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 Growing Use of Fabrication for Building Components
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.
Applications of Digital Fabrication
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:
- Structural steel framing
- Curtain wall elements
- Façade and building envelope features (for example, rain screens, shading features, and pre-cast panels)
- Mechanical systems and ductwork
- Piping assemblies
- Casework and furniture systems
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.
Advantages of Fabrication
Prefabrication and digital fabrication strategies typically offer many advantages compared to on-site piece-built approaches:
Fabrication of repetitive components typically brings economies of scale, enables optimized sourcing, and reduces waste and construction time.
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).
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.
Steps in the Digital Fabrication Process
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.
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.
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.
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.
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.
Best Practices when Modeling for Digital Fabrication
Plan for Precision and Tolerance Issues
Many materials have minor variations in thickness or stiffness. Design with reasonable tolerances in mind and plan for:
- Slop – allow for size variations (for example, oversize holes) at key locations to make assembly easier.
- Seams – allow for the imperfect interface between your digitally fabricated component and the rest of the world.
- Adjustment points – design in opportunities for on-the-fly adjustment during assembly.
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:
- Describe the advantages and limitations of digital fabrication strategies.
- Create assemblies and use assembly views to isolate production details for fabricated components.
- Use tools in the Autodesk Revit platform to encode machine-specific fabrication instructions.
- Describe a strategy for creating fabrication views to facilitate digital production of a custom building compoment or assembly.
Creating Assemblies and Assembly Views
In this exercise, students will learn how to:
- Use Revit assemblies to model prefabricated building elements.
- Create assemblies from groups of model elements and components.
- Create new assembly views.
- Add annotations, tags, and keynotes to assembly views.
- Work with sheets of assembly views.
Figure 7.4.1. Adding annotations, tags, and keynotes to assembly views
- Create a new Revit assembly for documenting the fabrication details of the stair and railing elements at the core of the building in the exercise dataset.
- Create another assembly containing all of the pieces of a custom window light shelf/sun shade design, which will also be fabricated.
- Add annotations, tags, and keynotes to the assembly views to document the pieces and fabrication details.
- Place the assembly views on sheets, as shown in Figure 7.4.2, and export DWF files containing all the sheets describing an assembly to be shared with the assembly fabricators.
Figure 7.4.2. Sheet containing the views and details needed to describe the fabrication of an assembly
Fabricating an Architectural Scale Model
In this exercise, you will learn how to:
- Add line types and styles to encode machine-specific fabrication instructions.
- Create fabrication views to isolate and display essential information for fabricating elements of the building model.
- Add instructions to fabrication views to create allowances and clearances for assembling the model pieces.
- Print or export fabrication views to create files of instructions for fabrication machines.
- Fabricate and assemble the scale model pieces.
Figure 7.4.3. Adjusting line types in fabrication views to indicate which lines are to be cut or scored
- Fabricate the pieces of an architectural scale model (at ½”=1’-0) for the Revit BIM model in the exercise dataset.
- Create fabrication views to isolate and create a 2D projection of each of the model elements.
- Adjust the line types in each view to indicate which lines are to be cut, scored, printed, or ignored.
- Add new lines to create the clearances and allowances needed for assembling the pieces.
- Produce the parts manually (by printing templates and hand cutting the pieces) or digitally (by sending the instructions to a laser cutter or similar device).
- Assemble the pieces of the scale model.
- Note any problems encountered and recommend changes for improving the fabrication and assembly process.
Figure 7.4.4. Sheet view with fabrication views ready for virtual printing
Fabricating Custom Building Components and Assemblies
In this exercise, you will learn how to:
- Convert models of custom building components and assemblies into machine-specific fabrication instructions.
- Use assembly views to isolate and display essential information for fabricating pieces of the assembly.
- Add instructions to fabrication views to create allowances and clearances for producing and joining the pieces.
- Export fabrication instructions to production machines.
Figure 7.4.5. Creating fabrication views for a custom component
- Create fabrication views for production of the custom shading component modeled in the exercise dataset using a 2D cutting process.
- Decompose the component into pieces that can be fabricated using 2D cutting.
- Add instructions for cutting the pieces to the fabrication views.
- Add tabs, and slots and connection features to the fabrication views, incorporating allowances for production tolerances and easy assembly.
- Export the fabrication instructions for digital production and assembly.
- Use 3D printing to create a scale model of the custom shading component.
- Prepare the model for 3D printing using STL.
- Use the Revit STL Exporter to create instructions for 3D printing with an STL device.
Figure 7.4.6. Example of a scale model produced by 3D printing
- What are the advantages of using assemblies versus groups?
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.
- What types of Revit elements cannot be included in assemblies?
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.
- What are some of the advantages of prefabricating building components?
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.
- What types of building elements are typically fabricated?
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.
- How are fabrication instructions for model elements typically encoded?
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.
- What kinds of model views are used to isolate parts for fabrication?
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.
- What types of elements are most difficult to fabricate?
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.
- What adjustments are needed when using 3D printing to create an architectural scale model?
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.
A category of Revit elements that supports construction workflows by letting you identify, classify, quantify, and document unique element combinations in the model. You can combine any number of model elements to create an assembly, which can then be edited, tagged, scheduled, and filtered.
Translation of a digital design into a physical object. The digital design is used to create a physical object from materials such as cardstock, foam, clay, resin, or metal.
Computer Numerical Control (CNC)
The automation of machine tools that are operated by abstractly programmed commands encoded on digital media (as opposed to manually controlled via handwheels or levers, or mechanically automated via cams alone). In modern CNC systems, modeling software tools produce a computer file that is interpreted to extract the commands needed to operate a particular machine via a postprocessor, and then loaded into the CNC machines for production.
Transforming a design model that captures design intent into a fabrication model by adding detailed information to facilitate machine production.
Creating views in a model to isolate the information needed for production. The views needed and information needed depend upon the machine requirements of the fabrication process to be used.
An additive manufacturing technology for producing models, prototypes, patterns, and in some cases, production parts. It uses a vat of liquid UV-curable photopolymer resin and a UV laser to build parts a layer at a time. The laser beam traces a part cross-section pattern on the surface of the liquid resin, and then exposure to the UV laser light cures the resin and solidifies the pattern traced, adhering it to the layer below.
Using manufactured or fabricated components, modules, or transportable sections to facilitate the production of buildings. These fabricated components are typically manufactured off-site under precise, controlled shop conditions and transported to the site for quick assembly and erection.