Lesson 4: 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 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 approache

    • 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 placement.

    • 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.

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.

    • 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.

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.

Additional Resources

Online

Publications

    • Eastman, C., Teicholz, P., Sacks, R., and Liston, K.(2008)
      BIM Handbook: A Guide to Building Information Modeling for Owners, Managers, Designers, Engineers and Contractors
    • Papanikolaou, D. (2008)
      Digital Fabrication Production System Theory: Towards an Integrated Environment for Design and Production of Assemblies
      Cuba, p. 484-488.
    • Sass, L. (2007)
      Synthesis of design production with integrated digital fabrication
      Automation in Construction, Vol. 16, No. 3, p. 298–310.
    • Sass, L., and Botha, M. (2006)
      Instant House: A model of design production with digital fabrication
      International Journal of Architectural Computing, Vol. 4, No. 4, p.109–123.
    • Sass, L.,  and Oxman, R. (2005)
      Materializing design: The implications of rapid prototyping in digital design
      Design Studies, Vol. 27, No. 3, p. 325–355.
    • Seely, J. C. (2004)
      Digital Fabrication in the Architectural Design Process
      Master Thesis, Massachusetts Institute of Technology.

WikiHelp

    • Assemblies

     o Creating Assemblies

     o Editing Assemblies

     o Disassembling Assemblies

     o Deleting Assemblies

     o Assembly Type Properties

     o Creating Assembly Views and Sheets

     o Assembly Instance Properties

    • Line Styles

     o Creating a Line Style

     o Deleting a Line Style

     o Modifying Line Styles in the Family Editor

    • Autodesk Inventor

     o 3D Print Service

    • Vasari

     o Exporting to STL

 

Objectives

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.

Key Terms

 

Key Term

Definition

Assemblies

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.

Digital Fabrication 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.

Rationalization Transforming a design model that captures design intent into a fabrication model by adding detailed information to facilitate machine production.
Isolation

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.

Stereolithography (STL)

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.

Building Fabrication 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.

BIM Curriculum

Learn how to fluidly design, communicate, analyze and plan using BIM models in an IPD framework. Use Autodesk Revit 2014 to access the functionality of all the Revit disciplines in one interface.  

Start here »

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