in Vol. 10 - November Issue - Year 2009
Roller Burnishing and Deep Rolling – New Applications for proven Methods
Fig. 1: Roller burnishing of a mandrel
Fig. 2a: Deep rolling of turbine and compressor blades
Fig. 2b: Tool FA for processing threads
Fig. 3a: Tool of the type RIOA for simultaneous counter boring, skiving and roller burnishing
Fig. 3b: Mirror - like inner surface of a cylinder tube
Since its formation the company ECOROLL AG Werkzeugtechnik has stood for innovative tool technology in the fields of roller burnishing, deep rolling and combined skive-burnishing. Especially in recent years a broad product range has been developed which covers a variety of applications.
The applications for roller burnishing and deep rolling range from general engineering, automotive and aircraft manufacturing to power plants and medical engineering. In all these areas ECOROLL tools are used for the surface treatment or to increase the durability or the realization of Lightweight. A closer look at the procedures should identify the source of many solutions and applications.
The procedures Roller burnishing and deep rolling are often called "rolling", however, it is useful to clearly distinguish them from each another. The main difference is the objective. During roller burnishing only a certain surface quality in the form of roughness should be reached, so during deep rolling fatigue is to be increased. Because the component life is very sensitive to parameter changes, it is clearly more important to the deep rolling to ensure reproducible process parameters. The most recent development in this area is software to monitor the relevant deep rolling power parameters.
A unique combination of the processes burnishing and skiving takes place in cylinder tube processing. In this way, very economical surfaces can be produced that meet the requirements of modern hydraulic pipes. The ECOROLL RDO "Omega" tools allow simultaneous skiving and roller burnishing of cylindrical tubes in accordance with good form tolerances. The latest product line RIO-A enables even the triple treatment "boring-skiving-burnishing".
Advantages of Roller Burnishing
Roller burnishing has been a proven method for decades, which offers great technical and economic advantages in an area of roughness of Rz < 10 µm. The very smooth surfaces, which are produced by roller burnishing, are characterized by a high profile bearing ratio. Other benefits of roller burnished surfaces are reduced friction, increased wear resistance and increased surface hardness.
There are no chippings or grinding sludge left by this forming process. This will reduce the environmental impact and reduce disposal costs. The economy is further increased by short processing times and the abandonment of expensive rework. When you switch from grinding, polishing, burnishing or honing to roller burnishing a reduction in manufacturing costs by more than 50% is possible. Another advantage is that this finishing operation, as well as deep rolling can be performed in one setting, for example, on a lathe.
Applications for Roller Burnishing
An application example for burnishing is the processing of a mandrel (Fig. 1). Here, surface finish to be achieved by Rz = 2 µm was required and achieved. In addition, the processing could take place in one setting after turning. This led to a significant time savings as well as an increase in hardness of the component.
In the past ECOROLL almost exclusively offered tools to roller burnishing, now the product range has expanded to machines for roller burnishing. Because there are always applications in which it does not seem useful to equip the lathe with a tool for burnishing, a separate machine was developed for this process. This can handle cylindrical parts with diameters ranging from 3 to 45 mm. As a compact machine it can be placed next to the lathes in the production, so that the worker can parallel burnish components in a two-machine operation.
Advantages Deep Rolling
Deep rolling has been successfully used for many years and is the only mechanical finishing process which combines three significant benefits, and thus leads to an increase in the strength of the life of the component. Deep rolled surfaces are characterized initially by a very low roughness. In addition, compressive residual stresses and dislocations that are caused by work hardening, are responsible for a deceleration of the crack and crack growth retardation. A lifetime increase by a factor of 10 can thus often be achieved. The fatigue strength can be partially increased by a factor of 4.
Compared to alternative methods such as shot peening, for example, deep rolling usually offers a larger increase in strength, and it is also cheaper to integrate this process into the process chain especially for rotationally symmetric components.
ECOROLL tools are characterized by the fact that they can be used on conventional machine tools. In this way, a component can be directly deep rolled after cutting in the same setting. Setup times and transport costs are therefore eliminated.
Applications for Deep Rolling
For the application in the aerospace industry a new tool was developed that allows processing of thin-walled components. The deep rolling of turbine and compressor blades (Fig. 2a) from aircraft engines is demonstrably a measure that has a significant impact on the life of such a highly stressed component.
For this very specific application, a pincer tool was built, in which no external forces act on the component, because the forces cancel each other out. So an unwanted deformation of the component can be prevented.
The best in years "ballpoint" tool principle allows the processing of such non-rotationally symmetric surfaces. It is based on a hydrostatic bearings ball, which is freely movable and is guided in such an area line by line over the surface. This patented tool principle also enables the machining of hardened components even up to a hardness of 60 HRC.
The FA tool for processing threads (Fig. 2b) is similar in the basic structure but much larger in scale. The component in this application is a part of a drill pipe as is used in oil and gas production. On this piece is a thread of the critical region because the thread root always leads to a stress concentration.
For a nominal diameter of 350 mm very high deep rolling forces are necessary to achieve the necessary change in the boundary layer in the thread root. Therefore, in this case, a pincer tool was developed, which deep rolls with three roles the radius of the thread root.
Combined Skiving and Roller Burnishing
The combination of machining and a transforming process is an extremely efficient method for finishing of pipes. For the use of hydraulic cylinders, there are special requirements for roughness and forming tolerance. At the same time it must be superior to competing techniques, such as honing. The combination of skiving and roller burnishing will meet these requirements.
The machining takes place at Ecoroll RDO "Omega" tool in the anterior part of the tool on three or six floating knives, which are supported on a common center. This arrangement ensures a very good circle. The chips produced during machining are flushed out to the front of the tube, because after the machining the roller burnishing process takes place. The resulting surface has Ra values of about 0.2 microns or less. This requires no further editing. Such combination tools can be used with diameters of 38 to 450 mm and lengths of pipe up to 15 m. The latest generation of these tools RIO-A also has an attachable boringhead. So the triple processing is possible for simultaneous counter boring, skiving and roller burnishing (Fig. 3).
Dipl.-Ing. Sirko Fricke
Research & Development
29227 Celle, Germany