No-leadscrew, single-motor, precision Z-axis design April 17, 2018 04:41AM |
Registered: 8 years ago Posts: 28 |
Re: No-leadscrew, single-motor, precision Z-axis design April 17, 2018 06:58AM |
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Re: No-leadscrew, single-motor, precision Z-axis design April 17, 2018 07:42AM |
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Re: No-leadscrew, single-motor, precision Z-axis design April 17, 2018 08:41AM |
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Re: No-leadscrew, single-motor, precision Z-axis design April 17, 2018 10:17AM |
Registered: 11 years ago Posts: 335 |
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MKSA
Perfect to demonstrate how to properly constrain, just that the "We assume we are using perfect wheels, no play bearings, non stretch cables, perfect flat and square surfaces..." foreword is missing.
Re: No-leadscrew, single-motor, precision Z-axis design April 17, 2018 01:43PM |
Registered: 8 years ago Posts: 5,232 |
Re: No-leadscrew, single-motor, precision Z-axis design April 17, 2018 02:02PM |
Registered: 8 years ago Posts: 28 |
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the_digital_dentist
Interesting. My first impression is that it looks like too many moving parts with too many critical relationships. In a 3D printer, the allowable error is very small because the error shows up in the print surface. I've seen a couple printers made with cantilevered beds and arrangements of crossed cables at the far end of the bed to try to keep it from bouncing. I have no idea if it actually does what it's intended to do.
It will work well if the bed/support can't flex, cables can't stretch, there's no play in the bearings, there are no losses in the bearings (friction), the guides have perfectly smooth surfaces, and are perfectly parallel to each other and the cables. Good luck getting all that working at once. Plastic? Hmm. Printed plastic? Double hmmm.
Do you have an example of any real machine that uses this type of mechanism to achieve 10 microns of positioning accuracy and precision (which would be sloppy for a printer expected to print in 100 um layers)? Are you going to build one?
Re: No-leadscrew, single-motor, precision Z-axis design April 17, 2018 02:59PM |
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Re: No-leadscrew, single-motor, precision Z-axis design April 17, 2018 03:43PM |
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Re: No-leadscrew, single-motor, precision Z-axis design April 17, 2018 04:16PM |
Registered: 8 years ago Posts: 28 |
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the_digital_dentist
I wasn't trying to s**t all over it, just pointing out some of the places where I think there are likely to be problems. The counterweight doesn't have anything to do with the "bed" flexing. The problem is trying to lift the bed from one corner. The edges/corners away from the lifted corner are going to lag the motion at the lifted corner, moving both up and down, even with the other cables. Another of your assumptions is that the guide planes are infinitely rigid. That would help keep the bed from tilting, but real guide planes are likely to have some give, especially near the center of the their length. 10 or 20 um is a very small error, but it's enough to cause a print quality problem.
It's an interesting mechanism, and I would like to see how a real machine built this way actually performs.
Re: No-leadscrew, single-motor, precision Z-axis design April 18, 2018 03:14AM |
Registered: 8 years ago Posts: 5,232 |
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Looking at the top diagram, if the bed rotates about +Y or -Y, the vertical cables will attempt to pull in towards the center of the carriage. Since the cables are under tension, this will be resisted and there is no rotation. To rephrase, any rotation should increase cable tension.
Re: No-leadscrew, single-motor, precision Z-axis design April 18, 2018 05:15AM |
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Re: No-leadscrew, single-motor, precision Z-axis design April 18, 2018 11:23AM |
Registered: 6 years ago Posts: 21 |
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o_lampe
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Looking at the top diagram, if the bed rotates about +Y or -Y, the vertical cables will attempt to pull in towards the center of the carriage. Since the cables are under tension, this will be resisted and there is no rotation. To rephrase, any rotation should increase cable tension.
The cables at the side only increase their tension when the rotation force is in CCW direction. Turn it CW and the tension will loosen.
I've seen similar cable tensioning, but there were always two cables on each side.
