feature, applied as a two-dimensional control.
14. Profile of a Surface
Profile of a surface is similar
to the profile of a line, except it is a three-dimensional control. Profile of a
surface can control size, form, orientation, and location of a feature.
GD&T Categories
What are the 14 symbols in GD&T? Categorizing the controls makes
it easier to remember and keep track of their applications. The traditional
categories are as follows:
Category Symbols
Form Straightness, Flatness, Circularity, Cylindricity
Orientation Parallelism, Perpendicularity, Angularity
Location Position, Concentricity, Symmetry
Profile Profile of a Line, Profile of a Surface
Runout Circular Runout, Total Runout
Feature Control Frames and Datums
Most GD&T symbols are applied
on a feature control frame that contains information about the application of the
tolerance. Datums define the necessary orientation or location for a feature and are referenced on the feature control frame.
Understanding the interaction between GD&T symbols, datums, dimensions, and feature control frames is crucial to defining and reading GD&T drawings.
Why Are GD&T Symbols Important?
GD&T controls enable
manufacturers and designers to define requirements that are applicable to the
function of a part rather than its dimensions. Applying GD&T controls makes
manufacturing and inspection easier and more accurate, and improves quality and
interchangeability between parts.
Conclusion
What are the 14 symbols in GD&T? The 14 traditional symbols are
straightness, flatness, circularity, cylindricity, parallelism,
perpendicularity, angularity, position, concentricity, symmetry, circular
runout, total runout, profile of a line, and profile of a surface. The controls
represent the different types of tolerance that can be applied with GD&T.
In order to truly understand the
controls, it is important that one learns about datums, feature control frames,
tolerance zones, modifiers, and inspection methods, as well. When learning GD&T
controls, it is best to practice reading and creating the feature control
frames and applying them to drawings. There is much to consider, especially
when it comes to datums, but through enough practice, the controls can become
second nature. If studying GD&T or the ASME standard, reviewing the symbols
in this article is a good start to learning.
Further reading:
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