Radial Engagement Research Articles

On the effect of radial engagement on the milling stability of modes perpendicular to the feed direction

This paper analytically studies the effect of radial engagement on chatter stability when the flexibility is perpendicular to the feed motion, with thin-walled part milling being the most relevant industrial case. By studying the mean directional factor and its harmonics, the superior stability of the up-milling strategy over down-milling for both Hopf and flip chatter is demonstrated. The optimal engagement for a theoretical infinite stable depth of cut Hopf is obtained, as well as the expressions for the critical depth of cut. For flip chatter, three different cutting zones with different lobe shapes can occur depending on the radial engagement. It is also shown that while flip chatter cannot be completely eliminated by tuning the radial engagement alone, an engagement can be found in up-milling that maximises stability. Finally, the findings are validated through experimental tests.

A Non-Uniform Offset Algorithm for Milling Toolpath Generation Based on Boolean Operations

In milling, the advancement of CAM strategies has increased the need for tailored algorithms for semi-finished phase computation. In some cases (e.g., thin-wall milling), variable radial engagement of the tool during the toolpath is desired, leading to the need of non-uniform machining allowance on the component that could be achieved only with a non-uniform offset algorithm, i.e., offset where the distance to the initial contour varies along that input. This work presents a general algorithm for non-uniform offset of polyline curves. The approach is based on 2D polygons and Boolean union operation, following these steps: (i) projection segments are generated, (ii) polygons (trapezoids and circular sectors) are created, (iii) Boolean union of all the polygons is performed, (iv) boundary of interest is extracted. The proposed algorithm is able to handle both internal and external offset and is robust for complexity of both the polyline and variable offset magnitude along that line, as proven by several examples and two applications to thin-wall milling provided.

Optimal cutting condition selection for high quality receptance measurements by sweep milling force excitation

In large machine tool applications, where machine's productivity is limited by structural chatter vibrations, an accurate dynamic characterization is vital for predicting reliable stability lobes. Motivated by the fact that the sweep milling force excitation method shows a great potential, this paper conducts a numerical stability analysis for optimizing the tool radial engagement employed during the characterization cutting tests. Additionally, an alternative spectra computation technique is proposed which considerably improves the signal to noise ratio over the existing technique. The receptance measurement is demonstrated using time domain simulations and applied in a large-scale three axis machining centre obtaining more accurate stability boundary predictions. The source code of the temporal milling simulation and the sweep milling signal processing is freely available to reproduce the simulation and experimental results. https://github.com/OierFranco/SMFE.

SEMI-ANALYTICAL PERIOD-DOUBLING CHATTER ANALYSIS IN THIN WALL MILLING

During thin-walled part milling, a dominant flexible direction perpendicular to the feed motion is most likely to exist, which allows to formulate the stability problem in the frequency domain in a very simple form. By these means, the existence of optimal engagements under up-milling strategy for achieving a theoretical infinite Hopf stability have already been demonstrated. However, period-doubling chatter can also pose a limit to the productivity in thin wall milling, but the knowledge on optimal engagements that can cancel this kind of chatter is inexistent. This paper discusses the effect of the radial engagement and number of flutes on flip stability in a dimensionless way and independent on the system dynamics. Up- and down- milling strategies are compared: a larger period-doubling prevalence is identified in the former, although in terms of absolute critical depth of cut, up-milling outperforms down-milling for most of the practical cases. It is also demonstrated that even though it is possible to find optimal engagements that minimise the flip likelihood, it is impossible to totally cancel the period-doubling chatter by simply tuning the radial engagement, which leaves the cutter helix tuning as the only way to completely eliminate flip chatter. Finally, the obtained results are validated through semidiscretisation simulations.

