Optimized Z-bed rail- and leadscrew-arrangement
Posted by WarriorMe
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Optimized Z-bed rail- and leadscrew-arrangement June 07, 2017 06:32PM |
Hello everyone 
I am currently designing my own 3D-Printer, and i would like the printbed to move in the Z-Direction with this one. At about 30x30 cm it will use a fairly large printbed, so cantilevering it seems like a suboptimal solution.
Most non-cantilevered Z-bed designs use 4 end supported rails (2 per side) with leadscrews between them, and one linear bearing per rail. I was wondering, however, if maybe a more efficient design could be achieved by instead using 4 bearing on 2 fully supported rails. Placing two bearings per rail would prevent the printbed from tilting around the X-axis, which i could imagine happening to a large printbed with 4 bearings placed within its plain, be it just minimally, while having a leadscrew on both sides would prevent tilt around the Y-axis.
Unless there is an obvious flaw to this idea that i have overlooked (
) i would like to try building a machine like that. I am not sure though about how to arrange the rails and leadscrews optimally. I have come up with 3 possible placement strategies:
Centering the whole motion design:
The bearings would always be loaded the same way, but the leadscrew force is offset from the bed's center of mass.
Crossing setup:
Both the center of friction from the rails and center of leadscrew force from the rails would align with the bed's center of mass, but opposing torques would be created on each end.
Offset setup:
The bearings are always loaded the same way, and the center of the leadscrew force aligns with the bed's center of mass, but the rails and thus the center of friction would be offset noticeably from the COM.
Maybe some of you can provide additional setup ideas, see additional advantages or drawbacks in my proposals, or have even tried using one of these designs, so i thought taking my concept onto the forums might be a good idea
I am currently designing my own 3D-Printer, and i would like the printbed to move in the Z-Direction with this one. At about 30x30 cm it will use a fairly large printbed, so cantilevering it seems like a suboptimal solution.
Most non-cantilevered Z-bed designs use 4 end supported rails (2 per side) with leadscrews between them, and one linear bearing per rail. I was wondering, however, if maybe a more efficient design could be achieved by instead using 4 bearing on 2 fully supported rails. Placing two bearings per rail would prevent the printbed from tilting around the X-axis, which i could imagine happening to a large printbed with 4 bearings placed within its plain, be it just minimally, while having a leadscrew on both sides would prevent tilt around the Y-axis.
Unless there is an obvious flaw to this idea that i have overlooked (
) i would like to try building a machine like that. I am not sure though about how to arrange the rails and leadscrews optimally. I have come up with 3 possible placement strategies:Centering the whole motion design:
The bearings would always be loaded the same way, but the leadscrew force is offset from the bed's center of mass.
Crossing setup:
Both the center of friction from the rails and center of leadscrew force from the rails would align with the bed's center of mass, but opposing torques would be created on each end.
Offset setup:
The bearings are always loaded the same way, and the center of the leadscrew force aligns with the bed's center of mass, but the rails and thus the center of friction would be offset noticeably from the COM.
Maybe some of you can provide additional setup ideas, see additional advantages or drawbacks in my proposals, or have even tried using one of these designs, so i thought taking my concept onto the forums might be a good idea
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Re: Optimized Z-bed rail- and leadscrew-arrangement June 07, 2017 09:50PM |
4 rails and bearings is over-constraint and will cause the mechanism to bind if anything gets even slightly out of perfect alignment. The rails are there to provide a smooth vertical path and to prevent lateral motion. Two rails should be enough to do that. It is very difficult to align 2 or 4 end supported rails that are widely separated to be parallel with each other and to stay that way.
Two rails at the middle of opposite sides of the bed should be sufficient, but you may need to use very long bearings to prevent wobble. End supported rails can be made to work but linear guides are much easier to manage and there is no play in the bearings at all.
Your drawings don't include any configurations where the rails are on the outside, the screws moved inward next to them, and the bed supported between them. I would try to put the bearings as close to the center line of the bed as possible, even if it means the screws have to be placed to one side or the other.
This is my belt lifted Z axis in my latest printer. It started as an experiment and I went through 3 revs of the design before I settled on one I liked, and it is working very well. It consists of two linear guides, two belts, and a 30:1 drive gear that prevents the bed from dropping when power is cut.
