Programmed electrode wear compensation device and method for 3D EDM scanning apparatus

A programmed electrode wear compensation device and method for 3D EDM scanning apparatus has a compensation device which receives raw data about a working path of a working apparatus from a CAD/CAM device, calculates therefrom compensated data which take into account electrode wear and transmits the compensated data to a control device which controls the working apparatus, so that smoothness and shape of a machined surface are maintained during electric discharge.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a programmed electrode wear compensation device and method for a 3D EDM scanning apparatus, particularly to a programmed electrode wear compensation device and method used in conjunction with a 3D EDM scanning apparatus, which ensures that smoothness and shape of a machined surface are maintained during electric discharge.

2. Description of Related Art

Machining by electric discharge is a working method which is slow, but applicable to working hard materials or alloys, which are usually difficult to work, and which is mainly used for generating micro-sized forms or cuts, with dimensions ranging from dozens of micrometers to several millimeters.

ReferringFIGS. 10 and 11, in conventional art, wire electric discharge grinding (WEDG), structures as shown inFIGS. 10are made by an arrangement as follows: A workpiece71is worked by a running wire72which is guided by a wire guide73. Working is performed by arranging a micro-sized electrode and is milling layers with the electrode by electric discharge. During working, however, the electrode undergoes wear, which needs to be tracked and compensated to maintain plane surfaces and homogeneous milling depths as well as to preserve smooth surfaces. Due to working, electrodes are usually not flat and become shorter over time, so that worked surfaces are not even and milling depths are not reached. Using small electrodes that run along linear tracks to work relatively large areas without compensating for wear results in uneven worked surfaces. Therefore, proper tracking and wear compensation are indispensable in EDM scanning.

Research has established relations between material, shape, dimensions, energy of discharge, working paths and electrode wear. In practice, however, working is still performed by individual judgement of users without clear rules. Generally, electrode wear is compensated only when proceeding to another layer. Programming support for EDM is provided only by regular computer aided manufacturing (CAM) and complemented by the experience of the user.

Conventional art is summarized as follows:1. Taiwan patent publication no. 520311 “Method for working holes with uniform depths by automatic compensation of electrode wear” teaches an automatic calculation of electrode wear, which is compensated in working subsequent holes. Although this method results in increased effectiveness of working, it is only applicable to working holes.2. U.S. Pat. No. 4,301,349 “Electrical machining apparatus for forming a three-dimensional surface contour in a workpiece” teaches a wire oscillating at high frequency, so that continuously changing sections of the wire are used for working to eliminate effects of wear.3. U.S. Pat. No. 4,386,248 “Electrical machining method and apparatus for forming a 3D surface contour in a workpiece with a travelling wire electrode” teaches a continuously running wire as discharge electrode, so that changing sections of the wire are used for working to eliminate effects of wear.4. U.S. Pat. No. 5,354,961 “Device and process for electrical-discharge machining of a three-dimensional cavity with a thin rotating tool electrode” discloses a controller which calculates wear and change of shape of a tool electrode and measures and adjusts the length of the electrode. The controller calculates volume changes, which possibly are not correct. Working paths are not controlled, so that working time is wasted.5. U.S. Pat. No. 6,454,930 “Procedure and device for the three-dimensional processing of a work piece by means of electroerosive or electrochemical machining” discloses a device moving with five degrees of freedom and working a workpiece by electroerosive or electrochemical machining. An angle between the workpiece and an electrode is adjustable, allowing for angular compensation between different layers. Working depths are small, and working and sensing are done simultaneously, which slows down working and increases manufacturing cost.6. U.S. Pat. No. 6,531,036 “Fabrication of micron-sized parts from conductive materials by silicon electric discharge machining” teaches an electrode which is made by engraving semiconductor material and depositing a metal layer thereon and used in conjunction with CAD/CAM for working plane surfaces. Therein, a microelectronic process and electric discharge are used for CAD/CAM working.7. U.S. Pat. No. 6,642,470 “Process and device for machining a three-dimensional piece by electroerosive milling” teaches a manufacturing process in which a regulation module compensates for electrode wear by simulation thereof and comparison with an actual electrode position. Compensation rules, however, are complicated, so that handling is difficult. Furthermore, crossing of working path sections is not taken into consideration.

