AAE
Learner
Like Santa?He has little helpers...
Like Santa?He has little helpers...
You could not have said it any better!The problem is not where you think it is IMO, and shrinking there will only cause more problems. The center looks like it is the right shape, but pictures can be deceiving. What kind of car is it?
That is one of the best posts I've ever seen...You could not have said it any better!
The OP has had "oil canning" because his roof sheet metal is now longer and wider than it was before blasting. It is no longer taut in the same "footprint" and has yielded in tension and become thinner somewhere. What yields and elongates in length also contracts and thickens due to conservation of mass somewhere. And it's not always evident where it might be on first thought. If something stretches in one or two dimensions then in the third it must contract and thicken somewhere--otherwise it separates. Carbon steel welds don't "shrink" except when they go from liquid phase to solid phase delta iron crystal structure at about 2700F and then from delta to gamma phase at around 1700F or so. They expand and grow in volume as they pass on cooling at about 1330F. The volume change at room temperature will be an increase in thickness whether filler metal is added or not. The thermal energy absorbed in heating something to about 1100F is simply to add in energy to lengthen something in three dimensions is just reversible on cooling back to room temperature because no crystal phase change has occurred. But go past red heat and it's going to be something else from a phase change. Find someone with a ultrasonic thickness (UT) meter and you will find where you are thin and thick. They are cheap now and mine is accurate to about 0.002" in 2"--- at least at room temperature
The picture of a 2" boiler tube bent in a hair pin on a 4 1/2" centerline is a damn tight bend and bend "cold" at room temperature. 40% of the wall thinned on the outside and that surface lengthened dramatically. The tube's wall measures 0.132" unbent and 0.080" thinned on the outside. It breaks in tension at about 48% wall reduction consistently. The inside surface grew in thickness and yielded in compression--essentially the crystals slid and piled up on one another--it got dramatically shorter. People focus too much on the stretch-out without realizing the thickening. You don't get one without the other. I sectioned that bend longitudinally to "open it up" to measure. That sample was about 100,000th one out perhaps 28 years ago in a production run. That same machine is well over 1.3 million bends on hairpins and 45 degree bends. It still bends the same if the carbon and manganese content of the tube is low.
The same fundamentals apply to the OP's problem. Since your roof panel is pinned from movement at the edges--that's likely where it has thickened since metal crystals slipping and moving had no more room to flow from blasting except to go the edges. Those are your fixed points. Nothing can move here at the drip line to increase in length from the blasting in width and the same for length with the windshield and back window framework . You need to get metal back to where it is thin from where it is thick. You can do that with "gathering with a hammer and dolly" or by what I described above. Shrinking discs are bad news since you can get to a phase change to thicken just from frictional heat alone from the discs on the very, very surface crystals of the sheet metal but not the through thickness. Then you don't have an even pull. Whole thickness of the sheet metal or nothing if you want to stay put out in the hot summer sun outside or a -30F day in the winter after working it.
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This roof dilemma has really set me back. In fact, I'm now scared to jack with it. Right now, it looks perfect just the freaking oil canning. I had a couple of guys on another forum tell me to leave it and stick sound deadener to the underside of the roof, something I intended to do anyway. Seems kinda halfazz but far better than me warping the crap out of it and using a ton of filler......arghhh! I just don't know!
I could not have asked for a more detailed explanation and how to than this!!! This should be put up somewhere for reference. Thank you Robert (MP&C), this is not the first time you've bailed me out!View attachment 29279
The support for your roof skin is similar to the support for an elliptical roof on a building structure. Once a dent occurs, it moves forces around and interrupts that support. You'll see tight areas and loose areas and, on a good day with the roof in front of you, can find it very much a challenge to work out the area to address and those to leave the hell alone. My suggestion in order to LOCATE the area that needs addressing.....
