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A new way to think about frame size and angle

I really like the work. What I like most is that it is an attempt to explicitly state the performance attributes (minimum available force) which drive a decision for frame geometry. In order to really get this right, all the potential constraints would have to be considered.

I think @dlang did a good job discussing a number of other potential limiting factors.

It is not clear to me that minimum available force is what constrains the Maslow. Here a few other performance attributes which may be constraining:

  • Stiffness:
    • Stiffness in the bottom corners is limited by chain-sag. If a cutting force is in the same direction of the chain’s tension, the chain’s tension is reduced; the chain sags more. This leads to a positional error.
    • Stiffness in the top center is limited by the chain’s elasticity. If a cutting force is downward, the chains will stretch, which leads to a positional error.

What I like is: if you create a script/model/function which can take the frame geometry as inputs, and calculate these you can quickly analyze a frame design.

My recommendation is to just be as meticulous as is reasonable, to include all the performance attributes and parameters. That way you will be more likely to create something that is broadly applicable.

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We have work in place to account for chain sag and stretch (it was developed as
part of the holey calibration work and has been at least partly ported and
merged into the main tree, but there hasn’t been a release yet that includes it)

David Lang

So what is the current recommendation? It seems like 12’ distance between motors and 30" above top of work canvas area with a sled of 20-25 lbs and angle of 10-15 degrees is best current compromise? between top center and low corner performance.

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That is my recommendation. However, that is not based on the rigorous analysis mentioned in this thread. I actually am not aware of all the historic work that was done to determine the optimal frame-angle. I can say that a 10’ top beam will be limited in the bottom corners of the worksheet. The top-center of the worksheet experiences quite a bit of chain-stretch; the kinematics equations become stiff, which makes calibration difficult. I think the 30" number is very good. Also, the 20-25 lb sled weight is very good.

the real question is will making the frame even bigger say 14’ wide and 40" from top of work piece be even better, but then we start to run into mundane issues like many ceilings are only 8’ tall and 14’ is taking up a lot of room in my garage. The larger the work piece becomes the more “even” the force measurements become over a 4x8’ surface area.

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take a look at the spreadsheet (I’m pretty sure it’s linked to earlier), try
different parameters.

right now it seems that the 12’ 30" is around the sweet spot for machine
dimensions, as you go wider you have more trouble reaching to the far side, even
as it improves the near side.

There has not been a real test on frame angle and sled weights, this is mostly
“it worked for Bar before he started the kickstarter”. There have been a few
tests, if you go too close to vertical, to far from vertical, or too heavy on
the sled, we know there are problems

we know that we are closer to the limit in terms of tilting back, 20 degrees
doesn’t work while 10 degrees does (the last testing had problems with a 5
degree tilt)

we have not had anyone do any testing since we added chain sag to the
kinematics, so I suspect that we handle lighter sleds and closer to vertical
angles better than we used to. But until someone gets a chance to do some
testing, it’s hard to be sure.

David Lang

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Yeah. This is a good question. For me, I think that as long as you increase the two dimensions at a similar ratio, you’ll end up with an improved machine.

If you just keep increasing the beam-length, you’ll eventually run into issues related to the weight of the chain, slow dynamics of the chain, chain-stretch, or frame rigidity. I don’t know at what point those issues become prominent, maybe a 20’ top-beam. To my knowledge, nobody has ever made a machine so big that its size became a problem.

I know i should be quiet when the grown-ups are talking. Just can’t help it. i know the .py script is just giving rough guidance and after re-framing to 11.5 ft again believe in a ratio in scale of a sheet.

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so going to 14’ 40" increases the max for by 2.58 pounds and decreases your min force by 1.7 pounds, so it’s worse in both places than a 12’ 30" frame

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That doesn’t make any sense. Something must’ve gone wrong in the comparison.


I put in 170" motor seperation and 40" work piece distance and got about 7.9 and 23.5 for min and max force which is better than stock and better than 144" distance

image
In this sample image the green rectangle is about 48x96", the distance between the motors is about double that or 196" and the height from top of rectangle to motors is about 40". Putting those numbers into the spread sheet gives min and max forces of about 10 and 26.5. a difference of about 16. Compare that to the “stock” set up which has a difference of about 30.5. the bigger the difference the more variability in the cutting forces.

Building a 16’ wide top beam is really not that hard to do, Most garages are 18 to 20 feet long.

On the other hand most people rarely cut out 8 ’ long parts and shifting the plywood towards the center is a lot easier to do.

I’ll point out that the constants in the v-plotter are not reliable, they were tweaked with the assumption that the stock dimensions were good (which we know is not the case now)

the problem isn’t the variability in the cutting force, but rather that:

if the peak force needed in the top center is too high, the motors won’t be able to operate there and the sled will fall behind (and droop, producing the ‘hook’ at the end.

If the minimum force available in the bottom corners is too low, the sled has trouble moving into the corners.

keep in mind that the sprockets on a 10’ beam are’t quite 10’ (120") apart they are more like 116"-118" apart

10’ 18" is min 3.3 max 33.7 (stock)
12’ 18" is min 6.4 max 40.2
12’ 30" is min 5.7 max 25.4
14’ 40" is min 7.4 max 22.8
16’ 40" is min 9.3 max 25.5

while garages are 20’ deep, most people don’t want to dedicate that much space. They also want a machine that’s short enough to fit through the door (or at least one that fits under the cealing) :slight_smile: we have quite a few people who haven’t had space in their workshop for a full 10’ top beam.

40" above the 48" workpiece, which is about 15-18" above the ground for almost 9’ of height. I don’t know about you, but I find that rather high to get up to for hooking up chains or measuring if the sprocket is really at 12 o’clock :slight_smile: stock doors are 80" high (so you want to stay under 79" tall to clear molding top and bottom)

that’s why I show my work and link to the tools, so others can catch when I make a mistake :slight_smile:

Yes I agree. The point I was making is the ideal frame size is too big for most to be practical.

If I’m looking for best overall accuracy, I’m gathering that right now the sweet spot is a 12’ distance between motors, 30” above top cutting surface, and a 10 degree angle is best? Dlang, thanks for all your insight on forum, it has helped me greatly.

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12’ & 30" seems to be a decent compromise with counterweights and not bungees, but not sure if anyone has posted side by side results.

Thanks Roland, I’ll be sure to use counterweights. Do you think it would be worth me going the 14 foot rout if I have the room? And 40” spacing between top cut surface?

I would not recommend it unless you routinely make parts longer than 6’ or routinely cut out an entire 4x8 sheet at once. . Shifting the plywood a couple feet to the left or right gives all the benefits of a bigger machine without becoming a guinea pig for testing.

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but if you like testing stuff, go ahead and try it and report back. Would be interesting to hear your experiences.

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