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'[OT] Holes get smaller'
2006\08\10@145939 by Gus S Calabrese

face picon face

When looking into the abyss of defeat,
change topics.  Nice move , Tony


When a piece of metal is heated .......
 as temp approaches infinity, the hole gets smaller.


Gus


2006\08\10@155534 by Tony Smith

picon face
> Subject: [OT] Holes get smaller
>
> When looking into the abyss of defeat,
> change topics.  Nice move , Tony
>
> When a piece of metal is heated .......
>   as temp approaches infinity, the hole gets smaller.
>
> Gus


Er, what?

Russell felt like talking about holes.

And if you think holes get smaller when you heat up metal, you've never done
any machining.  You need to Google coefficient of thermal expansion.

Sheesh, not another one who believes this.

Tony

2006\08\10@160328 by M. Adam Davis

face picon face
I would tend to think the opposite in this limit equation.  The hole
would become infinitly large, although not as infinitely large as the
rest of the metal.

-Adam

On 8/10/06, Gus S Calabrese <spam_OUTgscTakeThisOuTspamomegadogs.com> wrote:
{Quote hidden}

> -

2006\08\10@163957 by Mark Scoville

flavicon
face
>
> When a piece of metal is heated .......
>   as temp approaches infinity, the hole gets smaller.
>
> Gus
>

Maybe I'm missing something. Won't the metal just melt and turn liquid
before getting anywhere near infinity.

Maybe there is a joke in here somewhere that I'm too dense to get.

-- Mark



2006\08\11@033527 by Jon Chandler

picon face
Wrong.

Think about it this way.  Start with a sheet of metal.  Cut a disk out
of it.  The disk and the plate are the same material and the same
coefficient of expansion.  If you heat both pieces, the disk obviously
gets larger.....but it's still got to fit in the hole...so the hole must
get bigger!

Jon



Gus S Calabrese wrote:
{Quote hidden}

2006\08\11@102644 by Gerhard Fiedler

picon face
Jon Chandler wrote:

> Wrong.
>
> Think about it this way.  Start with a sheet of metal.  Cut a disk out
> of it.  The disk and the plate are the same material and the same
> coefficient of expansion.  If you heat both pieces, the disk obviously
> gets larger.....but it's still got to fit in the hole...so the hole must
> get bigger!

This all assumes that both pieces of material are in their entirety at the
same temperature so that no elastic deformation occurs.

The question is what happens if the material that contains the hole is
elastic -- which means that it can deform -- and the material that's
farther away from the hole is at the original temperature (i.e. at the
original size).

I'm pretty sure that there will occur some elastic deformation, and so far
nobody seems to have been able to really explain the nature of the elastic
deformations and their effect on the hole size.

Imagine a steel or iron cylinder (not cast iron, use more elastic iron), 20
cm diameter and 20 cm long (increase as necessary to avoid breaking), and a
hole of 1 cm diameter all the way through it along its axis. Now imagine
you heat up the inside of the hole. I don't think this is as trivial as
"all holes in materials with positive expansion coefficients expand when
the material is heated up". I'm pretty sure there happens some elastic
deformation of the material around the hole, and I don't think it's
completely unreasonable for the hole to become smaller under certain
conditions.

The expanding material has basically three ways of expanding: radially
towards the outside (that would be the "hole gets bigger" way), axially
(which may or may not increase the diameter of the hole, but probably not)
and radially towards the inside. The latter two, and especially the latter,
require some elastic deformation, whereas the first option requires that
the material compresses (or breaks). Elastic deformation is not unheard of
with steel or iron (again, I'm not talking about cast iron), and IMO more
probable than compression. The outcome probably depends on the strength of
the material around the hole (against breaking), the forces required for
elastic deformation in the different directions and the force required for
compression.

Gerhard

2006\08\11@111002 by Tony Smith

picon face
{Quote hidden}

That's a fairly contrived case, and is quite possible if you were aggressive
enough in heating the hole.  Don't forget, steel conducts heat very well and
is quite elastic, two factors woking against the shrinking hole.  Cast iron
would be the better choice.

I'm sure people making the main guns on battleships would have noticed
this... or even the people firing them...

I couldn't say with 100% certainty (I no longer work with a metallurgist),
but if it did happen, it wouldn't remain that way for long.

You might as well clamp it in a vice, then heat it.  That would work too  :)

You only work that way once, it's bad for what you are heating up.  Slow
learners do it twice, of course.

Tony

2006\08\11@122219 by James Newtons Massmind

face picon face
Jon, as the temperature approaches infinity, the metal melts. Then becomes
gas. Gus has an interesting sense of humor.

---
James.



> {Original Message removed}

2006\08\11@164217 by Gerhard Fiedler

picon face
Tony Smith wrote:

> That's a fairly contrived case,

I don't think so. I think this is a rather normal case when heating up a
localized area in a thick metal sheet or a block. Few people work on large
sheets or blocks that are at a uniform temperature when gas welding.

> and is quite possible if you were aggressive enough in heating the hole.
> Don't forget, steel conducts heat very well and is quite elastic, two
> factors woking against the shrinking hole.  

How is that? Elastic isn't the same as compressible. It's pretty common to
have steel almost melting around the welding area, and it still being not
even red or even only a bit warm far enough away from the welding area.

> Cast iron would be the better choice.

For reasons you already mentioned and I agree with, I don't think so. It
doesn't have the same elasticity. You may consider cast iron, I don't.

> I'm sure people making the main guns on battleships would have noticed
> this... or even the people firing them...

I'm not sure I made myself clear enough. A gun barrel does not fulfill my
criteria; it is too thin and probably would heat up fairly uniformly. It
probably expands (inside and out). They probably have found a way to deal
with that.

> I couldn't say with 100% certainty (I no longer work with a
> metallurgist), but if it did happen, it wouldn't remain that way for
> long.

Well... depends of course. I'm fairly certain that if you take a normal
sheet, 1 cm thick, maybe 1 m by 1 m or maybe bigger, and heat it with a
welding flame in the middle, the outside remains quite a bit colder than
the middle, even after a long time. I'm not sure it would not bend with
time, though.

Gerhard

2006\08\11@215047 by Tony Smith

picon face
{Quote hidden}

Unless you can come up with a good reason to rapidly heat the inside of a
20mm x 20mm cylinder with a 1cm hole drilled in it, it's a contrived case.
Getting the inside of a large gun barrel to size is about the closest case
you'll get.  Regardless, it doesn't matter.

Heat spreads out pretty quickly in metal when you heat up one end of it.  If
it didn't, I could throw away my welding gloves.

The metal outside the heating zone doesn't get compressed, it stretches.
Blow up a balloon, same thing.  Heating up the hole is the same as adding
more air to the balloon.  Everything gets bigger.  Elastic.  Cast iron isn't
as elastic as steel, so it is more likely to crack.

If you are trying to get a bearing into a casting, and you hit the hole with
too much heat, the casting will crack.  The hole may be red hot, the edge
may be ice cold, and it will still crack.  Heating the hole stresses the
metal.  The stress isn't relieved by material moving into the hole, it
propagates outwards.  It either expands or cracks.  Note that expanding
includes buckling.  It cracks because the outside can't expand fast enough.

If the material wasn't elastic, nor prone to cracking, then the hole would
get smaller as there is nowhere else to go.  (So cast iron is your best
bet.)  The universe wasn't built that way.  To get the hole to shrink, the
area surrounding the heating zone needs to be non-elastic.  If it's metal,
then it won't be.

