Sunset medallion

Initial Proposal

My proposed plan for one of my level 3 CAD projects was to create a more polished version of the Sunset High School medallion (used in previous school scavenger hunts) out of bronze. The process for creating this model would start with casting a bronze blank, most likely using greensand or styrofoam casting. I would also create a model in fusion 360 of the final medallion, and use the to CNC shape the blank disk. With the final shape of the medallion finished in the CNC machine, I could then polish and finish the model, and put it in the laser cutter to add etched designs to the bronze for the final product.

Project Plan

To create the bronze medallion, I modeled a concept in Fusion 360, keeping in mind the special criteria of using CNC and the realities of using circular bits when making inside corners. Then, I used Fusion 360 CAM features to create a toolpath which could be sent to the CNC mill. To test the model, I created an MDF version of the medallion as an initial prototype to polish out any design flaws. In addition to that, I planned to test a piece of bronze in the laser cutter to master the process and settings for laser etching bronze. With both of those steps completed, I could be confident in all the new processes I need to create the final medallion. I planned to cast bronze stock in my foundry (larger than the intended size of the medallion to make sure extra material could be milled away) and cut it to shape in the CNC. Then, with the geometry complete I would be able to put it in the laser cutter for the etching details, and have a final product.

Design Process

I started by designing one side of the medallion in Fusion. For the general shape, I knew I wanted it to be fairly large so it would feel substantial and be easier to machine, but also not so big as to be unnecessary. I settled on a diameter of 15cm, larger than the original wooden medallion just based on what I thought seemed right, and some brief polls in the classroom of fellow students. I also wanted to design some physical textures into the medallion so it is more interesting than a blank disc with some engraving on it. For this reason I created a raised lip on the outside, a couple millimeters high so as to give the medallion different faces. I also wanted there to be a large logo on one side, in the traditional sort of coin fashion, so I used the SVG of the schools logo and isolated the Sunset S. I resized this to take up the whole face of the medallion, and removed some of the edges to simplify the shape and allow it to be cut in the CNC. The S is also raised to add some more texture and make the surface more interesting.

The next step was to create a toolpath for the CNC to actually cut out the medallion from the stock. This was a completely new process for me, as I had only ever worked in the design and render workspace. In addition to that, I had only ever created models for 3D printing and laser cutting, so the design process had some snags in it due to the nature of CNC. However, after learning about the setting in fusion and playing around with the manufacturing workspace, I created a tool path for the first face of the medallion. This involved adding a facing function to cut the whole stock down to the thickness of the medallion. From there, I used the 3D flat function which creates an operation to cut down to all the flat faces detected on the model. Because all of the faces on the medallion are flat, this worked out very well for me and allowed for the rest of the tool path to be created with relative ease. This concluded the work on the first face of the medallion, and it was time to move to the other side.

Halfway through the project, I wasn’t quite finished with the back of the coin. By that point I had overlaid the Sunset High School Apollos text from the outside of the logo, separated it from the S, and placed it on the back face of the medallion. To keep a physical texture in my design, I was thinking about using Fusion’s engrave feature to have the CNC physically cut out the letters of the outside text. However, I chose to decide when I finished the design to go with the text on the back face of the coin.

All that was left of the design process was to finish creating both faces of the model. On the front face I decided to add some laurels surrounding the S logo because I thought it added more complexity. The laurels were from a creative commons licensed SVG file I found online, and overlaid onto the face of the coin. I left these as a sketch object, as they will be laser engraved later onto the final product.

The next part was finalizing the back of the coin. For this I chose to design a sunset to put in the middle. This was fairly simple, I put a circle at the center of the coin, placed some receding lines below it, and then used the offset function to give some thickness to the lines. Then, I trimmed off the excess lines, and added the sun’s rays at set angles to make them even from the center of the circle. The final touch was to filet the corners so they were left nice and rounded, giving a cleaner look to the final product.

The final step for designing was to create the toolpath for the pack of the coin. To do this I added another 3D flat operation using the ⅛ inch flat end mill. This completed the back of the coin, but there was one more issue I had to work through which was how to set up a new operation with a new orientation and stock on the same model. I did this by creating a new setup which allowed me to input the updated stock size and reorient the tool and axis. This marked the end of my design and toolpathing process, and from there it was all down to machining.

I created a first draft to test my design by curing the medallion out of some MDF. I used all the toolpaths I had made previously, and found that a feed rate of about 1200 mm/min worked great with the ⅛ in flat end mill and a 1mm step down. The facing bit required a slower rate of about 200 mm/min, but that’s to be expected given its larger size. The MDF model was mostly just to get an idea of sizing, and being the first project that had ever been fully machined to completion on the CNC mill, was a good proof of concept. I liked how it turned out, and figured that machining it out of bronze should result in a great final product when I’m able to machine it.

I didn’t get to make the final bronze medallion for my project, as both my friend and I were working on machining his project, an aluminum GoPro mount for a ski helmet. We experimented together, working with machining aluminum which was a completely different experience than MDF, and much more tedious. We eventually worked out that a step down of about .3mm worked best, along with feed rates that were below 400mm/min, depending on the size of the end mill we were using. We almost finished machining his part, but I’m sure with the knowledge we gained from his project I will be able to machine the final medallion fairly quickly out of bronze with the experience we gained from working with aluminum. Bronze is far more dense, so I may have to lower some of the feed rates but I anticipate the setting should remain fairly similar. 

Overall I had a lot of fun with this project, learning to use the CNC mill was a really great experience and I really enjoyed getting to know Fusion’s CAM functions. For future projects, I’m looking forward to machining bronze and seeing how that turns out. I think that in the future, the challenge will be finding new ways to secure the stock to the waste-board, which we could possibly accomplish using clamps. As we continue to work with new materials and narrow down the ranges for machining, our projects can only become better and more complex. I look forward to updating this post when I have a final product.