by sophie leibowitz and erick morales

introduction

Our assignment was to mold and cast 8 copies of a selected chess piece. The two of us immediately started looking at knights, as we thought they were the most artistic and would be the coolest to work with. After a brief search, we located this knight design on thingiverse:

3D Printing

A test 3D print of our knight revealed that the default size was very small, so we decided to scale it up by 1.5 times. 

We began by downloading the stl file and converting it to something that would be editable in Solidworks. We first used a program to decrease the number of elements in the stl to make it importable without overwhelming Solidworks.

Once in Solidworks, we cut the knight in half vertically, extruded the back of our half-knight and added a rectangle with peg holes. Our goal was to 3D print this half and then match the peg holes with the corresponding ones on our CNC half during casting. This half was now ready to be sent to the 3D printer. 

cnc machining

Once we had 3D printed our first half, next we went to make our CNC half. We used the small Nomad3 CNC mill for our cut. We prepared our cut in VCarve Pro, using a ⅛” end mill for the main cut, and a ⅛” ball end mill for the finishing pass. In all, the cut took about 45 minutes. The main issue we ran into was that the retaining tape was not strong enough to firmly hold our slab of wood in place during the cut. Due to this, we first tried securing it with a bunch of extra duct tape: 

This helped, but the wood still shook back and forth in place enough that we were concerned it would affect the alignment of the blade with the wood. As a last resort, we decided to hot glue the wood to the bed to secure it. This worked perfectly and even allowed us to speed our cut up to 200% speed for parts of it. The hot glue was easily removed from the cutting bed with a wrench when we were finished. Our final CNC knight can be seen below: 

mold making

For the molds, we went ahead and used silicone as the material. Before pouring our molds, we built walls of cardboard to go around the open edges on both the 3D print and the CNC cut. This way, we had an airtight and watertight way to pour our molds and let them cure. The negative image mold would then be used for casting.

Once the walls let no light in through a ton of duct tape, we poured the 1:1 Part A / Part B Silicone mixture until about an inch above the tallest point in the CNC and 3D parts in and let it sit overnight. The molds came out great with no air bubbles!

 

 

 

 

 

 

casting

Using urethane, we created a 1:1 Part A / Part B mixture to pour into the mold. When we tried to line the peg holes, the CNC mold presented difficulty due to the rough cuts and inaccuracies. We used rubber bands to put the molds together and held the mold halves closely and airtight. This was enough to secure the cast. We produced four colorless casts and four dyed casts. Overall, they came out solid! There was a discrepancy with the size of the CNC mold and the 3D Print mold that did not allow them to line up all the way, so we sanded down some of the rougher edges.

successes and failures

Overall, the project came out great. The CNC piece was not as detailed as the 3D print piece, but that was due to the machine itself having some smaller issues. The molds came out decent, although the pegs needed to be a thicker to be more useful in alignment. If the molds came out the same size of the piece, the alignment would have been a lot easier and sanding might not have been necessary. Very complicated experience, but lots of lessons were learned.

costs

Wood: ~$4

Nomad3 and Software: Provided

3D Print: ~$2

Silicone Mold: ~$20

Urethane Cast: ~$15

Time: 20 Hours X 2 People @ $13/Hour = $520

TOTAL: $561