Team: Mathias Adamu, Saad Yousaf
This final project was an amazing learning experience for both of us. Throughout the process, we gained a lot of exposure to preparing files, milling, pouring molds, and casting. This blog entry will take us through the different steps in the process that led to the final creation: a knight chess piece modeled after the Egyptian god Anubis.
Prototyping & Fabrication Process
To start things off, we found a cool knight piece from thingiverse (https://www.thingiverse.com/thing:1826018). After downloading the STL file, we imported it into meshmixer in order to isolate half the chess piece. Due to the complex geometries present in our initial STL file, we used meshmixer to reduce the percentage of triangles in our file and simplify the design. While some tiny features were blurred, the knight still maintained its general figure. With the STL file ready, we imported it into SolidWorks (shown below).
Within SolidWorks, the extra features were added to the knight STL which would be necessary for milling out the mold positive. Shown below, there is a back plate added along with holes and pegs (which will end up becoming pegs and holes respectively). There is also an air channel (highlighted) but in our failed attempt, we forgot to take the air channel all the way to the top. Thus, our casting process required us to clog the air channel with hot glue. The base of the chess piece is also lined up with one end of the base plate in order to create a pouring hole for the casting liquid. The second image below shows the dimensions used to create the pegs and holes on our base plate (note the difference in peg diameter (8.3 mm) and hole diameter (8 mm) in order to create some tolerance). The holes and pegs are filleted with a 2 mm fillet on each side.
With the SolidWorks file ready, we saved it as an STL and imported it into Fusion 360 to create a mesh. At this point, we will discuss the procedure we used to mirror our piece. While we attempted to mirror it in SolidWorks, the STL format was not conducive to it. Thus, our approach was to roll back features in SolidWorks to create a base piece knight, import its STL into Fusion 360 and create a body, mirror the body in Fusion 360 and save it as a STEP file, and import the STEP file into SolidWorks to add the holes and pegs again.
There are some reasons why we had to take this approach. First, the mirrored piece needs to have the knight shape mirror but the pegs and holes need to be in the same place (so that the final molds can fit into each other). If the pegs and holes could also be mirrored, then we could simply mirror the original file in Fusion 360 and work from there. Second, the reason the mirrored Fusion 360 file had to be saved as a STEP file is because SolidWorks doesn’t allow building features on STL surfaces. Third, we had to make sure that our units matched up. At one point in the process, the file saved the mm units as inches (for example, a 10 mm dimension became a 10 in dimension). We had to check our dimensions at different points and ended up scaling down our mirrored piece while it was in Fusion 360. The images below show the original knight base with features rolled back (notice the feature design tree on the left), the mirrored STEP file after it is imported into SolidWorks, and the mirrored STEP files with features on it.
With both files ready, we imported the STL files into Fusion 360 and followed the ENGI 210 instructions to create our machining g-code file. We unknowingly made the piece bigger than what was suggested which caused some adjustments later on with fitting bits into Carvey. We also adjusted the Z offset to be 2.5 mm (instead of 0 mm) so that our mold could be poured directly into the positive without the need for any external walls. We were also able to download a Carvey bit library from the link below which was super helpful! It brings its own settings for each drill bit. We would highly recommend checking it out and importing the library.
https://discuss.inventables.com/t/fusion360-tool-library-for-all-inventables-bit-sets/50579
Shown below is the CAM simulation of the knight piece in Fusion 360 as well as some practice pieces on foam. We first used the Carvey to mill out foam because its faster and allows us to get an idea about the final product.
After moving to wood, we encountered some challenges that provided learning experiences for us. Shown below are three pieces carved out of wood that were not used as our final pieces. Initially, we used a 1/8″ fishtail flat end mill to do the rough pass and planned to use the 1/8″ ball end mill for a smooth pass. In the first piece on the left, the rough pass looked beautiful but the smooth pass was misaligned a little, leading to a worse look after the smooth pass. We still held out hope and tried a smooth pass again, but the g-code messed up and wouldn’t even mill in the knight area (if you notice hole at the bottom right in the middle picture, that is where the bit would keep drilling). We also discovered the impact that grain can have on cutting. Ideally, we would use a slower cutting feed rate but that would have taken closer to five or six hours (our final cuts were ~3 hours). As seen in the picture to the right, the lateral grains caused the bit to shift (evident in the ridges). Thus, we decided to pick out some better wood with smoother grains.
For our final positive pieces, we decided to stick with a single rough pass with fine settings. We determined that the smooth pass was not worth the risk, and by using a 1/8″ ball end mill with a 2.5 mm rough step down (0.25 mm fine step up), we were able to produce some very high quality results. Next, we poured our molds into each positive and took them out after letting them sit for a few hours. We did struggle with trying to get the molds out (in this case, not having foam walls which we could simply take out was a downside).
Next, we embarked on our journey of casting. There was a lot to learn here too. In our very first attempt, we pushed the molds together with 3-4 rubber bands, and the results were less than optimal. Our mold actually started leaking from the bottom, partially from the air channel which we did not clog and also from the lack of force keeping the molds together. The result’s skeletons of the piece produced can be seen below. Thus, we learned that for future casting, we should clog the air channel using hot glue and hold the two molds together using some wood planks. We implemented these strategies as shown below, with our final method utilizing two wood pieces kept together using 8-10 rubber bands. This proved to be very effective. Also seen below is how we mixed dye into a single compound before pouring the second compound in. This results in a fixed color throughout the piece. We also used the dye to create a marble effect by first mixing the compounds and then pouring one or two drops of dye and swirling them just a bit.
Another issue we ran into was that the nose of our knight would sometimes not be completely filled in. To resolve this, we made sure to move around the mold while the cast was drying, shaking it around a bit to make sure the casting liquid filled all the corners. Due to experimenting so much, we ended up with many different knight chess pieces shown below. For post-processing, we sanded down rough edges (sometimes a bit too much accidentally) and applied a clear coat.
Cost Analysis
- 2″x6″ – Home Depot 2″x6″x8″ – $2.37
- 1 Gallon Silicon Rubber – $137
- Smooth On Casting Resin – $34
- Labor – $10/hr 20 hrs each, $400
Total: $539.37
For this final cost analysis one thing to note is that we aren’t going to calculate unit cost because this time around it actually makes sense not to. For the 2″x6″ we used more than 4ft of stock so even if we did by 8ft its worth it. Same for the rubber molding and casting resins, they were both used a considerable amount making a unit cost unnecessary (We might’ve finished one of the smooth ones by ourselves). Labor being that high of a cost also makes sense because we spent a lot of time perfecting our pieces, whether it be trying to concoct the perfect color, getting a clean nose job, or sanding for post processing.
