Model Based Definition: What is it and Why?

Model Based Definition (sometimes Drafting or Dimensioning or Design) is an iteration to mechanical design that uses a CAD generated 3D model with dimensions, GD&T information, annotations, and other design/fabrication information.

Erin Platvoet

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Article Summary

The 3D model with the embedded information becomes the defining document for the design and is the controlling document for communicating the design internally, to customers, and to fabricators. Shops read and measure directly from the 3D model as opposed to 2D engineering drawings. The MBD method has the potential to accelerate and sharpen manufacturing procedures from design to fabrication and quality control. MBD is slowly replacing other methods of mechanical design in some industries, but it is still rather unfamiliar within many industries. The purpose of this blog is to shed some light on MBD, discuss how it compares to more common design methodologies, and conclude by analysing whether it has a place in the fired heater and combustion equipment industries.

What does MBD replace?
As most in the engineering, design, and manufacturing world know, the status quo for communicating mechanical designs relies on 2D engineering drawings which can be viewed on paper or digitally. Engineering drawings are a graphic universal language, playing  a fundamental role in human progress from ancient pyramids and the Palace of Knossos to today’s geodesic domes and the space station. The fundamental purpose of an engineering drawing is to carry, control, and maintain a product’s definition in a precise and clear way with minimal risk of misinterpretation or faulting assumptions. 2D engineering drawings are usually produced by two means: 1. Direct interpretation on a 2D space (paper or CAD) and 2. As technology allowed, 2D views as projected (by a CAD program) from a CAD produced 3D model with little to no human interpretation of the views created.

Since the 1980s, the advent of solid modelling and specialised drafting software packages have contributed significantly to streamlining the production of engineering drawings. The solid model defines the product’s geometry, and the engineering drawing consists of only projections of that defined geometry for 2D viewing.

Despite its increased precision, there is a redundancy in effort to produce engineering drawings when they are created as a byproduct of a solid model, since these drawings only echo the information already defined in the solid model. Dimensions, material, and sometimes dimensional tolerances are built into the solid model. Assembly, subassembly, and weldment intent is often built into the solid model by means of nested assemblies and parts. If done correctly, the nesting (along with properties assigned to every part/assembly) results in a prebuilt BOM. Much of this information was assigned to the 3D model during creation, but it must be extracted later in a functional view within the 2D drawing. Most of the effort spent producing a 2D drawing from a 3D model is verifying that the embedded design information of the 3D model is clearly represented on a 2D drawing and configuring the drawing to have the expected aesthetic look.

However, not all of the time spent on the 2D drawing is redundant. The true complexity exists in the fact that most 3D models do not hold all the necessary information. Surface conditions, like those addressed in ASME Y14.5 GD&T sections, are not as easy to embed in the solid model. This also applies to heat treatment, weld notes, pressure testing properties, general notes, and many other properties that are considered during the design of equipment but are not stored directly in the model. These are often added to the 2D engineering drawing.

Room for improvement in the solid model to engineering drawing process is quite apparent. To be fair, there are solutions and workarounds that address some of the shortcomings. For example, it is typical to pull any dimension twice.  First, during creation of the solid model and second,  when a (ideally driven) dimension is created on the engineering drawing. To combat this, some software allows for “smart view” creation and auto-dimensioning for the 2D engineering drawing, thus  reducing that specific redundancy. Unfortunately, these tools often need a lot of massaging to work smoothly across similar geometries. Engineering and drafting departments often spend a lot of time creating drawing macros, customising properties within software tools, and creating complex rules in PDM systems to reduce time spent on each drawing.

For the sake of argument, let’s assume the 3D modelling process is usually more robust than total 2D design. There are still some great tools out there for 2D design --- people with years of experience, and specific applications and designs would argue that point --- but in a general sense the statement has proven itself true. Why shouldn’t we consider foregoing all 2D related design and drafting and find a way to better illustrate the defining properties that went into the creation of 3D models? It seems like the next logical step.

This is where several software companies have tried to enhance the solid modelling process to eliminate the need for 2D engineering drawings. This led to Model Based Definition. Going forward, these are the important questions:
- Is it really better than the status quo?
- What are the limitations or barriers for implementation?
- Does it work in most industries? What about the fired heater industry?
- What’s next?

What are the benefits of MBD?
Obviously switching to MBD requires a full commitment to 3D modelling. General benefits of 3D modelling include:
- Ability to visualise  clearances and interferences
- Enables parametric modelling and interactive assemblies
- Reduces a significant amount of room for misinterpretation of the design definition.
     1. Parts and assemblies within the 3D model are either accurate or not
     2. Orthogonal, isometric, and section views are exact either way
- Photorealistic renderings for marketing purposes can be created from models (Figure 2)
- Design considering specific machining equipment, tooling, and operations using CAM embedded features
- Excellent tools for difficult design processes like pipe routing, structural steel framing, and sheet metal bending

Obviously, the benefits of 3D modelling are limited regarding communicating the design to the client or fabricator. This is where the benefits of MBD begin to shine:
Going paperless! No need for shop drawings means no bulky filing cabinets or racks for storing drawings. Understandably this can be done in the 2D CAD world as well with digital pdfs, but that is a choice that is not always strictly followed. For MBD not-printing is a Hobson’s choice in that there is not anything to print.

Revision control is simplified in that only one document (or document set for assemblies) needs to be revised. There is no chance of making changes to a model/assembly and not producing the revised drawing.

Fewer documents means a simpler PDM system. One study1 showed a 25%-30% reduction in file sizes when switching from a drawing and non-annotated 3D model to a MBD 3D model.

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