Estimation of Minimum Uncut Chip Thickness during Precision and Micro-Machining Processes of Various Materials—A Critical Review

Evaluation of the phenomena characterizing the chip decohesion process during cutting is still a current problem in relation to precision, ultra-precision, and micro-machining processes of construction materials. The reliable estimation of minimum uncut chip thickness is an especially challenging task since it directly affects the machining process dynamics and formation of a surface topography. Therefore, in this work a critical review of the recent studies concerning the determination of minimum uncut chip thickness during precision, ultra-precision, and micro-cutting is presented. The first part of paper covers a characterization of the precision, ultra-precision, and micro-cutting processes. In the second part, the analytical, experimental, and numerical methods for minimum uncut chip thickness estimation are presented in detail. Finally, a summary of the research results for minimum uncut chip thickness estimation is presented, together with conclusions and a determination of further research directions.

Prediction of Nonlinear Micro-Milling Force with a Novel Minimum Uncut Chip Thickness Model

The minimum uncut chip thickness (MUCT), dividing the cutting zone into the shear region and the ploughing region, has a strong nonlinear effect on the cutting force of micro-milling. Determining the MUCT value is fundamental in order to predict the micro-milling force. In this study, based on the assumption that the normal shear force and the normal ploughing force are equivalent at the MUCT point, a novel analytical MUCT model considering the comprehensive effect of shear stress, friction angle, ploughing coefficient and cutting-edge radius is constructed to determine the MUCT. Nonlinear piecewise cutting force coefficient functions with the novel MUCT as the break point are constructed to represent the distribution of the shear/ploughing force under the effect of the minimum uncut chip thickness. By integrating the cutting force coefficient function, the nonlinear micro-milling force is predicted. Theoretical analysis shows that the nonlinear cutting force coefficient function embedded with the novel MUCT is absolutely integrable, making the micro-milling force model more stable and accurate than the conventional models. Moreover, by considering different factors in the MUCT model, the proposed micro-milling force model is more flexible than the traditional models. Micro-milling experiments under different cutting conditions have verified the efficiency and improvement of the proposed micro-milling force model.

Influence of Constitutive Models and the Choice of the Parameters on FE Simulation of Ti6Al4V Orthogonal Cutting Process for Different Uncut Chip Thicknesses

The constitutive model and its pertinent set of parameters are important input data in finite element modeling to define the behavior of Ti6Al4V during machining process. The present work focusses on comparing different constitutive models and the parameters sets available in literatures and investigating the quality of the predictions when varying uncut chip thickness (40 µm, 60 µm, 100 µm and 280 µm). In addition, temperature-dependent strain hardening factor along with strain softening phenomenon based reconstructed material model is proposed. The results from the numerical simulations are compared with experimental results available in literature. The comparison shows that the force values are highly influenced by constitutive models and the choice of parameters sets, whereas the chip morphologies are mainly influenced by the uncut chip thickness and constitutive models. This work justifies the need for an appropriate set of parameters and constitutive model that replicate the machining behavior of Ti6Al4V alloy for different cutting conditions.

FE analysis of the effects of process representations on the prediction of residual stresses and chip morphology in the down-milling of Ti6Al4V

Part I of these two-part papers will investigate the effect of three FEM representations of the milling process on the prediction of chip morphology and residual stresses (RS), when down-milling small uncut chips with thickness in the micrometer range and finite cutting edge radius. They are: i) orthogonal cutting with the mean uncut chip thickness t, obtained by averaging the uncut chip thickness over the cutting length, ii) orthogonal cutting with variable t, which characterizes the down-milling process and which is imposed on a flat surface of the final workpiece, and iii) modelling the true kinematics of the down milling process. The appropriate constitutive model is identified through 2D FEM investigation of the effects of selected constitutive equations and failure models on the prediction of RS and chip morphology in the dry orthogonal machining of Ti6Al4V and comparison to experimental measurements. The chip morphology and RS prediction capability of these representations is assessed using the available set of experimental data. Models featuring variable chip thickness have revealed the transition from continuous chip formation to the rubbing mode and have improved the predictions of residual stresses. The use of sequential cuts is necessary to converge toward experimental data.

FE analysis of the effects of process representations on the prediction of residual stresses and chip morphology in the down-milling of Ti6Al4V

In part II of these two-part papers, the effects of four FEM representations of the milling process on the prediction of chip morphology and residual stresses (RS) are investigated. Part II focuses on the milling of conventional uncut chip thickness h with finite cutting edge radius and flank wear, while part I of these two-part papers has reported on the results in the case of milling small uncut chip thickness in the micrometre range with finite cutting edge radius. Two geometric models of the flank-wear land composed of flat and curved wear land are proposed and assessed. The four process representations are: i) orthogonal cutting with flat wear land and with the mean uncut chip thickness h ¯; ii) orthogonal cutting with flat wear land and with variable h, which characterises the down-milling process and which is imposed on a flat surface of the final workpiece; iii) modelling the true kinematics of the down milling process with flat wear land and iv) modelling the true kinematics of the down milling process with curved wear land. They are designated as Cte-h, Var-h, True-h and True-h*. The effectiveness of these representations is assessed when milling Ti6Al4V with a flank-wear land of VB = 200µm.

Cutting Forces in Peripheral Up-Milling of Particleboard

An analysis of forces acting in the peripheral up-milling of particleboard is presented. First, a novel method of high-frequency piezoelectric force signal treatment is proposed and used to separate the original force signal from the vibrations of the previous cutting iteration. This allows for the analysis of single chip cutting force courses during industrial CNC (Computer Numerical Control) milling. The acting forces are compared with the theoretical, instantaneous, uncut chip thickness. The results show that, for a range of 40–60 m/s, the higher the cutting speed used, the higher the resultant and principal cutting forces. The method of cutting thrust force used was similar to that observed in solid wood milling, i.e., first using a pushing action, followed by a pulling action. The obtained average specific principal cutting forces for particleboard peripheral up-milling are equal to 32.0 N/mm2 for slow and 37.6 N/mm2 for fast milling. The specific cutting thrust force decreases with the increase in instantaneous uncut chip thickness.

Investigation of effect of uncut chip thickness to edge radius ratio on nanoscale cutting behavior of single crystal copper: MD simulation approach

Extremely small cutting depths in nanoscale cutting makes it very difficult to measure the thermodynamic properties and understand the underlying mechanism and behavior of workpiece material. Highly precise single-crystal Cu is popularly employed in optical and electronics industries. This study, therefore, implements the molecular dynamics technique to analyze the cutting behavior and surface and subsurface phenomenon in the nanoscale cutting of copper workpieces with a diamond tool. Molecular dynamics simulation is carried out for different ratios of uncut chip thickness ( a) to cutting edge radius ( r) to investigate material removal mechanism, cutting forces, surface and subsurface defects, material removal rate (MRR), and stresses involved during the nanoscale cutting process. Calculation of forces and amount of plowing indicate that a/ r = 0.5 is the critical ratio for which the average values of both increase to maximum. Material deformation mechanism changes from shear slip to shear zone deformation and then to plowing and elastic rubbing as the cutting depth/uncut chip thickness is reduced. The deformation during nano-cutting in terms of dislocation density changes with respect to cutting time. During the cutting process, it is observed that various subsurface defects like point defects, dislocations and dislocation loops, stacking faults, and stair-rod dislocation take place.