Sustaining Engineering on an Older FDM

Just a short update on one of the various fixing-up projects ongoing in the new shop! I’ve been breathing new life into an old Printrbot Simple I’ve had for a very long time – a better printer is in my future, but as I’ve had a lot of success with this machine in the past, I wanted to get it into some sort of working order. I’ve picked up a replacement UBIS 13s hotend, which is a drop-in replacement and should extend the life of the machine (especially since the original component has been discontinued, alongside the rest of the company 😢). I’ve upgraded to a larger power supply, and I’m hoping to bolt on a PCB or silicone thermal pad to the bed, improving the adhesion with non-PLA materials.

The gear/bearing pair is blocked by a cable, but you get the picture

But for now, I just need consistent parts. One of the issues that has prevented that, and has plagued my version of the machine, is a drive system which allows for too much play in the filament path (this was addressed on later versions). The extruder functions by compressing the filament between a driven gear and a passive roller bearing, pushing it into a hole to the extruder. The spacing between the drive gear and the feed hole to the extruder is larger than it needs to be, and thus allows for the filament to kink and twist out if dealing with some fast print speeds. This is especially challenging when printing flexible materials, which I explored with mild success a while back. At the time, I solved this with a printed insert that confined the filament to a much smaller space, but this was only a temporary fix, and the part broke through continued use.

I decided to make a permanent addition to fix this – I picked up a metal standoff with an ID/OD that roughly matched 1.75mm filament and the Simple’s extruder block, and bored out the inside to give the filament a bit more play. my plan was to simply adhere this into the extruder and move along with my day. However, upon disassembling the block, I discovered a second issue at play…

The kinking issue was a newer one, meaning something had changed in how the various components interact. I’d replaced the drive gear with one containing teeth that extended to the ends of the part, as opposed to the original which had a hobbed feature. I figured this was the main driver of the mis-alignment, but something strange caused me to take a closer look.

Before shim
After shim

It turns out that not only was the gear different, but the bore was also a bit too large, allowing for some play on the drive shaft. These extruders exert a constant compressive force on the members in order to grip the filament, so this must have been pushing the gear up against one side of the shaft. The set screw did lock the angular position of the gear on the shaft, but because of its position from the point of contact, a moment arm is formed, torqueing the gear back and forth for every rotation. Not only did this repeated loading damage the set screw and loosen up the connection, it also meant the gear was almost always off parallel with the drive bearing – pushing the filament sideways and eventually causing it to escape the extruder.

With the shim installed, the play was reduced significantly. I decided to still include the guide tube – It will help with making flexible things, and I do have future plans in that space. Another mystery solved with tape and superglue 👏💨.

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