A Quick Guide to Semantic References in Creo MBD

Semantic references for model-based definition (MBD) are CAD model surfaces that are included in the 3D annotations that explain how a part or assembly is to be manufactured and inspected.

Semantic references, such as those highlighted in green in the part shown above, can be read by computer-aided manufacturing (CAM) software for defining toolpaths and coordinate measuring machines (CMM) for performing inspection.

Incorporating MBD and semantic references can reduce the design-manufacturing-inspection cycle time by up to 74.8% according to a NIST study.

What is MBD?

Model-based definition is the practice of documenting and capturing a model’s product and manufacturing information (PMI) in the form of 3D annotations. These include details like dimensions, geometric tolerances, symbols, notes, and datum feature symbols that traditionally have appeared in 2D production drawings.

Image: 3D annotations attached to a 3D model help define dimensions and tolerances for manufacturing.

MBD provides numerous advantages to the design team. Creating 3D annotations in a model tends to be faster and easier than creating a 2D drawing for documentation. MBD simplifies the drawing checking and approval process since it allows for minimally detailed drawings; the model itself establishes basic dimensions. Detailing standards are not as strict as for 2D drawings. For example, a dimension can appear only once in a drawing, whereas the same annotation can appear in multiple combination states in a model.

What are semantic references?

I have spoken with engineers and managers who have implemented MBD in their organizations to reduce the time and effort necessary to create product documentation. When they do not extend MBD into their manufacturing and inspection processes, they are experiencing only a portion of the benefits.

For example, a dimension can define a distance between references or a diameter of a shaft or a hole. For CAM or inspection software to understand those dimensions, it needs to know those references. To understand a datum feature symbol or geometric tolerance, it needs to know the surfaces that they apply to. The same applies to other PMI like surface finishes.

When these surfaces are included in the 3D annotation and can be understood by manufacturing and inspection software, they are semantic references.

Since Application Protocol (AP) 242 of the STEP neutral file format supports both 3D annotations and semantic references, any software that can read STEP AP242 can take advantage of semantic references.

Recent Creo enhancements

PTC continues to improve functionality of 3D annotations in Creo to make it easier and more convenient. Recently, semantic references had added the ability to use surface collection tools.

In Creo, tools like Draft, Copy, Copy Geometry, and Publish Geometry often require the selection of multiple surfaces. Picking them one by one can be tedious and time consuming. Therefore, Creo has advanced selection tools to reduce the clicks necessary to pick multiple references. These include:

• Surfaces of all bodies: select all the surfaces from all solid geometry in the part.
• Seed and boundary: this method is like a champagne waterfall that you might see at a wedding. The champagne glasses are arranged in a pyramid. You pour champagne in the top glass (the seed), it overflows into the glasses below, until it reaches the tabletop (the boundary). The seed and boundary method is one of the most convenient methods for selecting a lot of surfaces in a model.
• Loop surfaces: pick all the surfaces around the edges of another surface.
• Intent surfaces: grab the surfaces associated with a feature like an Extrude, Revolve, Sweep, or Blend.
• Datum reference feature: collect a custom group of multiple surfaces, which are defined in this feature. Essentially the user defines the datum reference feature once and then can reuse this selection in other places, like annotations.

Similarly, certain 3D annotations require multiple surfaces as semantic references for manufacturing and inspection, like surface profile geometric tolerances, surface finishes, and even some symbols. These advanced surface collection methods can now be used to define sematic references. This recent enhancement makes 3D annotations easier than ever.