This page is under construction !
Product manufacturing information, also abbreviated PMI, conveys non-geometric attributes in 3D computer-aided design (CAD) and Collaborative Product Development systems necessary for manufacturing product components or subsystems. PMI may include geometric dimensions and tolerances, 3D annotation (text) and dimensions, surface finish, and material specifications. CAD application literature may also refer to PMI synonymously with Geometric Dimensions and Tolerances (GD&T) or Functional Tolerancing and Annotation (FT&A). See corresponding information on Wikipedia:
The information provided here is supported by AP203ed2 and to a big degree also by AP214.
Nonetheless that PMI is focusing on non-geometric information, geometric representations are required to link to.
Geometric product model
The geometric model of a product is represented by a shape_representation. STEP offers some 20 different specializations of shape_representation, but for the purposes of GD&T you should use advanced boundary representation only, see AM 1541 a wrapper for AIC 514. For the MIM/IR implementation we have to refer to these entities:
- advanced_brep_shape_representation, a subtype of shape_representation that requires a 3D coordinate system, established by a geometric_representation_context
- advanced_face, a topological bounded surface
- oriented_edge, a topological bounded curve
- vertex_point, the topological representation of a cartesian_point
- axis2_placement_3d that is used to define local coordinate systems within the 3D geometric_representation_context
Supplemental or constructive geometry are all the geometric elements that are needed to define the shape of a product, but that are not themselves part of the shape of a product. AM 1131 "Construction geometry" defines constructive_geometry_representation that is the collector of all these geometric elements. The valid entities are:
- placement, either axis2_placement_2d or axis2_placement_3d
- curve, bounded or unbounded
- point, in most cases cartesian_point
A constructive_geometry_representation is linked to the corresponding advanced_brep_shape_representation by a constructive_geometry_representation_relationship. Both representations must share the same 3D geometric_representation_context.
A draughting_model includes the shape_representations and constructive_geometry_representation by a mapped_item with an identity transformation. These representations must share the same 3D geometric_representation_context
Presentation and drawings
Throughout STEP the term shape is used to refer to geometric information without having any detailed geometric information.
- product_definition_shape represents the shape of a product. A shape_definition_representation links it to the corresponding geometric model
- shape_aspect that identifies a portion of the shape of a product. The attributes product_definitional defines on whether the shape_aspect is part of the item_shape.
- composite_shape_aspect, a subtype of shape_aspect that represents a collection of other shape_aspects
- derived_shape_aspect, a subtype of shape_aspect that is derived from one or more other shape_aspects. There are further specializations for apex, centre_of_symmetry, geometric_alignment, geometric_intersection, parallel_offset, perpendicular_to, extension and tangent