Development of High-precision Micro EDM System

Abstract : A micro EDM system was designed and developed. The system consists of horizontal axis layout V-type ceramic structure rotation spindle system, macro and micro servo servo system with piezoceramics, copying system for making micro tools, reading microscope and electrical control. The system has been applied. We have successfully machined micro shafts with a diameter of <4.5 μm and micro bores with a diameter of <8 μm.
Keywords : Microfabrication; EDM; Piezoelectric ceramic; Micro shaft; Micro hole

1 Features of Micro EDM

There is no essential difference between the principle of micro EDM and ordinary EDM. The surface quality of its processing mainly depends on the size and depth of pits, ie the energy of a single discharge pulse; and its processing accuracy is closely related to the discharge gap, process system stability, electrode loss and other factors.
Micro EDM is also the use of pulse power supply, the high-frequency discharge energy is discharged to the discharge gap, by the resulting high-temperature thermal effects and other comprehensive effects to achieve the removal of materials, so as to achieve the purpose of processing the workpiece. However, due to the fineness of the processed aperture, generally between <5 ~ 100μm, so to achieve the processing of dimensional accuracy and surface quality requirements, there are some special requirements. Micro EDM has the following characteristics:

(1) The discharge area is very small. The electrode for micro-EDM processing is generally between <5 and 100 μm. For a <5 μm electrode, the discharge area is less than 20 μm2. Discharge on such a small area discharges the distribution of discharge points. The range is very limited, and it is very easy to cause concentration of the discharge position and time, which increases the instability of the discharge process and makes it difficult to perform micro-EDM processing.

(2) Single pulse discharge energy is very small to meet the discharge surface of a small positive EDM requirements, to ensure the processing of dimensional accuracy and surface quality, the removal of each pulse should be controlled within the range of 0. 10 ~ 0. 01μm Therefore, the energy of each discharge pulse must be controlled between 10 -6 and 10 - 7 J or even smaller.

(3) The discharge gap is small because EDM machining is non-contact processing. There is a certain machining gap between the tool and the workpiece. The size of the discharge gap varies with the processing conditions, with values ​​ranging from a few micrometers to hundreds of micrometers. The control of discharge gap and its changing rule directly affect the processing quality, processing stability and processing efficiency. Especially in the micro-EDM process, the processing of micro-holes accounts for the majority, and the size and stability of the discharge gap are the key to the success of micro-hole processing.

(4) The tool electrode preparation is difficult to process tiny holes and tiny cavities with a small size, and a smaller tool electrode must be obtained first. In the conventional micro-EDM process, the micro-tool electrode generally adopts the method of special machining and secondary mounting on the spindle of the machine tool. At this time, there are obviously installation errors and deformation errors of the micro-electrode, and it is difficult to ensure the tool electrode and work. The verticality of the table and the coaxiality of the electrode and the rotary spindle. Prior to the emergence of wire electrode spark erosion (WEDG), the manufacture and installation of micro-electrodes has always been a bottleneck restricting the development of micro-EDM technology. Due to the problems in the installation process of the fine electrode, it is obviously not preferable to perform the electrode detection in an off-line manner. From the current application situation, the use of WEDG technology can well solve the problem of the preparation of micro-tool electrode. In order to obtain a very fine tool electrode, a high-precision WEDG system is required, and at the same time, the accuracy of the spindle rotation of the EDM system is required to reach a very high level, and it should generally be controlled within 1 μm.

(5) Difficulties in chip removal and difficulty in obtaining a stable spark discharge state Due to the small discharge area and discharge gap during micro-hole machining, it is very easy to cause a short circuit. Therefore, to obtain a stable spark discharge state, the feed servo control system must have sufficient Sensitivity, can quickly retreat in abnormal discharge, eliminate the abnormal gap state, improve the pulse utilization, protect the electrode from damage.

