Introduction: How to CNC a Skull
Really, everyone should have a skull on their desk. It's an age-old symbol, a memento mori that reminds us that we are not long for this world. So I set out to make one out of aluminum. This was my process.
Step 1: CAD/CAM
I designed the skull in Solidworks. I don't have a ton of images from this part, and surface modeling is a giant can of worms that I'm nowhere near good enough at CAD to properly explain. I got most of my help from the Solidworks Surfacing Bible, which can be purchased here. If you're interested in my methods, take a look at the attached Solidworks file.
I did the CAM setup in CAMworks. This was divided into a rough area clearance, a z-level, and a constant stepover finishing pass. I chose to do the rough clearance with a 1/2" hogging end mill, the z-level with a 1/2" regular end mill, and the stepover with a 1/4" ball end mill.
Step 2: Materials
CNC Mill (3-Axis Bridgeport)
1/2" Hogging End Mill
1/2" Regular End Mill
1/4" Ball End Mill
2" x 3" x 5" block of pink insulation foam
2" x 3" x5" block of 6061 Aluminum
I had access to the equipment and materials through my college's machine shop. The foam was just lying around, and the aluminum block was from a discarded bicycle welding fixture.
Step 3: Prototyping
To get a feel for my toolpaths, I ran a prototype in pink insulation foam. Foam is great for testing because it is cheap, readily available, and easily machinable. I modified the feeds and speeds to go for the maximum that the machine was capable of. This meant that I had a complete mockup in my hand in only 30 minutes. When machining foam, it is critical to vacuum during the process. I ran without coolant and held the shop vac next to the cutter for the entire operation time.
There are two prototypes here. The first one that I ran was done with a .050" step down, and is the second set of photos. The second one, which was the same code as the final metal version, was run with a 3/16" step down.
Step 4: Final Fabrication
Once I was satisfied with my CAD/CAM process and once the prototype looked successful, I started on the final fabrication. The piece of metal was ready without any modification, so I put it into the CNC, set up the zeroes, and went to work. The area clearance was terrifying. What had looked fine on the prototype simply wasn't in aluminum. The machine shook, the ground shook, and I was forced to slow the machine down significantly. The rough cut took well over an hour to complete. The z-level, however, was fine. Because I was using a finer cut depth, I did not get the same issues as the rough cut. My finishing pass, with the ball end mill was fine as well.
After I finished the CNC machining, I did some cleanup operations. The toolpaths left a high point that ran from the edge of the metal to the skull. I cleaned these off with a few passes on a manual mill. I also removed the large hole by milling down that edge of the finished piece.
Step 5: Surface Finish
After I finished the part, I decided to work with the surface finish. There were unsightly tool marks and surface weirdness, so I chose sandblasting as the quickest, most effective surface. Pretty much, if sandblasted aluminum is good enough for Apple, it's good enough for me. So I put the part in the sandblaster and cleaned it up.
Step 6: Finished!
It took me about 3 weeks from CAD to completion. I almost broke a lot of things, and there was much cursing. Hopefully you'll be able to learn something from my project, or at least be inspired to try your own version. Anyway, good luck!
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