Our final project was to make 8 molded chess pieces. We decided to make a Catty Cat Bishop as shown fellow found from thingiverse.com. While Catty Cat is a nickname, Catty Cat’s original name is Bisman Bishop.

 

There were several steps that went into casting 8 kind-of-perfect pieces. Follow the steps below to make your own 🙂

Step 1: 3D Print the bishop

To figure out the desired size of our bishop, we 3D printed 2 different sizes as shown below. The small one is at 25.25mm x 26mm x 47.50 mm, and the bigger one is at 40mm x 41.14mm x 75.15mm. After 3D printing them, we decided to go with the bigger one.

step 2: STL Preparation

In order to make two separate molds with different manufacturing processes, we needed to take the STL we downloaded from Thingiverse and make it into two separate positives for molding. To do so, we split the STL using Autodesk Meshmixer down the frontal plane to avoid any overhangs for future CNC efforts. We then took each half into Solidworks and extruded a 3.25”x3.5”x0.25” cube out of each missing half with pegs and holes for securing the mold. After these files were complete, we transferred them to 3DPrinterOS and VCarve for 3D Printing and CNC machining, respectively.

Step 3: 3D Print positive half

The 3D-printed half only took two attempts, as the first attempt was stopped for putting a raft on a flat part (dumb practice, we know). The second attempt printed without issue, giving us this positive for molding:

Unfortunately, the CNC portion was not so easy…

Step 4: CNC other positive half

While working on this positive, it felt as though VCarve was lying to us and the Nomad wanted to see us fail. In our first attempt, the bit immediately plunged as deep as it could into the stock, forcing us to emergency shut off the machine before causing any damage. After reloading the file and re-zeroing the stock, we ran the GCode again only to find that our positive was cut off roughly halfway, leaving only a half-embedded sculpture in the stock. On our second real attempt, the stock became unsecured from the double-sided tape below it, throwing off the milling path and ruining any chances of recovery. This happened again on our third attempt, when the mill dove too far in one corner, throwing the stock off the tape after pushing against it. Our 4th attempt was going perfectly, that is until the bit started getting too deep and we realized we didn’t make clearance for the chuck, which began to grind against the top of the stock. This once again forced us to scrap our stock, this time mere minutes from completion.The entire sequence of failures can be seen here:

Watch our big CNC failure #1 reveal below:

 

After this, we adapted to the clearance issue by pocketing down a layer of stock around the mold before roughing and finishing. We also transitioned to the Shapeoko machine, where we had more success with keeping our stock secure. After one last attempt, we finally produced the second positive of our mold, seen here:

Step 5: Molding

After both our 3D printed and CNC halves were ready, it was time to mold! While molding our 3D printed part, catty cat got featured in a how-to video by Dr.Wettergreen. It was a proud moment for us.

To create the negative mold of our positive half 3D printed catty cat, we used a 2-part silicone mixture. To create a base around the 3D printed part so the mold can sit in it, we cut 4 cardboard pieces approximately the same length as the edges. The cardboard pieces were hot glued to each of the sides until a housing the same size was created. To figure out the volume of the silicone mold to pour in, the volume of the 3D printed one piece was calculated by using the original dump-in-water method. The volume of the 3D printed one piece was 1.25oz (~37ml). Next, the volume of the 3D-printed positive mold was calculated (LxWxH). For the height, 1cm was added to the original height so the mold can be 1cm thicker. Half of the original one piece volume (~18ml) was subtracted from the total volume to get a volume of about 260ml. Part A mixture (130ml) was mixed with Part B. The volume of Part B was matched with the weight of Part A. Part A and Part B were mixed carefully together, poured in slowly to ensure no bubbles are created, and then left overnight until it hardened. The same process was repeated for the CNC positive part as well. Here is the math below (for 3D printed part):

Calculate volume of 3D printed part: 
   LxWxH = 10cm*10cm*2.78cm = 278ml
Subtract Total volume - half volume of 1 piece: 
   278ml - 18ml = 260ml 
Mix Part A and Part B: 
   260ml /2 = 130 ml
   Part A = 130ml 
   Part B = match the weight of 130ml

Step 6: Casting

We finally have both of our molds, and can complete the most exciting part of this project, casting! To cast, we poured polyurethane to create our solid part which also came in 2 parts. The process was to pour 18ml of part A (half of catty cat’s volume)  and 18ml of part B, mix them together very well, pour it into the mold, and wait 15 minutes before removing the mold. The 2 molds were held together with rubber bands, and lined up as well as they could be. This was repeated 8 times to make 8 birman bishops, and of course to make them less boring, we added different dye colors. The dye was mixed in part A, before mixing part B in. All 8 of them turned our perfect except there were some air bubbles by the whiskers. When we look at the final pieces, you can clearly see the CNC side and the 3D printed side. The 3D printed side had obvious ridges, and the CNC side was a lot smoother. 

 

 

 

 

 

 

 

 

Check out our army of Catty Cat’s below:

step 7: Calculate costs

Material Costs: 
   Full Bishop 3D Printed Prusa Print = $0.64
   Half Bishop 3D Printed Prusa Print (2): $0.75 * 2 = $1.5 
   25in of Wood for CNC @ $26.98/8ft = $7.02
   520ml of Silicone @ $63/947ml = $34.60
   288ml of polyurethane @ $54.47/1893ml = $8.29
   1 Pack of Rubber Bands = $1.08
   Total Material Costs = $53.13
Labor Costs:
   43 hours * $7.25/hour = $311.75
   Cost to use TXRX Labs that provides access to CNC & 3D Printing machines = $90/month * 2 people = $180
   Total Labor Costs = $491.75

Grand Total = $544.88