Factors Affecting Surface Topography in Diamond Turning

Abstract

Ultraprecision, single point diamond turning (SPDT) is a tool based machining technology that allows the ability to produce high quality surface finishes on the order of nanometers while meeting tight form tolerances on the order of micrometers. It is generally agreed that surface finish in SPDT is primarily affected by four factors: Tool edge quality, relative vibration between the tool and workpiece, material properties and microstructure, and tool geometry (nose radius and machining parameters) machining. To the author’s knowledge, no work has been done to combine all the factors to study their effect on surface generation in SPDT. This is important given that the factors are highly interdependent. Two diamond tools with nose radius of 12mm were used; however, one of them was chemically honed. Results suggest that the honed tool provides a much better surface finish with a significantly reduced amount of running-in stage tool wear. The cutting edge radius of the diamond tools was measured using a novel 3D confocal laser microscope to analyze the chemical honing process and to measure tool wear. The presence of built-up edge (BUE) is more prominent on the honed tool earlier in its life which results in unpredictable surface roughness to appear sooner than on the regular tool. To understand the dynamics of the machine, a redesign of the tool holder bracket was done to increase stiffness. Modal tests were then performed on it to verify performance improvement. With an understanding of the vibration and its effect on the cutting force, a 400Hz disturbance frequency was detected in the cutting forces. From a 3D scan of the surface, a total of 24 undulations on the surface of the part were observed when the spindle speed was set to 1000RPM The machine was instrumented and a rotordynamic investigation was carried out to determine the cause and nature of the vibration in an effort to reduce it and in so doing improve surface form accuracy.