EDM milling based on a 6-DOF serial robot

doi:10.21203/rs.3.rs-1592219/v1

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EDM milling based on a 6-DOF serial robot

https://doi.org/10.21203/rs.3.rs-1592219/v1

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Aiming at the problems of insufficient flexibility and limited size of workpiece in traditional EDM milling, an EDM milling method based on 6-DOF serial robot is proposed in this paper. To realize the rapid adjustment of machining gap, a tool electrode servo motion device was invented and installed at the end of a 6-DOF serial robot and a servo control strategy of tool electrode vertical trajectory fallback was proposed. Then, to achieve the controllable thickness of every layer in EDM milling based on 6-DOF serial robot, the servo control strategy of limited motion of the tool electrode on the basis of vertical servo control and a fix-length compensation method to compensate the tool electrode wear were proposed. Finally, to verify the correctness and effectiveness of servo control strategy and wear compensation of tool electrode, the trapezoidal groove structures on the plane workpiece and cylindrical workpiece were machined by EDM milling based on 6-DOF serial robot, respectively. Also, to verify the machining ability of complex trajectory on the space surface of workpiece of EDM milling based on 6-DOF serial robot, three complex trajectories, such as EDM, Chinese character ‘China’ and ‘Chinese knot’, were designed and machined on the hemispherical workpiece with the radius of 38 mm. The feasibility and effectiveness of EDM milling technology based on 6-DOF serial robot were verified, which has great market application potential.

Electrical discharge machining milling (EDM milling)

Six degree of freedom serial robot (6-DOF serial robot)

Servo control strategy

Wear compensation of tool electrode

Electrical discharge machining (EDM) can fabricate conductive hard brittle materials into complex shaped parts with high precision. Although the use of die-sinking EDM, where the tool is shaped electrode and the parts can be formed by the replication of the shaped tool electrode, is still popular in the industry today, studies to replace the electrodes that have complex geometric features with simpler tool electrodes have been active[1]. The main disadvantages of the die-sinking EDM are: a longer production time of the tool electrodes and necessity to machine mechanically shaped tool electrodes. In EDM milling, a simple cylindrical electrode is used as tool and the complex shaped parts can be generated by the movement of tool electrodes along a pre-defined path[2]. EDM milling simplifies the manufacturing process of complex shaped parts and reduces the production time and cost of tool electrode because of the usage of a simple cylindrical tool. Thus, EDM milling technology has been applied to fabricate complex shaped parts in both the macro- and micro-domains and attracted many researchers’ attentions.

In micro machining field, micro EDM milling has been widely used because micro cylindrical tool electrode with high aspect ratio can be fabricated precisely by the wire electrical discharge grinding (WEDG)[3]. Furthermore, due to the absence of cutting forces, micro cylindrical tool electrode can be as smaller as possible to fabricate much smaller structures. Different types of micro cavities such as circle, triangle[4], square[5], channel and circular pillars[6], hemisphere, complex recesses, inclined windows, tapered sockets, narrow slots with variable inclination and complex shaped cavity[7] are fabricated by micro EDM milling. However, the biggest challenge for micro EDM milling is the wear of tool electrode. A new method to address this is developed combining the advantage of EDM milling and wire EDM, which is referred to as wire electrical discharge machining milling (WEDM milling)[8]. But, limited by the complex mechanical structure, WEDM milling can only fabricate very simple structures. In the field of macro machining, how to improve the machining speed of EDM milling is the main concern of researchers. Therefore, scholars have developed electrical arc machining (EAM)[9–13]. EAM milling technology is a new machining method to remove workpiece materials by using the high temperature generated by arc discharge between tool electrode and workpiece. Compared with the spark discharge of EDM, arc discharge has higher energy density and electro-thermal conversion efficiency, so the efficiency of EAM is much higher than that of EDM. Then, new compound machining process employing super high-speed EDM milling and arc machining is developed for milling [9]. In summary, the researchers have conducted massive investigations on the EDM milling and has greatly promoted the development of technology and industrial application. However, at present, the EDM milling are mainly implemented on the 3-axis machine tool, which cannot machine complex structures on the free-form surface. Furthermore, the machining freedom and flexibility of EDM milling based on a 3-axis machine tool is poor, which limits the further application of EDM milling in high-end manufacturing.

