Abstract:
A method and apparatus of operating a wirecut electrical discharge machine for cutting a workpiece using a generated electrical discharge in a machining gap wherein the workpiece is opposing a wire electrode. The method comprises the steps of presetting a plurality of machining conditions in accordance with dielectric pressure and machined plate thickness combinations, storing the preset machining conditions in a memory, and automatically setting an optimum machining condition based on detected dielectric pressures and the machined plate thickness calculated in a numerical controller. The machined plate thickness may be calculated by dividing an area machining feedrate corresponding to present electrical condition parameters by the actual machining feedrate.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a wirecut electrical discharge machine (EDM) for the electrical discharge machining of a workpiece employing a wire electrode and, more particularly, to a wirecut electrical discharge machine for controlling electrical machining conditions in accordance with plate thickness changes in the workpiece and dielectric pressure changes in the wirecut electrical discharge machine. 
     BACKGROUND OF THE INVENTION 
     Wirecut electrical discharge machines are known which employ a metal wire of approximately 0.05 to 0.3 mm in diameter as an electrode. The metal wire is fed in X and Y directions relative to a workpiece to perform machining operations, e.g., cutting and contour shape forming. The wire electrode is usually controlled to be relatively fed at a stepped, constant speed in units of 1 μm per pulse, with the feedrate controlled so as to maintain a discharge of constant voltage in the machining gap without the need of controlling discharge energy, etc. However, when the thickness of the workpiece is not uniform, the workpiece is machined with the initial speed set at the speed corresponding to the maximum plate thickness (maximum area to be machined) in order to prevent short circuits between the wire and the workpiece or wire electrode breakage. In other words, the wire electrode is fed at the initially set low speed even though the plate thickness may have decreased during machining. Therefore, the overall machining efficiency is reduced. 
     A process for improving the above inefficiency was presented in Japanese Patent Publication No. 52890 of 1985. In this process, data combining the electrical conditions of a machining power supply suitable for various plate thicknesses of the workpiece and corresponding machining feedrates are stored in a memory. During operation, the data stored in memory is shifted to change the electrical conditions in correspondence with the machining feedrate so as to match the machining feedrate stored in memory with the machining feedrate during machining. 
     An example of this process will be explained in reference to FIG. 1, which illustrates a wirecut electrical discharge machine, wherein the numeral 1 indicates a wire electrode, 2 indicates a workpiece, 3 and 4 indicate upper and lower dielectric nozzles for injecting dielectric, respectively, 5 and 6 denote upper and lower wire guides for guiding the wire electrode 1, respectively, 7 indicates a feeder for feeding electrical power to the wire electrode 1, 8 designates a machining power supply, 9 denotes a table feed controller for controlling the movement of a table supporting the workpiece 2, 10 and 11 designate X-axis and Y-axis motors for driving the table in X and Y directions, respectively, and 12 indicates a numerical controller (NC) comprising a CPU, memories, a keyboard, a CRT, etc. The NC includes at least one memory which stores preset electrical conditions (ECs), i.e., peak machining currents I Pn , pulse widths τ Pn , pulse-off periods τ rn  and capacitor capacities C n , and the upper and lower limits of relevant machining feedrates F which correspond to various plate thicknesses. The memory stores data as generally shown in FIG. 2, where, for example, if the plate thickness is in the range of 0 to t 0 , for a feedrate between FO and FO&#39;, the optimal electrical conditions are given by EC 0 . 
     FIG. 3 illustrates the changing of the electrical conditions in response to changes in the plate thickness of the workpiece 2. Assume that the workpiece 2 of plate thickness t satisfying the condition t 3  &lt;t&lt;t 4  is to be machined using electrical condition EC 4 . When machining is to be effected using this electrical condition, i.e., with a peak machining current of I P4 , pulse width of τ P4 , width of τ r4 , and capacitor capacity of C 4 , the machining feedrate F is between F 4  and F&#39; 4 . Now assume that plate thickness t of the workpiece 2 changes to a thickness satisfying the condition t&lt;t&lt;t 2 , i.e., the plate thickness t decreases. Since the electrical condition was set to EC 4 , the actual machining feedrate F can be increased to exceed F 4 , the upper limit of the machining feedrate F of electrical condition EC 4 . Hence, a command is given to the machining power supply 8 to reduce the electrical conditions by one step, i.e., to EC 3 . 
