Investigation of Laser Drilled Pilot Holes and Additively Generated Pilot Holes as Drilling Guides for Single-lip Deep Hole Drilling

Abstract

Increasing demands on the performance and the design of components and the resulting requirements for design flexibility increasingly require the creation of bore holes with small diameters and high length-to-diameter ratios on complex shaped surfaces. Tools and implants in medical technology, cooling holes and oil channels for turbine construction in aviation or complex injection nozzles in the automotive industry are just a few examples. Typically, these bore holes are produced with single-lip deep hole drilling tools. Due to the asymmetric design and the displacement of the drilling tools on inclined or curved surfaces, a drilling guide in form of specially adapted drill bushes or pilot holes is usually required. Previous studies on laser drilling demonstrated that the laser beam is not displaced by the surface of the workpiece, thus the drilling method is suitable for the direct generation of pilot holes on complex shaped surfaces. Within the scope of the conducted investigations, pilot holes were produced with a helical laser drilling process in components made of the material X2CrNiMo17-12-2. In addition to a flat surface as a reference, pilot holes with an angle of 30° and 45° to the workpiece surface were also realized. In a second approach pilot holes were directly implemented in the production of samples with a selective laser melting process. The results show that both the additively and the subtractively generated laser pilot holes are suitable to guide single-lip deep hole drilling tools for drilling processes on complex surfaces. However, the surface quality in the pilot hole area doesn’t correspond to the quality of mechanical machining due to the thermal material removal during laser drilling, similar to additively produced holes. By using step drilling tools, it was possible to combine the flexible generation of pilot holes by means of the laser process with the excellent surface quality of mechanical drilling over the entire drilling depth.

Keywords: laser drilling; deep hole drilling; laser assisted processes; process combination; additive manufacturing