Kornel F. Ehmann

The Mechanics of Machining at the Microscale: Assessment of the Current State of the Science

This paper provides a comprehensive review of the literature, mostly of the last 10-15 years, that is enhancing our understanding of the mechanics of the rapidly growing field of micromachining. The paper focuses on the mechanics of the process, discussing both experimental and modeling studies, and includes some work that, while not directly focused on micromachining, provides important insights to the field. Experimental work includes the size effect and minimum chip thickness effect, elastic-plastic deformation, and microstructure effects in micromachining. Modeling studies include molecular dynamics methods, finite element methods, mechanistic modeling work, and the emerging field of multiscale modeling. Some comments on future needs and directions are also offered.

Machining Process Modeling: A Review

In this paper, a summary of work performed in the area of modeling of the dynamic metal cutting process is presented. A general view of evolution of the dynamic cutting process models is depicted. Specifically four modeling approaches including analytical, experimental, mechanistic and numerical methods are critically reviewed. A brief assessment offuture research needs is also given.

Volumetric Error Analysis of a Stewart Platform-Based Machine Tool

Abstract Striving for greater accuracy, some machine tool manufacturers are developing parallel structures for the next generation machine tool. Therefore, tools are needed to analyze the effect error sources have on this type of structure. A model of a Stewart Platform based machine tool is developed that provides the framework for inclusion of all relevant error sources. An error analysis is presented based on an error model formed through differentiation of the kinematic equations, and a sensitivity analysis is given as a design tool for tolerance allocation during manufacture. Finally, automated error analysis software is demonstrated that graphically depicts position and orientation errors along tool paths and throughout the workspace.

A CAD approach to helical groove machining—I. mathematical model and model solution

The problem of helical groove machining in practice is still predominantly approached from an empirical trial and error standpoint. For the analytical resolution of this problem through a CAD approach, a generalized helical groove machining model, utilizing the principles of differential geometry and kinematics, has been formulated. The approach is based on the establishment of the fundamental analytical conditions of engagement between the generating tool surface and generated helical groove surface. The general mathematical relationships established facilitate the determination of the resulting tool or helical groove profile for a given helical groove or cutting tool profile, respectively.

Calibration of a hexapod machine tool using a redundant leg

Abstract Parallel configurations are recently being applied to the machine tool with the hopes of greater rigidity, stability, and accuracy than conventional multi-axis structures allow. However, the many calibration methods presently available for serial machine tools are not applicable to hexapod type structures. A calibration method is presented that uses a ball–bar or other simple length measuring device to act as an ‘extra leg,’ allowing calibration of the hexapods true kinematic parameters. This method utilizes a total least squares minimization, does not require any special hexapod configuration or difficult six degree of freedom pose measurements, and is effective with as few as one additional length sensor. Selection of calibration pose sets is briefly discussed, as well as the influence of measurement noise on calibration accuracy. Simulations show the potential for this algorithm to significantly reduce errors to the point where machining errors are within 5–10 times the measurement errors.

Machining of Carbon Fiber Reinforced Plastics/Polymers: A Literature Review

Carbon fiber reinforced plastics/polymers (CFRPs) offer excellent mechanical properties that lead to enhanced functional performance and, in turn, wide applications in numerous industrial fields. Post machining of CFRPs is an essential procedure that assures that the manufactured components meet their dimensional tolerances, surface quality and other functional requirements, which is currently considered an extremely difficult process due to the highly nonlinear, inhomogeneous, and abrasive nature of CFRPs. In this paper, a comprehensive literature review on machining of CFRPs is given with a focus on five main issues including conventional and unconventional hybrid processes for CFRP machining, cutting theories and thermal/mechanical response studies, numerical simulations, tool performance and tooling techniques, and economic impacts of CFRP machining. Given the similarities in the experimental and theoretical studies related to the machining of glass fiber reinforced polymers (GFRPs) and other FRPs parallel insights are drawn to CFRP machining to offer additional understanding of on-going and promising attempts in CFRP machining.

Development of a virtual machining system, part 1: approximation of the size effect for cutting force prediction

Abstract In this three-part paper, components of a virtual machining system for evaluating and optimizing cutting performance in 2 1 2 -axis NC machining are presented. Part 1 describes a new method of calculating cutting-condition-independent coefficient and its application to the prediction of cutting forces over a wide range of cutting conditions. The prediction of the surface form error and transient cutting simulations, described in Parts 2 and 3, respectively, can be effectively performed based on the cutting force model with the improved size effect model that is presented in Part 1. The relationship between the instantaneous uncut chip thickness and the cutting coefficients is calculated by following the movement of the center position of the cutter, which varies with nominal feed, cutter deflection and runout. The salient feature of the presented method is that it determines the cutting-condition-independent coefficients using experimental data processed for one cutting condition. The direct application of instantaneous cutting coefficient with size effects provides more accurate predictions of the cutting forces. A systematic comparison of the predicted and measured cutting forces over a wide range of cutting conditions confirms the validity of the proposed mechanistic cutting force and size effect models.

Error model and accuracy analysis of a six-DOF Stewart Platform

Precision machining operations necessitate highly accurate, rigid, and stable machine-tool structures. In response to this need, parallel architecture machines, based on the concepts of the Stewart Platform, are emerging. In this paper considering major inaccuracy factors related to the manufacture, geometry, and kinematics, of such machines, first and second order error models are presented, and followed by a comparative assessment of these models in conjunction with illustrative examples. Furthermore, in order to understand the character and propagation of errors of 6-DOF Stewart Platform based machine tools, sensitivity analysis is adopted to describe the contribution of each error component to the total position and orientation error of the mechanism. An automated error analysis system that computes and graphically depicts the error distributions throughout the workspace along with the results of sensitivity analysis is developed and demonstrated.

A cutting force model for face milling operations

Abstract A procedure for the simulation of the static and dynamic cutting forces in face milling is described. For the static force model, the initial position errors of the inserts and the eccentricity of the spindle are taken into consideration as the major factors affecting the variation of the chip cross-section. The structural dynamics model for the multi-tooth oblique cutting operation is assumed as a multi-degrees of freedom spatial system. From the relative displacement of this system, based on the double modulation principle, the dynamic cutting forces were derived and simulated. The simulated forces were subsequently compared to measured forces in the time and frequency domains.

A Generalized Model of the Surface Generation Process in Metal Cutting

Machined surfaces are generated by a variety of processes, each of which produces a surface with its own characteristic topography. A method for the prediction of the topography of the generated surfaces has been developed based on general models of the machine tools kinematics and a generalized model of deterministic and non-deterministic cutting tool geometries. The model termed the surface-shaping system accounts for not only the nominal or global motions of the machine but also takes into account errors during machining such as tool runout, machine deformation and vibration, as well as higher order motions. Based on the surface shaping system model a computer simulation system has been developed which facilitates the 3D graphical representation and evaluation of the topography of the generated surface.