[dataway]

Further inspired by you guys I did a bit more work. Had some 3/16" x 3/4" tool steel bar lying around so I built an extended jimmy bar for getting up in the top of the wheel well from the trunk and quarter window holes. Was able to improve it quite a bit. I ground the edges of the end kind of spoon'ish, slid in there and used vise grips to twist it forcing the upper edge against the inner wheel house and the lower edge pushing out on the body. Was able to get the body proud of the well lip some, but I am left with some highs that I can't tap down to much without undoing my work from the inside I don't think.

Hoping the shrinking disc is cut out for this sort of thing.

A pic of the overall appearance now, and one of the rear corner I cut off and replaced.

In the first photo you can just see some of the original Verdora green hiding under the undercoat that is coming off.

[MUSLCAH]

Originally Posted by jww

Good filler, sand with half round DuraBloc with 80 grit. Repeat that a couple times till it “feels” right then prime /sand 2k primer a couple times till it “feels” right then prime one last time with 2k and move on or you will stress over this till summer is gone.
It’s a car, not the Mona Lisa

OCD at its finest…..

[Shiny]

Originally Posted by dataway

...I should probably worry more about using them and less about how they work....

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

[dataway]

Makes perfect sense, and explains why the metal doesn't have to get so hot.

In Materials class in Navy Nuke school we learned a lot about the crystalline structure of metal and often discussed how heat (like red hot) relaxed internal stresses and allowed the crystals to pack together more tightly, reducing dimensions compared to "as cast" or "as forged" dimensions. I assumed that was the process taking place, but I see it's not the only way to shrink a cat

Disc is using a totally different process ... using the "coefficient of thermal expansion" to raise the high to an exaggerated point where it easier to knock down or move, then as it cools and returns to the prevision linear dimension it also draws down the high.

[Shiny]

In general, there really isn't much true volumetric shrinking resulting from stress relief. But relieving internal stress can easily cause significant dimensional changes. The residual stress from a casting process can be very high and non-uniform. Think how far you can deflect a sheet of steel yet have it spring back without taking a permanent set. This gives you an idea of how much deformation can occur by relieving residual stress. Residual stress is all balanced by "elastic" or recoverable deformation within the part.

The crystalline microstructure does not really "pack closer", although a change in a crystalline phase can cause some volumetric change. Recrystallization is an extreme way to relieve stress and is not typical. It's more common to just use heat to accelerate "creep" or relaxation by dislocation movement within the microstructure.

For "shrinking", a torch will obviously heat the sheet metal much hotter than a disc. The mechanism is still the same - thermal expansion causes the heated region to deform because it is constrained by the surrounding cooler metal that does not expand. The "shrinking" still happens when the sheet cools and the thermal expansion relaxes and creates tensile stress that flattens the sheet.

The heat of the torch is high enough to cause a significant reduction in yield stress which increases the effect because the deformation is more pronounced.

[dataway]

I've always wondered about the mechanism at work when they torch arc an I=beam. Along what will end up the inside radius they heat triangular sections of the flanges, with the base of the triangle towards the edge of the flange ... I always thought it shrunk the metal enough at the triangle base to draw the beam into an arc. But, just researched it and it's exactly as you say .... the heated triangle expands but is constrained by the surrounding cool metal which forces it into a thicker cross section then when it cools it shrinks and draws the surrounding metal towards it, creating an arc along the beam. Volume of the heated triangle is the same but the thicker cross section changes the dimensions.

Very interesting, and something they didn't cover in school as we were more concerned with fatigue failures from repeated heating and cooling in power plants. So we learned mostly about expansion rates and their long term effects on welded joints and pipe fittings.

[Shiny]

Cool! I knew nothing about arcing I-beams but the mechanism you describe sounds identical.

As to fatigue, I never worked a lot with "high-cycle" fatigue risks/issues like you studied but spent years managing risk for solder-joint fatigue caused by heating and cooling electronic assemblies. This is usually called "low-cycle" fatigue. The cyclic stress in high-cycle fatigue stays below the yield stress of the metal. In low cycle fatigue, the stress actually causes the metal to yield. A combination of overstress and creep are involved. Bending a coat-hanger until it breaks is a good example of low-cycle fatigue. An axle that fails after a million revolutions is a good example of high-cycle fatigue.