My previous trepidation about buying a power supply stemmed from the desire to avoid a big unsightly cube with a bunch of unused wires coming out of it. But this pushed me toward DC power bricks, which seem to top out at 10 amps when they output 12 volts; probably not enough for my future build platform considering that my NEMA 17 motors aren’t the most power-efficient for the task. eBay offered up a great alternative: a PC power supply from a small-form-factor PC where space is at a premium. This HP power supply, for example, would fit perfectly beneath the build platform or off to the side, and 180 watts is bound to be plenty:
I just got the build platform done, and here’s where the Prusa’s at:
I think the blue painter’s tape on the build platform looks positively lovely next to the blue and white parts. The platform is not yet heated; it’s just another piece of MDF on top of the first one, covered in blue tape. But it’s benefited from several ingenious innovations pioneered by the RepRap community. First of all, it’s held up by only three screws rather than four or even six (as my Thing-O-matic’s is). Why? Because you only need three points to define a plane. Duh! I slapped my head in amazement at how simple the improvement was when I saw this layout on the build platform of my friend’s Huxley.
Another feature of his Huxley’s build platform is that the screws holding up the build platform are spring-loaded. This means that there’s a constant force pushing the platform up, so if you need to level the platform, all you need to do is tighten or loosen the appropriate screw, allowing the spring to do all the work.
The spacers are kind of wonky-looking because they’re just what I happened to have three of lying around, but they work fine. This mechanism allows for a truly dramatic improvement over my Thing-O-Matic’s platform leveling procedure, which consists of a much more complicated dance of manipulating a screw while holding constant one of its nuts after having loosened one of its other nuts, at which point they’re all tightened again. Ugh!
As you can see, it’s not a perfect fit on the threaded rods. That tall arch in the back is too high to fit on the rod beneath it; it was clearly designed for a Prusa assembled such that the threaded rod that runs perpendicular to the others is above them rather than underneath them—when positioned above, the rod tends to scrape the Y-carriage timing belt, which is why people no longer assemble it that way. Working around this by giving the arch something to sit on wasn’t difficult, but I’m on the lookout for a more elegant solution.
More progress! After unsuccessfully trying two other designs, I eventually went with some fairly simple LM8UU bearing holders for my Y-carriage, which is just a cut rectangle of MDF. Sure is lucky I had a jigsaw and a big ol’ piece of MDF lying around. I also incorporated a tensioner for the timing belt, which I’m really excited about. My Thing-O-Matic, like the Prusa, has two timing belts for X and Y axis motion, but neither of them are easily adjustable once the machine is bolted together. This has proven to be a big pain once they eventually worked themselves loose, and the tensioning mechanisms I’ve incorporated into the X and Y carriages should make this a non-issue for the Prusa.
Notice how far the X-axis smooth rods stick out on the left side of the picture. I’m hoping to use that to convince myself that I need one of these…
Things are going pretty well here on my kitchen table. The whole X-stage is assembled and working perfectly and the Z-stage is in progress. That said, while I finally managed to get the troublesome X-ends printed successfully, I do have some reservations about using them. The walls of the column that the LM8UU bearings fit into are awfully thin. How thin? This thin:
I had two prints fail or crack before I went for broke and printed new ones with 100% infill—totally solid! I definitely think the walls need to be chunkier. I decided to try my hand at modifying Greg or Josef’s OpenSCAD source files, and I’m still working on it, but in the meantime, hopefully the X-ends I have won’t self-destruct. And in the event that they do, hey, that’s what my other 3D printer is for. :p
It turns out that power drills and 3D printed parts with weak internal structures aren’t a good mix. Who knew!? So my Chinese bearings finally arrived and I was psyched to get going on my build again. Unfortunately, I hit a snag: the smooth rods for my X-stage were about 4 inches too long. Now, this wouldn’t normally be a problem, as I just had to stick them through the X-end idler and X-end motor mount. But in Josef Prusa’s new versions that incorporate LM8UU bearing holders, each of these parts only has holes in one side; you can’t stick a rod entirely through it. In fact, several of the comments are bemoaning this “feature”.