Read that paragraph again  :)

You could get the metal hot enough to melt or become vapor, but that's
cheating.

When welding, you need to compensate for the fact that a small area will be
overheated.  The metal will distort.  Every time.  Sheets buckle, tubing
twists.  (A 1cm plate is quite thick, you'll be there a while with that
torch.)  You don't point a gas torch as a point on the metal & leave it
there until it gets to red hot.  You need to heat up a wide area to reduce
the stress caused by expansion.  That's why you see welders waving the torch
about.  Jewellers too, but since their bits of metal are fairly small, they
have the luxury of heating up the whole piece.  Spot welders especially suck
for distorting metal.

I've bashed enough pieces of metal in my life (fitting & turning) and the
hole doesn't get smaller.  Your blacksmith friend was mistaken.  Don't
forget the numbers we are talking about are very small, I didn't realise
blacksmiths use micrometers to measure their work!  And theirs are better
than mine!

Tony

2006\08\11@222853 by Tony Smith

picon face
{Quote hidden}

Unless you can come up with a good reason to rapidly heat the inside of a
20mm x 20mm cylinder with a 1cm hole drilled in it, it's a contrived case.
Getting the inside of a large gun barrel to size is about the closest case
you'll get.  Regardless, it doesn't matter.

Heat spreads out pretty quickly in metal when you heat up one end of it.  If
it didn't, I could throw away my welding gloves.

The metal outside the heating zone doesn't get compressed, it stretches.
Blow up a balloon, same thing.  Heating up the hole is the same as adding
more air to the balloon.  Everything gets bigger.  Elastic.  Cast iron isn't
as elastic as steel, so it is more likely to crack.

If you are trying to get a bearing into a casting, and you hit the hole with
too much heat, the casting will crack.  The hole may be red hot, the edge
may be ice cold, and it will still crack.  Heating the hole stresses the
metal.  The stress isn't relieved by material moving into the hole, it
propagates outwards.  It either expands or cracks.  Note that expanding
includes buckling.  It cracks because the outside can't expand fast enough.

If the material wasn't elastic, nor prone to cracking, then the hole would
get smaller as there is nowhere else to go.  (So cast iron is your best
bet.)  The universe wasn't built that way.  To get the hole to shrink, the
area surrounding the heating zone needs to be non-elastic.  If it's metal,
then it won't be.

Read that paragraph again  :)

You could get the metal hot enough to melt or become vapor, but that's
cheating.

When welding, you need to compensate for the fact that a small area will be
overheated.  The metal will distort.  Every time.  Sheets buckle, tubing
twists.  (A 1cm plate is quite thick, you'll be there a while with that
torch.)  You don't point a gas torch as a point on the metal & leave it
there until it gets to red hot.  You need to heat up a wide area to reduce
the stress caused by expansion.  That's why you see welders waving the torch
about.  Jewellers too, but since their bits of metal are fairly small, they
have the luxury of heating up the whole piece.  Spot welders especially suck
for distorting metal.

I've bashed enough pieces of metal in my life (fitting & turning) and the
hole doesn't get smaller.  Your blacksmith friend was mistaken.  Don't
forget the numbers we are talking about are very small, I didn't realise
blacksmiths use micrometers to measure their work!  And theirs are better
than mine!

Tony

2006\08\12@005103 by Gerhard Fiedler

picon face
Tony Smith wrote:

> Your blacksmith friend was mistaken.  Don't forget the numbers we are
> talking about are very small, I didn't realise blacksmiths use
> micrometers to measure their work!  

I didn't realize you did use micrometers to measure red hot holes in metal
blocks.

So yes, if you measured them, you know how they behave.

Gerhard

2006\08\12@073220 by Peter Todd

picon face
On Sat, Aug 12, 2006 at 11:09:25AM +1000, Tony Smith wrote:
> Unless you can come up with a good reason to rapidly heat the inside of a
> 20mm x 20mm cylinder with a 1cm hole drilled in it, it's a contrived case.
> Getting the inside of a large gun barrel to size is about the closest case
> you'll get.  Regardless, it doesn't matter.

Well, you can heat up that cylinder's hole very rapidly by drilling
it... I do a fair bit of machining, and I've heard people say that while
doing high-speed drilling work with some types of plastics the heat from
the plastic was both unable to escape due to low termal conductivity,
and caused the plastic to locally swell enough to grap the drill bit.

This is unlikely to be the drill expanding as some plastics can have a
much higher COE than drill bits.

The cure is oddly enough to drill the hole even *faster* by increading
the rate, the feed rate, at which the drill plunges into the work. That
way more of the heat gets ejected in the thicker chips and there is less
time for the drill bit to rub against the plastic..

--
http://www.petertodd.ca

2006\08\12@080803 by Tony Smith

picon face
> > Your blacksmith friend was mistaken.  Don't forget the
> numbers we are
> > talking about are very small, I didn't realise blacksmiths use
> > micrometers to measure their work!
>
> I didn't realize you did use micrometers to measure red hot
> holes in metal blocks.
>
> So yes, if you measured them, you know how they behave.
>
> Gerhard


Sorry, you weren't the one who mentioned blacksmiths, but the idea of
someone measuring a lump of red-hot steel with a micrometer amused me.

The numbers involved are quite small, and you don't need to measure them to
know the hole gets bigger.  It's called an interference fit.

Say your 20cm lump of steel is a roller of some sort, and needs a shaft put
thru it.  How do you get a 1cm rod thru the cylinder, and have it as a tight
fit?

You heat up the cylinder.  

Or cool the shaft, or both.  It's far easier to heat the cylinder up to 500
degrees than to cool the shaft down to -500 degrees.

So the hole is 10mm, and the CTE of steel is 12 ppm per degree.

Heating the cylinder 1 degree causes the hole to expand 0.000012mm.  An oven
can heat it up 250 degrees, the hole will expand by 0.03mm.  12/1000000 * 10
* 250.  For the non-metric people, 0.03mm is about 1/1000".  A kiln will
easily get 500C, expansion is 0.06mm.  

As I said, small numbers, but enough so you can get the rod into place.
Once the cylinder cools, they'll lock together.  There's a formula to
calculate the tightness of the fit.

Another example is engine cylinder liners.  You bore out the engine block,
heat it up, and drop the liners in.  Hole getting smaller would be a bonus,
it would reduce piston slap.

Tony

2006\08\12@081811 by VULCAN20

picon face


Tony Smith wrote:

{Quote hidden}

They did. since the beginning of fire arms.  They stooped shooting until
the barrel cooled down.

But you cannot compare the hole in the middle of a hollow cylinder to a
flat sheet.  A cylinder or pipe or tube is just a flat sheet formed in
an endless circle, so to speak.

Think about when they made wood wagon with the steel rims or wooden
barrels.  They evenly heated steel rings and dropped them over the wheel
or barrel when the ring cooled it shrunk and  held the barrel or wheel
together.



{Quote hidden}

A balloon is totally different physics then  heating metal.  A balloon
is about pressure inside that exerts equal pressure in all directions
Here is an experiment that can be done at home that will prove out the
blacksmiths statement.
Take a metal cookie sheet or a piece of thin metal, put it in the oven
and heat to 400 deg F..
notice the bottom center of the object.

