Pounding and forcing thin metal sections into shapes that humans want and need has a long history. While there is disagreement about exactly when and where people began to work with metals, it was certainly in prehistoric times and began with soft metals like gold and copper.
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The discovery of how to control fire made extracting metals from mined ores more efficient than had been finding nuggets of almost pure metal. It also led to the ability to create alloys of various metals, by melting them. In many civilizations Copper Age developments were succeeded by Bronze Age advances, bronze being an alloy of copper and tin. Longer-surviving civilizations usually progressed from copper and bronze to iron and steel.
The qualities of metal, in particular its plasticity and strength, made it ideal for uses as varied as making ornaments, cookware, and weapons. In these and other uses, it had many great advantages over other materials like wood, bone, and ceramics. Various processes were applied to early metals: annealing, tempering, bending, stamping, rolling, casting, forging, cutting, soldering, welding, and many others. These were the precursors of many modern metal working processes still in use today.
The earliest metal forming techniques involved beating pure metals and alloys into small, flat formats. Then those sheet stocks were formed into useful or ornamental items like knives and pendants. We know that such ancient civilizations as the Hittites, Mesopotamians, and Babylonians were well along in using variants of some of those processes, thousands of years BCE.
Think about that the next time that you are at a car show, and admire some difficult-to-form body feature of a hot rod or custom car. The ability to produce it began thousands of years ago, with anonymous, ancient metal workers, beating copper into crude and unlovely bracelets or kitchen pots. The latest die stamping and rolling processes that produce modern automobiles are basically developments on those ancient metal arts. It’s kind of humbling, isn’t it?
In the modern sheetmetal fabrication and repair field, we use highly evolved versions of much of the knowledge, and many of the tools and techniques, employed by those ancient metal formers. But we have advanced greatly from where they left off. Every tool, device, and process that we use today is better than what they had. Our raw material, the sheet-metal itself, is pure and consistent beyond anything that they could imagine. Our knowledge is greater, and our results are often more daring and always more uniform and durable than their best efforts. For all that, we still beat metal with hammers, roll it through wheels, and weld it with heat. Some general aspects and principles of metal work have changed little over time.
Panel Types, Configurations and Reinforcements
Ancient metal workers may not have had a word for “crown,” but they certainly understood its significance. You need to understand this basic concept to work with sheetmetal. All formed metal shapes have some characteristic of crown—no or low crown, medium crown, high crown, reverse crown, or combination crown.
Flat metal has no crown. It may be bent, or formed into a simple arc, but it has no crown. Metal acquires crown when it is shaped in ways that cause it to fall away from a point, any point, in every direction. That is the essence of crown. The significance of crown is that it stiffens panels, and areas of panels, where it exists. This is because the stamping or rolling processes that are used to create crown in panels tend to harden them, and because an arched, three-dimensional structure is inherently stronger than a flat one. The more crown a panel has, the tougher it is likely to be in resisting the impact of a collision, or the hammer blows that a metal worker strikes to repair it. High-crown panels have more crown than low-crown panels. You can often move the metal in no-crown and low-crown areas of panels with your fingertips. This is not possible in highly crowned areas of panels.
Reverse crown is simply crown that faces away from the outside of a car. “Concave crown” would also describe this configuration. Combination-crown panels have different kinds of crown that work into each other, such as low into high crowns, or high or low crowns that work into reverse-crown areas.
All of this is important because crown imparts strength to panels, and therefore is more resistant to force applied to repair damaged areas where it exists. It is also important because crown is forgiving, up to a point, when you repair areas that have it. This is because stretched metal can be hidden in crowned areas. Since these areas are, by their nature, bulged shapes, a small additional bulge often fits undiscernibly into them. Very-low-crown and no-crown metal cannot hide stretches. They show as unsightly bulges and/or ripple distortions.
I am not exactly advocating autobody dishonesty here. However, this work involves reaching goals that are mostly judged on their visual merits. At times, and in some situations, a good practitioner uses characteristics of panel configuration to slightly trick the eye. (There will be more on this topic, later.)
Along with crown, how a panel is supported and attached to a vehicle is critical in understanding how it performs under impact, and how best to remove impact damage from it. Many panels have strengthening structures welded or bolted under them. Panels that are attached to vehicles by welding them to sub-structure perform differently from those that are bolted to substructure. Unless you deal with them, bent or damaged substructure reinforcements and fastening points that impart strength to panels, cause panels to resist restoration to their original formats. Always consider this factor when you plan panel repair or restoration work.