     Since the actual plate thickness is smaller than the plate thickness of electrical condition EC 3 , the machining feedrate F can exceed F 3  and the next electrical condition EC 2  is then output. The electrical conditions are thus changed until the actual plate thickness matches the plate thickness of the electrical condition. In this way, this process automatically changes the electrical condition in accordance with the actual plate thickness, allowing more efficient machining of workpiece 2. 
     Electrical condition switching for the known wirecut electrical discharge machine designed as described above changes the electrical conditions in accordance with a change in plate thickness when the plate thickness varies as shown in FIG. 4. However, referring to FIG. 1, it will be noted that since the positions of the dielectric nozzles remain unchanged, spacings between the workpiece and dielectric nozzles change in accordance with the change in plate thickness. This, in turn, causes the pressures of the dielectric injected into the machining gap to increase when the spacings between the workpiece and dielectric nozzles are small and to decrease when the spacings are large. In addition, when the dielectric pressures are low, sludge and other deleterious materials produced by the electrical discharge occurring in the machining gap cannot be fully removed. It will be apparent that without proper sludge removal, the focused electrical discharge will break the wire electrode unless the electrical condition is changed. 
     SUMMARY OF THE INVENTION 
     It is, accordingly, an object of the present invention to overcome the disadvantages in the prior art by providing a process of automatically changing machining conditions in accordance with the machined plate thickness and dielectric pressure. 
     Another object of the present invention is to provide a method for operating a wirecut electrical discharge machine wherein the optimum machining condition is selected based on estimated machined plate thickness and detected dielectric pressures. In particular, the estimated machined plate thickness advantageously is calculated based on the actual machining feedrate and an area machining feedrate corresponding to a present electrical condition parameter. 
     The present invention is intended to detect dielectric pressure changes corresponding to changes in spacing between a workpiece and dielectric nozzles by means of a plurality of pressure sensors, to detect a change in machined plate thickness by detecting the machining feedrate for the workpiece and to automatically select an optimum machining condition from a plurality of prestored machining conditions based on the dielectric pressure and the detected machining feedrate. 
     These and other objects, features and advantages are provided by a method of operating a wirecut electrical discharge machine for cutting a workpiece by means of electrical discharge generated in a machining gap wherein the workpiece is opposed to a wire electrode, the method comprising the steps Of presetting a plurality of machining conditions in accordance with dielectric pressure and machined plate thickness combinations, storing the preset machining conditions in a memory, and automatically setting an optimum machining condition based on detected dielectric pressures and a machined plate thickness calculated in a numerical controller. 
     According to a preferred embodiment of the present invention, the wirecut electrical discharge machine comprises a wire electrode opposing a workpiece, a plurality of nozzles disposed adjacent to the wire electrode for supplying dielectric to a machining gap formed in the workpiece by operation of the wire electrode according to one of a plurality of machining conditions, wherein each of the nozzles comprises a dielectric pressure detector, a control circuit for controlling a plurality of electrical condition parameters associated with wire electrode machining, and a device for selecting one of the machining conditions based on signals produced by the dielectric pressure detectors and an area machining feedrate corresponding to at least one of the electrical condition parameters. In the present invention, the electrical condition parameters comprise peak current, pulse width, pulse-off period and capacitor capacity. 
     According to one aspect of the present invention, each of the machining conditions represents a combination of at least one of the electrical condition parameters and upper and lower commanded machining feedrates. The wirecut electrical discharge machine also includes an actual machining feedrate determining device which, in conjunction with the selecting device, allows the machine to estimate plate thickness based on the actual machining feedrate and the area machining feedrate, wherein the selected one of the machining conditions is selected based on the estimated plate thickness and the signals generated by the pressure detectors. 