After the object cools  put it on a top burner and heat it. as the
center heats and the outer edges stay cool the hot spot in the middle
will bow up or down to allow for the expanding metal.

>> <>If you are trying to get a bearing into a casting, and you hit the
>> hole with
>> too much heat, the casting will crack. The hole may be red hot, the edge
>> may be ice cold, and it will still crack. Heating the hole stresses the
>> metal.
>
This is why blacksmiths heat the whole part in a forge.

>> <> The stress isn't relieved by material moving into the hole, it
>> propagates outwards. It either expands or cracks. Note that expanding
>> includes buckling. It cracks because the outside can't expand fast
>> enough.
>>
>> If the material wasn't elastic, nor prone to cracking, then the hole
>> would
>> get smaller as there is nowhere else to go. (So cast iron is your best
>> bet.) The universe wasn't built that way.
>
************************************************************

>> <>To get the hole to shrink, the
>> area surrounding the heating zone needs to be non-elastic. If it's metal,
>> then it won't be.
>
***************************************************************

>> <>Read that paragraph again :)
>>
>> I've bashed enough pieces of metal in my life (fitting & turning) and the
>> hole doesn't get smaller. Your blacksmith friend was mistaken.
>
The test above will prove out his statement to be true. As you stated
above.  take a 12" square by 1/4" piece of metal with a 1/2" hole in the
middle do the above test.

> <><> Don't
> forget the numbers we are talking about are very small, I didn't realise
> blacksmiths use micrometers to measure their work!And theirs are better
> than mine!
>
> Tony
>

2006\08\12@083544 by Tony Smith

picon face
{Quote hidden}

Yes, but the drill never gets caught in metal while you're drilling it.
Knock off for lunch with the bit still in the hole though...

For plastic, it's melting, that's why it grabs the drill.  You need to pick
just the right speed, never heard anyone recommend going faster before.
You'll get a hole, but not a very nice one...  

I've got a cordless screwdriver that does 500rpm, perfect for plastic.  You
can get bits designed for plastic, they are ground 'pointier' than normal
twist drills.  Grinding the edges off normal twist drills works too.

Tony

2006\08\12@095454 by Peter Todd

picon face
> Yes, but the drill never gets caught in metal while you're drilling it.
> Knock off for lunch with the bit still in the hole though...
>
> For plastic, it's melting, that's why it grabs the drill.  You need to pick
> just the right speed, never heard anyone recommend going faster before.
> You'll get a hole, but not a very nice one...  

I didn't say go faster, increase the speed, I said to increase the
*feed*, how far the drill is advanced for each revolution. It isn't
something that can be easilly done though, especially with a hand drill.

In any case, these guys made it very clear that this can be caused by
more than the plastic melting. After all, some plastics just don't melt,
they char or even burst into flames first. Try machining celluloid for
instance, once knew a guy who said he turned down a job doing that!

Think of it this way, you locally heat up the lining of a hole, the
material wants to expand, but the rest of the block is still cold, where
does it go? If it's not compressible, it'll expand inwards and catch
your drill.

> I've got a cordless screwdriver that does 500rpm, perfect for plastic.  You
> can get bits designed for plastic, they are ground 'pointier' than normal
> twist drills.  Grinding the edges off normal twist drills works too.

Very common mistake to think one type of drill bit will work on all
materials for sure... My tool catelog collection has drill bits for, in
some cases, even specific types of plastic, nylon, teflon, peek etc.
Hyper-optimised so much it's rediculous for anyone not making thousands
of parts on high-end machines.

--
http://www.petertodd.ca

2006\08\12@102016 by Tony Smith

picon face
> >> <>I'm not sure I made myself clear enough. A gun barrel does not
> >> fulfill my criteria; it is too thin and probably would
> heat up fairly
> >> uniformly. It probably expands (inside and out). They
> probably have
> >> found a way to deal with that
> >
> They did. since the beginning of fire arms.  They stooped
> shooting until the barrel cooled down.
>
> But you cannot compare the hole in the middle of a hollow
> cylinder to a flat sheet.  A cylinder or pipe or tube is just
> a flat sheet formed in an endless circle, so to speak.
>
> Think about when they made wood wagon with the steel rims or
> wooden barrels.  They evenly heated steel rings and dropped
> them over the wheel or barrel when the ring cooled it shrunk
> and  held the barrel or wheel together.


No, the hot barrel causes the shell to fire (or detonate) prematurely (bad
for staff morale), and increases wear.  I don't spend any time firing guns
so I could be wrong.

The wagon wheel example merely proves the case.  I can make the steel rim by
bending a strip of metal into a circle, or I can drill a very big hole in a
metal plate.  What's the difference?  Metal is metal.  Both methods give me
a metal ring.  There's no magic point when the thickness causes the hole to
shrink, unless it's big enough for gravity to play a part.

Consider a square hole.  I can file out the cylinder to get the square hole.
Would it shrink when heated?  Will a flat plate expand when heated?  If I
weld four of them together to match the cylinder, then what?

Tony

2006\08\12@104204 by Tony Smith

picon face
{Quote hidden}

Ah, feed, not RPM, you're right.

Machining plastic sucks anyway.  It either chips, melts or flexes.  I'll add
catch fire for celluloid.  What do you use celluloid for these days anyway?
Aren't table tennis ball celluloid?

But you can't compare plastic to metal.  The hole gets bigger when heated,
unless there's some other force involved, like clamping or stresses with the
metal.

Tony

2006\08\12@105853 by Peter Todd

picon face
On Sun, Aug 13, 2006 at 12:41:50AM +1000, Tony Smith wrote:
> > Very common mistake to think one type of drill bit will work
> > on all materials for sure... My tool catelog collection has
> > drill bits for, in some cases, even specific types of
> > plastic, nylon, teflon, peek etc.
> > Hyper-optimised so much it's rediculous for anyone not making
> > thousands of parts on high-end machines.
>
>
> Ah, feed, not RPM, you're right.
>
> Machining plastic sucks anyway.  It either chips, melts or flexes.  I'll add
> catch fire for celluloid.  What do you use celluloid for these days anyway?
> Aren't table tennis ball celluloid?

They are, only modern use of celluloid I've ever heard of too. The guy
said he was completely mystified as to why they needed the part out of
celluloid, some research project aparently, so who knows?

Can't blame him for turning it down, hell, machining some of the more
flammable metals, like magnesium and titanium, is bad enough as it is.

> But you can't compare plastic to metal.  The hole gets bigger when heated,
> unless there's some other force involved, like clamping or stresses with the
> metal.

But see, the clamping force is the *surrounding* material. If that
material stays cool, it's not expanding along with the hole, so it forms
a perfectly good clamp. It happens with plastic because of the very low
termal conductivity combined with the ability to agressivly drill the
stuff.

--
http://www.petertodd.ca

2006\08\12@131710 by Tony Smith

picon face
{Quote hidden}

It happens with plastic, well, because it's plastic.  Plastic is a different
substance to metal.  Lots of stringy stuff, not nice little atoms.

The circumference of the circle will increase.  The surrounding metal won't
act as a clamp, it will expand, or crack as many car mechanics have found.
The stress goes somewhere, but not into the hole.  By using uneven heating
and odd shapes you could get it to happen.

Arrange coins in a circle.  Replace them with bigger ones.  Hole gets
bigger.

I did find people selling celluloid pens, I wonder where they get their
stock from.  Next door to the bakelite factory?