Effect of Temperature on Tool Wear During Milling of Ti64

AbstractIn recent years, the development of new, increasingly resistant materials limit machining productivity. This observation is especially true for titanium alloys. The state-of-the-art shows that one of the phenomena responsible for tool wear is temperature. The high temperature is explained by the low thermal conductivity of the alloy and its high mechanical properties. Consequently, high temperatures generated when cutting speeds are increasing lead to very rapid wear phenomena. However in milling, the period during which the insert is not in contact with the material may allow it to cool but its effect is not clearly established. In order to correlate tool wear and cutting temperatures in milling, an experimental bench has been developed. In turning and therefore with a fixed tool, the milling conditions are recreated and allow to measure the temperatures on the cutting face. Two parameters were tested: (i) radial depth, which influences the tooth stress time, and (ii) the cutting speed, which is the fundamental parameter of the cutting temperature. Experimentally, it appears that increasing radial engagement and cutting speed reduces tool life and increases temperatures. However, the phenomenological analysis is not immediate. The relationship between these phenomena is based on a heat balance of the cutting process. The use of an infrared (IR) camera in this problem and a specific analysis method allow observing the temperature gradients on the cutting face making the analysis more robust compared to the thermocouple technic. It thus appears that the increase in radial engagement leads to a higher tool temperature, but the analyses show above all a higher temperature within the insert and therefore more difficult to evacuate.

The influence of radial engagement and milling direction for thin wall machining: a semi-analytical study

Milling stability prediction of regular milling processes has to be normally carried out by means of complex numerical methods due to the multidimensional nature of the limiting modes, hindering the direct study of the influence of the process parameters on milling stability. However, the dynamics of thin-walled part milling processes, especially flank milling operations, can often be described in a unique direction. Thus, the directional factor matrix is condensed onto a single coefficient whose mean value uniquely depends on the engagement limits and the radial-tangential cutting force ratio. By these means, this paper studies the influence of radial engagement and milling direction on thin wall milling stability. The superior performance of up-milling is demonstrated on account of its special geometric configuration, permitting both positive and negative mean directional factor situations. In addition, the expressions for optimal radial engagement for minimum Hopf chatter likelihood are provided, which serve as excellent tool for process planning regardless of the part dynamics or other process parameters. The obtained results are finally validated through semidiscretisation and initial value time domain simulations.

Effects of partial tool engagement in micro-EDM milling and adaptive tool wear compensation strategy for efficient milling of inclined surfaces

Abstract Compensation of tool wear in micro-EDM milling of inclined surfaces is a difficult task due to variations of the radial engagement conditions of the tool with the workpiece. Traditional compensation methods based on off-line wear prediction are likely to be inaccurate, while methods involving tool measurements are time-consuming. In this research, the process dynamics when the tool is not fully engaged with the workpiece are investigated by performing repeated milling tests and analysing the effects on the discharging process and tool wear behaviour. An adaptive tool wear compensation strategy is accordingly developed, combining traditional linear compensation method (LCM) and tool wear sensing. In this strategy, simultaneous monitoring of the normal discharge pulses and tool displacement is used for determining the tool engagement conditions. This allows to adapt the milling process to the expected tool wear behaviour when the tool is in partial engagement. The proposed control strategy has been tested for an industrially-relevant case, i.e. the shaping of a diffuser for a turbine blade. A machining accuracy in the range of the applied milling layer thickness was obtained without the need of tool measurements.

Effect of Workpiece Curvature on Axial Surface Error Profile in Flat End-Milling of Thin-walled Components

The advancements in the domain of CAD/CAM and CNC machine tools facilitated machining of complex geometric shapes to meet functional requirements and/or aesthetic appearances. These shapes combine several geometric features such as zero curvature (straight), constant curvature (circular) and variable curvature surfaces. The components often contain thin-walled sections having inherently lower stiffness that induces static deflections transforming into surface error and violation of error limits specified by the designer. This paper presents a comparison of deflection induced surface error profile generated during the end milling of zero and constant curvature thin-walled components. The proposed framework incorporates computational model to estimate cutting forces, Finite Element Analysis (FEA) model to compute workpiece deflections, and surface generation mechanism to derive error profile. The paper also investigates the effect of change in the radial engagement area due to workpiece curvature by introducing the concept of ‘Equivalent Radial Depth of Cut’. Further, the effect of change in workpiece curvature is investigated on the surface error profile in the paper. The proposed framework has been generalized to accommodate the variation of workpiece geometry, and the results are validated by conducting a set of end milling experiments.

