Direct drive.....Y axis. No belts. Am I crazy?

Building a mendel90ish style printer, and the Y axis motor is going to be placed vertically below the center of the bed. The bottom of the bed will have attached to it a belt glued vertically on a specially designed Y axis carriage (or just a printed part with the same tooth placement) Can anyone see any immediate disadvantages or deal-breakers? I do think I'll need a way to tension it....any Ideas? In the picture the motor doesn't have the pulley yet, it's just the shaft. Thanks guys!

If you use a straightened belt against a pulley you will get backlash because the pulley teeth are shaped to meet a belt wrapped around it. I.e. the gaps between the teeth get smaller, so the pulley has thinner teeth than the belt.

If you print a rack and pinion then you can get around that problem. You might want to spring the motor against it to avoid backlash due to imperfect alignment, etc.

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I planned on having a "pinching" tension system so that there would be no pressure pushing the carriage in one direction. I feel like it could get more accuracy than belts. Have you seen it done? Why isn't it more common? ...Hmm.

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Tempting to try it, and yah it would need a pinch style tensioner that applied equal pressure to both sides of the rack to really work well, that would have to float a bit on one axis, and be rock solid in the other.. not impossible. though i suspect it would have some of its own issues with loosening of components over time, likely resolved with some sort of tension adjuster on the pinch & the mount. but i also expect it would be very noisy too, unless you got the rack n pinion injection molded for smooth accurate movement.. I suspect anything less than laser sintering, would be less than ideal for gitter/noise but i could be proven wrong with the right post print smoothing technique.

Belts do have there issues, but there fairly easily over come on anything with a print volume 8" per axis or under, currently im working on a 12x12x12" now there belts get tricky with almost 3 feet of belt per axis.. after quite a few tweaks I am getting acceptable prints, but I am thinking of testing a Printerbot style Y axis to cut that belt length in half, but of course that adds its own issues as well to over come,..

If this is your first printer i might nudge you towards something proven rather than experimental,. if its anything but your first, go for it!

3D printing, its not a destination, its a journey, enjoy it, and try everything!

Edited 1 time(s). Last edit at 12/21/2013 10:55PM by AlexCphoto.

Looks like you've basically got half of a system I've seen described elsewhere (that I can't find a link for anymore) where a glued belt is used as a rack which is mated by another full length belt which is tensioned by a pair of idlers right by the pulley, just like a printrbot Y axis

[www.thingiverse.com]

pulling the motor out pulls tensions the teeth of the driven belt against the teeth of the rack so there's no backlash. Only the teeth around the idlers are engaged with the rack (the tails of the driven belt actually are loose) so the active portion of the belt is extremely short. They were driving much bigger heavier loads, but I don't see any reason why it wouldn't work on the scale of a desktop printer.

yup that's the Printrbot style,. Things to keep in mind with that set up is you are now moving the weight of the rods on that axis as part of the Y carriages momentum,. and your print bed platform now has to reach (in some way) (~1.9*YprintLength) so it can get a little bit un-weildly on larger bots for desk footprint. if i tried this id switch to polymer linear bearings and carbon fiber rods, that way the rod weight isent undoing the good you did with the ~1/2 shortening of the belt.

Routers from buildyourcnc.com have used a similar style for a while and move a gantry with router [buildyourcnc.com]