J. Cent. South Univ. (2013) 20: 378–384

DOI: 10.1007/s11771-013-1498-7

development of a one-axis manipulator for laser-assisted machining

KIM Dong-hyeon, LEE Choon-man

Department of mechanical design & manufacturing engineering, changwon national university, changwon 641-773, Korea

Central South University Press and Springer-Verlag Berlin Heidelberg 2013

Abstract: Laser-assisted machining has been considered as a new alternative machining method of difficult-to-cut materials. A laser module with one-axis manipulator is developed to focus on preheating laser beam effectively. First of all, the thermal characteristic analysis was performed to verify the importance of laser module location. Laser module should be moved within 1 mm. Analysis conditions of three positions in driving range of the one-axis manipulator are selected. And a C coupling is used as a connection device for spindle and laser module. An initial model has one C coupling, and the number of C coupling has been increased from 1 to 2 in an improved model. And the analysis is carried out again for the one-axis manipulator. The results of the static analysis, the maximum displacement and the maximum stress are decreased by 22% and 11%, respectively, for the improved model when the laser module is located at farthest away from the spindle unit. As a result of the modal analysis, the first natural frequency mode is increased by 13%, 18% and 12% at these positions of the improved model, respectively. The harmonic analysis of the improved model was performed by analyzing the results of the modal analysis. The maximum deformation was 0.33 mm in driving unit at 222 Hz. And the maximum compliance of the ISO axis direction was 0.23 mm/N. Finally, the one-axis manipulator has been fabricated successfully using the analysis result.

Key words: finite element analysis (FEA); laser-assisted machining; manipulator

1 Introduction

In recent years, the use of difficult-to-cut materials in high-quality has increased the development of automobile, aircraft, shipbuilding and semiconductor industries. Accordingly, high-efficiency is required in the machining of difficult-to-cut materials using a machine tool. Difficult-to-cut materials have been largely applied to various industries such as automobile and aircraft parts or mold machining, because they have much better high temperature strength, durability and corrosiveness than general metal [1–4].

The difficult-to-cut materials have great characteristics, but they are difficult to machine using general purpose machine because of their physical chemical characteristics such as brittleness. Therefore, laser assisted machining is the most suitable method in the difficult-to-cut materials machining.

The laser assisted machining is in the early stage of its applications and has only been used in limited fields including turning and micro end-milling throughout the world. Also, there are few studies on the research and development of laser assisted machining processing as a level of one-dimensional machining. But the industries of automobile and aircraft have required complex milling products [1, 5].

Melkote et al [1] performed study on laser assisted micro-milling of hard-to-machine materials. Rozzi et al [3] performed experimental evaluation of the laser assisted machining of silicon nitride ceramics.

Dumitrescu et al [5] and Zhang and SHIN[6] performed study on laser assisted turning. And Chang and KUO[7] studied laser assisted machining of ceramics planning.

Seo et al [8] performed evaluation of a dimple type EGR cooler using finite element method. And JUNG et al [9] performed study on thermal characteristic analysis of a ventilated disc using finite element method.

By combining the laser beam preheating and CNC milling, it could be named laser assisted machining (milling) as shown in Fig. 1, in which it performs a machining process after softening a workpiece through the emitted laser beam to a workpiece using a laser module installed at the upper section of the machine [10–13]. As shown in Fig. 2, when the laser module is located far from the workpiece surface or when the laser module is located near the workpiece surface, spot diameter gets bigger. Accordingly, laser module should be able to move to the laser radiation direction for machining of complex shapes.

Fig. 1 Laser assisted machining process

Fig. 2 principle of a laser focusing

In this work, the laser module with one-axis manipulator is developed to focus on preheating laser beam effectively. And the structural stability of the one-axis manipulator has been studied to reduce the static deformation and vibration using finite element method. Finally, the one-axis manipulator has been fabricated using the analysis result.

2 Thermal characteristic analysis of laser-assisted machining according to laser module location

The optimum distance between the laser module and workpiece surface is 138 mm. At this point, laser spot diameter is 3 mm. In this section, the thermal characteristic analysis was performed to verify the importance of laser module location.

Figure 3 shows the finite element model used in the thermal analysis of laser heat source. Table 1 represents the conditions used in this analysis. It was used in finite element analysis package ANSYS WB.

Figure 4 shows the results of the thermal analysis. And Table 2 represents the results of the thermal analysis with travel length of the laser module.

Fig. 3 Finite element model

Table 1 thermal analysis conditions

The surface temperature was decreased with the increase in spot diameter. The temperature was analyzed by 1 336°C, 1 245°C and 1 119°C at spot diameter of 3 mm, 3.14 mm and 3.28 mm, respectively.

Temperature difference of workpieces should be within 100°C in laser assisted machining to cut materials. Therefore, travel length of the laser module should be within ± 1 mm.

3 Finite element model of one-axis manipulator

A laser module with one-axis manipulator is developed to focus on preheating laser beam effectively in this work. For performing a static and dynamic analysis of the one-axis manipulator, a finite element model is modeled using finite element analysis package ANSYS WB.

An initial model has one C coupling, and the number of C coupling has been increased from 1 to 2 in an improved model. Machining error for the initial model can be produced by the static deformation and vibration. Figure 5 shows the finite element model for the improved model of the one-axis manipulator with 163 954 nodes and 89 618 elements.

