Selecting a Chess Piece

The first step in our process was selecting a chess piece. After looking on Thingiverse, a certain penguin caught out eye:

Penguin STL file on Thingiverse

When looking at the piece originally, we didn’t know that there was an “N” on the back of the penguin. It was kind of a random letter to have on the back of the penguin, but we thought it was funny and decided to name the penguin Norman based on the letter.

First, we had to decide what size we wanted to make our penguin. We made 3 different variations of the penguin – a larger version, a smaller version, and a compressed version. We talked to the TAs and Dr. Wettergreen, and they recommended using the largest size since smaller details can sometimes be difficult to cut on the CNC machine.

Varying the size of the penguin to see which size was the best

After getting it approved by Dr. Wettergreen, we decided that it would be best to split the piece as a front half and a back half. The front half would be 3D printed and the back half will be cut using the CNC Machine. 

Designing the Halves

Over the next two weeks, we followed the instructions posted on Canvas to attach the base to each half of our part using SOLIDWORKS. We used a 3.5” by 3.5” base. We then 3D printed both parts to have an idea of what they each looked like.

Left: front half of penguin; right: 3D printing both halves

CNC machining: rough pass

Next, we had to use VCarve to create the CNC half of our penguin. After finalizing our design, we realized that our cut would take 11 hours if we used the ⅛” drill bit. So, we decided that we would have to use the Shapeoko machine downstairs so that we could do our rough pass with the ¼” drill bit. After adjusting this, the rough pass was calculated to only be about 20 minutes. 

Before making our cut, however, we realized that our base was longer than the width of the wood. We had to go into SOLIDWORKS and cut about 0.25” off the end so that it fits perfectly within our stock piece of wood.

The blue line shows how much we cut off the end of the base so that it fits in our wood

We then used the CNC machine to cut our rough pass with the ¼” drill bit.

   

Left: setting up the cut; right: what it looked like after the rough pass

CNC machining: finishing pass

This was probably one of the parts where we struggled the most throughout this project. After having our rough pass done, we tried to use the CNC machine upstairs to complete the finishing pass for the back of our penguin to be smoother. However, within the first few minutes of the finishing pass, we noticed something was off – it looked like the finishing pass “zero” was starting in the middle of the penguin’s back rather than in the bottom left corner.

Finishing pass was off-centered

As a result, we had to recut the rough pass of our CNC half again. However, the Shapeoko machine downstairs was having issues, and we had to try again two times before we were able to successfully cut a rough pass again. In the image below, the wood marked with a smileyface was our final piece. The one right below was when the machine messed up so we had to stop it early, and the one at the very bottom was our first attempt with the finishing pass. Looking at these placed side by side, we noticed that the Shapeoko machine downstairs set the “zero” of our cut at a different location every time. This was why we had an issue with the finishing pass with the Nomad.

We asked the TAs for guidance. One of the options was to use a dremel instead of using the CNC machine for a finish. The second option was to alter our G-Code. In particular, we should redo our G-Code for the finishing pass so that it didn’t account for the full length of the wood; rather, the “zero” would be set at the left corner of the 3.5” by 3.5” square. We ended up going up with the second option and try it out (and it ended up working well).

Preview of our adjusted G-code

We then did the finishing pass using the Nomad. It was difficult to set the exact “zero” using the corner test, so we did it manually. It ended up being mostly in the right place, but it was off by just a little on the bottom. Specifically, the bottom of the penguin wasn’t flush with the edge of the wood. To solve this, we sanded off the bottom.

Finished product after finishing pass, before sanding the bottom off

There were a few rough areas in our finishing pass. Specifically in the top left of the penguin’s head and the bottom right of the penguin’s body. However, we didn’t really have time to redo the process again so we left them as is.

Rough patches in our cut

Silicone Molding

Our next step was to make our silicone molds. We used cardboard to create a box for the mold to be poured into. To measure the volume of how much silicone we needed, the volume of the box we calculated was 324 cm^3. We estimated that the volume of half the penguin was 62 cm^2. Thus, we needed 262 cm^3 of the silicone materials.

Constructing our box out of cardboard

We measured the volume of each part of the silicone mixture by ounces, which was about 4.4 oz for each part. We then mixed these parts together and poured them into our box. We repeated a similar process for our CNC part as well. We then let the molds set overnight to harden.

Allowing our mold to solidify overnight

Casting

The next day, we detached our mold from the cardboard and 3D printed/CNC machined parts of our penguin. Surprisingly, this turned out to be a more challenging task than we thought it would since we used a lot of hot glue to fill in the holes between the cardboard box. After a few minutes of letting the liquid version of our mold rest in the box, we noticed it started leaking and attempted to fix it as fast as we could. On top of adding heavy amounts of superglue, we placed a piece of cardboard underneath to stop the mold from spilling.

Our mold after hardening

Next, we lined up the molds against each other, and secured them together by using several rubber bands. We then prepared our urethane mixture. Since we calculated the volume of the penguin to be about 62 cm^3, we measured 1.5 oz of each part. For our first penguin, we added a drop of red dye while mixing the two parts together. We then poured it into the mold, slightly squeezing the molds together to ensure there would be no leakage. Three minutes later, we saw the urethane “bloom” or harden. It was really cool to see it with our own eyes!

Left: squeezing the mold together right after pouring; right: watching the mixture harden

After 10 minutes passed, we removed the rubber bands and peeled back one half of the mold. We were very satisfied with our result! While the piece was still warm, we used a razor to clean up some of the edges and seams.

One thing we noticed was that the left foot of the penguin seemed cut off at two of the “toes.” To solve this, we peeled back more of our silicone mold to ensure all the urethane was reaching every crevice of the penguin. This solved the problem.

   

Left: peeling back one half of the mold; right: penguin standing up!

We repeated this process several times until we had 8 penguins we were happy with, making sure to make 4 that were white. We tried to make a penguin with glitter at one point, but when we actually poured it into the mold, the glitter sunk to the top of the head and it looked odd. We ended up just keeping that penguin to ourselves.

Failed sparkly penguin 🙁

Final Result and Reflections

Below are images showing the side, front, and back angles of our final penguins!

Looking at each of the penguins, it’s really interesting to see the difference between the two halves and how the different methods affect the texture of the surface. We were especially impressed by how smooth the CNC half of the penguin was, while the 3D print side was more “layered.” Although it is a bit noticeable where the seam is, we were still proud of the final product given the issues we had with the CNC machine. 

If we had more time, we would have tried to do the CNC half again. Because of some of the issues with the finishing pass, there were some odd textures left on the CNC half, as shown in the images below.

Cost Analysis

• Urethane Part A and Part B = $164 x 0.20 = $8.20 (one gallon on Amazon is $164, and we used about 20% of the bottle)
• Silicone Part A and Part B = $115 x 0.10 = $11.50 (one gallon on Amazon is $115, and we used about 10% of the bottle)
• 6” x 3” x 0.7” wood planks (x4) = $8 (Lowe’s)
• Popsicle sticks = $5 (Amazon)
• Plastic cups = $7 (Amazon)
• Food dye = $3.69 (Target)
• Labor = $12 per hour x 8 hours = $96
• TOTAL COST = $139.39