Metal Selection, Gauge, Size and Composition

Metallurgy is simply the science of metal and metal alloy. Everything from selection of the correct type of metal, composition, gauge (thickness) and size, to welding, treatments, and working techniques will be better understood with a fundamental knowledge of metallurgy.

The two types of metal that you’ll be working with most frequently in automotive repair and customizing are steel (mild and, to a lesser degree, stainless), and aluminum. The difference between the various types of steel and aluminum involve the extra ingredients, or alloys, which essentially are other types of metal blended together with the base metal to enhance one or more aspects, such as strength, corrosion resistance, ductility or malleability. The weldability of metal can also be changed just by adding a percentage of one or more metals, so the first thing to remember is that a metal’s alloy content is an important factor to consider during the working stages of metal as well as for the structural integrity of its application.

At Customs By Eddie Paul, we often use 3003-H14 aluminum alloy (refer to the alloy definitions below) for much of our fabrication. The 3003-H14 has superior strength characteristics over pure aluminum and is easily welded with either TIG (tungsten-inert-gas) or oxygen-acetylene gas welders, yet remains malleable for shaping and bending. By comparison, a 6061-T6 aluminum alloy would yield even more strength than the 3003-H14, but the 6061-T6 is also more brittle and if welded, may develop stress cracks at the weld.

Following is a list of aluminum alloys defined by a four-digit numeric code to identify the alloy content. The first digit represents the main element of the alloy. The alphanumeric code that follows the four digits (i.e “H14” or “T6”) is the hardness and temper specification of an alloy. For example, a letter “F” in the temper code refers to fabricated, which is an aluminum that has not been treated for hardness. A letter “O” indicates annealed, or softened by a process of heating and cooling. A letter “H” indicates a strain-hardened alloy (hardened by cold-working), and a letter “T” means heat-treated. Generally speaking, the higher the number in the temper code, the harder and stronger the alloy.

1XXX (1000-series) is the designation for unalloyed (99 percent pure) aluminum. The 1000-series offers high corrosion resistance, excellent workability and welds easily; however, its low strength limits its use in certain applications. This is still a common alloy for use in automotive fabrication where strength is not an issue. Non-heat-treatable.

2XXX (2000-series) is an aluminum containing copper as its main alloy. 2000-series aluminum alloy provides a better strength-to-weight ratio than 1000-series and is also easy to work with. The trade-off, though, is that this alloy is not as ductile, meaning that bend radii must be fairly large and gradual, and joining pieces of 2000-series alloy must be accomplished by riveting or chemical bonding rather than welding. Heat-treatable.

3XXX (3000-series) indicates an aluminum with a main alloy of manganese. The addition of manganese yields a 20-percent increase in strength over 1000-series, yet it retains the working qualities of pure aluminum, and can be TIG or gas welded. For these reasons, 3000-series aluminum alloy is the most popular choice among automotive fabricators. Non-heat-treatable.

4XXX (4000-series) is an aluminum alloyed with silicon. Moderate strength.

5XXX (5000-series) is an aluminum alloyed with magnesium. Moderate-to-high strength. Non-heat-treatable.

6XXX (6000-series) such as 6061-T4 or 6061-T6 is commonly used in production due to its relatively low cost and excellent mechanical properties. Annealed 6000-series aluminum alloy (or 6000-series with an “O” temper code) also lends itself to forming. Heat-treatable.

7XXX (7000-series) is an aluminum alloyed with zinc. 7000-series offers the greatest strength, but is the least ductile. Heat-treatable.

Fortunately, the selection of steel suitable for automotive fabrication is more abbreviated than that of aluminum and therefore less confusing. Steel is an alloy of iron, of which there are two types: carbon steel and alloy steel. While some high-end customizers make liberal use of stainless steel, which is an alloy steel, the level of skill required to work with it is likewise at the higher end of the scale. Stainless steel is a corrosion-resistant steel commonly alloyed with a high percentage of chromium and nickel. There are many appealing structural and cosmetic qualities associated with the use of stainless steel, however, so you may want to consider advancing your skills once you have mastered the basics.

There are some fabrication jobs that we do in my shop that require the strength and weight of steel along with the corrosion resistance of aluminum. For example, the 14-foot mechanical great white sharks that I build for the Cousteaus and The Discovery Channel specials were framed entirely out of stainless steel. With constant exposure to the harsh salt water, any part of the shark structure made of carbon steel would corrode and fail within a few short days. By the way, not all stainless steel is “stainless.” Like aluminum, there are several stainless alloys with varying degrees of corrosion resistance, strength, etc.

For general automotive work, my use of stainless is usually limited to hardware items such as fasteners (bolts, nuts, washers, etc.). Occasionally, a job comes up where we fabricated portions of a frame or some brackets out of stainless. Stainless can be very easily machine-polished to a high chrome-like luster. But the cost factor for both material and labor usually keeps us working with carbon steel.

Carbon steel, a combination of iron and carbon, is used in most of the techniques in this book. But to avoid any confusion down the line, there are a few other terms that I may use in reference to steel. One is mild steel. Mild steel is simply a carbon steel that contains a maximum of 0.20 percent carbon. Mild steel cannot be hardened or tempered, but it can be case-hardened. Hot-rolled steel is a carbon steel that is brought up to a white heat during its manufacture and then passed through a series of rolls to reduce the cross section, thereby increasing its length. It is then cooled, cut to length, or coiled. Cold-rolled steel is a carbon steel that is manufactured by a process technically refered to as cold reduction. The cold-reduction process reduces, as its name implies, the thickness of steel by rolling or drawing the material without preheating it. This cold method adds strength as well as produces stock that is smoother and more consistent.

