View Full Version : Shrinking metal with torch
12-13-2005, 04:26 PM
Hey everybody, I finally got that hood about right. Now I have a high spot about four or five inches accross where the metal has stretched,and still wants to "oil can". Thought I would try heating to shrink. Of course I have a couple questions, do I heat the center of the high spot, or around the edges? How big an area do I heat? How hot do I get the metal, and how can I tell if it's hot enough? I have both propane and acetylene torch setups, which should I use, and if acetylene which tip? Should it take repeated heating an cooling? Also if I have forgot anything please tell me. The more complete and simpler the detail you can give the better as i don't want to ruin the metal,if that is possible. Thanks for your time. Ron p.s.special thanks to PhilV and MartinSr for your invaluable advice with this hood problem, it's almost back to 100%
12-13-2005, 06:05 PM
Ron, heat shrinking is very simple and can be done in a short amount of time ( maybe 5 minutes total). Your propane torch is definately not the right choice for metal shrinking. Get out your trusty oxy/acetylene torch with a #0 or #1 tip preferably (actually any tip will work if its used right). Before you light the torch you'll need to get some things together. You'll need a flat nosed body hammer or near equivalent and a dolly that reasonably matches the contour of the bottom side of your hood. You're also going to need a rag about the sized of wash cloth or a little larger. Next you're going to need a small bucket of water. Now your ready to light the torch. When you light it try and get to about a medium heat, not set real high but not real low either.
With the torch in one hand and your body hammer in the other hand heat a spot on the hood that is at the highest (most stretched point), concentrate your heat to one spot so that you will get the metal red hot in an area about the size of a dime. Pull the torch away and hit that spot with your flat nosed hammer while that spot is still red. If you need to then you can get an unwitting accomplice to hold the dolly on the under side of the hood (assuming there is dolly access under the hood for the area you are heating.
As soon as your done hiting that hot spot two or three quick hits with a hammer then douse your rag in the water and stick that wet rag on the spot you just heated and hit with a hammer. Thats it, thats all there is to shrinking covering the basics. Repeat heating as shrinking as many spots as it takes to get rid of the oil can effect. It should be pointed out that a dolly is not absolutely necessary in the heat shrinking process, but sometimes it can make a metal more flat while shrinking.
Obviously there are many variables involved that cover unique situations and there are side effect to the metal of work hardening from the heat and a hammer.
If you have a stud welder then shrinking is A LOT easier than shrinking with a torch. However if you're going to use a torch use oxy/acetylene as recommended by Phil because it can heat a small area without spreading the heat too widely.
You'll heat an area about the size of a quarter and the expanding metal will cause the metal to form a blister. You can then tap down the blister which drives the metal together the quench it with the water to stop it from re-expanding. It's much better to use a shrinking hammer because a flat face hammer can pinch the metal causing distortion and even have more problems than you started with. A shrinking hammer won't pinch as easily and makes the shrinking process much easier.
If you use a stud welder you don't need to quench the metal when you're done you just install the shrink tip and zap, zap, zap and you're done. Since we started using this method for shrinking metal we never went back to the hammer method.
Stud Welder Shrink Tip (http://autobodystore.net/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=2SW)
Lost in NJ
12-14-2005, 07:09 AM
Before you shrink how do you know it is a stretch?
It could be a shrunk area elsewhere. You can do more damage then you might imagine by trying to shrink when you really need to stretch.
For example. Take a 8 1/2 X 11 piece of paper and tear the middle of the long side about 1/2" in. Now just ever so slightly overlap the edges of the tear. Notice how the paper pops up in the middle. So how do you fix this?
You can shrink the middle, but does that really work??
The real problem is the edge is shrunk by a very small amount and needs some stretching.
So the real question you need to answer is where is the cause of the oil can?
Is it from some minor dent fixed on the edge of the hood? You do not say what kind of hood or what other possible damage it has. If the hood has any kind of a crown or raised bead then you have to consider the real problem could be in that area.
This is complex advanced body work that I just barely understand. I do know I have messed up some panels in the past by thinking they need to be shrunk to fix the oil can. In retrospect I now know the real problem was from some other body work and welding causing distortion quite a distance away.
Keep in mind I am not saying that the panel does not need a shrink or two. Just think a lot before you try.
It is very important to just do a little shrink at a time and you MUST be sure to let the metal to fully cool before you decided to do more shrinking. (yes more hard lessons as I learned)
12-14-2005, 08:21 AM
If you plan to shrink with a torch, try heating a small spot until it steams when you quench it, not so hot as to change the color of the metal to blue or red. You are controlling the heat so the metal does not get over worked and hard. Do not hit it with a hammer until it has cooled, and then only if it needs smoothing... and you back it up with a dolly. If you see steam when you quench, you are shrinking in a small controllable fashion.
