THICKNESS MEASUREMENTS OF DIAMOND COATINGS ON CEMENTED CARBIDE TOOL INSERTS
William F. Schmidt, Otto H. Zinke, Mahmoud A. Taher, Ajay P. Malshe Department of Mechanical Engineering, University of Arkansas
Keywords: AC magnetic bridge, Carbide inserts, Diamond coatings, Diamond thickness, SEM
ABSTRACT
Diamond coated tools can improve the productivity of machining highly abrasive materials such as high silicon-aluminum alloys used in the automotive industry. Cemented carbide diamond-coated tool inserts have not become an of-the-shelf product due to several difficulties including the reproducibility in the machining performance during use. One cause of the variation in the performance is the thickness of the diamond coating. This paper describes a technique for measuring the diamond thickness on cemented carbide inserts using an AC magnetic bridge. The output of the AC magnetic bridge, when used in the renull mode, gives changes in real and imaginary reluctance of the tool insert as a function of the space between the face of the tool and the face of the bridge. Calibration was obtained using plastic spacers and uncoated tools. With the bridge operating at 5kHz, real and imaginary reluctance of the uncoated tools were measured as a function of lift-off, lift-off values of 0, 12.5, 25.4, and 38.15 microns. The actual variables required to balance the magnetic bridge are resistance and capacitance. Theory shows that the imaginary reluctance changes are directly related to resistance changes and real reluctance changes are directly related to capacitance changes. Also from theory, the real reluctance depends on lift off and are therefore indicative of the diamond thickness. The imaginary reluctance changes are indicative of the composition of the tools. Resistance changes in the tests were essentially constant within a 5% reading error for insert values of 12.5, 25.4, and 38.1 microns. Therefore from the slope of the real reluctance curve, a calibration was obtained. The real reluctance of the coated tools were measured at these same values of lift-off and these values subtracted from the uncoated real reluctance at the same values of lift-off. At 12.5 and 25.4 microns the differences were divided by the respective slopes at these two lift-offs to obtain the thickness of the diamond coatings. No calculation was carried out at 38.1 microns because the polynomial through for which the slope of the calibration curve was obtained is not reliable at the end. To verify the results, a destructive technique was used to determine the diamond thickness. Five inserts were broken in half and the exposed face examined using a scanning electron microscope, (SEM). SEM examination enabled direct measurement of the diamond coating thickness. The measured thicknesses using the SEM ranged from 11.5 µm to 17 µm. The predictions based on the AC magnetic bridge varied between 9.3 µm and 22 µm with an uncertainty of ±5. Factors which contribute to measurement errors are discussed and techniques for improving the results are presented in the paper. The preliminary results indicate that the AC magnetic bridge can be used to non-destructively determine the thickness of diamond coatings in a production environment.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.full paper not received Publisher:Ashgate Publishing Company
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