The steel sheet stock that is formed into automobile panels is a truly amazing material. It is a complex alloy of iron, carbon, and other elements. It has been heat treated in its manufacture to disperse the carbon evenly into the steel’s granular structure. While steel has less carbon content than iron, the even dispersal of what carbon it does have makes it strong and somewhat plastic, or deformable, unlike various irons. Mild sheet steel, the stuff of auto-bodies, is roughly .25-percent carbon. Above that concentration of carbon, steels begin to fit into the medium steel classification. Between .6-percent and 1-percent carbon, steels are considered hard or high-carbon. Ultra hard steels, like tool steels, may contain between 1-percent and 2-percent carbon.
The softness of panel steel allows it to undergo the highly organized brutality of stamping it into complex three-dimensional shapes like doors, hoods, roofs, and fenders. Using heat and enormous pressure, automotive body steel is stamped into final sheet format. While it is primarily an alloy of iron and carbon, several other elements which, in some cases, have names that are hard to remember and difficult to pronounce are routinely added to it to give it the special characteristics that are needed to form it into automotive panels.
New car panels are presently in the range of 22-gauge to 23-gauge; that is, .0299 and .0269 inch. Note that as the gauge number increases, the thickness of steel sheet stock gets thinner. The way that this works involves an arcane formula that takes into account the weight of a cubic foot of the material involved. To make things thoroughly confusing, basing gauge on weight means that the same gauge number applied to different metals gives different thicknesses. For example, while 22-gauge sheet steel is .0299 inch thick, 22-gauge galvanized steel is .031 inch thick, 22-gauge aluminum sheet stock is .025 inch thick, and 22-gauge stainless steel is .031 inch thick.
The important things to remember are that as gauge numbers increase, thickness decreases, and that the same gauge numbers for different metals may translate into slightly different thicknesses.
Finally, there is a misconception that gauge designations involve the number of sheets of a particular gauge that can be fit into 1 inch. This, simply, is not true. Common gauge numbers for automotive outer-body steels are:
- 18-gauge .0478 inch
- 19-gauge .0418 inch
- 20-gauge .0359 inch
- 21-gauge .0329 inch
- 22-gauge .0299 inch
- 23-gauge .0269 inch
- 24-gauge .0239 inch
Thickness is important because, in part, it determines how difficult it will be to repair damaged body panels. In most cases, the thinner that body metal is the more problems it tends to present in repair. That is because the thinner body metal is, the more difficult it is to form and to weld. The alloys used in thinner panel sections tend to be harder than the older, thicker panels, because they contain more carbon. That makes them more difficult to deform with body tools, without taking them beyond their yield points (fracturing them). Their hardness also makes them very difficult to surface file for the purpose of leveling them. Welding thinner metal is always more challenging, due to the tendency of thinner sections to melt and “drop out” at welding temperatures. That outcome also can be very hard on a metal worker’s shoes.
Plasticity and Elasticity
When I speak of the hardness of metal, I am generally describing several significant characteristics, two of which are particularly important to anyone working in panel fabrication and repair: plasticity and elasticity. Plasticity is the ability of metal to deform without fracturing. The point of fracture is called the “yield” point. Automotive panels are stamped at the factory from flat stock into complex, three dimensional shapes. The fact that this can be done is proof of their plasticity. When a body repair technician works on them with hammers, dollies, and other tools, they are again deformed, courtesy of their plasticity.
Plasticity under tension is called ductility, and produces stretching when it occurs. Think of the bumper over-rider on a truck smashing into the door of your vehicle. It deforms it plasticity and it probably will put the metal under tension and stretch it ductility. When plasticity occurs under compression, as opposed to tension, it is called malleability, and produces the opposite of stretching by compacting or “upsetting.” In upsetting, metal is piled into itself.
Let’s go back to that unfortunate damage to your vehicle’s door that occurred when a truck hit it. After the accident, a technician removed the inner panel from the door. Then, the technician began to fix the damage by hammering the ridge near the center of the dent down and out against a dolly, centered under it on the outside of the door. If the technician had read this book, he or she would probably have had a better first move. The accident probably stretched the metal in the door’s skin because it was deformed while being held rigidly at both ends by the door’s substructure. The attempt to hammer it out put the area near the hammering under compression because the dolly was supporting the undeformed metal on either side of the ridge. The result of hammering down on the obvious ridge, with a dolly under it, was to compress the metal there latterly, or to upset it.
This is a critically important distinction in autobody work. When you stretch metal you are effectively exchanging some of its thickness for increased lateral dimension. When you upset metal, you are exchanging some of its lateral dimension for increased thickness. At various points in working with body metal, you need to create upsets, and even stretches, on purpose. At other times, you will need to avoid these dimensional transformations, or have to correct them. It is critical that you understand exactly what stretches and upsets are, and why and how they occur. Later, I will discuss how to purposely create them, and what situations call for creating them.
