English Wheel - Operation

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 over-stretching in the middle, as does 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. 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.

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 get in the way of very longitudinally curved panels, such as the cycle type mudguards (wings/fenders) used on motorcycles, pre-WW2 sports cars, and current open-wheeled cars like 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, with a hand operated adjuster close to the anvil wheel holder, instead of the horizontal and long vertical hand adjuster shown in the picture. A third type of adjuster moves the top wheel up and down with the bottom anvil wheel left static.

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 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 (or actually overlapping with low crown anvils), 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. Localised wheeling on one part of the panel is likely to cause mis-shaping in adjacent areas. Correcting that mishape, may itself cause knock on effects in other areas, this is because the tensions in the panel caused by stretching, affect the panel shape further away than might be imagined. This needs the operator to have to work over a large area of the panel, having to fix these side effects and causing more side effects in the process that also need to be fixed. It is a time consuming and fiddly iterative process, that is one of the most difficult and skillful parts of wheeling. Those new to wheeling frequently find this very frustrating. This is also a reason that very large panels can be very difficult to do and are made in sections. High crown panels/sections may need to be annealed due to work-hardening of the metal. After the correct basic shape has been achieved with the correct amount of metal in the right place, the edges of high crown areas have to be blended with the low crown areas, so that the surface contour goes from one to the other smoothly. After this the final wheeling stage is very light pressure wheeling to planish the surface to make it a smooth and cohesive shape. This stage does not stretch the metal but moves the already stretched metal around, so using the minimum anvil pressure and as wide an anvil as is possible with the panel shape, is essential.

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.

Five key limitations of the machine are:

• The thickness of the sheet that the machine can handle.
• Fitting the work piece in the 'throat' depth of the machine.
• The size of work piece that the operator/s can physically handle.
• The risk of over stretching/thinning an over-large high crown panel/section - it is no good having the correct contour if the metal is just too thin and weakened to be serviceable.
• As the size of the panel/section increases, the work involved and the level of difficulty increases disproportionately.

These limitations are the reasons why large high crown panels such as wings/fenders are often made in many pieces. The pieces are then welded together usually with one of two processes. TIG welding (Tungsten Inert Gas) 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.

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 or shrinking the flange, a hammer of the correct profile with a dolly of the correct profile is needed - matching the desired flange shape at the point of contact through the flange, (known as ringing the dolly) with the hammer.

An English wheel is a better tool for a skilled craftsman for low-crown applications than manually hammering. Planishing manually using dollies and slappers, after hammer forming is very labour intensive. Using a pear shaped mallet and sandbag to stretch the sheet metal (sinking), or by raising on a stake, speeds up the fabrication process for higher crown sections. A pneumatic hammer or power hammer is faster still. The English wheel is very effective when used for planishing, (for which it was originally patented in England), to a smooth final finish after these processes.