This paper aims at developing a unified approach to identify the cutting force coefficients together with the cutter runout parameters for general end mills such as cylindrical, ball, bull nose ones, etc. The cutting forces that are modeled using the instantaneous cutting force coefficients are analyzed and separated into two terms: a nominal component independent of the runout and a perturbation component induced by the runout. The nominal component enables the calibration of the instantaneous cutting force coefficients whereas the runout parameters are determined from the perturbation component. The validity of the present method is demonstrated with simulation and experimented data.

Efficient calibration of instantaneous cutting force coefficients and runout parameters for general end mills

During the milling operation, the cutting forces will induce vibration on the cutting tool, the workpiece, and the fixtures, which will affect the surface integrity of the final part and consequently the product's quality. In this paper, a generic and improved model is introduced to simultaneously predict the conventional cutting forces along with 3D surface topography during side milling operation. The model incorporates the effects of tool runout, tool deflection, system dynamics, flank face wear, and the tool tilting on the surface roughness. An improved technique to calculate the instantaneous chip thickness is also presented. The model predictions on cutting forces and surface roughness and topography agreed well with experimental results.

An improved cutting force and surface topography prediction model in end milling

This paper aims at developing efficient strategies for controlling the force-induced surface dimensional errors in peripheral milling of thin-walled structures. Emphasis is put on how to select the feed per tooth and depth of cut simultaneously for tolerance specification and maximization of the feed per tooth simultaneously. Three methods are presented. The first one proceeds by optimally selecting the maximum feed per tooth without tolerance violation. The second one is to find the appropriate cutting parameters by solving a linear programming problem. To show the efficiency of the first two methods, the third one, i.e. the so-called mirror error compensation method taken from references, is also addressed for comparison. Mechanistic model for cutting force estimation, cantilever beam model for cutter deflection estimation and finite element method for workpiece deflection estimation are used in all three methods. Besides, improvements on the calculation scheme of the surface dimensional error have been made and both numerical and experimental results are adopted for verification.

Strategies for error prediction and error control in peripheral milling of thin-walled workpiece

In this paper, new procedures are proposed to calibrate the instantaneous cutting force coefficients and the cutter runout parameters for peripheral milling. By combining with optimization algorithm, i.e., the Nelder–Mead simplex method, detailed calibration schemes are derived for a mechanistic cutting force model in which the cutting force coefficients are described as the exponential functions of the instantaneous uncut chip thickness. Three different cutter runout models are considered in the calculation of instantaneous uncut chip thickness. Only one or two tests are required to perform the calibration. Experimental verifications are also conducted to validate the proposed procedures, and the results show that they are efficient and reliable. To see the effect of different runout models on milling process, comparisons among the predicted results under a wide range of cutting parameters are made to study the consistency and limitations of different models. It is found that the radial cutter runout model is a recommendable one for cutting force modelling.

New procedures for calibration of instantaneous cutting force coefficients and cutter runout parameters in peripheral milling

This paper deals with parallel kinematics machine (PKM) applications for high speed machining. PKM behaviour is anisotropic, so structural deflection due to cutting loads generates different defects in the machined part, according to tool position and orientation in the workspace. The aim of the present study is the definition of a mechanical workspace where the part is machined to a specified quality level. Models of the machine tool structure, the load generated by the cutting phenomena and the machined surface are used to determine this workspace and are applied to a tricept structure with telescopic legs for grooving and drilling operations. A simulation shows that leg deflection gives rise to defects of the machined entity which are evaluated with respect to tool position and orientation. Finally, a mechanical workspace is defined for a drilling process using a tricept structure machine tool.

A study of the impact of machine tool structure on machining processes

The flatness defects observed in flank milling with cutters of long series are mainly due to the tool deflections during the machining process. This article present the results of an identification procedure of the coefficients of a force model for a given tool workpiece couple for the prediction of the defects of the tool during the cutting. The calibration method proposed meets a double aim: to define an experimental protocol that takes the industrial constraints of time and cost into account and to work out a protocol which minimizes uncertainties likely to alter the interpretation of the results (environmental, software or mechanical uncertainties). For that, the procedure envisages the machining of a simple plane starting from a raw part formed by a tilted plane, allowing for the variation of the tool engagement conditions. The tool deviation during the cutting process is indirectly identified by measuring the machined surface. The observed straightness defect conditions can be explained by the evolution of the cutting pressures applied to the cutting edges in catch during the cutter rotation. The precision was considerably improved by the taking into account of the cutter slope defect in the calculation of the load applied to the tool. After identification of the tool-workpiece couple, the prediction model was applied to some examples and allowed to determine the variations of form and position of the surface points with a margin of 5%.

/pdf/deviation-of-a-machined-surface-in-flank-milling-o9ukp0an5e.pdf

Deviation of a machined surface in flank milling

The flank milling of complex forms is a very effective process from the point of view of productivity and surface quality. Many works deal with research on the optimal positioning of the tool which is considered as a rigid body in order to minimize tool path errors. The purpose of our work is to integrate the compensation of the tool distortions in this optimal positioning calculation. In flank milling with long tools, the distortion of the cutter generates a significant wave (that can reach 0,6mm) on the machined surface due to the effects of the helical angle and the radial force which varies during the cutter rotation. After detailing an analysis of the force evolution and the associated model calculation, we will present a test protocol, that can be implemented in industry, in order to characterize the model parameters as a function of the couple tool-workpiece material. Then we will present a test to assess our prediction model of the straightness defects of the machined surface according to all machining parameters. These results make it possible to make up for defects by applying a translation to the tool in 3-axis and by applying a translation combined with a rotation in 5-axis milling.

/pdf/contribution-to-the-generation-of-tool-paths-in-a-cam-system-58dkppiu7p.pdf

Arnaud Larue

Papers

Deviation of a machined surface in flank milling

Contribution to the Generation of Tool Paths in a Cam System