Slab Flattening - Router Sled Jig - Design and Construction

This is fantastic!

Would you be willing to post the files in the community garden so that the project doesn’t get buried in the forums?

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Definitely will! I already tried to submit a project earlier this week for the tablet holder revision, and it looks like it hasn’t taken. I must be struggling with the whole garden/github thing still.

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It’s very much still a work in progress. I would love to know where the hang ups are so I can fix them :grin:

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how do you adjust how much pull there is on the bottom? or are you just trying
to increase force in the same direction gravity would apply?

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Please tag me too! I have been fiddling with this and I am waiting on some bungie cord and pulleys to something similar. Thanks!

That was a question I had too. It seems like it would either have too much tension at the top or not enough at the bottom.
I thought about trying something with counter weights like below.

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This seems like a perfect solution that could elimitate the bricks from the sled completely. Essentially moving the weight of the bricks to behind the work surface and making the sled easier to construct. And without needing to figure out the differential pull in the various corners.

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what is the advantage of removing the bricks from the sled?

For me, offhand:

  1. force is applied by the counter weight system into the lower corners at all times, helping to alleviate error due to the steep angle of one chain (bricks only provide force straight down)
  2. easier sled construction (replace the bricks with a couple of eye nuts to connect to the weights, do away with two countersunk holes and the bars to hold the bricks)
  3. no worry about bricks potentially falling out (this has never been reported, so probably not a concern)
  4. kit can ship with cord and pulleys for both chain tension and sled counter weighting, doing away with having to source bricks and providing a single solution (counter weighting) for both
  5. lighter sled to handle when not in use/placing stock to be cut

There may be other advantages

  1. force is applied by the counter weight system into the lower corners at all
    times, helping to alleviate error due to the steep angle of one chain (bricks
    only provide force straight down)

however the force is not in the right direction, when you are in the bottom
right corner and want to move right, you have all the force of the left weight
pulling you away from the direction you want to go, and only a small part of the
weight of the sled and the right weight working to move you to the right. This
is the opposite that you want

  1. easier sled construction (replace the bricks with a couple of eye nuts to
    connect to the weights, do away with two countersunk holes and the bars to
    hold the bricks)

if you are talking about similar amounts of force, an eyelet screwed into the
sled may pull out.

  1. no worry about bricks potentially falling out (this has never been reported, so probably not a concern)

as you say, since it’s never been reported, not a problem

  1. kit can ship with cord and pulleys for both chain tension and sled counter
    weighting, doing away with having to source bricks and providing a single
    solution (counter weighting) for both

so instead of bricks, what do you use for the weights? where do you source
those?

  1. lighter sled to handle when not in use/placing stock to be cut

countered by the fact that you have more things to detatch from the sled and you
can’t just hang the sled up out of the way to load plywood in, you have the
lower tension lines to work around.

If the lower lines were managed by the software to provide the right tension, I
can see value in them. As it is, I think they will make things worse in the
places they are needed the most (the bottom corners)

David Lang

the force pulling both down and to both lower corners will be constant with a counter weight system. While the opposite counter weight will pull away from the corner, it is balanced by the close counter weight, and the applied force from that close counter weight will be specifically in the intended direction. Locating the pulleys below the bottom of the work area will also aid in positioning the sled. The main advantage here is in adding an x-axis vector of force to the sled in addition to the y-axis vector, whereas the brick method only provides a y-axis vector and relies on that force to pull the sled in the x-axis by virtue of the sled acting as a pendulum on the nearly vertical chain. If it ends up being a concern, then the bricks and the counter weight could be used in tandem with less weight (one brick?) being on the sled.

I agree, which I why I suggest using an eye nut (rather than an eyelet or eye screw) on counter sunk (on the bottom of the sled) through bolts.

2 liter bottles filled with water? there are options galore. It’s not to say that getting bricks is hard, but some have expressed difficulty. Bricks are also fairly large and a little unwwieldy. I am currently using some lead ingots that I had lying around from another project, which allows me to keep the weight within the outline of the sled, which could be a bonus for some who need to limit the vertical height of the whole set up as bricks seem to extend the bottom of the sled a couple inches. I know it’s not a lot, but there are certainly people who have already expressed an interest in minimizing any additional height.

