Geometric Dimensioning and Tolerancing (GD&T)
Additional to dimensional variations in every part ever manufactured, there are also geometric variations. GD&T is a universally standardised way to define tolerances for the geometry of manufactured parts.
The example in Figure 1 shows how the parallelism of two surfaces can be defined.
The symbol attached to the bottom surface – the datum feature – sets that surface as the reference. The symbol attached to the top surface – the feature control frame – is divided into three boxes in this example. The first contains the parallelism geometric characteristic, the second shows the 0.1 mm tolerance and the third shows the reference surface A is being referenced.
What this means is that the top side must be parallel to the bottom side within 0.1 mm – the distance between the two blue lines in Figure 2. Note: this is a grossly exaggerated sketch to clearly show the slope.
The example above is just one of the geometric characteristics in GD&T, in total there are 14 tolerances can be defined for. These are shown in Figure 2: