Aspects of optimising the positioning and contouring precision of CNC laser cutting machines.

Biris, Cristina ; Deac, Cristian ; Tera, Melania 等

1. INTRODUCTION

Nowadays, the means and techniques employed for cutting various materials, and especially metallic materials, are very diversified, so both producers and users of cut parts are often faced with a difficult choice with regard to the right technique and machine to use for a certain application, the more so when the part has a complex shape.

The precision of parts realized by cutting can be improved significantly by using equipments for numerical control (NC) on the machine-tools used for this purpose. However, there are a variety of factors that can negatively affect precision even under these conditions (Biris, 2008), so special precautions have to be taken when programming a machine-tool for an operation that demands a high accuracy.

One of the most critical components for achieving a high precision are the feed systems, assimilated with electromechanical motion control structures that can be realised in a modular, reconfigurable structure. The possibility of using modular cutting systems also allows a significant cut of the costs of such an equipment, without implying a loss of performances (Tu&Xie, 2001).

The experimental testing of performances in the case of modular reconfigurable cutting systems implies the study of the dynamic behaviour of motion control systems on each axis as well as of the kinematic precision (viewed as positioning precision on each axis and as contouring precision in the case of multiaxial movement), thus determining the main causes for errors and the possibilities of compensating them (Wang et al., 2005).

When it comes to selecting a precise type of cutting system to analyse, there are nowadays several non-conventional choices: laser cutting, plasma cutting, abrasive jet cutting etc. Even if the laser cutting equipment is much more expensive than other equipment types, it still offers the best precision.

The present paper therefore aims to present some considerations and experimental results concerning the factors influencing the precision obtained by laser cutting steel sheet parts on NC machine tools.

2. THE LASER CUTTING TECHNOLOGY

Laser cutting uses a focused beam of high energy laser light to cut materials by selectively burning or melting a precise spot, while an assist gas is used to remove the molten material from the resulting cut. It is one of the fastest and most accurate methods for cutting a variety of metals and non-metals.

Nowadays, both gaseous (C[O.sub.2]) and solid-state (Nd:YAG) lasers are employed for cutting various materials. Since, in the case of C[O.sub.2] lasers, the type of gas flow can affect the cutting performance, several laser subvariants can be identified, such as fast axial flow, slow axial flow, transverse flow, and slab lasers.

Compared to other cutting technologies, there are several disadvantages to be considered (Olsen, 2008): high cost of the equipment; the thickness of materials that can be cut is very limited (10-20 mm at best); the cutting of metals with highly reflective surfaces can cause problems for the equipment (especially for the focusing lens); it can cause micro-fracturing in some materials; variations in the material's quality can affect the cutting results etc.

However, the major advantages of laser cutting over other cutting methods recommend its employment once the equipment is available and the thickness range is an appropriate one: it can cut a variety of metals and non-metals; it can produce part accuracies better than 0.08 mm; it can cut thinner metals at over 170 mm/s; it produces a narrower heat affected zone than plasma etc.

3. PRECISION OF CNC LASER CUTTING SYSTEMS

In the case of CNC machine-tools, the feed movements are realized in an automated regulation regime. The information on displacement and speed, needed for the part's processing, is generated by the machine's control unit and then transmitted to the numerical axes.

The precision of parts processed by laser cutting, on CNC machine tools, is significantly influenced by following two main factor categories:

--the configuration parameters of the position control system on each movement axis;

--the feed rate on each movement axis.

Previous researches presented in the speciality literature (Breaz&Bogdan, 2003; Bohez, 2002) indicate that errors introduced by the functioning of the numerical axes represent the main part of errors in the processing on CNC machine-tools.

The errors that appear in the functioning of the movement control systems are directly translated into contour generating errors. This is especially true for trajectories which require variable feed rates on the various axes, such as circles or curved lines.

The main factors influencing the precision of the CNC cutting machine-tool's machining feed can be grouped into dynamic constraints, uncertainty factors and non-linearities.

Among the dynamic constraints, there is the system's dynamic disturbance, which leads either to different speed amplification factors on the axes, leading to contouring errors in stationary regime, or to different time constants on each axis, which negatively influences the transitory regime. In the case of cutting a circular contour, for example, this contour will become rather an ellipse, with a "thinning" along the axis with a smaller amplification factor. Errors may appear especially during changes of direction or stops, which influences negatively the cutting of corners.

