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Tensioning system to improve performance in bottom corners



I’ve been wondering if something like this would work. Green are the motors, blue are free spinning sprockets, and the pinkish are the trolleys that ride around the sled ring. Still trying to figure it out in my head. I await for @dlang to point out where I’m wrong :wink: First, I’d think the weight(s) pulling on the slack would have to be greater than the weight of the sled.



The thing that comes to mind for me on that is there would be a pretty large amount of slack to take up. In the extreme lower right the left system would have around 9 foot of chain to the bottom left pulley, and 10 foot to the top left (estimations, dependent on your frame design and pully positions). Move the sled to the top left near that position, and you only have about 5 foot of chain to the bottom left, and 1 foot to the top left. That means you’d have to take up ~13 foot of slack, if I’m thinking about it correctly.


Yeah, I think you are right, but I believe there are solutions to taking up the slack (more sprockets, please!). I’m still wondering if the idea would actually work. It basically is a bottom driven maslow… i.e., calculations for how much chain to feed would be based upon distance of sled from bottom sprockets.

Two alternatives… left side is the previous diagram with extra sprockets for slack take-up but right side is an idea to keep the math the same (top feed design), however, I don’t know if either is really any different than any other idea that’s been suggested… i.e., I think it might just be constant force pulling on sled counteracting each other.



Maybe a better idea is to attach a spool to the motor sprocket (has this been mentioned?) with some fairly flexible string. As the motor turns to reel in chain, it would also reel in the string, thereby adding tension when you need it the most.



I was thinking of cross tying the bottom line to the opposite corner chain, it would decrease the amount of take up needed. But I’ll need to sit down and draw out a vector drawing to figure out if it’ll work. It the least it would be more downward force without adding any table friction.


The origninal post (all the way at the top) discusses this in some detail.

The main issue with the design above is that the exact same amount of force is being applied to the sled on both sides. If you are seeking to improve side-to-side cutting, this system would not really benefit you. However, there’s no arguing that some downward force is being applied to the sled with this system; but the question is, how is that different than just adding a bit more weight to the sled?


vertical lines don’t test the side chain angles, they test the slipperyness of
the sled as the near vertical chain is let out.

it’s possible to cut downward vertical cuts at max machine speed if the sled can
slide easily enough on the workpiece, but the same machine could have trouble
making the same cuts with the same sled on OSB (which has a much rougher
surface). incresing the length of the top beam will not help this at all.

the side cuts into the bottom corners are similarly depending on friction, as
well as the side chain angle. Here the side forces are very low (~3.3 pounds on
a stock machine) so increasing the near chain angle by extending the top beam
can have a very significant effect here, but since the threshold of having a
problem will vary from cut to cut (depending on how slick the sled is, and how
rough the workpiece is), it’s not something where a test will help lots of other


how do you make the paracord/chain have different lengths?

this sort of approach will apply the most sideways force to the sled when it’s
farthest away, which is exactly the opposite of what you want to have happen.

any tension applied towards the bottom corners needs to be active, springs or
weights won’t work. You need it to be applying the most force when the sled is
closest to the corner and the least force when its the farthest away.

people have tried this with springs and found that it made the problem worse.

David Lang


You know, another thing I’ve got to say about a design like you suggest, after looking at chain cost in order to make mine a 12 foot wide top beam…

Meh, the way their website loads this link doesn’t go directly to the chain. From that link I went to roller chain, then Ansi Roller Chain and Links.

Assuming I’m looking at the right chain, it’s fraking expensive lol. 5.14 a foot! No wonder only the bare minimum is included in the kit. A design like you were talking about would need like at least 25 foot of chain per side, $257 in just chain cost!


There are less expensive places to buy the chain…





Sorry, my link got munged…


Mcmaster-carr is expensive. eBay or Amazon is much cheaper.


Yeah, I see it is cheaper in some other places. I was looking for it by the foot somewhere so I could just get what I needed and was surprised by it’s price by the foot at McMaster lol.

With it being 15 for 11 foot on Maslow store, I figured it was only a dollar or so a foot.

Guess I’ll have to look into chain breaking and getting connecting links and such to. Haven’t really researched it at all.


it’s cheaper to buy 100’ of chain on e-bay than 20’ from McMaster


I’m late to the discussion of adding tension in the corners, but there are really three places where you might want a little more tension. At the bottom corners, as discussed, as well as at the top center. As @dlang stated, you want something that increases tension as the sled gets closer to those critical areas, which is the opposite of what springy materials do.


Well, actually if there is less string when the sled is in the corners, then tension would increase…

Looking at the diagram below, the maximum force would be applied at when the sled is at the top left (close to the motor) because that’s where the chain is the least. This is where you “calibrate” the stretchy string. You attach the string to the sled when its in the top left position, wrap it around the bottom idler up to the sprocketed spool and take up all the slack. You really want this taut at this position. As the sled lowers, it will naturally feed out more stretchy string and the force will decrease, but it will still provide tension (how much depends upon how much string is unspooled and the elasticity of the string). When the sled moves to the right, more string will be spooled-out and therefore it will stretch more to the point that the tension is really low when the sled is on the far right.

The amount of force applied will depend upon the elasticity of the string and the diameter/gearing of the spool. Finding the right combination will be the real challenge… assuming this works.

Additional thoughts… First, the sprocket for the spool may cause issues leading to chain slip (since it will want to push chain back toward the motor.) Second, I don’t know how chain sag calibration would be affected by this… but since it’s not consistent across the entire surface, it probably won’t be good.


what about the angle of the lines? when you are in the bottom right, the right
line is at a steep angle, so almost all it’s force is vertical, not horizontal,
while the left line is at a very shallow angle so it’s force is almost all
horizontal (in the wrong direction)

Also, even if you get this tension applied in the right direction, you are not
eliminating the chain sag (you could apply 1000 pounds of force and it would not
eliminate the chain sag, just reduce it.

It’s far better to have a simple chain sag that you can calculate and compensate
for than a very complex chain sag that you cannot.


The angle would depend upon where you place the bottom idler. The further away from the frame, the more horizontal the force becomes. As for the left line at a shallow angle, I hypothesize that with so much stretchy string extended, the force will be nil… so it doesn’t matter if the angle is shallow or not.

This is more “to scale”.


I agree. I just don’t know to what extent it would throw off the “simple” chain sag calibration factor is all. Is it insignificant?