Cylinder tube machining by skive burnishing
Skive burnishing is a very special process that was originally developed for the economical production of tubes for use in hydraulic cylinders. Today, however, various other applications are also conceivable in which a particularly smooth surface is required in bores.
In order to increase the cost-effectiveness of rolling, a combination of roller burnishing or deep rolling with existing machining processes is increasingly being sought. The roller burnishing or deep rolling process is an additional process step that extends the process chain and thus initially reduces productivity. In hybrid or process-related combination with the machining process, however, significantly higher productivity is generally possible.
One of these combination processes is so-called skiving and roller burnishing. This combines the "skiving" machining process with roller burnishing in one tool. This is particularly advantageous in the production of hydraulic cylinders. Hydraulic cylinders are very versatile components that are used in various applications to generate high forces. They are used in construction or agricultural machinery, in presses or lifting systems and, of course, in general mechanical engineering. A hydraulic cylinder is therefore an elementary component in mechanical engineering for generating high forces with little energy input.
These cylinders therefore have some very important functional requirements that must be met in the manufacturing process. First of all, the geometric quality must be very good, as must the shape, position and dimensional tolerances. Particularly in the cylinder-piston contact, the medium must not flow past the piston and thus lead to a loss of power or efficiency.
Friction is also very important. The piston should move easily and continuously, i.e. without a stick-slip effect, for which the surfaces must be friction-optimized. Very special requirements are therefore placed on the surface quality.
In addition, cylinders are also a mass product and must be manufactured economically and, above all, productively. In the past, the quality requirements were achieved by honing. This process offers very good component quality, the roundness is optimal and the diameter deviations within the tube are also extremely small. At the same time, we know from the combustion engine that honing makes the surface structure very suitable for frictional loads. Medium can collect in the honing grooves and be made available for frictional contact.
The tool for skiving and roller burnishing consists of two or three stages, which are arranged one behind the other. The first stage is boring with the boring head. This usually takes place as an additional process with a separate tool, but can also be integrated into the process. The next stage is the skiving stage. Here, the diameter is produced safely and very productively using an inner skiving process. Extremely high feed rates of up to 3 mm per revolution can be achieved thanks to the arrangement of two or three skiving knives distributed around the circumference and the minimal cdepth of cut during skiving. The skiving knives are arranged according to the OMEGA principle, in which the skiving knives influence each other. They therefore float in the skiving head and thus produce a very uniform shape. However, they follow the drill hole, which means that drilling sequences from the pre-process cannot be permanently influenced.
The third stage is the rolling stage. Here, several rollers are arranged around the circumference, which minimize the remaining surface roughness of the skiving and form it plastically, as is usual for the roller burnishing process. This produces a very smooth surface structure over the entire length of the tube.
The tools are used on deep hole drilling machines and are connected to the machine via the drill pipe. Machining lengths of several meters are therefore no challenge for the process. During the process itself, either the component, the tool or both rotate against each other. This depends on the machine and the machining concept.
The process settings for both processes (skiving and rolling) are very similar. The cutting and rolling speeds and the feed rates are in a similar range and can therefore be optimally combined. A particular advantage lies in the feed rate. This can be up to 3 mm per revolution for both processes. Combined with a cutting speed of 200 m/min, meter-long pipes can be processed in just a few minutes. Compared to several hours of honing, this is an enormous time saving.