Would you suggest a new user mount the linkage to the temporary sled or complete the basic build first?
I’m open to all input from everyone really.
That’s a good question, I think I would say that if you’re going to go the temporary sled route there is probably no need to mount linkages to it. The parts you cut using the temporary sled are not really dimensionally critical. The one thing I wouldn’t cut is the hole in the center of the final sled, it will be much easier to mark the mounting points for your linkage if there’s no hole in the center of the sled. (I thought someone was going to (or did?) update the AllParts.svg file so you have an option to drill linkage mounting holes with the temporary sled? IDK)
Maybe others have a different opinion. As someone who never made a temporary sled I can’t speak into this with much authority.
-Logan
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How about a totally different design?
Seeing the sprockets that put tension on the extra chain made me think of this design idea. Instead of a fixed ring, use 2 chains (of equal length) wrapped around a circular piece of plywood (or something) attached to the sled. The circular plywood would need to be precisely centered with the router bit, and it would have a double groove around the outside to retain the chains. The loops of chain might be secured with some clips or something, and the chain from the motor would be attached to an idler sprocket.
I haven’t built a prototype yet, but think this would be super simple and fairly inexpensive. The only parts are some plywood (which might just be lying around anyway) a couple idler sprocket pulleys, and some extra chain. The size of the sprockets, and lengths of the loops might have some effect on the performance, I still need to think about that a bit.
It could probably be done with some paracord and v-groove pulleys, but it might be difficult to get the loops exactly the same length.
Forgive my sloppy inkscaping - first time for everything 
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I was thinking about an idea sort of like this a couple weeks ago but I was thinking of attaching the chain loops to the sled at two fixed points - that was the critical flaw in my idea, with two fixed points the arc of the idler sprocket traces a section of an oval, not a circle. My idea was not a good one.
But the way you have it, with what appears to be loops of chain, allows the effective “end points” of the chains that attach to the sled to move as the angles change (at least I think that’s what you intended). I think it’s a great idea and totally worth exploring! Your wooden circle could be as big as you need to clear your router (maybe make it the same as your sled?). If it works this could be an incredibly cheap solution!
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Thanks -
Yes, the idea was that the points where the chain loops contact the ring/circle would move. If you envision the points of contact on the sled and on the idler sprocket, it is basically a 4 bar linkage with fixed lengths and moving attachment points. I feel like having a larger sprocket would be beneficial, but I can’t put my finger on why. ( it would be like having a larger piece on the top of the linkage. . .hmmmm 
)
One consideration is the weight, too long of loops or a heavy sprocket assembly would exacerbate the slack chain sag when at the horizontal edges of the work surface.
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how do you keep the sled from rotating when the chains are uneven?
That’s the problem we’re trying to solve (because letting it rotate introduces
pretty ugly math)
k
I hadn’t planned on keeping it from rotating. I don’t think rotation matters with this design. The sled could rotate freely, but the center point would not move. I kind of assumed that the vacuum hose and brick and wires would keep it more it less at the same angle.
I’m thinking short loops and larger diameter sprockets will maximize the lateral force at the sprockets, in case there are issues with jerky movement. I don’t think there will be if I can get decent idler bearings.
Actually it would be easy to get equal length with rope/cord, just make a single big loop and double it over. That would be really cheap to use with v-groove roller bearings instead of chain and idler sprockets.
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I’m not picturing how this would work that would allow it to rotate with the
chain around a ring and not have it affect the cutting position. could you post
some sort of sketch of this?
It is effectively the same as the bar/ring solution, except the bar is replaced by the path the sprockets follow. Since the chain loops aren’t fixed (within the range required for moving the sled), the distance from the end of the chain to the bit is fixed, and the angles change freely.
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This is a pretty poor example but it’s what came to mind when trying to think of a way to explain it: Imagine putting a rubber band around a glass, then stretch it out and put a pencil in the rubber band also. So now the band goes around the glass and the pencil. The glass and the pencil are parallel. Move the pencil away from the glass so there’s about 2 inches of space between the two. Imagine moving the pencil in a circle concentric to the glass. Because you’ve effectively made a geared system in order to keep the tension on the rubber band equal on both sides of the pencil the pencil will have to rotate around it’s own axis as it translates around the glass’ axis. A rubber band is a terrible example because it’s elastic.
Another example, imagine a two bike sprockets and a bike chain (loop). Imagine they are not on a bike but laying flat on a table with the chain around both of them. If the large sprocket was attached to the table so it couldn’t rotate or move you could imagine how the smaller sprocket could “orbit” around the larger one by rotating and translating. The chain would keep the orbit perfectly concentric to the larger sprocket.
This is the basic idea of this design.
The overall goal of any of these ideas (linkages, rings, bearings, chains, ropes, etc.) is to allow (force) the tip of the chain to move along an arc that is concentric to the router bit itself. This concept does that in a pretty elegant way… weather or not it works in real life is a different story but I believe conceptually it’s pretty solid.
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Thanks for the image, I was not getting that at all.
I don’t see any reason that wouldn’t work.
note that you would need to put the grooved plywood on standoffs, with the r2200 router you want about 3" of space between the chains and the top of the sled (2x4s ripped to slightly narrower width can do that well)
whatever you use for the connection to the idler sprocket needs to not stretch measurably under the 66 pounds max force. hat besides chain can do this well?
you don’t need teeth in the large ring, even with chain (who cares if it slips), but you would want the edges to be at a slight angle so that a slightly misaligned chain will fall into place.
#25 roller chain is 9mm wide, add a couple mm on each side and you are up to 1/2" or slightly more per ring
I agree it’s solid. I suspect that to get the ring strong enough will end up being more expensive than Bar’s steel ring, and more expensive than either linkage approach, just due to the amount of material involved.
But someone should try it 
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I also wonder if the offset between the two loops of chain would cause unwanted torsion on the sled? Ideally you want them to be on the same plane but that’s not really possible with this design (that I can see).
Do you think you could cut plywood rings with the maslow for the chains to ride on? I feel like that would be strong enough and pretty cheap. You’d still need chain, sprockets, and bearings (and maybe mounting hardware).
In my concept that was similar to this I kept getting stuck on the idea that they had to be on the exact same plane, I think that’s what prevented me from figuring it out like this. But now I’m wondering if the offset chains will cause a problem…
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Paracord is nylon and stretches - quite a lot.
As a kiter I would recommend using pre-stretched 200lb Dyneema braid, which has negligible stretch.The problems kiters face using Dyneema wouldn’t apply (friction from crossed lines melting the cord) as any movement would be very low-speed.
Pre-stretched marine braid stretches less than 1% over a 50M length.
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What about Kevlar cord? It’s used for ham radio antenna guying and available from lots of sources. Claims to be non-stretch
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the same question as with the pantograph kits. I really don’t think it matters
much. te different distances are not much and you have a large base.
David Lang
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you could, but you would need to do it in multiple pieces (the maslow doesn’t do
well in cutting groves in the side of something)
I would cut out the rings, either on the maslow or with a simple circle cutting
jig and a router, and then look at a table saw to cut the groves. I also wonder
if plywood is the right material or if this should be done with a more
consistant material (you are cutting away the inner layers, and ideally want a
taper on what’s left, that’s not great for standard plywood)
I don’t think a little offset will be a problem, but preventing chafing between the two loops will be important.
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