Tony

2006\08\12@141143 by Gus S Calabrese

face picon face

Arrange coins in a circle.  Replace them with bigger ones.  Hole gets
bigger.

Tony

^for someone who found discussing politics and holes equally tedious,  
you have
certainly written a lot of hole comments to the list.
Gus^


Gus S Calabrese
4337 Raleigh Street
Denver, CO
720 222 1309     303 908 7716 cell
adding "spam2006" bypasses my spam blocker.  Please use in subject  
line or in text
( i am hard to reach by phone )


2006\08\12@143514 by Tony Smith

picon face
> Arrange coins in a circle.  Replace them with bigger ones.  
> Hole gets bigger.
>
> Tony
>
> ^for someone who found discussing politics and holes equally
> tedious, you have certainly written a lot of hole comments to
> the list.
> Gus^


I can show the holes gets bigger.  Can be done in the comfort of your own
kitchen.

Politics?  Nah.  Those people can't take a joke.

How's that 'no government, let's all be free' manifesto going anyway?

Tony

2006\08\12@151725 by Gerhard Fiedler

picon face
Tony Smith wrote:

> Sorry, you weren't the one who mentioned blacksmiths, but the idea of
> someone measuring a lump of red-hot steel with a micrometer amused me.

Well, be that as it may be, you argue as if you had done it.

> The numbers involved are quite small, and you don't need to measure them to
> know the hole gets bigger.  It's called an interference fit.

I know it as press fit. This is not what I'm talking about.

> Say your 20cm lump of steel is a roller of some sort, and needs a shaft put
> thru it.  How do you get a 1cm rod thru the cylinder, and have it as a tight
> fit?
>
> You heat up the cylinder.  

You heat it up as evenly as possible, the whole cylinder. This is /not/
what I'm talking about. I thought I made that clear.

> As I said, small numbers, but enough so you can get the rod into place.
> Once the cylinder cools, they'll lock together.  There's a formula to
> calculate the tightness of the fit.

Of course. But this is not the situation I'm talking about. Try the same
calculation with a steep enough temperature gradient from the hole to the
outer end of the material. Take a material that's thick enough so that it
doesn't bend too easily (rules out the cookie sheet) and that's elastic
enough so that it doesn't crack too easily (rules out the cast block). The
formula you used doesn't apply anymore.

> Another example is engine cylinder liners.  You bore out the engine block,
> heat it up, and drop the liners in.  Hole getting smaller would be a bonus,
> it would reduce piston slap.

I agree. Another example of what I am /not/ talking of. Is it just me, or
do you really not understand that I agree with you on all these examples of
yours?

Gerhard

2006\08\12@155743 by Tony Smith

picon face
{Quote hidden}

I understand what you are saying.

It doesn't matter how you heat up the hole, nor how much mass is surrounding
the hole, it will still expand.  The only caveat is if you heat it enough so
it melts or some other factor is at play.

For interest, I looked up the gun barrel example to see exact why
overheating is a problem.  They all say the hole expands, no matter the
thickness of the barrel.  Hunters prefer lighter barrels because you can
swing them around faster, but heavier barrels perform better.  

Overheating problems include erosion of the barrel, ammo 'cooking off' from
the heat, and more telling, a reduction in range (gas goes past the bullet),
and accuracy as the bullet tumbles after it leaves the barrel.

A gun firing creates the conditions you describe.  If the hole in the barrel
shrank on the first shot, the second shot would jam.  That would make
machines guns a bit impractical.

People fit shafts & bearings every day by just heating up the hole rather
than the entire piece.  How aggressively they doing this depends on the
material.  The hole gets bigger anyway.

Tony

2006\08\12@191352 by Gerhard Fiedler

picon face
Tony Smith wrote:

> I understand what you are saying.

I'm not so sure. What you are saying doesn't match what I'm trying to say.

> It doesn't matter how you heat up the hole, nor how much mass is
> surrounding the hole, it will still expand.  

I disagree. What expands is not the hole (it's presumably air filled), but
the surrounding material. It only expands if it has a place to expand to.

> The only caveat is if you heat it enough so it melts or some other factor
> is at play.

I always assumed staying in the solid phase.

> For interest, I looked up the gun barrel example to see exact why
> overheating is a problem.  They all say the hole expands, no matter the
> thickness of the barrel.  

That's what I would expect. The gun barrel is not what I'm talking about. I
wrote this already. Why don't you just believe it?

A crucial difference is that the outside diameter of the gun barrel
probably will expand, in the same proportion as the inside diameter. If not
through heat, then through elasticity effects.

> People fit shafts & bearings every day by just heating up the hole
> rather than the entire piece.  How aggressively they doing this depends
> on the material.  The hole gets bigger anyway.

Just because there are conditions that make the hole expand is not a good
"proof" that it can't shrink under other conditions -- actually no proof at
all. You haven't even started to try to explain what happens when you start
heating up such a hole in a big block from the inside, second by second.
Where does the material go when the inner diameter increases? The outer
size hasn't yet increased...

Gerhard

2006\08\12@194817 by Tony Smith

picon face
{Quote hidden}

Says who?

There is no difference between a pipe, a rifle barrel, a cannon or your 20cm
cylinder.  There is no magic point when the wall thickess cause the holes to
get smaller.

Metal expand because to heat it, or you stretch it.  In this case (heat
source inside the hole), both happen.

Tony

2006\08\12@200918 by Tony Smith

picon face
> Just because there are conditions that make the hole expand
> is not a good "proof" that it can't shrink under other
> conditions -- actually no proof at all. You haven't even
> started to try to explain what happens when you start heating
> up such a hole in a big block from the inside, second by second.
> Where does the material go when the inner diameter increases?
> The outer size hasn't yet increased...
>
> Gerhard


Try this.

Don't heat up the hole.

If pressure is applied to the hole (e.g. via air pressure, hydraulic, etc)
what happens to the hole?  What happens to the rest of the cylinder?

The cylinder will expand because it's elastic.

If the force results from heat, what's the difference?

Tony

2006\08\13@041210 by Russell McMahon

face
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> Try this.
>
> Don't heat up the hole.
>
> If pressure is applied to the hole (e.g. via air pressure,
> hydraulic, etc)
> what happens to the hole?  What happens to the rest of the cylinder?
>
> The cylinder will expand because it's elastic.
>
> If the force results from heat, what's the difference?

BUT, how does the pressure from heat arrive at the hole?
Take a hole.
Surround it with a cold ring, not necessarily symmetrically.
Remove the cold ring to make a larger hole.
Locate the object relative to the centre of the initial hole (not by
touching but so that this is the origin).

Evenly heat the remaining material.
All agree so far that the larger hole will have grown larger and will
not touch its old circumference at any point.
Reinsert the removed material in its original position.
It will not be subject to thermally induced pressure as no part of the
heated material will touch the reinserted cold material.

HOWEVER.

All too often the difference between theory and practice is greater in
practice than in theory. [[Wish I'd thought that line up first]].

Odds are that uneven heating of a small and somewhat asymmetrical area
in the middle of a piece of unheated material near a hole can cause
stresses which cause forces to be directed in 'interesting'
directions. If an experienced black smith says that a hole to some
degree or other reduces its size in part under certain practical
situations, then he's quite likely to be correct.

Real world materials will make it more likely that hard to explain
things happen. 3 dimensional shapes are liable to complicate things
nicely.

