Is it true that there is an optimum ratio between width and hight of the frame?

Sorry for this one if it was already answered, under no Judgement, I hope it is OK.

Is it true that in an ideal world the optimum position for the placement of the motors would be on the lines of the X?
Minus the radius of the sprockets, or in more detail, something around the centre of the motor shaft, until perhaps half of a tooth, an average of pivot points?

Would that mean that there is an optimum ratio between hight and width of placing the motors, building the frame?

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not really, the width and heigh of the frame are driven far more by the size of
a standard sheet of plywood than anything else.

The steeper the angle to the chain on one side, the lower the tension to the
motor on the other side, which hurts accuracy (due to chain sag and the
possibility of friction being high enough to prevent the sled from moving as
desired)

but there wasn’t a whole lot of thought put into the exact details, just getting
the motors far enough out and up to work.

looking at it another way, whatever the ‘optimum’ angles are, as you move around the sheet you will end up being far from that optimum.

if you try to have the angle too flat, you need a lot of tension on the chains, so that means you want to have the motors high to avoid these angles

if you try to have the angles too steep, you loose control (because the portion of gravity that’s providing side force becomes too small), so you move the motors out to avoid these angles

Also, as the angles get flat, the effect of small amounts of chain movement get magnified, so if you don’t have a high enough resolution on your encoders, you can hit a situation where you have the torque to apply enough tension to move the chain higher, but you can’t position the chain accurately enough to achieve your desired resolution. With the stock motors @8148 steps/rev, we are FAR beyond the point where this matters, but if you were to try and go with a few hundred steps/rev (say what a stepper motor provides), you would run into this problem.

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I wonder whether an arrangement of stretchy string and pulleys from the bottom corners would help this?

I don’t think so, you would need it to pull more from the short side than from
the long side, which would be very difficult to arrange.

adding two additional motors in the bottom corners would do it, but the added
complication (and problems with getting the math right) would make this very
messy, as well as doubling the cost of the machine.

Do i understand it correct, that this pans out to a situation like…
When you dont actually need a full size machine it’s still makes sense to build a full size machine and just use the center of the workspace and have the best possible accuracy.
?

:slight_smile: that drawing makes me think of building an upside down machine.

yes, if you have the space for it, everything will work better near the center
where the chains are not especially steep or flat.

flat is less of a problem for accuracy than steep.

David Lang

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I have been putting some thought into this problem for a while and I totally agree that adding motors linked with chains could be quite complicated and difficult. But what if you added motors that were only roughly accurate and they were linked with chain and elastic for the last foot or two to the sled. This way they could always be kept under tension but not too much tension, and the tension would follow the sled. This solves the problem of the tension needing to be on the short side because the chain shortens when it needs to. It would be hard to solve that with a static piece of elastic but with a dynamic motor-driven piece of elastic it could work really well.

Also the math wouldn’t have to be perfect, or even that close. With the right lengths of elastic you could even use the same math and just “invert” it and allow the elastic to take up the error (of which there would be a lot in that case)

In this hypothetical motor-tension set-up it could even be run horizontally, which I realize defeats some of the purpose of Maslow but it’s an interesting idea.

I think there’s even a way to add appropriate tension opposite each motor without two extra motors. A passive mechanical solution only. In my head it’s pretty complicated but could work. I’ll try to boil the idea down to something prove-able at some point. There’s probably something huge and basic I’m overlooking in my daydreaming, I’m sure I’ll catch it sooner or later.

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Total n00b here but I am looking to start building the frame for my maslow and I think I am seeing some of my same questions being asked.
I am considering using a 12 foot top beam on the machine, to make the angles at the bottom corners less extreme. The obvious downside of this is that it adds additional chain sag. So I assume there is an optimum, height of the motors and also optimum distance. This would take into account both the angle of the chains and the additional chain sag.
Perhaps someone has already calculated that. Perhaps it was already taken into account in the original design. The biggest downside I see to adding more length to the top beam is that the machine will take up more space.
Thanks in advance for everyone’s replies.

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I think the original design was working around commonly available lumber and shop size. A longer top beam will need additional chain and that’s easy to do. Do pay attention to preventing twist on the beam, the motor mounts make a lever arm and any movement they cause really affects accuracy.
Sag hasn’t been thoroughly investigated, it might not be an issue for you. Will be interested to hear what you find. If it is a problem, coming up with a way to mitigate it would be an interesting study. @pillageTHENburn mentioned having an idea on the subject, it’s something others are starting to look at as well.

