Shear forming is a versatile process for manufacturing complex lightweight components which are required in increasing numbers by many different industries. Inherent advantages of the process are simple tooling, low tool costs, good external and internal surface quality, close dimensional accuracy, and good mechanical properties of the components. In times of free market economy, it is necessary to on the one hand fulfill the increasing demands toward the quality characteristics and on the other hand to reduce the development time needed to manufacture such a high quality component. Since shear forming is a complex and sensitive process in terms of deformation characteristics this is not an easy task. To assess the overall quality of a component several, mutually contradictory, quality characteristics have to be considered simultaneously. While conventionally each characteristic is considered separately, in this paper, a statistical approach is presented which copes with the above mentioned demands and provides the opportunity for an efficient, multivariate optimisation of the process. With a minimum of statistically planned experiments, mathematical models are derived which describe the influence of the machine parameters and their interactions on quantitative as well as qualitative component characteristics. A multivariate optimisation procedure based on the concept of desirabilities is used to find the best compromise between the mutually contradictory quality characteristics. With this statistical approach a workpiece for electrical industry is manufactured which requires a very good surface quality and close geometrical tolerances.
1. Introduction Shear forming, also called power-spinning or spin-forging, is a derivative of metal spinning. It is a competitive production technique enabling conical, rotationally symmetrical components to be produced to extremely close tolerances. By shear forming a circular blank is formed incrementally to hollow bodies by a roller tool which forces the blank onto a mandrel. Figure 1 gives a schematic overview of the shear forming process. The blank is clamped centrically against the mandrel by means of a tailstock. When the complete assembly is rotated the blank is formed continuously by the CNC-controlled roller tool which moves in one single pass parallel to the contour of the mandrel. The shear forming process can only be used for conical workpieces and the profile shape of the component has to be inclined between an angle of above 10° to 18° and below 80°, depending on the material used. The contour can be of linear, concave or convex nature. For more complex workpieces, different geometries are usually combined. Typical components formed by shear forming are, for example, funnels, lamp housings, reflectors, tank ends or music instruments. Some examples of shear formed workpieces are shown in Figure 2. The resulting wall thickness in shear forming is predefined by the cone angle of the contour and achieved by controlling the gap between the roller and the mandrel so that the material is displaced only axially, parallel to the axis of rotation. The required reduction of the wall thickness is described by the sine law which states that the wall thickness of a spun component is equal to the wall thickness of the blank multiplied by the sine of the half apex angle of the cone, sinα. Due to this law, the smaller the apex angle of the cone is chosen, the higher the percentage reduction of the wall thickness of the blank and the greater the degree of forming required.
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