Tony Smith wrote:

>>    
>>
>>>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
>
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.



>> <>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.
>
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
>
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