Cutting Edge Profile Characterizations by White-light Interferometry

Diamond coated cutting tools provide several advantages over traditional cutting tools, specifically in the machining of advanced materials such as composites. One of the main disadvantages of diamond coated cutting tools is the residual stresses induced from the diamond deposition, which lead to tool failure. Moreover, the cutting edge geometry can be critical to the deposition residual stresses of diamond coated tools. In this study, a white-light interferometer was used to acquire tool edge surface data, and a MATLAB-based algorithm was developed to accurately characterize the tool edge radius and the wedge angle. Commercial carbide inserts with five different edge radii were evaluated. In addition, carbide inserts before and after diamond deposition were evaluated to estimate the coating thickness. The insert edge geometry and coating thickness will allow for a quantitative relationship that predicts the residual stress level at the tool edge. The results demonstrate the ability of the MATLAB algorithm, both in its accuracy and efficiency. It is also found that large edge radius cases tend to have a greater deviation from the manufactured specification. Moreover, tools with a larger edge radius have a less perfect round profile at the flank face transition. BACKGROUND In an effort to replace costly polycrystalline diamond (PCD) cutting tools in the machining of advanced materials, technologies such as chemical vapor deposition (CVD) have been developed to apply diamond coatings to cutting tools [13]. Diamond coatings have been the subject of several previous studies [4] due to their unique properties and wide applications for the machining of advanced materials. Diamond coated tools have also been studied as to deposition residual stresses with various results reported [5]. Despite their economic advantages, the performance of CVD diamond tools is sorely outmatched by the PCD counterparts. The less ideal performance of CVD diamond tools is due to their main failure mode, coating delamination [6]. These failures can be catastrophic and are responsible for limiting the tool life of CVD diamond tooling [7]. The major cause for delamination (which occurs most often at the tool flank) is high stresses and degraded adhesion during machining. High stresses are induced during the diamond deposition processes of the tool’s manufacture. During the deposition process, the tool substrate (typically cobalt-cemented tungsten carbide, WC-Co) is heated to a high temperature, 800 to 1000 C, and after diamond deposition, the coated tool is cooled down to room temperature. Due to the differences in the properties of each material, specifically the thermal expansion coefficient, high stresses are imposed upon each material during cooling. The substrate experiences tensile stresses while the diamond coating, which possesses a smaller thermal expansion coefficient, experiences compressive stresses on the order of giga-pascals. Moreover, the change of geometry around the cutting tip area will generate stress concentrations and yet additional stress components. Information about the cutting edge geometry of a cutting insert is essential to predict the deposition residual stresses. In order to determine the maximum allowable stress endurable by the machining process, the deposition residual stresses must be taken into account. A larger cutting radius will alleviate the stress concentration and may increase the allowable machining stresses for the tool insert, also dependent upon cutting conditions [8]. Cutting edge radius is known as a significant factor to the chip formation and the thermo-mechanical states of the cutting tool, etc., in machining processes. Accurate measurements of cutting edge radius are desired for tool quality control as well as performance evaluation [9], especially for high precision machining such as diamond turning [10]. This is particularly important to diamond coated tools for the reason discussed above, deposition stresses [11]. Cutting edge geometry measurements have long been practiced and investigated. In general, there are contact and non-contact measurement methods, e.g., stylus profilometry and optical projection, respectively. Advanced techniques such as atomic force microscopy have also been developed to measure ultra-sharp edges (order of 10 nm) [12]. Recently, white-light interferometry (WLI) has been applied to surface/profile