The research objective of this award is to test the hypothesis that micrometer-amplitude vibration of a diamond cutting tool will reduce forces, improve surface finish and decrease wear of the tool. Material science and mechanical engineering faculty and students will measure and model the material removal process and define the parameters that control tool forces, tool wear and surface finish for the selected materials. Materials have been chosen to provide a range of physical and chemical properties that will test proposed concepts of material flow, temperature generation and wear mechanisms. Unique techniques for measuring the chip geometry, surface finish, tool edge sharpness and diffusion of the carbon from the tool to the chip/workpiece will be used to describe the details of the process.
If successful, this project will quantify the vibration conditions needed to create high-quality surfaces on materials such as steel, glass or ceramics that were thought to rapidly wear diamond tools. Steel is the most frequently used engineering material due to its excellent properties, availability and low cost. A national and global demand for ultra-precision steel parts and systems with sub-micrometer accuracy exists in automotive, medical and optical industries. Precision glass and ceramic components are also in high demand. In addition, this research project provides the opportunity for graduate and undergraduate students at North Carolina State University to learn the details and extend the capability of ultraprecision machining. The results of the work will be presented at technical meetings and published in scientific journals. The research experience provided to each of these groups will improve the quality of the science and engineering education.
Abstract Certain materials such as steels, nickel, and titanium alloys cause rapid thermo-chemical tool wear and poor surface finish when machining with a diamond tool (DT). However, optical quality surface finishes have been reported with these materials when vibration-assisted machining (VAM) is used. It was theorized that VAM reduces tool wear by reducing chip thickness, tool temperatures and forces. The goal of this research was to observe and measure tool wear during VAM, identify how this wear is related to process parameters and how it changes with VAM parameters. Conventional DT tests on thermo-chemical wearing material were carried out in conjunction with finite element (FE) simulations to develop a thermo-chemical wear model at the tool edge. To relate this model to VAM, kinematic calculations identified important parameters; namely relative tool tip velocity and instantaneous chip thickness. Tool temperatures were extracted from FE results and used to develop the thermo-chemical wear model. These predictions can be used to identify optimal EVAM conditions to minimize thermo-chemical wear. FEM Temperature Modeling FEM models of EVAM show average temperature is reduced, but peak temperature is increased when comparing it with conventional machining at the same tool speed. Chemical tool wear (calculated based on the Arrhenius model) is reduced with EVAM, but only when comparing conventional DT and EVAM at the same workpiece ( not tool) speed. When compared at the same tool/workpiece speed, the results are similar. This agrees with results from EVAM experiments on AISI1215 steel. Diamond Tool Wear in conventional DT Speed is the key component in determining thermo-chemical tool wear due to its direct relationship with temperature. Wear per machining distance is used as the definition of ‘wear rate’ rather than wear per time. The tool wear, while machining different ferrous alloys (AISI1215 steel and Stavax 420 stainless steel), follows an Arrhenius-type model previously theorized for diamond turning literature but unproven. Wear per distance exhibited a minimum at workpiece speeds of around 1.5 m/s for AISI1215 and 0.9 m/s for Stavax stainless steel. The Arrhenius coefficients, including activation energy, are much lower than those for graphitization or diffusion of diamond into metal. This indicates that diamond tool wear is a highly catalyzed reaction process exacerbated by the transport of diamond material with the chip. Diamond Tool Wear in Vibration Assisted Machining The relationship between VAM parameters and tool wear are based on temperatures from the FE simulations and measured tool wear. There are two key parameters 1) relative speed between tool and workpiece and 2) sliding distance of the tool during the experiment. For conventional DT, where the tool is fixed and the part moves, finding this speed and distance is straightforward. For VAM however, the tool is moving and it is the actual tool speed with respect to the workpiece and the total sliding distance between them that is used for comparison. Tool Wear Tool wear was measured in an Scanning Electron Microscope using the EBID technique to improve contrast. This technique allows quantitative measurements of the tool edge shape and the ability to perform additional experiments on the same tool and measure the change in shape. Elliptical VAM and Linear VAM are limited to workpiece velocities less than 0.5 m/s, even for ultrasonic vibration frequencies, based on geometric surface finish. Tool wear during the VAM experiments (with the exception of reduced wear during overlapping cuts on Stavax) was similar to conventional DT, a result that is contrary to previous research. When based on the actual tool speed and sliding distance for the two processes, the Arrhenius wear model describes the wear results for both. Parameters that minimize Tool Wear The conventional DT experiments showed tool wear when machining ferrous metals had a minimum near 1 m/s cutting speed. The experiments for VAM were not able to exceed this value so the results show that wear per unit distance decreased as speed increased. Based on these results, optimal EVAM process design should follow these steps: Use a horizontal ellipse or Linear VAM. This allows higher workpiece speeds based on the surface finish constraint and lower wear. Use highest frequency and largest amplitude available to approach the wear minimum at 1 m/s cutting speed. Surface Finish Despite the high wear rates machining steel, the unique EVAM cutting geometry seems to improve the surface compared to conventional DT. EVAM surfaces do not exhibit hard alloy particle dragging due to the unique tool motion and lower levels of metal pickup on the tool rake face were observed. Each EVAM and LVAM experiment resulted in tool wear with edge recession at least two orders of magnitude larger than the measured PV surface finish. It is hypothesized that at the start of each EVAM cycle, the worn tool ‘burnishes’ the surface and improves the finish where the EVAM chip thickness is zero.