Analysis and modelling of trochoidal milling in Inconel 718

Trochoidal path increases productivity, tool life and reduces cutting forces compare to classical slot milling. Consequently, this strategy is well adapted to improve the milling performance in refractory alloy such as Inconel 718. The tool path complexity affects the tool radial engagement and axes dynamic, so it appears difficulties to perform such strategy. Therefore, this article deals with an analytical approach of trochoidal modelling to determine the theoretical radial depth of cut, cut thickness, cutting forces and machine tool behavior in function of the different methods to program the trochoidal trajectory. Finally, this methodology allows the optimization of geometrical and kinematic parameters for trochoidal milling of Inconel 718.

Primena neuronskih mreža za predviđanje rešenja kinematskih grešaka kod trohoidne obrade petoosnom glodalicom matsuura MAX-330

The prediction of machining accuracy of a Five-axis Machine tool is a vital process in precision manufacturing. This work presents a novel approach for predicting kinematic errors solutions in five axis Machine. This approach is based on Artificial Neural Network (ANN) for trochoidal milling machining strategy. We proposed a multi-layer perceptron (MLP) model to find the inverse kinematics solution for a Five-axis Machine Matsuura MX-330. The data sets for the neural-network model is obtained using Matsuura MX-330 kinematics software. The solution of each neural network is estimated using inverse kinematics equation of the Machine tool to select the best one. As a result, the Neural Network implementation improves the performance of the learning process. In this work trochoidal trajectory generation formulation has been developed and simulated using the software Matlab Inc. The main advantage of the trochoidal path is to present a continuous path radius leading the machining process to take place under favorable conditions (no impact, less marking of the part, ...). Obtaining the toolpath is to allow programming of the toolpath according to ISO 6983 (which defines the principles of the G code). For this, numerical study of trochoidal strategy and experimental result are presented with aims to full milling and to ensure a control of radial engagement.

A Generic Multi-Axis Post-Processor Engine for Optimal CNC Data Creation and Intelligent Surface Machining

This paper focuses on the development of a multi-axis post-processor engine with a curvature-based feed adaptation module, capable of extracting generic CNC data for high precision machining. The motivation of this work stems from the drawback of standard and commercial post-processors to modify their internal source codes so as to be implemented to newly-developed functions which integrate modern CNC units. The multi-axis post-processor proposed in this work operates as a stand-alone function of an artificial intelligent module that optimizes machining parameters for standard swept cut multi-axis surface tool-paths. The post-processor developed receives APT source files previously been optimized by means of a genetic algorithm that handles cutting tool selection; radial cut engagement; maximum discretization step; lead and tilt angles. The algorithm optimizes the aforementioned machining parameters towards the minimization of the number of cutter locations found in a specific APT source file as well as the surface machining error as a combined effect of chordal deviation and scallop height. The final APT output is then properly handled by the post-processor engine so as to extract the final ISO code for a double-pivoted head 5-axis CNC machine and compute optimal values for feed rate in each NC block considering the interpolation error and curvature analysis given the surface properties. To simulate and verify our proposals, the MAZAK Vortex 1000 gantry-type 5-axis CNC machine tool equipped with a Fanuc 15i CNC unit has been selected as the manufacturing resource corresponding to the final CNC output that the proposed post-processor computes. A benchmark sculptured part is created and used for the virtual material removal simulation in CATIA® V5 R18. For that part, both the proposed post-processor engine and a commercially available post-processor were employed to extract G-code data whilst it was shown that identical outputs were obtained.

