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Slab Flattening - Router Sled Jig - Design and Construction



Using a ring linkage, if the bungees both attach to the same point on the vertical midline of the sled, the geometry will be correct.
I think we’d get an even better outcome if we could arrange to have the bungees (hot pink) pull in line with the midpoint of the angle between the chains (green).
bisect-thge-angle. That would take some additional steering of the trolley/traveller, though. An improvement beyond that would put more force on the bungee when the sled is close to the bottom of the workarea, less when near the top (more force when the angle between the chains reduced, less when the angle increased). Not asking for much, am I? :grin:


Yep, man I feell like that tail is being chased… all in trying to tighten up the bottom corners.


I set this up quick just to see what would happen. It did not work the way I had hoped. @dlang was spot on.


@Jacob so sorry to steal any of your thunder on the flattening jig!! Awesome idea and design!!! The pic of your setup just sparked that whole tightening up the bottom corners dilemma. The way I deal with them now is to manually apply force (think shaper origin)


that is a good question. It seems like the cord would want to always be the shortest distance possible, which would be straight down. This setup could be done with a single weight on the right (to choose a side) with the left side of the cord fixed. Then cord would then run through a left hand pulley on the trolley, up to a pulley on the sled, then back down to a right hand trolley pulley, over to the right hand side to go up to the weight. That might encourage the trolley to follow the sled better as any lag would be counteracted by the weight wanting to keep the cord between the trolley and the sled as short as possible. Starting to get into a lot of pulleys, though. Still an interesting idea.

Bummer, but thanks for verifying.


Hi all, in response to all of the discussion sparked by the tensioners in my post (@blurfl, @clintloggins, @dlang, @Keith, @ScrumdyBum, @bar), I figured I should shed some light on how they work.

Firstly, I want to establish the core relationships of certain parts of the maslow in relation to one another during normal operation, from which I derived the system.

At its core, the system relies on this:

  • When a chain is at it’s greatest extension, the maslow (gravity acting on sled) can provide the least amount of force in the opposite direction that the extended chain is pulling.

  • When a chain is at its greatest extension, the slack in the chain is at its minimum length.

From those two observations, I arrived at my current design:

The “Block and Tackle” setup uses the relatively small movements of the chain slack to greatly increase and decrease the length of the elastic cord depending on the sled’s position on the work area. One block (top one in diagram) is attached to the frame – the other to the blue paracord, and when the chain is fully extended, and the slack is pulled up, the pulleys pull apart, tightening the elastic cord on the opposing side.

If you mirror the mechanisms, you get this:

I understand this is all a bit confusing, but essentially you’re just using pulleys to magnify the chain slack’s movements on the opposing side of the frame to increase tension in the desired direction when it is in the corners.

As an added benefit, the tensioners also relax when the sled it towards the top of the work area where they are not needed, since the slack in the chains is longer when in the raised position.

I did some more testing this evening, and was pretty pleased with the results when running the calibration test code. I’ll post a video later this week to show the system in action in the comments here if anyone is interested.

@clintloggins No worries! I’m glad to add my drop to the magnificent bucket of effort here in the forums.

Thanks guys!

Tensioning system to improve performance in bottom corners

Here’s a few more photos for clarity:




I thought about just one weight, but that will pull the sled to the side towards
the weight


Awesome. I was going to make a single attachment point at the bottom that would be moved left and right by another motor and chain to give pull in the appropriate direction (down, or down/towards a corner). I was starting to think of making it not need a third motor and was starting to think about how to do it with block and tackle but looks like you worked it out already. I like the design and look forward to experimenting with it once I finally get to build my maslow! Nice work! I had all the same thoughts as dlang until I saw how you cross-linked them so the force grew greater towards the corner.

How many wheels are on your blocks to make it work out?

Tensioning system to improve performance in bottom corners

I still think I’ll also play with what I had in mind (have the parts). More control will be possible with the motorized bottom as the amount of pull to either side in the corners will be able to be changed to find optimal much easier than a system like this, but this is a pretty slick setup for not needing more electronics and motors.


The other concern that I have about this sort of thing is that you are replacing
predictable chain sag with a much more unpredicatable force on the sled.

We need to have measurements with and without the lower tensioners (on the same
machine to make sure they are apples-to-apples comparisons) to see which is
actually better.


Absolutely agree with you there.


Yep, I also agree. It will need a lot of testing to adjust/verify. Just an observation: the chain sag doesn’t appear to be much different, other than it goes in the intended direction down in the problem areas.

My main concern is the force on the motors. While the rotational force is actually reduced, the sideways loading on the shaft is probably about double what a standard tensioner would be. That could cause bearing failure if it’s too high.



I have considered the same arrangement… one fly in the ointment is that the top pulley on each side would need to be at least 10’ high to cover the full motion of the sled, top to bottom on both sides. Also, the design needs to create more pull towards the outside the closer the sled gets to the bottom corners… it seems like there will be no change in performance with this layout, as the weights just cancel each other out. Maybe using the slack chain with pulleys and bungees could create what we want, but the added complexity would likely create other problems. Perhaps the best answer is to make the top beam wider to increase the motor spacing by maybe a foot on each side? More chain, of course.


Block and tackle setup took care of the 10ft. But I agree, I think a 12 ft top bar may be the best chance.


I like the idea of making the top beam wider to give gravity a better shot at redirecting the sled to the desired location, I just have a fairly small space to work in currently. What’s the largest top beam someone has installed to date?


Hi All,

I’ve created a new topic to expand on the non-router-sled related discussion of the tensioning system that everyone is asking about.

It’s here if interested: Tensioning system to improve performance in bottom corners


Reply to this thought is in Tensioning system to improve performance in bottom corners


chain sag is different in that it’s more predictable when the only down force
is gravity.

The current calibration is able to make a fairly good guess at the correct
constant to account for it, and then it compensates for the sag consistantly.

With the additional tensioners on the bottom tied to the length of both chains,
the math to figure out how much force is being applied (and therefor how much
tension there is on the chains, and what that does to the resulting chain sag)
is FAR more complex, and I expect has lots of variables in what the lengths are,
how much tension the stretchy cord applies (which changes over time), etc.


I don’t need an elaborate answer to this but how is chain sag compensation done without knowing the weight of the sled and angle of the frame? Are just average values used?