Detection of Breakthrough in Laser Percussion Drilling
Nirmala Sankikommu, Ravi Bathe and A.S. Joshi
Laser percussion drilling is a process, which is of great interest in many industrial applications. If the components to be drilled have a closed cavity, as is typical in airfoil blade/vane, fuel injector nozzles etc., the laser beam must drill through the first surface but should be switched off before it can damage the underlying second surface. Therefore, it is important to be able, immediately, to detect when a hole has been completely drilled through the work-piece. The precise determination of the breakthrough process is, however, complicated and can lead to uncertainties in the optimisation of the laser drilling process, the parameters, unacceptable hole quality, wastage of time and the laser beam can also damage adjacent surfaces and back-wall. A precise and rapid method of determining when a through hole has been drilled in the work-piece is presented. The breakthrough detection device has been developed based on a photo sensor and a filter, and will detect only laser radiation. In this arrangement, one end of an optical fibre cable was held just below the work-piece being radiated and the other end of the fibre was coupled to a photo-sensor. The output of the photo sensor triggers an electronic circuit when it receives the “breakthrough signal” i.e. the laser radiation has passed through to the exit side. The electronic circuit can cause the pulses to be interrupted or switch off the laser. It is also possible to allow an additional predefined number of pulses after breakthrough has been achieved and then stop the laser pulses. We have observed that these additional pulses serve as finishing pulses and improve the quality of the drilled hole. Breakthrough detection has been demonstrated successfully and improves the hole quality and consistency by detecting the precise instant of breakthrough and then allowing an appropriate number of additional user defined pulses before switching off the laser power.