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The reason I posted the question was, I hope I can build a new frame soon to get a fresh start as to April 2017.
Not sure if this is true again, that if our plywoods where square it would be easier for the maths.
Be aware that your top beam should not bend when the sled goes up.
You could take a look at @dlang python script http://lang.hm/maslow/v-plotter.py and get a idea of how much space and hight you have and plan your motor positions.
I am still looking for the Holy Grale, oh…, sorry, for a ratio, may it be Fibonacci or logarithmic to get ahead with accuracy.
The Pantographs and circular sled mount testing is taking off impressively, with one part of the maths and accuracy,
I have realised, that if I want to keep up I need a accurate frame to start with.
For now, for me it is like: "How much of you plywood can you get centred in the PY script."
I am going to play with that script allot before I decide on the new frame dimensions.
Kindly let us know what you are planing and doing and keep us updated, oh yes, with pictures also.

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Agreed 100% on the twist of the top beam. I am planning to use dlang’s modified frame I will check out his script. I think I will use either some 8020 or a piece of unistrut as that top beam.
I may try to do some catenary curve calculations in excel to mess around. Love the almost instant feedback of this forum.

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That’s my approach, I also tied that top unistrut down to the bottom beam with a piece of unistrut on each end of the beam. The beast gets pretty heavy, though :grin:

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Had to look that up and decided that with beer this is not for me tonight.
Still exited of what there is to come

Not off topic I think. Is the py script based on triangular? If not how difficult would the adjustment be?

I am considering using a 12 foot top beam on the machine, to make the angles
at the bottom corners less extreme. The obvious downside of this is that it
adds additional chain sag. So I assume there is an optimum, height of the
motors and also optimum distance. This would take into account both the angle
of the chains and the additional chain sag.

I believe that the added angle of the short chain will add enough tension to
more than counter the extra lenth of the long chain.

Perhaps someone has already calculated that. Perhaps it was already taken into
account in the original design. The biggest downside I see to adding more
length to the top beam is that the machine will take up more space.

the added space is the biggest problem.

the machine was not designed with lots of complex calculations, it was designed
from a “I have an idea, let’s try it” iterative/experimental approach.

This means that there is room for things to be optimized, don’t assume that the
existing dimensions are ‘optimum’, everything is a trade off between size/cost
vs performance/accuracy.

Bar tinkered with stuff until it worked pretty well for him, then he setup the
kickstarter, as the triangular kinematics stuff is showing, we are finding ways
to improve it and make it easier to build.

David Lang

The reason I posted the question was, I hope I can build a new frame soon to get a fresh start as to April 2017.
Not sure if this is true again, that if our plywoods where square it would be easier for the maths.

square vs rectangular plywood should not matter

Be aware that your top beam should not bend when the sled goes up.

we should have a test that’s similar to the motor distance calibration that runs
the motors up to full power pulling directly against each other to see if the
frame bends in any detectable way (if so, the frame needs to be strengthened)

You could take a look at @dlang python script
http://lang.hm/maslow/v-plotter.py and
get a idea of how much space and hight you have and plan your motor positions.

I have realised, that if I want to keep up I need a accurate frame to start with.

That’s the beauty of the top beam approach, the rest of the frame doesn’t need
to be that accurate for cuts to be repeatable (now, if you have thing bowed, or
the top beam at an angle to the bottom plywood support, you will have trouble
using the full sheet of plywood, but the machine will be very repeatable)

For now, for me it is like: “How much of you plywood can you get centred in the PY script.”
I am going to play with that script allot before I decide on the new frame dimensions.

keep in mind that the outside cutoffs are fairly arbitrary, I took the maslow
dimensions and said “this machine should be accurate to the outer edges, so set
my accuracy limits so they fit on a 4x8 sheet of plywood” :slight_smile:

Kindly let us know what you are planing and doing and keep us updated, oh yes, with pictures also.

YES!!!

we had one person report on the sag they saw, and that indicated that our
accuracy in the bottom corners is around ± 1mm vs the ± 0.4mm that is the
target accuracy.

This isn’t bad, but it’s not quite as good as we wanted, adding weight, or
tilting it nearer to vertical would help this (as would moving the motors out
further), but we haven’t had a lot of experimenting to find out the shape of the
problem. I’ve been trying to think about how to measure the error, and finding
that a struggle.

Yes, the python script is based on triangular kinematics, I never got around to
trying to figure out the ‘normal’ kinematics.

It’s looks for accuracy limits based on
min and max tension on the chains
motor step size (can limit accuracy near the top center, but with the maslow’s
high resolution encoders, it’s not an issue)