For our final project, we were tasked with creating a chess piece through the process of molding and casting. Madison and I chose to make the knight. To begin we first found a CAD file of a knight that we liked on GrabCAD. In Fusion360 we cut the model into two halves, splitting them down the middle of the horse’s face. In each file we added a backing to the knight. To create keys so that we could align each half when making the mold, we extruded small squares on one of the blocks and cut away matching square holes in the other block. We then set the origin to be the lower left corner of the block on the upper face (as shown in the image at left). Once we had finished making the model, we inputted the cut settings, choosing a 1/8″ round bit. We then ran a simulation of the CNC cut to make sure it would work as we expected. Once we were satisfied with it, we exported the g-code for our model. In order to do so we had to download a specific post processor for easel.

With the g-code we were ready to cut the knight halves out of wood blocks on the CNC. We put the wood blocks in the vice, loaded in our g-code and set the origin of the machine. We had to delete one line of the g-code with an “M” instruction in order for it to load without errors. Once we got it all set up we stepped through the first lines of the g-code and then let it run continuously, blowing away the sawdust with the air hose and vacuuming it up. Both halves cut pretty cleanly, so we only had to lightly sand down the surfaces.

Once we had the two knight halves, we began the process of making our mold. Since each knight half was cut into the block and the block was larger than the size of the backing in our CAD file, this left walls around the part. We initially though this would be handy, since it meant we could pour the mold right into the wood block without having to build up a lego structure. When we went to make the mold there was unfortunately no more oomoo on the table, so we decided to use a different liquid rubber material instead, as we had seen another group do. We mixed this solution, poured it into the blocks and let them sit overnight.

When we returned to the OEDK to remove our molds we realized that it was going to be a challenge. Because the walls around the mold were not removable (the benefit of lego walls we finally realized) it was very difficult to pry out the mold. We started by trying to cut it out with an x-acto but didn’t make much progress. Realizing that this called for more drastic measures and that if we were to re-mold we wouldn’t want the walls, we decided to saw them off. This proved to be more effective, as we were actually able to get under the mold and pull part of it up. Unfortunately though the material that we used was quite fragile, and it cracked and tore when we tried to remove the mold.

In our second attempt at molding the chess piece, we built up a box of legos to surround our blocks. Since the oomoo reappeared on the ENGI 210 table, we mixed up some of the solution and poured it over the blocks. Pretty confident that this would work, we left the OEDK planning on coming back the next day. However, a couple hours later we got a message from Vidya saying that something had gone wrong because the knights on the faces of the blocks were now sticking up out of the liquid. Heading back to the OEDK yet again to deal with this, we found that the wooden blocks were more buoyant and had floated up above the oomoo. Unfortunately we couldn’t just push the blocks down to resubmerge them because the oomoo had already set and begun to cure. The only option was to just clean it up (which was a complete mess since it was so goopy) and try again. At least this process allowed us multiple chances to get the mold right without having to re-CNC.

Our third attempt finally worked. We rigged up a couple of lego stacks, which served as latches to hold down the blocks. This time when we poured in the oomoo the blocks stayed in place. After the mold had cured we were able to cut away the excess oomoo and carefully remove the molds from the blocks. We had to use an x-acto blade to clean up the keys and holes, but we were able to get the two halves to fit together.

The next step in the process was to cast the knights. We designed our mold to have a hole in the top to pour the liquid through. All we had to do was rubber band the two mold halves together, mix the Smooth-On 300Q and pour it into the mold. After it cured (~4 min) we removed the knight. It was super cool to see! We noticed that it was slightly misaligned, so we put toothpicks through the mold to better line up the two halves and poured again. Once we had two knights we were pretty happy with, we sanded down the join-edges and the excess cast we had poured. Finally, once the knights were cleaned up, we spray painted one of them black.

Overall I’m really happy with our final chess pieces. It was a really neat, albeit very time intensive and many-stepped process that I’m glad I learned. Seeing the knights emerge in the wood blocks on the CNC and then again in the cast was super cool. I think it was so amazing and surprising to us especially since it took so many tries to get it. I learned a lot throughout this process, through a lot of trial and error and ultimately some success.