I love rapid prototyping!
I’m working on an extruder design to reliably print with flexible filament on my heavily modified 2013 PrintrBot Simple (I call it “Grandpa’s Axe”). I think it might work on other wood models as well.
I started with http://www.thingiverse.com/thing:615465 , and veered off pretty heavily from there. The actual extruder block doesn’t have much left of the original design, but the lever has only been modified to use the larger spring I had on hand. The hot end mount lines up with the original hole in the wood part. I’m using an E3Dv5, but the stock Ubis should work as well.
I’ll reprint it (in a better color for my printer) and give it a try before I post it to Youmagine.com .
Did anything change regarding the ability to print flexible filament? Thomas, who did the original design, and i decided to try to use it for our Hackerspace printer, i would be interested if you were able to improve on the performance.
I did something very similar, helped considerably. I found a better solution, though. I switched to 3mm filament. My 3mm extruder setup has nearly a 10mm gap between the top of the hotend and the bottom of the gear, I can run the 3mm at 45mm/s without any issue. It is kind of crazy how much better it runs.
@Bjorn_Marl
Please tell Thomas thanks for the original! It was a great inspiration.
His design has a threaded mount for the hot end; I needed one to mount my E3Dv5 or an Ubis (same mount). I ended up getting tighter, and covering more area around the hobbed pulley. This design is, as yet, untested. I’ll probably be testing this evening.
We are testing similar changes to our Alu extruder 
@Brook_Drumm commented on my post! <swoon!>
LOL. Seriously, though, I really should just break down and buy one or two of those aluminum extruders if they’ll handle flexible filaments well.
The original ALU extruder does not handle flex well. Too much space between hobbing and feed hole into the extruder. The flex tends to make Dairy Queen ice cream cone like blockages in that space. Brook showed a mod, bending part to get the feed hole/guide almost touching the hobbing.
Damn. Looks like I need a 9th iteration.
I look forward to your 9th version. I’m still running my wood printrbot. It is my fastest printer.
@Alan_McNeil
I still had too much gap on the idler bearing side. I’ve designed the new one (I also love OpenSCAD!), but I’ll have to print and test it tomorrow.
I’m doing the whole thing in PLA, by the way. I think the combination of distance from the hot end and maintaining the wood mounting parts should help shield it from the heat.
Okay! Version 10 seems to be extruding reliably(-ish). It will slip if I try to push the filament too fast, but seems to push it out the hot end at an even rate. I’m running Polyflex black at 205C. I just have to work on adjusting things for good adhesion of the first layer.
I need to clean up my OpenSCAD code, and I’ll post the code and STLs soon.
I did my dual gear test in PLA also. No problem with PLA. Test failed because of too large a gap.