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Throwing my hat in the sled modification ring


#707

I looked at a couple examples earlier, it was several hundred lbs (700-800) for the smallest diameter line. (5/32" iirc) for prestreched.

Here is another promising option, made of the Dyneema line.
https://www.velasailingsupply.com/samson-amsteel-as-78-7-64-2-5-mm-1600-lb/


#708

so how much stretch is there in 1m at 66 pounds vs at 10 pounds (that’s about
the lowest tension, although we may want to check at 5 pounds


#709

If torsion is an issue, it could be solved with a 3 chain system (2 outer, 1 inner). I don’t think it will be an issue if the chains are within 1 cm of each other, but that’s just a gut feeling based on experience. Anyone with better math skills care to weigh in?

Looking at the chart here: http://www.smackdock.co.uk/rope.htm
In marine applications, a couple % of stretch is close enough to be “no/low stretch”. I’ve worked with 3" thick braided Kevlar mooring line, and the little bit of stretch only had to be accounted for in rare circumstances.

Even at fairly low stresses there will be some stretching. The linked graph is rough, but it looks like at a few percent of breaking load we will see > 0.1% stretch, and variable stretch as we go from 10-66 lbs.

Given that Kevlar is expensive, can have wear issues, and may stretch just a little, I think chain is the way to go.


#710

Any other options out there (I’m thinking a loop made of the plastic stuff tie-wraps are made of, or something)…

Of course chain is pretty straight forward… you could always cut wooden “teeth” all around the sled (JOKING!)


#711

Chain is easier to link ends into an accurate loop as well.

There are some very small chain sizes available, smaller than the #25, if link thickness is a concern.


#712

I know how torsion works. I’m just saying that it does not have such a big impact on what we are trying to do here. I’m talking about a cylinder that is Ø18 “x 6” with a minimal offset in the middle. There is a lot to be calculated. Chain will be affected as the thickness of the material that is being cut, where the sled is and so on. Moving the router in and out changes the center of gravity.


#713

I couldn’t resist
chaincenter

3 sprokets conneted to the sled
3 ‘idler’ sprockets
1 looped chain
1 brick

I don’t know if this works at all , just spitting out brainwaves


#714

I’m sorry I must have misunderstood you. I was not trying to imply that you didn’t understand torsion. When I said that I wonder if the offset between the two loops of chain would cause unwanted torsion on the sled and you said you thought “completely the opposite”…

…I (mis?)understood that as you saying that the offset would be an advantage. That concept didn’t make sense to me so I tried to explain my reasoning, it was not to specifically imply that your reasoning is incorrect.

I agree that it (torsion) does not have a big impact (that’s what I was trying to convey when I said “I’m not saying the torsion is a big problem”)

My only point in explaining the spray paint can idea is that if the attachment is on two different planes there will be torsion. Period. It might be tiny, perhaps even incalculably tiny, but the laws of physics say it will exist. I don’t see how having an offset is advantageous in this set up. I can see how it might be inconsequential but not advantageous.

Obviously I misunderstood what you were intending. I’m sorry if I sounded like I was attacking you.

As usual with everything I say, I will admit up front that I could be wrong. I don’t think I’m wrong in this but I could be, and I’ll be happy to concede when that happens. Thank you for being patient with me!


#715

to check if it works, draw it again with one chain 10 degrees from vertical and the other 10 degrees from horizontal. If everything still points at the center it’s good (note: in this case it won’t be good, the chain will be limited by the three sprockets next to the center and won’t be tangent to the outer edge of the blue dot)


#716

if we could go with lighter chain for this, we should be able to go with lighter chains for the main ones as well. I don’t think we are likely to be able to go lighter. (I also suspect that lighter chain is less common, so more expensive)


#717

the nice thing about this approach is that the chain slipping on the sled
doesn’t matter (although, screwing the chain to the sled may be a good way to do
things to hold it in place)


#718

note, we don’t need ± 140 degrees, we only need from 10-80 degrees on each
side. Remember, the chains are in different directions, so the anchor for one
chain can be in a place where the other chain is a good ways out. +/- 140 is a
HUGE range, and seems way too wide.

That’s 280 degrees, and a single side only needs to move 70 degrees, which would
mean that with the sled in one position 210 degrees need to be free to move.
Since that’s more than half a circle, that’s clearly wrong.

needing 140 degrees free with the sled in one position, making it need to be 210
degrees free to support the 70 degrees of rotation (leaving 150 degrees that
could be attached, or omitted) would be far more plausable.

you then don’t need a full loop, just enough to reach to the first point where
the screws can be attached and then run a screw through the chain. The fun part
will be doing this accurately. Still far easier than the stock design.


#719

if you move the three idler sprockets/spools/pulleys (whatever you want
to call them) insidentheboitline of the base, and pin their respective
axes to the base, then I think this should work for all practical chain
angles encountered during cutting.


#720

*inside the outline of the base


#721

How long is that piece of string? The one I looked at online was twelve-strand braided 12mm and rated at 2 tonnes. I’ve got 200lb line here which is about 2mm.

It’s easier to define the spec and find the component to fit it. And breaking strain isn’t the same as constant strain - iirc from kiting. No longer (and for some years) a boat owner, so I’m a bit out of touch.

DSM will have plenty of data on the subject: https://www.dsm.com/products/dyneema/en_GB/home.html


#722

Please do a diagram, there are enough things floating around that it’s really
hard to be sure we are talking about the same thing :slight_smile:


#723

Saw this on youtube, already addressed in this long thread?


#724

Yep, that’s Gero’s sled. He is a long way from ‘normal’ suppliers, so he put
that together and it tracks well, but expect him to have problems cutting
because the sled will act like a pendulum as things start and stop.

I’m building him a extra big ‘top pantograph’ kit that I plan to ship out
after thanksgiving (it should arrive early in December)


#725

I have software that can make it a big chain sprocket


#726

That’s easy to do, but not needed. The only advantage of fixing the chain to the
big ring is that you can use less chain, or if it helps alignment of the
remaining portion.