FRAC Exhibition Models - Fabrication Details

I, along with my colleague Bob Ongaro, completed the fabrication of some 3D printed models for the FRAC exhibition in France. These were designed by Dave Pigram of supermanoeuvre. The models are housed in tight fitting clear acrylic cases. Here are the models on display in France:

This is an image of the parts in their cases during a test fit - construction paper still present on the acrylic.These cases are about 8"x8"x20": 

Fabrication of the Cases

The cases were made of 3/16" thick acrylic. I used the extruded sheets. I've since learned it's generally easier to work with cast sheets. See this page for details.

The parts were cut on the laser cutters at Tabuman College. There are three of them which during the Fall and Winter semesters see nearly constant use. I experimented a bit but went with a single, slow, high power pass to cut them out.

Here are all the laser cut pieces.

Here are the basic supplies for attaching the parts. The most important is Methylene chloride and diacetone alcohol. It's a horrible, cancer causing, nasty liquid. But it works great! This isn't a glue. It essentially dissolves and then welds the parts together at the molecular level.

Gloves, eye protection, a needle tipped applicator bottle, and a MAP gas torch are also needed.

To accurately mill these edges I used a straight cutting bit on the router table.

The fence on my router can be set very accurately, to 1/1000th of an inch.

By using a feeler gauge against the in-feed fence  it's possible to set the offset between the fences very accurately. In this way were were able to mill off 0.005" per pass. Then using the digital calipers I could creep up on the exact size piece I needed.

We used the jointer table and fence, which can be set very accurately to 90 degrees, to glue the edges. The needle tip applicator rides along the inside corner as capillary action pulls the solvent into the joint.

The two pieces are butted up against a square to ensure they are perfectly aligned.

Fabrication of the Printed Models

The parts were printed at the University of Michigan 3D Lab. A ZCorp 510 printer was used. This printer is 12 years old, which is ancient in computer terms, but it still works well. Here a part is being excavated from the powder:

Here's the first part. It should measure 7.87" on the edge. It does! The printer is pretty accurate in XY but less accurate in Z.This introduced some problems.

 Here are the four parts printed, but not yet hardened.

The parts are drizzled in Cyanoacrylate glue (same stuff as Krazy Glue). This huge bottle, almost all of which was used, is about $90 US. But necessary because these parts are fragile!

Sanding was required to get the planar surfaces free of debris.

The inaccuracies of 3D printing made the fit poor enough that the seams were not perfectly flush. You can see that in the gap between parts below.

Assembly

Originally the parts were planned to be glued together, two per case. During fabrication the design was changed so the parts are separated by acrylic panels. This nicely hides the imperfections in fit shown above. 

For assembly, the case is laid on its side, paper is put in along the bottom, and the first part is slid in. Next is the acrylic sheet between them. Then the lower part is slid in. Next the case is tipped on its side and the paper can be slid out. Then the bottom is slipped on. 

Result

Here are the parts on display in France:

Here you can see the alignment between layers. Also visible is the rubber tubing on the edge of the support fin. 

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Pedestal Fabrication

I did some fabrication work for artist Lily Cox-Richards on some sculpture pedestals. Here's a summary of these (and earlier) pieces:

The Stand consists of carved plasters depicting tree stumps, wheat sheaves, and massive quartz crystals, all props that were once used structurally and allegorically in American Neo-Classical figure sculpture. The works in The Stand are all based on marble sculptures by Hiram Powers (1805-1873), once known as The Father of American Sculpture, whose works depicted idealized female figures that symbolized allegorical themes. By shifting the focus to the supporting elements and the contact points, Cox-Richard hopes to show a different allegory: I aim to create a new whole, not a fragment or ruin. In condensing these sculptures down to their supports, figure and ground conflate into new forms, revealing latent content.

Greek Slave by Hiram Powers

Here's a link to a video where Lily discusses a bit of her work.

These were modeled on the computer,  CNC cut from high density Styrofoam, then covered in a thin layer of plaster.

Here are a few of the CAD renderings, modeled in Rhino. They are all surface of revolution or one rail sweeps:

In order to get the plaster onto the pedestals we needed some templates that were 1/4" offset from the foam surface. Each template has two parts that ride against the foam. In between is the gap for the plaster.
They are cut on a old but functional laser cutter in the Taubman College woodshop. The kerf on this tool is 0.004".

The templates are made of 0.18" thick acrylic.

The foam used is 3#, C bead. That's the densest we could get. We ordered it from Arvron in Grand Rapids, Michigan. The foam is held in place using vacuum pressure from the table below.

Let the cutting begin. All the roughing is done from above. The bit is 1" diameter spherical tip that's about 10" long.

