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How it Works: Panel Beating

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Panel Beating

Panel Beating
These days, vehicle repairing on a commercial basis requires only a small but declining number of traditional panel beaters. The reasons for this decline are economic: panel beating is labour intensive and in the vehicle-repair industry it is far cheaper to strip off a damaged wing and replace it with a new one (unless the damage is very minor), than to employ a panel beater on time-consuming rectification. Indeed, so complex are some built up welded sections used' in car manufacture today, that traditional methods of repair by stretching and shrinking the metal are often impossible.

In the 1920s most automobile bodies had wooden frames of beech, oak, or ash. Relatively heavy sheet-metal panels were wrapped around this framework and fastened with nails or screws. As for the metal sheets, simple curves were the rule; curves that could be formed by passing the sheets through rollers. All the parts were assembled on a substantial channel-section chassis. In the 1930s, heavy press tools started to come into general use. The press tool stamped out a flat sheet of metal into intricate shapes - a door, a bonnet, a facia panel. Today, five-ninths of a typical modern car is built up from over 500 different sheet metal pressings, most of which are welded together.

From the 1970s onward pressings began to replace castings, and the old, heavy chassis virtually disappeared. Instead, the modern mass-produced car was of monocoque construction, that is, a structure in which the body is integral with, and shares the stresses with, the chassis. The monocoque gains its strength from the whole. Strength and rigidity are given to the sheet metal parts by the curves and angles pressed into the sheets. To understand the reason for this strength, consider a flat piece of sheet which can easily be bent by hand. Now fold up an inch of the sheet at right angles along one edge, and the piece will acquire rigidity in one plane; fold up the opposite edge to make a U shape, or channel, and it is stronger still. Now close across the top of the channel with another sheet lightly welded into place and the square metal section so formed will have attained immense strength for its size and weight.

Welded U sections such as these form the main frame-work of most cars today, the closure to the square section being completed by other components as they are welded into place. Today there may be one pre-prototype and several prototypes of a car before it enters production; it is in pre-prototype construction that the traditional panel beater can exercise their talents to the full. A new model usually starts its life as a scaled down replica in clay, which is repeated as a full size model in wood. The panel beater will then have drawings, wooden patterns and the full-size model to enable them to layout and mark up the metal sheet, form it and test it for fit. Intricate shapes can be made in a number of separate pieces which are welded together on a large steel layout table marked out with the design.

Malleability



For the prototypes, a number of low-cast aluminium or zinc dies, with hardened surfaces, will be prepared from the hand-made components already constructed, so that the hand working is reduced as far as possible. To understand why some metals can be easily bent, why others are tough or brittle, why some rust yet others are stainless, is to appreciate a vast subject. However; there are some basic properties of metal which the panel beater utilises to his advantage. The malleability of a metal is its ability to be hammered or pressed permanently out of shape without fracturing. This malleability is the opposite of brittleness - a brittle metal can be fractured with a heavy and sharp blow, but can possess great strength under normal usage.

Different metals possess different co-efficients of expansion - that is, the amount by which each metal expands under heat or contracts under cold. Bend a flat sheet of steel into a gentle curve and it will spring back; bend it still further and it will crease permanently. This is because it has been stretched beyond its elastic limit; its elasticity can, however, be restored by subsequent treatment. Differences between steels are caused largely by the addition of varying amounts of different elements in the smelting furnace - a high proportion of carbon will make the steel increasingly brittle and therefore less malleable, for example. Other differences are caused by the treatment given to metal while it is being worked - if cold metal is repeatedly hammered or bent in the same place, it becomes work hardened and brittle as stresses are set up.

A leather sandbag is used as support when using the blocking hammer
A leather sandbag is used as support when using the blocking hammer.


A planishing hammer is used to smooth out a rear light housing
A planishing hammer is used to smooth out a rear light housing.

Annealing



To remove these stresses and make the metal malleable once more, it is heated and allowed to cool slowly. This is called annealing. Steel sheets are available in various grades to suit different applications: the most useful is cold rolled commercial quality, which has a low carbon content and is highly malleable and has a good finish. The jvarious thicknesses of sheet steel commonly used in body building can be identified in more than one thickness gauge or measurement system. The British Standard Wire Gauge (SWG) used for sheet metal was introduced in 1894: the Birmingham Gauge in 1914. These two, apart from metric, were the only systems used in the UK. In the USA, the 'US Government Standard' is used. Therefore, although thicknesses in different gauges are very similar, if an exact match is required it is essential to quote both the gauge number and gauge system.

Aluminium sheets, half-hard grade, are still used in the automobile industry. At one time there were a number of aluminium-bodied vehicles on the market, but sheet steel is stronger and more easily worked into the intricate shapes required, so that this use of aluminium declined for mid-priced cars, although it remained widely used for coaches and vans requiring large, lightweight flat sheets - and has become popular with sports cars because of the weight savings. The hand tools used include at least seven different types of hammer, of which the most important are the blocking, planishing and the shrinking hammer.

The Blocking, Planishing and Shrinking Hammer



The blocking hammer is used for heavy blows, and the planishing hammer for smoothing and final finish. The shrinking hammer is similar in shape to the plenishing hammer, but its striking face is coarsely ribbed. The pick hammer has a long curved sharp end for lifting up a dented area. Bumping, dinging and flipper hammers have self-explanatory titles. Other essential tools include the mallet, left and right-hand panel cutters, snips and an assortment of dolly blocks ('dollies'). These are cast iron or steel blocks with flat or curved surfaces of various radii to match the inside shape of the panel. Their names are descriptive of the job for which they are intended heel, toe, low crown, beading, wedge, shrinking and button dolly, and the long-reach dolly for double skins.

