Introduction
In this post, we detail how we casted chess pieces using a combination of CNC machined and 3D printed molds. This is our final project of EDES 210: Building a Chess Set using CNC machining, molding, and casting.
Selecting An Object
First, we selected a chess piece from Thingiverse. The design we chose was a Minecraft Villager by user rival13, shown below.
Model from Thingiverse.
We then 3D printed the vanilla villager model. After receiving feedback regarding the overhang below the arms being difficult to CNC, we modified the file using CAD so that the arms are 90 degrees.
Modified file w/ 90 degree arms.
We liked the default scale of the piece. At about 3 inches tall, the villager is approximately the height of a bishop, while the dimensions of its base (1.5x1in) fits comfortably within a chess square.
Pre-Processing
Next, we modified the shapefile using Autodesk Meshmixer to split the villager model into two halves: one to be 3D printed and the other CNC machined.
Once we had our front and back halves, we used Solidworks to add a mold base to each. The mold base consists of four alignment holes to attach the two molds together. Constraints were used within solidworks to ensure consistent placement of holes with respect to both models. We also extruded a square to the back half model for use as a fill hole. This placement was chosen to ensure the arms (at 90 degrees) would not interfere with the casting process.
Challenges
We had first cut the villager into top and bottom halves rather than front and back halves. However, this caused issues when trying to make the CNC half, due to the CNC machine XYZ limits. We decided to restart our progress when the bit was too short to reach the bottom of the cut without hitting the portion of the model. Pictures of the initial model and setup are below.
3D Printing one-half positive
We 3D printed the first half of the positive with the mold base out of White PLA using the Bambu X1 printer.
3D-printed front half positive.
We then constructed a box of indeterminate size using cardboard and hot glue around the 3D printed positive, making sure it was air and water tight.
Constructed box of indeterminate size.
CNC Machining one-half positive
To CNC machine the second half, we first prepared the .stl file using VCarve Pro to create a toolpath for the CNC machine. We followed the provided instructions to setup and insert the model, and created a roughing path using a ¼ inch end mill drill bit and a finishing path using a ⅛ inch end mill & ⅛ inch ball nose drill bit.
Creating toolpath using VCarve.
We then CNC machined the second half of the positive using a wooden block and the Shapeoko Pro XXL CNC Machine. We changed the drill bit when prompted by the machine.
We again constructed a box of indeterminate size using cardboard and hot glue around the CNC machined positive, making sure it was air and water tight.
Constructing box of indeterminate size for CNC half.
Challenges
While CNC machined the second half of our correct model, it took three attempts to get it right. During the first attempt, the block came undone from the clamps due to a defective screw on one of the clamps, causing it to turn. During the second attempt, the toolpath of the first finishing pass was not correct. Finally, the piece was cut as intended on the third attempt. However, parts of the CNC cut half were still imperfect due to bit selection, leading to a lesser than ideal mold later on. Pictures of the progression of CNC cuts are below.
Failed CNC cuts.
Molding: Pouring the Silicone Negative
We then made two half negative molds by mixing and pouring the silicone mold solution (Part A and Part B at a 1:1 ratio). We calculated the volume to pour as follows:
(length of box) x (width of box) x (height of half chess piece + 1cm) – (volume of half chess piece)
1” x 2” x 3.3” – 2.4in^3 / 2 = 5.4in^3
Measuring the volume of Silicone to be poured.
We then poured the silicone mold solution.
Both silicone-negative molds are now ready for polyurethane casting!
Silicone molds ready to use.
Casting the Chess Pieces
To cast our chess pieces, we first put together our two silicone molds using six rubber bands in a grid pattern to secure them together.
Rubber bands used to secure the two molds.
We then mixed and poured polyurethane casts from our mold using Smooth-Cast 300. To create colored pieces, we dipped a popsicle stick into UV dye and inserted it into the Part A solution. We then mixed it with the Part B solution at a 1:1 ratio and stirred.
Casting process with Polyurethane. See fill hole in middle.
Finally, we repeated the process four times to create our four chess pieces of different colors.
Successfully casted model.
Challenges
Since we used the Smooth-Cast 300 polyurethane solution to cast, it set much quicker than we anticipated. We mixed our first pour for too long, causing the solution to harden while we were still pouring; this resulted in a half-casted piece.
In addition, our silicone molds were not completely aligned when casting. This caused the resulting cast to be misaligned.
Casted model misalignment.
Post-Processing
To post-process our pieces, we first used the band saw to remove the protrusion from the fill hole. We then sanded down our pieces using 150 and 400 grit sandpaper to remove imperfections from misalignment.
Sanding setup.
Final Product
Our final product is shown below!
Cleaned Workspace
Bill of Materials
- 3x 1.5” x 3.5” x 6” blocks of wood – $1.44 (8ft for $3.85)
- 35g PLA – $0.70 (20kg for $20)
- 5.4in^3 (3oz) Silicone Solution – $1.36 (64oz for $29)
- 11in^3 (6.1oz) Smooth-Cast 300 Polyurethane Solution – $6.29 (1gal for $132)
- UV Paint Set – $0 (Provided)
- Machine Use & Consumables – $0 (Provided)
Material Cost: $9.79
- Labor: 20 hours at $10/hr – $200
Total Cost: $209.79
Conclusion
While we thought we chose a simple piece to create, the process of molding and casting proved to be much harder than anticipated. Creating our chess piece not only brought together previous skills from the past semester, but also taught us new technical and soft skills. Technical skills included learning to process a 3D model in CAD, create a toolpath for a 3D model, using different bits for the CNC machine, and finally casting our model from a novel material from molds. Soft skills such as patience, communication, and organization were paramount as we ran into challenges at each step, needing to improvise and work around machine and material availability. Overall, we are happy with how our product turned out!
This project concludes the spring semester of EDES 210. It was filled with moments of joy, hardship, and learning, but we now walk away with invaluable hard and soft skills that we will undoubtedly use in future projects at Rice and beyond.