I'd sum things up (but people won't let me :-) ) by saying that for a
uniformly thick body made of the same "normally behaved" material
throughout then uniform heating will always cause a hole to grow in
size but that, in certain real world situations, variations in heating
gradients, materials properties and physical geometry may lead to a
hole reducing in certain dimensions.

No?


       Russell






2006\08\13@082915 by Tony Smith

picon face
{Quote hidden}

Yes, but the situation is clear cut.

I can made a hole in metal get smaller.  Take a square piece of metal.  Cut
out a curved piece from the top.  Now below the curve, cut out a square
hole.

__         __
| \_     _/  |
|   \___/    |
| __________ |
| |        | |
| |        | |
| |________| |
|____________|


Heat up the metal at the bottom of the curve (the thinnest bit).  Hole gets
smaller, everyone happy.  Until the whole piece heats up, of course.

Gerhard is saying the following:

1. A homogemous piece of metal, evenly heated, will expand proportionally,
incuding any voids.
2. A pipe, with uniform walls, will expand when the interior is heated (gun
barrel).
3. A pipe, with a wall thickness greater than $magic, the hole will contract
when the interior is heated.  At some point, the hole will start expanding
again.

#1 & #2 I agree with.
#3 doesn't happen, despite common sense saying otherwise.

In Gerhards case, $magic is < 95mm in steel.  (200mm cylinder, 10mm hole)

The idea behind #3 is the outside diameter of the pipe will remain a
constant size as it hasn't heated up yet.  Metal objects can expand without
being heated.

Tony


2006\08\13@113331 by Gerhard Fiedler

picon face
Tony Smith wrote:

> Gerhard is saying the following:
>
> 1. A homogemous piece of metal, evenly heated, will expand proportionally,
> incuding any voids.
> 2. A pipe, with uniform walls, will expand when the interior is heated (gun
> barrel).
> 3. A pipe, with a wall thickness greater than $magic, the hole will contract
> when the interior is heated.  At some point, the hole will start expanding
> again.

There is nothing magic about it. I don't know whether you are familiar with
situations where multiple effects cause behavior that depends on several
variables, and the behavior of the result WRT one variable changes as you
change the other one. This is something like that; no magic, just more than
one variable involved.

(BTW, IMO most "practice != theory" stories are incorrectly applied theory.
Theory is not that simple, most of the time... People tend to forget that
we work almost always with gross simplifications of known theory. As long
as we stay within the validity assumptions of the simplifications, we're
fine -- but only as long as we do :)


Maybe try this to imagine the effects I'm talking about... take a cylinder,
put a rod in the middle and fill the void with small steel balls. Add some
glue between the balls, which gets soft and less strong as temperature
increases. Now remove the cylinder and the rod. You have a contraption of
glued-together small steel balls. (This is, /very/ approximately, how you
could simulate such a situation.)

If you heat the whole cylinder, everything increases proportionally. No
stress on the glue. That's #1.

Now heat the steel balls from the inside of the hole, so we get to #2 and
#3.

If the wall thickness is small enough (#2), probably the growing balls from
the inside have enough force to just push the wall to the outside, until
they also heat up and everything is back to #1.

If the wall is thicker (#3), the resistance of the wall outside will
initially be larger, and the only direction the balls have to expand to is
towards the inside.

Where #2 starts to become #3 depends on a number of factors, and in our
simplification mostly on the strength of the glue and its softening with
temperature.

(BTW, whether and when the hole starts expanding again in #3 is also not
trivial, I think. Depends on temperature gradient, among other factors. Of
course, if you assume that in the end the whole piece will be at a uniform
temperature, we're back to #1 and it will be bigger. But as long as you are
heating from the middle, there will be a gradient. Whether it is big enough
to keep the hole small depends.)


> #3 doesn't happen, despite common sense saying otherwise.

See, you already said that you didn't try it. You didn't try to simulate
it. You didn't really explain what happens in detail. How can you be so
sure? Extrapolating a steady-state behavior to a situation where you have
gradients (over time or space) is not always guaranteed to work -- and this
is exactly what you are doing.


Check this out if you're interested in some practical experiences:
http://www.anvilfire.com/gurusden/index.htm (scroll pretty much to the
bottom of the Q&A section). For convenience, I copy my question and the
answers here:

---------------------------------------
Hole sizes in heated steel: We're having a discussion about whether holes
always increase in size or whether they in some situations may decrease
when heating steel or iron.

Take for example a 2 cm thick sheet about 30 cm square with a 1 cm diameter
hole in the middle. If you heat the whole piece evenly, the size of the
sheet increases, and the diameter of the hole increases in the same
proportion. So far we are in agreement (and I hope you are, too :).

Disagreement is about what happens when you heat only the area around the
hole. It would have to be a piece of material that doesn't crack and if
possible doesn't bend up (like a thin sheet would), and you'd heat the area
around the hole only, so that the outside stays cool (at least cooler than
the area around the hole). I'd love to make this experiment, but I don't
have any of the equipment necessary (and I'm not a blacksmith :).

Can you guys give us a definitve answer whether it is possible that a hole
in metal decreases in diameter when heating it up, given the right
circumstances?

Thanks a lot.
  Gerhard - Saturday, 08/12/06 14:35:46 EDT
---------------------------------------
Gerhard: In a material like steel that is a poor conducter of heat the hole
should close up when only the area around it is heated, as the rest of the
large plate will confine the expanding material, and since the hole is the
only place for it to go, it should become a little smaller. This is theory,
as I have not done the test, but if You want to try, I suggest drilling a
hole that will allow a ball bearing ball to drop through with verry little
clearance, then heat the area surrounding the hole to red hot with an Oxy
Acetylene torch. See if the ball still drops through.
  Dave Boyer - Saturday, 08/12/06 22:15:12 EDT
---------------------------------------
I agree with Dave: for a small hole when you heat just the surrounding area
the metal swells into the hole. A trick like this can be used to shrink out
a bulge in a plate. But if you heat the whole plate at once its going to
depend on the balance of the forces from all the plate. The size of the
hole, the shape of the plate and the position of the hole in the plate all
come into play. In some cases the hole might distort to oval growing on one
axis and shrinking on another or even become D shaped if its near an edge.
Predicting this from first principles is big math. I imagine that someone
who has experience with heating pieces of a particular size and shape could
win some money off the guys who think they can predict what will happen by
standing around and arguing about it :)
  adam - Saturday, 08/12/06 23:36:17 EDT
---------------------------------------
Hole in a plate: The hole will shrink, when the area around it is heated
and the plate as a whole is kept cooler. I actually tested this once, using
the ball bearing and close fitting hole method.

The same principle is used to shrink a spot in sheet metal.
  - John Odom - Sunday, 08/13/06 10:18:03 EDT
---------------------------------------


There's no proof in those messages, but when a bunch of blacksmiths in a
forum where they discuss issues seriously say that they agree with me and
one says that he already tried it and it works as I figure it would... you
tell me where my odds are :)


> In Gerhards case, $magic is < 95mm in steel.  (200mm cylinder, 10mm hole)

Nope. That was just an example number. No "$magic" here, even though there
is a number for every situation (but it probably depends on a lot of detail
factors). I didn't make any experiments or calculations. I just think that
if the wall thickness is /very/ thick (that is, thick enough, at a point
where thicker doesn't make much difference anymore), the shrinking effect
can happen. As the thickness decreases, the relationship between the
different forces involved (that work against each other; this is a material
stress situation) will change. If the thickness is small enough, the hole
will eventually increase. So there is a turning point in the middle, but no
"$magic": this all can be explained and probably to quite some degree
calculated. You probably can figure out how to heat up a sheet with a hole
so that the hole size does not change at all: just heat up a big enough
area with the right gradient, so that the contracting forces and the
expanding forces cancel each other out.