In the past three decades, robotic machining has attracted a great deal of research interest due to its cost advantages, high efficiency, high flexibility, and versatility of industrial robots[14]. Robotic machining are generally applied to performing tasks including drilling[15], milling[16], grinding[17] and deburring[18]. Because EDM can machine any electrically conductive parts regardless of hardness without contact force. EDM machining with robots means more advantages compared with cutting machining with robots. EDM milling with robot will not cause mechanical vibration and doesn’t need complex mechanical structures with great strength and stiffness. Furthermore, by using the flexibility of robots, it is possible to provide the workbench that can realize on-site processing. Mobile robot machining provides more flexible machining mode compared to the traditional machining with a fixed base. Thereby, this study proposed the EDM milling based on 6-DOF serial robot. To achieve this, an innovative servo control principle of EDM milling based on 6-DOF serial robot was put forward. The wear compensation of tool electrode in EDM milling was also considered. Finally, machining experiments of curved surface and complex trajectory were conducted to demonstrate the feasibility and superiority of EDM milling based on 6-DOF serial robot.

Figure 1 shows the schematic diagram of self-made EDM milling system based on 6-DOF serial robot. Figure 2 shows the picture of experimental equipment. The machining system is mainly composed of EDM control system, robot control system, a 6-DOF serial robot, end effector, workbench, and pulse power. The robot control system and 6-DOF serial robot are responsible for the machining trajectory motion of tool electrodes. The EDM control system and end effector are used to control the discharge state, which can be explained as follows. The end actuator is composed of a servo motor, a precision mobile platform, a connecting plate, and a fixture, as shown in Fig. 3. The cylindrical tool electrode is installed in the fixture of the end actuator. In the machining process, the discharge state detection circuit detects the voltage and current signals of the discharge gap in real time, and transmits the discharge rate, the short circuit rate and the open circuit rate to EDM control system. The EDM control system controls the servo motor of the end actuator according to the control strategy of EDM machining. Thereby, the discharge gap between the tool electrode and the workpiece electrode can be controlled and adjusted through the end actuator. Compared with traditional 3-axis EDM milling machine tool, in our EDM milling system based on 6-DOF serial robot, the tool electrode can be oriented in such a way that it is perpendicular to the sculptured surface, as shown in Fig. 4, which brings many benefits such as high machining speed and accuracy, convenient wear compensation of tool electrode, higher processing flexibility and stronger processing capacity of complex 3D surface.

3.1 Servo control strategy of limited motion of tool electrode

EDM milling is a machining method that uses simple electrode to do complex movement according to a NC predetermined track under the control of NC system. EDM milling can achieve complex three-dimensional structure by scanning two dimensional surface with equal thickness layer by layer, as shown in Fig. 5. Based on the servo movement direction of the tool electrode, EDM milling can be clarified into two types: EDM milling with servo motion direction along the machining trajectory shown in Fig. 6(a) and EDM milling with servo motion direction perpendicular to the machining trajectory (simply called vertical servo control) shown in Fig. 6(b). In EDM milling with servo motion direction along the machining trajectory, when a short circuit occurs in the machining process, the tool electrode retreats a certain distance along the original track to eliminate the short circuit, which has been widely used in the practical EDM milling. To eliminate the short circuit in time, the response speed of the machine tool should be fast enough. However, the response speed of 6-DOF serial robot is too slow to meet the application requirements of EDM. Thus, the servo motion direction along the machining trajectory cannot be used in EDM milling based on 6-DOF serial robot.

To solve the above problem, a tool electrode servo motion device was invented and installed at the end of a 6-DOF serial robot, as shown and discussed in Fig. 2. Then, the servo motion direction perpendicular to the machining trajectory can be used in EDM milling based on 6-DOF serial robot, in which the servo movement direction of the tool electrode and the scanning movement direction of the tool electrode are perpendicular to each other and their movements are independent.

