POLARITY REVERSAL PULSING METHOD AND APPARATUS FOR EDM

Abstract: An EDM method and apparatus in which a succession of machining voltage pulses of regular polarity is applied from a first pulse supply between a tool electrode and a workpiece to produce erosive discharges across the EDM gap in the presence of a water flushing medium. A second pulse supply is also provided to apply one or more voltage pulses of reverse polari­ ty during a time interval between successive machining voltage pulses of regular polarity across the EDM gap. A sensing circuit is provided to respond to a gap open-circuit condition and to provide an output signal representative thereof. The output signal is used to act on the second pulse supply so as to selectively trigger such one or more reverse-polarity pulses, or to temporarily modify the parameters of the reverse-polarity pulse or pulses, upon occurrence of the gap open-circuit condition.

Description: The present invention relates to electrical discharge machining (EDM) utilizing a water machining fluid and, more particularly, to a new and improved method of and apparatus and power supply circuit arrangement for performing such EDM operations. The use of water or a water-based machining fluid has become increasingly important in the EDM art. Such fluids are generally inexpensive and offer relatively high removal rate at some cost of electrode wear. In addition, they are free from the danger of a fire unlike conventional hydrocarbon EDM media such as transformer oil and kerosene. Thus, the practical use of water as the EDM medium first occurred and is now common in traveling-wire (TW) EDM machines where the gap is placed directly in the air and the electrode wear is less significant. The water is commonly deionized water which may incorporate one or more organic components to substantially enhance discharge stability and also to reduce the electrode wear. The capability of wear reduction has now proved the use of such a water fluid to be also advantageous in sinking or generic electrode EDM machines where the tool electrode is a formed or simple electrode to machine a workpiece into a desired three-dimensional shape. One significant problem arises, however, in the use of a water fluid in EDM processes and is the problem of corrosion or electrochemical attack of the machined surface. This is particularly noticeable when the workpiece is composed of a cemented carbide material such as WC-Co. The cobalt bider has a relatively high electrochemical solubility and thus tends to preferentially dislodge during an EDM operation. As a result, pits or localized irregular formations tend to develop on the machined surface and damage the quality thereof. Such phenome­ na are generally experienced when the workpiece is a metallic carbide, boride or alloy of high hardness in which a metal of relatively high electrochemical solubility such as Fe, Ni, Cr, Cu, Co, Zn, Al, Cd, Mn, Sn or Sb is a binder or alloy compo­ nent. It is known that this problem can be alleviated by providing a power supply circuit arrangement in which each of machining voltage pulses of regular polarity with the workpiece poled positive and the tool electrode poled negative to cause erosive gap discharges is followed by an auxiliary pulse of the reverse polarity effected across the gap to make the tool electrode positive and the workpiece negative. While such known arrangements have proven to generally limit the corrosion phenomena, however, it has been found that not only does the alternate polarity reversal reduce the power efficiency, the reverse polarity pulses applied in such a mode tend to adversely affect the regular polarity machining pulses and to unstabilize the resulting erosive discharges. Accordingly, the present invention seeks to provide a new and improved electrical discharge machining method whereby the problem of electrochemical actions on a machined surface in an EDM process utilizing a water machining fluid is overcome efficiently and effectively while maintaining the stability of erosive discharges and machining performance at maximum, and also to provide a circuit arrangement for carrying out the method described. The present invention is based upon the discovery that it is generally the mean gap voltage of the machining pulses which measures the magnitude of electrochemical action in a given time period causing the corrosion and localized irregular formations on the EDM surfaces in contact with the water fluid but that the mean gap voltage itself does not necessarily dictate the the EDM efficiency. However, a gap open-circuit condition can be picked up as a significant measure of both of a lack of the EDM action and a development of the adverse electrochemical actions. The gap open-circuit condition significantly contributes to the mean gap voltage and hence the degree of the electrochemical action, and also provides a proper indication of a lack or drop of the EDM action and efficiency. In accordance with the present invention there is provided, in a first aspect thereof, a method of machining a conductive workpiece by means of electroerosive discharges effected across a machining gap between a tool electrode and the workpiece in the presence of a water machining fluid wherein a succession of voltage pulses of regular polarity is applied across the gap to make the workpiece poled positive and the tool electrode poled negative to produce such discharges and at least one pulse of the reverse polarity having prede­ termined pulse time, amplitude and number parameters is in­ terposed during a time interval between such succeeding regu­ lar-polarity pulses, the method comprising the steps of: a) sensing occurrence of a gap open-circuit condition resulting from the application of at least one