Dollies are used to hold one side of a panel firmly while the other side is hammered during the process of forming or restoring shape to the work piece. Then there are flat and curved files for the removal of rough edges and surplus filling. Body spoons have their uses in confined spaces while dog clamps ('dogs') are used to straighten items such as bumper bars and brackets. Other common tools include cold chisels, pliers, hacksaws and punches. A variety of shapes can be produced in a steel sheet by the use of the wooden shaping block - typically 15 inches square with a 4 or 5 inch hollow, 1 inch deep, scooped out of the end grain.

Another tool is the leather sandbag. Both of these are used in conjunction with the blocking hammer. Even the most versatile of panel beaters will avail himself of the many machines designed to lighten his load and speed production. Such tools include the guillotine, for shearing off metal from the sheet; the folding machine, which will make both rectangular and curved folds of almost any radius, rollers for producing curved panels; swaging machines for raising moulds ('swage lines') along a panel; rotary shears for cutting out irregular shapes; and the burring machine, that will turn up the edge of a panel at right angles. The wheeling machine - a large and a small-diameter wheel mounted one above the other - is widely used for shaping, for example, a gently curved panel, which is passed to and fro between the wheels, stretching the 'wheeled' area until the required shape is achieved.

Foot-operated electric hammers, sanders, pneumatic chisels, stretchers and oxy-acetylene welding plant are among the many appliances available. Of materials, body solder is widely used to build up sunken areas that cannot be restored, while plastic filler is employed on smaller dents and cracks. Shaping a new panel may require cutting, shrinking or stretching. For cutting, shears or a guillotine are used. Often, a pattern will first be cut out of thick paper or card, perhaps removing wedges to form a rounded shape, and then used to mark out the metal panel. Corresponding wedges are cut out of the metal, then the edges are drawn together and welded. If only a small wedge shape needs removing, the piece can be shrunk by 'blocking', that is, laying the sheet over the cup-shaped depression in the wooden block already described, and striking one or more heavy blows with the blocking hammer until a crease is formed, running towards the edge of the sheet. This crease is now 'worked out' of the sheet, by lighter hammering.

Working with Aluminium



A reverse shape can be attained by forming the crease over a dolly. The panel can be stretched, again imagining a wedge shape, by beating out from the tip of the wedge and gradually increasing the width of the beaten area and the force of the blows up to the panel edge. When working with aluminium, it must be remembered that it is a much softer material than steel, and less force is therefore required to manipulate it. A smooth wooden mallet and sandbag are therefore the preferred tools to use with aluminium, unnecessary hammering being avoided. Aluminium can be both stretched and shrunk in the same way as sheet steel, but a sander is not used, as it tends to tear the metal away, leaving deep grooves.

With damaged body panels, it is essential to apply correcting forces in a directly opposite direction to the impact that caused them, and to take out ridges and dents in the reverse order to that in which they were made. If this procedure is not strictly adhered to, a well-finished job is virtually impossible. When a body panel is damaged by impact, the metal is stretched, usually inwards, leaving a depression with more or less well defined non-elastic edges. These edges can be gradually beaten out using a hammer and dolly, but the stretched centre will remain as a bulge, which must now be shrunk. One method used is to heat the centre of the bulge with an oxy-acetylene burner until it is red hot, then to hammer it causing the bulge to collapse but leaving a wrinkled area. Then, while the metal is still red, the ridges' are-hammered out with the planishing hammer and dolly-with a smoothing action, towards the scooped out of the end grain.

The Leather Sandbag



Another tool is the leather sandbag. Both of these are used in conjunction with the blocking hammer. Even the most versatile of panel beaters will avail himself of the many machines designed to lighten his load and speed production. Such tools include the guillotine, for shearing off metal from the sheet; the folding machine, which will make both rectangular and curved folds of almost any radius, rollers for producing curved panels; swaging machines for raising moulds ('swage lines') along a panel; rotary shears for cutting out irregular shapes; and the burring machine, that will turn up the edge of a panel at right angles. The wheeling machine-a large and a small-diameter wheel mounted one above the other-is widely used for shaping, for example, a gently curved panel, which is passed to and fro between the wheels, stretching the 'wheeled' area until the required shape is achieved.

Foot-operated electric hammers, sanders, pneumatic chisels, stretchers and oxy-acetylene welding plant are among the many appliances available. Of materials, body solder is widely used to build up sunken areas that cannot be restored, while plastic filler is employed on smaller dents and cracks. Shaping a new panel may require cutting, shrinking or stretching. For cutting, shears or a guillotine are used. Often, a pattern will first be cut out of thick paper or card, perhaps removing wedges to form a rounded shape, and then used to mark out the metal panel. Corresponding wedges are cut out of the metal, then the edges are drawn together and welded. If only a small wedge shape needs removing, the piece can be shrunk by 'blocking', that is, laying the sheet over the cup-shaped depression in the wooden block already described, and striking one or more heavy blows with the blocking hammer until a crease is formed, running towards the edge of the sheet. This crease is now 'worked out' of the sheet, by lighter hammering.

Welding and Riveting



Both welding and riveting methods are used to fix panel-beaten steel and aluminium sheets, although special techniques are required for aluminium. For welding, an oxy-acetylene torch is required to join, for example, two butting edges of flat plate, or one channel section to another. If the weld is visible, it must be sanded down, with any pits filled to the same level as the surrounding surface. Electric spot welding is used to join two overlapping surfaces or folded edges. Riveting is mainly confined to commercial-vehicle roofs and panels, since the: head of the 'pop' rivets mostly used today remain visible. However, the method has its advantages for the do-it-yourself car enthusiast, since pop-rivet-setting tools can be used in confined spaces and access is needed to only one side of the work. And the humble nut and bolt need not, of course, be neglected.

Also see: Panel Beaters
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