Story and photos by Eddie Paul
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.”
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.