The slack side of the chain will not have effect on the accuracy unless you pull to hard to introduce a backlash of the motor/gear.
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Ok so unless I have a problem with tensioner… I shouldn’t mess?
So far rubber and weight tension works without negative effects. Weight has the benefit of applying the same force no mater where you cut. Rubber is weakest where you need it most and strong where you don’t need it.
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So does rubber pull hard causing backlash since it is not consistent?
So far no one has used a rubber strong enough to cause that. To heavy weights (1/2 weight of your sled) would also cause a backlash.
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I suspect that in most cases, the '“trong where you don’t need it” is enough to
invoke the backlash error.
David Lang
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Actually, I think that nobody has checked for this. I’m pretty sure that the
stock bungee cords are more than 6.5 pounds of force at full stretch, and that
would be enough to cause backlash issues.
David Lang
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Not to add expense, and certainly not undue effort, but if we braded a bungee would this make the overall distance more consistent or just stronger? Think about it having less tension at first? Thoughts?
no matter what you do, any elastic/spring based mechanism is going to produce
more tension as it’s stretched, and we need the most tension when there is the
most slack (to keep the weight of that slack from pulling the chain out of
line), and the least tension where there is the least stack (as there is less
chain to keep control of)
Given how low the tension gets in the bottom corners, it’s really hard to find a
way to get something that has enough tension to handle the weight of 8’ of chain
when the sled is closest to the motor, that doesn’t provide too much tension
when you extend it an additional 4’ when the sled gets to the bottom corner.
In the stock machine, ‘too much tension’ is about 6.5 pounds of force
David Lang
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This is my solution to the chain tension problem
(after spooling my chain once).
I made two small rolling sleds that holds the chains and are have constant tension by a center weight. The center sled is free to move and redirects the line of pull over the top to weights behind. It uses (two) 2.5# weights (not shown) to pull on each small chain sled to keep the chain under constant a pressure. It keeps the chain on the same plane as the sprocket and out of the way of loading a sheet of plywood. I’ll post a video and the 3D printed pulleys to Thingiverse asap. Jon
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cool! Looking forward to the video. Can’t quite follow your description 
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Here is the link to the video…
and here is the link to Thingiverse for the 3D printed pulleys.
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That’s a very interesting rig! A very good video explanation, too.
Would the rig work with both nylon ropes going to one single weight?
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not being able to watch the video with sound at work, why have the center pully section move? why not have it fixed in place?
My first experiments used just one weight at the end of the two ropes and it did not distribute the tension equally. One side would pull more and decrease the tension on the other side causing the chain to sag. To be fair, I have also improved and changed to the square bottom pulleys since then so that may have been part of the problem. I also had to use a much heaver weight 7# as well and still had some unequal pulling. With the two weights it does not matter what happens to the other side as each line has it’s own independent tension. I always told my students to K.I.S. or keep it simple, however I feel in this design the K.I.S. train sorta left the rails at some time.
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With my design of the of the rolling sleds they would cross over the mid-point at extreme range and that would limit my use of the 4X8. That being said, I think if I would have moved the pulley to the inner side (same side as the rope attachment) and made the pulley bigger it might be possible to have a fixed center system, but it would be close. This way is more complex to be sure, but I’m no longer concerned about hitting anything.
Another varient to consider is to have the ropes cross and continue over to the
other side of the machine and drop down around the end of the beam. That way you
don’t have the big thing in the middle and the weights are clear of any wood
storage (you also need fewer pullys)
David Lang
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first make it work, then work to simplify it.
You have it working, so you can either use it as-is, or look at simplifying
things.
David Lang
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Great idea! My contraption has the weights in the center where they interfere with the wood storage.
Fantastic video. We need more video around here!
Thank you.