Anyway, I'm just applying physical principles and don't have any real
experience with that situation. But it seems that a number of people that
do have that experience agree with my theory.

Gerhard

2006\08\13@124219 by Dave Tweed

face
flavicon
face
Gerhard Fiedler <.....listsKILLspamspam@spam@connectionbrazil.com> wrote:
> Tony Smith wrote:
> > Gerhard is saying the following:
> > 1. A homogemous piece of metal, evenly heated, will expand
> >    proportionally, incuding any voids.
> > 2. A pipe, with uniform walls, will expand when the interior is
> >    heated (gun barrel).
> > 3. A pipe, with a wall thickness greater than $magic, the hole will
> >    contract when the interior is heated.  At some point, the hole
> >    will start expanding again.
>
> There is nothing magic about it. I don't know whether you are familiar
> with situations where multiple effects cause behavior that depends on
> several variables, and the behavior of the result WRT one variable
> changes as you change the other one. This is something like that; no
> magic, just more than one variable involved.

But what you're specifically proposing is that the elastic modulus of
the metal is a strong function of its temperature. I don't believe that
this is the case, at least not until you get close to its melting point.
I tried a quick Google on this topic, but couldn't come up with anything
immediately relevant. (Most discussion seems to be about polymers.)

As long as the modulus is reasonably constant, the strain will be
distributed monotonically throughout the metal, and all of it will
be in the direction of expansion, not contraction.

-- Dave Tweed

2006\08\13@140608 by Gerhard Fiedler

picon face
Dave Tweed wrote:

> But what you're specifically proposing is that the elastic modulus of
> the metal is a strong function of its temperature. I don't believe that
> this is the case, at least not until you get close to its melting point.

I don't know that either, but I didn't (at least didn't mean to) propose
that.

> As long as the modulus is reasonably constant, the strain will be
> distributed monotonically throughout the metal, and all of it will
> be in the direction of expansion, not contraction.

Expansion of each particle, yes. In principle, they don't seem to have a
preferred direction for expansion; I think they expand in all directions.
In what direction it will move because of the expansion is dependent on the
elasticity forces in the material and on the expansion (and resulting
elasticity forces) of the other particles.

 111112
 111112
 111112

Assume that each number is a particle. Assume that they all are bound
together by some elasticity forces. Assume that particles 2 will increase
in size. Where will they tend to expand into? (The "diagram" is of course
simplified because 2D, but you probably get the idea.)

And I have the blacksmiths on my side :)

Gerhard

2006\08\14@075603 by Dave Tweed

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Gerhard Fiedler <listsspamKILLspamconnectionbrazil.com> wrote:
> Dave Tweed wrote:
>
> > But what you're specifically proposing is that the elastic modulus of
> > the metal is a strong function of its temperature. I don't believe that
> > this is the case, at least not until you get close to its melting point.
>
> I don't know that either, but I didn't (at least didn't mean to) propose
> that.
>
> > As long as the modulus is reasonably constant, the strain will be
> > distributed monotonically throughout the metal, and all of it will
> > be in the direction of expansion, not contraction.
>
> Expansion of each particle, yes.

Forget the particles. If you heat a hole, the material around the
circumference is going to expand, both circumferentially and radially.
Cooler material surrounding this will be subject to the radial expansion,
and since it is also elastic, will end up being compressed radially and
stretched circumferentially, to a degree that is related to the temperature
gradient.

The only way the hole can get smaller is if the material gets considerably
softer at high temperatures, or if something nonuniform occurs -- the hole
wasn't round to begin with, the edge buckles, or there's an anisotropic
component to the expansion. Any of these might apply to your blacksmiths.

-- Dave Tweed

2006\08\14@102750 by Gerhard Fiedler

picon face
Dave Tweed wrote:

> Cooler material surrounding this will be subject to the radial expansion,
> and since it is also elastic, will end up being compressed radially and
> stretched circumferentially, to a degree that is related to the
> temperature gradient.

Hm... I see. This brought me on a track. In any case, this assumes that we
stay within elastic limits. Is this plausible?

> The only way the hole can get smaller is if the material gets
> considerably softer at high temperatures,

Check out http://midas.npl.co.uk/midas/content/mn049.html

Young's modulus of steel seems to decrease linearly up to ~600°C (~80% of
the modulus at 0°C), and then decrease sharply.

> or if something nonuniform occurs -- the hole wasn't round to begin with,
> the edge buckles, or there's an anisotropic component to the expansion.
> Any of these might apply to your blacksmiths.

Right. If any of these apply, don't you say the hole could have become
smaller?

---------------------

Check out this:
http://physics.uwstout.edu/Statstr/statics/Stress/strs38.htm, at the
bottom. This is approximately what I was talking about (constrained thermal
expansion).

They simplify the situation by saying let it first expand thermally, then
apply stress to bring it back into its original form. Applied to something
similar to my situation, we would have a smaller cylinder with the hole
that gets heated and a bigger constraining cylinder around it that remains
cold. (Simplified thought experiment; in reality there would a temperature
gradient, a stress gradient, heat conduction etc., probably with
qualitatively similar results.) So the smaller, heated cylinder expands in
all directions, its outer and inner diameters and its wall thickness expand
proportionally. Then the stress gets applied and the outer diameter gets
reduced to the size it was before.

What happens to the inner diameter is determined by the elastic or plastic
forces. Maybe it just shrinks back to its original size (probably the case
if we stay within the limits of Hooke's law, but I'm not sure this would be
guaranteed). Or maybe not. But it seems we have now a different angle to
look at it: what happens to the size of a hole in a cylinder when you
compress the outer diameter? Does it shrink proportionally? If so, what are
the limits of proportionality (there definitely are some)?

This approach doesn't take into account that the elastic modulus decreases
with increasing temperature.

Gerhard

2006\08\14@105241 by M. Adam Davis

face picon face
In explaining this whole thread to an officemate ("so now here's a
topic to get a bunch of engineer's in a room arguing about") I came up
with these thought models:

For this test case I assumed the "problem" was:
Given a sheet of metal with a hole in it, where the hole is subject to
local heating that is significantly greater (exponentially) than the
temperature of the remaining sheet, will the hole become smaller?  If
not then either the material will expand sideways, or the material
will expand outward and stress the rest of the sheet, or both.  (
alternately the material is pushed into another dimension :-)

So the two test cases are as follows:

Assume an inelastic material with a hole in it.  Place a ceramic liner
inside the hole.  place a metal washer/donut/etc into the ceramic
liner.

Now you have a hole in a chunk of metal inside a hole that's insulated
from the external material which will no expand.

I beleive this is the point that the "hole gets smaller" crowd is
trying to make:  That under certain circumstances a normal sheet of
metal will behave as this model.  In this case if you heat the metal,
then depending on it's thickness it may buckle inside the hole, it may
expand outward (along the axis of the hole).  If the buckling that
occurs is small enough and even enough then it may appear that the
hole has become smaller, but I doubt it will simply be a smaller
version of the original hole - it will have changed shape.
Regardless, a shaft that used to fit will likely no longer fit.