The process of hot-rolling produces a surface slag that, when compared side-by-side with cold-rolled steel, is quite obvious. The benefit to using hot-rolled is its lower cost. The more expensive cold-rolled steel is commonly used in precision sheet metal applications since it provides an excellent surface, material consistency and a more accurate thickness.

The same basic code system that defines aluminum alloys similarly defines steel. But before we get into coding, let me say that I seldom have to refer to or order my steel by code as I do with aluminum. The main reason is that I’ve developed a rapport with the metal supplier that I get all of my metal from and I simply refer to my carbon steel orders as either hot-rolled or cold-rolled. When it comes time for you to locate a metal supplier and place an order, keep in mind that a good supplier will have a catalog of the metal that they stock that usually contains a lot of useful information pertaining to sizes, gauges and alloys. And a knowledgeable salesman will also take the time to help you with your order based on your specific requirements. Still, it’s always good to know what you’re ordering so the following code definitions are part of this portion of custom bodywork. This is not a complete list of codes; I’ve narrowed it down to the basics to avoid any confusion.

1XXX (1000-series): Basic open-hearth and acid Bessemer carbon steel that is non-sulfurized. 1020-series cold-rolled steel sheet metal is a common material for automotive fabrication.

2XXX (2000-series): Steel alloyed with the addition of nickel.

3XXX (3000-series): Steel alloyed with nickel and about 1.25 to 3.50 percent chromium.

4XXX (4000-series): Steel alloyed with molybdenum or nickel-chromium-molybdenum. You’ve probably heard the term “4130 chrome-moly” a few times. 4130 is a steel alloyed with chromium and molybdenum. Stress-relieved 4130 chrome-moly is used where structure strength is most critical. Annealed chrome-moly is used for fabricating structures that require forming and bending.

The code series for steel continues up to 9XXX (9000-series) with different alloys and percents of additional metals being added that will enhance different features and characteristics of the base carbon steel. As you get more involved in sheet metal fabrication (as opposed to fabricating with bar stock or tubing), there are specific types of steel that you can use to enhance the working properties during the forming process.

My steel preference for general all-around customizing are two alloys referred to as AK and SK steel sheet. This is the metal we use at the shop for most of our metal fabrication. I would suggest you purchase AK or SK. “A” indicating the addition of aluminum during the killing process indicated by the “K” for “Killed” or in the case of SK, the addition of silicon. The metal supplier for my shop, M&K Metal in Gardena, California, has both AK and SK steel sheet stock and will sell single and partial sheets at a time, whereas some metal suppliers will only sell these metals in mega-pound quantities.

You will find this metal to be the best all-around alloy for metal fabrication of parts and panels. You will notice that if you work AK or SK steel it will not work-harden as quickly as regular cold-rolled steel does. This is a very big advantage for the fabrication of deeply contoured panels. If you cannot find the AK or SK near you at your local metal supply company, try calling any local customizer or stamping company. Many times they have already bought a few thousand pounds of it and may be willing to sell a few sheets to a fellow fabricator. I have, on many occasions, gotten together with someone else and placed a combined order; this will render a quantity discount on most items.

I use 18-gauge AK sheet metal for most of the customizing in our shop. As a rule of thumb, I try to match the gauge of sheet metal to that of the panels on the car that I’m working on. For a stand-alone project, 18-gauge is a little heavier than necessary, but this thickness does allow for deeper shapes to be formed into the metal. Although 20-gauge would be easier to cut and shape, 18-gauge sheet is perhaps the best for a beginning fabricator to start with.

Gauge: The Thickness of Steel and Aluminum

The gauge of sheet metal is a numeric reference that indicates thickness. It’s similar to the gauge scale for electrical wire in that a numerically higher gauge indicates a thinner material. This can sometimes be referred to as “the inverse law of logic as it pertains to sheet metal gauge.” Whether it’s sheet metal or electrical wire, this gauge system seems backwards to me, but we’re stuck with it.

If you make a side-by-side comparison, the same gauge number of a sheet of steel (ferrous) and a sheet of aluminum (non-ferrous) is different in actual thickness; in other words, the two sheet materials with equal measurements in thousandths of an inch will have different gauge numbers. For example, 20-gauge steel is 0.0359-inch thick while 20-gauge aluminum is 0.0320-inch thick; not a big difference, but enough to be confusing to some of the engineers out there. So 20-gauge aluminum is closer to 21-gauge steel, which is 0.0329-inch thick. Don’t ask me why, I have no idea. But it will mess you up when you are trying to match a gauge in a repair or when adding new metal, so be sure to specify the thickness, the gauge and the material when ordering your metal. Most of the material we work with in my shop is between 18 and 22 gauge.

Metal Shrinking: How To Shrink Metal and Why

So what exactly is metal shrinking? Well, to a fabricator, it’s when you literally pull or press a section of metal together into itself. Doing this doesn’t actually make any metal go away, but it reshapes it and makes that particular section of metal a bit thicker. This is one way to shape the surrounding area. By simply shrinking one section and having the surrounding area bend toward the shrink.

