Roller burnishing

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The burnishing is a forming process for producing a glassy surfaces or those with a defined surface structure. The process is used when a high surface quality is required for a metallic component or the desired surface quality cannot be reliably achieved through machining . The process is described, among other things, in the VDI guideline 2032 from 1975. This guideline, withdrawn in 2015, also dealt with a machining process called roller burnishing . In practice, however, roller burnishing almost always means roller burnishing.

In roller burnishing, a contact stress occurs at the contact point between the roller burnishing tool and the workpiece surface, which causes plastic deformation when the material yield point is exceeded . The roughness peaks are pressed down almost perpendicular to the surface and the material flow lifts the roughness valleys accordingly from below. The smoothing thus takes place through the flowing of the entire near-surface material layer and not according to a widespread false assumption through " bending " or " flanging " the peaks of the Rauiefengebirge.

advantages

Roller burnishing is a process that offers technical and economic advantages in the roughness depth range Rz <10 µm. The surfaces produced by roller burnishing are characterized by a high proportion of profile bearing.

Further advantages are:

  • low roughness (Rz <1 µm / Ra <0.1 µm) or defined roughness,
  • lower friction,
  • increased wear resistance,
  • rounded surface profile and
  • Increase in surface hardness due to work hardening.

Depending on the application, a change from grinding , polishing or honing to roller burnishing can reduce production costs by more than 50 percent. On the one hand, this can be explained by the fact that there is no need for expensive post-processing. On the other hand, significantly lower processing times and finish machining of the components in one setting z. B. possible on a lathe. This saves costs for additional machines, and main and set-up times can be extremely reduced. There are neither chips nor grinding sludge, which significantly reduces the environmental impact, disposal costs and the wear and tear of the machine's bearings and guideways.

Areas of application

The areas of application of roller burnishing range from general mechanical engineering, automobile and aircraft construction, engine construction to power plant and medical technology. The roller burnishing tools are suitable for use on almost any machine tool (e.g. conventional or CNC-controlled lathes, boring mills, machining centers, deep hole drilling machines).

Related procedures

Roller burnishing is a process related to deep rolling . As with deep rolling, the surface layer is plasticized and reshaped in the same way with the help of one or more rollers or balls. It essentially differs from deep rolling in terms of its objectives. While a certain surface quality in the form of a specific roughness value is to be achieved with roller burnishing, increasing the fatigue strength is in the foreground with deep rolling. Although this increase in component life is also based on smoothing the surface, the work hardening achieved and the induction of residual compressive stress in the edge zone have a significantly more significant influence on the increase in component life.

Another difference between the two procedures is the quality check. This is simple when 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 roller burnishing. The simplest variant is rolling in the grooving 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 and thus an early failure of the component would be favored. This kinematics is primarily used for deep rolling in order to z. B. to eliminate notch effects on shoulders of wave-shaped components by deep rolling.

By switching on a feed, the 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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  1. VDI guideline 2032
  2. Ecoroll: basics of roller burnishing