After attending Elecia White’s “On Cats and Typing” talk, I felt a little more motivated to look into robots. The robot arm used in her demo was the MeArm by Mime Industries. It is built out of commodity micro servos and laser-cut acrylic. I looked at it and thought I could get one up and running on my own. It is sold as a self-assembled kit but in the spirit of open source, people are also allowed to laser-cut their own pieces using the open-sourced DXF file available via Github.

Or at least that was the theory.

In practice, the DXF doesn’t work. Inkscape couldn’t load it. CorelDRAW couldn’t load it. Onshape couldn’t load it. Fusion 360 – from Autodesk, the people who created AutoCAD which is where DXF came from – couldn’t load it.

Well, that was disappointing.

Google results confirm I’m not alone. I found many reports of people failing to get this DXF to work for them, and not a single success story. Of course, there’s a little selection bias here: people who encounter no problems rarely go on the internet to announce they had no problems. But I would have expected a few of the forum posts from people having problems to get some positive responses, and I didn’t find any of those.

This is frustrating. I’m unlikely to go and buy the kit when I already had most of the pieces on hand and it is in theory open source for me to make my own. It’s impossible to tell if there’s a perfectly innocent explanation or if this was done maliciously to slap on the “open source” label without actually risking any cut into sales. Whatever the explanation for why this DXF is broken, it doesn’t change the fact that it is broken. When the publicly available source file is unusable. Is it still open source?

I vote no.

Continuing my self-examination for assumptions that might be holding me back, I started thinking about the fact that all the fixtures I’ve built so far for the box exercise are external to the box. This seemed like an obvious approach – tools are almost always outside of the object that the tools are working on.

But while working with various fixtures, I’ve occasionally wished for something inside the box to brace against. At various points I thought about building an internal component to mate against the external fixture, but for one reason or another that hasn’t happened. So let’s try that now.

It turned out far more successfully than I had expected. When the fixture is on the outside of the box, my hands performing assembly had to work inside the tight internal volume. But when the fixture sits inside, my hands have far more freedom to move around outside and everything is easier to do. The assembly of a test box with this fixture was far smoother than any of the test box assemblies built with previous fixtures.

Since it worked so well, I went digging into the pile of scrap acrylic and cut panels for more boxes. While putting together these boxes by hand, I thought about how I’d automate the various tasks involved. The good news is that the fixture is no longer the biggest blocker, other aspects of box assembly now demand some problem-solving time.

Task #1: Peeling the protective paper backing off the laser-cut pieces of acrylic. At the moment this is a very tedious task that demands strong fingernails and luck. If we want to make a production line of a laser-cut acrylic product, we need a solution.

Task #2: Dispensing Weld-On 16 acrylic cement. Acrylic cements like Weld-On 4, with low viscosity and flows like water, have been outlawed by the South Coast Air Quality Management District government agency. So any dream of production will have to figure out how to work with the legal but far more viscous Weld-On 16. Applying by hand resulted in inconsistent beads of cement and aesthetically ugly joints.

I took a pause from experimenting with fixtures for building a simple acrylic box, but it’s time to revisit the topic. While thinking about the external frames I had built, I started re-examining all the basic assumptions I had held during those experiments.

One assumption was that I must design the fixture to stay clear of the joints, lest I accidentally bond the fixture to the box with a bit of overflowing glue. So I started thinking what it might mean if i intentionally wanted to bond the box to its fixture. The result is the “exoframe box’, a box with an external support frame that also functions as the construction fixture for the box during assembly. This prototype led to the following observations:

Advantage:

It is stronger. Once everything is glued together, the external frame greatly increases the strength of the box. This either allows a box to handle a greater load compared to an equivalent frame-less box, or allow the box to be constructed of thinner acrylic.

It allows non-square profiles. The external frame in this prototype is rectangular, but there’s no reason why it has to be. The external frame can add contours to meet functional or artistic requirements. Say, make a network data storage computer look like an elephant, as a play on the urban legend that elephants never forget.

It lights up very nicely. The external frame adds a lot of edges and corners, which add light paths to any LED lighting in the acrylic construction. A quick test with an LED confirmed that the “bling factor” went up dramatically.

Disadvantage:

It took quite a bit of effort to keep the kerf compensation math straight. It probably shouldn’t count, though, as I assume I’ll eventually figure out a way to have the computer deal with the kerf compensation math for me instead of keeping it straight in my head.

Increased complexity in assembly since we have 9 interlocking pieces instead of 5. Somewhat mitigated by the fact that the external frame assisted in alignment of the box internal pieces, just like fixtures are supposed to, but overall a box with external frame is still much less friendly to automation than a plain box.

After being humbled by my ambition overreaching my skills, I abandoned the idea of an articulated build fixture. To keep tolerance variations under control I wanted to build a simplified version just to make sure I can do at least the simple thing. Also, doing simple fixtures will be an useful skill for times when I want to build a one-off project that needs a fixture but doesn’t justify the investment for a complex fixture.

The simplified fixture is a stack of acrylic plates, made of a mix of designs depending on the task for that layer. The common thread along all the plates are strategic cutouts to keep away from the cement surfaces. This ensures any overflowing cement will not seep into the gaps between the box and the fixture and ruining everything.

The box is built upside-down with the side pieces going into the fixture first. Once they are in place and glued together, the bottom of the box is added last. The bottom-most plate in the stack keeps the box panels aligned vertically. The top-most plate locates the square panel that serves as the bottom of the box.

This fixture tells me the kerf compensation I had been using is a tiny bit on the aggressive side. In the previous fixture, the various errors masked this fact, but in this simplified fixture there is no escaping the truth. The four side pieces of the box inside the fixture have a very tight fit. So tight, in fact, that capillary action was unable to wick enough cement into one of the joints, which promptly fell apart after the box was removed from the fixture.

Which brings us to the advantage of the simple design: I could make an adjustment, cut replacement pieces, and have a better-fitting fixture in a fraction of the time of building the overly complex articulated version.

So after the successful kerf compensation and the reminder that thickness is important, the resulting construction fixture was much better than the 3D printed version but sadly still not good enough.

The holder for each of the four sides worked well – I’m especially happy at the fact they can grip the panel with just enough force to hold it in place. This was a super encouraging result of the kerf compensation math. If I were a tiny bit off one way, the side piece would be loose. If I were a tiny bit over the other way, the side piece would be gripped too hard and cause scratches. (Or wouldn’t fit at all.) Feeling the pieces fit “just right” was very satisfying.

The problem came from the multi-piece articulated design. Even though the kerf compensation was close to exact fit between two pieces (+/- 0.1 mm) the overall dimension of the fixture depends on perfect alignment of acrylic pieces across assemblies of 5-10 pieces. I was close, but each little error adds up and the resulting box built by this fixture has errors of up to 0.5 mm. Easily detectable by the eye.

And, as should be obvious from the pictures, this fixture took a lot of work to assemble. Generally speaking, it is OK (and actually fairly typical) for mechanical design of a fixture to be more complex and time-consuming than the mechanical part itself, so the complexity itself is not a problem. The problem is that I have yet to learn all the ins and outs of designing the fixture so the desired tolerances can be maintained when my fixture starts getting complicated.

But that’s OK, learning from experiences like this is exactly why I’m doing it.