Research concerning machine-tool accuracy based on dynamics behaviour.

Ispas, Constantin ; Anania, Florea Dorel ; Zapciu, Miron 等

1. INTRODUCTION

High speed machining (HSM) is a key which enables technology to be used in an increasing number of industries. In the aerospace industry, structural components are increasingly being machined as monolithic structures from a single billet. (Ispas C-tin et al. 2007) The results are drastically reduced part counts, assembly costs, and even maintenance costs.

In the present paper, an approach is developed to optimize NC programs by implementing the machine tools errors on each axis into algorithm. Therefore, the CAD (Computer Aided Design) systems can generate optimized surfaces based on real and ideal pieces surfaces for CAM (Computer Aided Manufacturing) advanced systems.

2. MACHINE TOOLS ACCURACY

Numerous error origins affect tool tip position. Among the key factors that affect the accuracy of this relative position are the geometric errors of the machine tool and thermal effects on the machine tool axes (Marinescu et al. 2002). Other error origins are the resolution and accuracy of the linear measuring system, elastic deformation of drive components, inertia forces when braking/accelerating, friction and stick slip motion, the servo control system and cutting force and vibration (Anania at al., 2007).

For a multi-axis machine, the calibration should include each axis and its roll, pitch, yaw, squareness and positioning error in the workspace (Ispas et al., 2006). The static working load and the mass of the workpiece being machined produce distortions that result in positioning errors in the machine tools.

The following Fig. 1 shows the error origins of multi-axis machine tools and their high level relationships. Broadly, machine tools errors can be divided into two categories: systematic errors and random errors. Systematic errors can be described and are predictable based on some mathematical models. Random errors are difficult to model and to compensate.

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The real position of tool tip in space will be translated and rotated after each axis of Cartesian systems. The tool tip position will be translated on X with [x.sub.r] (cumulated machine tool linear errors on X axis), on Y with [y.sub.r] (cumulated machine tool linear errors on Y axis), on Z with [z.sub.r] (cumulated machine tool linear errors on Z axis) and rotated with [PSI] (cumulated machine tool angular errors on X axis), with [phi] (cumulated machine tool angular errors on Y axis) and with [theta] (cumulated machine tool angular errors on Z axis). In reality this position can not be measured by machine tools command system so it can not be corrected.

For transformation from Cartesian system OXYZ into Cartesian system O'X'Y'Z' the homogeneous transformation matrixes were used.

The transformation vector from point O to point O' I results as:

[T.sub.OO'] = [R.sub.x] x [R.sub.y] x [R.sub.y] x [R.sub.Z] x [R.sub.r]. (1)

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The C++ software takes in account the geometrical errors (linear and angular) measured.

The errors generated by other factors can also be easily introduced into calculus. The generated elements are used to change CAD and CAM model.

Based on this compensate surface an NC program should be generated for high speed surface machining. The compensate errors plus machine tool errors should be nearly 0 (ideal machine tool).

3. STUDY CASE OF A GANTRY MACHINE TOOL

In this paper, it is presented a study case of a GANTRY milling machine tool. There are established some methods to study the dynamical behaviour the machine tool into working environment. There were measured some systematic errors and random vibration using a Vibroport41 devices.

Under continuous machining conditions, two types of vibration occur as a result of movement between a work-piece and tool: external vibration (result of interference force transmitted through the foundation into the machine); self-excited vibration, (the machine system oscillates, basically, at one or more natural frequencies, when no external forces are acting). (Valdes et al. 2006)

For this study case were used the frequency spectrum" and "Transfer" functions. The acquisitions were made with a vibroport 41 device and two piezoelectric accelerometer Schenck type AS-020 S/N: 0022FE7H (fig.1). This equipment is located to the National Research Centre for Performances of Technological Systems--Optimum into University POLITEHNICA of Bucharest.

The measurements points are presented in figure 3. The numbers represent the position of the accelerometer and the arrows represent the direction of measurement. The letter "A" represents the point of the impact for transfer functions measurement.

An example of data obtain a for transfer functions measurements on the machine structure (accelerometer in position 2, direction on Y machine axis and impact in A after Y direction) is presented in fig 4. After analysing all measured data it was identified the next frequency proper 12.5Hz; 25Hz; 50Hz, 62.5Hz, 75Hz.

The dynamical study was made for the machine tool in the next working condition: machine stopped; the main spindle speed:1500 rot/min, 3000 rot/min, 5000 rot/min, 7500 rot/min (fig.5), 10000 rot/min, 20000 rot/min of and for linear movements with speed:: 1000 mm/min, 5000 mm/min and 15 0000 mm/min..

After analyzing the data it was identified a number of frequencies which are repeating almost into all measurements. So this frequency could influence directly the performances of the machine tool. The frequencies, their amplitude in [micro]m and direction are presented in table 1. These frequencies were measured on the machine structure in different points on different direction, but their effects are on the tool.

The amplitude value of the measured frequencies can be used as imput data into C++ correction software.

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5. CONCLUSIONS

The solution that we proposed in this paper has the advantage of making a CAD correction of the pieces surfaces, function of the machine tool cumulated errors.

In the dynamical study presented in this paper it were established the direct influences of the external frequencies in combination with the frequencies proper to the machining accuracy. Combining experimental measures with a theoretical study, the dynamical errors of the machine tool can be accuracy defined.

The errors correction by modifying the CAD surfaces is not depending of machine tool CNC system or the machine tool systems. It can be easy implemented on all type of machine tool, even for the high speed. The errors values used for calculus were measured with high precision instrument, independently from the machine tools.

The results of this study were used as input data for tool correction software of the machined surface of the pieces.

6. REFERENCES

Anania F.D., Ispas, C.; Mohora, C.( 2007) An Algorithm for CAD correction of the work pieces based on machine tool errors, Proceedings Annals of DAAAM for 2007, 24-27 octomber, Zadar, Croatia, issn1726-9679, ISBN 3-901509-58-5,

Ispas C-tin. Anania F.D., Mohora C.(2007) Experimental research concerning machining accuracy for a gantry machine tool, The 5th international conference of advanced manufacturing technologies ICAMaT , 12-14 iulie 2007, sibiu, romania, ISSN 1843-3162 ISBN

Ispas, C-tin., ANANIA, F.D., Dogariu, C-tin., TILINA, D. C-tin., (2006). Contributions about Gantry machine tool geometric accuracy improvement by laser alignment--The International Conference of the Carpathian Euro-Region Specialists in Industrial Systems--BAIA MARE

Marinescu I., Ispas C., Boboc D.(2002)--Handbook of Machine Tool Analysis, United States of America, ISBN 08247-0704-4, 002, 2002

Valdes E., Greffioz A, Dequidt A.(2006)--Differents modeles de comportament dynamique d 'une machine-outil durant sa phase de conception, 4-emes Assies Machines et Usinage Grand Vitesse, 8,9 juin 2006, ENSAM, Aix-en-Provence, www.lsis.org/AssisesMUGV, Accessed on: 2008-08-09

Table 1. Important frequencies

Frequencies Displacement Accelerometer
 position and
 direction

20,00 Hz 2,1237 [micro]m 2z
33,75 Hz 0,2152 [micro]m 1x
35 Hz 0,0683 [micro]m; 1x
41,25 Hz 1,3785 [micro]m 2z
55 Hz; 1,5505 [micro]m 1x
55,75 Hz 1,5732 [micro]m 1y
57,5 Hz; 1,0719 [micro]m 2z