GD&T - Part 5; Location
In this post we are taking a closer look into:
- Position
- Concentricity
- Symmetry
Linear Dimensions vs Location Control
Although linear dimensions can be used successfully dimension feature on a 2D technical drawing, location control features can elevate our, the designer’s, intentions to a whole new level.
Location control features have the following advantages compared with linear dimensions:
- There is no accumulation of tolerances. Location control features can be located with theoretical exact dimensions (TEDs) allowing for a more simple tolerance calculations.
- Functional relationships to one or more datums are directly indicated
- There is the option to indicate cylindrical tolerance zones. This can reduce positional error by 39%
Position
In the method of positional tolerances, in order to determine the location of features, theoretical exact dimensions and a control feature frame can determine the locations of any feature (points, axes, planes and median surfaces). The tolerance zones is located symmetrically about the theoretical centre.
Concentricity... or Co-axiality
Both concentricity and co-axiality share the same control feature symbol which can cause confusion.
Concentricity is a 2D tolerance zone feature around a theoretically exact centre point of the datum. Typically used to control position of one circle to another, however as form is not considered, concentricity can be used on other shapes too
Co-axiality is a 3D tolerance zone a theoretically exact centre axis. Typically used to control one cylinder to another cylinder. However, like concentricity, form is not considered so other shapes can be considered.
In most cases where depth is considered i.e. a hole. Co-axiality should be used over concentricity
Symmetry
For symmetry control, the tolerance is applied to the theoretically exact centre (axis or plane) of the datum shown in the tolerance frame.
