Messing With My CNC

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TL;DR: I learned how to use Kiri:Moto in CAM mode to<br>run basic jobs on my CNC router. I tried to cut a spoon and a stamp in<br>pine, but failed several times. This was a disappointing outcome, but at<br>least now I have a minimal CNC workflow for non-planar parts.

🔗Kiri:Moto

It appears that Kiri:Moto is<br>the best open source solution for computer-aided manufacturing (CAM). I<br>admit I didn’t look that hard for alternatives, but this<br>software met my needs, so I didn’t have much reason to keep searching. I<br>used the version served at the link above, but eventually I plan to host<br>a copy on my home network.

CAM is mostly new to me. It’s kind of like a puzzle where the goal is<br>to create a sequence of operations and settings that will cut away<br>everything in the stock material except the desired part. This usually<br>means starting with a big tool to remove most of the material around the<br>part (a “rough” pass), followed by more passes with smaller tools to get<br>more detail. Some parts require multiple “setups” with the stock in<br>different orientations relative to the machine. The most common of these<br>is a “flip” along either of the horizontal axes, which allows milling on<br>both sides of the stock. A problem with having multiple setups for a<br>part is that the action of moving the stock destroys the relative<br>orientation between it and the machine, so it’s necessary to re-index<br>the part to recover the alignment between the machine’s expectations and<br>reality. This is a challenging problem (for me) that I haven’t been able<br>to solve yet in my workflow.

My first goal was to mill a spoon<br>out of some scrap 1×4 pine. I thought this would be a good level of<br>difficulty because it requires a flip operation and exercises my CAM<br>knowledge somewhat. On my first attempt, I made it through the first<br>rough pass but found that the spoon was too small, so I tried again<br>after scaling it up. The second rough pass also worked, so I continued<br>with the contour pass.

After the first rough and contour passes

After the flip and second rough pass

Then I attempted the flip, and I realized I’d messed up. Kiri:Moto<br>had dutifully placed my part in the center of the stock material, whose<br>dimensions I’d estimated from the nominal size of the lumber scrap. It<br>turned out that the actual stock was a bit narrower, and since I’d<br>zeroed the machine on a corner of the stock, the part was off center. I<br>tried to compensate for this with some arithmetic, but it didn’t work,<br>and the second rough pass tore the spoon apart. I also learned that my<br>goal was probably not reasonable anyway because the spoon handle was too<br>thin to remain rigid when made from pine. Even applying light pressure<br>to the handle, which was partially freed from the surrounding stock,<br>caused it to bend quite a bit. I guess there’s a reason that silverware<br>is typically made from metal, and wooden spoons are generally chunkier<br>and made for stirring stuff in pots instead of eating.

I experimented a little with using registration holes to index across<br>the flip. This means taking a separate bit of material, putting it under<br>the stock, and drilling two holes through the whole stack. The holes<br>need to be symmetrical across the axis of the flip. Then, when making<br>the first setup, put pegs into the holes to register the parts and clamp<br>them down separately. Make all passes on the first setup, then remove<br>the stock but leave the other part in place. Flip, align holes, insert<br>pegs, and re-clamp. The stock is now flipped but has retained its<br>position relative to the machine. I didn’t explore this as much as I<br>want to, but I think it’s the most reasonable approach to the problem in<br>my case. Another option might be to 3D print some kind of fixture, but<br>I’m not sure.

I decided to try something that I thought would be simpler. Even<br>though we’ve lived at our house for almost five years now, we still get<br>a fair amount of mail for people who no longer live here. It’s annoying<br>to have to write “RETURN TO SENDER / NOT AT ADDRESS” on all these bits<br>of mail, so I thought I’d try milling a stamp in pine. I created the<br>design in Inkscape, then converted the text to paths and mirrored them<br>so the stamp would show correctly on the paper. Then I imported the SVG<br>into Blender, extruded it by two millimeters, and added a block<br>underneath to act as a crude handle.

After importing the model into Kiri:Moto, I couldn’t figure out how<br>to get the software to produce a tracing path around the letters in a<br>way that I wanted. I think this is almost certainly a knowledge issue,<br>although I did notice some weird selection behavior with some of the<br>letter edges. I settled for making two perpendicular contour passes,<br>which didn’t achieve the same level of smoothness in the curves, but I<br>figured it was probably fine.

As usual, the rough pass went fine, but I quickly realized that I’d<br>made the stamp a few millimeters thinner than the stock, so I spent a<br>lot of extra time milling stock for no reason....