The "hole gets bigger" crowd is simply saying that it would be a very
contrived and improbable situation for a regular sheet of metal to get
in.  It simply would not happen under normal circumstances.

I think of it as an archway in a castle.  The arch may become hot, and
the archway will become larger regardless of the heating of the
adjacent walls.  Either the arch will twist as it expands if it no
longer fits in the wall, or the wall or the arch will fail.  Either
way, the archway does not get smaller unless some sort of structural
failure occurs.

But perhaps I'm taking too simplistic of a view...

Please note that when I say the localized heating is greater, this is
not a linear function.  the heat can drop of very rapidly from the
hole to the edge and still be "even" in the sense that the entire
metal structure can easily withstand the expansion created near the
hole without forcing the hole to become smaller.  The temperature
difference must be huge to create the buckling of the metal structure
near the hole, which I view as a difficult situation to achive, if not
impossible, before reaching a liquid state at the hole.  Maybe if one
circulated liquid nitrogen around the entire sheet of metal about one
inch away from the hole...

-Adam

2006\08\14@105434 by M. Adam Davis

face picon face
In explaining this whole thread to an officemate ("so now here's a
topic to get a bunch of engineer's in a room arguing about") I came up
with these thought models:

For this test case I assumed the "problem" was:
Given a sheet of metal with a hole in it, where the hole is subject to
local heating that is significantly greater (exponentially) than the
temperature of the remaining sheet, will the hole become smaller?  If
not then either the material will expand sideways, or the material
will expand outward and stress the rest of the sheet, or both.  (
alternately the material is pushed into another dimension :-)

So the two test cases are as follows:

Assume an inelastic material with a hole in it.  Place a ceramic liner
inside the hole.  place a metal washer/donut/etc into the ceramic
liner.

Now you have a hole in a chunk of metal inside a hole that's insulated
from the external material which will no expand.

I beleive this is the point that the "hole gets smaller" crowd is
trying to make:  That under certain circumstances a normal sheet of
metal will behave as this model.  In this case if you heat the metal,
then depending on it's thickness it may buckle inside the hole, it may
expand outward (along the axis of the hole).  If the buckling that
occurs is small enough and even enough then it may appear that the
hole has become smaller, but I doubt it will simply be a smaller
version of the original hole - it will have changed shape.
Regardless, a shaft that used to fit will likely no longer fit.

The "hole gets bigger" crowd is simply saying that it would be a very
contrived and improbable situation for a regular sheet of metal to get
in.  It simply would not happen under normal circumstances.

I think of it as an archway in a castle.  The arch may become hot, and
the archway will become larger regardless of the heating of the
adjacent walls.  Either the arch will twist as it expands if it no
longer fits in the wall, or the wall or the arch will fail.  Either
way, the archway does not get smaller unless some sort of structural
failure occurs.

But perhaps I'm taking too simplistic of a view...

Please note that when I say the localized heating is greater, this is
not a linear function.  the heat can drop of very rapidly from the
hole to the edge and still be "even" in the sense that the entire
metal structure can easily withstand the expansion created near the
hole without forcing the hole to become smaller.  The temperature
difference must be huge to create the buckling of the metal structure
near the hole, which I view as a difficult situation to achive, if not
impossible, before reaching a liquid state at the hole.  Maybe if one
circulated liquid nitrogen around the entire sheet of metal about one
inch away from the hole...

-Adam

2006\08\14@113704 by Dave Tweed

face
flavicon
face
Gerhard Fiedler <.....listsKILLspamspam.....connectionbrazil.com> wrote:
> Dave Tweed wrote:
> > The only way the hole can get smaller is if the material gets
> > considerably softer at high temperatures,
>
> Check out midas.npl.co.uk/midas/content/mn049.html
>
> Young's modulus of steel seems to decrease linearly up to ~600°C (~80% of
> the modulus at 0°C), and then decrease sharply.
>
> > or if something nonuniform occurs -- the hole wasn't round to begin with,
> > the edge buckles, or there's an anisotropic component to the expansion.
> > Any of these might apply to your blacksmiths.
>
> Right. If any of these apply, don't you say the hole could have become
> smaller?

Please forgive me for jumping into this thread late. I assumed that you
were talking more or less theoretically about round holes in uniform,
isotropic materials, and applying heat in the general region of the hole.

Sure, if any of those things apply, some dimensions of the hole will get
smaller. What's your point?

> They simplify the situation by saying let it first expand thermally,
> then apply stress to bring it back into its original form.

Ah, yes, hypothesizing a container that has no elasticity at all kind of
changes the universe of discussion. In that case, shrinkage of the hole
requires a reduction of modulus, as I said before; otherwise, the hole is
simply returned to (or maintained at) its original size.

I'll jump back out of this thread now. Please carry on.

-- Dave Tweed

2006\08\14@120413 by Tony Smith

picon face
{Quote hidden}

> -

2006\08\14@120527 by Tony Smith

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{Quote hidden}

Everyone is happy with the conditions, just not the outcome.  

We've settled on steel, no weird stuff like case hardening, in the form of a
cylinder.  The cylinder is 200mm in diameter, and 200mm high.  The hole
bored thru the middle is 10mm.  Being thick it won't warp like sheet metal
will.  (200mm = ~8", 10mm = 3/8")

The hole is heated aggressively, but not to the point where the steel loses
strength, melts, etc, so 500 degrees celcius is probably enough.  A metre of
steel will lengthen by 6mm at 500C, so the hole diameter will only change by
0.06mm.  An eyeball test isn't going to cut it :)

Everone agreed that if heated evenly, the hole gets bigger.  The question is
if the hole is heated rapidly, can the tensile strength of the cold steel
(the outside of the cylinder) overcome the pressure caused by the hole
trying to expand.  Does the hole say - well, got to go somewhere, might just
fill up the hole.  Or does the cold steel stretch?

There's nothing except the metal and heat.  No clamps etc.  No cast iron
because it would crack and so on.

The cylinder seems to have become only 20mm thick lately, but that should
still be ok.

Tony

2006\08\14@120545 by Tony Smith

picon face
{Quote hidden}

Everyone is happy with the conditions, just not the outcome.  

We've settled on steel, no weird stuff like case hardening, in the form of a
cylinder.  The cylinder is 200mm in diameter, and 200mm high.  The hole
bored thru the middle is 10mm.  Being thick it won't warp like sheet metal
will.  (200mm = ~8", 10mm = 3/8")

The hole is heated aggressively, but not to the point where the steel loses
strength, melts, etc, so 500 degrees celcius is probably enough.  A metre of
steel will lengthen by 6mm at 500C, so the hole diameter will only change by
0.06mm.  An eyeball test isn't going to cut it :)

Everone agreed that if heated evenly, the hole gets bigger.  The question is
if the hole is heated rapidly, can the tensile strength of the cold steel
(the outside of the cylinder) overcome the pressure caused by the hole
trying to expand.  Does the hole say - well, got to go somewhere, might just
fill up the hole.  Or does the cold steel stretch?

There's nothing except the metal and heat.  No clamps etc.  No cast iron
because it would crack and so on.

The cylinder seems to have become only 20mm thick lately, but that should
still be ok.

Tony

2006\08\14@120547 by Tony Smith

picon face
{Quote hidden}

Everyone is happy with the conditions, just not the outcome.  

