By Sammie Mahung and Eva Woodman

When we first paired up for this project, we knew we wanted to make something fun. So, we began scrolling through Thingiverse in search of something truly magnificent. We happened upon a squirrel 3D model that we absolutely adored, but we wanted to give it our own little twist, so we decided to give it a chess-piece crown chopped off from another Thingiverse model.

After 3D printing it and speaking with the TAs, we realized the divots in the crown wouldn’t come out right on the CNC half, so we made sure to smooth them out later.

 

Sammie: CAD & 3D Printing

Once we had a feasible model of our piece, we tried splitting it to make the two halves of what would become our mold, but it was hard to work with in Fusion or Solidworks since it was a mesh. We tried troubleshooting together but still weren’t getting anywhere, so we consulted Bisesti, who had us put the file in Rhino to troubleshoot since both she and Eva had experience with Rhino. Eva had to leave for dance rehearsal, and I had no prior Rhino experience, so Bisesti walked me through how to make the file we needed. 

I got to learn a lot about Rhino. It’s both kind of annoying and also pretty helpful. We did a lot of things in Rhino that I cannot precisely recollect (especially since I am not familiar with Rhino), but the file I made with Bisesti’s help had what we needed for both the 3D print and CNC sides of our mold. To export only the 3D-print side, Bisesti had me put its features on a separate “layer” from the CNC parts so I could export just that layer. From there, I was able to 3D print it to use for making our silicone mold later.

 

Eva: CNC

What a learning curve taking the model to CNC was.

First I modified the original Rhino file Sammie created. I deleted the half we already 3D printed and I used Boolean difference to subtract the holes from the base, which will become the “pegs” in our final mold. I then exported the model as an .stl file, which took many attempts as our mesh originally had over 200,000 faces. I experimented with reducing the mesh. I also learned from research that meshtoNURBS is unnecessary if the final export is an .stl file, since it will have to reconvert to a mesh anyways and polysurfaces are actually even bigger files then mesh. 

I imported the .stl file into VCarve on the CNC computers and followed the instructions in the homework document for creating toolpaths. I had a lot of difficulty clamping the wood down to the Shapeoko bed. The screws were not long enough to use the clamps as shown in the CNC Homework instructions. I configured the clamps in a similar fashion to the clamp on the Nomad. When beginning my first 3D Roughing pass, the bit began picking the wood up, creating jagged and deep cuts. I pressed the emergency stop on the machine and reassessed.

For my second attempt, I covered both the spoil board and my stock wood in duct tape, then used superglue to attach the taped surfaces together. I added additional clamps on the sides to keep the piece from shifting. The first pass went smoothly, but the wood began to shift on the second pass. For my third attempt, I modified my VCarve toolpaths so that the cut began in the middle, leaving space on both sides. I put a long strip of duct tape over each end, securing it to the table, then placed clamps over where the tape met the spoilboard. This prevented the wood from lifting up, which was the main issue as the bit cut in a vertical motion. The green stoppers helped keep my wood from shifting horizontally. 

Finally my 3D Roughing pass went smoothly! I changed the bit to a 1/16” ballnose for the finish pass, but not long into the pass the bit snapped, even though I had lowered my feedrate. I made a new G-code toolpath using a ⅛” ballnose, replaced the bit, and rehomed the Shapeoko. The cut went smoothly and after a little hand sanding, I was done with the wooden half! I put the clamps away and vacuumed the CNC machine.

 

Together: Molding

Once we had our 3D-printed and CNC-ed halves, we prepared them for molding. First, we made cardboard boxes of indeterminate size around the pieces and secured them with hot glue. Before mixing and pouring the silicone for the 3D-printed half, we calculated the required volume to be 100ml based on measurements and our STL file. However, that wasn’t enough, so we mixed an additional 30ml to top it off and let it cure overnight. Then, when we poured the CNC half later, we made sure to mix 130ml of silicone from the start. That was also somehow not enough, so we ended up borrowing some of James and Caroline’s extra silicone to finish filling it up. Once it cured, our silicone mold was complete!

 

Sammie: Casting

Next, it was time to cast our pieces. We cast the first one in class, securing the sides of our mold using 7 total rubber bands, 4 horizontal and 3 vertical. Referencing the volume of our STL file, we anticipated our pieces to need 100ml of polyurethane, but when we poured it in, we had 60ml left over, which we gave to another group. 

For the rest of our squirrels, I repeated the process using 40ml of polyurethane, mixing 20ml of each part, using the kind with the 30-minute cure time. It actually took more than 30 minutes to cure, so the squirrels’ heads were still a little soft when I took them out. I just put them back in the mold and let them sit a little longer, sanding each previous one while the current one was curing. The two sides of the mold were already lined up pretty well, so it didn’t take too much work to clean them up, mostly just smoothing out the seams.

Eva: Painting

I used the belt sander to even the base of the squirrel. I then used masking tape to block off the crowns and painted them with metallic spray paint. For the fur, I mixed brown, indigo, and ivory acrylic paint. I used photos of real squirrels to replicate their fur patterns accurately. I had to paint in layers to get the details of different hairs without the colors bleeding together. I then taped off the base and spray painted it black to make the pieces look more finished. I finished the post-processing with a clear matte spray coat.

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Cost Breakdown:

• 3D filament ~50g * $0.05 per g = $2.50
• Wood block ~ 3.43 x 5.95 x 1.48” = $0.55
• Silicone ~300ml = 10.14oz * ($60/140oz) = $4.35
• Polyurethane ~ 160ml = 5.41oz * ($50/72oz) = $3.76
• Acrylic paint ~ 1oz * ($5.99/4oz) = $1.50
• 3D printer & CNC access = $50 (Makerspace price for one month)
• Labor ~16 hours between two people * ($15/hour) = $240

Total = $302.66