Our first step was to select a chess piece from Thingiverse. After much trial and error and talking to the instructors, we chose the hex pawn. Kate Mischlich and I started working on a unicorn knight chess piece but stopped when we were told there were too many delicate pieces at different angles that would make removal from the mold difficult, and there would be some overhang that the CNC machine would be unable to handle. So, instead, we decided to move forward with a pawn with a spiral design.

Fig 1: 3D printed versions of both pieces

Once we decided on the hex pawn, we 3d printed it to understand better what it would look like.

Next, we did CAD to create both halves. Many thanks to Rafe for being really helpful on this one. Once we found a design that would work, we downloaded the file from Thingiverse and sliced it in meshmixer. We sliced it directly down the middle and reduced the number of meshes. One-half of the file was for 3D printing, and one-half was for CNC machining. We continued by adding the back of the positive mold along with pegs/holes to hold the mold halves together in Solidworks. We had to ensure that the dimensions matched on both pieces so that we would be set up for success later on in the process when we combined halves of the mold.
Once the files had been made, we 3D printed one-half of the positive mold.

Fig 2: CNC positive mold half

Next, we glued the cardboard to the 3d printed part in order to pour the mold.

Fig 3: Gluing the cardboard barrier

Fig 4: Mixing the halves of the silicone solution and pouring the mold

5 hours later …

Fig 5: removing the negative mold from the positive half

Next, we used the instructions to make the mold and pour it. We mixed the silicone and poured it into the positive 3D piece in order to make one-half of the negative mold. We had some issues with leaking, but the silicone solidified before it got too bad. The fun had just begun.

Once the negative had been made for the 3D printed half, we proceeded with the CNC positive half. We made the CNC stl as well using Meshmixer. We used vcarve to convert the STL file to gcode- a roughing pass with a 1/8″ endmill, and a finishing pass with a 1/8″ ball nose bit.

Trying to figure out the CNC machine was the hardest part of the project, and we had several failed attempts before we finally were successful in getting the mold cut. First, we ran into an error saying “limit switch hit”. We ended up coming back later with a new piece of wood, but now the base on the back of the piece was too large to fit in the wood. We had to restart the whole process by editing the piece in Solidworks, sending the STL to vcarve, and making it into gcode. Eventually, we got the cut to fit into the wood, but again ran into the limit switch hit error. No one in the class groupme nor any of the lab assistants knew what the error really meant, but Danny and Fernando did suggest we go into vcarve and mess with the settings until the error went away. Thankfully, this worked! We ran into still more issues when the block of wood came unstuck during the middle of the cut. We had to quickly pause the cut, remake the gcode with the dimensions of a new block of wood, and start the cutover.

Fig 6 : Roughing pass

Fig 7: After finishing pass and removal of excess wood

Finally, we were successful with the CNC-positive mold. We realized that there was too much excess wood on the ends of the mold, so we used the band saw to cut the ends off. We then made a cardboard box around the block of wood so that it would be watertight. Next, we poured the green mold onto the wood piece. We mixed and poured the silicone, then waited for it to cure.

Once I finished the silicone mold for the CNC half, I realized that it was a different size than the 3D half of the mold, so it wouldn’t sit flat on the table. I used a box cutter to cut away the excess silicone so that the mold would sit flat and allow the urethane to cure with a flat base and not spill.

Fig 8: Creating Mold with a watertight barrier.

Fig 9: Silicone mold after removal

Next we mixed the urethane to make positives 8 times whilst using rubber bands to maintain seal. We used the two halves of the mold to make 8 chess pieces. We combined the two silicone negative halves of the mold, bound them tightly with rubber bands, and poured the mixed urethane into the completed mold. Then, after 10 minutes, the piece was ready to be removed from the mold, so we took the rubber bands off and carefully separated the halves of the mold and took the piece out from the middle. We had some trouble getting the halves of the mold to align, but got better at it as we repeated the process more. We figured out it was because the pegs and holes were really small compared to the rest of the mold, so they didn’t provide much security for the piece overall. We also found that if we used a box cutter on the chess piece right when it came out of the mold and was still warm, We could cut away most of the registration error pretty easily. It was also a really messy process! The first time we poured the urethane, I wasn’t used to the consistency and spilled a bit of it all over the mold and the surrounding workstation and then it cemented a lot of things together. All the dye containers in the bucket were covered in dye that had leaked, so our gloves got covered in color that we then proceeded to smear on other things without noticing. The white chess piece ended up with a little bit of rainbow tie dye pattern because of this, but I liked the way it looked! After a lot of cleanup, we were done.

Fig 10: Pieces after mold removal

I am super happy with the way the pieces came out! It was really cool to go through all the different steps of this process, especially after having so much trouble with the CNC part of it, and get to see the final products. There were some misaligned edges from the molding and casting process and some ridges from the CNC machine and the 3D printer that were visible on the final piece, but I think the pieces still looked really good!

Cost analysis
materials
Wood- $6 (home depot)
Silicone- $35 (amazon)
Urethane- $32 (amazon)
cups/stirrers- $2
Filament- 7 in^3 at $4/in^3
machine use
3D printer- 1 hr at $2/hr
Nomad 3 CNC machine- 6 hrs at $40/hr
labor
20 hrs at $15/hr
Total- $643 ($80.38 per piece)
Pretty expensive, but the bulk of the cost came from labor time and use of the CNC machine, both of which could be cut down drastically after learning more about the process. Also, most of the cost came from making the molds, which could be reused to make many more pieces.