The arrival of machine hammer peening opens the way to fast finishing of large milled injection and deep drawing moulds. Instead of manual polishing, an industrial robot does the work in a shorter time. This process also offers a number of other advantages.

Milled moulds are traditionally polished manually. A labour-intensive job that, especially with complex pieces, can take up to 20 man-days. Moreover, the quality of finishing is not always the same. German engineering company Sematek, in cooperation with 3S Engineering and robot builder KUKA, has developed a new finishing method, aimed to achieve a production time reduction of 60 percent. By means of a CNC robot the surface of moulds can be smoothed in a relatively short period of time. The process is flexible, precise, reliable and cost-efficient.

Characteristic of the robot that is used is its range, flexibility, position repeating and track accuracy. For components requiring five-axis machining, this robot represents a significant cost advantage.

Pneumatic tools are attached to the flange of the robot: a linear oscillating hammer head for machining surfaces. With the CNC software CAD drawings can be converted directly into G codes. Once the robot has measured the workpiece (via a 5D-measuring system), it guides the hammer head over the surface. This pushes the cutters into the milling grooves. The diameter of the ball, feeding speed, track distance, angle of movement of the feed and angle of pitch between the hammer and the surface must be selected and set.

Advantages

The process not only smoothens the milling surface, but also increases the surface hardness and optimises the distribution of residual stress. The surface becomes uniform and reproducible. The process allows a surface roughness Ra of less than 0.1 µm, which would be impossible using milling alone. Due to the pressure, the surface hardness can be increased by 30 percent, depending on the material being machined. As a result, in some cases additional surface finishes may become completely superfluous. Manual grinding and polishing can be completely omitted. The finishing time is considerably reduced and even complex free forms can be finished. It is often possible to machine the entire component, which means that costly and time-consuming re-clamping can be omitted. The ultra-precise planning of the machining path allows the robot to work at high speeds with minimal overlap.

The cutter head makes the surface smoother and harder by pushing the milling cutters into the milling grooves (Source: Kuka)

Application and future

The technology can also be applied to very large workpieces, such as wind turbine parts or ship propellers, by using a larger robot or placing a robot on a linear axis. Scaling up the process is also possible, because the software control remains the same. This solution is also suitable for high-end applications in the plastics industry.

Car manufacturer Daimler is the first to use this technology in a pilot project in two robot cells on its production line: for finishing moulds for the production of metal and plastic body parts.

The technology will be further developed and optimised in the coming years. For the time being, the robot still has to be perpendicular to the piece, which is not easy if certain areas of the piece are difficult to reach. Reproducibility in applications in the plastics industry also remains a challenge for the time being.

The operation and advantages of machine hammer peening can be discovered in the following video https://www.youtube.com/watch?v=BOkQoYVVAIE