Solid subtraction model for the surface topography prediction in flank milling of thin-walled integral blade rotors (IBRs)

Current CAM and milling verification software packages are not ready to predict form and surface errors derived from non-ideal conditions such as runout tool error or tool–workpiece flexibility. As a consequence, high-end parts like integral blade rotors (IBR) suffer from this lack of estimation to achieve a sound manufacturing process preparation. This work describes the implementation of a cutting force prediction model that introduces the radial engagement reduction caused by (i) tool runout and (ii) workpiece flexibility. The model is used as the seed for a calculation module based on solid modeling to obtain the surface roughness and topography applying the cutting force–wall restitution equilibrium, making use of the power of CAM solid modelers for the final topography definition. This idea takes advantage of the good accuracy of current solid modelers in comparison with the z-map topographical approach for obtaining the cutter–workpiece engagement (CWE) at each tool path step. Several flank milling tests were carried out to validate the simulations. The results demonstrated a significant level of accuracy to predict shape errors. The model may be integrated in a CAM procedure, and it is a useful utility for reducing deformation on the manufacturing of blisks and impellers.

Effects of cutting conditions on forces and force coefficients in plunge milling operations

The modeling of milling forces is a crucial issue to understand milling processes. In the literature, many force models and experiments to identify force coefficients are found. The objective of this article is to develop a new approach, based on the traditional average force method, able to measure and compute the cutting coefficients for end mills used in plunging operations. This model has been used to evaluate the effect of the radial engagement on the cutting coefficients themselves, proposing a new strategy to update these values for different cutting parameters. This dependency of the cutting coefficient is particularly important for the determination of the stability lobe diagrams, used to predict the chatter conditions. In this article, the method to assess the cutting coefficients, the results of the experimental tests, and the effect of condition-dependent cutting coefficients on process stability are presented.

Control of the contact force in a pre-polishing operation of free-form surfaces realised with a 5-axis CNC machine

The challenge is to control the radial force applied during pre-polishing operations realised with milling machines. This permits managing the contact pressure between the tool and the machined surface. Since CNC machines are controlled in position but not force, a flexible tool is used to obtain a smooth connection between contact force and tool position. A Design Of Experiment demonstrates the major rule of the radial engagement. A 5-axis toolpath is proposed to optimise pre-polishing operations of free-form surfaces. It accounts for the real tool shape, thus improving the stability of contact force. Experiments are realised to validate the developments.

On cutter deflection surface errors in peripheral milling

Cutter deflections induce significant amount of surface error on machined components and it is one of the major obstacles towards achieving higher productivity in peripheral milling operation. These surface errors do not take one particular form and their shape and profile measured along axial direction, varies significantly with cutting conditions. The understanding and characterization of all possible surface error types is of immense value to process planners as it forms a basis for controlling and compensating them. This paper presents a methodology to classify surface error profiles and to relate the same with cutting conditions in terms of axial and radial engagement between cutter and workpiece. The proposed characterization scheme has been validated using computational studies and machining experiments. The importance of proposed characterization is further demonstrated in understanding peripheral milling of curved geometries where workpiece curvature influences radial engagement of the cutter that often changes surface error shape both qualitatively and quantitatively. Computational and experimental studies undertaken to study machining of curved geometries underline the importance of proposed characterization scheme in identifying correct cutting conditions for a given machining situation.

Evaluation of 5-axis HSC dynamic behavior when milling TiAl6V4 blades

Gas combustion represents the second principal way of electricity generation due to its disposal, and smaller dimension and pollution capacity when compared with diesel engines. The conversion of natural gas in electricity is made through gas turbines, and the rotors of its compressors are components that present significant difficulty in its manufacture. This paper focuses on an optimization approach of the 5-axis milling of titanium blisks, based on all the CAD/CAM modeling chain. Special interest is pointed to the systematic analysis of the correlation between machining parameter and surface integrity properties. The experimental part of this work is composed by two tests. The first one analyses the roughing operation of a TiAl6V4 impeller when using a tool radial engagement of 30% and 68%. The second is the simultaneous 5 axes milling of a test-piece, a TiAl6V4 integral bladed disk (BLISK) section containing five blades, with which the best milling strategy for this purpose is sought. The results showed the fundamental importance of CAD modeling in order to achieve simultaneous 5 axes milling without intermittent feed speed. It was observed that milling a blade following its parametric curves in the BLISK axis direction is the best choice, among the strategies proposed.

