This chapter covers shaping, fitting, and smoothing sheetmetal with hand tools and power forming equipment. These are the operations that turn flat stock into the finished shapes that you need and want.
This Tech Tip is From the Full Book, AUTOMOTIVE BODYWORK & RUST REPAIR. For a comprehensive guide on this entire subject you can visit this link:
SHARE THIS ARTICLE: Please feel free to share this article on Facebook, in Forums, or with any Clubs you participate in. You can copy and paste this link to share: https://musclecardiy.com/bodywork/automotive-bodywork-how-to-form-fit-and-smooth-sheetmetal/
Simple Tools and Equipment
Most autobody metal work is performed with relatively simple and traditional tools like body hammers and dollies. Add a few really straight-forward but clever tools that speed work and enhance capabilities, and you have the basis for tackling most projects in this field.
Beyond that, there are several expensive, specialty tools and machines that greatly increase the speed of working with sheetmetal, and add capabilities that are beyond what is possible with simple tools. For example, the use of expensive tensioning machinery to pull out and to straighten large panels, like the sides of vehicles, can produce results that are either impossible or prohibitively time consuming if you try to achieve them with simple tools.
While the old masters of the metal crafts were able to hand hammer some very complex panels, or at least parts of them, and join those parts together with welds, this kind of work is so skill intensive and time consuming that there are only a handful of shops left on our planet with those capabilities. Advanced, modern metal-forming equipment has made it possible to accomplish what those old masters did in a fraction of the time that they required, and with no loss in quality. But that kind of equipment is exotic and very expensive, and it only applies to advanced projects.
Machines like Pullmax formers, fitted with Steck and Eckold shrinking/forming heads, cost many tens of thousands of dollars. They can accomplish truly wonderful things with incredible speed, compared to simple tools. They also require tremendous skill to operate. Sure, anyone can shove sheetmetal into one of these devices. But to know when and where to use them, at what settings and with what material movements, and for how many cycles, are issues that require a great deal of experience to get right.
While there are limits to what can be done with the simple tools I mentioned, those limits are often beyond what many metal workers assume. It is hard to communicate these limits in specific terms because they vary with individual skills and aptitudes, but I’ll take a stab at it: Generally, if you have to make something like a complete fender, or most of one, for a 1930s automobile, or the nose section for an AC or Ford Cobra, that work is best left to people with advanced skills and equipment. However, if you need to form one side of a cowl for that 1930s auto-mobile, it should be possible with fairly basic tools and devices.
In general, remember that there are limitations to what can be done without highly advanced skills and equipment, but that those limitations are pretty far out on the scale of projects that people working with autobody metal usually encounter. Before you pine for some expensive metal working machine for a specific job or task, consider if it can be done acceptably with the tools that you already have, or can access.
Applying Plasticity/Elasticity, Work Hardening and Annealing
The characteristics of plasticity, elasticity, and work hardening were discussed, in detail, in Chapter 1. Here, we will see how they apply to actual metal work.
Plasticity is the characteristic of sheetmetal that allows it to be deformed without breaking. This characteristic comes into play whenever its shape is changed. If metal’s plasticity is exercised under tension, such as die stamping it into a panel, it will be stretched. This is normal. However, if metal is deformed in a collision, or if it is stretched beyond its plastic limit in the process of fabricating it, this must be counteracted. That is, the metal must be shrunk. This is done by upsetting it, literally compacting it into itself, so that some of its lateral dimension can be exchanged for increased thickness.
The opposite is also true. If metal is shrunk in a collision or in fabrication, by being deformed while it is under compression, then upsetting occurs. This amounts to shrinking. Areas affected by this kind of shrinking must be stretched back to the point where they can assume their correct shapes.
Elasticity is the ability of metal to bend, up to a point, and then return to its original format by simply releasing it from the force(s) that bent it, or that are holding it in its modified shape. This ability of metal to remember its last stable configuration is an important ally for anyone working with sheetmetal. It is often referred to as memory.
Work hardening is the characteristic of metal that causes it to become progressively harder to deform in those area(s) where its elastic limit is exceeded as its shape is changed.
The applications of the rules of plasticity, elasticity, and work hardening are critical in any but the simplest work with body metal. Each of these factors would become an insurmountable obstacle if it could not be counteracted relatively easily fortunately, it can be.
