Our final project for ENGI 210 tasked us with creating 8 identical chess pieces using molding and casting techniques. To accomplish this, Hawk and I teamed up and began to brainstorm ideas for what we wanted our chess piece to be.
After a few minutes scrolling through Thingiverse, we decided to create a grub, which is an enemy in the game Hollow Knight. In addition to liking the design, it was also vertically symmetrical about the middle which would make it easier for us to create the mold.
Once we had our design and an STL file of it, we began to process it using meshmixer. This program allowed us to slice our design into two halves – one that would be molded using a 3D print and one using a CNC’d piece of wood. Also, we reduced the size of the files by approximating the design using triangles. After this, we imported each half into SolidWorks and attached them to a flat base with pegs on them. Finally we scaled the pieces to a reasonable size and began to create each positive half using a different method.
Part 1 – 3D Printing
For the first positive half, we just directly 3D printed the piece made in SolidWorks and it came out pretty well, but we later realized that the base on the bottom of the grub was not needed so we broke it off before molding this half.
With the positive half made, we could now create a negative mold of it using a 2 part silicone mix. To do this, we made a box around the positive half to contain the silicone while it cured, combined both parts in a 1:1 ratio by mass, mixed them thoroughly, then slowly poured the result into the box.
After a few hours we came back and after removing the 3D printed half from the silicone, we had our first negative mold half. There was a slight issue however which was a hole in the mold caused by not pouring enough silicone in. This could be easily fixed though by just hot gluing some cardboard over the hole.
Part 2 – CNC Machining
For the other positive half, we had to use the CNC machine to cut our design out of a piece of wood.
We first had to find a suitable piece of wood. We were unable to find one large enough to fit a carve out, so we just glued together a couple of scraps into a piece of wood approximately 5.5” x 5” x 2.3” large.
We then used these dimensions and imported the 3d STL into VCarve. After positioning the model in the right spot, we created two toolpaths – a roughing toolpath and a finishing toolpath. The roughing toolpath would use a .25” square endmill, while the finishing toolpath would use a .125” ball nose endmill.
After securing the wood to the Shapeoko with double-sided tape, as well as some extra wood and the green holders, we set zero using the probe and ran the machine. There were a few issues with the machine having issues cutting, not being able to move, and then becoming offset and losing the zero, but after setting the machine to a higher RPM (approximately 25000) and lowering the feed-rate to 50%, it cut well.
We had a couple issues with the finishing pass. The machine would stall and at times friction would burn the wood. Initially we thought it was because the machine was struggling to carve out the base, so we removed that section of the toolpath. However, it turned out the actual issue was that the ball nose was not long enough to reach the bottom, so the holder of the router was spinning against the wood. As such, we had to only finish the center, which was fine as we had a large gap. This did mean that we were unable to cut the pegs, as well as a bit of the bottom. Additionally, instead of our grub having feet, it just has a flat bottom. However, this was due to the finishing bit not being small enough to fully add that specific detail.
To fix the tail, we just sanded it as much as we could to make it smooth. As for the pegs, we figured we could line it up by hand, as the mold would have a gap at the bottom, and one side was semi-clear, so we could see if it was about right. Here are the molds.
Part 3 – Casting the Chess Pieces
We made 8 pieces, 4 white, 4 dyed green. We used 2.5 oz of each of the liquid plastic parts for a total of 5 fl. oz. for each piece. After taking them out of the mold, we sanded off any excess material. Here are the final pieces, before and after sanding.
