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CAx-IF Recommended Practices
Representation and Presentation of Product Manufacturing Information (PMI) (AP242)
Version 4.0, October 13, 2014
© CAx Implementor Forum http://www.cax-if.de/ 9
http://www.cax-if.org/
5 Implementation Guidelines for Dimensional Tolerances
5.1 Associating Dimensions with Features or Geometry
Dimensional tolerances are attached to the items of geometry or topology that the dimension ap-
plies to. This is done by means of a subtype of the shape_aspect_relationship Entity, i.e.
dimensional_location, or by means of a dimensional_size entity. The identification of the
type of tolerance is done via a specified string in the “name” attribute of the Dimensional Location
or Dimensional Size entities. Note that Angular Dimensions are an exception to this, being defined
by the Angular Location and Angular Size entities. The mappings for the Dimensional Tolerance
types to the required string are shown in Table 1 and Table 3.
Note the combination of the shape_aspect entity, geometric_item_specific_usage entity
and the geometric_representation_item, i.e. advanced_face, are unique. That is to say
if another geometric dimension or tolerance references the same advanced_face entity, the
same shape_aspect entity and geometric_item_specific_usage should be used as well.
This rule should be followed for all such relationships throughout this document.
Note: In AP242, there is a uniqueness rule on each of shape_aspect, dimensional_loca-
tion, dimensional_size and shape_aspect_relationship which requires the attribute
pair (id, of_shape) to be unique if the id attribute exists. There is also a global rule requiring
uniqueness of the id attribute across population of a collection of the above entity types if the id
attributes exist. These rules have been introduced in the context of the Semantic PMI Representa-
tion capabilities and External Element References (EER). The second rule is more restrictive as it
requires coordination amongst several entity types. For backward compatibility reasons, AP242
does not formally require the id attribute to exist.
Since the id attribute is derived, an instance of id_attribute must be populated, which has the
id string as its attribute_value and any of the aforementioned entity types as identi-
fied_item.
While adding the id_attribute is allowed but not required in the formal AP242 document, omit-
ting it in an AP242 file will violate the business agreement for Semantic PMI and EER. Also, in or-
der not to have to make the decision what purpose a shape_aspect is used for, it is recom-
mended to add an id_attribute to all instances of the above entities, with an attribute_-
value string that is unique among all instances of id_attribute in the context of the respective
product_definition_shape, i.e. if there are 8 id_attribute that reference a combination of
the above types which all reference the same product_definition_shape in their of_shape
attribute, there shall be 8 distinct values of attribute_value.
Figure 4 illustrates the instantiation of the id_attribute:
shape_aspect
id_attribute
identified_item
attribute_value
product_
definition
of_shape
‘unique id’
Figure 4: Unique Id Instantiation
Note if a single feature is actually made up of multiple geometric_representation_item enti-
ties, the structure illustrated in Figure 5 should be used. An example use of this concept is a cylin-
drical hole represented by 2 faces, where each face represents half of the cylindrical hole.