Thursday, May 16, 2013

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.


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. 


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.