Table 3 represents the material properties. In the analysis, the upper part of the spindle unit is fixed and its self-weight is considered in the whole system. The maximum speed, positioning repeatability, stroke of the one-axis manipulator are 250 mm/s, ±0.2 mm and 200 mm, respectively.

Fig. 4 results of thermal analysis under different conditions:

Table 2 results of thermal analysis with travel length of laser module

Fig. 5 Finite element model for improved model of one-axis manipulator

Table 3 Material properties

Figure 6 shows the analysis conditions according to the position of laser module. When the laser module is located at the nearest spindle unit, the stroke is 0 mm, and when the laser module is located at farthest away from the spindle unit, the stroke is 200 mm.

4 Finite element analysis

4.1 Static analysis

The static analysis is carried out by considering the self-weight of the one-axis manipulator. Figures 7 and 8 show the maximum deformation and maximum stress at position 3 of the initial model, respectively. The maximum deformation of 0.36 mm and maximum stress of 92.72 MPa occur at the end part of the driving unit. The deformation should be decreased within 0.3 mm to focus preheating laser beam exactly. The initial model has one C coupling. Accordingly, the number of C coupling has been increased from 1 to 2 for improved model. Analysis results are just shown for position 3, because position 3 is most dangerous state among these analysis conditions.

Figure 9 shows the maximum deformation at position 3 of the improved model. The maximum deformation of 0.28 mm occurs at the end part of the driving unit. The deformation was decreased within 0.3 mm to focus preheating laser beam exactly. Figure 10 shows the maximum stress at position 3 of the improved model. The maximum stress of 92.72 MPa is decreased to 82.75 MPa at C coupling of the manipulator.

Fig. 6 Analysis conditions according to position of laser module:

Fig. 7 Maximum deformation at position 3 of initial model

Fig. 8 Maximum stress at position 3 of initial model

Fig. 9 Maximum deformation at position 3 of improved model

Fig. 10 Maximum stress at position 3 of improved model

Table 4 represents the summary of the maximum deformation and maximum stress at three positions.

Table 4 Result of static analysis

Figure 11 represents the results of comparison of the static analysis for the initial and improved model.

4.2 Modal analysis

The objective of a modal analysis of the structure is to investigate resonance regions and vibration frequency bands. The priority thing to do in the design of the structure is the modal analysis.

Table 5 represents the results of the modal analysis. The values of natural frequency occur quite differently depending on the positions. The first natural frequency mode is computed by 29 Hz at position 3 of the initial model and the first natural frequency mode is computed by 33.24 Hz at position 3 of the improved model. Also, bending mode for both models occurs along longitudinal direction.

Fig. 11 Comparison of static analysis results

Table 5 Natural frequency by modal analysis

Figure 12 represents the results of comparison of the modal analysis for the initial and improved model. The 1st natural frequency of the one-axis manipulator increased about 13%, 18% and 12% at three positions of the improved model, respectively.

Fig. 12 Comparison of modal analysis results

4.3 Harmonic analysis

The objective of a harmonic analysis is to analyze the frequency response of structures under varying cutting forces and the stiffness of structures [14].

1 000 N of cutting force was applied to the spindle and the harmonic analysis was performed in a frequency range of 0–250 Hz. Frequency bands were referred to the results of the modal analysis. The cutting force applied to the spindle is the exciting force of the harmonic function shape.

Figure 13 shows the maximum deformation by harmonic analysis at position 3 of the improved model. The maximum deformation was 0.33 mm in driving unit at 222 Hz.

Figure 14 shows compliance of X, Y, Z and ISO axis directions. The maximum compliance was 0.23 mm/N of the ISO axis direction at 222 Hz.

The one-axis manipulator has been fabricated based on analysis results of an improved model. Figure 15 shows the one-axis manipulator installed in a CNC machining center.

Fig. 13 Maximum deformation by harmonic analysis

Fig. 14 Compliance by harmonic analysis

Fig. 15 Photograph of fabricated one-axis manipulator with a CNC machining center

5 Conclusions

1) In the results of the thermal characteristic analysis of laser assisted machining, laser module and workpiece surface should be maintained at the optimum distance for effective laser assisted machining. Laser module should be moved within 1 mm.

2) Analysis conditions of three positions in driving range of the one-axis manipulator are selected. And after analysis of the initial model, improved model is suggested to maintain the maximum deformation at the end part of the driving unit within a certain value.

3) The maximum deformation and maximum stress are decreased by about 22% and 11% at position 3 of the improved model, respectively, by the results of the static analysis.

4) The natural frequency of the one-axis manipulator is increased by about 13%, 18% and 12% at the each position of the improved model, respectively, by the result of the modal analysis.

5) The harmonic analysis of the improved model is performed by analyzing the results of the modal analysis. The maximum deformation is 0.33 mm in driving unit at 222 Hz. And the maximum compliance of the ISO axis direction is 0.23 mm/N.

As a result, the laser module with one-axis manipulator is judged to focus preheating laser beam stably. In this work, structural stability of the one-axis manipulator has been studied. In conclusion, stable structure has been obtained by the static and dynamic analysis of the manipulator. And the one-axis manipulator has been fabricated successfully using analysis results.

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(Edited by HE Yun-bin)

Foundation item: Project(2012–0005688) supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MEST)

Received date: 2012-05-31; Accepted date: 2012-12-01

Corresponding author: LEE Choon-man; PhD; Tel: +82-55-213-3622; E-mail: cmlee@changwon.ac.kr