Waterjet cutting - feasibility and limits

A complex process chain

The basic units of the Microwaterjet systems are manufactured to order by a Swiss precision engineering company. © Klaus Vollrath

"We quickly realized that it was far from enough to simply trim the dimensions of the jet downwards and the path accuracy of the machine upwards," reveals Maurer. Waterjet cutting is a complex process in which fluid dynamics, mechanics, mineralogy and pneumatic and mechanical conveying technology interact in many different ways. The decisive factor here is the flow behavior of the jet after it passes through the outlet nozzle, when it picks up the cutting material particles in the mixing chamber and then in the focusing tube and accelerates them towards the workpiece. The size of the mineral particles and their shape play an important role. The more angular and sharp-edged they are, the more effectively they cut. Another criterion is their distribution in the jet: high cutting performance is best achieved when the particles are accelerated in the center of the water jet. If, on the other hand, they tend to be entrained around the circumference of the jet, they will better reflect the diameter of the focusing tube on the workpiece. Although this slightly reduces the cutting performance, it increases the edge sharpness and also reduces the roughness of the cutting surface.

Schematic representation of a waterjet cutting head. © Wikimedia Commons, Zureks

"We have invested a lot of time, money and effort in researching these relationships together with university partners," says Maurer. The current fourth generation (F4) of the in-house developed waterjet cutting machines is based on a machine frame whose axes achieve a positioning accuracy of ±0.5 µm. The jet head, which has also been revised several times, can achieve a radial accuracy of the jet diameter of up to ±1.5 µm at a diameter of 0.2 mm, depending on the boundary conditions, so that the nominal final accuracy can be up to ±2 µm. Depending on the material and cutting width, cutting surface roughnesses down toRa 0 .5 µm can be achieved. This means that the self-imposed target of exceeding the accuracy of conventional system technology by a factor of 10 has been achieved. The control system also plays an important role, with functions developed in-house such as a correction of roundness deviations of the beam and a control option for the blasting medium throughput.

Garnet sand in various grain sizes is the preferred abrasive material for waterjet cutting. © Klaus Vollrath

"With our machines, micro waterjet cutting with accuracies of up to 10 µm and slit widths down to 0.2 mm can be safely mastered," says Walter Maurer. However, due to the numerous parameters and boundary conditions, running in new processes requires personnel with experience, care and conscientiousness. This is why Waterjet attaches great importance to providing comprehensive support services for its customers, regardless of whether it is simply a matter of cutting services or the purchase of machines. The spectrum of cutting orders ranges from research prototypes and small series to large series. If the customer wishes to set up their own production, the required system is supplied either as a stand-alone machine or, on request, with the necessary automation. Depending on the agreement, we can also cover the running-in of the production process up to series production. The decisive factor here is that the customer is supported in every situation with all the know-how accumulated in-house. All tasks are taken care of, from advice on new product developments to the provision of reserve capacity in the event of breakdowns or bottlenecks. Continuous in-house research ensures that this service is always at the cutting edge of technology.

Technologies for further miniaturization

Separating incision made with a water jet to examine the integration of implant and bone. © Waterjet

"We are often asked by customers whether or when there will be a further step in the miniaturization of waterjet cutting," adds Maurer. Of course, the company is continuing its research in this direction and has already developed the corresponding components. However, the hurdles are high, as considerably finer abrasive material is required for this. The biggest handicap is the significantly lower removal rate, as the smaller abrasive particles can transfer less kinetic energy. This reminds him a little of the situation with the miniaturization of laser processing. Femtosecond lasers can be used for much finer processing than the systems commonly used in industry today, but here too the ablation rates are so low that the market for corresponding applications is extremely limited. Whether and when there will be a market breakthrough for ultra-micro waterjet cutting technology cannot yet be estimated.

Klaus Vollrath, freelance journalist / ag