This past week, I participated in a STEAM event at one of my local libraries by teaching the kids about 3d printers. I used my PrintrBot pretty heavily in preparing for the event, because I had to print out over 200 items to bring with me. During that time, I decided that after the event was over, it would be time to convert the PrintrBot from a direct-drive extruder to a bowden style. Anyone who owns a PrintrBot Simple can attest to why you might want to do this. There tends to be a considerable amount of sag in the Y carriage in the Simple, and (at least for me) it seems to have gotten worse over time. There’s so much sag in mine, that I can put some pressure near the back of the carriage and move the hot end over 2mm in the vertical direction. A lot of this sag is due to the heavy stepper motor mounted on the end of the Y carriage.
A bowden-style extruder can help us with this problem by moving the extruder stepper motor off of the Y carriage. Instead, we can run a narrow tube directly into the hot-end, and have the extruder motor sitting off to the side, pushing the filament through the tube. This concept is similar to how a bowden cable works, thus the name. I did a little bit of research and found that Ed Sells was the first to talk about using this setup on his RepRap blog a few years back. I have a bowden extruder on my Kossel Mini already – here’s a picture of what that looks like. Notice that the hot-end has the Teflon tube (PTFE) running into it, with the extruder mounted on the side of the printer.
For the PrintrBot bowden setup, I used thing #281113 from Thingiverse. I had a couple of M5 push fits that didn’t fit into the 4mm fitting, so I drove an M5 tap into the hole and it fit fine. Here’s a shot of the bowden attachment on my Simple.
By converting the Simple to a bowden set up, we actually incur another problem. This other issue is another one of those things that’s fairly well known in the PrintrBot community. In normal PrintrBot operation, in order to drive the filament into the hot end, the potentiometer for the extruder’s stepper motor needs to be cranked up a bit. Otherwise, the extruder motor will be under-powered and skip if there’s a snag in the filament spool. When you do this, the extruder motor gets hot. Sometimes, too hot. I’ve measured my extruder motor in excess of 130 degrees Celsius. And when it gets too hot, the filament that it’s driving into the hot end gets soft and is prone to buckling and causing a jam. I didn’t notice this at first because my prints were all pretty short (an hour or less). But when I started doing longer prints, I would start getting filament jams and the prints would never finish. As a stop-gap solution, I ended up pointing a 70mm fan at the extruder motor and just kept that on all the time. That solved the problem temporarily, but it’s a hassle. Frequently, I see pictures of people’s PrintrBots in the forums with heat sinks attached to the extruder motor in an attempt to dissipate the heat. This will help, but it does add even more weight to the end of the Y carriage. The issue that you’ll encounter in a bowden set up is that there needs to be more torque to drive the filament into the hot end, because there is some distance between the extruder and the hot end. So you’ll have to crank up your e-stepper pot even more, which will most assuredly produce too much heat and soften the filament.
The solution to this is to do a gear reduction on the stepper motor. Basically, we’d be trading in speed to gain torque, which is what we need on an extruder motor. Extruder motors don’t need to be incredibly fast because we only want to drive too much filament into the hot end – it will either goop out or jam the hot end. By doing a gear reduction, you can run the motor at a higher speed and get more torque as a result, thus allowing you to lower the voltage going to the motor and keeping the motor cooler.
In order to do a gear reduction, you have two options – build or buy. You can purchase a stepper with a planetary gearbox attached for about $35.
Or you can print out the gears yourself and attach them to the stepper that comes with the PB. I have a planetary gearbox on my Kossel, and it works great. For this build, however, I wanted to do it myself, so I opted for the DIY approach.
I did some research on Thingiverse, and ultimately settled on thing #338620, mostly because of the simple design that accommodates the bowden setup without modification. Most of the other stepper gears I found were for direct drive extruders. I was fresh out of 608 bearings, so I ended up order a set of 30 from Amazon for $18.50, as well as a package of 100mm M5 threaded rods for $9.30, which I’ll cut down to size with my dremel. Right now, I have the printed pieces mounted and am just waiting for these parts to arrive to finish it up:
The next thing I need to do after that is create a mounting bracket so I can attach it to the Simple somewhere. I think I have the perfect spot, so I’ll get it created in OpenSCAD and share it when it’s done.