Thought this was interesting. I’ll give it a go once when I’ve got a spare moment. Would certainly save some costs vs $30 leasdcrews.
http://www.thingiverse.com/thing:112718
I doubt this will work as intended - what will happen is that the position will become less repeatable the softer the whole assembly is (think plastic). So when you drive the thing back and forth a couple hundred times, like in a typical print, you’ll end up in a different spot than you started in, leading to skewed prints and potentially crashes.
@Thomas_Sanladerer Well, the proof of the pudding etc etc. Do you have bandwidth to build a couple and run an x hour torture test? I’d sure like to see someone test this. The two sided versions with an adjustable tension screw look most promising. Occured to me you could use a stiff spring on the tension screw to accommodate for some wear.
@Tim_Rastall i’ll be pretty busy for the next months, so i doubt i’ll get around to proving my thesis.
A spring would definitely prevent the assembly from loosening up, but would introduce even more inaccuracies.
You see, the issue i’m fearing here is that the bearings can tilt under load and therefore - even just for the slightest bit - change their drive angle, which leads to a slightly different steps/unit for varying loads (like accelerating/cornering without acceleration/constant speed).
Since this is more a replacement for the Z than X or Y, if you don’t have Z lift the repeatability is much less of a concern.
@Thomas_Sanladerer I see your point and I’m aware of how even microscopic angle changes can effect z accuracy and how noticeable the resultant artefacts are. All the same, I’m keen to see what the limits are. Imagine if it was a reliable solution and you could use it on a Tantillus style configuration for x/y motion. Granted, the rods would have to spin crazy quickly to get good speeds.
It looks like there are companies selling these, and they’re claiming some decent numbers. Eg: http://www.amacoil.com/rolling-ring-how.html
Making them out of plastic may not be the key to reliability, but it might be workable. It certainly seems to me like it might be possible to eliminate some vitamins in a rep-rap with this…
If the 2 come out “exactly” the same then they’ll move the same amount per turn of the rod…this can be controlled through machining. A little less exact with 3D printing…of course it’s probably possible to clean them up within the tolerances required. I might try printing 2 and seeing if after a few hundred revolutions of the rod back and forth if they both end up at the same place.
this is pretty cool i guess Im going to print one out and see how it works
Closed loop control might be the order of the day if repeatability is impacted too much. Maybe an encoder reading true height with some kind of rack-like engagement on a structural member would do the trick?
@Jeffrey_Landrum If you’re gonna put in a rack, why not just use it with a gear to move the axis? Seems redundant. I’d rather do something like use an encoder wheel, ultra-sound distance measurement, or an optical mouse attached to the x-axis idler…
@Richard_Betel I should clarify - the rack was to provide surface you could rig an encoder to with gearing for high-resolution positioning, though it’s certainly not the only mechanism capable of providing good position data.
@Richard_Betel @Jeffrey_Landrum it occurred to me that you could potentially hack a digital micrometer to provide a true height value.
@Tim_Rastall I’ve seen that done on a taig lathe.