2 Overall design of micro-EDM system

Based on the analysis of the characteristics of the above micro-EDM machining, based on a large number of domestic and foreign researches on micro-EDM and related technology research results, a prototype of micro-EDM machining system was designed and developed. The system is divided into two parts: mechanical and electrical. The mechanical part is mainly composed of four parts: horizontal axis rotary spindle, stepper motor and piezoceramic servo feed device, micro-electrode copying system and reading microscope. The block diagram is shown in Figure 1.


Fig.1 Schematic diagram of the mechanical structure of the micro EDM system

2. 1 Horizontal axis layout ceramic V-support rotating spindle system

Under normal circumstances, the EDM machine tool spindle is vertical, so when the hole is processed, the removal of the erosion material is difficult, and the discharge gap is moved from the bottom of the hole to the top of the hole to overcome the gravity discharge, during micro and deep hole processing. The situation is even more serious. Because the micro-EDM process, the discharge energy is small, the spark's explosive force is small, the discharge gap is also small (the gap can be controlled below 1μm), the discharge of its own power is difficult to discharge the erosion of the product. Due to the difficulty of chip removal, the discharge is not easily stabilized, and the short circuit increases, resulting in a decrease in processing efficiency and deterioration in processing quality.

In order to improve the micro-EDM processing conditions, the system adopts the horizontal axis layout scheme proposed by Prof. Zawaji Takahisa of the University of Tokyo in Japan. It is the horizontal machining method of the spindle horizontal layout. The erosion material does not have to overcome the gravity, and even can use its own weight. Ejection, chip removal effect is good. Experiments show that with the horizontal axis layout of micro-hole processing, the ratio of depth to diameter can reach 10:1 or more. Rotation of the electrode not only helps to improve the roundness of the hole processing, but also improves the processing efficiency by 2 to 5 times [2]. At the same time, the spindle rotation is also a need for on-line production of fine electrodes using wire electrode grinding.

In conventional EDM machines, the C-axis (rotary axis) is generally mounted on a Z-axis moving part to drive the electrode to perform a rotary motion. Therefore, there is inevitably a parallelism error between the C axis and the Z axis. In order to overcome the above design defects, the spindle of the micro EDM system adopts high-precision V-type ceramic support, and the DC motor drives the spindle through the belt to make a smooth and high-precision rotation to realize the C-axis rotary motion; the piezoelectric ceramic compound stepper The motor serves as a feed servo mechanism to push the spindle to slide on the V-shaped support to achieve the Z-axis movement, thereby realizing the combination of the C-axis and the Z-axis. Micro-EDM requires that the spindle's rotation accuracy should be within 1 μm, so the scientific and rational design of the spindle system is crucial. The spindle of this system adopts high-precision double V-shaped block support, which is easy to realize high-slewing precision, and the structure is simple, economical, and the measured rotating accuracy can reach 0. 75 μm. If the spindle system is designed using a bearing structure or other methods, it is not only difficult to achieve, but also has a complex structure, a large volume, a large inertia of the system, and difficulty in servo. In addition, the use of ceramic V-block support can also achieve electrical insulation between the main shaft and the bed, which helps to reduce the parasitic capacitance during EDM, thereby further improving the machining accuracy.

2. 2 Macro-micro servo feed system with piezoelectric ceramic

Stepper motor and piezoceramic servo feed system structure shown in Figure 1. The stepping motor controlled by the single-chip microcomputer rotates in micro-steps, and then generates micro-infeed through the screw and nut. In the range of 0 to 5 mm, the spindle resolution is 0. 6 μm, and the macro feed is controlled by the single-chip microcomputer. The ceramic driver can achieve a micro feed of 3.42 nm in the range of 0-14 μm. Due to the high response speed and high feed-in resolution of piezoelectric ceramics, it can well meet the requirements for high-precision feed and short-circuit retreat of micro-EDM. When the workpiece and the electrode are short-circuited during processing, a command is issued by the microcontroller to allow the piezoelectric ceramic control electrode to quickly retreat. After the short-circuit is eliminated, the normal machining feedrate is restored. Therefore, the EDM spindle servo feed system composed of a stepper motor and a piezoelectric ceramic can not only achieve a large stroke EDM, but also can achieve high feed resolution and positioning accuracy, and can be quickly realized. Responsive to achieve efficient and stable EDM machining.