In EDM milling, the thickness of every layer must be controllable. In EDM milling with servo motion direction perpendicular to the machining trajectory, the servo motion of tool electrode can affect the thickness of every layer. Thus, the controllable thickness of every layer is the key to EDM milling based on 6-DOF serial robot. To guarantee that the layered thickness is controllable, we proposed the servo control strategy of limited motion of tool electrode on the basis of vertical servo control, as shown in Fig. 7. In this servo control strategy, we set the upper and lower limit planes of the tool electrode motion, which were the UU’ and DD’ planes. That is, the tool electrode can only do the servo motion in the area between the UU’ and DD’ planes whose distance is$\Delta G$. After the tool electrode finishes one layer, the tool electrode feeds down one layer thickness$\delta$and the UU’ and DD’ planes also feeds down one layer thickness$\delta$.

3.2 Wear compensation of tool electrode

EDM milling faces a well-known problem: the wear of tool electrode. Due to long machining path, the electrode wear becomes important. It appeared to be necessary to compensate for this wear in order to generate an accurate geometry in the machining process. Thus, this study used a fix-length compensation method to compensate the tool electrode wear. In this method, the tool electrode compensates downwards for a constant length $\xi$ every time when it mills a fixed distance L, as shown in Fig. 8.

According to the geometric relationship, the compensation distance L can be expressed by Eq. (1).

$$L=\frac{T}{\Psi }\xi v$$ 1

Where v is the scanning speed of tool electrode, T is the total machining time, $\Psi$ is the wear length of tool electrode, which can be expressed by Eq. (2).

$$\Psi =\frac{{V\cdot \text{R}\text{E}\text{W}\text{R}}}{S}$$ 2

Where V is the workpiece material removal volume, S is the sectional area of tool electrode, REWR is the relative electrode wear ratio, which is the ratio of tool electrode wear volume to the workpiece material removal volume.

Then, combining Eqs. (1) and (2), the compensation distance L can be expressed by Eq. (3).

$$L=T\frac{S}{{V\cdot \text{R}\text{E}\text{W}\text{R}}}\xi v\;$$ 3

The machining parameters used in this study were shown in Table 1.

Table 1

Experimental conditions
Open voltage	110 V
Discharge current	15 A
Pulse frequency	2 kHz
Pulse duration	25%
Tool electrode	Copper (+)
Dielectric liquid	EDM oil
Workpiece	High speed steel, 45 steel, 304 stainless steel (-)
Scanning speed of tool electrode	0.2 mm/s
Layer thickness $\delta$	100 µm

4.1 EDM milling of trapezoidal groove

To verify the correctness and effectiveness of servo control strategy and wear compensation of tool electrode, the trapezoidal groove structures on the plane workpiece and cylindrical workpiece were machined with 3 mm diameter tool electrode by EDM milling based on 6-DOF serial robot, respectively, as shown in Fig. 9. Firstly, through the experiment of EDM milling plane, the REWR, which was 8.2% under the experimental conditions shown in Table 1, was obtained. In the machining experiment of trapezoidal groove on the plane workpiece of high speed steel, the sectional area of tool electrode S was 7.065 mm², and the workpiece material removal volume V was 275 mm³, and the total machining time T was 5008 s, and the compensation value $\xi$ was 5 µm. Then, the compensation distance L can be calculated by Eq. (3) and L was 1.57 mm. The EDM milling results of trapezoidal groove on the plane workpiece without and with wear compensation of tool electrode were shown in Fig. 10(a) and 10(b), respectively. From Fig. 10(a), it can be found that the distance between the bottom surface and the top surface of trapezoidal groove was only 3.2 mm, which was far less than the actual 5 mm. Furthermore, the included angle of the side of trapezoidal groove was 35.7 °, which was also far less than the actual 45°. But, from Fig. 10(b), it can be found that after considering the wear compensation of tool electrode, the distance between the bottom surface and the top surface of trapezoidal groove was 5.1 mm, which was close to the actual 5 mm. Furthermore, the included angle of the side of trapezoidal groove was 45.6°, which was also close the actual 45°. This experiment verified the correctness and effectiveness of servo control strategy and wear compensation of tool electrode.

