Designing Process | Making the Wawona Sculpture
The Wawona sculpture, which is currently installed at the Museum of History and Industry (MOHAI) in Seattle, was a work heavily derived from the invention of process. The structural design work of ARUP was as unique as the process used by Studio Fifty50 to design the geometrically complex components. Through innovative design, and collaboration among designers and fabricators, a process was developed to meet the challenge of creating the necessary components. This article walks through the intricacies of design, fabrication, and assembly that make this work unique.
PHYSICAL TO DIGITAL
The digital design process began by digitizing a scale wooden model provided by the artist. Using a digitizing arm, in conjunction with Rhinoceros 5 modeling software (Rhino), the geometry of the physical model was translated by capturing points on the model’s surface (image 1). These points were then used to generate section curves, which were lofted to create the initial 3-dimensional model. This method of information capture proved highly effective for maintaining the artist’s vision, while providing starting geometry for digital design and structural analysis.
Knowing that the geometry would continue to evolve throughout design, parametric modeling was employed to streamline the digital process. Initial geometric refinement was dependent on several key parameters, including sculpture weight, center of gravity, and surface curvature. (These parameters were evaluated using Grasshopper, a generative design plug-in for Rhino.) For example, an initial study of weight distribution in the sculpture shed light on two issues. First, the top hang point of the sculpture fell outside the envelope of the geometry, and second, due to its own center of gravity, the inner geometry wanted to hinge open generating additional stress on the structural system. Because the geometry was tied to a parametric definition, the center of gravity of both the inside and outside geometry, as well as the composite hang point could be updated in real-time to correct each issue. Ultimately, the geometry of the sculpture was modified, pulling the inner form outward and extending a ridge three tiers higher along the flat face for stiffening.
Once the overall geometry for the sculpture had been solidified, it was broken into individual panels for fabrication. The panelization process began by dividing the geometry into nine tiers. Tier heights were governed by fabrication and material constraints as well as the desire for tier heights to decrease relative to their position from the ground. For example, the machine envelope of the 3-axis CNC router used to manufacturing the sculpture’s components defined the largest panel that could be milled, and thus set the tallest tier height. Material quantity and size also provided defining criteria for panelization. The wood stock was initially assessed and documented. This information was imported into Grasshopper and compared to several different tier height variations to determine what strategy would yield the most efficient use of the limited stock. Once the heights were set, sections were taken through the geometry to define the top and bottom of each tier. These section curves became the foundation for all other parametric definitions (image2).
Section curves that defined all model components.
Although the section curves established the underlying geometric relationships of the sculpture, structural requirements established the panelization of each tier. Hanger rod spacing, mechanical fastening patterns, and the panel fastening strategy provided rules that became adjustable parameters. The ability to adjust these parameters in a real-time saved hundreds of hours in the digital design process, as they defined more than 200 complexly shaped wood panels, 87 hanger rods, 24 collars, 261 bolt locations and clearance pockets, and 724 panel fasteners.
The initial material assessment provided a rough estimate of material consumption; however, material efficiency was evaluated further for individual board selection. By running a custom “best fit” script in Grasshopper, the minimum volume required for each panel, while held in its proper orientation for machining (top and bottom surfaces vertical), was determined. The optimal stock dimensions were given to the artist, and stock selection was then made based on optimal dimensions and aesthetic quality. Due to the dynamic curvature of the inner tiers (image 3), a second material strategy was employed to aid in efficiency. Boards in this area were sliced into as many as three individual pieces, machined, and glued back together in the dynamic form (image 4). This strategy decreased material use over a monolithic stock part by more than 60 percent.
Twisting geometry of inner tiers. | Splayed boards for twisting form.
DIGITAL BACK TO PHYSICAL
The required accuracy of the hundreds of custom parts necessitated the use of digital manufacturing. The wood fabrication process was designed to accommodate the use of a 3-axis CNC router instead of a 5-axis machine, which is more commonly used for complex geometry online casino machining. Although limiting at times, the selection of the 3-axis machine allowed the artist and the fabrication process to happen within a few miles of one another. This allowed constant engagement between artist and fabricator, and enabled material selection efficiencies to occur that would be impossible if working with a remote fabricator.
Since the machining process was limited to 3-axis movement, undercuts of varying degrees were not possible. Due to this constraint, a 4th axis was designed which allowed for rapid CNC driven adjustment of part support making it possible to lift each board to an exact angle (image 5). Once located at this angle, the top and bottom connection channels could be cut to their exact dimensions. When each panel was completed 5 of the 6 sides had been machined while the 6th side was profiled and scribed to indicate the final surface depth.
