Rollieren und Glattwalzen – zwei im Grunde unterschiedliche Verfahren zur Oberflächenbearbeitung

If you want to find out more about the "roller burnishing" manufacturing process, you need to realise that this term is generally used differently today than it was actually defined. The term roller burnishing is now usually used to describe the localised forming of roughness peaks by a rolling rolling body. Or simply put: Roller burnishing is equated with roller burnishing or deep rolling.

Historically, however, roller burnishing and roller burnishing are two completely different processes that even compete with each other. However, in the many publications, texts and process descriptions available today, it is almost impossible to find a good definition or clarification of these terms. We want to make up for this at this point.

When is roller burnishing used?

Both processes have been used successfully in the past to produce components with an extremely high surface quality. Roller burnishing was used in particular in the production of small parts, e.g. in the watch industry. The diameter range for roller burnishing is D = 0.1 - 8 mm. With roller burnishing, on the other hand, very large diameters of up to D = 400 mm and more can also be processed. There are also a large number of different tools available today that can machine a wide variety of geometries.

However, the main difference between roller burnishing and roller burnishing is the type of smoothing. Roller burnishing involves minimal material removal, which is why it is categorised as a machining process. With roller burnishing, on the other hand, the surface is smoothed by forming. The roughness peaks are deformed locally by a rolling roller, which reduces the roughness. In order to describe the differences between the two methods in more detail, the processes are described separately below.

Roller burnishing - a machining process for smooth surfaces

The roller burnishing process corresponds to the kinematics of external cylindrical grinding. Specially shaped discs made of a hard material are used as tools. These are usually made of tool steel, carbide or oxide ceramic. This so-called roller burnishing disc rotates at a peripheral speed of _vR = 180 - 300 m/min and is pressed onto the component surface with a low force. The workpiece rotates at a significantly lower rotational speed of _vWst = 3 - 7 m/min in the opposite direction. This results in a relative movement between the two surfaces.

It is important that the workpiece does not rotate freely, but is supported in a type of sleeve called a brooch. Figure 1 shows a schematic diagram of the roller burnishing process.

The very low metal removal rate during roller burnishing is generated by the abrasive and roughened surface of the roller burnishing wheel. This means that, in contrast to grinding tools, a roller burnishing wheel does not consist of the three components abrasive grain, bond and pore, but only of a hard material.

Roller burnishing process for different workpiece contours

Roller burnishing is usually used for the final machining of small shaft journals. The cylindrical outer surface is machined by the roller burnishing disc. In order to produce the material removal, the roller burnishing wheel is pressed onto the surface to be machined with a force of only_FR = 10 - 30 N during roller burnishing. This is known as grooving or mould roller burnishing. The roller burnishing disc is fed purely radially. By profiling the disc, non-cylindrical components can also be roller burnished.

In longitudinal roller burnishing, an axial movement of the disc is added to the radial infeed. Similar to external cylindrical grinding, the disc moves at a feed rate in an axial direction over the component in order to machine a larger surface.

End faces or shoulders can be machined with face roller burnishing. Similar to longitudinal roller burnishing, an axial movement is used here. However, in this case it only serves the purpose of pressing the roller burnishing disc axially against the shoulder.

Two somewhat more exotic processes are copy roller burnishing or roller burnishing with hand roller burnishing steel. In copy roller burnishing, the tool is guided over the crest using special kinematics in order to burnish the crest as well. All the processes mentioned so far are usually used on roller burnishing machines. In contrast to this is roller burnishing with hand roller burnishing steel. The tool here is a rod, similar to a file, which is pressed onto the surface by hand.

In the early days, roller burnishing was a manual process, but it became increasingly automated as the number of parts increased. In the end, this process was almost exclusively carried out automatically on machines, replacing manual roller burnishing. Figure 2 shows an overview of the different roller burnishing processes and their specific kinematics.

Roller burnishing tools - design and structure

Roller burnishing tools have a very simple overall structure. As a rule, they consist of discs with a cylindrical or slightly conical outer surface. Different hard materials are used, for example tool steel, carbide or oxide ceramics.

The surface of the disc is produced by a grinding process, whereby a specific roughness depth of Rt = 6 - 15 µm is created. It is interesting to note that the grinding grooves have a specific orientation. They tend to be orientated axially, but have an angle of attack of 10 - 30° to the axis. This orientation of the roughness structures causes material to be removed from the component surface, thereby smoothing it. With this kinematics, the mechanical load of the process increases with increasing smoothing, which slightly changes the edge zone. Specifically, this leads to a hardening of the surface.

