EDIT 23/09/2014
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I have just uploaded a new 2.01 version with minor updates and I'm unchecking the "work-in-progress" checkbox, since I have been receiving lots of positive feedback about this project, from which I understood that it is working as expected. YAY!
From my personal experience, I can say that replacing and removing hotends for maintenance is a piece of cake, and I have been swearing a lot less lately.
I still need, though, to write a step-by-step guide and a complete hardware list as well. But time is short, and I apologize if I can't dedicate enough time to this thing or answer your private messages as quickly as I'd like to.
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This is the new version of my "universal" extruder (version 1 is here http://www.thingiverse.com/thing:378330)
After the initial success of that project I decided to upgrade it and tried to address some problems:
• the extruder is attached directly to X carriage, so a table is no longer needed; it is far easier now to remove and re-attach the extruder;
• with less parts, it takes less time to print and saves printing material;
• without the table sides, the bottom part is free for every add-on you need: fans, fan ducts, servos etc
• extruder stepper is over the extruder body, improving balance and keeping gears towards the user;
• better wire management: there is lots of room to run wires for your servo and extruder and fans between the extruder and the x-carriage
• gears included: for this project you have to use kinda small bolt gear, otherwise it will get on the way of the hotend adaptor.
The fan duct holder must be developed, it's not as sturdy as I wanted.
For now, I abandoned the idea of using the hotend as a probe for the auto bed leveling system.
This project doesn't replace the previous, it's just an upgrade (although in a near future I might leave the other one, since this is very improved)
Special thanks to everyone who contributed with valuable ideas for this development, specially Thiago Peixoto http://www.thingiverse.com/thiagopeixoto16/about and the whole reprapbr group.
Teaser - http://youtu.be/Slvnd0vNulA (sorry, no subtiltes yet)