2. 3 tool electrode copying system

The micro-holes and micro-three-dimensional structure processing with micro-electrodes with simple shapes has become one of the mainstream technologies in current micro-EDM machining. Therefore, the precise and efficient production of micro-electrodes occupies an extremely important position in the micro-EDM process. There are two traditional methods for producing fine electrodes: one is to straighten fine wires that have been drawn by cold drawing; the other is to use cutting and grinding methods. It is obviously difficult to meet the requirements of micro-EDM processing by using the off-line method for micro-electrode fabrication. This is because the electrode will inevitably produce a rotational accuracy error and its verticality with the work surface during the second installation. The operator's technology is highly dependent on accuracy and repeatability. In practice, the electrode diameter that can be used in this way is limited to ф50 μm. Therefore, it is imperative to adopt an on-line method for fine electrode fabrication. At present, the commonly used electrode online production methods mainly include block reverse copy machining and wire electrode electric spark grinding (WEDG). The invented reverse copy electrode system includes the above two methods and can be used to prepare various sizes and shapes on line. Fine electrode.

2.4 Electrical Control System for Micro EDM System

The electrical control part of the micro-EDM system adopts a single-chip microcomputer-based control system under the control of a PC, including the core control system, micro-energy RC pulse power supply, DC motor and stepper motor drive control circuit, and piezoelectric ceramic drive control. Circuits, discharge process monitoring circuits, and contact sensing and measurement circuits are shown in Figure 2.


Figure 2 The composition of the electrical control system of the micro-EDM system

The single-chip computer as the core computer controls the real-time operation of the entire system, and its operation instructions are derived from the RS-232 communication of the host computer. The micro-pulse power supply adopts the original low-voltage technology. The single-pulse spark discharge energy is reduced from the normal 10 -6 to 10 - 7 J to the minimum of 10 - 12 J, which is very good for micro-EDM machining. Needs. The discharge state detection circuit consists of several parts including voltage divider, photoelectric isolation, sample and hold, and A/D conversion. The function of the voltage divider circuit is to reduce the high voltage at the open circuit by an appropriate ratio to the range that the A/D converter can convert. The spark discharge signal is a very complex source of interference. The function of the opto-isolation circuit is to use a linear optocoupler to completely isolate the spark gap gap circuit from the A/D circuit and the computer circuit to minimize the spark discharge to A/D conversion. And the interference of computer work. The A/D converter converts the analog signal of the gap voltage into a digital signal that can be recognized by the computer, so that the computer can judge the gap discharge state according to the A/D converter reading value and then make a decision on the next execution action.

3 Micro EDM machining experiment

The limit processing capability of the micro-tool electrode is an important index to evaluate the performance of the micro-EDM system, and it is also a precondition for machining the micro-holes and cavities of the corresponding sizes. Taking the machining of micro-axis as an example, the method is to install the tool electrode on the spindle as the workpiece, using the block reverse copy or WEDG method to reduce it to the required size, or to the minimum size that can be achieved by the system's limit processing capability. . Figure 3 is a scanning electron microscopy image of an example of a micro-axis machined using the micro-EDM system developed in this paper. Its diameter is only ф4.5 μm [3].


Figure 3 ф4. 5μm micro shaft

The size of the tiny holes is also a very important indicator for evaluating the performance of the prototype. Figure 4 shows an example of tiny holes machined by this system. Its diameter is only ф8μm [3].


Figure 4 ф8μm pores

4 Conclusion

This paper focuses on the characteristics of micro-EDM machining, and designs and develops a micro-EDM system. The system is mainly composed of horizontal axis layout V-type ceramic structure rotating spindle system, macro and micro servo servo system with piezoceramics, copying system for making micro tools, reading microscope and electric control part. The system has successfully machined micro-diameters with diameters of only 4.5 μm and micro-holes with diameters of only 8 μm, which have now reached leading domestic and internationally advanced levels. This indicates that China has made significant progress in micro-discharge machining systems and applications, and will greatly promote our research in micro-EDM processing.

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