Typically you'll cycle the oil can in and out in order to see the outer perimeter of the affected area. You've already done that and identified the perimeter, whether you know it or not... But lets continue for identifying the areas not so easy to pick out... Once the perimeter of the oil can is located, mark the outer perimeter using tape, sharpie, etc. Now that the outer perimeter of the oil can is located, using your thumb on one hand, apply slight thumb pressure along the outer perimeter and while holding this pressure, use the other hand to cycle the oil can in (and back out if it doesn't do it by itself) Then move the thumb pressure around about an inch on the perimeter, hold the same pressure, and cycle again. What you are looking for is a location along this perimeter that will LOCK the oil can from cycling. If you locate one, continue around the entire perimeter, repeating this process as there may be more than one "sweet spot" that needs work. This (these) are the area in need of work to resolve the issue and not compound things by guesswork. But before starting down that rabbit hole, read on.....
In looking at your initial video about two thirds of the way in, it appears in (picture) that your perimeter cycles just shy of that body filler area (as one would expect) I don't feel you can address anything oil can related without starting here first. A dent is a STRETCH. Aggressive media blasting is a STRETCH. Any area welded is a SHRINK. Whatever caused your issue, the opposite will need to occur to fix the situation. I would start by removing the filler and bumping/shrinking that dented area as needed, then see how the oil can has responded. A dent has introduced a "lock" and caused an interruption of forces across the entirety of the panel, as can be seen in your video. As a side effect, the dent may also pull from other adjacent areas, possibly making them loose. In the interest of showing why you don't fuck around with guessing on an oil can, IMO the majority of your issue was caused by that dent and NOTHING should be touched until the dent is fixed and the results of said repair are assessed. Anything else done before addressing that dent just risks changing forces in the panel unnecessarily.
Does the roof have a specific spot where it cans. A tiny bit of manipulation there may be in order, the most active place may very well be the most stretched, Adding anything extra always seems cheesy to me. Take your time and try to correct the issue is better than treating the symptoms. JMOSo, I've not messed with the roof yet other than to remove the filler that I put on it, sand it, and apply a couple coats of epoxy. Tonight, I plan to block sand it again in order to find the highs and lows.
Since my original post, I've continued to research, ask questions and weigh my options. Can't tell you how many stories I've read where nightmares were created trying to fix such a problem, even by supposed skilled individuals. Spoke with a local guy who worked in the business and taught in a Vo-Tech for many years. I sent him my video as shown above. He's also a big Mustang guy and knows these cars in and out. He weighed out three options for me:
A bad go at #1 may lead to #3 and I want neither the cost or the hassles in replacing aftermarket panels. Just don't want to risk it. However, option #2 sounds like a good alternative, as the roof looks great right now. That said, I'd sure like your thoughts on putting a brace under that roof? Again, the roof has no major issues, just cans a little and pops right back into shape.
- Attempt to fix and hope for the best. Not knowing exactly what caused this makes the fix scary.
- Put a brace in the roof, either welded in or glued in with panel bond.
- Cut the top off and put another on.
Something noted and not sure it matters, but I was in the shop last night and pushed down on that roof and it seem more taught, or rather difficult to push down on. Still cans, just not as easy. The shop is cold, in the 40's, as I've not been out there working and the heat is off. This makes me think that the roof is indeed stretched through the media blasting and therefore needs shrinking. Thoughts???
I get that, but what I was trying to relate to was a possible fix, as I'm not 100 percent certain what caused it. I believe the media blasting was the cause, but not certain. The cold made the metal shrink and stiffened the roof, so I'm thinking shrinking in the right places would remove or decrease the canning??? The media blasting, according to MP&C stretched the roof, makes sense that it needs shrinking. There are a few small dents in the roof that I plan to address first, then will go from there......slowly.Your roof panel contracted in length both across the roof and over its fore to aft dimension with a drop in temperature in your shop. Steel contracts or expands 8 millionth of an inch per inch of length for every 1 degree F temperature change. Thickness contracts too--but its dimension relative to length dimensions is insignificant. That's why your looseness feels less when colder. But by the same token--take your roof out as oil canned in the hot OK summer sun where you live and let it go to 125F or so and your problem will be even more--horrendously more.
(2.) isn't going to solve that problem, it will break the panel bond or depress the brace or shift the growth to can or distort another part of the sheet when exposed to the sun.