The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post

MP&C · 03-31-2021, 10:27 PM

Near impossible to weld without warpage unless you can planish (stretch) the weld afterward in a low crown area like that. The weld tacks will shrink when they cool down, and that shrink pulls from all directions. So as the weld shrinks, about 6" or so out from the weld area, the panel remains cool and unaffected by the heat. The area between these two areas, caught between two differing forces (one shrinking, one staying the same) results in a buckling, a sinusoidal wave of distortion. To remove the wave adjacent to the weld, you stretch the weld to eliminate the pull. If ANYONE suggests using a shrinking disc, it is the incorrect process. You don't fix a shrink with more shrink. You may remove some of the wave but you will only be shrinking the center even more, resulting in a low area for more oil canning. Fix the cause, not the result.

When you cut around the perimeter of the welded area, the roof returned to normal as you "disconnected" the pull of the welds...

More helpful hints:

A 90* tight corner concentrates the shrinking effects on the inside of that corner for a noticeable pucker. A radius in corners helps to balance the shrinking effects on either side of the weld for an easier task of planishing out the shrinking deformity. For that small circle I would have used a round patch to fill the hole, not a square one. (food for thought)

The tighter the patch (as close to zero gap as one can get) the better. A gap offers no resistance to the panel pulling closer together when the weld cools and shrinks, so any gaps will result in the outlying area pulling in more than with tight joints.. Where the crown of any panel is there to provide support and stability for the panel across its entire length, any disruption of that support risks oil canning. So for a low crown area like a roof skin, this means any additional shrinking/pulling in the center from gaps results in an even larger low area in the middle of the roof as that arc (cross section depiction) turns into more of a straight line. Which results in loss of that structural support that the crown gives, and an even greater likelihood of oil cans..

03-31-2021, 10:27 PM	#4
MP&C Registered User Join Date: Jan 2013 Location: Leonardtown, MD Posts: 1,633	Re: Roof patch causing warp / oil canning Near impossible to weld without warpage unless you can planish (stretch) the weld afterward in a low crown area like that. The weld tacks will shrink when they cool down, and that shrink pulls from all directions. So as the weld shrinks, about 6" or so out from the weld area, the panel remains cool and unaffected by the heat. The area between these two areas, caught between two differing forces (one shrinking, one staying the same) results in a buckling, a sinusoidal wave of distortion. To remove the wave adjacent to the weld, you stretch the weld to eliminate the pull. If ANYONE suggests using a shrinking disc, it is the incorrect process. You don't fix a shrink with more shrink. You may remove some of the wave but you will only be shrinking the center even more, resulting in a low area for more oil canning. Fix the cause, not the result. When you cut around the perimeter of the welded area, the roof returned to normal as you "disconnected" the pull of the welds... More helpful hints: A 90* tight corner concentrates the shrinking effects on the inside of that corner for a noticeable pucker. A radius in corners helps to balance the shrinking effects on either side of the weld for an easier task of planishing out the shrinking deformity. For that small circle I would have used a round patch to fill the hole, not a square one. (food for thought) The tighter the patch (as close to zero gap as one can get) the better. A gap offers no resistance to the panel pulling closer together when the weld cools and shrinks, so any gaps will result in the outlying area pulling in more than with tight joints.. Where the crown of any panel is there to provide support and stability for the panel across its entire length, any disruption of that support risks oil canning. So for a low crown area like a roof skin, this means any additional shrinking/pulling in the center from gaps results in an even larger low area in the middle of the roof as that arc (cross section depiction) turns into more of a straight line. Which results in loss of that structural support that the crown gives, and an even greater likelihood of oil cans.. __________________ Robert Last edited by MP&C; 03-31-2021 at 10:55 PM.