I’d already printed the part and felt that modifying it would be easier than printing a new one. All I really had to do was drill holes in four thin walls, right? What could be so hard about that?
Oops. I guess a power drill wasn’t the right tool for the job. In retrospect, this should have been obvious, and I had even been successfully using my Dremel to file down some bits earlier, which would have almost certainly worked due to its greater precision and lower power.
In the meantime, I took droftarts’s suggestion in the comments section and compiled Greg Frost’s own version that fixes this problem. I’ve become a great fan of Greg’s work. His Github is just kind of a magical wonderland of 3D printing goodies. My new Greg Frosty X-end motor mount is printing now, but it’s another annoying delay to getting this Prusa up and running. On the other hand, once it’s done, it’ll be state-of-the-art, with the latest parts!
That’s my kitchen table. I’ve gotten the frame of my Prusa Mendel built, and I’m just waiting on some LM8UU linear bearings to arrive so I can move on. You might wonder why one of the parts is red… yeah, there’s a story behind that. I’ve opted for a few deviations from the “standard” Prusa—if you could call anything open-source a standard.
Mainly, I’ve decided to use LM8UU linear bearings on all the smooth rods rather than the standard printed bearings. I’ve heard stories of the printed bearings wearing out over time, and especially for the build platform and extruder carriage, replacing the bearings will mean recalibrating for the new Z-height every time. That seems like a pain in the neck, and I managed to order 12 LM8UU bearings from some Chinese company called “CoolCheapWorld” on eBAY for only $20! The downside is slow shipping; I ordered them over three weeks ago and they’re still not here yet. Guess that’s what I get for ordering stuff halfway across the world.
In order to use the bearings, I had to print different X-ends that were just recently created by Josef Prusa himself. They’re a very clever new design. I also printed a copy of Greg Frost’s neat LM8UU-compatible X-carriage.
All in all, I’ve spent $487 so far, which I consider to be a fantastic deal considering that I bought my Thing-O-Matic for $1,300, and it’s got a smaller build platform and can’t print even half as fast as the Prusa will be able to.
I finally got around to making Jetguy’s Thing-O-Matic extruder mod to make it print in PLA better. The primary motivation was a frustrating experience with a multi-hour PLA print that was beautiful until a bulge jammed in the barrel when it was 93% done.
All you need to do is cut some slits in the barrel below the heat sink to retard the upward flow of heat. It turned out that a jeweler’s saw is unnecessary and in fact insufficient for cutting through the stainless steel, but if you have a vise and a hacksaw, you can do this in about 15 minutes. I probably spent more time removing and reattaching the hot end than I did cutting the barrel.
The result? It helped! …a little. Not as much as I would have liked, unfortunately. The mod did retard the flow of heat, but not enough that I could touch the top of the barrel without feeling like I was gonna burn myself. The tube really needs active cooling. Unlike most Thingiversians and Reprappers, I don’t have a seemingly endless supply of randomly-sized DC computer fans lying around, available for bolting onto electronics projects, so I took the only fan I had and and aimed it at the bot:
It’s a shotgun solution, and it’s not perfect because it cools the heater while it’s trying to heat itself, but it sure keeps the tube nice and frosty! and I’ve been able to print for hours without any jams. You just really need to cool that tube to have any hope of printing anything large in PLA on your Thing-o-Matic.
The awesome thing about a 3D printer is that you can use it to mock up a new design at close to zero marginal cost. In designing my new super-small NEMA 8 extruder, I’ve been able to come up with an idea, model it on my computer, and then bring it into the world. Here’s my quick-and-dirty first draft of a bracket to connect the motor with a worm drive, printed in a festive blue ABS:
It’s a pretty magical experience. It also lets me quickly and easily identify design problems and correct them. You can see the progression:
Alas, the screw holes are unfilled because nobody local has any M2 screws. Also, I didn’t actually design the worm drive’s gears; I grabbed ‘em off Thingiverse because of coursesomeone has already made a printable worm drive! It’s a 16:1 gear ratio, which is probably too much for my needs: the motor’s maximum torque of 40 mNm works out to 640 mNm when geared down 16:1. That’s definitely overkill, and it will also limit the speeds I can rotate the drive shaft that feeds the filament.