Stretching is the most common problem in auto body work. Correcting it requires shrinking the metal in the affected area. There are various approaches to doing this. The traditional method is to heat a small area of the stretched metal an area roughly between dime and nickel size with an oxyacetylene torch to red hot. It is heated until a combination of its expansion, and its being bounded by the unheated and unyielding metal that surrounds it, causes it bulge up. The bulged area is then quickly hammered down with a body hammer, while it is supported by a dolly that is held behind the un-bulged metal surrounding the heated spot. The bulged spot is hammered back to level with the panel, but no further, as this would cause the metal to be hit directly against the dolly, which would re-stretch it. In the traditional torch-shrinking method, a damp sponge may be applied to the shrink spot to quench it. That stops the shrinking, and controls the result of the operation. When using this shrinking technique, it is common to use a pattern that groups five spots (four of them around one in the center).
This method takes some practice, but works well. Other methods of shrinking include serrated spinning discs mounted in body sanders or grinders. The disc’s serrations impact and heat metal at high spots. These combined actions, heating and impact, tend to upset and shrink these high spots—but not necessarily stretched spots. There are also numerous shrinking attachments for MIG and resistance (spot) welders that work with varying effectiveness to shrink metal. For mild shrinking, there are hammers and dollies that are patterned, or that actually move parts of their surfaces, to pull metal together as it is hit and to upset it.
The hardest things about shrinking are to know where to shrink, and how much to shrink. Unfortunately, something called false stretch compounds this difficulty. Basically, where you see a bulge or wave in metal may not be the origin point of an apparent stretch. What appears to be a stretched area in a panel may be set up by an actual stretch that is many inches away from an apparent deformation. Your experience will help you to learn to deal with this issue. For now, be aware that it exists. In metal work, shrunk or upset metal probably is not as common as stretched metal, but it can cause similar havoc in the shape of a panel. Shrunk metal is often a result of the upsetting of an area(s) of a panel in the course of dinging dents out of it. Removing upsets of this sort is refreshingly simple, again involving an exchange of lateral dimension for thickness. And, again, knowing where to stretch metal is more difficult than stretching it because stretching metal only involves making it thinner and, thereby, laterally broader. You can do this by hitting it between a body hammer and a dolly, or with a planishing hammer, or other type of power hammer.
Work hardening may progress to the point that metal becomes so hard and resistant to further movement that it fractures rather than yield to your attempts to change its shape with tools like hammers. When this happens, the solution is to anneal it. This reforms its crystalline structure to make it soft and workable again. It is done with steel panels by heating the metal with an oxy-acetylene torch to a temperature between 1,550 and 1,600 degrees F (between bright red and salmon red), and allowing it to air cool. Sometimes, depending on the metal and the situation, it may be advantageous to apply a damp sponge to the annealed surface, after it has cooled substantially, to slightly enhance its stiffness, and to give it structural strength.
You will need to experiment with annealing to master when and how to use it. When you have done this, you will find that annealing will join shrinking and stretching as one of your best allies in metal work.
Hammering Techniques that Work
Hammers and dollies are the basic tools of sheetmetal work. Hammers vary in size and configuration. They range from configurations that are flat, to those that are highly crowned, and from square faces to round faces. There are also pick hammers, designed to raise small areas of metal in very small increments, and specialty hammers for accessing hard-to-reach areas, or for performing special jobs like door skinning.
Hammers should have smooth, clean striking surfaces, to avoid scarring what they hit. Good hammers have a feel and balance that makes them natural and comfortable to swing. They are best swung with the arm, from the elbow, with a slight flexing or unwinding of the wrist. The motion against the metal for most procedures should be a slapping action that allows rebound, sometimes with a little bit of sliding thrown in. This is not like driving nails.
Hammers should be held somewhat loosely, and with a limber wrist behind them, to allow them to rebound. You should pay attention to that rebound because it contains information about what is happening to the metal that you are hammering. The sound that a hammer makes also communicates information about what its impacts are doing to the metal. Most beginners, and a few professionals, tend to hit too hard with body hammers, expecting one or a few master blows to move the metal. In most situations, it is far preferable to use several lighter blows. Good metal workers develop distinctive rhythms and timbres to their hammer blows.