Now you’re probably wondering how do you shrink metal? There are a number of ways to do this. One of the basics is to use a pick hammer with a padded dolly. Then there’s the shrinking hammer and shrinking dolly, a shrinker, or an oxy-acetylene torch (hot-shrinking). In some ways a dent will shrink a surrounding panel by stretching it in a small area! Confused? Well imagine a rock hits the center of your door and puts a hemispherical dent in the metal. That rock has stretched the door metal at the area of impact, but the surrounding area has been pulled toward the impact therefore shrinking the door skin in general. Now if you use a pick hammer (which is a small body hammer with one end pointed) and back the panel with a padded or rubber-coated dolly (or a block of wood), as you pick the panel you are basically shrinking the metal inward toward the work area but with greater control than using a rock.

The reason for using a block of wood instead of a steel dolly is that if you used metal for a backing then you will be stretching the metal, not shrinking it. Pounding metal between a hammer and a steel dolly tends to thin the metal and since metal has to go somewhere it expands outward into the surrounding metal. By using a soft block and a pick hammer, you allow the metal to form small peaks, thereby pulling the outer metal toward the small peaks.

I know that I might be oversimplifying this process, but doing so will make it easier to understand the process of shrinking, and the more you understand the better you can work metal. Just remember that for every action there is a reaction, so moving metal in one spot will cause the metal to move somewhere else. The trick is to know where the reaction will be and in what direction the metal will react. Then, and only then, will you have become the master over metal.

The process of shrinking metal with the use of a torch is well known and pretty standard — except for a new twist, which metal fabricator Ron Covell pointed out to me. He no longer recommends quenching, or accelerated cooling of the metal after shrinking it. The accepted method is (or was?) to find the area that needs to be shrunk, which would be a high spot in an otherwise smooth panel, then heat a small spot about the size of a silver dollar. Then, as it turns cherry red and raises to form a small bump, simply tap the bump down slightly until the spot is level with the surrounding metal. After which you would, but may not need to, dip a shop rag into a bucket of water and quench the spot. But what Ron told me, and I agree with him on this point, is that by quenching the spot, you will harden the metal at that spot. But if you do not use the quenching method and let the area cool off naturally, the metal can be worked later without having to anneal it again. On the other hand, Brian Hatano, the main fabricator in my shop, prefers to quench the metal and is able to attain the shape that he wants.

Stretching metal is the opposite of shrinking and produces the most common mistake that body men make when working, or overworking, a panel. It is the hammer-on-dolly work that thins and stretches the sheet of steel that requires shrinking it into its proper shape.

Remember, you can shrink metal a lot, it will only get thicker; but if you stretch it too much, it will then tear as it becomes too thin to have any tensile strength.

Whether shrinking or stretching a sheet of metal, you’ll notice, if you’re observant, that the worked area actually hardens. Why does the metal get hard after shrinking or stretching? We call this “work hardening” and it’s the direct result of squeezing the molecules of metal so close together that the metal gets tougher and harder to work. If you run into this while working with metal, which I am sure you will, you can simply anneal the metal to soften it again.

In writing this book I came across the dilemma of explaining to the reader the problem of knowing when to shrink and when to stretch metal. Let’s just say, for example, that you have an area in the front part of the hood that you just extended with a section of metal and, after welding it, wound up with metal that looks like the Pacific Ocean during a storm. You now have large sunken-in areas and you know you have to shrink or stretch them to pull them back to the original shape. Well which will it be: shrink or stretch?

So to solve this dilemma I can break it down to a simple example and a rule to help you remember: If the panel is flat and it has a large “oil can” in the center indicating too much metal, you would shrink the center area, pulling the excess area together into itself and as a result “tightening the metal” and removing the oil can effect. On the other hand, if the area has a large compound curve and the oil can is in the center of this curve you would stretch the metal, forcing it to stay in one direction as opposed to canning in and out.

So the simple rule would be: for flat panels shrink; for curved panels stretch.

Now, what if a section of metal has a very slight curve or is almost flat? Then I would start by slightly shrinking and if it gets worse then resort to stretching. Shrinking will thicken the metal, which can be stretched later, but stretching the metal will decrease the thickness and make it harder to work with if you need to shrink it later. If in doubt, shrink it first. Or, as I like to say, “error on the side of thicker.”

Metal Stretching

Once again, for every action there is a reaction, and stretching metal in one spot will result in a buckling in another. To better understand the reaction concept I like to carry the example to the extreme with a whimsical example of a car that is hit in the front fender, now this car was absolutely perfect to start with in every dimension so that all the seams were 1/4-inch exactly. The average person would see a dent from the impact. But, the true scientist would notice that the small hit on the front fender closed the fender seam between the fender and the door a few thousandths of an inch and upon further measurement the rear door seam was slightly closed as well and no longer was exactly 0.2500000 inch, but 0.24999992 inch.

This is just for example and no car is that exact, but the point is that a simple tap in the front fender will result in every other part of the car being affected. So as you stretch metal in one area you should notice that the surrounding area is affected. In most cases this is just what you want so your action will cause the “desired effect” somewhere else. Once you understand the action/reaction concept you will have mastered working with metal. Many times the area you need to work is not the area with the damage, but the result of the damaged area. Or it is near the damaged area and by working this area you will relieve the stress on the damaged area.

There are stretching tools on the market—I know because we manufacture a good metal stretcher—and they do a fine job around the edge of a panel, but sometimes you will need to stretch the center of a panel and the end stretchers just don’t reach in far enough, so different methods are required. Among the ways to stretch metal, the most basic method is the “hammer-on-dolly” technique. This method requires that you place the dolly directly under the point where the hammer will strike so that each hit of the hammer compresses, or stretches, the metal.

