Walter Maurer, founder and Chairman of the Board of Directors of Waterjet in Aarwangen, Switzerland, explains: "The previous limitation of the working area of our F4 machines resulted in both technical and economic disadvantages. This began with the time required for the multiple cutting of the panels, which are usually supplied in the format 2,000 × 1,000 mm. In addition, a 200 mm wide residual strip remained afterwards, which often enough ended up as scrap. If parts with larger dimensions had to be produced despite the size restrictions of the work area, a sheet with the corresponding oversize was clamped and offset by the required distance after the first machining pass. However, reclamping inevitably leads to offset and angle errors and a corresponding loss of quality on the workpiece.

Even more serious, however, is the loss of material due to the "edge effect" when nesting parts on the work surface. The larger the parts are in relation to the dimensions of the work surface, the more the material-saving arrangement of the parts on the sheet is restricted. In unfavorable cases, these grid losses can reach values of over 50 %, which is well above the usual average of 30 %. And last but not least, depending on the job characteristics, a larger machine can run unmanned for much longer, even after the end of a shift or into the weekend.

Precision mechanical engineering for maximum accuracy

"With the considerably larger dimensions of the new system, we had to come up with some ideas to ensure at least the same accuracy values as the previous F4 version," reveals Maurer. This starts with the machine bed: this consists of a chrome steel construction that has been filled with 8 tons of vibration-damping mineral concrete.

In work preparation, the CAM programmer can mark certain contour areas such as sharp corners or drill holes with macros. © Klaus Vollrath

The three main axes have high-precision ball screws, and the position is measured using temperature-insensitive glass measuring rods with a resolution of 10 nm. The workpiece carrier is mechanically decoupled from the water basin and the vibrations that occur in it. The repeat accuracy of the axes is 0.5 µm. The new nozzles with a cutting jet diameter of just 150 µm are also used on the system. This allows cutting gap widths of just 170 µm to be achieved with correspondingly thin material. Accuracies of up to 5 µm can be achieved on the workpiece. For mechanical processing, the working head can also be equipped with a high-frequency drilling spindle with its own Z-axis. This means that start holes can also be drilled in laminated materials such as glass or carbon fiber composites. If an attempt were made to drill such holes with a high-pressure water jet instead, material damage could occur due to delamination.

More precise thanks to shape analysis of the water jet

"Based on precise investigations, we know that the water jet is never exactly round. The roundness deviations are up to 3 µm," says Maurer. To take this into account, the exact geometry of the water jet is determined using a patented process after a new nozzle has been installed. To do this, a hole with a diameter of 2 mm is created in a metal sheet, the geometry of which is precisely measured under a high-precision measuring microscope. The roundness deviations recorded in the process are stored in the control system as a function of the angle and compensated for during use. For this purpose, the actual contour of the water jet is guided tangentially to the desired outer contour of the workpiece. The prerequisite for this is that the CAM software can differentiate between the workpiece side and the residual grid side.

Contour-dependent adjustment of the working parameters

"With the CAM software, we can adapt the working parameters to the specific requirements of certain areas of the part contour," says Maurer. One example is the cutting of sharp corners or narrow gaps, for which the cutting speed is reduced in order to be able to follow the part contour really cleanly. However, this slowdown would result in an undesirable widening of the cutting gap if the working pressure of the water jet remained the same.