AbWaHo

Absorber for waterjet machining of hollow bodies

Aim of the development

The water jet is one of the most universal tools for cutting materials. It can cover a range of materials from rubber, paper, fabric and foodstuffs to glass, stone and ceramics. To achieve this, water jets with high pressures are required, which also result in a low jet divergence. However, this has a negative effect on the circumferential machining of pipes or circumferentially closed geometries. The emerging high-energy water jet also interacts with the opposite side and removes material. Particularly in the case of glass materials or components with an internal diameter of less than 100 mm, this leads to restrictions in use.

The aim of the project was therefore to develop and qualify a jet catcher system. To this end, model tests were carried out on transparent process chambers developed for this purpose, which made it possible to observe the course of the beam in the cutting process in order to better understand the mechanisms for energy dissipation of the beam and to be able to exploit them in a targeted manner.

Based on this basic understanding, various materials and geometries were evaluated with regard to their interaction with the water jet and their suitability as an absorber material. Based on this material selection, various absorber geometries were developed, designed and manufactured. These were tested and evaluated in cutting tests, including on glass materials. Further work focused on integrating the absorber into the movement system of the waterjet cutting system in order to be able to constantly align the absorber in an optimum position under the waterjet. Devices were developed for this purpose and integrated into the system control. In addition, the integration of a sensor system for detecting the jet penetration into the workpiece was evaluated.

As a result, a jet catcher system was developed and evaluated that uses ceramic spheres as an absorber material to ensure reliable catching of the water jet when cutting hollow bodies. This opens up new fields of application for the processing of geometries that could not previously be cut with a water jet.

Advantages and solutions

To develop the absorber system, the necessary requirements were first compiled. These included the operating conditions for tube processing, the dimensions of the internal geometries and the service life of at least 500 operating hours. The cutting behavior on quartz glass was then investigated for various beam parameters and the exit beam was measured. High-speed images of the cutting process were taken for this purpose. The images were used to measure the important scattering angle of the beam behind the workpiece. In principle, it was shown that the exit angle of the beam is particularly large when the cutting quality is also low. The grooving process was also observed. Here it was shown that the water jet does not continuously drill straight, but meanders in the depth of the workpiece. Hydrodynamic effects cause occasional changes in the direction of flow during drilling, so that the deepest point is not always machined the most.

After investigating the water jet behavior, various materials were examined with regard to their suitability for jet attenuation. Ceramic balls, steel balls, sand, quartz and sheet steel were used in the experiments. Despite their high ductility, the tested steel balls did not show any significant improvements compared to ceramic balls. However, they were removed more quickly by the jet. In addition, they tended to rust at the removal points. It proved to be fundamentally sensible to arrange the balls compactly. If the balls are loose, they yield to the jet and the effective thickness of the ball layer is reduced.

Small ceramic spheres have the advantage that they form fewer gaps and the beam is deflected more frequently over the same distance. Large spheres, on the other hand, have the advantage that they tolerate a larger beam entry of the absorber and still remain compact.

In order to be able to observe the beam attenuation, a partially transparent vessel was manufactured. The high-speed videos recorded show that, depending on the angle, the beam needs 2 to 3 contacts with the spheres before it can no longer ablate the glass window and base. At very shallow angles of incidence, more contacts may also be necessary.

Two versions of an absorber were produced: Firstly, a very cost-effective KG tube with ceramic ball filling and secondly, a steel tube with hard metal plates and ceramic ball filling. In addition, a classic version made of pressed tooth plates was produced for comparison.

Both the KG pipe and the steel pipe can withstand the maximum pressure of 3,500 bar. The ceramic balls show moderate wear and are easy to refill.

As part of the project, the absorber was then to be integrated into the movement system of the cutting machine. This ensures that the relative position of the cutting beam and absorber can be kept constant throughout the entire machining process and that the same impact angle is always guaranteed.Initially, the connection of the absorber directly to the cutting head or the gantry system was considered.However, the machine axes are not designed to accommodate the additional weight of the absorber.The attachment and movement of the additional mass would impair the precision of the machine and could even lead to damage to the movement system.In the second step, the originally planned implementation of the absorber movement in the machine bed was therefore considered.The advantage of the absorber movement is that the absorber does not have to cover the entire length of the workpiece and can therefore be made smaller.However, the mass to be moved and the additional control engineering effort limit the area of application.

The results have shown that there will be no single absorber that can be used for all cutting applications, but with a selection of a few absorbers, there will be a sensible and simple solution for every cutting task.

Finally, the piercing process was also measured in the project.The process must start at a low pressure so that brittle workpieces are not destroyed right at the start by the impact of the water jet.By default, a fixed time is programmed in the machining program for the piercing process and the machine waits for this time before starting the actual cutting program.If the breakthrough can be detected, the machine could automatically run the contour immediately and thus save processing time.Acceleration sensors were used for detection, as they are robust and available at low cost.The experiments carried out showed that the piercing process can be detected using acceleration sensors.Sensor-supported process time can be saved here.

Target market

The newly developed absorber allows significantly smaller hollow bodies, such as pipes, to be waterjet cut than was previously possible.

The project results are primarily aimed at users and contract manufacturers for waterjet cutting tasks.The project results should enable service providers to process sensitive hollow bodies made of glass and plastics.More complex parts can be processed with the new absorber than is possible with conventional glass processing.This provides design freedom for the system concept, which makes it possible to build component groups from fewer individual parts and thus save on production costs right from the design stage of the glass equipment.

The development developed in the project will also increase the overall demand for waterjet cutting services and will probably also result in investments in new machine technology in order to cope with the increased order volume.