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.

Geometric tolerancing

Figure 1: Geometric tolerancing.

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.

Parallelism

Figure 2: Parallelism.

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:

Geometric characteristics

Figure 3: Geometric characteristics.

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