Elasticity in metal is its ability to flex to a limit its elastic limit and still return to its original shape, on its own. Some call this characteristic memory, or spring back. You might have encountered this when you slammed the hatch on a minivan or SUV, and had the queasy sensation of feeling your hand deform the hatch metal where you were pushing against it. But then, as you released the panel, you felt the metal under your hand return to its rightful shape. You can thank elasticity for that good outcome. If the metal didn’t spring back, it was because you exceeded its elastic limit.
Elasticity is critical because damaged panels usually contain a small minority of surface area that has been pushed, or deformed, beyond its elastic limit. Most of what may look like damaged metal because it is out of position has not been deformed beyond this limit, and will return to its pre-accident shape when you release the small areas of badly deformed metal that are holding it out of place in the damage. I don’t want to sound excessively rosy about these matters but, to the untrained eye, panel damage almost always looks worse than it is.
Work Hardening: The Metal Remembers
The great elephant hiding discreetly in this sheetmetal living room is called work hardening. This is the tendency of metals, like mild sheet steel, to become progressively harder as they are deformed beyond their elastic limits.
Doubtless you have already performed experiments involving this factor, although you may not realize it. If you, like most people, ever tried to straighten out a paper clip with your fingers, you encountered work hardening. What you discovered was that it is all but impossible to get the three bends out of a paper clip with your bare hands. What happened when you tried to do this probably under the cover of a pile of books or a knapsack, so that your teacher would not see you performing this metallurgical experiment was that before any of the three bends in the paper clip could be straightened, the metal stopped moving in the bends and bent on either side of them, leaving shapes like saddles between two opposite-facing humps, in kind of a camelback configuration. The saddles were what was left of the original bends. The humps were new bends, in the opposite direction, that occurred when the metal in the original bends stiffened as you bent it, and approached its elastic limit. Then, the opposite-facing humps were made as you continued to apply pressure.
That poor paper clip began its life as a straight piece of wire. Forming it into a paper clip work hardened the metal in its bends. When you tried to straighten it, you made some progress, but work hardening made complete straightening impossible, so the metal bent on either side of the work-hardened area. This is not trivial. Work hardening is terrifically important in body work. You must learn to identify it, predict it, and deal with it, because it tends to be a factor in almost all of your collision damage and fabrication efforts.
For the record, work hardening occurs because steel has a granular structure. Bending it rearranges and distorts its grains. Beyond a certain point, this becomes difficult, and somewhere beyond that, the steel will fracture; that is, it will reach and exceed its plastic limit. Maybe in your frustration, when you couldn’t straighten that paper clip, you bent it back and forth until it broke. Do you remember that it felt warm at the place where you were bending it, before it broke? That heat was generated by the friction of the grains in its structure deforming and riding against each other as you bent the paper clip back and forth.
Heat also has the ability to rearrange those grains for important purposes. Beyond certain temperatures different ones for different metals and alloys of metals—the grain structures of metals rearrange themselves and eliminate work hard ening effects. This process is called annealing, and only works if sheet-metal air cools slowly, after being heated to its critical temperature. In the case of autobody steel, that temperature is roughly 1,600 degrees F, which appears as a color between bright red (1,550 degrees F) and salmon (1,650 degrees F). How steel cools, after it has been heated, determines many of the characteristics of the hardness that it exhibits. For example, quenching it (cooling it rapidly with air, water, or oil) after it has been heated to its critical temperature, tends to rearrange its grains in ways that harden it.
There is more discussion of the effects of heat on sheet steel in later chapters, with particular regard to using annealing and quenching to solve problems caused by work hardening from moving cold metal, and hardening softened areas near welds.
At the Factory and Afterward
Autobody panels begin their lives in near-ideal conditions. Clean, uniform sheet stock was stamped or rolled into shape. Huge machines accomplished this work by exerting many tons of pressure on flat sheet stock that was inserted between the drawing and rolling dies of stamping devices. In such operations, flat metal is deformed by enormous force that stretches and shapes it. The metal is clamped at its edges by “binder rings,” and then acted on by dies that force it into desired shapes. Later, it is trimmed and pierced at attachment points.
For the metal worker, the important thing about these processes is that the stretching and forming of sheetmetal between dies work hardens it. That is one of the reasons for stamping it; to make it stronger. The other reason, of course, is styling. If cars were fabricated from unstamped sheetmetal, their panels would literally flutter in the wind, and from road vibrations. Stamping imparts strength, and helps to eliminate most flutter. Besides, no one would want to drive a car that looked like a steel box.
