A new format for 5-axis tool path computation, using Bspline curves

doi:10.1016/j.cad.2003.12.002

Computer-Aided Design

Volume 36, Issue 12, October 2004, Pages 1219-1229

https://doi.org/10.1016/j.cad.2003.12.002 Get rights and content

Abstract

This article presents a new format of tool path polynomial interpolation in 5-axis machining. The linear interpolation usually used produces tangency discontinuities along the tool path, sources of decelerations of the machine tool whereas polynomial interpolation reduces the appearance of such discontinuities. The new format involves a faster tool path and a better surface quality. However, it imposes a modification of the process so as to take the interpolation format and the inverse kinematics transformation (necessary to 5-axis machining) into account. This article deals with the geometrical problem of tool path calculation. Validation tests are detailed. They show that profits concern the reduction of machining time as well as the quality of the machined surfaces. Indeed, the trajectory continuity avoids the appearance of marks and facets.

Introduction

The purpose of the machining of free-form surfaces is to carry out complex shapes according to a given level of quality, while minimising machining time. This method of machining relates to mould and dies, aeronautics, or car industries.

The manufacturing process consists of four main activities. The first one relates to the design of part surfaces using a CAD system in order to respect design requirements.

The second activity consists in calculating the machining tool trajectory, according to the chosen machining strategy by means of a CAM system [1]. The machining strategy defines the geometrical characteristics that are necessary to carry out the machining. In addition to the geometry of the part surfaces and that of the tool, one chooses the machine tool, 3 or 5 axis, the machining type, point milling or flank milling, the machining direction and the longitudinal and side step values. The latter are calculated according to the geometrical specifications, essentially form deviation and surface roughness. Then, the calculation algorithm calculates a set of tool positions defining the tool path by calculating tangent tool locations at the surface and by arranging them according to the longitudinal and side steps. The tool path is calculated considering the given type of tool path description format, to be read by the Numerical Control unit (NC unit).

During the third activity, the post-processor ensures the translation of the APT program file to the ISO program file, in order to be treated by the NC unit.

The last activity is the realisation of the part on the machine by the movement of the cutting tool. The machine tool is a mechanical system, generally using 3 or 5 axis of displacement. A NC unit controls the machine. In particular, the NC unit ensures the reading of the program, the calculation of movement and speed orders of each axis of the machine tool.

The communication between these four activities goes through the definition of geometrical languages for the surface description (IGES, STEP, geometrical model of surfaces) and for the machining tool path (APT, ISO, interpolation format of the tool path).

To improve part quality and to decrease machining time, the process must henceforth be adapted to High Speed Machining (HSM). In particular, the tool path calculation and its communication to the NC unit must take the dynamical behaviour of the machine tool and the real-time behaviour of the NC unit into account. Heisel and Feinauer indicate that the form of the calculated tool path and the interpolation format used generate constraints on the real-time follow-up of the tool path and may cause decelerations of the machine tool or appearance of chatters during machining [2]. If the tool path calculation and its communication to the NC unit are optimised, machining time is thus minimal while respecting quality requirements.

To solve this problem, we consider that the CAM system must compute a C² continuous tool path using polynomial curves. This improvement should increase the effective speed of the tool during machining in 3-axis as well as in 5-axis machining of free-form surfaces.

To evolve in this direction, NC units now propose to read polynomial or Bspline curves in 3-axis milling (Heindenhain, Siemens, Fanuc and Num for example). It is thus necessary to define a dedicated format for the communication of the tool path between the CAM system and the NC unit, for the APT format only considers linear and circular interpolation formats. CAM systems must evolve to propose tool path computation methods well adapted to this new format. However, in 5-axis the problem is more difficult since calculation methods must integrate the problem of the inverse kinematics transformation.

This article presents a new format of tool path computation and interpolation for 5-axis machining, based on the Bspline interpolation of the tool path in the part coordinate system, respecting accuracy requirements. The goal is to define a 5-axis tool path description format, adapted to the communication between the CAM software and the NC unit. In this case, the CAM output is directly expressed through Bspline curves. We propose a calculation method allowing solving the computation problem.

Generally the Bspline interpolation is used to build curve for CAD systems. Some works are related to the curve interpolation for NC machining, such as papers presented by Erkorkmaz, Cheng, Xu, Yeh, and Yong [3], [4], [5], [6], [7]. Authors suggest to use tool path described as polynomial curves, to enhance the acceleration, speed and chord error control, during the real-time NC follow-up of the tool path. The main results consist in a smoother variation of the speed axis, a reduction of machining time and a diminution of the chordal error.

