Sylvain Lavernhe

Optimization of 5-axis high-speed machining using a surface based approach

This paper deals with optimization of 5-axis trajectories in the context of high-speed machining. The objective is to generate tool paths suited to high speed follow-up during machining in order to respect cutting conditions, while ensuring the geometrical conformity of the machined part. For this purpose, the optimization of the tool axis orientations is performed using a surface model for the tool path, which allows integrating kinematical limits of the machine tool as well as classical geometrical constraints. The illustration of the optimization through an example highlights the gain in machining time, thereby demonstrating the feasibility of such an approach.

Characterization of 3D surface topography in 5-axis milling

Within the context of 5-axis free-form machining, CAM software offers various modes of tool-path generation, depending on the geometry of the surface to be machined. Therefore, as the manufactured surface quality results from the choice of the machining strategy and machining parameters, the prediction of surface roughness in function of the machining conditions is an important issue in 5-axis machining. The objective of this paper is to propose a simulation model of material removal in 5- axis based on the N-buffer method and integrating the Inverse Kinematics Transformation. The tooth track is linked to the velocity giving the surface topography resulting from actual machining conditions. The model is assessed thanks to a series of sweeping over planes according to various tool axis orientations and cutting conditions. 3D surface topography analyses are performed through the new 3D roughness parameters proposed by recent standards.

Feed drive modelling for the simulation of tool path tracking in multi-axis high speed machining

Within the context of High Speed Machining, it is essential to manage the trajectory generation to achieve both high surface quality and high productivity. As feed drives are one part of the set Machine tool - Numerical Controller, it is necessary to improve their performances to optimize feed drive dynamics during trajectory follow up. Hence, this paper deals with the modelling of the feed drive in the case of multi axis machining. This model can be used for the simulation of axis dynamics and tool-path tracking to tune parameters and optimize new frameworks of command strategies. A procedure of identification based on modern NC capabilities is presented and applied to industrial HSM centres. Efficiency of this modelling is assessed by experimental verifications on various representative trajectories. After implementing a Generalized Predictive Control, reliable simulations are performed thanks to the model. These simulations can then be used to tune parameters of this new framework according to the tool-path geometry.

TOOL PATH GENERATION AND POST-PROCESSOR ISSUES IN FIVE-AXIS HIGH SPEED MACHINING OF HYDRO TURBINE BLADES

As a core part of aerospace, space, and steam turbine plants, blades are generally machined via 5-axis linkage processing to satisfy the high precision requirements of the rigorous surface. To save costs in blade machining, many small- and medium-sized enterprises often combine standard 3-axis computer numeric control machines with the automatic indexing turntable. The traditional 4-axis machining method adopts a constant feed rate, which causes overcutting near the leading and trailing edges of the blade because of the rapid changes in tool orientation. To solve this problem, we propose a speed optimization method that utilizes variational speed to ensure that the decomposition velocity and acceleration of each axis do not exceed the allowable values. First, we guarantee the correct tool lead angle. Second, a corrected speed model is established to obtain the component speed of each axis and to determine the constraint conditions of maximum and accelerated speed. Third, a 4-axis post processor for blade processing is developed using Java advanced language combined with the optimization algorithm. The cutting experiment reveals that our proposed speed optimization method effectively controls the precision of the surface profile and overcomes the overcut phenomenon that often occurs in traditional 4-axis machining.

Performance evaluation of CUDA programming for 5-axis machining multi-scale simulation

5-Axis milling simulations in CAM software are mainly used to detect collisions between the tool and the part. They are very limited in terms of surface topography investigations to validate machining strategies as well as machining parameters such as chordal deviation, scallop height and tool feed. Z-buffer or N- buffer machining simulations provide more precise simulations but require long computation time, especially when using realistic cutting tools models including cutting edges geometry. Thus, the aim of this paper is to evaluate Nvidia CUDA architecture to speed-up Z-buffer or N-buffer machining simulations. Several strategies for parallel computing are investigated and compared to single-threaded and multi-threaded CPU, relatively to the complexity of the simulation. Simulations are conducted with two different configurations including Nvidia Quadro 4000 and Geforce GTX 560 graphic cards.

