Capacitance Sensors Offer Advantages for Thread Inspection Applications - MTI Instruments - #1

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MTI Instruments, Inc. 325 Washington Avenue Extension Albany, NY 12205 PH: +1-518-218-2550 OR USA TOLL FREE: 1-800-342-2203 FX: +1-518-218-2506 sales@mtiinstruments.com www.mtiinstruments.com APPLICATION NOTE Capacitance Sensors Offer Advantages for Thread Inspection Applications Introduction Many manufacturers today, especially suppliers to the automotive industry, insist on 100% inspection of the quality of internal threads on critical parts and components. This reduces the probability of adding future “value” to a part that, in the end, may need to be scrapped. In addition, improperly tapped holes must not reach consumers to avoid warranty concerns, assure customer satisfaction and reduce potential liability. Proper inspection can also pinpoint optimal tool change intervals, saving thousands of dollars per year. The following paper outlines the use of capacitance sensors and their advantage over screw and eddy current systems. The Problem Manual inspection is slow, not always reliable and tedious for the inspector. Because of this manufacturers have struggled to come up with ways to reliably automate the process at a reasonable cost. Automated systems are available that use “screw sensors” which consist of a motorized stage that monitors torque as a test piece is screwed into a bore hole. An abnormally high torque value indicates a crossed thread, an insuffi cient thread depth or possible debris in the hole. Low torque may indicate missing threads and/or a larger than desired hole diameter. Unfortunately the test screw can quickly wear or be cross threaded, requiring operator attention and a tooling change. Additionally, the motorized torque stage is not cheap, often putting the price tag beyond reach for typical applications. And fi nally, a worn screw can provide false rejections leading to the unnecessary scrapping of parts. Eddy current systems can also be automated. Their output is compared to the signature of a stored “ideal master” as the probe is stroked in and out of the hole being inspected. If the signatures match within preset limits the parts are accepted, otherwise they fail. Unfortunately eddy current technology relies on the magnetic properties of the material being measured and if fl uctuations occur the accuracy can be affected. In addition, for proper operation, eddy current probes should be calibrated to the specifi c material being measured for optimal performance, which is not always easily accomplished. MTII appnote: threads.pdf - Page 1 of 2