We've settled on steel, no weird stuff like case hardening, in the form of a
cylinder.  The cylinder is 200mm in diameter, and 200mm high.  The hole
bored thru the middle is 10mm.  Being thick it won't warp like sheet metal
will.  (200mm = ~8", 10mm = 3/8")

The hole is heated aggressively, but not to the point where the steel loses
strength, melts, etc, so 500 degrees celcius is probably enough.  A metre of
steel will lengthen by 6mm at 500C, so the hole diameter will only change by
0.06mm.  An eyeball test isn't going to cut it :)

Everone agreed that if heated evenly, the hole gets bigger.  The question is
if the hole is heated rapidly, can the tensile strength of the cold steel
(the outside of the cylinder) overcome the pressure caused by the hole
trying to expand.  Does the hole say - well, got to go somewhere, might just
fill up the hole.  Or does the cold steel stretch?

There's nothing except the metal and heat.  No clamps etc.  No cast iron
because it would crack and so on.

The cylinder seems to have become only 20mm thick lately, but that should
still be ok.

Tony

2006\08\14@155355 by Gerhard Fiedler

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Dave Tweed wrote:

> Please forgive me for jumping into this thread late. I assumed that you
> were talking more or less theoretically about round holes in uniform,
> isotropic materials, and applying heat in the general region of the hole.

We started out with the quiz question "what happens to a hole in metal when
you heat the metal?" and there was the obvious answer "it grows, of
course".

I brought up the idea that the "it grows, of course" requires a number of
assumptions to be fulfilled, and claimed (at this point pretty much
shooting almost in the dark) that there are situations where the hole may
become smaller.

At that point this wasn't yet theoretical, and the material was assumed to
be steel or iron or similar. As we proceeded to discuss, we elaborated some
conditions and for lack of proper tools for executing enlightening
experiments, the discussion became more and more theoretical.

> Sure, if any of those things apply, some dimensions of the hole will get
> smaller. What's your point?

More or less this is my point. Maybe this is obvious to you, but it isn't
to me, and it doesn't seem to be to many others. So far I seem to have been
unable to convince Tony Smith of that possibility, but this may well be
because I don't really understand that much about the details of the
effects involved.


> Ah, yes, hypothesizing a container that has no elasticity at all kind of
> changes the universe of discussion.

I didn't mean to change the universe with that; it's just one aid for
explaining things. I think it doesn't have to change things. If you
simplify the temperature gradient to a cold cylinder outside and a hot
cylinder inside, there will be stress on both (on the outside cylinder
towards increasing size, on the inside cylinder towards decreasing size)
and the inside cylinder will likely grow less than it would if both were
heated uniformly. What the maximum of that reduction is I can't tell... I'm
sure I could calculate it if I spent the time.

> In that case, shrinkage of the hole requires a reduction of modulus, as I
> said before;

Which happens (see the posted link). One question is whether the reduction
is enough.

> I'll jump back out of this thread now. Please carry on.

No matter whether you jump out or stay in, thanks a lot for your
contribution. It inspired me to research which cleared a few things up in
my head.

Gerhard

2006\08\14@163157 by Gerhard Fiedler

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Tony Smith wrote:

> Your ball bearing & glue model sounds good.  I'd replace with glue with
> rubber though.  Anyway, under heat, the atoms are behaving like little
> children holding onto their parents hands - bouncing all over the place, but
> not getting very far.  They're very determined to go somewhere.

Rubber wouldn't show any plastic deformation, and elastic deformation is
already part of the metal balls. There's a point where metal undergoes
permanent deformation. I'm not sure we would get there; considering the
small distances, possibly not.

> Still, the ball bearing test is a good one.  Downside is it sounds easy to
> do, but isn't.  

I agree with that, and I think all tests to confirm/deny any of the
affirmations here are difficult. There are so many other possible effects,
and the changes are really small.

I'm still not really convinced there isn't a situation where it won't grow.
There may not be a situation where it shrinks. But I think it is
non-trivial to find that out.

Gerhard

2006\08\16@030128 by Tony Smith

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{Quote hidden}

I remain unconvinced  :)

The only time I can see the hole getting smaller is if you heat it to around
550C or more.  At this point it loses it's elasticity, and is very
malleable.  It's also lost a lot of its strength (half?  40%?) and goes
downhill rapidly after that.

You could argue that the metal is no longer homogenous, as a phase change
has occurred in the area around the hole.  This why I said heat the hole to
500C.  (Strength starts dropping at 220C.)  

I used to know these numbers, but at least I kinda remember the tempering
colours.  (There's always Google.  These are close enough.)

The steel will deform, and thus the hole may get smaller.  But it won't
shrink evenly, it'll collapse, and remain that way once the heat is removed.
Sheet metal warps, meaning the hole circumference has gotten bigger.

The only material I can think of that behaves the way you describe (shrinks
then expands when heated) is water.  Ice heated up shrinks, and then starts
expanding after 4C degrees.

Making the steel plate with a hole in it to drop a ball bearing thru is
possible, rapid heating of the hole make be troublesome.  Lead or tin (or
solder) might prove to be good test subject, as it has a high COTE (30-40 vs
steels 10-12), but offset by a fairly low melting point.  I can see a lot of
solder sticks from here...

The main reason I remain unconvinced is no-one (save that guy on the
blacksmiths site, but he said a couple of other odd things too) has actually
seen it happen.  There are lots of bits of steel with holes drilled in them,
and surely someone would have gotten a shaft wedged (temporarily) in one by
now, or had a bush/bearing get squished.  Scientists also love the weird
exceptions to rules, and they seem to be fairly quiet.

Tony

2006\08\16@070105 by Gerhard Fiedler

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Tony Smith wrote:

> The only time I can see the hole getting smaller is if you heat it to
> around 550C or more.  At this point it loses it's elasticity, and is
> very malleable.  It's also lost a lot of its strength (half?  40%?) and
> goes downhill rapidly after that.

This may be what the blacksmith is talking about -- that's what they do :)
This is also what I have imagined: the steel around the hole starting to
get red.

> You could argue that the metal is no longer homogenous, as a phase change
> has occurred in the area around the hole.  This why I said heat the hole
> to 500C.  (Strength starts dropping at 220C.)

Hm... steel isn't really a "simple" material :)

Look at this:
http://www.mace.manchester.ac.uk/project/research/structures/strucfire/materialInFire/Steel/HotRolledCarbonSteel/thermalProperties.htm

All throughout this discussion, we assumed a rather constant thermal
expansion coefficient (I think you call it COTE). But according to this,
around room temperature it seems to be close to 0, and grows with
temperature.

There is a phase change around 700°C, but even before and after that it
doesn't behave that linearly as we assumed. ("Linearly" would be a constant
COTE.)

> The steel will deform, and thus the hole may get smaller.  But it won't
> shrink evenly, it'll collapse, and remain that way once the heat is
> removed.

This is what I've been trying to say. The shrinking of the hole -- if it
happens -- would probably be a plastic effect, not an elastic effect.

Gerhard

2006\08\16@122135 by Tony Smith

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{Quote hidden}

The hole doesn't shrink in that case.  It becomes a funny shape, just how
funny is a bit unpredictable.  Plastic deformation has already been
mentioned, and dismissed with "so what?".  You could heat it up even more
until it melts and closes up the hole.  Or just thump it with a hammer.

I remain unconvinced that heating up the hole causes it to contract  :)

Tony

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