As described in,12 the adjustment of the distance from the position of the focus of the laser beam to the surface of a part can be used to shape a longitudinal micro-hole to be drilled therein. The exact knowledge of this position in relation to the workpiece is therefore crucial for the reproducible production of a larger number of micro-holes. This parameter is easy to control at a normal incidence on flat surfaces. However, if the micro-holes are to be drilled at a certain angle of incidence or normal to a curved surface, the effort involved in maintaining the mentioned distance increases. Most laser micro-machining systems are equipped with linear translation stages, which enable the positioning of usually smaller workpieces in relation to the stationary focus of the laser beam. Drilling at a certain angle therefore requires additional fixtures to adjust the part’s rotation.
In the course of the research project reported in this paper, a number of tools with different shapes and sizes were laser-drilled. These included jaws used for flat strip drawing tests,13 the radius tools used for stretch-bending tests,14 and finally the forming tools used in the feasibility studies to produce rectangular cups, as described in “Endurance Investigations on Tool Wear Using Volatile Media” section. The laser-drilling of the forming tool for the production of rectangular cups includes all of the above-mentioned difficulties, since it features inclined micro-holes on the main surface of the tool as well as on the draw-in radius. The three-dimensional positioning of the laser beam in respect to the surfaces of the blanks of the forming tool is therefore critical.
Laser Beam Positioning with a Multi-Axis CNC System
Laser-drilling of the forming tools used in the endurance tests described in “Endurance Investigations on Tool Wear Using Volatile Media” section were performed with a multi-axis CNC system. In this case, one of the inserts of the forming tool was fastened to a flat machining table and a galvanometer-scanner was moved with the help of three linear and two rotational axes. Such a setup is shown in Fig. 6, where a galvanometer-scanner in combination with a telecentric f-Theta lens can be angled at any orientation to the surface of the forming tool.
A global coordinate system was defined in the center of the tool blanks, in order to enable the positioning and orientation of the galvanometer-scanner for more than 672 micro-holes. Furthermore, each micro hole was defined in the three-dimensional space based on its entry point at the top of the tool’s surface as well as its exit point. These two points were used to calculate the orientation vector of each micro hole and to deduce the inclination angle, α, and the rotation angle, β (see Fig. 6).
The positioning of the focus of the laser beam for the individual micro-holes at any position and angle on the surface of the forming tool was significantly simplified by the use of a five-axis coordinate transformation. This feature furthermore allowed for a precise movement in the direction of the beam with static inclination and rotational angles of the galvanometer-scanner during the drilling process by compensational, simultaneous movement of all three Cartesian axes. It was therefore possible to readjust the focal position during laser-drilling with respect to the surface of the forming tool for any required orientation of the micro-holes.
Depth-Adapted Drilling Strategy
The use of a depth-adapted drilling strategy in combination with a moveable galvanometer-scanner has two main advantages. On the one hand, the gradual shifting of the laser focus into the material allows the delaying of the usual stagnation of the drilling rate that occurs in conventional percussion drilling. An analytical model describing this effect was recently introduced by Holder et al.15 On the other hand, the continuous movement of the laser beam in the plane perpendicular to the beam propagation by a galvanometer-scanner enables the shaping of the micro-holes with a significantly higher precision. The setup as well as the drilling strategy used for the drilling of the forming tools is explained in the following.
A kW-class ps laser of the IFSW was used for this purpose in combination with a galvanometer-scanner and a telecentric f-Theta lens with a focal length of 163 mm, resulting in a focus diameter of approx. 50 µm. The laser was operated at a pulse energy of 2.8 mJ and a repetition rate of 30 kHz, resulting in an average power of approx. 80 W. Thermal damage caused by heat accumulation is particularly problematic when drilling in hardened tool steel. Heat accumulation can occur, especially during laser-drilling with high repetition rates combined with the use of high pulse energy, and must be avoided.16 For these reasons, the repetition rate was reduced to 30 kHz for the experiments described in the following.
The further properties of the laser system and the focusing optics are summarized in Table I.
The experimental setup is schematically depicted in Fig. 7. The linearly polarized raw laser beam was converted to circular polarization by means of a λ/4-wave plate before entering the galvanometer-scanner. In combination with a telecentric lens, the laser beam could be deflected in the x and y directions, while remaining parallel to the original beam propagation direction. In the example shown in the lower part of the figure, the laser beam is moved along a spiral path on the sample surface, which describes the initial step of the processing of each micro-hole. Subsequently, the galvanometer-scanner is gradually moved towards the tool surface, shifting the focal position below the surface and deeper into the material.
The scan radius, rscan(z), required to shape the micro-holes according to specification was calculated using the model for the ablation radius, rabl(Ep), described in.12 The parameters used for the drilling process are listed in Table II. Approximately 1.8 × 106 pulses were used for the vertical micro-holes. For drilling the inclined micro-holes, an additional processing with a pulse energy of 2.8 mJ was added and the overall number of pulses was increased to approximately 2.2 × 106.
The outer radius rscan(z) of the spiral scanning geometry as well as the number of pulses were adjusted for the individual steps in one-millimetre increments, beginning on the surface and shifting the focal position into the material. Furthermore, the pulse energy Ep was gradually increased from 100 µJ to 2.8 mJ, forming an energy ramp to provide a smooth drilling start, avoid sharp burrs, and reduce smoulder marks around the micro-hole's entrance.
Results of the Laser-Drilling Process
To inspect the reproducibility and quality of the micro-holes, samples made from the same hardened tool steel (1.2379) were drilled and subsequently cross-cut and ground. The thickness of the samples was 6 mm and thus approximately corresponds to the average material thickness, since the micro-holes in the forming tools to be drilled are between 5 mm and 6.7 mm deep. The total number of pulses was set to approximately 2.2 × 106. A cross-section through such a micro-hole is depicted in Fig. 8. The opening of the gas outlet (facing towards the incident laser beam) has a radius of approximately r2 = 300 µm, and that of the gas inlet (opposite to the incident laser beam) has a radius of approx. r1 = 100 µm. These meet the requirements for the very precise delivery of the lubricant. The conical shape of the micro-hole is very smooth and therefore ideal for the transport of the volatile lubricant.
A total of 672 micro-holes were drilled into the two forming tool inserts which included holes in the drawing radius, as shown in Fig. 9.