For some, the theoretical benefits of moving towards releasing Model-Based Definition rings true enough for them to make a change. A frequent adoption of MBD involves transitioning from fully annotated drawings to fully annotated models, essentially replicating the traditional effort of adding dimensions in order to convey the complete and detailed geometric form of a component. In this case, however, the 3D model is annotated with PMI where it can be used directly by software, rather than only read by a human.
This single deliverable fulfills all of the functions of engineering documentation. The following examples illustrate how it is used.
- Tooling Design: Tooling designers can develop molds, dies and casting tooling based on a model and its PMI without consulting a second definition of the design.
- Generating NC Toolpaths: Machinists can generate toolpaths directly from the model and the associated machine-readable PMI, which includes tolerancing. Speeds, feeds and tools can be determined using the model only.
The advantage of this method lies in the fact that there is a single and unambiguous definition that both conveys the complete and detailed geometric form of a component and defines the measures and thresholds within which a component meets quality standards. Theoretically, this method should address the flaws associated with releasing a 2D graphical drawing and a model. With only one deliverable, there is no divergence of the definition from which errors can emerge. Findings from this research report do, in fact, verify that is the case, as a higher percentage of organizations releasing only MBD deliverables cite reduced change orders (30% vs. 21%) and reduced scrap (35% vs. 15%) by wide comparative margins. The benefits, in this case, are strong for the company.
Table 2: Comparison of benefits realized across the Model-Reliant and Model-Based cohorts. Details on calculations of the estimate, assignment of respondents into cohorts and the drawing datasets are detailed on The ROI of MBD Study page.
The shortcoming using this method, however, lies in the effort invested in annotating the model as if it were a drawing. This effort essentially replicates the traditional effort of adding dimensions in order to convey the complete and detailed geometric form of a component when the 3D model already inherently does so through its geometry. As a result, one would expect no reduction in the amount of time it takes to annotate the 3D model compared to creating a 2D graphical drawing. Interestingly, however, findings from this research show it actually requires more time to replicate the effort, with estimates to develop the fully annotated model averaging 11.7 hours compared to 8.8 hours to create the fully annotated drawing dataset.
Figure 3: On the left is the average of the estimate to create the fully annotated drawing dataset by All Respondents cohort. On the right is the average of the estimate to create the fully annotated model dataset by the Model-Based cohort. Details on calculations of the estimate, assignment of respondents into cohorts and the drawing datasets are detailed on The ROI of MBD Study page.
Why would it take longer to fully annotate a 3D model than create a fully annotated drawing? The reasons are many. Developing a Model-Based Definition involves more complexity, including deciding upon how to orient dimensions and tolerances on presentation planes as well as grouping such annotations into sets that can be turned on and off. Furthermore, the additional time may also be the result of a transition between technologies, as there is a learning curve that must be navigated. Over time, however, improved proficiency in using the new technologies will reduce this as well.
Another important point to note is that creating fully annotated models represents a duplicative effort. The geometric representation of the model already conveys the complete and detailed geometric form of a component. Adding dimensions adds a different means of doing the same thing. The risk in this approach is that these two duplicative methods of conveying the complete and detailed geometric form of a component may fall out of synch as the dimensional values can be changed without a corresponding modification of the geometry. The only additional effort that is truly required is to define the measures and thresholds within which a component meets quality standards through tolerancing and other annotations.
Ultimately, transitioning from fully annotated drawings to fully annotated models yields benefits for the company overall, with fewer ECOs and scrap, but not for engineering, who must invest more time to create such deliverables.