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2004 |
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Precision Measurement of Small Diameter Hole in a Metallic Object |
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Equipment, Machines & Instruments: Design & Testing |
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| Content |
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There are quite a few industrial application such as in aerospace and disk drive industry require precision measurement of small diameter holes with high aspect ratio. Some of the standard measurement systems such as optical techniques while capable of characterizing holes on the surface of the workpiece, would face some difficulties to characterize the interior of a hole especially if the hole is a non through hole. A novel vibroscanning method has demonstrated the possibility measurement of 125µm micro hole with level of precision better than 0.5µm [1,2]. However, this technique requires some special probe, which is not widely available. So quite often, CMM using mechanical probe is still commonly used in industry. But using CMM for small hole measurement has certain drawbacks. It is expensive, time consuming, and it is a contact measuring technique. Also it is difficult to use CMM for the measurement of very small diameter hole. Therefore it is desirable to develop a more widely available cost effective precision alternative approach.
Capacitance sensor may be a good solution as it is cost effective, widely available, and it has been used for measurement of fastening hole for the aerospace industry [3]. It is the objective of this paper to refine this technique for the measurement of small diameter high aspect ratio hole. It is also the objective of this paper to see the level of precision of measurement can be achieved using the capacitance sensor.
There is a challenging issue facing capacitance measurement system for small diameter hole with high aspect ratio. As the hole is small and deep, it is not practical to make the capacitance sensor much smaller than the diameter of the hole. As a result, the capacitance sensor inside the hole has a tendency of picking up signal from various region of the hole. Therefore, efficient way of deciphering the integrated capacitance signal for the profile of the hole is necessary. Different types of capacitance sensors have been designed, and the test results have been compared. Also the test results have been compared with CMM measurement. Our preliminary result shows that the level of precision of 0.1µm can be achieve by this technique. This level of resolution is more than an order of magnitude better than the small measurement technique using CMM. It is also more than an order of magnitude of improvement over the previous recorded result [3].
[1] Masuzawa T., Hamasaki Y and Fujino M “Vibroscanning method for nondestructive measurement of mall holes” Annals CIRP 42, p589-92, 1993
[2] Kim B., Masuzawa T., Bourouina T “ The vibroscanning method for the measurement of micro-hole profiles” Meas. Sci. Technol. Vol 10, 697-705, 1999.
[3] Hammond, J L, J R; Glidewell, S R, “Design of algorithms to extract data from capacitance sensors to measure fastener hole profiles” IEEE Transactions on Instrumentation and Measurement (0018-9456). Vol. IM-32, pp. 343-349, 1983 |
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