Now that I have an adequate mounting solution, I need to design a drive for myself that’s probably somewhere between 4:1 and 6:1, and I need the circular gear to firmly hold an axle that will either hold a toothed pulley for feeding the filament, or will be hobbed to itself feed the filament. I’m super-happy with the toothed pulley on my Thing-O-Matic’s extruder, but Makerbot is charging $12 for it, and I’d still need a 6 mm axle for it to sit on. Add up the cost of the motor ($24), the pulley ($12), the four LM8UU bearings I’m planning to use for motion ($8), and a few extra bucks for the fasteners, and the filament drive part of the extruder would approach $50. Of course, as a point of comparison, that’s the price Makerbot charges for just the motor on their extruder.
It might be more cost-effective to just buy an appropriately sized bolt and somehow hob it myself…
Even though I haven’t actually started building my Prusa Mendel (half of the parts are still in transit), I’m already thinking about what’s next. It seems to me that one of the biggest challenges in FDM 3D printing is the issue of how to make the extruder move faster. The current trend is to make the print bed stay in one place (or just move in the Z-axis) while the extrusion head zips around. Of course, it can’t do that very well if it’s really big and heavy, and the current solution involves separating the big heavy motor that pushes the filament from the hot nozzle that melts it, connecting the two with a flexible cable that the filament travels down. This is known as a Bowden extruder system. The Ultimaker uses one, and you can see it pretty clearly:
*Drool* Oh how I want one…
But the Bowden extruders have a major problem: hysteresis. That is, when the motor does something with the filament on one end, it can take a bit for that change to reach the nozzle, especially with springy or compressible plastics. That reduces the motor’s ability to control how much filament is extruded with any degree of precision, depending on the length of the Bowden cable and how well-tuned the mechanism is.
I got to thinking: what if you could design an extruder with a motor that’s light enough that it could be mounted on the moving mass with the hot end? This would of course be the much sought-after small motor with high speed and high torque. In the real world, generally you have to settle for only one of those characteristics, or maybe two if you’re willing to pay a lot.
I just picked up a cute little NEMA 8 motor. Here it is next to the much larger NEMA 17 motor that drives my Thing-O-Matic’s Z-axis:
It’s a quarter the size! And with that reduction in size comes a reduction in torque. The little NEMA 8 is only capable of outputting 40 mNm (that’s milli-Newton-meters) compared to 240 or so for the NEMA 17. But just how much torque do you actually need to push 3mm filament sandwiched between a toothed pulley and a ball bearing?
…It turns out that nobody’s really sure how much torque it takes. There are ballpark estimates, of course. Ed Nisley estimates that it’s between 100 and 700 mNm, with maybe 300 mNm being a good place to start.
300 mNm is a lot of torque to ask from a motor that can only put out 40. I’d have to slow it down with a 7.5:1 gear ratio. That’s certainly possible, but it limits how fast the feed pulley can go if the motor itself has to be spinning 7.5 times faster. The motor on my Thing-O-Matic’s extruder can go a maximum of 50 RPM, so I’ll use that as a theoretical maximum for my little NEMA 8. That means the feed pulley would be able to turn at a maximum of 6.66 RPM (50 RPM / 7.5). Is that enough? That’s what experiments are for!
I’m still tweaking my PLA setup, but it’s getting better. Here’s a 70 mm tall Yoda I just printed with a nice small layer height of 0.15 mm:
There’s still work to be done around the face and ears, but I know what the problem is. I was extruding this PLA at 225ºC, but I think that’s too hot. It was taking too long to cool, so by the time the next layer was being laid down in areas with overhangs—such as the chin and face— the earlier layer was starting to droop. I’ve since dropped my extrusion temperature to 215º and the drooping has been much better. I’m also going to try directing a desk fan at the in-progress print until I can manage to hook a computer fan up to the extruder assembly.
You can get a sense of the detail my bot is physically capable of by looking at the rear of his cloak:
It’s like a real high-resolution rapid prototyping machine!
…
to file down some bits earlier, which would have almost certainly worked due to its greater precision and lower power.
