HI Folks, just got on board this great project, and like any newby I have a few ideas! Feel free to set me straight :-). Attached is an idea that I think addresses a few issues. With this design both chains are acting on the centre line of the cutter as is preferred, only one chain attachment rotates so the router rotates with the base and resists the turning motion of the router (the long arm helps this process). Fabrication is relatively easy with an angle grinder, hole saw and welder. Materials are a bit of steel plate, some steel pipe for the coupler and a single cheap bearing. 
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I’ve experimented with a symmetrically weighted sled using a top-bar linkage, with poor results. Your arrangement might work better, it would be interesting to hear how it does. The target weight for sled plus router is somewhere in the 20 to 25 pound range, the weight helps to overcome friction and sag of the chains.
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How much weight are you using on your sled now? I had a steel sled cut (1/8" steel plate 18" diameter) and it is fairly heavy, so I am wondering how low I can go on added weight and keep everything upright.
Welcome! I like the outside-the-box thinking but I don’t think your design ensures the chain on the left side will point to the bit. This is key to the alternative linkages giving better accuracy.
The direction that the long protrusion will point will be driven as much by the weights pulling down on the sled as the chain force. Nevertheless, keep the ideas coming. It helps those of us who have been looking at things too long to see the problems from a new perspective.
My setup is 26.5 pounds, using bricks.
I agree with that 100%. Lately there have been a lot of new people with new ideas here, helps to keep everything fresh.
While to me it doesn’t look like it would necessarily be an improvement, it doesn’t mean that it wouldn’t work. Try it, and let us know, i’m honestly very interested.
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this was one of the very early alternate designs discusse.
The dealbreaker ended up being the bearing, finding one that was large enough to
clear all bits, strong in non-axial loading, and that didn’t get gummed up with
sawdust was the dealbreaker for this design.
When we started doing the ring design, I was advocating a smaller ring segment
with exactly this approach of having one side fixed (see the very long ‘throwing
my hat into the ring’ thread)
you would have to have the weight on the sled symmetrical, and we found that if
it is, it tends to rotate under the router torque until it hits a limit (and
then flexing the chain a bit) so in practice, we want both sides to rotate and
have the weight on the sled be asymmetrical to keep it from rotating too far.
David Lang
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Ok, thanks for the feedback, interesting points
How about this redesign of the bearings? These bearings are 44x38x5mm so lots of room for the router bit etc and a very low profile, The chains now connect to the bearing holders and are both on the same plane.
The bearing holders are press fitted alluminium. The sled can now be weighted with a counter weight like the original design to counteract the torque of the router.
These bearings are only $5 each
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I also looked at slew bearing designs and it would not be to difficult to make a triple one.
The challenge is that it needs to be big enough to go around the router.
If you look at chain mount systems around, you will see that we need the height above the sled to keep the sled vertical, if we hang it in mid air. All this designs will make the sled tilt backwards pulling the bit out of the cut.
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Hi Gero, not sure why you would need a triple bearing? This design has three independent rotational faces already. The sled is connected to the inner race of both bearings and the chains are each connected to the outer race of a bearing. The only restriction is that the chains cannot cross each other (and they never should)
The sled will be asymmetrically weighted like the existing design and the whole router/bearing assembly can be place higher on the sled as well. With the chain position being quite close to the cutting surface plane, bricks may not be the best option for weighting the sled as they would move the center of gravity out. A heavier steel base or flat lead weights may be better.
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how do you mount the router? do you mount it to the sled? (if so, how do you
clear the bearings and chains?)
If you mount it to a third surface, is that surface going to be rigid enough?
how do you keep the router from spinning from it’s torque.
David Lang
Hi David If you look at the first drawing you will see that the router is mounted to the smaller top disk (router mount). The sled base is 3mm (or thicker if more weight is needed) mild steel, the coupler is a piece of mild steel pipe welded to the the sled base. The bearing assembly (bearings and bearing holder) is then place over the coupler and the router mount (5mm mild steel plate) is secured to the coupler with 4 (or more if required) 4mm taper head machine screws.
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If i understand it correctly, you’re mounting your chaind waaay under the centre of gravity of the sled. That could work, but is not advisable.
I plan to make the sled out of steel plate and the router I plan to use is a small hand held so the centre of gravity will be much lower than the normal setup. Also the bricks that are normally used don’t help the centre of gravity, I will use lead plate if I need it.
For the record, I am not trying to make the machine as cheaply as possible, I am trying to make it as good as possible with the best possible materials first, then I’ll look at how it can be done with alternative materials.
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That is definetly something someone has to do, glad you volunteered
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just so you know, there is some debate on if the ring approach is the best
possible approach or if some sort of pantogaph approach is best.
There is a long post in the middle of the 900+ post on frame design that
explains how the side forces on the ring are small, so it’s far more likely to
not roll smoothly (instead jerk, stick, jerk, stick…) while with a pantograph
approch, the same small forces are working at the end of a long lever arm, so
the force on the joints is much higher, and so less likely to stick.
We see the sticking in the videos that Bar posts (but he also hasn’t scraped the
paint off the inside of the ring), and when I saw him at maker faire we did note
that the edges of the parts that he cuts tend to have a slight wave in them
(which would correspond to the effect on effective chain length from the
sticking)
But nobody has done a head-to-head comparison of a ring and either of the
linkage kits.
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Hello, SimonD!
Did You succeeded in implementing this central ball bearing sled? How was the cutting error at the corners of the full sheet work area?
Where did You sourced these ball bearings, 44x38x5mm, are they from bicycle bottom bracket, (central/pedal axis)? What brand/model? The biggest i could find are 42x30x7mm: https://wheelsmfg.com/products/bearings/enduro-6806-2rs-abec-3-sealed-bearing.html
Please post some pictures of the implementation. I’m currently only waiting for shipment of Maslow cnc and meanwhile contemplating how should i fit a big router, Hitachi M12V2 inside the ring kit. Your solution with central bearings sled could be the answer.
PS found that SKF, Craft bearing and other manufactures are making 65x50x7mm size bearings (size 61810) that could be found for even less than 5$/euros. Maybe they are better suited as they have bigger internal diameter - 50mm?
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