CUT-OFF PROTECTION SYSTEM FOR ELECTRICAL DISCHARGE MACHINING APPARATUS

Abstract: During electrical discharge machining the electrical discharge machining apparatus must be protected from the excessive currents which occur when the gap between the electrode (22) and the workpiece (24) is short circuited. This protection is provided by a programmable system for controlling machining current magnitude and reducing it in a step function as gap voltage (10) falls indicating gap short circuit condition. A set of off-time numbers are computed and stored in an off-time counter. These numbers are recomputed each time a new on or off time is entered in the pulse generator (18). The original off time is the normal cutting parameter. As the gap voltage drops, indicating problems, the off-time starts doubling or increasing in a like manner until it has reached many times its normal duration.

Description: CUT-OFF PROTECTION SYSTEM FOR ELECTRICAL DISCHARGE MACHINING APPARATUS Technical Field The field to which the present invention relates is that generally known as electrical discharge machining, sometimes hereinafter referred to as EDM, in which material is removed from an electrically conductive workpiece by the action of electrical gap discharges occurring between a tool electrode and a workpiece. A dielectric coolant fluid is circulated and recirculated through the gap, usually under pressure, throughout the machining operation. An electrode or a workpiece servo feed system is used to provide relative movement and thus maintain an optimum gap spacing between the electrode and the workpiece as the workpiece material is being removed. Background Art It is important to the process of EDM that the machining power pulses provided at the gap are of closely and precisely controllable on-off time and frequency to insure repeatability of results and to provide appropriate cutting action for the type of operation being carried on. Various types of pulse generators which have this capa¬ bility have this capability have been developed and are in commercial use for EDM. One commonly used type of EDM power supply includes as a principal part of its machining power pulse generator an astable multivibrator in which on-off time and frequency are controlled and preset by a ganged capacitor and resistor arrangement. One example of this type of pulse generator and an associated protection system is shown and described in Kurt H. Sennowitz, U.S. Patent No. 3,64*9,802, issued on March 14, 1972 for "Protective System for Electrical Discharge Machining Power Supply Circuit", which patent is of common ownership herewith, A further arrangement for digital type EDM pulse generator is shown and described in Oliver A. Bell, Jr., U.S. Patent 3,809,847, issued on May 7, 1974, for "Method and Apparatus for Electrical Discharge Machining". A still further type of digital multivibrator is shown and described in Oliver A. Bell, Jr., U.S. Patent 4,071,729, issued on January 31, 1978, for "Adaptive Control System and Method for Electrical Discharge Machining". This patent shows an on and off time generator which receives inputs from a programmable computer and from this general arrangement provides machining power pulses to the machining gap. All the above noted patents are of common ownership herewith. The present invention is particularly designed for use with a digital type pulse generator that is controlled by a programmable computer or similar input device. Ref¬ erence is made to our co-pending U.S. Patent Application #068,328 filed on August 21, 1979 for "Programmable Pulse Generator for Electrical Discharge Machining Apparatus". Disclosure of the Invention Our invention provides a cut-off protection system for EDM which operates in a step function to lengthen off time " responsive to drop in gap voltage level indicating gap short circuit conditions. A set of cut-off times is ■ computed each time the off-time is reset by the operator or entered by the operator or entered by computer control. Each successive off time in the set is doubled or multiplied by four or some other integer to reduce machin¬ ing current. 