GD&T: Presentation of Dimensional Tolerances
From WikiSTEP
This section deals with the additional entities required to take a dimensional tolerance and associate it with a visual display of that tolerance in the 3D model workspace.
Note that currently only the 3D model workspace is specified, mainly because of the lack of 2D implementations, but there should be very little additional work on top of this specification to associate the display of the tolerance with a specific 2D view of the model, once the 2D viewing mechanism has been implemented.
The information and mapping in this section has been drawn from a number of documents, notably the “Recommended Practices for model viewing, basic drawing structure and dimensions�, dated 5th October 1999 and co-authored by Rogerio Barra, Markus Hauser, Bettina Neuhof and Linus Polikaitis. This Document is available at the Cax-IF website (www.cax-if.org or www.cax-if.de). One Major difference is the removal of the Structured_Dimension_Callout entity from these practices. This was done as there did not appear to be any semantic value added by this entity to the exchange.
Relating the Presentation with the Representation
The Representation of the Dimensional Tolerance is covered in GD&T: Implementation Guidelines for Dimensional Tolerances. The full description of Presentation will be covered in a later section, however, for the purposes of this section, it is enough to know that the associated Presentation information for the Representation is collected by a Complex Structured_Dimension_Callout Entity, as shown in Figure 24. For this example, a Linear Dimension with a Plus-Minus tolerance is used.
Figure 24. Associating Representation and Presentation
The Dimension_Text_Associativity entity is a subtype of Mapped_Item, hence the links to Representation_Map and Structured_Dimension_Callout, and also a subtype of Text_Literal, being able to hold the textual representation along with placement information and alignment. For presentation in a 3D context, the placement should be an Axis2_Placement_3D, with the text positioned in the X-Y plane, as for 3D associative text.
Note that is any of the subtypes of Text_Literal are required, e.g. Text_Literal_with_Blanking_Box, then this entity will have to be instantiated as a complex entity.
The Dimension_Callout will be a complex instantiation of two subtypes of Draughting_Callout, the particular subtype of draughting_callout which defines the type, such as linear_dimension, and Draughting_Elements. The dimension curve and witness lines are also defined in this Entity (See Presenting the Associated Graphics).
Presenting the Tolerance
The tolerance value is presented using the STEP entities shown in Figure 25, attached to the Structured_Dimension_Callout defined in Figure 24.
Figure 25. Presentation of Dimension Value
The Structured_Dimension_Callout entity gathers together the components of the Dimension value string, i.e the value, the tolerance and the symbol separating them. The linkage of the value back to the representation of the Dimension is done by attaching to the " Text_Literal" part of the Dimension_Text_Associativity entity. There is no equivalent linkage for the Tolerance value, although it is possible to navigate through the entities to pick up the true value. In fact, the entire Value and Tolerance string can be derived from the Representation, without the need for the textual presentation entities. However, if the receiving system does not support Dimensional tolerances, then at least it can make use of the textual information to present it on the screen to the user.
Presenting the Associated Graphics
The other part of visualising the Dimension is to apply the dimension and witness lines for the presentation. These do not exist in the Representation of the dimension, and are phyically placed in the View or 3D Model space, without any linkage to the defining geometry of the part by specialisations of the Draughting_Callout entity.
An instantiation diagram of this associated geometry is shown in Figure 26.
Note that in this figure, the Linear_Dimension Entity is the same entity as the draughting_callout in Figure 25.
Figure 26. Associated Dimensional Geometry