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Two rails at the middle of opposite sides of the bed should be sufficient, but you may need to use very long bearings to prevent wobble. End supported rails can be made to work but linear guides are much easier to manage and there is no play in the bearings at all.
Your drawings don't include any configurations where the rails are on the outside, the screws moved inward next to them, and the bed supported between them. I would try to put the bearings as close to the center line of the bed as possible, even if it means the screws have to be placed to one side or the other.
This is my belt lifted Z axis in my latest printer. It started as an experiment and I went through 3 revs of the design before I settled on one I liked, and it is working very well. It consists of two linear guides, two belts, and a 30:1 drive gear that prevents the bed from dropping when power is cut.
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
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Re: Optimized Z-bed rail- and leadscrew-arrangement June 08, 2017 08:31AM |
Quote
the_digital_dentist
Your drawings don't include any configurations where the rails are on the outside, the screws moved inward next to them, and the bed supported between them.
How much of a difference do you think it would make to put the rails and screws all in one line like that? The frame of my design is already almost half a meter wide, and i would need to extend it by another ~5cm to make space in between the bed and rails for a leadscrew on the right side...
Quote
the_digital_dentist
This is my belt lifted Z axis in my latest printer. It started as an experiment and I went through 3 revs of the design before I settled on one I liked, and it is working very well. It consists of two linear guides, two belts, and a 30:1 drive gear that prevents the bed from dropping when power is cut.
Your Z-Axis looks a lot like my Y-Axis, i even have a 1:2 belt reduction gearing in the center as well
I am not particularly worried about gravity suddenly changing direction and my Y-Axis dropping though 
What material are you using for the bar connecting the two pulleys? Overexcited about building my own printer i already bought an aluminium rod, but theoretical calculations show that it would twist enough to create noticeable artifacts... theoretically...
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Re: Optimized Z-bed rail- and leadscrew-arrangement June 08, 2017 11:46AM |
The shaft is made of some unknown grade of tool steel, 8mm in diameter. Using an on-line calculator, I determined that the torsion under a 8 kg load will contribute about 50 um of vertical displacement in the Z axis (IRIC).
I measured the combo of belt stretch and torsion up to 4 kg load and found a total of 42 um/kg of print mass, measured at about the middle of the Z axis. It may be slightly larger at the bottom of the Z axis, and will be smaller near the top of the Z axis due to the varying length of belt being stretched. That amount of displacement is meaningless in any application for which a 3D printed plastic part is an acceptable solution. When printing 200 um layers in PLA, the maximum per layer thickness error is limited to under 1 um, based on covering the entire bed surface with a layer of plastic (a pretty rare thing in a 3D print).
As far as placement of screws and bearing go, I am not concerned about bed wobble when using linear guides, as evidenced by the placement of the belts and guides in my printer's Z axis design. My experience with round, end-supported rails and linear bearings tells me that there is usually significant play in them which, coupled with an offset screw location, will result in the edges of the bed tilting each time the Z axis reverses direction (such as z-lift on retraction), unless the bearings are quite long and the end supported rails quite large in diameter.
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
As far as placement of screws and bearing go, I am not concerned about bed wobble when using linear guides, as evidenced by the placement of the belts and guides in my printer's Z axis design. My experience with round, end-supported rails and linear bearings tells me that there is usually significant play in them which, coupled with an offset screw location, will result in the edges of the bed tilting each time the Z axis reverses direction (such as z-lift on retraction), unless the bearings are quite long and the end supported rails quite large in diameter.
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
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Re: Optimized Z-bed rail- and leadscrew-arrangement June 08, 2017 11:58AM |
I will be using fully supported 16mm round rails, so i am not worried about them flexing, and the bearings don't feel like they have any play in them, they seem to instead have some slight pre-load to them. I think i will just place my leadscrews behind the rails then, and hope for the best
I think i will go and buy a steel rod too, because the Y axis, unlike your Z-axis, will have to be rigid under the dynamic load of accelerating my gantry back and forth. From what i can find online steel seems to perform multiple times better in that role than aluminium, and i don't think the extra rotational inertia of the heavier rod will make a big difference
I think i will go and buy a steel rod too, because the Y axis, unlike your Z-axis, will have to be rigid under the dynamic load of accelerating my gantry back and forth. From what i can find online steel seems to perform multiple times better in that role than aluminium, and i don't think the extra rotational inertia of the heavier rod will make a big difference
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