Non-contact inspection of internal threads of machined parts

During the manufacturing process of mass-produced automotive parts, millions of internal threads are produced annually by each automaker. The manufacturers invest a lot of effort and resources to improve the quality of the threads by inspecting their features, mainly by using manual contact gages. The majority of manufactured threads are used in threaded joints that assemble two mechanical parts. However, there are threads that have additional functionality, for example, an automotive spark plug. Recently, automotive designers have found that the angular direction of the spark plug inside the combustion chamber affects the efficiency of the engine. Hence, the manufacturers are interested in measuring the exact position of the thread starting point, allowing them to position the plug in the right direction during the plug assembly process. Our technique allows precise measurement of the thread starting point and meets this requirement. In order to improve the quality of internal threads, it is essential to measure the threads dimensional features. It is also important to inspect its internal surface for defects such as the existence of burrs, thread discontinuities, or internal porosity defects. The paper introduces two novel non-contact electro-optical techniques for inspecting small-size internal threads. The first one enables in-process internal thread geometrical measurements using a high-precision laser sensor which utilizes a motorized periscope. The second technique is based on image processing and uses innovative machine vision; a line-scan technique which uses a CCD camera with a "sight pipe" panoramic optical device. The two methods are complementary; together they allow the practical inspection of all needed features of the internal thread. The methods were tested successfully.