Accuracy studies on non-destructive metrology of small hole
The use and formation of small holes in different materials is critical to the efficient operation of many industrial processes. The application of such technology spreads across a very broad spectrum of problems, from the efficient ejection of diesel spray to the quality of drawn synthetic yarns. However, there is a lack of suitable non-destructive methods available for evaluating the quality of such holes below the size of 200 μm. This work has been further stimulated by evolving manufacturing technology which has both increased production throughput and the need to assess the quality of the hole. The thesis concentrates on the application of digital image processing to conventional microscopy. In particular random edge detection and internal surface shadow image reconstruction algorithms have been devised to achieve an internal view of the hole. This has been achieved by selectively filtering the infocus- strip formed by the microscope objective. Measurements have been made of the diameter, straightness and interior surface roughness. The method has been applied successfully to the following applications: The internal 3-dimensional visualisation within a 0.2 mm diameter diesel injector hole. The objective is to give a complete account of the hole diameter and straightness at the same speed as a spark erosion tool formed the hole. The internal surface roughness has also been measured. The measurement of a spinneret holes used in the production of synthetic yarn. In this case the hole size varies from 0.07 to 0.5 mm in diameter. Investigations have been performed for Rolls-Royce where an accurate measuremenits required of the straightness and diameter within a typical turbine coolant hole. The exercise has also measured the wall thickness between the hole and the blade surface. A research exercise has been tried for Marconi Electronics, their research specification being to construct the equivalent of a RS232 connector for fibre optic inter-connects. To achieve this they required a series of 0.1 mm holes through which to run a series of fibre optic cables. Again research has been performed to assess the straightness and diameter of these holes. Further investigations has been made to investigate more general applications of the technique for the interior mapping of large production components. Finally although the initial stages of this study were performed using a high cost research microscope an exercise to make a low cost dedicated automated inspection process has been successfully been completed.