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Why does paper fold so well?

bbc.co.uk

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Re: Why does paper fold so well?

#41

Post by foobarian »

srean wrote: post #6

Oh no! woe is me, they don't highlight my absolutely, ridiculously favourite fact/curiosity about a sheet of smooth paper: If you fold it clean, the crease is a straight line. In fact I don't know of any other good way of obtaining a straight edge from scratch quickly, meaning without transporting one existing straight edge to another (*). I remember spending a lot of enamored time coming up with different geometrica…

One thing I find interesting about paper is that wetting and drying it turns it uneven. Even when drying it under a press.

And then another ridiculous process not involving paper, but super cool nonetheless is creating a flat surface by grinding 3 not-flat objects against each other in round-robin manner.

Re: Why does paper fold so well?

#42

Post by mauvehaus »

bena wrote: post #28
wongarsu wrote: post #27

Earlier quoted context omitted.

If you want to use a rope to get a straight line, your best bet is to turn the rope itself into the pencil. Coat it in chalk or other powder, then put it under tension and snap it on to the desired surface

This is actually a tool used in construction. A chamber filled with chalk and a coiled line. You hook the line to one end of your item, pull the chamber across, make it tight, snap the line. https://www.homedepot.com/p/Milwaukee-100-ft-Bold-Line-Chalk...

The Tajima ones [0] are phenomenal, though the hook leaves a longer blank stretch than I'd like. They make a super nice snap knife too. Highly recommend Tajima for anything they make. Annoyingly, they don't sell a rip saw, only crosscut.

[0] https://www.tajimatool.com/product_category/mt/#chalk-rite

Re: Why does paper fold so well?

#43

Post by jayd16 »

srean wrote: post #6

Oh no! woe is me, they don't highlight my absolutely, ridiculously favourite fact/curiosity about a sheet of smooth paper: If you fold it clean, the crease is a straight line. In fact I don't know of any other good way of obtaining a straight edge from scratch quickly, meaning without transporting one existing straight edge to another (*). I remember spending a lot of enamored time coming up with different geometrica…

rub 3 surfaces together and they end up smooth and flat. Fast in the across the world sort of way but not fast like folding a sheet of paper.

Still a sheet of paper is already made smooth and consistently flat. I'm not sure how well it works from hand made paper.

Re: Why does paper fold so well?

#44

Post by boothby »

dotancohen wrote: post #30
globular-toast wrote: post #20

Earlier quoted context omitted.

> In fact I don't know of any other good way of obtaining a straight edge from scratch quickly A string made taut between two points is surely a better way? And works at much bigger sizes too (people build walls and foundations using this technique all the time). The paper is less useful in practice because any paper you find is probably straight and square anyway. Still, I had fun thinking about this as I definitely…

Over long distances the string will sag in the middle. That's one of the reasons (not the only reason) construction uses lasers today.

It depends what you need that line for -- if you're projecting onto a wall, that sag matters. If you're projecting onto the ground, it doesn't.

Re: Why does paper fold so well?

#45

Post by boothby »

gacgacgac wrote: post #39
dotancohen wrote: post #30

Earlier quoted context omitted.

Over long distances the string will sag in the middle. That's one of the reasons (not the only reason) construction uses lasers today.

Folded paper won't?

If anybody has ever tried folding a very large paper (or, bedsheets, tarps, etc), they'll realize the wisdom of this comment. Our intuition from folding paper on the order of several to tens of centimetres does not scale to arbitrary size and precision. Paper is relatively rigid, but its rigidity is finite and ensuring local-to-global flatness becomes a painstaking endeavour.

Re: Why does paper fold so well?

#46

Post by mjanx123 »

foobarian wrote: post #41
srean wrote: post #6

Oh no! woe is me, they don't highlight my absolutely, ridiculously favourite fact/curiosity about a sheet of smooth paper: If you fold it clean, the crease is a straight line. In fact I don't know of any other good way of obtaining a straight edge from scratch quickly, meaning without transporting one existing straight edge to another (*). I remember spending a lot of enamored time coming up with different geometrica…

One thing I find interesting about paper is that wetting and drying it turns it uneven. Even when drying it under a press. And then another ridiculous process not involving paper, but super cool nonetheless is creating a flat surface by grinding 3 not-flat objects against each other in round-robin manner.

Three negatively curved surfaces (saddles) mate despite not being flat. You need to rotate the surfaces also when lapping. (See the famous Robrenz video)

Re: Why does paper fold so well?

#47

Post by flybrand »

yread wrote: post #4

And why does it sink. Its basically squished wood!

because the fibers are denser than wood and hydrophilic - so the pockets between the fibers fill with water that displaces the air.

Air pockets become water pockets = neutral.

Fibers denser than water = sink.

Re: Why does paper fold so well?

#48

Post by srean »

boothby wrote: post #45
gacgacgac wrote: post #39

Earlier quoted context omitted.

Folded paper won't?

If anybody has ever tried folding a very large paper (or, bedsheets, tarps, etc), they'll realize the wisdom of this comment. Our intuition from folding paper on the order of several to tens of centimetres does not scale to arbitrary size and precision. Paper is relatively rigid, but its rigidity is finite and ensuring local-to-global flatness becomes a painstaking endeavour.

Then there is the puzzle of coming up with folds to intentionally increase the rigidity of the surface. Essentially, using Theorema Pizzarium

Re: Why does paper fold so well?

#49

Post by srean »

mjanx123 wrote: post #46
foobarian wrote: post #41

Earlier quoted context omitted.

One thing I find interesting about paper is that wetting and drying it turns it uneven. Even when drying it under a press. And then another ridiculous process not involving paper, but super cool nonetheless is creating a flat surface by grinding 3 not-flat objects against each other in round-robin manner.

Three negatively curved surfaces (saddles) mate despite not being flat. You need to rotate the surfaces also when lapping. (See the famous Robrenz video)

I did not know this, very interesting.

Re: Why does paper fold so well?

#50

Post by srean »

foobarian wrote: post #41
srean wrote: post #6

Oh no! woe is me, they don't highlight my absolutely, ridiculously favourite fact/curiosity about a sheet of smooth paper: If you fold it clean, the crease is a straight line. In fact I don't know of any other good way of obtaining a straight edge from scratch quickly, meaning without transporting one existing straight edge to another (*). I remember spending a lot of enamored time coming up with different geometrica…

One thing I find interesting about paper is that wetting and drying it turns it uneven. Even when drying it under a press. And then another ridiculous process not involving paper, but super cool nonetheless is creating a flat surface by grinding 3 not-flat objects against each other in round-robin manner.

I think wetting and drying of paper is a bit like heat treatment of steel. The fibres find a new local minimum stable position, prompted by the swelling and the shrinking.
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