     These and other objects, features and advantages of the invention are disclosed in or apparent from the following description of preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments are described with reference to the drawings, in which like elements are denoted throughout by like or similar numbers, and in which: 
     FIG. 1 is a schematic diagram of a wirecut electrical discharge machine known in the art; 
     FIG. 2 illustrates the memory contents of the machine of FIG. 1; 
     FIG. 3 indicates relationships between machined plate thickness and electrical conditions; 
     FIG. 4 shows the profile of a workpiece having thickness variations which is used to explain the operation of the wirecut electrical discharge machine according to the present invention; 
     FIG. 5 is a schematic diagram of a wirecut electrical discharge machine according to an embodiment of the present invention; 
     FIG. 6 is a flowchart illustrating a sequence of operation of the wirecut electrical discharge machine of FIG. 5; 
     FIG. 7 is a table illustrating electrical condition parameters comprising combinations of electrical parameters and area machining feedrates; 
     FIG. 8 is a table illustrating machining conditions comprising combinations of electrical condition parameters and upper and lower limits of machining feedrates; and 
     FIG. 9 is a table illustrating machining condition matrices comprising combinations of machined plate thicknesses and dielectric pressures used in the flowchart of FIG. 6. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will now be described with reference to the drawings. In FIG. 5 the numerals 13 and 14 indicate pressure sensors provided in dielectric nozzles or piping (not shown) for detecting pressures of the dielectric in upper and lower nozzles 3 and 4, respectively. The other parts illustrated in FIG. 5 are identical to those shown in FIG. 1 and described above. 
     FIG. 6 is a flowchart illustrating an operating sequence of one embodiment of the present invention. FIG. 7 illustrates the relationships between electrical parameters, generally denoted E, and defining, for example, peak current I P , pulse width τ P , pulse-off period τ r  and capacitor capacity C combinations corresponding to area machining feedrates, generally denoted S. There may be several hundred E combinations. It should be noted that these relationships are already defined and stored in the memory of NC 12. As shown in FIG. 8, the relationships between the electrical parameter sets E and the upper and lower limits of the machining feedrates F are defined as machining conditions, generally denoted K, which are also stored in the memory of NC 12. 
     It will be apparent that the optimum machining conditions are dependent not only on the thickness of the workpiece 2 but on the pressures of the dielectric ejected from nozzles 3 and 4. For example, given a workpiece 2 of constant thickness t, when the pressure in nozzles 3 and 4 decreases, a condition exists where the sludge produced by the machining may not be properly removed. On the other hand, if the pressures in nozzles 3 and 4 increase, the increase may be indicative of an increase in workpiece 2 plate thickness. As shown in FIG. 9, the optimum machining condition advantageously is set for a particular workpiece thickness t and a predetermined dielectric pressure range P. It will be appreciated that since the thickness t is actually a range of values, corresponding upper and lower pressure ranges, P ul  to P u2  and P d1  to P d2 , respectively, are advantageously established for each thickness range t 1  to t 2 . As shown in FIG. 9, the machined plate thicknesses and dielectric pressures are listed as matrices together with the corresponding machining conditions, and stored in the memory of NC 12. Actual maximum values of P are in the vicinity of 25 kg/mm 2 . 
     The machining operation according to the present invention for a workpiece 2 having a thickness t profile as shown in FIG. 4 will now be described while referring to the flowchart of FIG. 6. During step S1, a section 0-A defining an approach zone of workpiece 2 is machined starting with the initial machining conditions consisting of the electrical condition parameter set, machining feedrate F, etc., corresponding to K 100 . In section 0-A the table is fed at the machining feedrate between F 100  to F&#39; 100  corresponding to the area machining feedrate S 100 , as shown in FIG. 7. When machining of section 0-A is finished, electrical discharge machining is initiated in section A-B. 
