Sunday, June 2, 2013

A better filament drive?





Ever since I saw Sublime's Tantillus 3D printer using spectra fishing line, I've ben really intrigued by the idea, in fact I built 2 Hbot prototypes using the idea.
The fundamental issue that comes up is that the fishing line "walks" as it wraps around the pulley, this changes the angle of the line, which in turn affects the tension. The problem is worse on a HBot because of the single long belt the number of times that the belt wraps around the pulley to go extent to extent can be large. In fact on my first prototype where I didn't try and address this there is a noticeable change in tension.
There are basically two way to approach the drive Tantillus passes the line through the "pulley" and wraps the filament on and off, this makes it impossible for the line to slip, but it's also difficult to address the walking filament, on Tantillus the combination of short axis and linear line path make this a minor issue that doesn't seem to impact the print quality.
The other option is to have sufficient friction on the drive pulley to prevent slipping, this requires a significant contact area between the line and the pulley and significant tension on the line.
The idea of the figure 8 line drive above is to use an idler to continually reset the filament position, basically the drive pulley in this case has 4 grooves, and the idler 3 that are offset by 1/2 the groove spacing. The filament always rolls onto the top groove on the drive pulley and off the bottom one.
There is no filament walking, and the equivalent of multiple wraps around the drive pulley to provide additional friction.
You can see the prototype running here


As a point of reference I'm running it off the LPC1768 based electronics discussed in other posts.
The idler has 2 9mm bearings in it's center.
I stole the idea for this from somewhere, I think the first time I ever saw it suggested was on the CNC Zone forums regarding wire drives for routers.
I'm pretty happy with the friction from the 3 figure8's, I might reduce the groove spacing on the drive and idler from 1.5mm to 1mm, but 1.5mm seems to work well.
My intent is to use this mechanism on my 4th HBot prototype, though I intend to use the CoreXY mechanics rather than the HBot mechanics, I have most of the X/Y Gantry laid out in CAD, hopefully I can get the CAD work complete this week.




31 comments:

  1. Excellent work. Do you think they have to be aluminium or will printed pulley work?

    I have come up with another solution to the issue of the filament walking. Make a drum with a circumference equal to the distance required for the cable to move (or 50%) and then a planetary gear set inside to give you back the steps per mm wanted for resolution. This way the drum only has to turn once (or twice).

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    1. Yes I think you could print them, the actual dimensions aren't critical. I happen to own a lathe and it was the easiest was to get something workable.

      I've also considered using a thread that moves as the filament rolls on and off, but I can't see how to do that without adding at least some slop which would end up as backlash.

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    2. A friend had the idea to make the drum slide on a keyed shaft so the entire drum could wander instead of the cable alone.

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    3. Yes that's one of the two Ideas I had for the moving thread, but the keyed shaft would have at least some rotational play for it to slide nicely.
      The other way to use a moving thread is to mount the stepper so that it could move as the thread travels through a fixed nut, but again there would be play in that.

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  2. You could try and make something like this http://www.rpmechatronics.co.uk/shop/products.php?10427&cPath=656_657_660 from a rod out of a set of window blinds and some printed parts (or milled delrin, nylon). Or you could go fancy try something like these http://www.euro-bearings.com/lin18.html which you may be able to make using a cheap LM6/8uu bearing and a hex shaft.

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  3. Nice design :)
    My main concern would be that the grooves on the pulley would probably have marginally different depths, this would cause continous slippage of the filament and unnecessary wear I guess?
    Have you noticed this on your prototype or are your pulley grooves just that good? :)

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  4. I haven't noticed accumulated error on the trivial prototype above.
    The depth of the grooves ought to be at lest consistent to +/- 0.05mm and should be probably closer to +/-0.01mm. The line having some stretch will eat some of the slippage you'd see.
    The other thing to note here is that even if you get some continuous slippage, the direction of that slippage would reverse when the direction of motion reversed, so I don't think it would accumulate.
    I have a 90% complete X/Y stage using the system, if I get a chance it should be functional this weekend, and I can try running some more interesting tests including long runs to measure and accumulated error. My best guess is that over the few hours involved in a print, the error is probably not significant.

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    1. I didn’t mean that the issue would result in an error; the main concern would be wear on the filament.
      If the grooves have different diameters it would mean that they would drive different length of filament per revolution. The line would slip in some groove(s) to conform to the length drawn by the groove with most friction.
      But I think you have excellent tolerances and that this probably is a nonissue for you.
      I on the other hand would not come near this on the old manual lathe I have available.
      I'm about to build a delta robot from scratch and been researching different filament solutions, this is by far the most interesting.
      I’m looking forward to hearing more about this. :)

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    2. I machined them on my manual lathe which is about as crappy as lathes get. When I say the tolerances will be good, it's not because I think the accuracy is high, but because all the grooves are cut relative to each other, so it becomes about the repeatability and given the short length of the part, the only thing that would throw that off (significant runout) is mostly a none issue.
      The most painful part of the machining is getting the outside diameter right, but that's all about cut, stop, measure and creeping up on the correct size.

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    3. Ah,I don't think I can get that kind of repeatability on the lathe I'm using. I think I could probably get within 0.1mm, I think I'll give it a try and see how it turns out. :-)

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  6. Why not have spiral cut grooves in the drive pulley? The line won't creep/walk if it's constrained from doing so. A simple worm gear pulley would prove this concept.

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    1. It creeps up the spiral, unless the spiral is held like a thread in a captive nut and moves as it turns, as discussed with Sublime above.
      Try it with a screw thread and notice how as the screw turns, the line moves along the length of the screw, in fact this is exactly the same issue as seen in the not spiral cut case, but it's easier to see why it happens.

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  7. Hi Rob,
    Happy new year!

    Do you know if spectra fishing line is superior to kevlar wire to this application?

    I think the fishing line could expand/compress a little bit, right?

    Best Regards,

    Alan

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    Replies
    1. The line is supposed to have minimal stretch I think they claim <1% but it does stretch noticeably when tensioning, it doesn't seem to become looser over time, but I don't have a huge number of hours on the test system.

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  20. Hey Rob,

    Im building an x-axis control aiming to use this sort of system. Is the length between the pulley and the idler variable? would i get less friction/more slippage if the pulley and idler were sat close together? I am theorising about having both on the same NEMA 17 stepper. the pulley in the centre and the idler in a corner. there would only be about 3cm between the centres of the pulley and idler. I have metal cylinders which i can have grooves cut in, but there's about 5mm gap between the two cylinders when mounted. you have a good 5cm between pulley and idler. do you think there will be any issue having them close together?

    Thanks

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    1. **sorry, you don't have 5cm space but i meant you have more space than 5millimetres!

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