Using BIM for Fabrication

In this lesson, students explore how BIM models can be used to support prefabrication strategies and enable digital fabrication of custom building components and assemblies.

Students will learn how to use Revit features, such as assemblies and assembly views, to isolate production details for fabricated components. They will also learn how to use tools in the Autodesk Revit platform to create fabrication views and encode machine-specific fabrication instructions, use BIM models to create architectural scale models, and facilitate digital production of custom building components and assemblies.

The integrated design and fabrication of building components is becoming more prevalent in architecture, challenging our notions of what is possible and expanding our understanding of how project information is created and consumed. Offsite fabrication of building components is becoming increasingly common and driving the need to apply advanced processes that have traditionally served the manufacturing industry, such as digital prototyping, to construction.

The use of BIM models to support digital prototyping is a natural fit. Using BIM models, project teams can experience a project digitally before it's built, simulate performance and constructability, and communicate and interpret design intent. The information contained in these models can also be used to create instructions for digitally fabricating building elements, and this enables cost-effective production of design-intensive custom components.

A digital design-to-fabrication workflow creates opportunities for improved collaboration among architects, fabricators and builders. Using coordinated data within a fully informed building information model can improve the constructability, reduce the cost, and enable design innovations for creating unique and repetitive building components.

BIM models are increasing being used to facilitate a variety of related building activities, including digital fabrication of building components. The use of BIM enables digital design-to-fabrication workflows for many creating many types of building elements, including:

Digital fabrication can be used during many phases of the project lifecycle, supporting both design activities (3D printing of scale models of design options) and production tasks(creating actual building elements).

While BIM-based digital fabrication is just starting to gain traction in construction, it has the potential to bring productivity gains and advantages seen in the manufacturing sector to the building industry.

Prefabrication and digital fabrication strategies typically offer many advantages compared to on-site piece-built approaches:

Cost Savings

Fabrication of repetitive components typically brings economies of scale, enables optimized sourcing, and reduces waste and construction time.

Schedule Reductions

Fabrication of components off-site reduces on-site interferences and location availability bottlenecks. Components can be manufactured in advance and delivered just-in-time, reducing lead times and enabling quicker erection and placment.

Improved Quality and Control

By using information extracted directly from the project model, digital fabrication reduces errors resulting from miscommunication of misinterpretation of design intent. The quality of fabricated components produced in controlled environments and using machine tolerances is typically better than elements constructed on-site.

Better Coordination and Clash Detection

Digital fabrication model can also be used by the project team for 4D modeling and clash detection with other building disciplines and models (such as MEP and architectural).

Informed Design

Using design models for digital fabrication creates a natural feedback loop between fabricators and designers that brings fabrication considerations forward to inform design decisions as alternatives are being evaluated.

Computer-automated fabrication tools, often called CNC (Computer Numeric Control) machines, use a variety of construction methods—some cut (for example, lasercutters and waterjets), some carve (for example, mills and routers), and some build up (for example, 3D printers). And any of a combination of these methods can be used to fabricate the pieces of a design.

Digital fabrication typically involves these steps:

Inspiration, Tempered by Practical Considerations

Digital fabrication enables great creativity and inspired design through mass customization (every part can be customized, because the cost of producing custom components is reduced) and mass production (custom parts can be produced, because it is inexpensive to produce lots of them).

When designing for digital fabrication enables many possibilities, it is important to consider the practical limits of the materials and the machines to be used. Most materials come in standard size sheets, and exceeding those sizes can be cost prohibitive.  Similarly, overly complex designs may exceed the capabilities of the machine to be used for fabrication.

Configuration

Configuring is the process of determining the individual elements and parts that are needed to build the fabricated component. This step typically starts with building a design model that represents the design intent. These design models are often placed in project models as placeholders for more-detailed fabrication models that will be created later.

Rationalization

Transforming a design model into a fabrication model requires adding a lot more detailed information that factors in the limitations of the fabrication machine. Rationalizing a design model is similar to adding detail to a BIM model as it moves from the design development to construction document phase. The assembly and connection details must be worked out, the interaction between the parts must be designed, and the model may need to be adapted.

Isolation

Transferring a model to a system that can fabricate it requires isolating the information that is needed for production. For example, if all of the parts will be produced using a 2D process, views must be set up to isolate the 2D profiles, so they sent to the CNC machine.

In many cases, you can work within Revit and use section views to isolate and create 2D projections of the parts, making sure to crop the view and use visibility graphics overrides to display only the needed information.

These isolated views can be exported as DXF or DWG drawings (a format that is used by many CNC, laser, and waterjet cutting devices) or printed directly to a virtual printer driver that control a fabrication device.

Fabrication

The final step is the actual fabrication of the parts. This often involves set up and coordination with the machine or service that you will be using for fabrication to work out the machine-specific instructions or required file formats.