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The post Metalworking 101: Fundamentals of Fabrication appeared first on Old Cars Weekly.

By Matt Joseph
Excerpted with permission from “Collector Car Restoration Bible.”

Sheet metal work falls into two realms: manual arts and technical skills. The strategies for and operations of removing complex deformations (a.k.a. “dents”) from sheet metal body parts are so variable as to make choosing between them an art in itself.

There is usually no single correct approach to such a complex task, and various approaches may produce virtually equal results. Of course, there are also numerous substandard or incorrect approaches to this work that can hide original damage while actually producing further, hidden damage.

Because elements of judgment, efficiency, experience and even inspiration are possible in sheet metal work, it borders on being an art. However, other aspects of sheet metal work, such as hammering, welding and knowing the effects of heat on this material, are highly technical and require a clear understanding of cause and effect before you can understand and perform them successfully. These are really technical areas that can be demonstrated scientifically. The result of all of this is that good sheet metal work requires a study of basic technical factors, experience in the actual work and imagination and ingenuity in approaching some of the more difficult problems posed by sheet metal repair.

There are several textbooks that deal with automotive sheet metal and refinish work in particular, and that devote one or more chapters to sheet metal repair in general. Some of these books are fairly useful for beginners, but many of them are designed for use in conjunction with classroom instruction. The latter really don’t work very well without it.

While it is possible to approach some mechanical repair operations with “the book open” on a fender, this will never do for body restoration and refinishing. As well, classroom guides will tend to tell you just enough to make you dangerous, and will often overlook much that is basic. That’s the best of them. The worst of them tend to describe procedures, operations and materials in ways that are completely and perfectly understandable, as long as you understood these things before you read the descriptions of how to do them.

There are two books that I think really do provide useful insights into how to deal with various aspects of steel sheet metal repair. Unfortunately, one of them is too cursory for most people and the other is too detailed. However, if you are new to this work, you should have a look at both of them.

The Key to Metal Bumping by Frank T. Sargent was first issued in the late 1930s and was basically a user’s guide to the body tools made by the Fairmont Forge Company. Various revisions and editions followed the original issue, and by the third edition (1953) this book had become a pretty good treatise on the “Fairmont Method” of dealing with sheet metal repair. The third edition also included all sorts of helpful hints regarding welding and other skills. The basic premise of the book is that you must employ a specific method to straightening sheet metal. You cannot just go in with a hammer and start banging ‘out’ things that seem to be ‘in’, or vice versa.

The method proposed in The Key… involves distinguishing between permanently deformed metal and metal held out of place by permanently deformed metal. The prescription for repair is to analyze the order in which damage occurred during the impact that caused it, and to remove it in the reverse order. The Key… is a short book and leaves a lot unsaid, but it is a good basic guide to the field of dinging out and metal finishing sheet metal.

At the times of its issue and revisions, The Key… was almost revolutionary in proposing a method of analysis and plan of attack to confront sheet metal repair work. I would suggest that the proposed plan is useful, but not the only way to approach these problems. In any case, The Key… is a good place to start the study of sheet metal work. It is also readily available from a number of old car hobby booksellers and from suppliers of autobody tools and supplies.

Automobile Sheet Metal Repair, by Robert L. Sargent (Chilton), and its newest revision, Chilton’s Mechanics Handbook, Volume 3: Autobody Sheet Metal Repair is the most comprehensive general book that I know of on this subject. Whereas The Key… makes this work sound wonderfully easy and simple, Sargent confounds the reader with the full complexity of every aspect of the analysis and remedial operations involved. This sure isn’t bedtime reading if you want to sleep at night, but if you take the time to read it and understand it, you will gain a good grasp on the theory and the practice of this work. I recommend it highly for those dedicated to learning how to perform this craft.

One thing that you will get from reading these books, or the rest of this chapter, is the concept that sheet metal repair involves more than just beating or pushing out a dent. Beyond that, there are approaches that will efficiently yield a repair that looks good, is permanent, uses no or very little filler and restores the basic integrity of a damaged panel. That, of course, is the object. However, no matter how many articles, books, pamphlets, videotapes, CDs, DVDs and seminars you absorb on this topic, experience is still essential to perfecting your sheet metal technique.

NEVER attempt to do work like this solely based on book knowledge. The best approach is to find some old body panels: doors, fenders, hoods, etc., and damage and repair them yourself to get the feel of the thing. Armed with a basic knowledge of the craft, you will learn more in five or six hours of experimentation with real sheet metal parts than you would have thought possible. I stress this point because I have seen body panels and whole cars ruined by people who thought that body work was as simple as skilled practitioners or glossy tool sales pamphlets make it look. It isn’t. Scrap panels are cheap, but the repair of the damage you can do to a treasured car will be expensive.

In addition to practicing your technique on scrap panels during your early learning, you can often try new or alternative strategies out on them. Sometimes, it’s easy to duplicate in scrap approximately the actual damage in something that you are working on. Then, you can experiment to determine what the most effective repair strategy will be. Scrap panels also provide a wonderful inventory of formed metal sections for repair purposes. It’s amazing how often you can find an area or part of a scrap panel that can be modified for a specific place or purpose that you have. This can save hours of work with rawhide mallets and shot bags.