Optimization of surface roughness in an end-milling operation using nested experimental design

This paper presents an experimental study to optimize the surface quality of an end-milled surface on a Vertical Machining Centre using Taguchi’s nested experimental design. The effect of various machining parameters on surface roughness was investigated on two different work piece materials, Aluminium alloy and Plain Carbon Steel. Other control factors, namely, feed rate and spindle speed, depth of cut and radial engagement of tool were varied in the experiment to measure surface roughness at four different positions on the work piece. Position was taken as an uncontrollable noise factor. Depth of cut was observed to be the most significant factor that affecting the surface roughness. Also, better surface finish was obtained while machining aluminum alloy as compared to plain carbon steel. Spindle speed and feed rate were the other two significant factors while machining aluminum alloy parts, although these factors did not significantly affect the finish for steel. Radial engagement of tool had no impact on the surface finish for aluminium alloy, while it had a significant impact for plain carbon steel. Further, the analysis of results shows that position P2 (middle of the milled surface) had the best surface finish while position P1 (at beginning of the cut) had relatively poorer finish.

Time domain model of plunge milling operation

Plunge milling operations are used to remove excess material rapidly in roughing operations. The cutter is fed in the direction of spindle axis which has the highest structural rigidity. This paper presents time domain modeling of mechanics and dynamics of plunge milling process. The cutter is assumed to be flexible in lateral, axial, and torsional directions. The rigid body feed motion of the cutter and structural vibrations of the tool are combined to evaluate time varying dynamic chip load distribution along the cutting edge. The cutting forces in lateral and axial directions and torque are predicted by considering the feed, radial engagement, tool geometry, spindle speed, and the regeneration of the chip load due to vibrations. The mathematical model is experimentally validated by comparing simulated forces and vibrations against measurements collected from plunge milling tests. The study shows that the lateral forces and vibrations exist only if the inserts are not symmetric, and the primary source of chatter is the torsional–axial vibrations of the plunge mill. The chatter vibrations can be reduced by increasing the torsional stiffness with strengthened flute cavities.

A prediction of the machining defects in flank milling

In peripheral milling with great axial engagements, the tool deflections generate some geometrical defects on the machined surface. This article present a prediction method of these defects which is applicable on every ruled surface. The cutting forces are estimate with the cutting pressure notion. The parameters of the tool/workpiece material couple are identified by a test part. The prediction of the tool deflections requires controlling the tool immersion angle for each angular position of the tool. The deflections can be significant. An original procedure which is based on an engagement cards avoids an iterative calculation of the radial engagement. The experimental checking of the method of prediction is presented in a test.

Parameter Space Decomposition for Selection of the Axial and Radial Depth of Cut in Endmilling

Feeds and speeds for conventional endmilling operations have been empirically investigated and extensively tabulated 1. However, the selection of the geometric cutting parameters, the axial and radial depths of cut, remains an inexact science. Observation of mechanistic process simulation predictions reveal a relatively complex topology resulting from the multiple cutting flutes of conventional endmilling cutters as the axial and radial depths of cut are varied. A partitioning approach is presented that explicitly enumerates the transition events due to the entrance and exit of the individual cutting flutes. The resulting simplified optimization formulation permits selection of the axial and radial depth of cut that most efficiently satisfy critical simulation predictions such as maximum cutting force or form error. Case studies are presented illustrating the application of the method to select the cutting parameters in climb milling. The optimization objective in the case studies is to maximize the material removal rate, subject to the process induced constraints. Results suggest that operating at the extremes of either axial or radial engagement may in various instances be preferable to more conventional combinations of depth and width of cut. Certain regions of the parameter space are observed to be necessarily sub-optimal relative to particular planning constraints, while other regions are found to contain particularly attractive operating points.