Eliminating Hadron z wobble

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Re: Eliminating Hadron z wobble

Postby brnrd » Thu Dec 20, 2012 6:01 pm

A bent or off-center lead screw would not try to rotate the motor shaft if the lead screw is free to move sideways. The motor would not be forced sideways and it will not move up and down. The change in height of the x carriage should be insignificant for small bends in the lead screw or misalignment between the motor shaft and lead screw. The other advantage of this method is that you no don't have to make sure that the z-nuts on the x-carriage line up perfectly with the z motor shafts.
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Re: Eliminating Hadron z wobble

Postby cvoinescu » Thu Dec 20, 2012 9:48 pm

Sorry I wasn't very clear. A threaded rod with a rigid coupler will be constrained in the XY plane at two points: the nut on the Z carriage, and the motor mount. So, those points can't move along X or along Y. If the rod is bent, like this (, the bottom will be at an angle from vertical. (If it were vertical, the bend in the rod would move the lead nut sideways; the lead nut can't move sideways, so the bottom of the rod must twist out of vertical, just like the bottom of that parenthesis.) The rigid coupler can't bend, so this twists the motor too. It's not a linear sideways push (the Z lead nut isn't nearly stiff enough to constrain the rod to vertical at that point), it's a twist around a horizontal axis perpendicular to the radius of the bend. With a motor that tries to twist to a side, the relatively flexible mount twists too, and that results in a raising and lowering of the motor.

I have this habit of being long-winded and less than clear, so let me try a different approach.

Imagine I removed the Z carriage entirely and left the threaded rods pointing up, attached only to the motors. Push the tip of one rod horizontally toward the center of the machine. What happens? The rod, rigid coupler and motor move as a unit, and flex at the point of least resistance, which is the motor mount. That is attached to the rest of the machine to one side, so the motor lifts a little. Push the rod the other way, the motor lowers a little. Use a bent rod and hold its tip fixed, and it's the same as pushing it sideways in one direction or another as it rotates.

I think the best solution, in addition to a straight(er) rod, would be to replace the rigid coupler with a flexible coupler that allows the rod to bend out of vertical, but not move up and down (e.g. a regular flexible coupler with a bearing ball in it and the tips of the shaft and rod resting against that ball; or file the tip of the rod slightly round and have it rest directly against the shaft of the motor inside the coupler). That, and much stiffer Z motor mounts. A Z nut that allows some off-axis twist would also be good.
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Re: Eliminating Hadron z wobble

Postby Turbo442 » Thu Dec 20, 2012 10:33 pm

I am assuming the stiffer z mount has the benefit of correcting the bow in the z lead screw. I think spending a little time in correcting the lead screw bow could go a long way.
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Re: Eliminating Hadron z wobble

Postby gyrogearloose » Fri Dec 21, 2012 2:38 am

Ok guys, I have solved the problem. After battling with uneven layers ever since I had the printer and thinking it had to do with the extruder settings, it is now fixed. Check out the picture of the 2 buddhas. They are 44mm high and printed with 0.25 layer thickness. The one on the right has been printed with the wobbling Z axis. The problem is the coupling itself. 8mm threaded rod is not 8mm in diameter, it is about 7.8mm. This allows some clearance of the rod in the coupler hole. The 2 grub screws, once done up, will push the rod slightly sideways or even at a slight angle. If the grub screws were spaced at 120deg, it would probably help. If the coupling is not 100% parallel in line with the rod, the motor will wobble. If the motor moves up and down, that amount of movement is taken away from the z axis movement. If you can see the motor even move slightly, you will not get a decent print. 0.25 of a mm is not a lot. Even if the motor bracket only flexes by half that amount, you loose 50% of your resolution for that layer. The layer at 180deg motor rotation further (0.5mm) will be different by the same error, but the other way. This is where the periodic lines come from.
Here is what I did to fix it. I cut 2 delrin couplers on the lathe with a tight push fit on the motor side and an M8 thread halfway down the other side. The threaded rod screws into the internal thread and is fixed with a locknut and a washer. This way, the rod stays in line with the coupling and for the first time, I was able to print a decent buddha.
Attachments
SANY1040.JPG
SANY1039.JPG
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Re: Eliminating Hadron z wobble

Postby fma » Fri Dec 21, 2012 6:05 am

I do have flexible couplers, and I still have Z wobble...
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Re: Eliminating Hadron z wobble

Postby gyrogearloose » Fri Dec 21, 2012 6:28 am

fma wrote:I do have flexible couplers, and I still have Z wobble...

Of course you do. Flexible couplers are not the answer. The couplers I made are rigid. The secret is having the rod axis and motor shaft axis exactly in line to start with.
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Re: Eliminating Hadron z wobble

Postby fma » Fri Dec 21, 2012 6:41 am

They are! I carefully aligned them.

I can see on the left budha that there are still ondulations... Not much, but still visible. At least as much as I always get on my Prusa. I was hopping to get better results with ORD Bot; this is why I buy one...
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Re: Eliminating Hadron z wobble

Postby gyrogearloose » Fri Dec 21, 2012 7:20 am

fma wrote:They are! I carefully aligned them.

I can see on the left budha that there are still ondulations... Not much, but still visible. At least as much as I always get on my Prusa. I was hopping to get better results with ORD Bot; this is why I buy one...


Actually, what you see are the layers. It has been printed with 0.25 resolution and the figurine is only 44mm high. Where the slope of the belly flattens out on the top, the steps of the layers become more visible, of course.
Attachments
SANY1045.JPG
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Re: Eliminating Hadron z wobble

Postby fma » Fri Dec 21, 2012 7:52 am

Yes, right.

Could you make a cube or so, to have a vertical face? This is on such parts problems are most visible...
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Re: Eliminating Hadron z wobble

Postby gyrogearloose » Fri Dec 21, 2012 11:10 am

fma wrote:Yes, right.

Could you make a cube or so, to have a vertical face? This is on such parts problems are most visible...


Here is a cylinder I just printed. 30mm dia, 10mm high.
Attachments
SANY1059.JPG
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