Our approach does not consider real-time interpolation but it is only located at the CAM stage, and concerns the geometrical calculation of the tool path. To ensure the follow-up of the tool path, we use an adapted NC unit Siemens 840D. Many papers study the computation of tool path with curves in 5-axis milling. Only Fleisig and Spence presents an off line interpolation of the tool path [8]. In Section 3, we study the main difference between our both approaches.

Section 2 exposes problems linked to 5-axis machining. Section 3 is dedicated to the presentation of the interpolation format we propose, and the method of calculation adopted. Section 4 relates comparison tests, which show that polynomial interpolation leads to important profits in terms of quality and machining time.

Section snippets

5-Axis machining

Five axis machining is carried out on machine tools which have two rotation axes (A,B or A,C or B,C) allowing the tool to be oriented relatively to the part in addition to the usual axes of translation X, Y, Z. Bohez presents the various types of machine structures [9]. In the present work, we are only interested in continuous 5-axis machining, the most generally used for aeronautics parts. Continuous 5-axis machining is used for the surface milling with ball end or toroı̈dal cutter, and flank

Polynomial interpolation in 5-axes

When using polynomial interpolation in 5-axis machining, two geometrical calculations are necessary, at the CAM and Post-processor stages:

•
problem A: computation of the tool path using a polynomial curve,
•
problem B: calculation of the inverse kinematics transformation.

Here, the essential difficulty concerns the relative position of these two problems within the general process. In other words, does the calculation of the tool path have to be carried out in the part coordinate system (problem A

Applications

Several test parts were defined and machined in order to estimate benefits brought by the polynomial interpolation. They are representative of different type cases in five continuous axes machining:

•
machining of a smooth surface with a toroidal cutter to maximise the covering rate,
•
machining of a cavity with a hemispherical tool to avoid collisions with the part,
•
machining of surfaces having the shape of a turbine blade.

Machining was carried out on the Mikron UCP710 milling centre of Lurpa,

Conclusion and prospects

HSM requires to guarantee a high speed of the axes of the machine tool all tool path long. This depends on both the numerical controller capacities to calculate laws of optimal accelerations and the CAM system capacity to calculate accurate tool paths that are adapted to the NC unit treatment.

This article presents a new description format of the tool path in 5-axis machining. This format uses non-uniform cubic Bspline curves and expresses the tool movement in the part coordinate system

Acknowledgements

The work presented in this article relates to the 5xNurbs project, which gathered Dassault Systèmes, Siemens, Cenit and Lurpa about the development of a new format of interpolation in 5-axis machining.

Jean Marie Langeron, graduated in 1986 of the French Aerospace Engeneering school of ENSMA (Ecole Nationale Supérieure de Mécanique et d'Aérotechnique) he joins the (Numerical Control) NC department of DASSAULT SYSTEMES in 1987. In charge of the Tool Path generator development team of CATIA NC for a long time, he is now Consultant Research and Development, directly attached to the Director Research and Development of the DELMIA and CATIA Machining Solutions.

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Emmanuel Duc is Assistant Professor at the French Institute for Advanced Mechanics (IFMA-Institut Français de Mécanique Avancée) of Clermont Ferrand. His research interests include the tool path computation for High Speed Milling, particularly in 5-axis machining and pocket machining. Computational methods are developed using Bspline curve, and experimental validations are tested on various machine tool. He has conducted many studies in aeronautics aera.

Claire Lartigue is Professor at the department of Mechanical Engineering of IUT Cachan-University of Paris XI. She is in charge of research activities in the fields of CAD/CAM and inspection of sculptured surfaces. These activities focus in particular on the assessment and the improvement of the geometrical quality of machined parts. An experimental laboratory consisting of a 5-axis HSM center and digitizing means contributes to the success of this research.

Pierre Bourdet member of CIRP is a Professor at Ecole Normale Superieure in Cachan. This institution is a main place in France, which educate new teachers. Pierre Bourdet is doing researches at University Research Laboratory in Automated Production (LURPA). The domain of research concerns the tridimensional geometry for activities of design, manufacturing and inspection. The team's research activities concerning three topic areas: algorithms for the assessment of measured points in co-ordinate metrology, geometrical quality of machined parts and model functional tolerancing for mechanism design.

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A new format for 5-axis tool path computation, using Bspline curves

Abstract

Introduction

Section snippets

5-Axis machining

Polynomial interpolation in 5-axes

Applications

Conclusion and prospects

Acknowledgements

CIRP Ann. Manuf Technol

Int J Mach Tools Manuf

Mach Tool Manuf

Comput-Aided Des

Comput-Aided Des

Comput-Aided Des

Comput-Aided Des

Int J Mach Tools Manuf

J Mater Process Technol

Ann CIRP

Comput-Aided Des

CAD

Comput Ind

Comput-Aided Des