Improved chain tensioning for top beam configuration

try going with as light a weight as you can to prevent the tension on the motors from every hitting zero (balance between the bricks and the sled)

Please let us know how light you can go.

I measured with water and since 1 litre is ~ 1kg I can say that it’s ‘ok’ with 1.5kg and perfect with 2kg (~4.41 lbs). I assume that gets divided by 2 because the chain is double.

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Thank you this is a fantastic mod for the bolt together frame setup listed on the home page. I used a 33 inch bunge and it performed the test cut (4 cornters) flawlessly without any slack or too much tension. Much better than the constant babysitting and multiple misadventures with the design on the instruction set.

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That sounds good and below the stock frame tension.

I’ll bet that we see a slight accuracy improvement with weights like this vs
bungee cords on the same amchine (recalibration should be done between them as I
expect the bottom width cuts will vary just a smidge as a result)

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This looks like the simplest counterweight design ive seen, what pulleys are you using on the top and where did you get them?

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I claim the title of simplest and cheapest, as it’s all in the kit (except the carabiner hooks). :slight_smile:

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I got the top pulleys at a local Home Depot or Menards, they are actually rollers for a pocket door. I still think a larger diameter pulley would be better but once again , these are what I could find.

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I saw some people using extra idler sprockets. Where can I buy them that won’t cost an arm and a leg? https://www.mcmaster.com/#idler-sprockets/=1e6pxyr

McMaster want $26 for ONE. My normal Chinese searchs aren’t turning up anything. There are a few on ebay for $10, but I’m thinking these should be pretty cheap commodity type items?

I think these will do.

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So… Does tension on the other side of the sprocket give accuracy? Are we otherwise gradually pulling it out of calibration? Forgive me, I just want to understand risk/reward on this. I have had no problems with the included design and yet I haven’t had to deal with precision just yet.

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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