Two finish passes are done. Each at 45 degrees, one from each side.

Um, ah, errr... lots-o-stuff to clean up. That blizzard is after 3 pieces have been cut.

Here are three pieces, still covered in dust. All the templates are visible on the table.

Here's a YouTube video of the Cutting Process:

Here's a close-up of a template. A "key" slips in to a notch in the template. This lets the template slide over the molding and lock into place in a groove routed into the top and bottom of each pedestal.

These have been exhibited at the Vox Populi Gallery in Philadelphia. The exhibition is called The Stand (Possessing Powers)

More photos of the finished pieces:

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Robotic Fabrication of Parametric Chairs

I just finished the design and fabrication of a series of chairs. The chairs were made by hot wire cutting Expanded Polystyrene (EPS) foam using the 7-axis robot at Taubman College of Architecture at the University of Michigan.

To do the cutting the robot holds a rigid frame with a wire stretched between the ends. A voltage is applied to the wire which heats it up allowing it to move through the foam. A spring is used to allow the wire to stretch a bit as it cuts.

Here's a a video of the cutting process. The first section shows the cross sections of the chair being cut. Those cross sections are glued together and then additional operations are done. The video is sped up by a factor of 3 or 4  depending on the operation shown.


For me, the question was – how can a straight line be used to achieve pleasing, comfortable curvature in the design of the chair? To understand the answer requires a bit of study of ruled surface geometry.

Ruled Surface Geometry - Definitions
• Ruled Surface: A ruled surface is a surface swept out by a straight line as it moves through space. For example, a cylinder is formed by moving a straight line around a curve in a plane, keeping it perpendicular to the plane at all times.

A cone is formed by moving a line so that it stays fixed at one point.

A helicoid is formed by moving a straight line along another straight line, keeping it perpendicular but rotating it as it moves.

• Doubly Ruled Surface: A surface is doubly ruled if through every one of its points there are two distinct lines that lie on the surface. The hyperbolic paraboloid and the hyperboloid of one sheet are doubly ruled surfaces.

• Hyperboloid:  A doubly-ruled surface generated by a set of straight wires, whose ends span two parallel circles rotated relative to one another.

Parametric Chair Development
I began by creating a basic chair form I liked by manually modeling the chair section curve in Rhino. This is the main generating factor for the chair. The section can be edited to alter the shape of the chair. Thus the section is another parameter.

Next I decided what properties of the chair were going to be parameters. The principal ones are:

Seat Height, Seat Angle, Seat Center Depression, Back Height, Back Angle,
Back Pattern Scale, Cutter Angle, Cutter Bulge, Cutter Height, Cutter Width, Cutter Z Start

Variations
A variety of chairs – these all have the same initial cross section. Only the parameters were altered to create these variations:

Two chairs with surface variations applied to the back – this creates a stark contrast from front to back and makes for some dramatic ruled surfaces:

Parametric Model - Grasshopper
 I developed the parametric chair using Grasshopper. Grasshopper is a graphical algorithm editor integrated with Rhino’s 3D modeling tools. All the modifications are done using simple geometric transformations: translation, rotation, scaling.

A variety of user-interface controls are available. I used all sliders - here are a portion of them used to affect the chair:

Shaded model – typical of what’s seen while adjusting the model in Grasshopper:

Shown below are the original generator curve (green) and the final section curves:

Grasshopper also does an initial layout of the parts on the foam block. The block is correctly position in world space for cutting on the robot:

The parts are manually rotated to optimally position them on the foam for cutting:

Additional geometry is used to make the cuts possible by removing material that would hit the frame. Also additional cuts are used to free the parts from the foam.

Mastercam/Robotmaster Setup
The next step is to import the geometry into Mastercam/Robotmaster and to establish the sequence of cuts and position of the robot during cutting. It takes experience to know how to rearrange the position of the arm during cuts to make it work. The motion of the robot can be simulated to test if interference will happen. The robot has no knowledge about the shape of the tool it is holding. Therefore it is critical that I carefully verified the paths in the simulation prior to cutting. I needed to make sure the frame never hit the robot nor touched the foam block in areas which hadn't been removed yet.

Fabrication
First the four section pieces are cut.

These are then glued up using polyurethane glue.

After that cures the chairs are placed on a vacuum table and extra cuts are made to trim the sides and holes. It's necessary to raise the chair on blocks so the robot can reach all the way to the bottom of the chair.

Completing the second side cut:

Here the robot is preparing to make the hole cut:

Here are the chairs after all cuts have been made. These are just prototypes - to make a fully functional chair they'd need to be covered in a more durable material. They are strong enough to sit on and test however.

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