In the machining experiment of trapezoidal groove on the cylindrical workpiece of 45 steel with the experimental conditions shown in Table 1, the workpiece material removal volume V was 374.8 mm³, and the total machining time T was 6965 s. Then, the compensation distance L can be calculated by Eq. (3) and L was 1.558 mm. The EDM milling results of trapezoidal groove on the plane workpiece without and with the wear compensation of tool electrode were shown in Fig. 11(a) and 11(b), respectively. From Fig. 11(a), it can be found that the distance between the bottom surface and the top surface of trapezoidal groove was far less than the actual 5 mm. But, from Fig. 11(b), it can be found that after considering the wear compensation of tool electrode, the distance between the bottom surface and the top surface of trapezoidal groove was 4.8 mm, which was close to the actual 5 mm. It should be noted that limited by the installation accuracy of worktable fixture of self-made equipment, in the machining of curved workpiece, lower positioning error of workpiece resulted in lower machining accuracy than the plane workpiece shown in Fig. 10. However, this problem can be solved by designing and manufacturing high-precision fixtures in the future.

4.2 EDM milling of complex trajectory on the space surface workpiece

To verify the machining ability of complex trajectory on the space surface of workpiece by EDM milling based on 6-DOF serial robot, three complex trajectories, such as EDM, Chinese character ‘China’ and ‘Chinese knot’, were machined with tool electrodes of 2 mm diameter, 2 mm diameter, 3 mm diameter, respectively, under the experimental conditions shown in Table 1. The material of hemispherical workpiece with the radius of 38 mm was 304 stainless steel. The machining result presented in Fig. 12 show that in the EDM milling of complex trajectory on the space surface using a 6-DOF serial robot, the tool electrode can be always perpendicular to the tangent plane of the workpiece surface, which ensures the machining accuracy of complex trajectory. The experimental results demonstrated that the EDM milling based on 6-DOF serial robot is feasible and effective on machining of complex trajectory on the space surface.

This paper proposed EDM milling based on 6-DOF serial robot. The research results were summarized below:

(1) To solve the problem that serial robot cannot meet the rapid response requirements of EDM for real-time adjustment of discharge gap, a tool electrode servo motion device was invented and installed at the end of a 6-DOF serial robot and a servo control strategy of tool electrode vertical trajectory fallback was proposed to realize the rapid adjustment of machining gap.

(2) To achieve the controllable thickness of every layer in EDM milling based on 6-DOF serial robot, the servo control strategy of limited motion of the tool electrode on the basis of vertical servo control and a fix-length compensation method to compensate the tool electrode wear were proposed.

(3) To verify the correctness and effectiveness of servo control strategy and wear compensation of tool electrode, the trapezoidal groove structures on the plane workpiece and cylindrical workpiece were machined by EDM milling based on 6-DOF serial robot, respectively. This experiment verified the correctness and effectiveness of servo control strategy and wear compensation of tool electrode.

(4) To verify the machining ability of complex trajectory on the space surface of workpiece of EDM milling based on 6-DOF serial robot, three kinds of complex trajectories were designed and machined on the hemispherical workpiece with the radius of 38 mm.

Conflicts of interest/Competing interests

The authors declare no competing interests.

Ethics approval

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Consent to participate

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Consent for publication

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Funding

The authors would like to thank the National Natural Science Foundation of China (No. 51905311).

Authors' contributions

Xiaoming Yue: Conceptualization, Investigation, Validation, Visualization, Writing - original draft, Funding acquisition. Zhiyuan Chen: Data curation, Visualization. Zuoke Xu: Formal analysis, Methodology. Jing Liu: Project administration, Supervision, Resources, Writing - review & editing.

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Editorial decision: Major Revisions Needed
24 May, 2022
Reviews received at journal
01 May, 2022
Reviewers invited by journal
27 Apr, 2022
Editor assigned by journal
27 Apr, 2022
First submitted to journal
25 Apr, 2022

You are reading this latest preprint version

EDM milling based on a 6-DOF serial robot

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental Setup

3. Servo Control Principle Of Edm Milling Based On 6-dof Serial Robot

3.1 Servo control strategy of limited motion of tool electrode

3.2 Wear compensation of tool electrode

4. Machining Experiments Of Edm Milling Based On 6-dof Serial Robot

4.1 EDM milling of trapezoidal groove

4.2 EDM milling of complex trajectory on the space surface workpiece

5. Conclusions

Declarations

References

Status:

Version 1