such regular-polarity pulses; and b) in response to said condition, temporarily increasing at least one of said parameters of said re­ verse-polarity pulses subsequent to occurrence of said condi­ tion. According to another general feature of this aspect of the present invention there is provided a method of machining a conductive workpiece by means of electrical discharges effected across a machining gap between a tool electrode and the work­ piece in the presence of a water machining fluid wherein a succession of voltage pulses of regular polarity is applied across the gap to make the workpiece poled positive and the tool electrode poled negative to produce such discharges and at least one pulse of the reverse polarity having predetermined pulse time, amplitude and number parameters is interposed during a time interval between such succeeding regular-polarity pulses, the method comprising the steps of: a) sensing occurrence of a gap open circuit condition resulting from the application of at least one such regular-polarity pulses; and b) in response to said condition, selectively impressing at least one such reverse-polarity pulse across said machining gap. The invention also provides, in a second aspect thereof, an apparatus for machining a conductive workpiece by means of electroerosive discharges effected across a machining gap between a tool electrode and the workpiece in the presence of a water machining fluid, power supply means having a first pulse supply for applying a succession of voltage pulses of a regular polarity across the gap with the workpiece poled positive and the tool electrode poled negative to produce such discharges and a second pulse supply for interposing at least one pulse of the reverse polarity during a time interval between such succeeding regular-polarity pulses, said means further comprising: setting means operatively associated with said second pulse supply and having pulse time, amplitude and number parameters of said reverse-polarity pulses adjustable; sensing means connected to said gap for sensing occurrence of a gap open-circuit condition resulting from application of at least one said regular-polarity pulse and producing an output signal representative of said condition; and a control circuit coupled to said setting means for temporarily increasing at least one of said parameters of said reverse-polarity pulses in response to said output signal. According to another general feature of this aspect of the present invention there is provided an apparatus for machining a conductive workpiece by means of electroerosive discharges effected across a machining gap between a tool electrode and the workpiece in the presence of a water machin­ ing fluid, power supply means having a first pulse supply for applying a succession of voltage pulses of regular polarity across the gap with the workpiece poled positive and the tool electrode poled negative to produce such discharges and a second pulse supply for interposing at least one pulse of the reverse-polarity during a time interval between such succeeding regular-polarity pulses, said means further comprising; setting means operatively associated with said second pulse supply and having pulse time, amplitude and number parameters adjustable; sensing means connected to said gap for sensing occurrence of a gap open-circuit condition resulting from application of at least one said regular-polarity pulse and producing an output signal representative of said condi­ tion; and a control circuit coupled to said second pulse supply for selectively impressing at least one said reverse-polarity pulse across said gap in response to said output signal. These and other features of the present invention as well as advantages thereof will become more readily apparent from the following description of certain embodiments thereof then taken with reference to the accompanying drawing. The sole FIGURE is a circuit diagram illustrating power supply means according to the present invention. Referring now to FIG. 1, a power supply circuit arrangement shown incorporates a tool electrode E and a conduc­ tive workpiece W . It will be appreciated that the electrode may be of any conventional EDM electrode form and can ben a wire, tape or like traveling electrode, or a solid formed, or simple or generic electrode. As shown, the circuit arrangement includes a first (main) circuit 1 comprises a first voltage source 1 which is connected in series with a current limiting resistor 3, a power switch 4 (shown by a transistor), the tool electrode E and the workpiece W and a second (auxiliary) circuit 5 comprising a second voltage source 6 which is in series with a current limiting resistor 7, a switch 8 (shown by a transistor), the tool electrode E and the workpiece W , the two series circuits 1 and 5 being connected parallel to each other across an EDM gap G defined by the tool electrode E and the workpiece W . Of course, a single power source may be used to provide the two DC supplies 2 and 6. The power switch 4 in the main circuit turned on and off alternately by a pulsing circuit 9 to apply a succession of voltage pulses of regular polarity across the gap G with the workpiece W poled positive and the tool electrode E poled negative to produce intermittent electrical discharges between them. Such a succession of pulses may when originated in the pulsing circuit be in the form of a series of periodically interrupted trains of pulses so that groups of erosive dis­ charges or composite erosive discharges are generated in succession and with internals across the EDM gap G . The DC source 2 has an adjustable voltage output to enable adjustment of the peak level of the machining voltage pulses. The resis­ tor 3 is also adjustable to enable presetting of