Well met Edouard! 
Seems like the ring could be cut such that the mounting holes for the z-axis mounts are included at three points along the ring (in lieu of the hand cut plasma edge 
), then, using Edouard’s paper template to locate the ring on the sled base (a simple circle of the same inner radius as the ring with a center mark, which is easily reproduced at home. Wouldn’t even need a whole circle, as a semi circle would suffice), the mounting holes can then be traced onto the sled and the z-axis mounts placed over those marks and secured. Then the ring is centered when it is attached to the z-axis mounts.
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What is the reason for the double bearing trolley? Doesn’t this just introduce more drag on the system?
WIth a single bearing, there is a pivot point, and the bearing will pivot until overcoming any resistance to rolling on the ring. This can cause jerky motion. With two bearings, more of the force is applied to rotating on the ring, smoothing out the movement.
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The chain still acts as a pivot point though, and there’s more friction and rolling resistance to overcome with two rollers vs one.
I agree with @ImpetuousWombat that:
This is an issue because we want a straight line from the motor to the center of the router bit, and if the attachment point is pivoting around the bearing instead of moving it adds a “dog leg” to the path. It’s probably a pretty small effect, and the second bearing does add cost and complexity but I think it’s worth it.
I’ve been working on the paper template idea and I think it’s a good one. Here’s what I’ve drawn up so far:
Are there any more features we would like to see?
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Do we know that there’s less pivot at the chain with two bearings than there is at the bearing with only one?
I can say from using both, the two bearings runs much smoother!
All that the double bearing does is move the pivot point from the large bearing
to the chain attachment point.
Could someone please show the math that makes this better? otherwise, we should
stick with a single bearing for cost and simplicity.
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That’s true, but the chain attachment point being further out means that the rigid metal piece acts like a lever, exerting more force at a given angle, to overcome resistance to movement.
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so does that mean that making the brackets longer will help more?
With all of these options, we need to consider the costs involved. And there I
think the linkage approaches will be a big win, simply because they use less
material (and pack tightly onto the material so there is less waste)
David Lang
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It would apply more force, though I suspect pretty diminishing returns and issues with weight.
I agree, which is why the metal ring is a sticking point for me. I’d personally want the top pantograph over the ring & bearing setup.
The template looks good. Maybe some reference measurements in inches and millimeters? For those that don’t have a round sled, some horizontal base lines may also be welcome.
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So I recently switched to this style and I love the idea but i’m having some trouble with the calibration. I ran the built in calibration and a few things just were not right. I have the machine to where it will cut a nice straight (checked with a string) horizontal line. I also have is so the horizontal dimension is pretty spot on. I cut a 64x30 inch (1625.6x762mm) rectangle. The 64 inch dim is right on the money (measured with a measuring tape. However the 30 inch dim is 3/16 long. What am I missing. My ring is on center.
Recheck the measurement from chain end to centr of bit.
Make sure you are set to triangular kinematics, and then you only have to
dimensions to worry about
-
the distance between the motors (check that what the system finds is sane)
-
the extra length from the chain to the center of the bit.
If you are filling any any other dimentions, something is wrong.
Edit I remembered this post of @bar: Newsletter Ring Development
I can’t open it until I get back to my personal laptop, but I am imagining that the final layout pattern for the mounting holes is in those CAD files. Would that be correct? end Edit
@bar is this the final mounting layout for the rings that will be shipping next month? I am looking at setting up my final sled and wanted to have everything ready for the new ring system so swapping it in will be trivial.
Thanks!