Multi-pass metal spinning from metal disc blanks (the metal spinners blank)
Process variables and limits within the spun metal
Many parameters require consideration when spinning from a metal disc blank. These are critical for metal quality, such trueness of shape, dimensional accuracy, wall thickness and surface finish.
The following deals with the effects of the most important factors as well as limitations of the process. The main requirement in metal spinning is to remove metal from the blank or pre-form to a smaller diameter until finally the diameter of the spinning chuck is achieved. This is only possible by introducing compressive stresses at tangents in the metal. As a result, the metal is compressed in a tangential orientation - increasingly towards the edge of the (yet to be) spun metal disc. However, there are precise limits to this process. As the loads increase, the metal's resistance to buckling is overcome, leading to the formation of wrinkling. The trick is to apply load within limits by progressively forming the metal blank rather than in a single pass.
Wrinkle tendency is dependant on the relationship between bland up-stand and metal thickness, “up-stand” relating to the area of the blank which is to be formed and is therefore un-clamped.
In order that the metal can withstand higher loads/requires fewer intermediate steps, the roller is traversed to produce a series of dished shapes, resulting in the area at the edge of the spun metal remaining stronger.
When the roller moves towards the edge of the spun metal, the metal receives additional stress in a radial direction. The magnitude of this depends largely on the path taken by the roller and its radius of nose. The combination of a tight arc radius with a small nose radius tends to produce high stress. If the stresses are too great for the metal, this leads to splitting along the circumference, particularly in the area close to the chuck. And so radial stresses introduce a further limit on the metal spinning process, even if this is a rare occurrence.
When the roller travels in a reverse direction (towards the centre of rotation), the spun metal is subjected to an overall load of compression. Except for the outer edge, a wave of metal forms ahead of the roller. This adds rigidity to the flange and counters wrinkling potential.
Along with failures already described, edge splitting experienced by metal spinners tends to be of lesser significance. It is usually caused by pulling the edge of the blank back during metal spinning or by overworking on severe wrinkling, formed earlier. Within limits imposed by wrinkling and splitting, the roller path can be freely set by the metal spinner. This path can be used to influence the pattern of generated stresses as a means of achieving, for instance, a specific wall thickness. Tensile loads reduce the wall thickness and so counter the thickening caused by axial compressive loads. A reduction in wall thickness can also occur. Because of limitations within processes imposed by metal failure during metal spinning and metal flow forming, there are physical limits to the degree to which metal wall thickness can be influenced. Typically 30% can be quoted as a guideline to achieve reductions. An increase in wall thickness of up to 10% may be achieved in metals which have natural resistance to wrinkling.
After progressively spinning the metal blank down, it is usual to planish and smooth the part. This results in further thinning of the spun metal, provided it can flow in an axial direction, and is only possible with metal spinning chucks which are more or less parallel-sided.
In addition to the metal spinning and metal flow forming techniques mentioned above, a blank support attachment is used to delay wrinkling. It is best to use a metal disc which makes contact with the blank over a large area and with adjustable pressure. Together with a special dished roller path, the risk of metal buckling is therefore minimised. A blank support is of benefit whilst metal spinning deep-drawing-grade mild steel up to 2mm thick, 99.5 aluminium, or aluminium alloys up to 3 mm.
Metals with pronounced anisotropy (the property of being anisotropic; having a different value when measured in different directions) tend to lobe during working. An un-uniform metal blank edge means that stresses around it's circumference cannot be uniformly distributed, which in turn encourages buckling of the spun metal. Employing a trimming knife can be useful in these cases.
As the blank edge is moved axially by the metal spinning roller it progressively touches the knife and is trimmed round. Trimming knives are usually used on aluminium, but thin-gauge mild steel up to 1 mm can be trimmed also.
Surface finish when metal spinning is a function of the specific technique applied to each metal spinning. The external finish in the micro range will be similar to that of the metal spinning roller. In the macro range, however, the finish is created by grooves in the metal surface made by the hardened steel roller. The depth of these grooves is attributable to roller shape, feed, and speed
Due to the high compression loads underneath the roller, internal finish is determined by the surface of the metal spinning chuck. Light-gauge metal tends to imitate external feed lines on the inside. A typical spun metal surface can be improved by finishing runs using a lower feed and lower speed, or by changing to a smoothing roller, specially designed for metal finishing.
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