For our final project we were asked to mill an asymmetrical object split in half and then make oomoo molds from the milled molds and finally produce plaster molds from the oomoo molds. We (Danielle and Tibbie) chose to mill a knight from a chess set. We found a pre-split knight stl on thingiverse:

We chose the knight because is was solid, with no hollow or overhang points and it is slightly asymmetrical because you have to mill two separate halves. If you merely milled one half and produced the same oomoo mold from that twice, the nose would be facing opposite ways (I found that out while I was doing an alignment check on Solidworks, and it was a real bummer). Next we generated the two knight halves in a box with posts in solidworks:

We decided to make it quite small so it would fit on an average chess board and we added alignment posts and holes. The holes were 2 mm in diameter (turns out that’s too skinny for the mill to make so these alignment holes were not actually milled, however due to the box construction, the alignment was still quite easy). Next came the hardest and longest part of this entire project: Generating G-code. I watched about 5 tutorials on how to use the post processor in Fusion 360. None were particularly helpful. Also Fusion 360 is a very confusing program that I think would take about a semester to really understand because it has soooooo many options and menus and it’s very powerful, but I never felt like I really understood it. They all advised to use different post processors and had pre decided on all of their settings without expanding on how they got those numbers. All I knew was I wanted to use a ball end mill of 1/8 inch diameter. At this point I talked to Brian and Lucas a lot and asked them about how they set it up. They gave me a ton of super helpful advice about removing G28, G54, M9, M8, and M7 and which post processor to use and what numbers to set etc. However when I ran the code, I kept getting an arc center error. At this point I asked Grant to help me (shout out to him because he spent like an hour and a half helping me when he was not working as a lab tech so he’s the best). He was super helpful and advised me on a bunch of stuff. At this point I got my first set of G code to run. I wound up using the built in post processor in Solidworks which I didn’t know existed. I highly recommend the HSM tools in solidworks, they eliminate all toolbed errors and set a default origin in the center of your stock which does not get translated over like it does when you input the g code from Fusion 360. Here’s the tool path:

I had cut some wax earlier and Joe helped me face mill it so it was beautiful so I started up my first run and it went terribly…. Basically my step downs were too aggressive and my feed rate was too high so the mill literally pushed the wax block out of the vice on the table and wrecked it:

At this point I left and I decided to come in later in the day to fix it all up. I changed my feedrate (600 mm/min) and step down (5 mm max per pass) to be less aggressive and the first half milled. I would like to note here that the arc error did unfortunately come back, however the flash cut program on the cnc mill computer is the source of this arc error and for whatever crazy reason if you turn off predictive time (under the g code tab) the arc error goes away. I found this out after reading a few message boards about it. So in short, we got the mill running:

CNC Milling the Knight Piece by dnr21

Here’s the finished product:

As you can see the top face has melted edges. This is due to the mill bit not being placed 20 mm from the chuck when I had a 20 mm deep extruded cut. I fixed this for the next milling but I could not fix this problem while the first set of G code ran so I just had to go with the melted edges. This piece took roughly 2 hours to cut.

Next I had to generate G code for the second half of the horse. I had previously made this in solidworks to perfectly align and perfectly align with the pour spout. I had also put it in an assembly to ensure an aligned output so I was confident that this piece was ready to go. Generating this G code was again a pain. For some reason Solidworks kept setting the top of my stock to be the bottom and vice versa, it took about 20 minutes to sort out but then the G code came out just right. And here’s the toolpath:

This mill had a much finer stepdown (3 mm) so it came out smoother and with no melting in roughly 4 hours:

We cleaned up both of the milled pieces and finally the hard part was over!! Next we just had to pour the oomoo which was very straight forward:

And then we had our final molds:

We then made a variety of knights in a several colors and styles using 7.5mL of Parts A and B of Smooth-On 300Q and variable amounts of dye. First, we created a transparent knight without dye:

Satisfied with the result, we created a white knight and a black knight to match the usual colors of chess pieces. Unfortunately, on the first try, the black knight piece did not come out correctly because we completely filled the mold as well as the pour spout which led to a large unwanted protrusion at the bottom of the knight. Learning from this mistake, we were able to make a correct white and black knight:

We then created a blue knight by mixing So-Strong white dye with blue dye:

Next, I (Danielle) experimented with various color combinations by creating an orange knight by mixing yellow, red, and white dye, a light violet knight by mixing violet with white, a light green knight by mixing green with white, and a dark blue knight by mixing green, blue, and white dye.

I also tried a marble effect by not completely mixing white, brown, and flesh colored dye together and a black and white knight by using 3.5mL of Smooth-On dyed black and 3.5mL of Smooth-On that was dyed white:

There was an issue with excess materials on the knights but I used pliers and sandpaper to remove it:

The final results:

All in all, we learned a lot through this project about the CNC mill, which can incredibly finicky but powerful, as well as the process of casting and molding.

Thank you for reading,

Danielle and Tibbie