Model-Based Definition > Model-Based Definition > Creating Various Annotation Types > Tolerances > Working with Geometric Tolerances > Creating Geometric Tolerances > About Geometric Tolerances
About Geometric Tolerances
Geometric tolerances (GTOLs) provide a comprehensive method of specifying where on a part the critical surfaces are, how they relate to one another, and how the part must be inspected to determine if it is acceptable.
They provide a method for controlling the location, form, profile, orientation, and run out of features. When you store a Creo Parametric GTOL in a solid model, it contains parametric references to the geometry or feature it controls—its reference entity—and parametric references to referenced datums and axes. As a result, Creo Parametric updates a GTOL’s display when you rename a referenced datum.
In Assembly mode, you can create a GTOL in a subassembly or a part. A GTOL that you create in Part or Assembly mode automatically belongs to the part or assembly that occupies the window; however, the GTOL can refer only to set datums belonging to that model itself, or to components within it. It cannot refer to datums outside of its model in some encompassing assembly, unlike assembly created features.
* If you need to define set datums and basic dimensions you must define them before you place the GTOL.
You can add GTOLs in Part or Drawing mode, but they are reflected in all other modes. Creo Parametric treats them as annotations, and they are always associated with the model. Unlike dimensional tolerances, though, GTOLs do not affect part geometry.
When adding a GTOL to the model, you can attach it to an edge, existing dimension, or existing GTOL, as well as display it as a note without a leader. You can also attach GTOLs to designated areas.
When you place a new GTOL, Creo Parametric automatically creates parameters for each tolerance value that you specify while creating that GTOL. You can edit the values of these parameters using the Parameters dialog box that opens when you click Tools > Parameters.