CAD-Based Calibration of a 3D DIC System

The aim of this study is to predict the strains induced by welding-brazing of automotive thin metal sheets using finite elements simulations. In order to validate the simulations, experimental tests on industrial parts are carried out. One goal of these tests is to determine 3D surface displacement fields during welding. The measurement technique chosen for these tests is 3D surface DIC (stereocorrelation). To take into account the specificity of these tests (object size about 1 m2) a new calibration method of the stereovision system has been developed. Currently, calibration methods are based on the knowledge of the calibration target geometry and its projections onto both right and left camera frames. In the approach proposed herein, the calibration is performed directly on the measured part, which is modeled by a Bezier patch. A first step consists in finding the best 3D (CAD surface) to 2D (camera picture) transformation.

PERFORMANCE OF OFF-LINE POLYNOMIAL CNC TRAJECTORIES WITHIN THE CONTEXT OF HSM

The objective of the article is to present various off-line calculation methods to calculate polynomial tool trajectories, format well adapted to High-Speed Machining. In particular, we are interested in comparing machining performances of various polynomial calculation algorithms such as interpolation, association or inter-approximation with energy minimization. This comparison is achieved using a test part through simulations and machining tests to justify the efficiency of the calculation methods. Measurements of velocity and position during machining highlight differences between the different association methods. Attention is also paid to the visual and geometrical quality of the machined surfaces.

A physically-based model for global collision avoidance in 5-axis point milling

Although 5-axis free form surface machining is commonly proposed in CAD/CAM software, several issues still need to be addressed and especially collision avoidance between the tool and the part. Indeed, advanced user skills are often required to define smooth tool axis orientations along the tool path in high speed machining. In the literature, the problem of collision avoidance is mainly treated as an iterative process based on local and global collision tests with a geometrical method. In this paper, an innovative method based on physical modeling is used to generate 5-axis collision-free smooth tool paths. In the proposed approach, the ball-end tool is considered as a rigid body moving in the 3D space on which repulsive forces, deriving from a scalar potential field attached to the check surfaces, and attractive forces are acting. A study of the check surface tessellation is carried out to ensure smooth variations of the tool axis orientation. The proposed algorithm is applied to open pocket parts such as an impeller to emphasize the effectiveness of this method to avoid collision. A new approach is proposed for collision avoidance in 5-axis end milling.The method is based on a physical modeling to compute the tool axis orientation.The ball-end tool is considered as a rigid body moving in 3D space.Repulsive forces are deriving from a scalar potential linked to check surfaces.Check surfaces tessellation ensures smooth variations of the tool axis orientation.

Offline gain adjustment with constraints for contour error reduction in high speed milling

In machining complex parts with high speed milling, small contour error (CE) and high surface quality are usually two major concerns. Many papers have focused on reducing CE by online contouring adaptation in the presence of disturbances and/or uncertain nonlinearities. Besides, calibration methods on pre-machining process such as smoothing the setpoints with optimized feedrate are also preferred. In contrast, less efforts have been done to analyze the relationship between profile geometry, control tuning or adjustment and effective CE in order to obtain CE as small as possible. To effectively deal with this challenge, an Offline Gain Adjustment (OGA) algorithm is developed in this paper. The simulated comparisons with existing fixed gain controller prove the efficiency of the proposed OGA method.

A tool path patching strategy around singular point in 5-axis ball-end milling

In 5-axis high-speed milling, large incoherent movements of rotary axes around the singular point are known to be a problem. Correction methods found in the literature deal mostly with the collision that may happen between the tool and the part but not with the feedrate slowdowns which affect surface quality and machining productivity. The method proposed in this paper addresses both geometrical and productivity issues by modifying the tool axes orientation while respecting maximum velocity, acceleration and jerk of the machine tool axes. The aim is to detect these behaviours and replace the considered portion of the tool path by a patch curve respecting kinematical constraints of the machine tool. Compared to previous works, the inserted patch curve is not constrained to pass through the singularity but respect tangential constraints to ensure the monotony of the tool path and is also connected with the rest of the tool path to ensure a continuity up to the third derivative in order to fulfil jerk limitations. For that purpose, the initial articular positions of the rotary axes around the singular point are fitted with B-spline curves, modified and finally discretised for linear interpolation. Experimental investigations on a test part are carried out to show the efficiency of the method in terms of feedrate and surface quality.