Alternately, I had a similar idea to have weights that could set on the floor when chain is short or be lifted when chain is long:
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Welcome to the Forum!
Oh wow! That looks great!
The issue I see is that you would want all colours attached to the sled the same height over the sheet to keep the sled parallel. Same height should be at the 2 motors and the first 2 rollers at the bottom counting from the sled. With some chain guides to avoid them touching at the crossings, could work. Some more hardware I guess. Thanks for sharing.
Kind regards, Gero
remember, you need the most tension when the lines to the bottom corner are the shortest and the least tension when the lines to the bottom corner are the longest.
so I think your second proposal has problems
the first may work, please do a more complete analysis and show what the actual lengths are in the 4 corners and letâs see if this works.
This is the part that makes a passive system like this difficult.
My opinion is also that a closed 4 chain system like the first one you proposed also requires too much tension to be safe or achievable on a wooden frame. It seems to me like this would only work if you drastically decreased the weight of the entire system including the chains.
An idea that I have bantered around for 3+ years, but never achieved, is a 3 chain system with a passive counterweight. In theory with three chains, nothing is ever over constrained. I have always thought of this design as a âvectored gravity approachâ in a way moving the angle of gravity on the sled to keep both chains in tension. My hunch is that an angle in between the two chain angles is ideal, but I am not smart enough to know how to calculate this.
Anyways, here are two drawings of the idea. I am happy to discuss it if anyone wants. The vector of the passive counterweight is controlled by a 4th motor on a slide.
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I like it. Alot of thought went into that, but Im not sure I could handle a build like that though I agree there are probably those on these forums that could. As far as the tension for my idea, I would definately select cording with enough stretch to be safe and requiring as little tension as possible. My initial thought is that the chain as well as attached cords, represented in dark blue and yellow will need minimal stretch properties so that the 2 sided pulley movement distance up and down is predictable. Because this part is doubled over, when 8-9ft of chain is let out or retracted to cross plywood it should move up or down around 4-4.5ft. The tension will be applied by stretch of the orange and light blue cords of the apparatus. These will require a double pulley system at top and bottom so that when not stretched the cord measures about 8-10ft and can reach across the work surface(orange). and when pulley moves up 4-4.5ft, they will stretch to 16-20ft(light blue). In my head it makes sense, but I will get some better measurements and keep updated. For safety factors I can initially hide some additional slack on the bungee side until I can get some initial tension readings.
The problem with this approach is positioning the slide across the
bottom (remember sawdust and chips will be falling on it) and how much does this
complicate the math of figuring out where the sled actually is based on the
motor positions (since you now donât just have gravity pulling on the sled, you
have this additional force)
our movement speed is limited by the available force of gravity in the bottom
corners, but our accuracy is limited by the accuracy of our mathmatical model of
the machine. Iâm not convinced that we are properly modeling chain sag/stretch,
adding an additional force complicates the model more.
David Lang
BeeDub
the problem is that weights provide constant tension, if you have constant
tension, then lines to the bottom corners wonât work because the angle to the
far corner is going to be flatter than the angle to the near corner.
This will produce more pull towards the far side than to the near side (break
the tension down into a vertical and horizontal component and you will see what
I mean), this will pull the sled in the wrong direction.
David Lang
Correct. My idea with weights is for weight to be lifted only when sled approaches the bottom corner. The opposite weight would be resting on the ground at this point and be out of play providing tension only to side that requires it. But you are right at the point when the weight is lifted, it would be constant tension
how do you lift the weight only when there is less line fed out?
Also, the point where you start lifting now introduces an abrupt change in the
tension, how do you measure exactly where that happens so that you can add it to
the model and know when you need to run the motors more to compensate for the
change?
David Lang
Please refer to apparatus with weights. The coupled pulleys shown off to the sides are free air similar to boat rigging. The normal operation of the Maslow will cause this free aired rigging to move up when chain is lengthening toward sled and it will move down as chain shortens from motor to sled.
The change in tension does not need to be abrupt. For example I have shown a square solid weight, but the weight could be added gradually by using a more flexible elongated weight with even density distribution. Maybe like a piece of chain. Then, as the weight is lifted only partly as it comes off the ground leaving the floor to support still some of the weight until it is completely suspended. Surely someone on here could come up with a better idea, but this is just for example.
Just realized I may not have completely answered your first question. In addition to lifting the free air pulley you will also have to adjust ratios by doubling up on the side attached to the weight. This will allow you to âstoreâ extra line when free air pulley is further away and less is fed out from the bottom.
So it works. Kinda. I was only able to quickly put together one side. And take a couple pictures. A couple ideas that could quickly improve. First better pulleys. The ones I have arenât very smooth and are kinda jumpy. Second I need a cord with less stretch than paracord. Any one have any recommendations? I took a couple images for you guys. You can see that the chain is lifted higher therefore more tension when sled is closer. May need to triple or quadruple wrap on the bottom. It didnât absorb quite as much of the paracord as I had hoped, but maybe if the cord didnât stretch so much it would also be better
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Nice work!
