Ph.D. Thesis Defense by Ty G. Dawson
Friday, June 14, 2002

(Dr. Thomas Kurfess, advisor)

"Machining Hardened Steel with PCBN Cutting Tools"

Abstract

Machining hardened steels has become an important manufacturing process, particularly in the automotive and bearing industries. Abrasive processes such as grinding have typically been required to machine hardened steels, but advances in machine tools and cutting materials have allowed hard turning on modern lathes to become a realistic replacement for many grinding applications. There are many advantages of hard turning, such as increased flexibility, decreased cycle times, reductions in machine tool costs, and elimination of environmentally hazardous cutting fluids. Despite these advantages, implementation of hard turning remains relatively low, primarily due to concerns about the quality of hard turned surfaces and a lack of understanding about the wear behavior of polycrystalline cubic boron nitride (PCBN) cutting tools.

Because PCBN tools are expensive, excessive tool wear can eliminate the economic advantage of hard turning. To address this concern, this research investigated the effect of changing process conditions on wear behavior when turning hardened AISI 52100 steel with both ceramic-coated and uncoated PCBN cutting tools. Traditional techniques of characterizing tool condition (optical and scanning electron microscopy) provide important information about tool wear, but are generally restricted to qualitative analysis. For a quantitative study of wear, a white light interferometer was used to obtain three-dimensional images of the wear scar on cutting tools. Data from these measurements allow quantification of flank wear, crater wear, and tool life, and empirical wear models describe the behavior well. Using three-dimensional wear data, a new technique was developed to determine volumetric loss in the crater and flank regions by localizing the interferometer data to a CAD file of the tool, then integrating deviations between the localized data and the model.

High quality surfaces are also required if hard turning is to replace any grinding process, so the effects of cutting conditions and tool wear on surface integrity and surface finish were also studied. A major concern in hard turning is the generation of undesirable changes to the surface microstructure and the formation of tensile residual stresses. Additionally, surface roughness must be comparable to grinding if hard turned surfaces are to be accepted. The results of this work indicate that proper selection of machining conditions produces acceptable surfaces in addition to providing adequate tool life for most applications.