To determine whether or not you need to shrink, Lost in NJ has given you some good advice. I would add that you should make templates of the curves in both directions so you actually know where the high spot is. If the panel is not smooth and fitting the templates everywhere, you should not shrink indiscriminately. If you have smoothed the panel with a slapper or hammer and dolly, and it is in its original form (correct curves in both directions) then you can shrink any high spots that are there. Shrinking methods in order of preference (the further down the list, the more damage to the metal, in my opinion):
Shrinking disc or wornout sanding disc (friction shrinking)
Tuck shrinking (no heat, not usually for the center of a panel, but it can be done)
Stud welder (a skilled operater may push this up the list)
Shrinking hammer (only if you like making marks on smooth metal)
If you are planning to use filler, none of this matters much. Choose whatever method is easier for you.
When chasing an oilcan, I never concern myself about overdoing one process or another. You can reverse the effects of shrinking or stretching if you choose the right methods....without damaging the panel. This makes it possible to experiment until you get it right. You will have it right when the surface area is correct, and the metal is in it's proper form or configuration...meaning the curve front to back and side to side are correct. Too much surface area, you need to shrink, too little, you need to stretch. Too wavy, you need to smooth.
Have you tried shrinking with a stud welder? There isn't much skill involved and there is no need for water. You just press it against the metal and pull the trigger.
Also, a shrinking hammer doesn't really do the shrinking it just stops the metal from expanding as you hammer it against the dolly. A shrinking hammer works much better than a standard body hammer. I would think that most DIYers will use a little filler to finish the job just like most pros do.
I find that the torch method to be the most difficult and the stud welder to be a no brainer with much less time and effort involved than other methods.
04-19-2006, 12:07 PM
Another post that I somehow missed...sorry! Nope, I've never used a stud welder. I have no problem with any metal working method, I just think some are better than others. I tend to harp on my favorite methods, because they seem so easy once someone takes a little time to learn them, but people do really nice work using other methods all the time..don't mean to sound like another tool or method snob.
I had to do tons and tons of shrinking on my latest project truck, probably a 100 shrink spots, because it had been hit hundreds of times by big rocks, bullets, a pick axe through the roof, and it was kicked and beat up by goats who lived in it while it sat in a field for about 50 years. I chopped the top and had to make and weld in a lot of patch panels, so there was lots of straightening to do. So I had a lot of time to practice doing heat shrinks, and learned by my mistakes what not to do too. One thing you never want to do is hit the hot spot with the edge of the hammer, because that just makes a big crescent shaped deformed spot in the metal that's hard to get rid of. I figured out that when the torch hits the right spot on an area that needs to be shrunk, the red hot dime-sized spot bulges out instantly. If it doesn't seem to want to bulge out as it turns red, you might be in the wrong spot. Rub a smooth almost flat-faced dolly on the surface when everything is cool and the high spot that needs to be shrunk usually shows up as a shiny spot, and that makes it easier to hit the right spot with the torch. Circle the spot with a white pencil to make it easier to see if you're wearing welding goggles. It helps to have someone else heat up the spot so you can concentrate on doing the hammer and dolly because it only stays red hot for a few seconds. Hold the hammer perfectly square with the face of the metal and knock it flat. Some people like to try to mash it flat in two or three solid blows, but actually I found it works better to make a ring pattern of smaller quick blows around the edge of the hot spot rather than right in the middle of the hot spot. It comes out flatter when you're done. And another trick is to use an air gun to cool down the metal when you're done rather than a wet a rag. The air gun takes longer to cool down the surface, but it doesn't turn the metal all ugly and corroded and pitted like what happens when you slap a wet rag on it. For thicker metal like the 16 gauge on an old pickup bed, I used about a ten pound brick of steel with a flat face held tightly behind the high spot and a 2 pound hammer with a low crown instead of a body hammer, because a body hammer isn't really heavy enough.
The main idea with shrinking is to make the metal bulge up dull red in that ~1/2 diameter spot, then hammer it flat again while it's still hot, which displaces the metal into a smaller area, then as it cools down, it shrinks back even tighter. If the heat doesn't cause the metal to bulge out like a little dome, then you might be in the wrong spot. Often you'll have to do two or three shrinks in a row if you have a long oval shaped high spot.
I really noticed after a while though that when the metal bulges up very obviously right away when you hit it with the torch, the heat shrinks usuallly work:cool: like a charm. If it barely bulges or just gets sort of deformed and twisted looking when you heat it up, it doesn't usually work, and sometimes just makes it worse.
Geez Tom, you probably could have saved a lot of time and effort if you used a stud welder with a shrink tip. You could have pulled the dents with the pins and shrunk where needed with the tip. Both of these types of repairs are much quicker and easier using this type of tool.
Shrink Tip (http://autobodystore.net/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=2SW)
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