Modeling of metal cutting and ball burnishing - prediction of tool wear and surface properties

Abstract

The main theme of the proposed research was centered on a broad scope of surface finishing processes, including conventional finish cutting (turning) to ball burnishing, which is relatively new and used to provide modifications to the machined surface quality.In finish cutting, the effects of edge preparation and tool wear of the cutting tool are considered most critical, as they directly determine surface finish and properties of the subsurface layer (residual stress, microstructure, microhardness).In addition, different tool life criteria based on surface finish, excessive burrs, or dimensional run-out are all related to tool wear.Therefore, understanding of the tool wear behavior and the capability of predicting it are the key to successful process control and optimization.To work towards this goal, the effect of different cutting edge designs (hone and chamfer geometries) on the cutting variables and process mechanics was first investigated using Finite Element Method (FEM) cutting simulations.Then, an FEM-based methodology involving nodal wear rate calculations was developed to predict the progression of tool wear geometry during cutting, initially for uncoated carbide tools.The predictions of the proposed model were verified with the data obtained from the tool wear cutting experiments.After refinements of the incorporated tool wear model, this simulation method may be extended to other solid uncoated tool materials such as PCBN.Today, cutting inserts with multilayer coatings are used in every category of industrial cutting applications.However, the analysis of the wear behavior of coated tools largely relies on extensive experimental tests.To supplement reliable process data and hence iii reduce the required experimental runs/costs, a general FEM simulation model for coated tools was developed by modeling the coating structure as an effective composite layer (vs. the individual layer model).The thermal insulation effect of the hard coatings (ex.Al2O3) was evaluated using the proposed models and qualitatively compared with the experimental data in the literature.The developed analysis model for coated tools was then applied to a selected industrial case study in which a comparative analysis of tool wear under different cutting conditions was required.The wear characteristics of the coated tool used (1mm-TiN/9.5mm-Al 2 O 3 /4mm-TiCN) against two different workpiece materials (AISI 1080 & 8219) were analyzed through conventional turning tests at semi-finishing conditions.Correspondingly, an approximated 2-D simulation model was developed based on fresh tool geometry.This model predicted the initial wear rate at the start of cutting and allowed differentiation of the tool wear under different cutting conditions as well as between the two materials.Another focus of this research was on the surface enhancement generated by ball burnishing, which is used following machining to improve surface finish and provide a surface layer of compressive residual stresses.For a successful ball burnishing process, the selection of process parameters (burnishing pressure, ball diameter, speed, and feed rate) needs to be optimized.In this research, a full 3-D FEM analysis model and a simplified 2-D model were developed for this purpose and the predictions of residual stresses were evaluated with limited experimental data.For the 2-D model, the strong elastic recovery of the burnished surface caused contact with the ball around its trailing end during unloading, making the simulation like a series of "indenting" cycles.Furthermore, the effects of the initial plastic strain and residual stresses in the machined surface, as opposed to uniform bulk material, were analyzed.Preliminary results showed that they did not have a significant effect on the predicted residual stresses after burnishing.