Metrological evaluation of cutting edges and cutting clearance geometries in the laser cutting of refractory metals.

Bliedtner, Jens ; Buerger, Wolfganf ; Guddel, Yvonne 等

Abstract: Within the framework of the research project "Process development for laser precision machining of refractory metals" different measuring procedures for the evaluation of cutting edges and cutting clearance geometries were investigated.

Key words: laser cutting, cutting edge quality, cutting clearance geometry, measuring technique

1. INTRODUCTION

The cutting edge quality can be described by the roughness of the laser cut and the geometry of the cutting clearance. The roughness is calculated from the surface profile (see figure 1). The parameters of the cutting areas for laser cutting are defined in the VDI guideline 2906 sheet 8 and illustrated in figure 2.

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In order to describe the cutting edge quality by means of quantitative parameters, different measuring procedures can be applied. The investigated measuring procedures are divided in figure 3 according to the two criteria "roughness of the laser cut" and "geometry of the cutting clearance".

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2. EXPERIMENTAL PROCEDURE

In the research project "Process development for laser precision machining of refractory metals" the above mentioned measuring procedures were investigated. Sheets with different thicknesses and material combinations (such as MoCu or WNiFe) were used as test materials. An octagon with incisions at the corners was chosen as test geometry. This geometry is to enable statements on the influence of rolling direction of the material as well as the polarisation of the laser radiation. The eight cutting areas and the eight cutting clearances are defined in figure 4. The roughness of the cutting areas is measured with sufficient material thickness in three positions (see figure 5).

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3. RESULTS

An influence of rolling direction of the material as well as the polarisation of the laser radiation on the roughness of the laser cut cannot be detected (see figure 6). In this case sides 1 and 5, 2 and 6, 3 and 7 as well as sides 4 and 8 should show comparable roughness values. However, figure 6 shows that the roughness increases with increasing cutting depth (from t1 to t3).

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In the laser cutting of WNiFe the influence of different cutting gasses and feed rates on the cutting edge roughness was investigated. Figures 7 and 8 show the results for the cutting gasses air, oxygen and nitrogen.

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All specimens showed that the burr formation or the shape of the burr depends significantly on the feed rate. With the slowly-cut specimens (100 mm/min) the melted material is blown out cleanly and the burr accumulates on the bottom side in slender, long shape. With the fast-cut specimens (500 mm/min), however, the melted material mainly accumulates on the top side and in the kerf.

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4. CONCLUSIONS

The contact-free optical measuring procedures with autofocus, confocal and chromatic sensor are suitable for measuring the roughness for the evaluation of the cutting edge quality of refractory metals. This statement is based on successful test measurements with different device configurations of different manufacturers (OPM GmbH, Ettlingen (N.N. 2007 a); (N.N. 2007 b); NanoFocus AG, Ettlingen (Brodmann, 2007); FRT GmbH, Bergisch Gladbach (Koglin, 2007). They are suitable within certain limits in order to define the cutting clearance geometry. The kerf width can be defined on the laser entry and exit side. From this conclusions on the geometry can be made. White light interferometry (N.N., 2006) is neither suitable for defining the roughness of the cutting edge nor the geometry of the kerf of the investigated refractory metals.

The cutting clearance geometry can be well-defined by means of light microscopy on micro-sections. For this an appropriate software has to be used which can measure the cutting clearance according to the VDI guideline parameters as shown in figure 2.

The Surface profilers "Form Talysurf Serie 2 Laser" and "Form Talysurf 120 induktiv" as well as the light microscopes "Zeiss Stemi 2000-C" and "Zeiss METAVAL" provided sufficient facility for the University of Applied Sciences for evaluating the cutting edge roughness as well as the cutting clearance geometry.

5. REFERENCES

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