Hey everyone, I haven’t played around with a Maslow kit yet but I’ve been reading the forums for a little bit and had an idea that doesn’t appear to have been brought up yet.
Why not use cable instead instead of chain? It wouldn’t stretch like a chain does over time and the lighter weight would significantly reduce any sag, right? Garage doors have grooved spools that could be used to feed accurate lengths of cable out. I think this one has room for 16’+ before the cable would wind on top of itself: http://ddmgaragedoors.com/parts/part/DR5250-018.html
This would also open up the possibility of using a small diameter pulley at the corners for the cable to wrap around. Thoughts?
Those spools are a very cool thing that I didn’t know about. I played around with the idea of using cable for a little while. I was thinking of using regular spools with pinch rollers with encoders to measure the length, but I was worried about the amount that the woven steel cable would stretch under load. All of the stuff I played with was fairly elastic
it would still say, and cable is more likely to try to retain a coiled shape.
Also, the calbe will end up at a different distance from the workpiece as it
spools out
Sag is a problem, but I don’t think cable would be any better
you would have to have a small diameter idler for the cable to pivot on, as the larger diamiter spool will chage the geomitory depending on which direction the cable is leaving the spool. this could also remove the issue of the different distance from the workpiece issue
0.125" looks like it would be the largest diameter cable you could use on those garage door spools I linked earlier. Has anyone run the numbers to see what the max load would be on the sprokets?
@dlang
I’m sure there’s always going to be some amount of sag unless we rigged up some type of telescoping rod or something. We could get around the cable retaining a coiled shape by storing it straight when not in use.
Like @pyrosrock mentioned, an idler pulley could be used to control the distance from the workpiece. This could also be a great way to adjust for different thickness workpieces.
using 1/8" 7x19 cable of 302/304 stainless over a distance of 110 inches would see a 0.0829% stretch, or about 0.09119 inches. That would be a little more than 2.3mm, probably more than we want to see.
using 3/16" cable would see a 0.0369% stretch, or about 0.04 inches, about 1mm - that’s much better, but stretch is another source of inaccuracy . We’d be trading sag for stretch?
@dlang
I’m sure there’s always going to be some amount of sag unless we rigged up some type of telescoping rod or something. We could get around the cable retaining a coiled shape by storing it straight when not in use.
where would you store it? anything that requires that the machine be
disassembled after each use is going to end up adding inaccuracy (not to mention
just the hassle of using the machine
Like @pyrosrock mentioned, an idler pulley could be used to control the
distance from the workpiece. This could also be a great way to adjust for
different thickness workpieces.
In my experience, you need to be most careful how you guide the chain onto the
spool, you can’t just add an idler and expect it to spool nicely.
@dlang
To store the cable, I was thinking of unhooking the cables from the sled and just hooking them on to something on the opposite end of the frame. Having to take things apart could certainly add more inaccuracy into the mix though.
remember, we are aiming for ~0.4mm of accuracy, and currently, chain sag is
thought to cause ~1mm of inaccuracy, so any ‘set’ or stretch in the line, or in
the connection needs to be substantially less than that. I don’t think it’s
likely to be a significant improvement over the current ± 1mm problem of sag
Gotcha. The only other thing I could think of along these lines would be some type of worm drive. I did some digging on McMaster and found this: https://www.mcmaster.com/#6542k64/=1aft47n
Its basically a worm drive hose clamp except the slotted material comes in a ~100ft roll for $60 and you cut it to the length you want. The only tensile information given says that it will fail at 550 lbs.
that stuff is designed for tighten once and leave, it wont last any consistent use.
why not just use a regular lead screw rod? sure its not flexible and thus would more room around/above/below your work area, but could be a way to prove/eliminate the source of the inarticulacy.
is that likely to be as flexible as the chain? I would expect it to start
holding it’s shape, which will be far more error than the chain sag.
remember, once we can figure out how to measure the amount of chain sag, we can
allow for it in the software. The big problem is figuring out how to measure the
amount of sag that we have.
With a 20 pound sled at the top center of the work area each chain (or cable) will have about 33.26 pounds of tension. (reference)
David Lang and I have a bit of a long-running disagreement on the forces produced on the sled. The 66 pounds that he is referring to is the stall force for the motors - it’s what the motors are capable of producing; it is not the force on the chains in a static system at the top center. The actual tension in the chains (33.26 pounds) is easy to calculate and is explained quite clearly in this post.
