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Definition
Flow forming is a metal forming process, classified under DIN standard 8583, used for the production of hollow components with nonuniform thickness distribution. During a flow forming process, a hollow metal blank, a tube, or a disk is fixed on a rotating mandrel and plastically deformed by one or more rollers due to the fact that the distance between the mandrel and axially moving rollers is at least in some areas of the workpiece smaller than the initial thickness of the workpiece.
Theory and Application
Flow forming comprises processes during which rollers are moved along the axial direction of a rotating preform. The movement of the rollers leads to a reduced thickness and increased length of the part. Longitudinal sections of the created products can be cylindrical, cone-shaped as well as convex or concave curved. Processes leading to cone-shaped or convex or concave curved longitudinal sections are covered by shear forming. Cylindrical shapes of longitudinal sections can be achieved by cylindrical flow forming. This variant of flow forming is described in the following.
Typical products made by cylindrical flow forming possess good surface quality and high geometrical accuracy. They can be found, e.g., in hydraulic or pneumatic cylinders, wheels, and gearing mechanisms.
The principle of cylindrical flow forming is shown in Fig. 1. The blank or tube is fitted into the rotating mandrel. The rollers move along the workpiece axis and deform it plastically in the vicinity of the contact zone. In doing so, the initial wall thickness S0 is reduced to S1 and material flows in the axial direction. At the same time, the initial length of the workpiece L0 is increased. The final workpiece length can be calculated under the assumption of volume constancy. By adapting the parameters feed rate v as well as the attack angle γ and the trailing angle δ, a desirable material flow can be achieved. Small attack angles lead to a large circumferential material flow. In consequence, bulge formation in front of the rollers can be observed. Too large attack angles on the other side induce surface waviness (Wong et al. 2003).
Improvements of cylindrical flow forming are achieved by usage of several rollers which are shifted in axial and circumferential direction. Since this measure is very effective, it is often applied. It also allows for different shapes of the rollers. This can help to overcome the limitations of material flow caused by the selection of a specific value of the attack angle.
According to the direction of the axial flow, two variants of cylindrical flow forming can be distinguished: forward and backward flow forming. Figure 2 shows the kinematics of these two variants. During a forward flow forming process, the material flow possesses the same direction as the feed rate of the rollers. During a backward flow forming process, the material flow is directed in the opposite direction to the roller’s feed rate. This is provoked by stoppers which enforce a movement of the spun tube or blank on the mandrel. The main advantage of backward flow forming is the state of stress in axial direction. Since compressive stresses act in axial direction, materials with low tensile strength can be formed. The main disadvantage of backward compared to forward flow forming is the need for relative movements between the deformed workpiece and the mandrel.
Recent developments of flow forming technology aim at more complicated geometries. Gears can be produced by using mandrels with gearings. The main challenge in these applications is the durability of the mandrels. In order to improve mandrel lifetime, Rachor proposes to use a ring-shaped tool instead of rollers (Rachor 2003).
References
Rachor C (2003) Investigation of different principles of flow forming for the manufacture of internal gears (in German) Shaker
Runge M (1993) Spinning and flow forming (in German). Leifeld Metal Spinning
Wong CC, Dean TA, Lin J (2003) A review of spinning, shear forming and flow forming processes. Int J Mach Tool Manuf 43:1419–1435
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Groche, P. (2015). Flow Forming. In: The International Academy for Produ, ., Laperrière, L., Reinhart, G. (eds) CIRP Encyclopedia of Production Engineering. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-35950-7_16750-1
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DOI: https://doi.org/10.1007/978-3-642-35950-7_16750-1
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