The wood components were machined in three sets of operations. In the first, each board was placed on the router bed and two flat datums were cut to allow the board to be securely held in the subsequent operations. This portion of the process also verified that any twist or warp in the reclaimed material had been accurately accounted for in the initial measuring. Once leveled, the 4th axis and base support were installed on the router bed. As mentioned earlier, the custom 4th axis lifted the board to an exact angle for machining, while the base support indexed the bottom of the board. The base support also provided additional vertical adjustment if router gantry clearance became an issue.
Four areas of the panels were machined while supported on the lifting bar. First, the inside face of the panel was roughed to within 0.0625” of the final surface geometry (image 6). This surface was then smoothed by hand rather than machined, since the material was soft and could be shaped more quickly with a skilled hand and an angle grinder. Next, the top and bottom 3 inches of the panel was defined and the channels in which the steel collars would eventually connect were milled (image 7). Lastly, the clearance holes, for the threaded stud and accompanying nut, were pocketed into the bottom of each panel where necessary (image 8). Once these operations were complete the panel was removed from the router bed and any excess edge material was harvested using a band saw. While relatively small portions were removed in this manner, the material yielded was later used where certain board required additional width or thickness.
The final set of CNC machining operations included machining the tapering edges of the panels and scribing the outside surface edges and profiles. Because the edges of the boards were not parallel to one another, and due to the use of a 3-axis router, the edges of each board had to be cut while supported on custom laser cut templates (image 9,10). More than 800 custom templates were generated using a custom Grasshopper script. This script not only generated the laser cut template profiles and labels but also generated optimized toolpaths and g-code for the edge machining. Once both sides had been trimmed to their final geometry, a profile that denoted the final panel thickness was machined on the exterior face of the board, followed by two or three randomly placed surface scribes indicating the final 1.5 inch board thickness. Additional material was left on each panel to provide the artist with additional stock material to use for shaping the panel face.
As mentioned previously the steel components of the sculpture were also generated and coordinated using parametric modeling. Certain components, such as the hanger rods, were simply defined and only required a length to be generated by the parametric definition. While other components, like the steel collars, contained significantly more information for post-production fabrication.
The ¾ inch and 1 inch steel collars were CNC waterjet cut, allowing very tight tolerances to be achieved while minimizing any negative impact the cutting process might have on the final part (image 11). Once cut, hanger rod attachments to each collar required a compound angle hole to be drilled and tapped in the steel collar at each hanger rod location. This was manually done using a Bridgeport mill, as the process of realigning the parts on a large 5-axis CNC mill proved to be cost prohibitive. The waterjet accurately scribed all the information required for the manual drilling and tapping process into the surface of the steel collar without compromising the integrity of the part
The information scribed at each hole location indicated; the angle at which the hole was to be drilled, the hanger rod vector projected to the collar surface providing angular alignment, and the point at which the angled hanger rod intersected the top surface of the collar (image 12).
Once the parts had been cut and marked by the CNC waterjet the post-process operations could take place. First, the milling head was set to the angle indicated on the collar. Next, to properly align the steel collar on the table a transfer punch was used to trace the hanger rod vector line scribed on the part. Once alignment was achieved, the part was clamped down and the table was locked in place. The punch was removed and replaced with a carbide end mill, which was used to create a flat surface to ensure that the drilling operation could be done without the bit drifting from alignment. Next, the hole was drilled at the correct angle with a pilot point drill bit. Finally, a gun tap was used to cut the final threads for the hanger rod connection (image 13). This process was then repeated for each of the 86 remaining hanger rod holes.Compound angle hanger rod hole being tapped.
Due to the size of the sculpture the components could only be assembled once they had been moved into the main hall at MOHAI. Since the sculpture had to be built from the top down, a scaffolding tower was erected around a platform that was used to lift each tier into place (image 14). The wood panels, steel collars, and hanger rods were then assembled into their final form and staged to be lifted (image 15). As each tier was lifted into place the top collar of the lifted tier was fastened to the bottom of the hanging artwork (image 16,17). The platform was then lowered to accept the next assembly. This process was repeated nine times until the last tier was suspended (image 18). Because of precise planning and digitally controlled machining, the 1000 components that make up the tiers were able to be quickly assembled and installed.
Written by Jeff Hudak, Part-time UW Lecturer / Partner, Studio Fifty50
Artist: John Grade
Digital Design / Digital Fabrication: Studio Fifty50
Consulting engineers (Sculpture): ARUP
Consulting engineers (Building): CPL
Building architects: LMN
Lighting design: Candela
Steel Fabricator: Pegasus NW
Wood Fabrication: UW BE Fab
Images 1-18: Studio Fifty50
Images Cover image, 19, 20: John Grade