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How to CNC machine a Giant Statue

Giant statue of a big blue bear at the Denver convention center

On June 23rd 2005, Kreysler and Assoc, based out of American Canyon, California, installed a new piece of public art: the "Big Blue Bear". The parts have been created using a 3-axis CNC milling machine, heavily utilizing the DeskProto CAM software.

The work was designed by artist Lawrence Argent and commissioned by the city of Denver, CO to interact with the newly constructed Colorado Convention Center.

Computer-design of the sculpture
Scale model in EPS foam
Left a computer generated rendering of the design, right the 40" (1 m) high construction model.

Argent began the immense project with a small maquette, very different than the 42 ft (12.2 m) finished piece which he watched materialize in Denver with the help of four semi-trucks and a large crane.

The original form was first scanned using a Cyberware 3D scanner and then animated into position using Newtek Lightwave. The model was also decimated down at this stage to produce a polygonal surface texture. FDM models where produced at equivalent scale to the architectural model of the building it would adorn which assisted the artist in securing the commission.

Rhino, segment numbering
Rhino, segments in 3D
Construction of the 94 different segments in Rhino CAD.

It was at this stage that Kreysler and Assoc took over the fabrication of the bear from the mesh data.
Their first step was producing a 40-in tall construction model in Urethane foam, to be used for construction planning. The tools used were Rhino, DeskProto and Kreysler's custom 10-ft long 4-axis cnc mill.

DeskProto, toolpaths
DeskProto, simulation
Left DeskProto toolpaths for one segment, right DeskProto simulation of the mold.

In order to meet the target budget of $450,000 a technique of direct milling unique molds in EPS foam was chosen. Kreysler then segmented the model into over 90 parts which were machinable within the envelop of their 30-ft long custom built CNC gantry. Mold geometry and part flanges were constructed using Rhino which generated stl files for use in DeskProto.

Machining a mold
The resulting mold
Left Kreysler's 30 ft long CNC milling machine, right one of the molds in EPS foam.

Toolpaths were generated exclusively using Deskproto. After milling the molds were sprayed with gel-coat and hand layed with chopped strand mat. They were finally sprayed with a top-coat of pigmented polymer concrete and finally with urethane clear-coat to combat graffiti.

Gel-coat applied
Polyester part in the mold
Left: the blue gel-coat has been applied on the mold. Right: the part has been created in polyester by hand layup.

Keeping track of all the parts and relaying this information to the technicians was aided using a point numbering system to track all vertices and a unique identifier for each part. These identifiers were transcribed onto the construction model to visually assist shop technicians who laminated each part to its neighbors.

Two polyester parts
The assembled upper body of the bear
Left two of the total 94 parts, right one of the five sub- assemblies, before coating.

The parts were then laminated together into five large pieces which could be transported via truck to Denver. Once there they would be fully assembled for the first time.

The artist was concerned about the appearance of the seams in the five components, so seam locations were carefully chosen to occur on flat, non angled, surfaces, to ensure a solid butt joint without any gaps. An interior structure of steel, which extends a third of its height, served to strengthen the structure and tie the components together as well as to its concrete anchor.

The resulting bear statue
Assembly of the bear's head
Left the bear, looking into the building, right assembly on location of the final part.

The competed piece arrived in Denver and was installed in just 2 days. Total weight is roughly 10,000 pounds. The name of the sculpture is "I See What You Mean".
You can find more facts, reviews and great pictures at the Kreysler website.

Bear and building
The Bear in front of the Convention Center.

Local newspapers have commented: "You can't miss one of downtown's newest public sculptures: It's blue, it's gigantic and it's a bear."
The new sculpture has received by many positive comments, and is predicted to become a symbol of Denver.