There are many neat tricks in bodywork that can save time and promote quality, but there are also some very bad “dirty tricks.” In each case, it is important to know why something is supposed to work rather than just taking someone’s word for it. Over the years, manufacturers have come up with many tools and materials that don’t work at all, or that work only to a limited extent or in limited situations. Take, for example, panel flanging tools. There are very few applications where these tools can be used appropriately and to advantage. Mostly, they are used to save time and to reduce the skill levels that otherwise would have been required to fit panels properly for butt-welding. When used improperly, these tools stop being neat tricks and become devices of destruction. In these cases, either experience or common sense, or both, should guide you away from such misuses.

Then, there are the really dirty approaches that should never (as opposed to “almost never”) be used. Drilling holes and using body hooks or welding studs to sheet metal to pull it when it could have been pounded out from behind come readily to mind. I realize that you will see so-called “professionals” doing this stuff and, in fact, I see several examples of these and other barbaric “techniques” on display at automotive trade shows every year. They may work well enough to meet the needs of low-end commercial work. That doesn’t make them right for restoration work. Intuition and common sense should tell you which approaches are damaging and which are in the interests of the preservation of old automobiles.

One of the nice things about sheet metal work is that simple tools and simple approaches are often best suited to the needs of repair and restoration. Seemingly complex problems can often be subdivided into a series of simpler problems and tasks, and solved simply. While fancy clamping, pulling, pushing, and bumping tools are available, a few good hammers and dollies, along with the skill to use them properly, will almost always provide the best basis for restoration repair work on sheet metal.

This is not to argue against some of the sophisticated equipment and techniques out there, but just to state that knowledge and experience are always the starting points in this work, and that much of what passes for sophistication in the modern repair sector has very little application to old car restoration.

Sheet Metal: Composition, Fabrication and Basic Characteristics

The sheet metal used for automobile panel fabrication, and for some panel support structures, is a highly evolved and complex series of alloys based in the steel family. Sheet steel uses several alloying components to achieve desirable characteristics. The most important of these is carbon, which is added to steel in concentrations of between 1/4 and 3/4 of 1 percent, (usually near 1/4 percent for automotive sheet metal). Because many operations are involved in converting a basic slab of raw steel into what we call “sheet metal,” the choice of characteristics that alloying is designed to accomplish must begin with these transformations in mind. Beyond that, automotive sheet metal has to be die-formed into complex shapes, trimmed and sometimes flanged. In many cases, it also has to be weldable for attachment purposes. These needs dictate the specific constitution of the steel used in automobiles.

Numerous technical terms define the physical characteristics of steels. These include elasticity, hardness, ductility, plasticity, yield strength, toughness and so forth. Each of these terms, and several others, has a specific meaning when used to describe steel.

The descriptive terms that are of most interest to us are plasticity and elasticity. The first, plasticity, describes the ability of steel to be formed by pressure (dies) without tearing, cracking or otherwise failing. The second term, elasticity, involves the ability of steel to deform and subsequently spring back to its original shape without any change in that shape. In both cases, the key phenomenon is the presence or absence of something called “work hardening.” This phenomenon is of crucial interest to those who work with sheet metal. It involves the fact that as sheet steel is deformed (by die stamping, accidental impact or a repairman’s hammer), its crystalline structure changes with the effect that it becomes harder and thus more resistant to further change. The classic example of this is a demonstration with a paper clip, which begins life as a piece of straight wire and is then bent into its customary shape.

Yet, if you attempt to straighten one of the bends in a paper clip by grasping its straight sections 1/2 inch back from a bend and applying force in the reverse direction from which it was applied to make the bend, the wire will not straighten completely. Instead, the metal on either side of the original bend will ultimately deform before the bend is completely removed. Photos that accompany this chapter make this point with regard to a 1/2-inch-wide strip of 22-gauge body steel.

What has happened in this example is that the original bend that I put in the strip of mild steel has work hardened it to the point that, when I apply counter-pressure to it to remove the bend, I create two more deformations on either side of the original one. It is easier for the metal adjacent to the original bend to yield than it is for the metal in the original bend to yield, because that metal has been work hardened by its original deformation.

The phenomenon of work hardening is critical in the design and fabrication of sheet metal automobile panels. It is both a problem for, and an asset to, anyone who has to repair sheet metal. The asset is that the areas where dies have deformed sheet metal from its original flat state provide much of the necessary panel strength in body design. The problem is that when a panel must be straightened due to impact damage, it will have hardened in several places and in ways that may make it difficult to straighten it without inducing additional deformations.

It was hardened in the original stamping process of its manufacture. It has been further hardened by road vibration, which is particularly prevalent in configurations like pontoon fenders. Finally, impact damage has further hardened it. Now it may be difficult or impossible to get the panel bumped back into shape without dealing with the work hardening of the metal that is holding it in its deformed shape.

Sometimes you can work around work hardening by adopting a repair strategy that forces things back into place in spite of it. In the case of the infamous paper clip, it is possible to bend it almost back into a straight wire if the work-hardened legs of the bend are supported close enough to the center of the bend during the reforming operation. It is also possible to hammer it flat on a vise or anvil. In other cases, the effects of work hardening are so severe that the metal involved will readily fracture before it can be hammered or forced back into its original shape.