When you repair damaged sheet-metal, you must deal with the work hardening that occurred in the original stamping or rolling process that turned flat stock into finished panels, and with the additional deformations that occurred when it was damaged. There is also the factor of road vibration, which, over long periods, hardens panels as they travel down the road. It is important to keep all of this in mind when you find a panel resisting your best efforts to change its shape and restore it to its original configuration.
One of the worst forms of damage that you will ever encounter is bad repair work. A range of people, from the truly clueless to the dedicatedly inept, may have tried to repair the damage before you. Their misguided efforts, often with very large hammers and other destructive devices, may have made things worse or much worse than they were. Collisions deform and work harden metal.
They also may stretch or upset it. Bad body work, the kind that roughs out damage and then gobs filler over crude work, tends to make these problems more severe. These situations will tax the full range of your abilities, talents, and patience.
Impact is not sheetmetal’s only enemy as it ages. The other major problems gather under the brown banner of corrosion, a.k.a. rust. Rust is birthed by a chemical reaction between water and metal. Road salt, an electrolyte in dirty water, enhances the speed of this reaction. Rust occurs when moisture gets through or around paint and other anti-corrosion surface treatments. Since water is very adept at infiltrating small spaces (through capillary action) and at penetrating coatings, it is a cinch to attack vulnerable areas like door seams and panel attachment points. A great deal of body work on cars involves repairing the ravages of rust. Sometimes, small areas of perforation damage can be welded shut. More often, panels require the excision of diseased areas, and replacement with sound metal.
Necessary Tools and Equipment
Somewhere between having the basic tools for autobody work, and having the latest, most exotic, and most advanced metal-forming devices ever made, there is a happy medium of being reasonably well equipped for most of what you encounter. An el cheapo starter body tool kit, with three unbalanced body hammers and a crudely cast dolly, probably won’t take you very far in this work. On the other hand, roaring out and acquiring the likes of a good English wheel, a Pull-max power forming machine, and a high-quality TIG welder is almost certainly way beyond the needs of novice- or intermediate-level auto-body metal work.
The best approach is to acquire tools and equipment as you find the need for them, not just because they are there. When that need arises, it is a good idea, in most cases, to stick to top-quality tools—ones that come from reputable vendors and that will last for the rest of your working life. There are exceptions to this. Some air tools, like the die grinders and air disc sanders that are so useful in autobody work, largely have become disposable tools. Buying good ones with name brands probably is a waste of money. Most people I know buy cheap ones and replace them as needed. Since the prestige versions of these tools cost between three and five times more than the throw-aways, and the repair (tune-up) kits for them cost as much as the generic versions of these tools, this makes great sense.
However, items like cheap body hammers or tin shears tend to create bad results and should be avoided. My general rule is: If something makes direct contact with metal, like a file, hammer, or dolly, it should be top quality. Otherwise, evaluate the economics of replacement strategies for tools that don’t contact the metal. To get started in autobody work, you need some basic hand tools for shaping metal. I recommend an assortment of hammers and a few dollies. The hammers should have faces of various crowns, sizes, and shapes. A set of soft hammers, say plastic and/or rawhide mallets, is a great addition. A shot bag is a good item to have to back up hammering various shapes, and a good anvil is essential.
Small items, like sheetmetal pliers and an assortment of hand shears, are essential when you start this work. A few good body files of differing tooth count and some rigid and flexible holders for them are necessary for many jobs.
A good electric disc sander is a must for doing this work; 7 or 9 inches will do. A small electric or air hand grinder, 4 or 41⁄2 inches, will be endlessly useful. Some way of cutting metal with rotary abrasive wheels is very desirable. A 3-inch air muffler cutter can be bought for less than $10, and fitted with more-useful 4-inch cutting wheels. Air nibblers, shears, and small reciprocating saws have become very inexpensive in recent years, and are extremely useful in this work.
A good MIG welder and an oxy-acetylene torch are highly desirable for performing many bodywork tasks. Likewise, a good plasma arc cutter is a great asset. You might want to put these items on your wish list, if you do not already own them.
As you do more fabrication work, you will want a metal shear, a slip roll, and a metal brake. These devices vary in quality from expensive to very expensive. There is even a common unit that embodies all three functions in one tool and, while it is a bit clumsy, it provides an economical approach to doing reasonably good work.
