Measurement of Small Holes by Confocal Microscopy

Drilling of cooling holes in the manufacture of modern jet engine components makes use of Electrical Discharge Machining (EDM), Laser and Waterjet technologies. Quality control of the size of the holes and the resultant coolant air flow is a time consuming step usually carried out by labor intensive manual pin-gage testing. While novel Automated Vision Inspection (AVI) systems has been developed that can optically acquire hole size information, analyze the data individually, statistically or collectively, and interface with the laser drilling workstation in a rapid, on-line fashion. Based on the proprietary feedback software, the system is capable of altering the laser drilling program in such a way as to change the size of the next holes to be drilled, keeping the overall quality requirements within specified limits and bringing the drilling process into control in real time. This procedure allows for the inspection and quality control operations to occur automatically without human intervention, and shortens the overall cycle time considerably. The system can automatically process holes of arbitrary shape for straight thru, angled or blind holes. BENEFIT: The benefits of our proposed approach are numerous and both technical and economical in nature. From the technical point of view, automated inspection of holes and feedback to the drilling station allows the system to directly alter the drilling parameters on-the-fly, and keep final flow within the desired range. This automated feedback loop replaces lengthy manual operator interventions, increases accuracy, speed and repeatability. We aim at reducing cycle times by a factor of two, and flow variability from +/- 10 % to +/- 2.5%. In addition, we cam achieve higher accuracy of individual hole measurements 0f ~0.1 µm. This is more than two orders of magnitudes smaller than the present pin gage measurement capability of 0.001” (25 µm). Financially, the approach presents a realistic potential for reducing the cost of the JSF program (primary market) by ~$200M. When combined with the potential applications in commercial avionics, electronics and medical hole drilling applications (secondary markets) the potential cost savings are estimated at ~$2B over the next thirty years.