Geometric Tolerancing

Geometric Tolerancing is the art of applying GD&T. Geometric Tolerancing differs from GD&T which is a mechanical engineering language, GD&T, or Geometric Dimensioning and Tolerancing, represents a way to define the size, location, orientation, and form of a part feature. The definition includes a symbol to communicate design intent, a tolerance that specifies the permissible variation, and coordinate system, i.e. datum reference frame, for inspection and manufacturing purposes. Using GD&T, either manually or with GD&T software, the design engineer can precise communicate a great deal of information that is useful throughout the product development process. An example GD&T callout is shown below.

geometric tolereancing

Geometric tolerancing is governed by two standards.  One standard is ASME Y14.5 and the other is an international standard, ISO 1101:2004.  While both standards have a great deal in common, some differences exist and should be understood when applying to drawings to ensure accuracy in communicating design intent.

Using geometric tolerancing imparts a well defined control for the variation of a part feature .  Functional features of a part, those features that are used for assembly, manufacturing, or product function, are critical to the quality of an assembly.  When a functional feature on one part varies, the corresponding mating parts must also vary.  Mating conditions between parts dictate how the variation will propagate through the assembly. This variation is propagated through the assembly.

For example, in this case the surface deformation causes an orientation error on the mating part.  The tolerance applied to a feature of size controls bending, twisting, or warping of the plate, as shown in the figure below.

sigmetrix geometric toleancing

 Too much variation at a critical feature location can cause problems when assembling the parts or can cause the assembly not to perform as intended. This will result in scrap, rework, field failures, loss of productivity, and warranty and liability costs. Therefore, it is critical that tolerance analysis, or assembly variation analysis, be conducted as a routine part of the product development process.

To conduct a tolerance analysis that includes geometric tolerancing, the GD&T callout must be defined in terms of the implied translational and/or rotational variation. In the example above, the tolerance assigned to the rotational variable, (θ), of the green block could be expressed by the following expression:

sigmetrix example engineering geometric tolerancingWhere –
BW = the tolerance bandwidth

                                                            L = the length of contact of mating parts

By mathematically quantifying variation and understanding how part features cause shifting in assemblies, it is possible to perform not only one-dimensional (1D) tolerance stack-ups, but also two-dimensional (2D) and three-dimensional (3D) statistical tolerance analyses. And, through direct integration with the CAD model, the process of creating tolerance stack-ups has never been easier.

For more information on how to create analyses directly from your CAD assembly models, please contact