A hot, high-precision dance

© Photo | TRUMPF

Micro fusion cutting is the alternative to cold ablation with scanners.

Mention micromachining and many people will think of ultrashort pulsed lasers. For many applications, however, cold ablation with galvo scanners cannot achieve the required level of precision and edge angle (taper angle). Steep edge angles occur when the amount of energy (fluence) delivered to the material at the edge of the kerf is low, or when material is re-deposited there. In this case it Is advisable to work with fixed optics, process gas nozzles and high-precision axes.

However, the accelerations and feed rates of the axes are not as high as those of galvo scanners with their much lower moments of inertia. Working with fixed optics and axes therefore leads to the formation of a melt phase – and it is essential to ensure that this does not negatively affect the quality of the part.

Good edge quality and a steep taper angle

5-Millimeter-großes-Zahnrad

A gear wheel measuring approximately five millimeters. Micro fusion welding can be used to achieve good edge quality and a steeper taper angle. (Photo: TRUMPF)

Unlike traditional fusion cutting techniques such as those based on disk lasers and CO2 lasers, ultrashort pulses ensure that heat is applied only to a very limited area for a very limited period of time. This ensures that the parts do not suffer any thermally induced damage. Figuratively speaking, this is equivalent to a highly controlled, hot dance with highly precise movements in a very confined space.

If the process is carried out properly, micro fusion cutting can achieve good edge quality and a steeper taper angle than cutting processes based on cold ablation using a scanner. Optimized nozzles deliver process gases under high pressure to drive the melt out of the cutting kerf. Several key factors have an impact on the result, including the nozzle geometry and position as well as process gas type and pressure.

Measurement technology is the key to ensuring quality

Another important aspect is the development of suitable mounting devices or holders. They should be capable of holding the part firmly in place while simultaneously enabling free cutting without requiring any direct support beneath the part. The design should also prioritize suitability for use with lasers in order to avoid what are known as piercing points.

Mikroschmelzschneiden-fuehrt-zu-erhoehter-praezision

The supply of process gas through nozzles (top) and suitable part holders (bottom) are key factors in achieving high quality. (Photo: TRUMPF)

At the same time it is essential not to underestimate the importance of measurement technologies in the context of process development and quality control. Systems must be in place that are capable of reliably measuring tolerances of shape and position to an accuracy of five micrometers or better and measuring average roughnesses (Ra) of less than one micrometer.

Also suitable for ceramics

Mikroschmelzschneiden-von-Keramik

Micro fusion cutting of ceramics. The part shown – a test geometry for the watchmaking industry – has a diameter of approximately five millimeters. (Photo: TRUMPF)

This special kind of micro fusion cutting can be used to machine not only metals but ceramics, too. It is also possible to use multiple processing optics in parallel to make the processes cost-effective. This kind of ‘beam splitting’ can be achieved using powerful ultrashort pulsed lasers from TRUMPF’s TruMicro range. Various finishing techniques can be used to enhance the machining results even further.

TruMicro Series 5000

TruMicro Series 5000 lasers are picosecond lasers with power outputs of up to 100 watts and pulse energies of up to 250 microjoules. The extremely short pulse durations of less than 10 ps vaporize virtually any material so quickly that no heat-affected zone can be detected.

Once the micro fusion cutting process has been completed, heat can be applied to the material to obtain a specific degree of hardness or strength or remove residual stresses from the material, similar to the heat treatment typically used after electric discharge machining and micro milling. The roughness of the surfaces can also be further reduced by polishing.

Another option is to electroplate the parts after the laser machining process. These methods are already state of the art, for example in the watchmaking industry. This sector is also one of the most important fields of application for micro fusion cutting and drilling of parts such as jewel bearings, watch hands, and other design elements.

90 degree edge angle

90 degree edge angles are a common requirement in microtechnology. This can already be achieved for smaller holes by using what are known as trepanning optics in the drilling process. The trepanning optic gives the laser beam an inclination angle and rotates it around a wobble point to correct the taper angle.

Testgeometrie-fuer-die-Uhrenindustrie

Micro fusion cutting can also be used to machine sapphire and silicon. (Photo: TRUMPF)

This approach is increasingly showing promise for more complex geometries, too. The highly precise feed rate once again relies on the mechanical axes. However, most trepanning optics still require further development and must be better tailored to these demanding applications in order to make them more user-friendly and suitable for industrial use. An alternative solution that is mechanically demanding but worth considering is to compensate for the taper angle by using flexibly tiltable sample holders.

High precision

In summary, micro fusion cutting is an interesting alternative to cold ablation with scanners for applications that require high levels of precision. There is, however, still plenty of potential for further development, for example in the fields of part holders, part handling and process gas feed systems. Improvements could also be made to special-purpose optics, for example by creating a user-friendly special-purpose trepanning optic for beam positioning to compensate for the taper angle in complex contours.

TRUMPF contact details

Athanassios Kaliudis
E-mail: athanassios.kaliudis@de.trumpf.com
Phone: +49 7156 303-31559

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