English Wheel


English Wheel

The English Wheel (or Wheeling Machine - British English usage), is typically a manually operated metalworking apparatus, (used in car restoration, NASCAR racing car construction, and aviation amongst other uses), that allows a craftsman to form smooth, compound curves from flat sheets of metal, such as aluminum or mild steel. When used on thicker sheet metals such as boat or ship hulls the machine may be powered and be much larger than the one described here.

Construction

The machine is shaped like a large, closed letter "C". At the ends of the C, there are two wheels. The wheel on the top is called the rolling wheel, while the wheel on the bottom is called the anvil wheel. (Some references refer to the wheels by their position: upper wheel and lower wheel.) The anvil wheel usually has a smaller radius than the rolling wheel. Although larger machines exist, the rolling wheel is usually 8 cm (3 inches) wide or less, and usually 25 cm (9 inches) in diameter, or less.

The rolling (top) wheel is flat in cross section, while the anvil (bottom) wheel is domed.

The depth of the C-shaped frame is called the throat. The largest machines have throat sizes of 120 cm (48 inches), while smaller machines have throat sizes of about 60 cm (24 inches). The C stands vertically and is supported by a frame. The throat size usually determines the largest size of metal sheet that the operator can place in the machine and work easily. On some machines, the operator can turn the top wheel and anvil 90 degrees to the frame to increase the maximum size of the work piece. Because the machine works by an amount of pressure between the wheels through the material, and because that pressure changes as the material becomes thinner, the lower jaw and cradle of the frame that holds the anvil roller is adjustable. It may move with a hydraulic jack on machines designed for steel plate, or a jackscrew on machines designed for sheet metals. As the material thins, the operator must adjust the pressure to compensate.

A properly equipped machine has an assortment of anvil wheels. Anvil wheels, like dollies used with hammers in panel beating (which are also known as anvils) should be used to match the desired crown or curvature of the work piece.

Operation

The operator of the machine passes the sheet metal between the anvil wheel and the rolling wheel. This process stretches the material and causes it to become thinner. As the material stretches, it forms a convex surface over the anvil wheel. This surface is known as crown. A high crown surface is very curved, a low crown surface is slightly curved. The rigidity and strength in the surface of a workpiece is provided by the high crown areas. The radius of the surface, after working, depends on the degree that the metal in the middle of the work piece stretches relative to the edge of the piece. If the middle stretches too much, the operator can recover the shape by wheeling the edge of the piece. Wheeling the edge has the same effect in correcting mis-shape due to overstretching in the middle, as shrinking directly on the overstretched area by the use of heat shrinking or eckold type shrinking. This is because the edge holds the shape in place. Strength and rigidity is also provided by the edge treatment such as flanging or wiring, after the fabrication of the correct surface contour has been achieved. The flange is so important to the shape of the finished surface, that it is possible to fabricate some panels by shrinking and stretching of the flange alone, without the use of surface stretching or shrinking at all.

The pressure of the contact area, which varies with the radius of the dome on the anvil wheel and the pressure of the adjusting screw, and the number of wheeling passes determines the degree to which the material stretches. Some operators prefer a foot adjuster in order to be able to maintain a constant pressure over the varying sheet metal thickness for smoothing, while using both hands to manipulate the work piece. This style of adjuster is also helpful for blending the edge of high crown areas that are thinner, with low crown areas that are relatively unstretched. A drawback of the foot adjuster, is that it can foul very longitudinally curved panels, such as cycle type mudguards (wings/fenders), as used on pre-WW2 sports cars and on the Lotus / Caterham 7. To address this problem, there are wheeling machines that have a hand adjuster close beneath the anvil yoke in order to allow such panels to curve underneath unobstructed. This type of machine typically has a diagonal lower 'C' shaped frame, that curves lower to the floor, instead of the horizontal and long vertical adjuster shown in the picture. A third type of adjuster moves the top wheel up and down with the bottom anvil wheel left static.

The operator needs a great deal of painstaking patience, to make many passes over an area on the sheet in order to form the area correctly. He may make additional passes with different wheels and in different directions, (at 90 degrees for a simple double curvature shape, for example), in order to achieve the desired shape. Using the correct pressure and appropriate anvil wheel shape and pattern of accurate, close to overlapping wheeling passes, makes the use of the machine something of an art in order to produce a piece of steel, aluminium or other sheet metal with a particular physical shape. Too much pressure results in a finished product that is undulating, marred and stressed, while too little pressure causes work to progress very slowly.

Typically, only small high crown panels, (such as repair sections) or large low crown panels (such as roofs), are made in one piece. Large low crown panels need two skilled craftsmen to support the weight of the panel.

Two key limitations of the machine are: The size of workpiece that the operator/s can physically handle and also the risk of over stretching/thinning the panel section - it is no good having the correct contour if the metal is just too thin and weakened to be serviceable. This is why large high crown panels such as wings/fenders are often made in many pieces and then TIG welded or oxy-acetylene gas welded together. TIG produces less heat distortion, but produces a harder, more brittle weld that may cause problems when planishing/smoothing by hand, or in the wheeling machine. Oxy-acetylene welding joints don't have this drawback, provided they are allowed to cool to room temperature in air, but do produce more heat distortion. Panel joints may be achieved using "autogenous" welding - that is welding without filler rod, this is useful when finally smoothing the welding joints as it reduces the amount filing/grinding/linishing needed or almost eliminates it altogether. It also, more importantly reduces heat distortion of the surface contour, which has to be corrected on the wheel or with hammer and dolly. Shrinking the edge prior to wheeling, aids the formation of shape during wheeling, and reduces the amount of stretching and thinning needed, to reach the final shape.

At every stage during fabrication, constant reference needs to be made to the shape that the operator is trying to reproduce. This may involve the use of template paper, section templates (made using paper or thin sheet metal), station bucks, formers, profile gauges, profile templates and of course an original panel. Wheeling machines that feature a quick-release lever, which enables the operator to drop the anvil wheel away from the upper wheel, so the work can be removed and inserted quickly without losing the pressure setting, are great time savers during this part of the process.

The final process in the fabrication of a panel, after the correct surface contour has been achieved, is provided by the edge treatment such as flanging (sheet metal) or wire edging, this is to finish and strengthen the edge. There will be too much or too little metal in the flange, this will pull the panel out of shape after the flange has been turned, so it needs to be stretched or shrunk in order to pull the surface shape back to the correct contour. This is most easily done using Eckold type shrinking and stretching, but can be done using heat shrinking or cold shrinking, (by tucking and beating the tucked metal into itself), or by using a cold shrinking hammer and dolly. For stretching, a hammer with a dolly of the correct shape - matching the desired flange shape at the point of contact with the hammer.

Working with an English wheel is easier for many applications than manually hammering the steel, and is usually more appropriate for smooth curves than using an pneumatic hammer or power hammer, it may used for planishing to a smooth final finish after these processes.

References

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External links

* [http://www.fournierenterprises.com/MetalQA.html#10 Ron Fournier Metal Q & A at fournierenterprises.com]
* [http://tinmantech.chainreactionweb.com/html/articles.php Articles and 'How to's by Kent White on tinmantech.com]


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