(saw whirring)
- This is a cast saw.
It's used to extract you from a cast
after your bones have healed.
The blades are sharp enough and fast enough
to cut through the hard outer shell of the cast,
and that seems pretty dangerous
given how close the cutting blade
gets to soft human skin parts.
And it would be a terrible idea
for someone with no medical experience
to attempt such a procedure on themselves.
Like what if my hand slipped and the blade touched my skin?
(saw whirring)
Well, it turns out nothing would happen actually,
because a cast saw is able to cut through a cast,
but it's not able to cut through human skin.
To see how that's possible,
we need to film it in slow motion.
My first thought was that because the movement is cyclic,
I could use the stroboscopic effect
to fake having a high-speed camera,
like I did in my gravitational waves video.
If you want an explanation
of how the stroboscopic effect works,
watch to the end of that video,
link in the comment in the description.
But when I actually tried that,
the rolling shutter artifacts were just off the charts,
so I hired a Phantom high-speed camera
that can do 2,000 frames per second in full HD.
Straight away,
you can see that the blade isn't in fact spinning.
It's oscillating backwards and forwards.
And looking at the footage,
it's only moving back and forth about three millimeters,
a tiny distance, really.
Actually, you don't need a high-speed camera to show that.
If I draw a mark on the blade and turn it on,
you can see the mark doesn't spin round.
It becomes blurry around the edges,
which tells us that it's oscillating, not spinning.
We'll get to the slow-motion footage
of the blade on my skin in a moment,
but first, here's a clue as to why a cast saw
can cut through a cast but not through human skin.
Here's an under-inflated balloon
and here's an over-inflated balloon.
The cast saw can't pop the under-inflated balloon.
But look what happens
when I press it into the over-inflated balloon.
It pops easily.
The difference is when you over-inflate a balloon,
you're pushing the balloon to its elastic deformation limit.
It can't stretch any more after that.
So when the teeth of the blade touches the rubber,
the rubber can't stretch and move underneath it.
The blade scrapes along the rubber,
which cuts a hole through it.
The under-inflated balloon
has a lot of stretch still left in it.
The rubber just moves and stretches underneath the teeth,
so no hole can be cut.
And it turns out that human skin
is like an under-inflated balloon.
You can see that really clearly here.
As the teeth bite into my skin,
they proceed to just casually move the skin back and forth.
It's fun to see the ripples propagate outwards
from the point of contact there.
This is probably a good opportunity to point out
that although you may have a tool at home
that is superficially similar to a cast saw,
it may very well operate in a different way.
For example, it may be a rotary tool
as opposed to an oscillating tool.
This Dremel is a rotating tool,
and look what it does to the under-inflated balloon.
The rubber can only stretch so far.
Hopefully, I've got that warning out
before anyone paused the video
to run and get their angle grinder.
Even if you know for sure
that the tool you're holding is an oscillating tool,
it may be that the distance over which it oscillates
could be beyond the elastic deformation limit of your skin,
in other words, it could cut you.
And in fact, if the tool was sharp enough,
it could cut you anyway.
What I'm saying is don't try this at home.
I wonder about hairs.
I wonder if I could use a cast saw to shave.
Before we find out, it's interesting to see in slow motion
how the cast saw interacts
with the different types of cast you can get.
The traditional cast is made from plaster of paris.
So first you put on this sleeve,
then you wrap your arm in padding.
I mean, you don't do it, the doctor does.
The roll of fabric,
which has all this plaster of paris powder on it,
is dipped in water.
That's then wrapped around your arm,
and it dries fully in 24 to 48 hours,
which is quite a long time to wait,
but it's really cool
to see plaster of paris being cut in slow motion.
Because it's rigid,
the teeth of the saw scrape along the surface of the cast,
chipping away little bits of it
that you can see flying through the air.
Another cool thing I noticed is the oscillating blade
actually moves the whole cast back and forth a little bit.
But look, the plaster of paris shavings
kind of float above the surface
while the cast jiggles about underneath.
So that's a plaster of paris cast,
but if you broke your arm today,
they would most likely put on a synthetic cast like this.
I mean, they wouldn't put it on like this.
The fabric is fiberglass
and it's coated in a water-activated resin.
So the process starts in the same way,
you dip the roll in water,
but once it's applied to the arm,
it only takes about 15 minutes to dry,
and actually it creates much less mess
when you're cutting it off.
Something interesting about the waves
that radiate out from the contact point,
they represent the speed of sound of my skin.
It's interesting to see how the speed of sound
is different on different parts of my arm.
Here on the back of my arm,
I estimate it's about 10 meters per second,
but on the back of my hand it's about six meters per second.
Interestingly, from that,
I can calculate the shear modulus of my skin,
and from that I can calculate
the Young's modulus of my skin.
But to get from the shear modulus to Young's modulus,
I need to know what the Poisson ratio of my skin is.
I talked about the Poisson ratio in my auxetics video,
link in the comment in the description.
When I Googled Poisson number of skin, I got this result.
The Poisson ratio of cow teat skin
was measured uni- and biaxially
and is shown to be anisotropic
and dependent on aspect ratio.
So there you go.
But I also found out that the Poisson ratio
of human skin is about 0.48,
and it turns out that the Young's modulus
for the back of my hand is about 100 kilopascals,
and the Young's modulus
for the back of my arm is about 300 kilopascals.
Which is good to know.
(saw whirring)
I really don't like vendor lock-in.
For example, email doesn't have vendor lock-in.
Like I could use Gmail as my vendor,
you could use Hotmail,
and we could still communicate with each other,
whereas I couldn't use Facebook to DM you on Twitter
because those services use proprietary protocols
that don't talk to each other.
It even happens in the physical world.
Like I have this power tool
and a bunch of batteries that go with the power tool,
but the connection is proprietary.
So if I wanted to switch to a different brand of power tool,
I'd need to buy a bunch of new batteries to go with it.
Even razors have proprietary connection technology,
so once you've bought a particular razor handle,
you have to continue buying the cartridges
from the same manufacturer.
But of course, there's been an open standard
for razor blades for over 200 years, the safety razor.
Why would you buy into a proprietary standard
when you could get a beautifully engineered safety razor
from the sponsor of this video, Henson Shaving,
and then buy quality blades
from whoever you like for pennies?
Compare that to a cartridge razor where the handle is cheap
but the cartridges are sold at a huge markup.
It might seem like an economical option at the beginning,
but it's a false economy.
The total cost of ownership of the Henson AL13
is lower than a cartridge razor after just a few months.
The Henson AL13 is what you get
when a family-run aerospace machine shop
pivots to making their own products.
The blade is well supported by its all-metal body.
It protrudes exactly 30 microns from the plane
at an angle of tau 112 radians for a really nice shave.
The nice thing is you only have to buy
a new handle once a year.
Wait, once a lifetime.
It's something you literally buy once ever.
But what's the actual shave like?
Well, I shaved this side of my face with the Henson AL13,
and I shaved this side of my face with a cast saw,
and the difference is unbelievable.
Go to hensonshaving.com/stevemould
and use promo code STEVEMOULD at checkout
to get 100 free blades with your purchase of a razor.
Just make sure both items are in the basket
when you apply the code.
The link is also in the description,
so check out Henson Shaving today.
I hope you enjoyed this video.
If you did, don't forget to hit subscribe,
and the algorithm thinks you'll enjoy this video next.
(upbeat jazzy music)