Our final project was to CNC two halves of a blank for a mold, cast a mold from that using oomoo, and then create a positive object from that. Because we had so many issues along the road for this project, I’m going to include more detail than a simple outline of the process would to hopefully save someone a headache along the way if they decide to read this.

So to start with, we wanted to make a cool-looking knight, so we found a cool dragon knight piece and started preparing it for the CNC. What took us an embarassingly long time to realize was that the space between the horns wouldn’t be able to be CNC’d, so the dragon wouldn’t end up looking very cool at all because instead of horns it’d just have a huge block on its forehead. So when picking your model to CNC, make sure that there aren’t any big gaps in the design that the CNC won’t be able to get to.

So anyway, once you have your model, it’ll probably need to be simplified. We did this by using meshmixer, which you can download for free. Once you open up your file in it, you just need to hit Ctrl+A to select your model, then hit Edit, then Reduce on the menus that pop up. On the Reduce menu, you can simplify it to a set number of triangles by selecting Triangle Budget under the Reduce Target tab. It needs to be reduced to fewer than 10,000 triangles, but Fusion 360 runs better when there are fewer. We ended up reducing it to 2500 triangles, which didn’t crash Fusion 360 every few seconds.

Once we’d reduced our file, we exported it as an stl then imported it into Fusion 360. You have to do this in a very specific way to be able to manipulate it in Fusion 360. The way you do this is, under the Create tab, select Create Base Feature. Then you use the Insert tab to insert it as a mesh. When you orient it, make sure it’s oriented the way it’ll be machined, with the side of it facing out along the z-axis. Otherwise the g-code won’t work. From there, you just need to select your piece, right click, and select Mesh to BRep. This makes your model compatible with anything you add to it in Fusion 360.

Once our model was compatible with shapes in Fusion 360, we added a box onto the back to hide half of the model, then cut down that box in depth. Then we added some pegs to the box so the sides would line up. As a side note, if you try to mirror the object so you only have to make the halves once, it doesn’t seem like you can get rid of the original, so we had to make the two halves of the model separately. You also don’t seem to be able to make a g-code for unconnected shapes, and Dr. Wettergreen advised us against machining the two halves at the same time, even though we tried to do that at first. So we just repeated the process with the horse facing the opposite direction and with holes instead of pegs and with a pour spout.

From here, you select CAM on the grey drop-down tab on the left of the toolbar in Fusion 360. We opened up the menu that popped up when we selected Adaptive clearing under the 3D tab on the toolbar then set our tool to 1/8″ flat even though we planned on using a rounded one. Then under the second tab in the box we went into tool orientation and set the origin as the bottom left corner of the stock box. From there we used Actions to generate the g-code. To post process it so the CNC could read it, we went onto Easel and downloaded the Fusion 360 post-processor from the import g-code menu (under file). Then we just selected that from the folder we downloaded it to on the Post Process menu, named our g-code, and hit post. From there, we had two working g-codes and only had to CNC them.

That’s when another problem cropped up. We didn’t do enough reading through all of the resources, so while we figured out we needed to delete the M7 and M9 lines, we didn’t realize that we also needed to delete the G54 line from the g-code for a really long time. If you don’t delete this line, your code is offset a lot and probably outside of the toolbed. But once we got that sorted out, we machined one of our halves no problem, in about 40 minutes. Unfortunately, we then used the wrong g-code for the second half, which means that they didn’t line up properly and it didn’t have the posts that would keep them aligned. Instead of taking the CNC again when other people needed it, we decided to just go ahead and make the molds from these after a bit of sanding to get rid of any rough bits.

 

 

 

 

 

 

Pouring the oomoo in was pretty easy, but as a side note, filling the cutout in the wood isn’t the best idea for two reasons. For one, it was pretty hard to get out – to remove one from the wood, we had to break the sides because of the trench our g-code inexplicably decided to have around the box. The other problem was that the outside (top, here) of the mold is extremely thin because it couldn’t get that deep above the knight, making it very easy for the molds to deform when we filled them. So the way other people made their molds, with a lego cage, might be a better call because you can get that added depth. Just make sure it doesn’t float up, apparently.

Anyway, once we had the molds out we had to cut out a pour spout (again, because we didn’t CNC the right file). Then we just struggled to get the halves to line up nicely so we had to keep trying. Eventually we got some that we were happy with (as a side note, our black knight ended up being very grey because we didn’t add enough dye because we basically ran out). Then we just sanded them down a lot to correct for any mismatch, then spray painted the grey one black. Our final horses ended up looking like this:In all, this project was pretty convoluted and complicated for the end result, though it really only feels that way because of all of the difficulty we had. It was nice being able to practice CNCing and molding/casting, even though it was a bit more of a hassle than it really needed to be. But seriously, we wouldn’t have had any of these problems if we had gone through all of our resources, because all of the questions were answered in previous blog posts, videos, etc. that we had at our disposal. But hopefully this will help anyone who has problems and hasn’t looked through any of those resources.