the peak current level of electrical discharges resulting from these pulses. The pulsing circuit includes a setting unit (not shown) having the on-time and off-time of the machining voltage pulses or the resulting discharges selectively adjustable. When a series of periodically or interrupted trains of control pulses is employed, the number of such pulses in each train and the time duration of each interruption are also made adjustable in the setting unit. The switch 8 in the auxiliary circuit 5 is turned on and off by a pulsing circuit 10 to apply reverse-polarity voltage pulses across the gap G to make the tool electrode E poled positive and the workpiece W poled negative. A timing line 11 connecting an output of the pulsing circuit 9 to an input of the pulsing circuit 10 is provided so that each reverse-polarity pulse if or when generated occurs during the time interval between successive machining voltage pulses and thus may not overlap with any of these pulses. The timing line 11 may include a delay network 12 so that the leading edge of the reverse-polarity, auxiliary pulse is spaced from the trailing edge of the preceding regular-polarity, machining pulse. The auxiliary DC source 6 has its output voltage adjustable so that the peak voltage of the reverse-polarity pulses may be variably set. The resistor 7 is also variable to enable setting of the peak current level of the reverse-polari­ ty pulses. Shown at 13 is setting means for variably establishing parameters of the reverse-polarity pulses, which include the duration and number, the voltage level and the peak current thereof. Thus, the setting means 13 can be divided into three units. A first unit 13 a is associated with the pulsing circuit to permit setting and change of the duration of each re­ verse-polarity pulse which is, and the number of such pulses which are, to follow each or a given regular-polarity pulse. A second unit 13 b is associated with the auxiliary DC source 6 to permit setting and change of the voltage level of the re­ verse-polarity pulses. A third unit 13 c is associated with the variable resistor 7 to make the current level of the reverse-polarity pulses adjustable. The first, second and third setting units 13 a , 13 b and 13 c are associated with the pulsing circuit 10, the adjustable DC source 6 and the variable resistor 7, respectively, to permit setting and change of the duration and the number, the voltage level and the peak current of each reverse-polarity pulse which is, or such pulses which are, to follow each or a given preceding regular-polarity pulse. A sensing circuit 14 is operably coupled to the EDM gap to provide an output signal representing a gap open-circuit condition. There are two basic forms in embodying the principles of the present invention. In the first form, the output signal of the sensing circuit 14 is furnished to a control circuit 15 which provides a control signal that selectively actuates and triggers the pulsing circuit 10 to provide one or more prede­ termined number of reverse-polarity pulses in response to the gap-open circuit condition. In this form, no reverse-polarity pulse will not follow a regular-polarity, machining pulse which is free from the gap open-circuit condition. In the second form, the output signal of the sensing circuit 14 is applied via the control circuit 15 to the setting units 13 a and/or 13 c to increase the time duration and/or number, the voltage level and/or the peak current of one or more reverse-polarity pulses in response to occurrence of the gap open-circuit condition. In a normal condition free from the gap open-circuit condition, such one or more controlled parameters of the reverse polarity pulses are held at minimum. The term "gap open circuit condition" is used herein to refer to a condition in which an applied machining voltage pulse fails to effect an electrical discharge substantially at all across the machining gap and also include a condition in which only a small proportion of regular-polarity voltage pulses applied in an unit time results in electrical discharges across the gap. Such a condition, which is characterized by a relatively high mean gap voltage, provides an adequate indication of both a lack of the EDM action and development of the adverse electrochemical action. The gap open-circuit condition can be measured for each single applied machining voltage pulse, or alternatively over a number of such pulses applied per a unit time period. A gap open circuit pulse is a pulse which fails to produce an erosive discharge and can be ascertained to be so if the voltage fails to drop to a discharge voltage level. This can be achieved, for example, by providing a monitoring time period in each applied voltage pulse, e.g. at the end portion thereof and monitoring the voltage in such a period with respect to a certain threshold level above the discharge voltage level. If the voltage remains above the threshold level, the pulse is determined to be a gap open-circuit pulse. Monitoring can also be carried out for a series of applied voltage pulses to determine occurrence of a gap open-circuit condition. In this case, each of such pulses is monitored as to whether it is a gap open-circuit pulse. A counter may be provided in the sensing circuit 14 to count such pulses and to provide an output signal when the number of these pulses per the total number of the applied pulses for a given monitoring time period exceeds a preset level. It is also possible that the sensing circuit 14 measure the average gap voltage of the regular-polarity voltage pulses or integrate these voltage pulses in a given monitoring period to provide an output signal representing a gap open circuit condition when the measure average voltage exceeds a preset threshold level.

