Quality Control What is Model Based Definition?
Using model-based definition (MBD), 3D models become the source authority that form the basis for all processes in the product creation process. Find out how it works and what the advantages are.
Everyone is talking about digitalisation — and processes in design and development can benefit from digital technologies as well. One example is the traditional 2D drawings of models, which still often serve as the basic document for production. However, a CAD file contains much more than just geometry: in addition to design history and parameters or dimensions, it also contains metadata such as material, weight, centre of gravity and many other properties.
Media break between model and drawing
When creating a drawing, a 3D model is converted into two dimensions, which means that a lot of information is lost. In addition, the designers record a lot of information about the component and its production only on the drawing, such as materials, tolerances or other detailed information, parts lists, configurations and general notes. This information is therefore not available in the digital process, which is based on the 3D model.
The so-called model-based definition provides a solution: here, all product information is defined on the model itself. In this way, 3D models are equipped with all the data needed to define, manufacture and test a product. This model can be used downstream by suppliers as well as across organisations within a company.
The history of MBD
- MBD's roots go back to the introduction of 3D CAD systems, especially with parametric solid modellers like Pro/Engineer in 1989. The idea was to describe the product entirely in digital form. The 3D model is the basis for the 2D drawing, which contains further manufacturing information such as dimensions, descriptions and tolerances. At PTC, this is referred to as the "model-centric" approach with the drawing as the legal exchange format.
- In the mid-1990s, several experts met at Boeing in Wichita, Kansas, to propose a new methodology for capturing PMI directly in the CAD model: MBD. The technical standards for MBD are set out in ASME Y14.41-2019 and ISO 16792:2015.
- With the ISO GPS standard system, published starting in 2011, contradiction-free rules were introduced to describe geometric features completely and unambiguously. This is the basis for the unambiguous description of the product within the 3D model, i.e. without a drawing. This approach is called "model-based" at PTC. The company has been following this approach with PTC Creo since version 2.0.
What information is attached to the model?
Ideally, the information is no longer attached to the drawing as a derivative of the 3D model, but rather attached directly to the model. This includes all the geometric features such as size dimension, shape, direction, location or run in technical specifications. In addition, the model should contain so-called product manufacturing information (PMI). This refers to all manufacturing-related information that can be attached to a 3D model as an annotation. Examples are: Dimensioning tolerances, shape and position tolerances, surface information and other texts or annotations.
Why is MBD more efficient than the traditional method using 2D drawings?
3D annotations in a CAD model are semantic: they are machine-readable information - in contrast to human-readable information on a drawing. Other software can therefore understand them: If a native MBD part file or a CAD-neutral MBD step file is passed on to subsequent steps instead of a PDF or a printout of a drawing, the CAM or CMM software recognises the 3D annotations and their geometric references. This eliminates the need for translating the information from a drawing to downstream software, increasing efficiency and reducing errors.
According to a study by the National Institute of Standards and Technology (NIST), MBD can shorten the process from design to manufacturing to testing by 78.4 percent.
This benefits all processes downstream of the design. In particular, production with work preparation and NC programming. The greatest advantages can be seen when connecting the measuring machine for quality control to the finished component. Furthermore, there are advantages in the automation of planning and production processes - not only within the company, but also in the connection of supply chains.
Conclusion: What is the benefit of MDB?
MBD enables digital drawingless processes, as the drawing is no longer the primary source of information. Nevertheless, users can also create drawings if the supply chain requires it. Instead of a PDF, the primary work output of MBD is the technical data package (TDP), a set of electronic files that can contain:
- The model either in its native format or in the CAD-neutral Step-AP242 format,
- the parts list in Excel or CSV format, which can be used by other software and
- other associated electronic files such as notes, schematics and First Article Inspection (FAI) documents.
The main elements of MBD in a CAD model are:
- Annotation layers to define the placement and orientation of details.
- 3D annotations for product and manufacturing information.
- Combination states to clearly arrange and organise the 3D annotations. These also include exploded states, cross-sections, simplified representations, display states or style states.
Further information about MBD and ISO GPS and sources