This shows that by precisely controlling the elements of the feed kinematic chain, and especially the parameters of the control system of the numerical axes, we can significantly improve the CNC laser cutting machine-tool's performance.

However, even when using an optimal feed rate, recommended by the speciality literature, there can still appear contouring errors due to a dynamically imbalanced control system. This underlines the importance of using mathematical models and experimental researches for an optimal tuning of the working parameters of laser cutting machine-tools.

4. EXPERIMENTAL RESEARCHES

The experimental researches regarding the cutting precision were carried out on a NTX-48 Champion laser cutting machine manufactured by Mazak (Japan) using a CO2 laser and having following characteristic working parameters:

--maximal power output: 1.5 kW;

--impulse frequency: from 0 to 2000 Hz;

--maximal machining feed rate: 10 m/min;

--positioning precision on X and Y axes: [+ or -]0.01/100 mm;

--positioning precision on Z axis: [+ or -]0.01/500 mm;

--NC control unit: Mazak L-32B.

The blanks used for these researches were sheets of OL 52 2k steel, STAS 500/2-88, with thicknesses of 3 mm, 6 mm and 8 mm, respectively. The aim was to obtain, on the one hand discs with a diameter of 60 mm, and on the other hand rectangular shapes with sides measuring 40 and 60 mm, respectively, to assess the dimensional errors and thus the machine's precision, for different working regimes and NC setups.

Figure 1.a shows the result of cutting a disc from an 8 mm thick OL 52 steel sheet, with a feed rate v = 700 mm/min, while figure 1.b presents the contouring errors in the case of cutting a disc from the same type and thickness of steel sheet, but with a feed rate v = 1500 mm/min.

[FIGURE 1 OMITTED]

Figure 2.a presents the contouring errors in the case of cutting a rectangular shape from a metal sheet of 3 mm thickness with a feed rate v = 800 mm/min, while figure 2.b shows the result of cutting the same part from the same steel type and thickness, but with a feed rate v = 2000 mm/min.

[FIGURE 2 OMITTED]

5. CONCLUSIONS

The researches presented in this paper have underlined, on the one hand, the benefits of laser cutting systems with regard to precision, even if the high costs of the laser cutting equipment may pose some start-up problems. On the other hand, the importance of a precise control of the feed rate with the help of rotation transducers has been proven and an optimal feed rate has been found for the studied case.

As a conclusion, for both part types, at lower feed rates the contouring errors are relatively small compared to the case of higher feed rates, a phenomenon that aggravates with decreasing sheet metal thickness. This indicates that the optimal setting for this laser cutting machine is achieved when cutting alloyed steel types or harder materials in general, whereas for softer and/or thinner materials, which can be processed at higher feed rates, a modifying of the machine's dynamics is needed for ensuring an acceptable precision.

In future, it is sought to use the outcome of the experimental researches presented here for generating and optimising mathematical models that could take into account the above-mentioned error-inducing factors and thus lead to a more precise cutting technology.

REFERENCES

Biris, C. (2008). Proiectarea unui sistem de comanda si control aferent echipamentelor de debitare (Design of a command and control system for cutting equipments), Ph.D. paper no.2, "Lucian Blaga" University of Sibiu.

Bohez, E.L.J., (2002), Compensating for systematic errors in 5-axis NC machining, Computer Aided Design no. 34, pp. 391-403.

Breaz R., Bogdan L. (2003). Automatizari in sisteme de productie (Automations in production systems), Publishing House of the "Lucian Blaga" University of Sibiu, Sibiu.

Olsen, C. (2008). Waterjets.org: Waterjet Web Reference, http://www.waterjets.org/index.html

Tu, Y. L., Xie, S. Q. (2001), An Information Modelling Framework to Support Intelligent Concurrent Design and Manufacturing of Sheet Metal Parts, Intl. J of Advanced Manufacturing Technology, 18 (12), pp. 873-883;

Wang, S.-M., Yu, H.-J., Liao, H.-W., (2005) A new high efficiency error compensation system for CNC multi-axis machine tools, Intl. J of Advanced Manufacturing Technology 16, pp. 623-631.