Friday, September 09, 2011
"Hains" Prusa Mendel RepRap: Printing Prusa Parts
The "Hains" Prusa Mendel RapRap 3D printer has begun to replicate. It's printing parts for the "Desmonda", the second machine I will build. As you may be able to tell from the photo above, the quality is very good, and it's about time!
That quality was hard won and well earned. I've been lax in posting to the blog because I've spent weeks and weeks tweaking, fixing, modifying, calibrating, and test printing, often putting in 14-16 hours a day. You could say there have been some bumps on the road.
Here are the highlights of the challenges I've been dealing with over the last month and a half.
The first prints were such a disaster (see Movember ring first print, above), I now know, because I was setting the temperature too high. I set it to 210 at first, the default temperature for PLA plastic. Then I read that setting it at 185 was a good starting place. Even at 185 deg. Celsius, though, the plastic was oozing out of the nozzle like crazy when idle, and printing a real mess. Sometimes it would even burp plastic, which I came to realize was gas released from the plastic when the temperature is too high. Essentially, it was boiling. Lesson one: the thermistor (temperature sensor) is not calibrated. So, when the computer is set to 185, that doesn't mean the nozzle is actually 185 deg. I started lowering the temperature, and until I got down to 149, the prints got better every time I did a test print. I now print with it set to 152.
Bearing Upgrades and Modifications
The pulley design for the RepRap turns out to be quite problematic. The metal fender washers don't move in the default design, so the belt rubs up against them and causes a lot of friction. You have to turn the motors WAY up to counter it. It also makes things irregular and generally hard to manage. I used a combination of techniques to fix this problem. For the front and undercarriage pulleys I loosened the nuts on either side of the fender washers so the washers could rotate freely, but cemented the nuts in place with loctite. On the back pulley next to the motor I cut a nylon washer in half and glued it back it together inside of the fenders. They are low friction and rotate when they need to. But that's an ugly hack. The X-end idler could be taken apart, so I upgraded the pulley there. On Thingiverse.com there is a bearing upgrade object, and it comes standard on machines from MakerGear and others. Lesson two: these upgrades will replace the fender-bearing design on every machine I make from here out.
As I start to print my own RepRap parts, and after having worked with the parts I got from eBay all summer long, I've come to kind of loath them. So much of my trouble has come down to poor quality parts that I couldn't guess how many hours they took up in testing and tweaking. Notice the crack in the pulley gear in the photo above? Yeah, now you start to see why this has taken me so long, right? In addition to bad gears, the X motor mount and X end idler are really quite poor, causing the smooth rods to be out of alignment with each other, and I suspect, not entirely perpendicular to the Y axis. The extruder idler came small and out of shape, and I've had to bore it out a few times to make it so the extruder won't bind. Lesson three: get good parts! The guy I got the parts from is well intentioned, I have no doubt, but I think these Makerbot people aren't the best candidates for producing quality parts. They're the type that are attracted to the instant gratification (or as much as can be had right now) of an easy kit they can throw together in a weekend. The RepRap people, on the other hand, tend to be kind of obsessive about their machines because it takes so much to get them working right. You have to know a lot more to use a RepRap. So, I would say that parts made on a RepRap will tend to be higher quality, as a rule, than parts made on a Makerbot. MakerGear has some really fantastic parts, and if I had to start over again I would have bought their parts. I would have bought their kit, frankly, but that's a point I'll come to shortly.
This was probably the biggest problem of the entire project, from start to finish causing me headaches the whole way. First there was the issue with the stepper motors. The parts list I got from the guy on eBay included a link to a supply of stepper motors, except they were the wrong motors for a RepRap. They weren't the right size (dimensionally) and they didn't have enough holding torque. Then there was the matter of fasteners. I didn't know what I was doing, for a start, and then the parts list I got from the eBay seller had parts for an SAE Prusa, but a metric Wade's extruder. I got all confused and now have a large assortment of various screws and nuts that weren't of any use to me in the project. The pulley gears were at the heart of it, in the end. I spent weeks of calibrating after the first print, all of it wasted because the eBay seller included a link to the wrong belt. It's the kind of belt that is used overseas, and is metric. So they didn't match the printed pulleys that came with the parts. I first bought replacement printed pulleys of the same size because I thought the problem was bad parts, and then bought the manufactured pulleys when I finally figured out the problem was wrong parts. Lesson learned: don't believe the parts list supplied by the eBay seller. Do your homework, hard as it is, and understand which nuts and bolts you'll use by walking through the instructions one step at a time and counting it all up.
While trying to hunt down the backlash problem caused by using T5 pulley gears and XL belting, I learned a lot about belt tension. In the process I managed to pull off a short and messy print I could turn around and put to use right away. It's called a Y-tensioner, and is probably the first thing I downloaded and tried to print off of Thingiverse after the Movember ring and test cubes. It allows me to adjust the tension of the Y axis belt, the most inaccessible one, with a hex key without having to take the print bed off. The X axis belt has to be done the old fashioned way, and boy what a pain that is! Lesson learned: yes, there is such a thing as too tight on the belts. I'll leave it at that. Suffice is to say that you're shooting for a Goldilocks zone when it comes to belt tension: just right.
My prints were pretty good, but there was room for improvement. Early on someone had pointed out to me that it appeared that I had quite a bit of wobble in the prints, meaning that the layers didn't stack right on top of each other exactly. That can happen when the leadscrew for the Z axis (up and down) orbits a bit because the motor shaft doesn't line up exactly with the center of the leadscrew and the nut in the X ends. In my case, you may have noticed in an earlier post I mentioned that the Z clips I got from the eBay seller snapped during assembly. Well, they did, and I replaced them with rubber tubing (thanks Tonokip) but the leadscrews definately were orbiting. ScribbleJ, who has helped me a ton via IRC, designed these Z axis constraints that I printed up and added to the machine. Lesson learned: use MakerGear modifications and their design to eliminate Z wobble from the start.
That's a taste of what it took to get to printing well enough to make Prusa Mendel parts for "Desmonda" on the "Hains". I'll tell you about my software calibration journey another time. The test cubes! So many test cubes!!!