Posted by Paul (in NY) on Friday, January 31, 2003 at 10:41AM :
In Reply to: sheet metal shrinking posted by Greg on Friday, January 31, 2003 at 0:16AM :
Back in December when I did the first panel on my Carryall, I won a oil can on the left side. I did not know what caused it, nor how to fix it. I asked a lot of people, professional body people. I got a lot of conflicting answers, 180 degrees from each other. I finally went up to visit a Custom Street Rod Builder near the Canadian Border. I got there at the right time, he was welding in body panels on a new street rod, and had a small oil can to deal with. He took me in like family and explained what causes them and how to get rid of them.
He first gave this little demo. He had me hold the edge of a 8.5 x 11 inch piece of paper with my thumb and finger of both hands spaced about 4 inches apart. He did the same on the other long edge. We both pulled, stretched the paper and Walla we made a oil can in the paper. He then tool a steel frame he made from 3 inch channel iron, about 12x12 inches. He built this
frame many years ago when he was teaching body work. Think of it as a picture frame. He then laid a piece of 18ga sheet metal on the frame and put a lot of BIG tacks around the edge. Yup, he made the sheet metal, oil
can. He explained that the heat from the spot welding makes the metal shrink when it cools. Then when you grind the weld, you introduce more heat and more shrinkage. Then we went out to the street rod body he was building with the oil can. He has a large Air Powered plenishing air tool. Looks like a big C clamp, (think big). On one end is a Replacable dolly, the other end you can put different shaped body hammer ends. He went to work plenishing the welds (Banging them flat). He went back and forth for a few minutes. He explained that this stretches the weld, and the oil can will go away. I watched him do this, and it worked, the oil can went away. He told me to go
home, take a handheld dolly and a body hammer and go back and forth on the top and bottom of panel welds. He cautioned not to plenish to much or the weld will thin and crack. Its a art. I went home with this technique, AND IT WORKED !!! the oil can went away.
Now he said to prevent this or minimize it. Spot weld with small spots, spot weld far apart, LET IT COOL to hand touch. Plenish them, then go back and add more spot welds, cool and plenish. This process is repeated till the panel is welded in place with no oil can. He also cautioned to keep the weld beads small, less grinding, means less heat and less shrinkage = less oil can. He said NEVER NEVER try to heat a panel, cool it to get rid of a oil can. I spent about 5 hours with him, and he would not even let me buy him lunch. HE was just a nice helpful sheetmetal artist. I am new at working with
sheetmetal as such. But I used his instruction when doing the right side panel
oon the Carryall and other sheetmetal and had great results. I am sold on this
technique, I saw it demonstrated, and then I did it, and it worked both times. Just my
.02 cents from a new guy learning sheetmetal work.
Paul
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