Similar to conventional milling, conventional working by electric discharge is performed-using procedures and parameters that are the result of experience. Material, dimensions, roughness of surfaces and user experience decide on working parameters like waveform, voltage and computing parameters. Usually, a groove is made by clearing away material layer by layer, with plane of worked surfaces being determined by working paths and compensation of electrode wear. Optimal speed of working depends on electrode wear and is decided by experience. So far there is no CAD/CAM technique on the market that considers working paths. Any working program still has to overcome this problem.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a programmed electrode wear compensation device and method for a 3D EDM scanning apparatus.

The present invention is a programmed electrode wear compensation device and method for 3D EDM scanning apparatus has a compensation device which receives raw data about a working path of a working apparatus from a CAD/CAM device, calculates therefrom compensated data which take into account electrode wear and transmits the compensated data to a control device which controls the working apparatus, so that smoothness and shape of a machined surface are maintained during electric discharge.

The characteristics of the present invention include:1. Raw data from a CAD/CAM device about a working path of a working apparatus are used and compensated for electrode wear.2. A complete working path and process is calculated based on the raw data from the CAD/CAM device, yielding electrode wear, a process working path and a working result.3. Stored parameters are easily modifiable according to case-by-case demands.4. No modification of the control device beyond manual input of commands is required.

The present invention can be more fully understood by reference to the following description and accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown inFIGS. 1 and 2, the present invention has a programmed electrode wear compensation device30connected with an storage device10afor roughness of workpiece, electrode dimensions, electrode structure and material, and a storage device20afor discharge and empirical parameters. Furthermore, a CAD/CAM device40agenerating raw working data is connected with the compensation device30. The compensation device30generates compensated data, which are fed into a control device50controlling a working apparatus60. The compensation device30comprises an input interface31, a computing device32, and an output interface33. The input interface31is connected with the CAD/CAM device40. The computing device32computes compensation data according to compensation rules. The output interface33is placed behind the computing device32, providing output data. Raw working data generated by the CAD/CAM device40are input, compensated, and a working path is calculated and transmitted to the control device50for controlling the working apparatus60.

Thereby, due to compensation of electrode wear and a corrected working path, flatness and roughness of working surfaces are ensured.

The method of the present invention comprises the following five working ways:

1. A 3D groove (as shown inFIG. 10A) is worked by clearing away various 2D layers (as shown inFIGS. 10B and 10C).

2. Each 2D layer is worked in a plane, using crossing working path sections. In an example shown inFIG. 3, the first cut (A) proceeds leftwards, working along the Y-direction, the second cut (D) proceeds downwards, working along the X-direction, the third cut (C) proceeds upwards, working along the X-direction, and the fourth cut (B) proceeds rightwards, working along the Y-direction.

3. For each 2D layer, a ratio r of an electrode diameter and distances of neighboring working paths is determined by 0<r<1, preferably r=0.7.

4. Electrode wear under various conditions is calculated, and compensation data for working are determined, as explained further below.

5. If electrode wear has exceeded a threshold that requires replacing of the electrode, working is halted. Subsequent working after replacing of the electrode is again performed with compensation of electrode wear.

Referring toFIGS. 4A–4D, four calculation methods are employed. In the figures, P denotes a workpiece, Q denotes the electrode, R denotes a displacement of the electrode Q, RM stands for R×0.7, and G denotes a gap.

(1) Electrode wear in Z-direction (vertical): As shown inFIG. 4A, When working in Z-direction, electrode wear compensation needs to be performed according to empirical data and proportional to working depth. As shown inFIGS. 5A and 5B, with vertical working in Z-direction having a depth ΔZ and a wear rate R1, the electrode wear ΔL equals ΔZ×R1. The programmed position in the Z-direction Tzhas to consider the electrode wear ΔL, so that Tz(ΔZ)=Z−(ΔZ+ΔL)=Z−ΔZ−ΔZ×R.