In these cases, heating the affected area to its “transformation temperature” is usually the best solution. This process is called “annealing.” Auto body sheet metal will lose the effects of work hardening if it is heated to temperatures of about 1,600 degrees F. and air cooled. The application of such heat allows the crystalline structure of the metal to rearrange itself in ways that undo work hardening effects. The problem is that this solution may produce a panel, or areas of a panel, that have little of the hardness that was stamped into them originally. Since the original stamping was probably designed to induce work hardening into the panel’s critical areas as an element of its structural strength, annealing can create structural weaknesses. Heating followed by water quenching (rapid cooling) is the most common solution to selectively re-hardening metal in ways that maintain some of the original hardness of the die-stamped panel.

The die-stamping process is a wonderful thing to behold in an automobile stamping plant. When you see it, you can appreciate the enormous forces at work when automobile panels are manufactured. In the stamping operation, huge dies (108 inches long dies are pretty standard for large panels) that weigh many tons are forced together under enormous pressure with sheet metal between them. The dies are often lubricated if they are the “deep draw” variety. The first action of their closing is for “binder rings” to clamp the metal at its edges before the dies deform it. If this were not done, metal would be pulled into the die and would wrinkle under the pressure of the closing die faces. More recent stamping technology employs even more massive and complex tri-axle transfer presses that literally roll shapes into metal.

Following the stamping process, trimming operations and (sometimes) flanging operations occur. In almost every case, the areas of high deformation, such as creases that run the length of a panel, are put there to give the metal strength by purposely work hardening areas that will bear stress or load in service.

The sculpted and ridged sides of automobiles are usually as much accommodations to the needs of structural design as to the whimsies of styling. Of course, some areas of great deformation are there for the necessities of function, as, for example, the formed ends of panels on a car that wrap around so that the car can end!

The die stamping operation produces three types of panel area, and infinite combinations of these three. The three basic types are: high crown, low crown and reverse crown. It is critical to distinguish between them when you repair damaged automobile panels.

High crown panels are those with a great deal of curvature in all directions. They have a rounded appearance and fall away from a point both north and south, east and west. These are, of course, panels that have been substantially deformed in the die stamping process. They usually are much easier to work with than low crown panels because they have fewer tendencies to buckle under heat or when they are hammered after they have been deformed or mildly stretched by impact or by previous repair. When high crown panels are properly finished, they tend to reflect light in a way that is forgiving, even if their exact original curvatures are not retained in repair.

In contrast, low crown panels are quite flat and have very little curvature north, south, east and west. They may have curvature in one direction, like the top of a door or fender, where the format is usually a simple bend in one direction. The slab-sided doors on Lincoln Continentals in the early 1960s are another example of low crown panels. Low crown panels have little of the internal strength of high crown panels because they underwent very little deformation and work hardening in their die-stamping process. Strength is often added to low crown panels by adding supports, or sometimes by forming them in the pre-stressed (monocoque) construction that is occasionally borrowed from aircraft design for advanced automobile design.

Low crown panels can be very hard to work with because, if they are large, any stretching will make them buckle when they are returned to their correct shapes, unless the stretched extra lateral dimensions of the panels can be chased to their edges or hidden in high crown areas somewhere else. Otherwise, they have to be shrunk accurately when they have been stretched. This can be a very difficult repair procedure.

A particularly common variant of this problem occurs in restoration work when cars with very flat doors have had those doors fill with water and rust out for several inches along their bottoms. Any welding process that is used to section-in new metal will produce some degree of heat distortion in the door skin. This must be painstakingly eliminated.

In four-door cars, the back doors usually must have contours that match the front doors, thus continuing the body lines. The door pairs on each side of the car will have to reflect light in a way that indicates that the panel match is uniform and continuous. If this cannot be done, I would suggest that the car always be parked in the middle of a large field or unlined parking lot, and away from anything distinctive that may reflect light off of its sides and indicate the problem! Good luck.

Reverse crown panels are simply high crown panels in concave configurations. Reverse crown areas are sometimes found between fenders and trunks, among other places. Like high crown panels, they are usually easier to work with than low crown panels, but they often present unique access problems.

Obviously, most old car body panels are combinations of high and low crown areas with an occasional reverse crown thrown in. When a choice is available as to where to weld a patch seam or where small amounts of stretched metal should be relocated, high and reverse crown areas are good bets, as long as they are not weakened by annealing or by changes in curvature in the process.

A final characteristic of auto body sheet metal that should be considered is its basic gauge or thickness. There are half a dozen gauge wire and sheet steel gauge standards running around out there, but automotive material is generally described by the “Manufacturers Standard Gauge for Sheet Steel” standard. In this system, the gauge number is the number of pieces of steel of a specific thickness that can be fit into an inch. Thus, 2-gauge would be 1/2 inch thick; 4-gauge would 1/4 inch thick, and so forth.

Automotive sheet metal once ran in the range of 18-gauge, which was 48 thousandths of an inch thick (actually 0.0478 inch). 20-gauge became common in more recent times, and this meant 0.0359-inch-thick metal—still a lot to work with in-bumping and metal finishing. However, more recently, 22-gauge (0.0299-inch) has become common, and now 23- and 24-gauge (0.0269- and 0.0239-inch, respectively) have appeared on the scene under the euphemistic name, “high-strength steel.” This dreaded (by real metal men) and miserable stuff contributes slightly, I suppose, to lightening automobiles, but carries with it a host of problems. The first is that the alloys used to make it are difficult to form in repair situations because they are relatively hard (high carbon) and have very little elasticity. Check out the decklids on some modern minivans and hatchbacks in any parking lot and note the dents and creases left by people’s hands when they have been overly energetic in slamming them shut.