     Assume that the plate thickness t i  in section A-B is t 2  &lt;t i  &lt;t 3 . Since the average voltage is controlled to be constant after the start of machining, the machining feedrate Fc decreases. During step S2, the machining feedrate Fc is detected and is compared with the upper limit F 100  and lower limit F&#39; 100  of the reference machining feedrate set in section 0-A during steps S3 and S4. During step S7 the machined plate thickness t i  is calculated by dividing the area machining feedrate S 100  retrieved from memory during step S6, which feedrate was found from the electrical condition parameter E 100  set while machining section 0-A, by the machining feedrate Fc, which was detected during parallel operating step S2, according to the formula: 
     
         t.sub.i =S.sub.100 /Fc (&gt;S.sub.100 /F&#39;.sub.100) 
    
     If the result of the calculation of step S7 satisfies the condition t&lt;t i  &lt;t 2 , the corresponding machining condition k nm , for example, machining condition K 101 , is selected from the machining condition matrices shown in FIG. 9 during step S9 based on the combination of the upper dielectric pressure P u  (here, between P u1  and P u2 ) and the lower dielectric pressure P d  (here, between P u1  and P u2 ) detected separately by the dielectric pressure sensors during step S8. According to the selection of, e.g, K 101 , corresponding parameters E 101  and S 101  are set as indicated in FIGS. 7 and 8. However, the machining feedrate corresponding to the machining condition K 101  is the machining feedrate range of F 101  to F&#39; 101  and, thus, if Fc &gt;(F 101  to F&#39; 101 ), the machined plate thickness is calculated again: 
     
         t.sub.i =S.sub.101 /Fc (&gt;S.sub.101 /F&#39;.sub.101) 
    
     This sequence of operation is repeated until the machined plate thickness t i  satisfies the condition t 2  &lt;t i  &lt;t 3 . When the actual machined plate thickness approximately matches the calculated machined plate thickness t i , i.e., when the machining feedrate commanded by the machining conditions approximately matches the detected machining feedrate, machining using the corresponding machining condition, e.g., machining condition K 102  here, is effected in section A-B. In other words, once the actual machining feedrate is approximately equal to the commanded machining feedrate (i.e., is within the predetermined range), the machining is performed according to operational steps S3, S4 and S10, which are performed repeatedly until the actual machining feedrate no longer matches the commanded machining feedrate range. 
     It will be noted that in section A-B of FIG. 4, the machined plate thickness is constant. It will also be appreciated from examination of FIG. 9 that if the dielectric pressure P u  or P d  changes for some reason, the machining condition K 102  shifts to another machining condition K n02 . It will be apparent that for section B-C, where the machined plate thickness increases along with the progress of machining, the machining conditions are continuously changed during machining as described above. 
     In section D-E, since the machined plate thickness is reduced as machining progresses, the machining feedrate increases and exceeds the upper limit Fnm of the reference machining feedrate set for section C-D in step S3. During Step S5, the machined plate thickness is set to the minimum plate thickness of the machining condition K 100 , the machined plate thickness calculation of step S7 is repeated, and the machining condition is shifted to another machining condition K nm  until the calculated machined plate thickness matches the actual machined plate thickness. When the actual machined plate thickness approximately matches the calculated machined plate thickness, the machining condition K nm  is selected in accordance with the combination of the machined plate thickness and dielectric pressure separately detected. 
     It will be apparent that the present invention, as described above, allows changes in dielectric pressures, which occur due to changes in spacings between a workpiece and dielectric nozzles, to be detected by dielectric pressure sensors and the machining condition to be automatically determined in response to both dielectric pressure and machined plate thickness. Therefore, when the dielectric pressures are low, the present invention does not provide high electrical discharge energy, thereby protecting the wire electrode from breakage. The present invention advantageously responds automatically to any unexpected machined plate thickness change. 
     Other modifications and variations to the invention will be apparent to those skilled in the art from the foregoing disclosure and teachings. Thus, while only certain embodiments of the invention have been specifically described herein, it will be apparent that numerous modifications may be made thereto without departing from the spirit and scope of the invention.