For our final project, we were tasked with casting eight chess pieces using a combination of CNC machined and 3D printed molds. Raniyah and I decided to create a typical king piece using a design/file from Thingiverse.

Milestone 1: Prepare to Print 1:1 Representation

The first step after choosing our design was to create a 3D printed 1:1 representation of our piece. This would allow us to visualize what our final pieces would look like, as well as provide some guidance on calculating the volume of our piece later on. The original piece on the file was about five inches tall, which was larger than most of the examples we were shown in class. Based on measuring a similar piece that was successfully made using the same methods we were going to follow, we chose to scale down our piece to be three inches (60% of original).

We sent this file to the 3D printers, and were able to successfully get our king on the first try!

Milestone 2: Create Half Piece for 3D Printing

The next step was to create a file to create the 3D printed half of our chess piece. To do this, we first imported our file to Autodesk Meshmixer. Here, we used the plane cut function to cut our piece in half in an orientation where we could get two identical halves. This was then saved as an .obj file which was then used in Solidworks to add the base to the piece. On Solidworks, we created a rectangular base that the piece would sit on and added two pegs and two holes (where the pegs would be inserted to close the mold). The file was then saved as an .stl to be sent to 3DPrinterOS to get our 3D printed half.

After one try, we were happy with our result, and were ready to move onto the CNC half.

Milestone 3: Create CNC Molded Half

To use the CNC machine, we needed to first create a Gcode file. We did this using the .stl file we had in Vcarve. It took us a little while to get the model to be the right size, but after a few attempts, we were able to get our piece in the correct orientation and of the right dimensions (96.94 mm X 56.72 mm X 28.55 mm for x, y, z respectively). We then put the model at the top of the modeling plane  before moving on to creating a toolpath.

We needed to specify the dimensions of our piece of wood, and positioned the model in the material with a 0.05 inch gap above the model. The first file that we made was the Rough Machining Toolpath, where the majority of the material will be removed, but no details will be visible. We set this to use a ¼” end mill and set the limit boundary to model boundary. We then created the Finish Machining Toolpath in the same way and set our tool as the ball nose ⅛”. The files were then saved both toolpaths and were ready to start the cut.

We decided to use the Shapeoko CNC machine in the basement, which was the same as the machine upstairs which we had used before, only larger. We calibrated the machine following the computer instructions, secured our piece to the base, and started the rough cut. Our cut was going well, but we noticed that the wood started to wiggle around a little. We ran into a problem when the wood came unstuck, and the end mill made a hole through the side of the wood, making it unusable for our mold since the silicone would spill out.

We got another piece of wood that was the same width and height, but a little longer in length, and were able to restart the cut. This time, we successfully completed the roughing pass and then followed the same procedure to do the finishing pass. With two minutes left, we again ran into a problem. The end mill was too short, and the metal piece holding it in the machine ended up running into our wood, so we stopped the machine as a preventative measure. Upon further evaluation, we decided that this small flaw in our wood would not cause a problem with our mold since it was at the top that we would need to cut off to pour in the polyurethane.

Milestone 4: Molding and Casting of Parts

With our two parts made, we were now ready to pour the silicone to make our mold.

For the 3D printed part, we created a box of indeterminate size to make the walls for our mold. We then calculated how much silicone we needed to make before pouring. We measured the volume of the box that we had (1 cm above the top of the chess piece) and then subtracted half of the volume of the piece to get the correct volume of silicone. 

With these measurements, we planned to mix one half of the total volume by weight of Part A, add the same weight of Part B to get our silicone mixture, pour it into our box, and let it dry overnight. Thanks to Tiffany and Anjitha who had some extra silicone and were able to pour it into our mold for our 3D component. We followed the same procedure to find the volume of silicone for the CNC part, and let it dry overnight. When we returned, we had our two functional halves that fit together really nicely and made a very convenient seal at the bottom.

Our final step was mixing the polyurethane. We used the same volume measurement to determine the volume of each part needed to mix and then prepared our mold using two pieces of wood and rubber bands to keep it together. We also decided to mix blue and red food coloring to make our chess pieces purple. We mixed the purple with Part A, and then mixed in Part B, and immediately poured this into our mold. We waited ten minutes for the polyurethane to set and then removed the cast from the mold to get our completed piece.

We repeated this process 8 times and had our final project completed! As a final step, we sanded our chess pieces for a smooth finish using a 250 grit paper and then using an 800 grit paper. Specifically, we sanded the bottom edges, around the cross, and underneath each overcast. 

Cost Analysis

Raw material cost: 

Material	Ticket Price	Spent
PLA	$49.95 (750 grams)	$13.32 (about 200 grams)
Wood	$17.19 (4 pieces of wood)	$4.30 (one-piece of wood)
Silicone	$51.99 (70.56 oz)	$29.47 (40 oz)
Polyurethane	$54.47	$13.62 (25%)
PolyColor	$4.75 (4 dye)	$2.38 (2 dye)
Total		$63.09

Labor cost:

Assuming minimum wage in Texas of $7.25:

Task	Time	Spent
CAD modeling	2 hours	$14.50
3D Printing	0.5 hours	$3.63
CNC	2 hours	$14.50
Mold making	1 hour	$7.25
Cast making	1.5 hours	$10.88
Post-processing	1 hour	$7.25
Total		$58.01 $116.02 (for 2 people)

Machine time/machine hour cost: 

Machine hour cost can be calculated by dividing the cost of the machine by the total hours it should function, and multiplying that result by the time that the machine was in use.

The Original Prusa i3 printer at the OEDK costs about $1,099, and is expected to last for about 800 print-hours before maintenance. For this case of 800 hours, and since we used the printer for about8  hours (4 hours for the entire 3-D printed King chess piece and 4 hours for ½ printed King chess piece) , the total machine time cost was $16.49.

$1,099/(800 hours) = ($1.37/hour)×8 hours = $10.96

The CNC machine at the OEDK costs about $2,300, and is expected to last for several hundred hours (we will assume 500 hours in use). For this case of 500 hours, and since we used the printer for about 2 hours, the total machine time cost was $9.20.

$2,300/(500 hours) = ($4.60/hour)×2 hours = $9.20

Project total: $199.27

Overall, we really enjoyed this project and thought it was a great way to wrap up our prototyping course!