The high-strength steels are also so thin that in areas where salt and moisture are a problem, they exhibit rust perforation alarmingly soon after their manufacture. The elaborate, much ballyhooed and highly advertised anti-corrosion treatments being applied to them are, in fact, necessitated by the thinness of the material from which cars are fabricated. There is some hope, however, because some manufacturers have begun to increase panel thickness slightly on some of their newest cars.

The gauge of the metal with which you are working may determine, in large part, the best repair approach. If, for example, at some future date people decide to restore some of the econoboxes that graced our streets and roads as new cars in recent years, they had better locate a good supply of NOS body panels before they undertake such projects. Many contemporary panels are too thin and too hard to hammer straight when they are seriously deformed. Traditional metal finishing techniques are out of the question because files tend to skate over their high carbon metal or, if they do cut, they weaken the panels grievously or cut right through them. Even disk sanding them can be a hair-raising experience if you are not super careful.

The good news is that the thick, relatively soft metal in most collector car bodies is very susceptible to straightening, welding and metal finishing. When some of the newer technologies, such as MIG (properly GMAW) welding, are applied to them, repair becomes so easy that it is permissible to listen to the radio while you are working.

Basic hammer and dolly work, shrinking operations and welding operations applied to old cars are attainable skills, not the impossible dreams that they sometimes seem to be when you attempt to apply them to most contemporary auto body sheet metal.

Acquired Characteristics in Old and Damaged Sheet Metal

The types of damages that can occur to collector car sheet metal are just about unlimited. The most common, by far, are corrosion damage and impact damage. Beyond this, each car that you work on is likely to exhibit some daring innovations in the field of possible sheet metal defects. Stress cracking occurs routinely in some areas of some cars. Wood-framed bodies often exhibit structural shifting that deforms sheet metal, while swelled framing wood can bulge sheet metal in ways that are difficult to resolve. In cars with welded and spot welded attachments, a combination of vibration and corrosion can cause things to break loose and move in ways that produce major messes.

Yet with all of these possibilities, the damage that I most dread is that done by people armed with minimum knowledge, bad attitudes, heavy hammers and the misconception that they are in the body repair business. When these types and their minions add acetylene torches, plasma arc cutters and pop rivet guns to their basic repertoire of chipped hammers and hardened-screw-tipped slide hammers, they become a definite menace to the welfare of sheet metal everywhere.

It is sometimes difficult to fathom the degree of imbecility and the resulting destruction that some of these Bondo artists have done to the panels of the poor automobiles that have had the misfortune to come under their hammers. Instead of carefully analyzing the nature of the panel damage that confronts them and repairing it in non-destructive ways, these minor thinkers apply the heaviest hammers or biggest pry bars that they can wield against damaged areas of metal, literally bashing things back toward their right places. In that barbaric process, they produce stretching, further deformation and work hardening that are difficult to correct later.

When confronted with rust or torn metal, sectioning and butt-welding are usually beyond their limited skill levels, so out come the flanging tools, brazing rods, and pop rivet tools. More damage inevitably follows.

These guys buy plastic filler by the 55-gallon drum and the only apparent limit to their use of this stuff seems to be that they never allow the weight of the filler to exceed the weight of the original automobile. Aside from the fact that this kind of work has a life expectancy of between 6 months to 2 years, it always produces severe problems when it has to be reworked by someone who wants to do it right. OK, you’ve been warned. Also, as always, avoid seeing things in stereotypes.

The two most common forms of sheet metal damage, corrosion and impact, should be dealt with in very specific ways. Corrosion damage must be detected by investigation that employs physically picking and probing, in addition to visual inspection. This may seem brutal, but all kinds of corrosion can be lurking under seemingly sound paint. Certainly, where paint has bubbled and/or blistered, there is good cause to suspect underlying corrosion. A scratch awl is your best guide to its extent. Where body contours appear to be modified, or where panels are 1/8 inch thick, or more, you will often find rust, fiberglass bandages, pop riveted roofing tin and any manner of other mischief underneath the surface.

Flanged and brazed panel patches are also frequently found under bubbling paint. Sometimes, and this is almost a pleasant surprise, filler will be used to cover dents and other impact damage because the attempted repair involved difficult access to the back of a panel or the individual making the repair lacked the skill and/or commitment to bump the panel to correct its contours. Alas, more often than not in these cases, a slide hammer and hardened screw, body hooks, or welded studs were used to pull dents out crudely, and what lurks under the Bondo is serious corrosion damage, made worse by this kind of attempted repair.

The drift of all of this is that the only proper way to repair corrosion damage that perforates sheet metal is to weld in new metal, and the only proper way to deal with impact deformation is to beat it back out in ways that produce the least stretching and buckling of the metal.

Sometimes, small amounts of filler are necessary. When this is the case, body lead (actually an alloy of tin and lead that is now commonly available in a 30/70 ratio) is really the only way to go in restoration work.

In addition to the work hardening that occurs in body panels when they are stamped and later subjected to road vibration and flexing forces, there are several other changes in autobody sheet metal that occur when there is impact damage and the attempt to repair it. The most important of these is stretching. When a panel is severely deformed in an accident, it is sometimes stretched. This means that the pressure exerted on it has caused it to become longer or wider, or both. When this happens, it also has become thinner somewhere. Unfortunately, the act of straightening a deformed and stretched panel involves hammering on its ridges and channels, either directly over a dolly block or adjacent to one. This often results in further stretching the metal because metal is made thinner when it is hammered on. Bad repairs often work harden and stretch metal. This can create a difficult combination of defects to address with proper repairs.