In addition to the roller burnishing disc, a workpiece support is required. This so-called brooch is usually adapted exactly to the diameter of the workpiece and ensures that the component is inserted into the brooch on one side. The brooch is often used as shown in Figure 1, so that it can be used for longer and the tool life can be increased. To optimise this even further, the brooch is also made from a hardened material, usually carbide.

Surface quality after roller burnishing

The result of roller burnishing depends on various parameters, such as the pre-machining, the roller burnishing time, the roller burnishing force and the lubrication used.

Pre-machining should be carried out by fine turning or, alternatively, by fine grinding. Depending on the machining diameter and material, a roller burnishing allowance and a pre-machining roughness depth should be available. In the withdrawn VDI guideline 2032 "Roller burnishing and roller burnishing", the roller burnishing allowances and roughness depths are specified in Table 2. On average, the guideline speaks of a diameter allowance of 15-25 µm, which is ideally produced by fine turning. The roughness depth before roller burnishing should be Rt = 1 - 3 µm. Overall, the component should therefore already be suitably smooth. Roller burnishing can then produce a roughness depth of Rt = 0.1 - 0.8 µm, and this for different materials from brass to free-cutting steels with a hardness of up to 62 HRC.

It is important that seizing of the component is prevented during this process so that the surface is optimised and not deteriorated. This is achieved through the essential use of cooling lubricants. In addition to cooling the process, this is the most important task of the medium. A mixture of lubricating oil and petroleum is often used.

Roller burnishing, the "new" roller burnishing?

The process relationships are completely different in roller burnishing. Here, significant smoothing is achieved by reshaping the peak-to-valley roughness. The process has already been described in detail in another article. Therefore, only the most important properties are described below.

In roller burnishing, a roller body is pressed onto the surface with a defined rolling force. The rotational movement of the workpiece or the tool then causes the rolling element to roll on the surface. However, the roller itself is not driven separately. There are different ways of pressing the roller onto the surface. A spring force or a tool mechanically set to oversize is often used. However, a hydrostatically mounted rolling ball can also be used.

The shape of the roller body is arbitrary and can be designed differently depending on the component geometry. The roller can also be mounted directly or indirectly and must always be selected to suit the application. Figure 4 shows three different types of tools used for roller burnishing.

The mechanical pressure of the mould on the surface results in a plastic flow of the material in the edge zone, i.e. under the surface. This results in a clearly smoothed surface with average roughness depths well below Rz = 1 µm and a noticeable homogenisation of the surface. In addition, the material properties in the edge zone also change. This results in work hardening, an increase in dislocation density, an increase in surface layer hardness and the introduction of residual compressive stresses. All of this has a positive effect on the service life and wear resistance of the component.

Depending on the objective of the process, the process is referred to as either roller burnishing or deep rolling. In the case of roller burnishing, the primary objective is to reduce surface roughness. In deep rolling, the process always focusses on achieving the correct consolidation. Depending on which objective is in the foreground, more or less attention must be paid to the process parameters during process control. Various articles and videos have already been published on this topic.

Comparison of roller burnishing and roller burnishing

Looking at the text described so far, the first thing that stands out is that both processes significantly smooth the surface and that both processes are surface finishing processes. These processes are in direct competition with grinding, fine grinding, polishing or fine turning, but have some significant advantages over them.

Compared to grinding processes in particular, the processes are significantly more productive and result in lower tool and machine costs. In particular, roller burnishing and roller burnishing bring further benefits for the component because they also improve the edge zone properties due to the mechanical component.

In a direct comparison between roller burnishing and roller burnishing, however, the range of applications for roller burnishing is significantly greater and the process is much more reliable and efficient. This can be seen, for example, in the required pre-processing. The initial roughness depth for roller burnishing can be _Rz = 20 µm and an Rz _value of less than 1 µm is still achieved.

In purely linguistic terms, the two process designations are now used synonymously. While both processes were still known in the 1970s and were also treated differently in textbooks, the term "roller burnishing" in its original form can no longer be found anywhere. As a result, the process is probably only rarely carried out in reality.

Anyone who talks about roller burnishing today is actually referring to roller burnishing. This term has become established on the Internet anyway, and it is also being used more and more frequently in journal articles and scientific publications. For this reason, ECOROLL AG, one of the leading manufacturers of tools for roller burnishing and deep rolling, also refers to roller burnishing, the roller burnishing tool or the roller burnishing process, even though this is of course not entirely correct from a historical point of view. In our documents and texts, we use the terms roller burnishing, roller burnishing and deep rolling synonymously. However, we often emphasise deep rolling in particular, as its objective goes beyond simply smoothing the surface and therefore places higher demands on the process.

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