Great tip. Thanks

 

Amen on the Henrob! I do all my own patch fabrication and welding and have got to the point of almost no rod.
Just a shame it was invented in Aussieland!!

I even bought a 2nd one to use for cast iron and cast aluminum repair as in blocks, heads and intakes for flatheads.

 

The biggest mistake novices make when trying to remove dents and such is trying to hammer them down with the dolly behind the dent, otherwise called "on dolly" hammering. Picture what is happening this way: if you take a lump of pizza dough (the metal panel) and set it on a counter (the dolly) then repeatedly punch it with your fist (the hammer) what happens? The dough thins out where you punched it and the diameter of the ball grew in size right? Same happens in "on dolly" hammering! The metal is pinched between the hammerface and dolly so it is made thinner and the surface area gets larger. Unless you are hammering all the way out to the edges (in which case the entire panel will get bigger) that extra metal is trapped by the surrounding metal and can only do one thing: dome up forming what is commonly called an oil can. since there is more surface of metal than space for it it will just pop in and out. The more you try to flatten it by more on dolly hammering the worse it will become since you are continuing to stretch the metal larger yet.
When you locally heat metal the opposite happens. The red hot plastic state metal wants to expand in all directions but is confined by the cooler metal around it so instead it gathers to the center and thickens, reducing or "shrinking" the surface area. If it cools at that point it is locked in that state and the panel has been "heat shrunk". Since the forces involved are so great and the hot metal so plastic a very small shrink spot can reduce a substantial amount of surface area surrounding it.
Common problems and solutions: 1. I tried shrinking out a "bubble" like I've read/heard/seen, and it got worse instead of better! Solution: Everyone has seen or read how to do a shrink. You heat a spot about the size of a dime red hot with a small hot torch flame until it bubbles up, you put the torch down and hammer the bubble flat with hammer and dolly while still red hot. That is the correct technique, but it's very very easy to mess it up! Let's go back to the pizza dough for a moment. suppose you've stretched it too much for the pan size so when you fit the edges to the size of the pan it puffs up in the center ("oil cans") you now need to shrink it. You work a small area in the center of the pan by pushing the dough together at that spot which pulls in the excess dough flattening over the entire pan all but the lump in the very center that's now thicker than the rest and sticking up so the sauce will run off. If you very gently pat that lump down you can get it flat and just slightly thicker than the rest of the dough where it's hardly noticable and won't stick up above the sauce. However if you stat pounding down the lump in the center to the thickness or thinner than the rest of the dough all you'll accomplish is undoing the shrink or even stretching it more, compounding the original problem! Moral of the story: you just want to TAP the bubble down flat while it is still red hot so it doesn't stretch back out! If it remains a little thicker than the surrounding metal that's perfect!!! All you need to do is file off the extra thickness or blend it out with a sanding disc. If you wait until it starts to lose the red color or hammer too hard you'll have only succeeded in making more work.
2. Butt welding in a patch can make it seem like you've created an oilcan in the center of the patch, but if you examine the contour carefully what you'll find is you have heat shrunk the edges around the patch in the welding process! To correct you need to stretch the shrunk metal back out by hammering on dolly (pinching the metal between the hammer and dolly to thin it) along the seam, stretching more where there is more shrinkage and less where there is less until the seam is back up to contour. This stretching across a welded seam process is called "hammer welding" and if done with sensitivity can result in little to no filler being needed! You can minimize the amount of shrinkage hench stretching by doing very short tacks no more than 1/2" each and allowing each tack to cool to room temp before doing another anywhere near the last one. The welding will take a lot longer that way, but the remedial work needed will take many times any time saved by welding the entire seam at once.
Ordinary mid wire is pretty hard and brittle compared to the sheet metal and the bead is thick so stretching the weld bead is somewhat more difficult and seam cracking can be a problem compared to gas or TIG welding beads. Two ways to help that is to grind the bead down nearly flat before hammering and use a special wire made by ESAB called Easy-grind wire. It is softer and less prone to cracking than commonly used wires. It's unlikely your ESAB dealer will have it in stock unless he is especially well stocked or enlightened, and may not have even heard of it, but insist he check his ESAB supplier and order it for you! DON'T accept any substitutes as being the "same thing" or "it's what everybody uses". It must say EASY-grind on the spool!

 

AX, excellent info (as usual) I had always wondered why you were supposed to hammer on dolly along the seam. Thanks for clearin that up

Bobby

 

One more thing: If you have an oilcan in a panel SHRINK IT OUT! Do NOT push it in and fill it with body putty!!!!! It is unstable and sooner or later the metal will pop back out, making an unsightly body putty zit in the panel or even popping the body putty right off. Not what you want to happen to your freshly painted truck.
For more info on dealing with a "bag of walnuts" panel from someone wailing away on a dent with a hammer do a search on "shrinking disk" here.

 

AXracer,
Great desccription of the techniques. Very easy to understand for us
Metally challenged.

 

Lee - Thank you for the great info and pics. I just love learning new old stuff like that. Way cool....(pun intended)

Ax - As usual, your input is also most edifying. I will almost surely need that info when doing the bodywork on my cab hood, which will be soon.

 

Ax, thanks for sharing your knowledge, I'll be reading this over and over probably for a while. It's taking awhile to grasp the movement of metal as it's being worked on, that's what makes me the novice that I am and I appreciate that you and others are willing to share your knowledge, I'll be leaning on it for quite a while. I don't know what a henrob is, I suspect that it's not a pain in the butt seeing as it is spelled differently, is this specialized type of hammer or dolly? John

 

A Henrob is a special Ox Ac torch that's of a unique design that is supposed to be far superior to the traditional design or even a mig welder, especially for sheet metal welding. I've not actually seen one in person or used one myself, but I do know that those who have swear by them, even the pros. Biggest negative? They are ~ 2-3x the cost of a traditional welding/cutting setup and handle a little differently so there is a learning curve FWIU.

 

Thanks Ax, I don't think I'll be running out to buy one, I have enough to learn now. John

 

John, I suggest you do a search under my user name: AXracer, I have made a number of similar posts over the last year or so on other aspects of metal shaping and body work you may find useful as well. If you don't find the answer to any questions you might have feel free to ask them on the forum or of me personally, I'm always glad to help.
I'd also suggest to anyone contemplating any body work that you order yourself a copy of Ron Covell's video "Basic Techniques for Working with Steel" www.covell.biz it's the best 35.00 you can spend!

 

Maybe when comparing the Henrob to an import O/A but a full Made in USA Harris or Victor kit runs about the same as the Henrob when it is on sale (fairly often)
Check out "http://www.cut-like-plasma.com/"

 

Sam's has a complete Harris torch set with hose and regulators for <130.00. I got mine band new on ebay (genuine Victor not knock off with 2 stage regs) for <80.00. So my price comparison is valid IMHO, I may still invest in a Henrob some day tho, but I already have the Victor, MIG welder, plasma cutter, stick welder, 3 mini OA torch sets, and 3 (two brand new never used) Prestolite Ac air torch setups, so I don't really NEED another torch.