How an Event devastating to Morale resulted in a New Appreciation of Lasers.
The Quote Question
So armed with manufacturing diagrams of the parts I needed made, I first paid a visit to mfg.com, which was suggested to me as a good place to find manufacturers who would bid on my parts. After some investigating, I determined that, in fact, the most basic buyer's account costs US$100 per month, renewed automatically unless you send mfg.com an email. I just couldn't justify the expenditure.
Instead, I went to thomasregister.com, and looked for CNC-capable small manufacturers. I chose a few from Pennsylvania (i.e. local), and a few from other places around the country. In total, I chose 11 shops. I sent each of them the diagrams, explaining what I needed, and asked for a quote with a deadline of 15 days for a response.
Out of the eleven, three wrote back declining to quote. Another three gave quotes, and the other five never responded. Of the three that quoted, the total prices were $19,000, $30,000, and $101,000. I felt that the $19,000 quote was reasonable, and also came from a shop recommended by a good friend. I let the shop know that they were chosen.
Then I asked them whether they could give me some insight into the costs of their own shop, and maybe the other shops. Why were they a third lower than one shop, and 80% lower than another? A day later, I got the answer: they had just gotten a large two-year contract, and they had to clear their calendar completely for it, which included declining my job.
The Morose Maker
I spent the next day moping about. What could I do? I calculated how big a Kickstarter project needed to be assuming that gifts cost 15% of the pledge, fees from Kickstarter and Amazon ate up 8.5%, and federal and state income taxes took away another 20%. The answer was, add 77% to what you need, to get what you really need. So $30,000 would end up requiring $53,000: a huge Kickstarter project. Even if you somehow managed to class your project tax-wise as a business, you'd need to add 63%. I couldn't even imagine how Ibuilding a steam-powered all-mechanical computer could be justified as a non-profit business (so, no taxes).
So I sat around at NextFab, aimlessly looking at drawings of my parts, turning them upside-down, looking at prototype parts, fitting them together, fiddling with screws, and generally feeling sorry for myself. I went home with no solution in sight. I went to bed, and woke up the next morning with an idea.
The Illuminating Insight
Apparently all that fiddling around at NextFab primed my brain to think subconsciously about the problem when I was asleep. This seems to happen often to me: I'll be sleeping, or taking a shower, generally minding my own business, not thinking about a problem, when all of a sudden a new path of investigation will occur to me. You know how dreams often incorporate thoughts and objects encountered during the previous day? I'm convinced that these flashes require priming the brain so that the problem takes on a sort of prominence in one's mind. That way, the subconscious gets to chew on the puzzle while the conscious mind does something less stressful for a change.
That's my theory, and I'm sticking with it.
I woke up with the idea that I should take another look at the casing for each module, and try to make it thicker and out of acrylic. If it were made out of acrylic, I could use the laser cutter to cut the pieces, and not have to rely on an outside shop. My rule is that I can use any material as long as it doesn't have some essential property that was not available in Babbage's time. Acrylic is fine, as long as I'm not relying on its transparency or weight for its essential function.
That day at NextFab, I made this on the laser:
This is half-inch acrylic, which seems to have the strength I need. The large notch in the center is new: I determined, using the sample rod that you see above, that the rod would bend under its own weight (plus the weight of the bumps attached to it) if it had to span 16 spaces, but not 8 spaces. So I had to add supports crossing the center. Here's a drawing I did before I came up with the acrylic solution, showing what I mean.
The perspective kind of makes it look like part of an Aztec pyramid. By looking at this diagram, I discovered that I didn't need alternating notched and un-notched pieces: I could make a single piece.
Anyway, I started calculating. I can fit about 14 acrylic side pieces on a single 18" x 18" x 1/2" acrylic sheet. That sheet costs $44, so figure $3.15 per piece for materials. I can cut one piece every 4.5 minutes on the laser, or 13 pieces per hour. At $25 per hour, that makes $1.92 per piece for cutting. Then I need to postprocess the pieces in the CNC mill because the laser doesn't cut nice and straight, and I need to drill holes that the laser can't cut.
I haven't tried it yet, but I think postprocessing the pieces will likely take something like 3 minutes per piece, or 20 pieces per hour. At $30 per hour, that makes $1.50 per piece for postprocessing.
The total becomes $6.57 per piece. The cheapest shop wanted $16.87 for the equivalent (one notched and one un-notched piece). I had just cut the largest cost in my project down to 39% of the original cost.
The next largest cost in the project is the rods, and I'm not sure how much lower I can go with that. There's no way I can do it in-house: the CNC mill doesn't do steel, I need 34 holes drilled per rod, and I need about 1300 rods.
The Ridiculous Roland
For some reason, the Roland CNC mill is built such that the toolhead cannot reach to within three inches or so of the bed. This means that every piece has to be raised up off the bed quite a bit, which also requires ensuring that whatever is supporting the piece is flat relative to the spindle.
This, to me, is ridiculous.
It means that one has to make some kind of crazy elevated fixture for every piece one mills. So I doodled a bit, and came up with an idea for an elevated tooling plate, atop which one could put custom fixtures, attached to the elevated plate with slip-fit pins. This means that once you determine the machine coordinates for your fixture, you wouldn't have to zero the machine again if you took the fixture out and put it in again some other day.
My idea requires use of the lathe to create supporting pillars for the plate. The Roland's bed has only screw holes, and if I screwed pillars in, then the tops of the pillars would change height depending on how tightly they were screwed in. Instead, I figured I would have shorter screw-in pillars, with cylinders that slip-fit over the pillars. This way, it didn't matter how tightly the pillars were screwed in: the cylinders would determine the height. Then, pin a flat plate on top of the cylinders, and we have an elevated tooling plate.
Alas: the lathe at NextFab somehow got its spindle out of center, meaning that I can't make round things. They come out like ellipses. The repair guy is due to show up some time in the next few weeks :/
So you probably won't see much progress on the Engine over the next few weeks. Making the tooling plate and the fixtures will take up a lot of time, and once that is done, I will slowly start working on making the casing pieces. But I will blog my progress on the tooling plate.