Week 8 -- Make Something Big
This week we were tasked with 'making something big'. We were trained on the large bed shopbot and the waterjet cutter and supplied with rigid insulation foam (2' X 8') and OSB panels. We were encouraged to use other materials (aluminum, glass, plywood) if we could get a hold of it.
After passing up on molding a dinosaur a few weeks ago, I decided this would be a good chance to go back to that idea. Walking by the excellent dinosaur skeleton Andy Bardagjy made last year, I decided I'd build an 'living' dinosaur to differentiate a bit (i.e. not a skeleton). I found a .STL model of a nice herbivore and used AutoDesk's 123D-Make software. The software allows you to upload a 3D cad model and presents options for you to segment the design for either layered or interlocking pieces. I decided to use the insulation foam and to make a layered design. The foam is 2" thick, so it was thick enough to easily fill the full volume of the 3D model. 123D-Make output a .eps file which I imported into AutoCad and exported as a .dxf file. The .dxf can then be imported into Partworks which builds a toolpath for the shopbot.
Starting the shopbot was fairly straightforward and Tom provided a lot of guidance. My initial design fit onto about 1.2 pieces of foam when packing the pieces closely together. Tom advised using two full pieces of foam though and spacing the parts loosely. After zeroing the x, y, and z axes and screwing down the foam boards, I started the job. Everything went smoothly until my first small piece was cut out. After the piece was fully cut, the vacuum used to pick up sawdust and debris had enough suction to lodge the small piece loose and pull it up a bit. When this happened, the piece would usually get hit by the end mill and get stuck between the end mill and the plastic dust cage. The first time this happened, I hit the emergency stop button as a first reaction. Apparently what I should have done though, given that this wasn't necessarily an immediate safety issue, was to click the software stop button. By clicking the emergency stop button, the shopbot job lost it's place and I had manually edit the .sbp file to get back to the point when I hit the emergency stop button. From this point on, I had to quickly hit the software stop button at the end of each small piece, remove the piece (hopefully before much damage was done to it) and then resume the job.
To assemble the pieces I began by using a 3M spray adhesive. I tested the spray on a small piece of foam first, to ensure that it wouldn't erode the foam and to test the strength of the bond. The test did erode the foam a small amount, but the bond was sufficiently strong that I decided to move forward. Assembly went smoothly until the tail portion. The balance of the body was such that it was hard to hold the pieces in place while the glue set. I eventually began using small dabs of hot glue on the underside of the joints to create a stronger bond. The hot glue would prove useful again when I got to the neck portion as these pieces were strongly off balance. On some pieces of the neck I also opted to stick small stiff wires from the bottom through a few of the pieces. At the end, the model seemed to hold together quite well as I carried to around and moved it about.