Anyone around here played with spectra/fishing lines as a belt replacement yet? I wonder how your experiences were? Any special pulley designs you prefered? I got a printer design in mind where fishing lines could be a good first choice but i never played with it.
maybe i should as there’s a 1000-meter-spool laying around in my basement 
Would be interested in any experience you have to share.
It’s cheaper than belt. You can get good results from it, some say better than belt in some use cases.
However it is a pain to setup and tweak properly wrt steps/mm, tension and walk.
@Nicholas_Seward used them a lot and his designs and CoreXZ (http://www.youtube.com/watch?v=vp-ECU6ZiiU) is best to show how to use it. With guitar tuners as tensioners.
I like spectra line because of flexibility, which belt doesn’t have (bending, changing directions, etc.)
@Helmi I would argue that almost any RepRap could benefit (speed, acceleration, cost, sound, size, artifact reduction, etc) from a switch to fishing line. Any belts or lead screws are ripe for replacement. 3D printing is such a dainty thing. Using belts and screws is akin to using a pickaxe to make a intricate wood carving. The fishing line I use is 100lb test and I estimate that I usually only have less than 5lbf of tension.
I use V grooved bearings for redirecting the string. Those can be 623vv, 624vv, 625vv, 628vv, or a double stack of 608s. The double stack of 608s is the cheapest but not by a lot. 625vv are my go to. They are very light in comparison and weight on the gantry matters. RobotDigg has them in bulk.
For actuation, I highly encourage you to use a technique like the spool/pulley used for the 3DR/Kossel. http://trains.socha.com/2013/12/building-3dr-simple-delta-printer-part.html . I get my pulleys from here http://tridprinting.com/Mechanical-Parts/#3D-Printer-Spool . With this technique, you don’t rely on friction so there can be no slipping. One slight problem, is that the string will walk up and down the spool. I always design it in such a way that there are static redirect pulleys more than 100mm away for both strings leaving the pulley. This will keep the error from walking well under 100 microns. However, even if you completely ignore the 100mm static redirect advice you probably would never notice any errors. (Assuming you are working with a loop of string, a walking error will do more to change the tension of the system than it will to change the position.)
Now for the best part, tensioning. Amazon and ebay sell some guitar tuners for about $1.50 a piece. Figuring out how to mount them can take some forethought but it is worth it. When you first install a string, you will need alternate between tensioning and running through the range of the axis. It will take 3 or 4 cycles of this before the tension isn’t lost during operation. You might even start doubting that string is a good choice at this point. Don’t worry. After the initial tensioning on my CoreXZ, I have printed 4kg+ with zero retensioning.
Additionally, I am not nice to my printer and have accidentally grabbed/pushed the string with much force. I have yet to have the string hop off one of the 625vv’s that has an admittedly shallow groove. I once pulled a string off on purpose and it took more effort than I expected. To put it back on I just had to loosen the tension a bit and pop it back on.
Lastly, string can do every thing that a belt can except for deal with continuous rotations. For instance, you can’t use string to drive a filament drive. However, string can do tons of things belts can’t do. The routing can be as crazy as you want it to be. It can fit through a 1mm crack. I am still discovering ways I should use it.
Sorry for the long post. I hope this helps.
This is an amazingly detailled reply, @Nicholas_Seward thanks so much. Also thanks to the other guys for their answers.
@Nicholas_Seward that’s really valuable input. Though my thoughts are not so much reprap-style and on first sight i’m a bit unsure about the guitar tensioners but i’d definitely have a look into that too. I’m basically quite ok with belts so far but i’m looking into buying something more compact and will see what fishing line can do here.
Also thanks for your links. Didn’t know that there are aluminum spools around already.
@Helmi I do have to admit that I would hesitate to put string in a non-DIY product and putting guitar tuners on a product would look like a hack. (Even though any other solution to make it look more professional will be much more expensive and most likely less effective.)
Syncromesh Cables http://www.sdp-si.com/web/html/newprdbelts6.htm as seen on the Bukito http://deezmaker.com/bukito-portable-3d-printer/ might also be of interest to you. It has some of the advantages of string while being appropriate for a finished product.