Some specialized forms of fabrication, such as automated cold-rolled framing machines, HVAC duct sheet metal unfolding, and structural curtainwall fabrication have suites of dedicated software tools for transforming Revit exports into production jobs.

Plan for Precision and Tolerance Issues

Many materials have minor variations in thickness or stiffness. Design with reasonable tolerances in mind and plan for:

Use an Appropriate Level of Detail in Design and Fabrication Models

Think explicitly about the level of detail needed at every stage in the design and fabrication process and model accordingly.  One recommended strategy is to meet with the fabricators to understand their processes and get answers to questions about how the model will be used.

Anticipate Compatibility and Translation Issues

Moving data between software tools often introduces compatibility issues and errors that must be fixed. Plan and fully test the complete chain and flow of data between all of the software tools in the planned production process, and then prototype and test to find and resolve the unanticipated issues.

Don’t Overestimate the Advantages of Digital Fabrication

While digital fabrication offers incredible design flexibility and can time and money, don’t automatically assume that it will be cheaper or better. Building elements that can be easily and cheaply produced in conventional ways using standard members need not be digitally fabricated. Digital fabrication typically offers the greatest advantages when every part needs to be unique or when the parts are too complex to produce using other methods.

Don’t Underestimate the Time/Cost of Creating the Fabrication Details

Creating a design model is merely the first step in a complex process. The digital fabrication details required vary with the specifics of the process and machine planned for use in fabrication.

Carefully consider that actual construction sequence and assembly order to avoid creating designs that look great on the screen, but are impossible to build.

After completing this lesson, students will be able to:

Assemblies and groups are alike in many ways. When elements are added to either assemblies or groups, your work with and modify them using similar techniques. Two essential differences are: assemblies enable you to isolate and create a set of views specific to the details of that assembly; assemblies can be scheduled, whereas groups cannot.

Some examples of elements that cannot be included are: annotations and detail items; complex structures (trusses, beam systems, curtain systems, curtain walls, stacked walls); elements in different design options; groups; imports; linked elements; masses; MEP-specific elements (ducts, pipes, conduits, cable trays and fittings, HVAC zones); model lines; rooms; and structural loads.

Prefabrication enables constructors to efficiently build components off-site, then install the fabricated assemblies quickly. The advantages of prefabrication are multi-fold.  Components can be precisely built in a controlled work environment, insulated from on-site weather concerns and work area interferences. These prefabricated components are often built to tighter tolerances with less material waste. And the fabrication operation can be located in an area that optimizes the labor and material costs for production. On-site operations can also be significantly improved. Managing and installing the prefabricated assemblies simplifies and speeds up the construction process, which in turn frees up work areas and enables the overall workflow on site to proceed more smoothly. One of the few caveats is that working with prefabricated assemblies typically requires greater planning and coordination to ensure that the parts can be easily placed (without interferences) as assembled units.

Digital fabrication is rapidly expanding to a greater variety of building elements. Traditionally, this approach has been used for elements that required piece-by-piece customization—such as structural steel framing—and for assemblies of parts that would be more efficient to create off-site and install as integrated units—for example, mechanical system ductwork or piping assemblies.  The flexibility that digital fabrication affords is now being applied to realize very creative designs for curtain wall elements, building facade and envelope features, wall and ceiling panel systems, furniture, and interior design elements.

Fabrication instructions are often encoded by applying specific line colors, weights, and styles to the boundaries of fabricated pieces in 2D views. These views are used to generate instructions that drive the fabrication machines. For example, some laser cutting systems use a color-based scheme to convey instructions. A red line can indicate edges to be cut, while a blue line can indicate lines to be scored on the surface of the material.

The type of model views needed depends on the fabrication process to be used. For example, for a 2D process such as laser or water-jet cutting, 2D views are needed to isolate each part to be cut. Plan and elevation views are typically used to create 2D views of surfaces perpendicular to the line of sight. In these views, the linework tool can be used to apply specific line styles to edges that should be cut or scored. You can also use the line style or visibility graphics overrides to hide elements that won't be used in the fabrication. Oblique elements create more of a challenge. But Section views can typically used to create a 2D view that is perpendicular to any surface.

The degree of difficulty when digital fabricating elements depends on how closely the machines capabilities match the shape that you are trying to fabricate. For example 2D fabrication techniques, such as laser or water-jet cutting, work very well for fabricating planar surfaces. But, fabricating curved shapes—such as cylinders or spheres—with these techniques can be very challenging, because the shape must be flattened or unrolled. 3D printing techniques, such as STL, can easily fabricate most any shape. But, they limit the scale of the objects that you can create.

The biggest adjustment that is typically needed is to adjust the thickness of the elements that will be included in the 3D print job.  Elements that are quite thin at full-scale—for example, the walls of the aluminum tubing in curtain system mullions—may be too thin to be modeled at small scales, given the limitations of the 3D printing technology. For these elements, you can thicken the walls without changing the external size to enable them to be included in your 3D scale model.