The opposite of stretching is “upsetting,” which sometimes occurs in impact damage but more often is the result of bad repair strategy. This phenomenon involves making an area or areas of the metal in a panel thicker and laterally smaller than it or they were originally. Hammering down a bad buckle directly over a dolly block can produce an upset because the metal may have no lateral place to go. The result is that the upset part of the panel becomes thicker and laterally smaller than it was. This defect must be corrected for the metal to assume its correct original contours. Upsetting can be dealt with in a repair situation and is, in fact, sometimes purposely induced to overcome the effects of stretching. In that case, it is called “shrinking.”

Impact Repair Approaches

Impact and corrosion damage are sometimes so severe that it is necessary to find replacement panels or to fabricate and section new metal into damaged areas. An example of a small panel fabrication and of section welding are shown and described in the photos and captions that accompany the text of the next chapter. Much of the bodywork that a restorer is likely to encounter involves minor crash damage—dents, scores and the like. It is the complete removal of such damage that can distinguish a very well restored car from one that looks like a near miss.

The most important aspect of repairing this kind of damage is to understand the material with which you are working—sheet metal—and to have some general and specific notions of how it got deformed and what kinds of actions will be necessary to return it to its original shape with a minimum of distortion, stretching and upsetting. Remember, a dolly block and hammer used the wrong way can be as destructive as the events that caused the damage that you are trying to repair.

Proceed in these matters with a very definite plan of attack. Part of that plan should be based on the known sheet metal theory that is described in this book and in the books mentioned at the beginning of this chapter. Another part of your plan will come from your experience, gained from experimentation with scrap panels. The point is, when you swing a body hammer, or decide where to begin to remove a dent, or whether to work “on dolly” or “off dolly,” your knowledge will guide you and your experience will give you an intuitive sense of what the results of a given action will be.

Prior to the publication of Fairmont Forge’s The Key to Metal Bumping in 1939, such texts that existed in the field of body repair tended to be vague and to stress the black magic aspects of the craft. Sheet metal skills tended to be passed on by oral tradition, which meant that there were some awfully good practitioners and some who were pretty bad. The Key… was a major contribution to the craft because it proposed a simple and very understandable format for sheet metal defect analysis and repair.

The nugget of the “Fairmont Method” was to logically distinguish between “direct” and “indirect” damage. Direct damage includes areas that have come into direct contact with an impacting object or objects. Indirect damage describes areas that are deformed and locked in by the results of the direct damage, but which were not actually directly impacted.

Most indirectly damaged areas will spring pretty much back into proper shape if the adjacent areas of direct damage are removed and the forces holding the indirectly damaged areas are thus released. Stamped steel has a memory that promotes this return to original format. Typically, briefcase-sized dents involve mostly indirect damage in terms of the amount of effected surface area. The Fairmont Method prescribes unlocking large expanses in sheet metal that are not deformed beyond their elastic limits by working only on those areas that are. A small “key” unlocks a big puzzle. The revelation of the Fairmont Method is that you don’t have to get a big hammer and pound mindlessly on everything that seems to be pushed in or out in a process that inevitably stretches and work hardens metal unnecessarily and counter-productively.

Instead, inspection and analysis will indicate which areas involve direct damage and therefore should be dealt with first. In addition to inspection, the application of logic will yield an understanding of the sequence in which direct and indirect damage occurred. If direct damage is repaired in the reverse order that it occurred, most of the indirect damage will be released as you go along.

More recent approaches to body damage analysis and repair strategy tend to pay more attention to what is there and less to exactly how it got there. I tend to side with the latter approach but hasten to add that, if you can determine the order of deformation of a particular damaged area, removing the constituents of the damage in the reverse order of their creation is always a good approach. It is not, however, a good idea to waste half a day theorizing about the order of creation of damage, since this is not absolutely necessary information to have in-head before you proceed with corrective measures.

In any theory of damage analysis and repair strategy, the damage itself is reduced to one or a combination of three possible constituent parts. These are V-channels, ridges and buckles (also called “rolled buckles”). These three categories, and their almost infinite combinations, cover the field. Ridges, as the name implies, are areas of raised metal, which stand out in a linear formation. V-channels are depressed areas formed into lines, the opposite of ridges. Buckles are areas that are forced and locked into the metal by the waveform created in the metal by the original impact.

Unlike ridges and V-channels, which are either results of direct damage or fairly gentle extensions from it, buckles are formed by the collapse of the metal when it is under pressure and literally has no alternative other than to collapse. Buckles often involve substantial upsetting, which is not the case with ridges and V-channels.

When you recognize and understand the genesis of these three components of damage, you will be in a position to execute an effective strategy for their removal. In large part, your actions should unlock what are usually large areas of indirect damage.

In a sense, the test of a good strategy is how little hammer and dolly work is necessary to remove damage. The analysis method works because breaking damage into components, and attacking those components logically, represents an efficient attack on the causes of the problem. The alternative, to mindlessly attack the symptoms of damage, ends up as the “bigger hammer” approach and usually fails to recognize even such obvious components of damage as bent substructure. It substitutes damaging counter-force for intellect and skill. For that reason, it usually fails.

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