Some jobs are best performed with specialty hammers like those with rubber, rawhide, or plastic heads. Choosing the right hammer for a task involves both experience and personal preference.
Dollies are used to support metal that is being hammered. In some cases, such as in tight-access situations against the backs of fenders, they are also used as hammers. Most dollies are made of cast iron, and present several different and useful contours for working surfaces. When hammering metal that is supported by a dolly, there is the critically important distinction between on-dolly and off-dolly techniques. Work on-dolly means that the dolly directly supports the metal that you are hammering and is placed exactly under and in contact with the area that is being hammered. This means that you are hitting the metal between the hammer and the dolly. The inevitable result is to stretch that metal. Sometimes this might be your object, or part of it, but sometimes it produces the unwanted result of stretching.
Hammering off-dolly is much more common, and usually more useful. In this technique, the dolly is not held directly under the metal that is hammered, but offset from it. An example would be holding a dolly under one or the other side of a ridge that is being hammered down. The result is to level the ridge to the panel. There may be some unwanted upsetting of the metal that is hammered this way but this can be corrected easily, later.
Hammering off-dolly makes good use of the rebounding action of the dolly, after it is impacted by the metal that is being struck against it with a hammer. After the hammer blow is struck, the dolly rebounds against the metal and acts to push it out, toward the hammering force. For this to work, the dolly must be pressed against the back of what you are hammering. You can easily imagine that driving a configuration like a ridge down at its center, while holding a dolly, alternately, under each side of the ridge, tends to level the panel, and remove the ridge. As the ridge goes down, the metal bordering it is kept level by the dolly’s rebound action. Various specialty dollies are available in many different shapes and, in some cases, are clad in relatively soft materials, like rubber, to give them resilience, or dampening.
Shot and sand bags are very useful for hammer forming three-dimensional shapes. These bags can be filled with steel or lead shot, as well as sand or other materials. They are used to back up metal in a somewhat yielding manner. As you hammer metal on a shot bag, it dishes out. This provides relatively smooth forming and controlled stretching in the same operation. Shaped plastic mallets, used with shot bag backing, is a particularly effective hand-forming combination.
Every autobody practitioner has some favorite backing surface for hammering metal. These can range from anvils to blocks of various woods, and even plastic materials. One of my favorite backup surfaces is between one and three thicknesses (layers) of heavy, corrugated cardboard.
Bending, Beading and Prying
The fastest way to move a lot of sheetmetal in a broad area is with devices that bend and bead it. Bending and beading apply more to fabrication than to repair. Prying, another form of mechanical bending, is used mostly in repair work. The mainstays of equipment for bending body metal are brakes and slip rolls. Brakes are used to make straight-line bends, in sheet stock, to very precise angles. They also can be used to radius flat material by applying numerous, successive, small bends to it. Finger brakes, or box and pan brakes, are useful for making bends in local areas, with standing metal on one or both sides of those bends. Slip rolls are used to impart permanent curvature in one plane to panel materials. Bead rollers are specialty tools that are capable of rolling shallow beads or other shapes into flat or slightly curved sheetmetal.
Picks and pries are used locally to move metal, particularly in poor access areas, where hammers and dollies cannot reach it.
All of the tools that are used for bending, beading, and prying represent non-impact methods of moving and modifying metal.
Eventually someone realized that the action of striking metal with a hammer on a dolly could be mechanized, thereby greatly increasing the amount of force and frequency of its application. This realization led to some pretty violent devices for forming metal. The most famous of the early versions of these were the Pettingell and Yoder power hammers. These were huge, noisy devices that used a wide variety of shaping/stretching dies to greatly speed the process of custom forming metal.
Over the years, power hammers evolved into much more compact, quiet, and effective machines. Fore-most in the modern crop of such devices is the Pullmax, a machine used widely in prototype and advanced metal restoration shops.
In contrast to the earliest power hammers, modern machines, like the Pullmax, are as often used with the likes of Eckold shrinking dies and Steck shrinking/shaping dies. These are general-purpose heads that can form and/or shrink metal very locally and with no fuss. They are relatively quiet and easy to use. The tricky part of the proposition is to know when, where, for how long, and at what pressure settings to use them. Before you add a Pullmax or other power hammer to your want list, you should know that these are very expensive machines that are in the province of professional, not amateur, use.
