Even though the OEDK is immortal and will forever live in our hearts, all human constructs like classes and semesters must come to an end someday. That is why today, I present to you my final ENGI 210 project: molding and casting a chess piece.

The process of molding and casting acts very well as a culmination of everything we’ve learned in the latter half of 210. Successfully casting a piece requires the following procedure:

• Choose a 3D file of the piece
• CNC mill a positive of the file
• Pour molding material into positive to create rubber negatives
• Combine two negative halves to form a full piece, and pour casting material into the mold to create the final piece

In partner teams, we were assigned a chess piece to mold and cast. Fernanda and I got the bishop as our piece. To kick off the process, we spent some time browsing Thingiverse, in search of the perfect bishop design. We found a few interesting candidates (including this really cute seal bishop and this slightly creepy Easter chess set) but ultimately, we decided on an art deco bishop design. The art deco geometry looked cooler than a typical smooth bishop, and the file had few overly detailed/fragile parts or overhangs that would cause issues during the CNC/molding process.

With our file chosen, we processed the file in Meshmixer to cut the piece in half. In Solidworks, I added a rectangular base for the bishop half to rest on, which would serve as a platform to add extra features like lock keys. Then, I imported the .stl file into Fusion 360, where I’d be able to orient the model and specify the settings for CNC milling. And now the nightmare starts.

At one point, I probably spent a solid ten minutes clicking random points on the piece, desperately trying to rotate the x and y axes the right way. Once the orientation was finally correct, I waded through all the confusing CAM settings, set some variables based on the Fusion 360 tutorial recommendations, and converted the file to g-code. I had a feeling that I was typing random numbers the whole time and could only hope that the file wouldn’t make the Carvey explode.

Once the g-code was prepared, we exported the file to Easel and ran our first test cut using foam. As expected, our first cut did not go so well. Our file included a parallel and perpendicular smoothing step at the end, which left many deep and jagged marks on our piece. Also, our piece was 2 inches long and took 45 minutes to cut, so that wasn’t too optimal either. After removing the smoothing step and increasing the feedrate, we obtained a satisfactory foam positive.

This showed us that our bishop file could actually work!!

Next, we attempted to modify the bishop for cutting our actual positive. In Fusion 360, we scaled up the model to 8 cm in length and added holes and cylinders to the rectangular base to act as lock keys for the molds. Fusion 360 is not user-friendly. It’s stupid how you can’t go back and change the dimensions of the holes and cylinders after they’re created. It’s also stupid how Fusion 360 doesn’t recognize objects sometimes even when they’re plainly attached to the piece but that’s a story for two more paragraphs down the line.

We cut our modified positive using wood. It turned out decently, though several of the features (the key holes, the diagonal indentation on the bishop head) showed up very indistinctly or not at all. We attempted to incorporate a second pass using the smoothing feature (we uploaded separate g-code files for the original and smoothing passes). Though that cut made the diagonal indentation more distinct, it left the piece with many ugly deep lines, and left the Carvey with one less usable drill bit. The second cut also didn’t cut out the key holes. During this time, Fern and I took turns babysitting the Carvey. For me, it was a mostly boring but at times anxiety-inducing job.

We soon reached a dilemma. How are we supposed to align our mold without the key holes? We realized that our bishop was not symmetrical because of the diagonal indentation on the head. If we wanted to keep the indentation, we would need another positive with a mirrored image of the bishop. But since the indentation wasn’t showing up in the first place, we thought about packing sawdust into the groove of the first positive and forcibly making our piece symmetrical.

Before we resorted to sawdust, I opened up Fusion 360 one more time (a mistake for my mental well-being?). It turns out that Carvey wasn’t carving out the holes because … they don’t exist?

I did a double take when I looked at the toolpath simulation closely for the first time. What’s going on here? I asked at least 5 different people for help and went through 40+ versions of Fusion360 files. I tried:

• Changing the file from brep to mesh (didn’t help)
• Downloading the Carvey tool library and selecting the specific bit for milling (selecting 1/8” ball, the longest drillbit, got the holes to show up, but the Carvey still would not mill to the correct depth)
• Changing feedrates and randomly checking boxes in the CAM settings (not too useful)

Finally, Jeremy told me about one simple setting that changed my life. By changing the ramp type from “ramp” to “plunge,” the drill bit would plunge straight into holes and crevices without circling around them first. Even though the 1/8” bit was small enough to create the 5 mm diameter holes, it could not carve them easily with the ramp setting on! Jeremy also showed me a different way to watch simulations, which was pretty neat.

After messing around with some more minor settings (checking the smoothing option box for smoother cut, unchecking stock to leave, with may have made the drill bit go deeper than intended), we cut both the bishop and a mirror using just one pass. They were beautiful. We sprayed the positives with the mold release spray. We measured around 60 g of each part of the silicone mold, mixed them, and poured them on top of the two positives. The next day, we removed the molds from their enclosures. The mold for the original half was beautiful too. The mold for the bishop mirror was not so good.

The mirror mold had several air bubbles along its base and top, and was sticky to touch. After removing the mold, there was also sticky residue leftover on the positive itself. We attempted to cast a piece anyways, by binding the two molds together with wood supports and rubber bands, and then pouring a mix of the easy-flow through the base (dye added for marble effect). When we removed the piece, one half was extremely smooth and clear, but the other half was rough and bubbly. We also slightly misaligned the two halves.

At first, we thought the air bubbles were from not waiting long enough after applying the mold release before pouring the silicone. We poured a second mold for the mirror, this time making sure that we wait 5 minutes after spraying. Though the second mold was better, it still had several air bubbles in the base. I dug up a vacuum pump from the molding/casting closet, after hearing that it worked to remove air bubbles for students in the past. I poured a third mold and watched the bubbles burst to the surface. The next morning, I removed the mold from the vacuum and witnessed a monstrosity.

I decided that mold wasn’t going to cut it either. As a last attempt, we decided to CNC an entirely new mirror positive. The old positive had some silicone stuck to it still. Because the mirror positive also had a knot in the wood, we thought that some of the sap might have leaked out and combined with the mold. I babysat the Carvey again as it produced a clean mirror positive. I poured a fourth mirror mold and left it overnight. When we returned the next day, the mold was a success, with few air bubbles in sight!

After all the pain, it was time to cast and have some fun. We experimented with many different dyes and color patterns. To get the marble effect, we first combined the A and B components of easy-flo into a large cup, then poured some of that into a smaller cup where we mixed the dye. We then drizzled the dye into the large cup again with minimal mixing and poured the contents of the large cup into the mold. For the blue and green Earth bishop, we swirled mixtures of blue and green dye with the blue and green in separate cups. We sanded down our bishops afterward with the belt sander and fine grit sandpaper. We finished the pieces off by coating them with a layer of acrylic spray. To proudly display our final project, Fern used her extreme laser cutting skills once again to create two display stands for our bishops.

And now this is it. ENGI 210 has conluded. While this class has given me frustration at times and too many excuses to spend my days and nights at the OEDK, I’ve had a blast this semester becoming more experienced with different machines and prototyping techniques. This class has given me a ton of ideas and projects to work on in the future, so you’ll definitely be seeing more of me around the OEDK next semester. Thanks for reading, and until next time.

Cost Analysis:

Wooden Board: $5

Silicone Mold: $100

Easy-Flo: $60

Dye: $10?

Acrylic clear-coat spray: $8

Hours of frustration from Fusion360 spent by an inexperienced engineering student: $0

Babysitting fee for Carvey: $10/hr * 6 hrs = $60

Machining costs (Carvey, belt sander, etc.): $20/hr * 6 hrs = $120

Total: $363