Quote:
Originally Posted by dataway
...I should probably worry more about using them and less about how they work....
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Thanks for sharing your progress, it is most impressive and I am always amazed at your creativity and homemade tools.
As to knowing how stuff works, you seem to want that so it must be important!
The disc does not have to heat the metal super hot to "shrink" it. It works by generating localized thermal expansion while constraining the metal. This expansion causes stress that makes the metal yield. The disc only has to heat the metal enough to cause it to expand and generate high internal stress.
Technically, you cannot shrink metal. You can deform it, but the volume is always maintained. Think of a small steel cube exactly the same dimension in X, Y, and Z. Compress it hard enough in one direction to deform (yield) and it will permanently expand in the other two directions.
Sheet metal shrinking, whether by disc or by torch is all about manipulating the internal stress in the sheet.
Here's my take on how the shrinking disc works:
1. Frictional heat causes a localized area in the metal sheet to expand.
2. The spot cannot expand in the plane of the sheet because it is constrained by the surrounding cold metal. This constraint causes compressive internal stress acting on the hot spot. This in turn causes the hot region to try and buckle.. it cannot move in the plane of the sheet so it tries to bulge out of plane.
3. The pressure you apply through the disc does not allow the spot to bulge. It is now constrained in X, Y, and Z.
4. With no-where to go, the internal stress builds up high enough to exceed the yield stress of the metal within the hot spot. It doesn't have to recrystallize nor get super hot. This is low carbon steel and has a low yield stress. You can yield it easily, so can thermal expansion.
5. The deformation relieves the internal stress. The sheet will actually get a little thicker where the compressive stress exceeds the yield stress.
6. When the spot cools down, thermal expansion now puts that same spot under tension. The surrounding cold metal now "pulls" on the spot you heated with the disc.
7. This internal tensile stress pulls the bulged area which makes it flatten out.
Hope this makes sense..
Mike