I haven’t thought the connection all the way through, but a couple of ideas spring to mind including a single attachment point to the sled (which would allow the counter weights to balance themselves when removed), or having a cleat at each bottom pulley to secure them on removal. It would have to be worked out for sure, but this is just an initial run at it, so certainly time to collaborate with those interested to find a good solution.

That would be pretty cool. I’d be interested in that solution, though their is certainly an added complexity and cost associated with that as well.

That is fair, but it still seems like something to explore. There’s always the chance that there’s no benefit in it, but aside from speculation at this point, there’s no way to know without further study and/or experimentation. And it may not be for everyone (after all, there are a lot of different frame designs out there), but for some it might be ideal for one reason or another.

also, apologies to @Jacob, I did not mean to hijack your thread. When I have a little more time I will pull this conversation out to a new thread. I’d also appreciate a tag in your follow-up

(perhaps that is where this discussion should go? thoughts?)**

the force pulling both down and to both lower corners will be constant with a
counter weight system. While the opposite counter weight will pull away from
the corner, it is balanced by the close counter weight, and the applied force
from that close counter weight will be specifically in the intended direction.
Locating the pulleys below the bottom of the work area will also aid in
positioning the sled. The main advantage here is in adding an x-axis vector
of force to the sled in addition to the y-axis vector, whereas the brick
method only provides a y-axis vector and relies on that force to pull the sled
in the x-axis by virtue of the sled acting as a pendulum on the nearly
vertical chain. If it ends up being a concern, then the bricks and the
counter weight could be used in tandem with less weight (one brick?) being on
the sled.

the close line will be at a fairly steep angle, so only part of it’s force will
be in the X direction while the far line will be at a shallow angle so that most
of it’s force will be in the (wrong) X direction.

so the net effect of the two lower lines will always be to pull the sled towards
the middle.

I agree, which I why I suggest using an eye nut (rather than an eyelet or eye screw) on counter sunk (on the bottom of the sled) through bolts.

which doesn’t eliminate the need for countersinking on the bottom :slight_smile:

2 liter bottles filled with water? there are options galore. It’s not to say
that getting bricks is hard, but some have expressed difficulty. Bricks are
also fairly large and a little unwwieldy. I am currently using some lead
ingots that I had lying around from another project, which allows me to keep
the weight within the outline of the sled, which could be a bonus for some who
need to limit the vertical height of the whole set up as bricks seem to extend
the bottom of the sled a couple inches. I know it’s not a lot, but there are
certainly people who have already expressed an interest in minimizing any
additional height.

you can mount the bricks at any angle, nothing says they need to be at the 45
degree angle that they are by default, make them sit at 90 degrees and they
won’t extend past the bottom of the sled.

I haven’t thought the connection all the way through, but a couple of ideas
spring to mind including a single attachment point to the sled (which would
allow the counter weights to balance themselves when removed), or having a
cleat at each bottom pulley to secure them on removal. It would have to be
worked out for sure, but this is just an initial run at it, so certainly time
to collaborate with those interested to find a good solution.

you also want to check how this will rotate the sled.

That is fair, but it still seems like something to explore. There’s always
the chance that there’s no benefit in it, but aside from speculation at this
point, there’s no way to know without further study and/or experimentation.
And it may not be for everyone (after all, there are a lot of different frame
designs out there), but for some it might be ideal for one reason or another.

I’m all for throwing out ideas and working through the implications, that’s why
I asked for more details on the tensioners.

I’ve made lots of suggestions for changes, many of which have ended up being
improvements and made it into the standard design, some of which seemed like
good ideas at the time, but later on they ended up not being as good due to
factors that none of us were thinking of at the time.

David Lang

Here’s a variation of your idea, using a unistrut trolley to direct the force down all across the workarea.
IMG_0352

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That has a better chance of working, the question is how well the trolly will
follow the sled.

@blurfl I remember a thread long ago that pertained to this very topic. Cant find it :thinking: please correct me if Im wrong but I think the key was to have the bungie pulling in line with the bit (same as the chains)
I remember something about possibly having a full ring with 2 additional rollers at the bottom (router/dust would prob get in way) with the end of th left and right chain attacched to bungee cords routed in similar fashion as @Jacob ‘s, connected to the added bottom rollers. The reason bungees would be used was due to the precision there would need to be if going with all chain.
Whew!! With all that said, I like your idea because it does seem like the force would vary at X on each side

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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:

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

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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)

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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.