Deep rolling

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The hard rolls is that converts method to positively influence the peripheral zone properties of a component. The process is characterized by the fact that it is the only mechanical process to increase the service life of the component that smooths the surface, induces residual compressive stresses and works hardening of the edge layers. This method is particularly suitable for components that are subject to dynamic stress under operating conditions and can therefore be destroyed by material fatigue. The interaction of a reduction in the roughness depth and the work hardening results in an up to fivefold increase in the use of deep rollingVibration resistance and thus a noticeable increase in the service life of a component.

advantages

Compared to other methods (e.g. shot peening ), deep rolling is an extremely economical process, the field of application of which extends over almost the entire spectrum of metallic materials. Deep rolling tools are characterized by the fact that they can be used on conventional machine tools. In this way, a workpiece can be deep-rolled in the same set-up immediately after machining. Setup times and transport costs are therefore eliminated.

Areas of application

The areas of application of deep rolling range from general mechanical engineering, automobile and aircraft construction, engine construction to power plant and medical technology. This method can be used whenever it is necessary to increase the operational strength of a metallic material or to implement lightweight construction solutions.

Related procedures

Deep rolling is a process related to roller burnishing . As with roller burnishing, the surface layer is plasticized and reshaped in the same way with the aid of one or more rollers or balls. It differs from roller burnishing essentially in the objective. While a certain surface quality in the form of a specific roughness value should be achieved in roller burnishing , the fatigue strength must be increased in deep rolling. Although this increase in component service life is also based on smoothing the surface, the strain hardening achieved and the induction of residual compressive stress in the edge zone have a significantly more significant influence.

Another difference between the two procedures is the quality check. This is trivial in roller burnishing by z. B. perform a tactile measurement of the surface quality. With deep rolling, however, parameter influences can only be verified through service life tests, measurements of residual stress depth profiles, etc. by destroying the component.

The quality correction also differs between the two methods in this regard. In roller burnishing, a surface parameter that has not been achieved can in most cases be achieved by a repeated machining process. With deep rolling, the repetition of a machining process would basically have to be verified by tests and investigations, which can only be achieved using destructive testing methods and thus deep rolling cannot be repeated. For a series process, it is important to ensure that previously defined processing parameters (deep rolling force, feed rate, etc.) are strictly adhered to. Process monitoring is recommended, if not absolutely essential, especially when deep rolling safety-relevant components .

variants

Different process kinematics can be implemented for deep rolling. The simplest variant is deep rolling in the groove process. Here, the surface is contacted at an axial position with the roller or ball, the rolling force is built up over a few revolutions and then transmitted over several revolutions at a constant level. At the end of machining, the rolling force is reduced again over a few final rotations. The build-up and decrease of the rolling force is of great relevance, as otherwise stress gradients can occur in the edge zone of the component, which would encourage early failure of the component. This kinematics is primarily used to z. B. to eliminate notch effects on shoulders of wave-shaped components.

By switching on a feed, the deep rolling is carried out in the feed process to, for. B. to edit cylindrical surfaces quickly and easily.

Tools with hydrostatically mounted balls also enable the machining of flat and free-form surfaces. Here, the rolling element is guided in the form of a ball via a tracking system. This enables the user to compensate for a wide variety of component tolerances and machine elasticities in a defined area without having to forego a continuously constant rolling force on the surface. This enables the machining of complex geometries with constant process quality. Especially for components that are subject to the highest safety requirements, only force-controlled tools for roller burnishing or deep rolling can be used.

The principle of these hydrostatic tools for roller burnishing and deep rolling also enables components with a high initial hardness to be machined. Tools with mechanically mounted rolling elements are generally only used up to an initial hardness of 45 HRC (Rockwell hardness). Beyond this hardness, among other things, wear would be a disadvantage that should not be neglected. With the tools, which have a hydrostatically mounted ball, initial hardnesses of up to 65 HRC can still be machined. Even under such conditions, surface smoothing, work hardening and the induction of internal compressive stresses in the component edge zone are still possible.

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