Checkout Amsteel Blue for a super low stretch cable. I was looking into a cable design for a long time and that seemed like the best option.
Thanks. I was also wondering how much weight to add? Has anyone ever tested how much tension would need to be added towards the bottom corner to correct cutting errors? I know there are some variables such as friction, but Im looking for a starting point. Keep in mind, because the way it is set up, that the tension I add to the bottom will also add counter tension to the backside of the chain on the motor. I think it will be equal, but Iâm not 100% sure on that. My pulley system knowledge is pretty weak. I should really measure to be sure.
you donât want more tension on the slack side of the chain than on the sled side
(you donât want backlash to come into play)
adding more force towards the corners is less an accuracy thing than a helps
move the sled better thing.
no matter how much tension you put on the sled towards the bottom corner, you
will still have the chain sag, 100 tons of tension would not keep the chain
perfectly straight.
the motors are only designed to apply 66 pounds of force, so if you have too
much pull towards the bottom corners, the motor wonât be able to move the sled
away from it (but by default, the tension is <4 pounds on the chain towards the
motor in the bottom corner
adding force towards the bottom corners is going to mean that the calculations
done in the firmware that do not account for this force will not be accurate. It
may be that the calibration routine will set things well enough to result in
better accuracy, but it my also be that different calculations will be needed.
Until someone does a very detailed analysis or actually build such a setup and
compares it to the stock approaches, we wonât know for sure.
What if you put a 4th and 5th motor on that would adjust amount of tension based on absolute sled position? the system always knows or at least has an approximation of the sled position. When it gets into one of the corners, have the motor activate and pull it into the corner. the motor could be current controlled to indirectly control torque or have a tension idler and strain gauge ($$$) to indicate down force. Knowing the exact position, combined with the length of the chains enables calculation of the angle of the chains and with a fixed point lower chain, the vertical and horizontal force vectors could be calculated by knowing the chain tension, controlled by the lower motor depending on the location and not exceed the max force and using the chain mount location to control the force applied as mostly a lateral force rather than gravity enhancement. This applied force could be a map of different forces across the cutting area with none in the upper center, and more in the lower left and right corners by the left and right motors respectively.
Second idea and this one is more out there. Computer mice have been around a long time. They track movement by using a laser and a camera. Would it make sense to track sled position with a mouse or even 2? Once centered the position is known and then with any sled movement the mouse could track direction and distance. having 2 mice could account for any sled âtwistâ on the ring and then that could activate the lower motors. This could also allow a home position to be located without resetting the chains, though that ups the complexity factor to a whole other level.
Maybe Iâm better off explaining what facts I currently know and you guys could offer up a solution.
I have a 9â6" beam. This is my limit to fit the Maslow into a nook in my garage.
As expected due to a shorter beam length, the slack on the long chain causes erratic cutting as the sled approaches the bottom corner.
If I manually grab the router as it approaches the corner while the Maslow is running and apply force to remove excess slack(I understand that physics says I cannot remove it all), the result is a cut that I am happy with. Though I am sure there are people trying to get 1/1000" accuracy, I am not.
Adding additional motors a doing some programming is beyond the amount of time I want to spend and likely beyond my capabilities.
Is there a solution out there that solves all of this? I am certain I am not the first in this situation.
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I thought about paying my kids to stand there and push it into the corners. If that isnât an option, then maybe you could mount the cut piece so the edge is closer to the middle of the maslow and cut the ends, then slide it over and cut the middle, so the sled never has to go to the extreme corners, but this only works if you have smaller cuts.
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keep coming up with ideas, but these are ones we have talked about.
We have a couple people (including Bar) who have been working on the idea of a 4
motor version for years. Itâs significantly harder to get right than you think
(and it roughly doubles the cost of the machine)
unfortunantly we do not have a mathmtical model that completely matches the
physical machine (if we did, it would be accurate everywhere, even without
additional chains), and that makes controlling the additional motors hard. We do
not know the tension on the chains, just the position fo the sprocket (which is
a close match to, but not identical to, the distance from the sprocket to the
sled)
The mouse idea is one weâve thought about, but mice loose counts as they move,
so they are not reliable over long distances.
David Lang
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with a narrower top beam there is not a good way to cut all the way into the
corners.
One idea that we came up with was a telescoping, constant-force mechanism that
is hinged in the center of the top beam, so it applies constant force to push
the seld away from that point. But nobody has built one yet to see how well it
works.
you can improve accuracy in the bottom corners by not trying to move as fast,
and by reducing friction between the sled and the workpiece (sand the bottom of
the sled smooth, them wax it or something similar to make it very smooth)
David Lang