English Wheel
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The English Wheel is a manually operated metalworking apparatus that allows a craftsman to form smooth, compound curves from flat sheets of metal, such as aluminum or mild steel.
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/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 screw jack on machines designed for sheet metals. As the material thins, the operator must adjust the pressure to compensate.
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 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.
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 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.
The operator makes several 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.
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, it may used for planishing to a smooth final finish after these processes.
[edit] References
- Tim Remus. Ultimate Sheet Metal Fabrication. ISBN 0-9641358-9-2.
- Ron Fournier. Metal Fabricator's Handbook. ISBN 0-89586-870-9.
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