Geometric Dimensioning and Tolerancing – GD&T
GD&T – Geometric Dimensioning and Tolerancing refers to the Geometrical aspects of the part being drawn. Engineers communicate among themselves through Engineering Drawing. When reading an Engineering Drawing it is important to read the following three aspects.
- Geometrical Features
The Dimensions refer to the size description of the parts. For example the length, Diameter etc.
Geometrical Features refers to the shape of the parts. For example Perpendicularity, Roundness,etc
Notes refer to the information that are given in text where the same cannot be explained in the drawing format. For example seal swept area, Casting non clearance allowed etc. You can find such notes in the Product Drawing.
The Geometric characteristic are mostly used to represent Form, Profile , orientation, Location and Run out.
Let us Understand the definitions of the Geometric characteristics in simple terms
Straightness is is used to ensure that a part is uniform across a surface or feature.
Flatness is a condition of a specified surface having all elements in one plane.
Circularity is used to describe how close an object should be to a true circle.
Cylindricity describes how close an object conforms to a true cylinder.
Profile of a surface
Profile of a surface describes a 3-D tolerance zone around a surface.
Profile of a line
Profile of a line describes a tolerance zone around any line in any feature, usually of a curved shape.
Angularity describes the specific orientation of one feature to another at a referenced angle.
Perpendicularity controls how Perpendicular a specific Surface or an axis needs to be to a datum.
Parallelism describes a parallel orientation of one referenced feature to a datum surface or line.
Position is defined as the total permissible variation that a feature can have from its “true” position. The term True Position refers to the exact coordinate, or location defined by Drawing Specification.
Concentricity is a tolerance on the axis of a feature versus an ideal axis specified.
Symmetry is a 3-D tolerance to ensure that two features on a part are uniform across a datum plane.
Runout is how much the given reference feature or features deviate with respect to another datum when the part is rotated 360° around the datum axis.
Total Runout is how much the entire feature or surface deviates with respect to a datum when the part is rotated 360° around the datum axis.
We hope this blog including the simple definitions would have helped to under the the meaning of the GD&T Symbols that are used in Engineering Drawing. Do let us know the key take away from this content in the comment section Below.