(2) Within each 2D layer, electrode wear compensation at first cut of the 2D layer: As shown inFIG. 4B, with working taking place in the XY-plane, electrode wear compensation needs to be performed according to empirical data and proportional to working path length. As shown inFIG. 6, with working in X-direction having a path length ΔS and a wear rate R2, the electrode wear ΔL equals ΔS×R2. The programmed position in the Z-direction Tzhas to consider the electrode wear ΔL, so that Tz(ΔZ)=−ΔL=Z−ΔS×R2.

(3) Within each 2D layer, electrode wear compensation for distances of neighboring paths being smaller than the diameter of the electrode: As shown inFIGS. 4C and 4D, with working taking place in the XY-plane, but distances of neighboring paths being smaller than the diameter of the electrode, a third method of electrode wear compensation needs to be performed, considering that working paths cross and are according to empirical data and proportional to working path length. As shown inFIG. 6, with working in X-direction having a path length ΔS and a wear rate R3, the electrode wear ΔL equals ΔS×R3. The programmed position in the Z-direction Tzhas to consider the electrode wear ΔL, so that Tz(ΔZ)=Z−ΔL=Z−ΔS×R3, similar to the second method of electrode wear compensation. The third method of electrode wear compensation, as compared to the second method thereof, takes into account that working paths cross when setting a value of R3. An empirical value for R3is R2multiplied by 0.7 to 0.8.

(4) Electrode wear compensation during fine cut: A fourth method of compensation accounts for relatively small, but not negligible differences and is done in the same way as the third method of compensation. As shown inFIG. 6, with working in X-direction having a path length ΔS and a wear rate R4, the electrode wear ΔL equals ΔS×R2. The programmed position in the Z-direction Tzhas to consider the electrode wear ΔL, so that Tz(ΔZ)=Z−ΔL=Z−ΔS×R4.

Referring toFIG. 7, fine cutting is performed downward and in X-direction and upward.FIG. 8shows electrode wear (Y-axis) versus working path length (X-axis) for various electric discharge energies, with M denoting the largest and N denoting the smallest electric discharge energy in the figure.

R1, R2, R3and R4denote electrode wear rates and are dimensionless numbers, being defined as length changes per working depth or working path length. The electrode wear rates are determined according to working conditions. ΔS denotes the length of any working path in the XY-plane.

FIG. 9is a flowchart showing calculation of compensated data for electrode wear by the programmed electrode wear compensation device30. Steps therein comprise: determining crossing of working path sections81; determining working depth and electrode wear of each layer82; determining distances of neighboring working path sections83; for each layer, determining electrode wear due to plane working of the layer, considering overlap of working path sections84; determining length of electrode85; input of raw data from CAD/CAM device86; and input of 3D EDM scanning apparatus data87. The raw data from the CAD/CAM device are modified to account for electrode wear compensation. Compensated data are fed into the control device50for controlling the working apparatus60.

Referring again toFIG. 2, in an embodiment of the present invention, parameters are selected according to data from a storage device10for roughness of workpiece, electrode dimensions, electrode structure and material, and a storage device20afor discharge and empirical parameters. The CAD/CAM device provides raw data for controlling a working path. The compensation device30generates compensated data from the raw data to be transmitted to the control device50for controlling the working apparatus60.

While the invention has been described with reference to a preferred embodiment thereof, it is to be understood that modifications or variations may be easily made without departing from the spirit of this invention which is defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1is a schematic diagram of the programmed electrode wear compensation device for a 3D EDM scanning apparatus of the present invention.

FIG. 2is a schematic diagram of the programmed electrode wear compensation device for a 3D EDM scanning apparatus of the present invention in an embodiment.

FIG. 3is a schematic illustration of plane working paths of the present invention.

FIGS. 4A–4Dare schematic illustrations of various ways of electrode wear.

FIGS. 5A and 5Bare schematic illustrations of electrode wear compensation with a vertical working path.

FIG. 6is schematic illustration of electrode wear compensation with a horizontal working path.

FIG. 7is a schematic illustration of a fine cutting working path of the present invention.

FIG. 8is a plot of electrode wear against working path length for various electric discharge energies.

FIG. 9is a flowchart of the programmed electrode wear compensation method of the present invention.

FIGS. 10A–10Care schematic illustrations of cutting a groove in layers.

FIG. 11is a perspective view of a WEDG device.