There are hundreds, maybe thousands, more tools and devices that may be helpful in pursuing metal work. The key to your tool and equipment program is to figure out what you may need regularly, versus what you will probably use no more than once a year. Purchasing the latter class of tools can be put off for a long time. Hey if you only need to use something once a year, you might consider borrowing it.
The main point in acquiring tools is to avoid the extremes of under equipping and over equipping. If you only have three chipped hammers in your repertoire, and one of them is a carpenter’s hammer, your metal work will show it. At the reasonable middle ground, a quality planishing hammer is a very good investment for a wide range of projects. At the extreme, an old Yoder or Pettengell power hammer, hulking in the corner of your workplace, taking up the space of a BMW Mini, gathering dust, and sagging the floor under its enormous weight, will do you little good if, through ignorance or lack of opportunity, you never use it. In fact, it will do you no good at all. In some cases you will fabricate special tools for special tasks as you go along. This is particularly true where the right tool does not exist, or is too expensive. Always keep your mind open to making tools when you need them, particularly in areas like fixturing. Devices to hold your work can often be easily fabricated from scrap metal.
Tools and equipment tend to be as good and useful as the person using them. Don’t waste time spooning after expensive and exotic stuff. Great equipment hardly ever makes it possible to do a job. Usually and at best, it increases the efficiency of doing it. Keep that in mind when you peruse tool catalogs. Many great sheetmetal fabrications were completed with very simple tools, in very simple settings, with planning, skill, and patience.
Find a happy medium. If you realize that you badly need some-thing to work more efficiently and to get better results, lay your plans to acquire it. But, if you find that you have tools and equipment that you never use that’s why there are garage sales, classified columns, Craig’s List, and eBay.
As you pursue autobody metal work, you can often find comfort zones in many of the varied tasks that you perform. That is, you find specific ways of doing things that “just feel right,” and that feel better than other ways of performing the same tasks. Never undervalue that sense of some-thing feeling right, it is not absolutely infallible, but it is usually important.
Beyond that, each metal worker brings to this work his or her own personality, character, and experience. Attributes like keen observation, sensitivity, logic, and the ability to plan are all helpful. If you do it right, this work will concentrate these traits, as well as your attained skills.
Traditionally, humans are considered to have five senses. You will need to use four of them, and to effectively interpret what they tell you, to do good work in this field. Hmm, let’s see—sight, sound, touch, smell, and taste. Sight and touch are obvious. They directly inform you regarding the contours, dimensions, and surface characteristics of the metal on which you work. Sound is critical in things like how a hammer sounds hitting metal, or how a panel resounds when you tap on it. Smell is useful when you heat metal. It helps to inform you regarding its temperature. Okay, I don’t have any use for the sense of taste in metal work. I’m still working on that one.
Each of the four senses noted above can provide you with useful information, if you interpret what it tells you in the sheetmetal way. For example, your sense of feel means different things in sheetmetal work than it does in refinishing. To be good at this work, you need to train your senses to comprehend things in ways that are appropriate to and useful in this work.
Most sheetmetal tasks can be performed in many ways. Some give better results, or are more efficient, than others. A few of them are just plain wrong, and fewer are indisputably the only way to do something. As you pursue this work, you will learn which ways give you the best results.
The best way to learn what works best for you is practice. Experience is more valuable when it is attained without ruining valuable metal. Before you strike with any hammer or other device, always try to practice what you are planning to do. Practice hammering out dents on junk fenders, before you try it on a repairable or restorable fender. Practice welding on metal that is similar to the metal you want to weld, and get your materials and settings right, before you ruin a good panel. Try out new tools or processes on scrap, before you try them out on something important. You get the idea.
Some of the tools, equipment, and processes that you will use in this work are inherently dangerous. There are sharp edges, caustic chemicals, flying abrasive grits, electric shocks, and many other hazards to consider.
Always consider safety first. No sheet metal creation is worth the loss or impairment of sight or hearing, or worse. Read manufacturers’ warnings about their tools and supplies, and take them to heart. Some hazards, like those posed by sheet-metal brakes and welding torches, are pretty obvious. Other hazards, like those posed by lead filings and airborne zinc fumes, are less obvious but just as serious. If you have any questions about safety, ask them. It will be worth your effort.
I try to note some of the safety hazards in this work as I go along, but I do not know and am not able to mention all of them. As I said, if you have any doubts about the safety of some tool, procedure, or process, ask questions about it. Don’t become a victim of something that could have been avoided. You are responsible for your own safety. While I try to inform you about relevant safety hazards as you read this book, the author, editors, publisher, and agents of this book cannot ensure your safety in this work. Only you can do that.
Written by Matt Joseph and Posted with Permission of CarTechBooks