Claims: 1. A method of machining a conductive workpiece by means of electroerosive discharges effected across a machining gap between a tool electrode and the workpiece in the presence of a water machining fluid wherein a succession of voltage pulses of regular polarity is applied across the gap to make the workpiece poled positive and the tool electrode poled negative to produce such discharges and at least one pulse of the reverse polarity having predetermined pulse time, amplitude and number parameters is interposed during a time interval between such succeeding regular-polarity pulses, the method comprising the steps of: a) sensing occurrence of a gap open-circuit condition resulting from the application of at least one such regu­ lar-polarity pulses; and b) in response to said condition, temporarily increas­ ing at least one of said parameters of said reverse-polarity pulses subsequent to occurrence of said condition. 2. A method of machining a conductive workpiece by means of electrical discharges effected across a machining gap between a tool electrode and the workpiece in the presence of a water machining fluid wherein a succession of voltage pulses of regular polarity is applied across the gap to make the workpiece poled positive and the tool electrode poled negative to produce such discharges and at least one pulse of the reverse polarity having predetermined pulse time, amplitude and number parameters is interposed during a time interval between such succeeding regular, polarity pulses, the method comprising the steps of: a) sensing occurrence of a gap open circuit condition resulting from the application of at least one such regu­ lar-polarity pulses; and b) in response to said condition, selectively impress­ ing at least one such reverse-polarity pulse across said machining gap. 3. In an apparatus for machining a conductive workpiece by means of electroerosive discharges effected across a machin­ ing gap between a tool electrode and the workpiece in the presence of a water machining fluid, power supply means having a first pulse supply for applying a succession of voltage pulses of regular polarity across the gap with the workpiece poled positive and the tool electrode poled negative to produce such discharges and a second pulse supply for interposing at least one pulse of the reverse polarity during a time interval between such succeeding regular-polarity pulses, said means further comprising: setting means operatively associated with said second pulse supply and having a pulse time, amplitude and number parameters of said reverse-polarity pulses adjustable; sensing means connected to said gap for sensing occurrence of a gap open-circuit condition resulting from application of at least one said regular-polarity pulse and producing an output signal representative of said condition; and a control circuit coupled to said setting means for temporarily at least one of said parameters of said re­ verse-polarity pulses in response to said output signal. 4. In an apparatus for machining a conductive workpiece by means of electroerosive discharges effected across a machin­ ing gap between a tool electrode and the workpiece in the presence of a water machining fluid, power supply means having a first pulse supply for applying a succession of voltage pulses of regular polarity across the gap with the workpiece poled positive and the tool electrode poled negative to produce such discharges and a second pulse supply for interposing at least one pulse of the reverse-polarity during a time interval between such succeeding regular-polarity pulses, said means further comprising; setting means operatively associated with said second pulse supply and having pulse time, amplitude and number parameters adjustable; sensing means connected to said gap for sensing occurrence of a gap open-circuit condition resulting from application of at least one said regular-polarity pulse and producing an output signal representative of said condition; and a control circuit coupled to said second pulse supply for selectively impressing at least one said reverse-polarity pulse across said gap in response to said output signal.

Inventors: INOUE, KIYOSHI
Applicants: INOUE-JAPAX RESEARCH INCORPORATED
Application year: 1985
Main classification: B23H1
IPC codes: B23H1