Hybrid servomechanism for micro-electrical discharge machining

Details Hybrid servomechanism for micro-electrical discharge machining Hybrid servomechanism for micro-electrical discharge machining

1999-04-16

2002-05-07

Grant, William (Department: 2121)

Data processing: generic control systems or specific application

Specific application, apparatus or process

Product assembly or manufacturing

C310S316030, C219S069110

Reexamination Certificate

active

06385500

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention generally relates to electrical discharge machining and, more particularly, to a hybrid servomechanism for micro-electrical discharge machining.
BACKGROUND OF THE INVENTION
Electrical discharge machining (EDM), or spark erosion, is a method of machining conductive materials by applying a series of electrical sparks in the presence of a dielectric. It was serendipitously discovered by B. R. Lazarenko and N. I. Lazarenko in 1943 in the process of trying to remove a stuck drill bit from a hole by means of pulsed electrical discharges. As shown schematically in
FIG. 1
, a spark discharge is produced by the controlled application of DC voltage pulses between two electrodes, namely, the work-piece
10
and the tool (electrode
12
), which are separated by a distance of approximately 0.01 mm to 0.50 mm (spark-gap). A dielectric fluid
14
is present in the spark-gap. Upon pulsed application of a high voltage, the dielectric
14
in the gap is partially ionized, thus causing a spark discharge between the tool
12
and the work-piece
10
.
Each discharge produces enough heat to melt or vaporize a small quantity of the work-piece
10
material, and this material is ejected at the end of the discharge, creating a tiny pit or crater that is left behind on the surface of the work-piece
10
. This is the mechanism of material removal. Even though in EDM tool wear is high and the machining rate is much smaller than in turning, milling or grinding, it has still found a wide range of applications. The facts favoring EDM over conventional machining processing in some applications are its ability to: (1) remove machine materials of high hardness, high tensile strength and poor machineability; (2) machine complex or irregular shapes and intricate cavities; and (3) fabricate parts that are too thin and fragile to withstand the forces produced in conventional machining. Furthermore, the manufactured component is free of burrs. The largest application of EDM is in the machining of dies and molds, either before or after hardening; machining of carbides, tungsten and more recently conductive ceramics such as titanium di-boride, boron carbide and silicon carbide composites. Die-sinking and wire-cut EDM are the commonly used configurations for these applications.
A more recent EDM process that was developed in the late 1960's is micro-hole drilling, henceforth referred to as a micro-EDM process. An important application of this process is in the drilling of the small diameter (~150&mgr;) orifice holes of fuel injector nozzles in diesel engines. Holes of diameter ~100 &mgr;m to 250 &mgr;m and with an aspect ratio greater than 5 (aspect ratio being the ratio of the depth of the hole to its diameter) are very expensive to drill by conventional means. Frequent tool re-sharpening, excessive drill breakage, the poor ability of hard alloys to withstand machining and formation of entry or exit burrs with mechanical drills make conventional drilling almost impractical as a production process for producing such micro-holes. But with micro-EDM, the machineability is more a function of the melting point rather than the hardness of the work material, and it is inherently a burr-free process. In conventional drilling, the hole diameter is primarily determined by the diameter of the drill and the operator has little control over the size of the resulting hole. But by the suitable selection of process parameters, it is possible to control, within bounds, the amount over-cut in EDM. Hence, for a given diameter of the tool electrode
12
, the operator can control and adjust the diameter of the hole. The dimensional accuracy of the holes (i.e., their size and taper) produced by micro-EDM is usually superior to that produced by other unconventional processes such as electro-chemical machining (ECM) and laser machining. Hence, micro-EDM has become an established production process for the drilling of small holes.
Much effort has gone into understanding the physics of the EDM process and to relating the instantaneous gap conditions to the process performance. In this process, the machining is carried out by a series of electrical discharges which are applied between the tool
12
and work surfaces
10
in the presence of a liquid dielectric medium
14
. A relaxation type, or a pulse generator type, of power supply provides a DC voltage of 100 to 200 volts between the tool electrode
12
(usually the cathode) and the work piece
10
electrode (usually the anode). The tool electrode
12
for hole drilling is in the form of a thin circular wire which is guided through closely matched ceramic guides. Tungsten or a tungsten-copper alloy is commonly used as the tool electrode
12
material because of its low rate of wear. The dielectric
14
is usually de-ionized water, which is drip-fed into the gap between the tool
12
and the work-piece
10
surfaces. At a critical value of the applied voltage, the dielectric
14
breaks down, causing an electrical discharge to occur between the tool
12
and the work surface
10
. During every such discharge a small volume of material is removed from the work-piece
10
surface as a consequence of localized melting and ejection of the molten material. The crater produced by the localized melting is usually small, typically a few micrometers in width. The cumulative effect of a succession of such discharges spread over the entire work-piece
10
surface leads to its erosion, or machining to a shape which is approximately complementary to that of the tool
12
.
As machining occurs, a servo system
16
advances the wire (tool)
12
in order to maintain a preset gap of about of 0.01 mm between the tool
12
and work surfaces
10
. The action of the servo
16
in micro-EDM is based on a measurement of the average gap-voltage between the tool
12
and the work
10
. In micro-EDM, exceptionally low energy pulses with a small pulse duration are used to obtain the high accuracy required. Furthermore, discharge repetition rates are high, as over a million discharges are required to machine a hole of diameter ~0.006 inches to a depth of ~0.030 inches. The electrical pulses that are used to initiate discharges are much smaller; the objective is to have discharges of small energy, ideally of the order of 10
−7
to 10
−5
Joules, removing smaller increments of material from the work-piece
10
.
To compensate for a possible fall in machining rate, because of lesser material removal, the frequency of the pulses is increased to a few orders of magnitude greater than die-sinking or wire EDM processes; for example, typical current pulse widths are 150 nanoseconds to 250 nanoseconds, and at rates of a million discharges a second. This causes the gap conditions to change rapidly. At such high discharge rates, the reliability of discharge repetition suffers with the use of oil-based dielectrics, conventionally used in EDM; to increase the reliability, de-ionized water is used as a dielectric
14
.
In order to obtain holes of good quality with a smooth and damage-free surface, and to maintain consistency of dimensions from one hole to another, it is desirable that the sparking discharges occur in a controlled and uniform manner. While certain types of discharges produce surfaces with a good finish, other types of discharges are known to cause work surface damage or not remove material at all. In a typical machining cycle it is desirable that the fraction of “good” machining discharges be kept as high as possible. The state-of-the-art EDM machines for microhole drilling are not adequately equipped to discriminate between the various types of discharge pulses. They are only equipped with a servo
16
which controls the feed of the tool electrode
12
in such a way as to maintain a constant gap between the front faces of the electrodes
10
,
12
. Such servo systems
16
respond to the average voltage in the spark-gap which is not a sensitive indicator of the “instantaneous” gap conditions or the efficiency of individual discharges (instantaneou

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