In order for a 20 pound sled to put 66 pounds of force on the chains the internal chain angle would have to be about 162.55˚, that puts the sled just over 217mm (8.54 inches) above the top of the work piece. So it’s easy to see that the system should not see 66 pounds of force under normal operation. I do agree that there will be brief, momentary, spikes in the forces due to variables such as knots, sled friction, path direction change, dull bits, etc. but this will not be constant and therefor any error will also be momentary. (this math is assuming a 20 pound sled with motors that are 440mm above the workpiece and 2900mm apart)
I have not only done the math but I’ve also done real world tests with inline spring scales and come to the same conclusions (some of which is documented here).
Again, if anyone can show me how they are calculating 66 pounds per chain I would love to see the math. I could be wrong and I would be happy to concede if that’s the case. My math, real-world tests, and logic shows me ~33 pounds per chain, and I’ve shown all my work and explained the reasoning so hopefully someone can show me where I’m wrong if I’m wrong.
@pyrosrock
I can’t speak to the long term reliability of that strap but I agree that the one I linked would probably have issues since it uses ridges rather than an actual slot that is cut through the material. I thought about using a lead screw but I figured that the extra space requirements would keep it from being a viable option for many people who have their Maslow setup in a smaller space. It certainly makes a lot of sense from a precision standpoint.
@dlang
Yes, the strap would be less flexible than a chain and it would retain the shape that it is stored in. But it wouldn’t have to be wound up into a coil at any point after shipping. It could be straightened out before being used. The advantage over a lead screw is that the flexibility of the strap would allow it to deflect if pushed into a wall when the sled was in the upper left or right hand corner.
Cool! I didn’t know the chain sag was something that could be factored into the software. Before I ask any obvious questions on that front, is there a post or an article discussing that topic? Thanks!
@pillageTHENburn
Its hard to argue with theory that’s reinforced by empirical data. Using the wire rope stretch calculator, 33 pounds on a 1/8" diameter equates to 0.0155% stretch. If there’s 33 pounds on a 6 foot length of cable, that’s about 0.28mm of stretch. Additionally, the load on the motors could be used as a way to quantify just how much tension there is in the cables and use that to compensate for whatever stretch there is, right?
Cool! I didn’t know the chain sag was something that could be factored into
the software. Before I ask any obvious questions on that front, is there a
post or an article discussing that topic? Thanks!
there are posts talking about it (somewhere), but no articles
Its hard to argue with theory that’s reinforced by empirical data. Using the
wire rope stretch calculator, 33 pounds on a 1/8" diameter equates to 0.0155%
stretch. If there’s 33 pounds on a 6 foot length of cable, that’s about 0.28mm
of stretch. Additionally, the load on the motors could be used as a way to
quantify just how much tension there is in the cables and use that to
compensate for whatever stretch there is, right?
Chain Sag and cable stretch both depend on the same two factors
the length of the chain/cable
the tension on the chain/cable
In both cases, if you know these two factors, then you can calculate the
sag/stretch and compensate for it in software.
The problem is exactly how to determine the tension in the chain/cable since it
depends on the weight of the sled, which we don’t have a way to measure to any
accuracy. We can measure it indirectly by measuring how much the chain sags and
calculating how much weight there would need to be to have that much sag.
But I haven’t been able to think of any other way to measure this. We are trying
to detect very small differences in length (on the order of one part in 10,000)
The problem is exactly how to determine the tension in the chain/cable since it
depends on the weight of the sled, which we don’t have a way to measure to any
accuracy
Just jumping in here (with all intentions to be very respectful), but how accurate do you need to be on the weight of the sled? I’d think that if you had calculations that could account for sag based upon the weight of the sled (along with length of chain which you should know at any given time) that even coming close to the real weight would be an improvement rather than a detriment. Can’t you just tell people to put their sled on a scale and measure to the nearest lb/kg? If you measure it as, idk, 20 lbs. and the real weight is 20.34432 lbs, I can’t see that the delta would throw off the very small sag adjustment so that it makes Maslow less accurate than before.
Might be able to get by with a ‘rule of thumb’ value based upon a sled with a R22002 router and two bricks.
Question though, is weight of the sled the only thing important? Just thinking about it (and not sure how much of a factor it would be), the chain that’s “pulling” the sled toward it would seem to me to have less sag than the chain that is just supporting the sled’s weight. And if it’s cutting something, there’s more resistance and therefore less sag? Again, not sure how much of a factor it would be in comparison to the weight.
. We’d be trading sag for stretch?