I was playing with putting a cross inside a small ring yesterday and found
out that steel expands and shrinks when heated quite a bit more than I
thought it would. Made the cross fit and hold by heating the ring and
making it shrink around the cross.
Then today, I was drawing out the ends of a rod to equal lengths by marking
the center and then measuring how far I drew each taper out so I was sure
they were the same. Again, I was surprised to see how different the
measure was between when it was hot and cold.
So I just went out to the shop and did a little more careful testing with
digital calipers and a 1/2" square rod of mild steel.
What I found was that a 10" rod will lengthen about 1/8" when heated to a
bright orange (around 1000C or 1800F). That's a simple enough number to
remember and adjust for.
That value matches the documented thermal expansion numbers of steel I
found on the net.
It never occurred to me that it would stretch that much. I assumed the
expansion was more like a few thousands - something that would be hard to
measure with a ruler and below the level of accuracy a typical blacksmith
project would need to care about (aka one tap with a hammer to adjust).
But it's a lot more than that becuase we are working with such large
temperature changes. At these amounts of change, trying to make hand
forged parts a specific length (like fitting a hot scroll inside a cold
frame) becomes a fun challenge if you don't keep in mind the difference in
length between hot and cold. Make the inside part snug when it's hot, and
it will be very loose once it cools if you don't adjust for the change
12 years ago