Pulling Approaches to Moving Metal
So far, most of our attention has been directed toward hitting metal down with a hammer, or using a dolly to hit or rebound it out. There are also times when it is desirable to pull metal. These situations are some-times encountered in repair work. In the most elaborate processes, pulling plates are soldered, brazed, or welded to areas that require massive pulling force to return them to something close to their original positions. Then mechanical or hydraulic force is used to pull them out by the plates. This is very heavy duty repair work that requires considerable equipment.
Smaller scale pulling is commonly performed to remove dents, where most of the displaced metal is locked out of position by a very small area of metal, and where access issues prevent using impact tools to push out that small area. Manual and mechanically activated suction cups can be used for very light duty pulling. Some shops employ the barbaric practice of using a slide-hammer to push or puncture (or shoot) a hardened screw through an area of a panel that is to be pulled. Then, the screw is tightened into the metal by turning it, and the slide-hammer is operated in the other direction to pull the metal out by the screw. Avoid this rough approach.
A more refined version of this practice is to use a stud welder to weld a steel stud to a depressed area of a panel, and then to use a slide-hammer with a special clamp, that holds the stud’s head, to pull the metal out. When this operation is finished, the stud can be ground level to the panel.
Smoothing, Stretching, Shrinking and Forming Operations
Two of the simplest machines made for metal working, the English wheel and the planishing hammer, are extremely useful for basic fabricating jobs. These exist in both relatively inexpensive and high-end versions.
English wheels were among the earliest applications of machines to metal forming. While these devices are only powered by human muscles, knowledge, and imagination, they are almost always larger items than can be hand held, and are incredibly useful for stretching, forming, and smoothing metal for fabrications.
The basic device is a C-clamp-shaped unit with two opposing wheels that can be incrementally tensioned against each other. The wheels usually differ in diameter, while the tension between them is adjustable. The top wheel is generally flat, and much larger than the bottom wheel. The bottom wheel usually has varying degrees of lateral curvature, and is almost always available in different contours.
The principle of the English wheel is that as metal is pushed and pulled between the tensioned wheels, the pressure stretches and forms it. The curvature and thus the contact patch area and resulting pressure of the shaped wheel helps to determine the contour that is worked into the wheeled metal. Stroking the metal through the wheels at different angles makes it possible to form almost any curved or dished shape. It takes considerable practice to know where, with which wheel combinations, with what pressures, and for how many strokes to use an English wheel. When you begin to learn how to determine and combine these variables, it is amazing what you can accomplish with this simple device. Wheeling is often performed after some kind of impact procedure, like hammering metal into a shot bag, has been used to rough out a shape in it. In these cases, wheeling can fine tune the format of the metal, and smooth out the results of the impacts used to form it before it is wheeled.
Unlike the power hammering devices mentioned earlier in this chapter, English wheels vary from inexpensive to very expensive. Even if your use for one is only occasional, you still may be able to justify buying a less-expensive version of this very versatile and useful tool. For serious jobs, the larger and more stable English wheels work far better than the cheap ones.
Planishing hammers are relatively inexpensive air-driven power hammers that first appeared on the scene as devices intended for removing dents from fenders and from the turret tops of some automobiles that arrived in the mid 1930s. They are basically C-clamp-shaped frames that hold two opposing working surfaces: a small anvil, and a forming hammer. The hammer is operated as a pneumatic percussion device, with a rapid cycle rate. Put simply, metal hammered between a planishing hammer’s members gets pounded, often. The force of that pounding is adjustable by varying either the air pressure supplied to the device, the length of the hammer’s stroke, or both.
As autobody tools, the original planishing hammers were pretty poor because they stretched metal badly. However, some genius figured out that if you mount a planishing hammer on a stand, and supply a foot control for its air supply, you have a device that is capable of stretching and forming metal very quickly. Modern planishing hammers vary from being very inexpensive tools that use muffled zip guns to drive their hammers, to being very precise and somewhat expensive tools that are easier to control, and quite predictable. Again, somewhere on that continuum, there may be a planishing hammer that fits your needs. These tools can do forming very quickly and without rough
Written by Matt Joseph and Posted with Permission of CarTechBooks