Re: No-leadscrew, single-motor, precision Z-axis design April 18, 2018 07:13PM |
Registered: 8 years ago Posts: 28 |
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o_lampe
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Looking at the top diagram, if the bed rotates about +Y or -Y, the vertical cables will attempt to pull in towards the center of the carriage. Since the cables are under tension, this will be resisted and there is no rotation. To rephrase, any rotation should increase cable tension.
The cables at the side only increase their tension when the rotation force is in CCW direction. Turn it CW and the tension will loosen.
I've seen similar cable tensioning, but there were always two cables on each side.
Re: No-leadscrew, single-motor, precision Z-axis design April 18, 2018 11:52PM |
Registered: 8 years ago Posts: 28 |
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Melty
Instead of being fixed in place, could the anti-rotation cables be auto-tensioned with suitably strong springs to eliminate some of that risk?
Re: No-leadscrew, single-motor, precision Z-axis design April 19, 2018 02:33AM |
Registered: 6 years ago Posts: 1,007 |
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Melty
....
Instead of being fixed in place, could the anti-rotation cables be auto-tensioned with suitably strong springs to eliminate some of that risk?
Re: No-leadscrew, single-motor, precision Z-axis design April 19, 2018 04:20AM |
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Re: No-leadscrew, single-motor, precision Z-axis design April 19, 2018 04:24PM |
Registered: 8 years ago Posts: 28 |
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o_lampe
e.g. the spring loaded nesting wheel ist at it's highest load when the bed is horizontal, the spring will always try to push the corner up/down if possible.
Re: No-leadscrew, single-motor, precision Z-axis design April 20, 2018 02:39AM |
Registered: 8 years ago Posts: 5,232 |
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Oh, true that. I think this could be solved by setting the nesting force vector at a slight Z angle (no longer parallel to the bed plane), with the wheel's pivot hole drilled not straight down along Z, but with a small X/Y component. That way, the nesting spring's normal operating load would be slightly less than maximum, and it would have to 'climb' over the maximum when rotating in the bad CW rotation. So the nesting spring, in this case, would provide nesting force for all 5 DOF constraints. Depending on the angle, it would provide slight additional tension on the wires, along with the carriage weight. I'm having a brain fart moment though and can't determine whether it should be angled up, or down (up, right?). Is this a crazy idea?
Re: No-leadscrew, single-motor, precision Z-axis design April 20, 2018 04:05AM |
Registered: 8 years ago Posts: 28 |
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o_lampe
You could do that, but it's against the rules of exact constraint theory. Tilting the axis of the wheel could introduce banding.
You could also use linear bearings running on a smooth rod to replace the nesting wheel, but it's the same rule breaker.
Let's face it: You'd need two sets of anti-roll cables on X and Y-plane to obey the rules.
Re: No-leadscrew, single-motor, precision Z-axis design April 20, 2018 04:58AM |
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Re: No-leadscrew, single-motor, precision Z-axis design April 22, 2018 03:05PM |
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Re: No-leadscrew, single-motor, precision Z-axis design April 23, 2018 01:48PM |
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Re: No-leadscrew, single-motor, precision Z-axis design April 23, 2018 02:32PM |
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Re: No-leadscrew, single-motor, precision Z-axis design April 24, 2018 01:04AM |
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Re: No-leadscrew, single-motor, precision Z-axis design April 24, 2018 04:47AM |
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Re: No-leadscrew, single-motor, precision Z-axis design April 28, 2018 06:55PM |
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Re: No-leadscrew, single-motor, precision Z-axis design April 29, 2018 01:36AM |
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Another way is to use ground cylindrical rods as guide surfaces, with flat wheels.
Re: No-leadscrew, single-motor, precision Z-axis design April 29, 2018 04:24AM |
Registered: 8 years ago Posts: 28 |
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o_lampe
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Another way is to use ground cylindrical rods as guide surfaces, with flat wheels.
That's not the same.
When the flat wheel rotates around Z-axis ( center of bed ) and the cylindric rod is fixed, the contact point wouldn't be tangential to the z-axis. That's another constraint.
I hope, I made my point clear.
Re: No-leadscrew, single-motor, precision Z-axis design April 30, 2018 11:28AM |
Registered: 8 years ago Posts: 5,232 |