Prototypes Are Not Products

If you’ve never started a hardware company before, it’s very hard to understand the vast differences between a prototype, a product, and a company which supports it. A product is so much more than just the machine. It’s also the packaging, the quality assurance plan, documentation, the factory, the supply chain, vendor relations, testing, certification, distribution, sales, accounting, human resources, and a health plan. It goes on and on. All things you probably weren’t thinking about in the rush to get the Kickstarter launched.

Jonathan’s MTM Snap connector.

Our obstacle was that Jonathan had never intended MTM Snap for production. It was a design challenge experiment: to see if he could build a machine without screws. This is a great feature if you’re trying to save money as a graduate student, but from a manufacturing standpoint it had two major disadvantages. The first was that if you imagine the edges of the frame as a coastline, and you unravel and stretch it into a straight line, there’s a lot of edge. Cutting out the edges of the tiny frame would take a long time. The second, and larger problem, was that the buckles tended to come apart over time. Sure, you could just hit the loose side with a hammer if you knew what to look for, but that’s not something we felt we should be asking customers out in the world to do. In the end, we arrived at a hybrid design. One that used the locating pins, but reverted back to using more traditional fasteners.

The next challenge was that at 4"x3", we felt that the bed size was just too small. The only issue was that increasing the size of the rails, even by just an inch, would create a whole cascading host of interdependent design problems. To understand why, you need to know something fundamental about machine design.

Everyone has heard of 3D printing by now. There are lots of different kinds of machines, but the most common, an FDM printer, works by extruding hot plastic through a rapidly moving printer head over a platform. A 3D printer has very different requirements than a milling machine. When a printer is extruding hot plastic, it only has to worry about the forces generated by moving the weight of the printing head. The lighter the printer head, the less force the 3D printer has to overcome to move it around.

A milling machine, on the other hand, has a different challenge. It works by moving a rapidly rotating cutting tool through material at high velocities, cutting off tiny pieces as it travels. This generates an equal and opposite force that moves back through the cutting tool, through the spindle bearings spinning at 10,000 RPM, along every bushing, rail, and fixture contact point. Basically the force moves all through the machine and back to the part being cut. This is called the structural loop, and to be effective, it needs to be able to counter these forces by being as rigid as possible. As the size of the mill goes up, each one of these parts in the structural loop needs to be stronger and stiffer to keep the machine accurate while it’s cutting. As the mill gets larger and heavier, it needs stronger motors to move the carriages around, and those motors need more power. This is one of the reasons why you don’t see a lot of small milling machines.

That’s just part of the problem, and it’s the easy part. The harder part is something called tolerance stackup. To be accurate, the cutting tool needs to be very precisely located in space, which means all of its parts need to line up. The larger the machine, the harder it is to get everything to line up. Problems like these are just the tip of the iceberg. Everything is connected. Change one variable, and all the rest change with it. Like a game of chess, a designer needs to see several steps ahead to keep from getting backed into a corner.

Jonathan’s mill had two very important features we wanted to preserve. It was portable, and it was accurate. That portability was important to me. A mill has always been a tool you go to visit, not one that comes to visit you. We wanted to change that. What kinds of places could a machine go if you could take it with you on public transit? Accuracy was also paramount. We set ourselves the goal of achievable repeatability in the range of +/- 0.001", and then pushed it even farther.

We added handles, dust windows, an internal power supply, a new motion controller, bigger bushings and rails, a solid metal Z block to hold a newly designed industry-standard ER-11 spindle, a quieter belt drive, and a new Z-axis motor with better holding torque. Then we put the first model together and it took too long to assemble. We added holes to access internally buried screws, and zip-ties for cable routes. The engineering team polished and refined.

The software team doubled in size. Getting our desktop application, which we called Otherplan, ready for launch was proving to be more work than anticipated. We were pushing the controller board farther than it’s designer, Alden Hart, had intended. We added new features to the firmware, like tool touch-off and probing.

Perhaps the most controversial feature of the Othermill is its plastic frame. It’s the only commercially available mill to use plastic for the main structural loop. This allows us to incorporate many hidden features milled directly into the frame, creating a highly integrated mechanical system. Cable guides, wire routing, bearing pre-load flexures, alignment pins, lighting mounts, motor mounts, back panels, handles, tool holders, all get incorporated into the frame. The plastic sheets we use are lightweight, strong, and low cost. Best of all, the plastic machines quickly.

One of the things I’ve learned about manufacturing is that the way you make something is tightly coupled to how many you plan to make. The process that’s right for 200 is not necessarily right for 20,000. If what you want to make is folded out of sheet metal, injection molded, and filled with printed circuit boards, then the manufacturing world is your oyster. This is why so many products use these three technologies. They’re heavily optimized for low-cost, high-volume product runs. If the thing you’re building is not made using these technologies, then you’re going to have a harder time. The tooling needed to injection-mold the frame would have cost more than our entire Kickstarter funds, and we would have had to get it right the first time. By milling out our frames, we could stay flexible and fix problems as they arose.