1^1- R Brief Description of Drawings The present invention, its operation, features and advantages, and the advancements which it affords will be understood from the following specification together with the drawings in which like numerals are used to refer to similar or like parts where they occur, and wherein: Fig. 1 is a block diagrammatic showing off the system with which our invention is used. Fig. 2 is a graph showing the operation of the cut-off system and clarifying the relationship between gap voltage and pulse off-time; and Fig. 3 is a block diagrammatic drawing showing one programmable computer or microprocessor used with the present invention. Best Mode of Carrying Out the Invention Figure 1 shows the basic parts of a programmable computer 25 and an associated electrical discharge machin¬ ing apparatus. Gap voltage is sensed by a voltage sensing network 10. The gap voltage is passed through an analog to digital co'nverter 12 and then to the CPU 14 of the programmable computer. The memory 16 of the microprocessor is also shown. One example of a microprocessor suitable for use in connection with this invention is the micro¬ processor currently manufactured and sold by the Mostek Corporation, 1215 West Crosby Road, Carrollton, Texas 75Q06 and known as the Model MK 3880. Also shown in Fig. 1 is a digital pulse generator 18 of the counter type as shown and described in our co- pending application No. 068,328 referred to above. One counter is set to represent machining pulse on-time. The other counter is set to represent machining pulse off- time. The EDM power output module is indicated by the numeral 20 and includes the main DC source and an output power switch turned on and off by the pulse generator 18 to provide machining power pulses to the machining gap. The machining gap is defined between a tool electrode 22 and a workpiece 24. The gap voltage signal is shown at the left hand side of the drawing. It is used in the pulse generator 18 to control the off time in a manner 5 that will be further explained in the section "Descrip¬ tion of Operation", hereinafter. It will further be clarified by inclusion of a program for the micro¬ processor. Figure 3 is a Block diagram of the major elements in 10 the CPU 14. These include the CPU Control 30, internal register 32, data bus control 34, arithmetic logic unit 36, CPU registers 38, and address control 4Q. The several interfaces and data busses are also shown in the drawing. While the described embodiment of the invention includes 15 elements of a microprocessor, the invention is not limited to this type of computer. A variety of different program¬ mable computers can be used. Description of Operation The machining pulse off-time and on-time are entered 20 from memory 16 or from an operator keyboard. The protectio system operates by modifying the off-time only responsive to drop in gap voltage. The on-time is not altered. This is because the off time is not a factor in overcut or finish. The on time controls peak current and does affect 25. these factors. The gap voltage signal is passed through A/D converter 12 to get an 8 bit representation of gap voltage. We derive a table of references by multiplying the off-time by two and loading it in the cut-off table. This redoubling is repeated until we have a multiplication 30 by 64 times the original off-time. The invention is _not limited to the use of a particular ratio. The factor could be four. It is important that the cut-off be controlled in gradual steps rather than abruptly. We have found that this type of cut-off improves the v- ' C- " stability of cut. The table of numbers starts with the lowest number. The table is loaded into CPU Register 38. The first number is subtracted from the 8 bit digital representa- tion of gap voltage level. If the number resulting from the subtraction is positive, i.e. the reference voltage is smaller than the gap voltage, then we go on to the next step. The original off time is used as the normal cutting parameter. ' As the gap voltage decreases, the off- time starts doubling until it reaches 64 times its normal amount. Thus, if the off-time were 10 microseconds, we we could load up to 640 microseconds of cut-off. This provides a very broad latitude of protection. When cutting is normal and relatively stable, the cutoff values used most often are the times 2 and times 4. If the gap is in a dead short condition, the off time would go to 64 times. As shown in Figure 1, the control signal from the CPU 14 and from the 8 bit data bus is passed to the off-time control counter of the pulse generator 18 to change the off time. Reference is made to the above noted co-pending application for a detailed explanation of the manner in which that operation is handled. Reference is also made to the explanatory routines in the following program: CUTOFF 01017 0201 OEOO CUTOFF LD C,0 SET UP FOR OFFTIME 01018 0203 D9 EXX SWAP REG 01019 0204 3E00 LD A,0 CLEAR 01020 0206 328120> LD (ELOP),A SAVE IT 01021 0209 7A LD A,D ALT. DE = ARC AND 01022 020A D9 EXX SWAP BACK C1023 020B CB3F SRL A DIVIDE BY 2 01024 020D 47 LD B,A SAVE IT 01025 020E 3A7B20 > LD A,(CUREF) GET BIAS 01026 0211 80 ADD A,B OFFSET THE ARC NU 01027 0212 47 LD B,A SAVE THE ORIGINAL 01028 0213 210211 > LD HL,NORMV SET UP FOR NORMAL 01029 MTEST HIPON,DRIVE WAS HIPOL ON 0216 3A6B20;> + LD A, (DRIVE) 0219 CB47 + BIT HIPON,A 01030 021B.7.8 LD A,B GET ARC VOLTAGE B 01031 021C CA2202> JP Z CU1 Z FLAG SET IF HIP 01032 020F 21FC10> LD HL,HIV SET FOR HIGH VOLT 01033 0222 96 CU1 SUB (HL) IS ARC < FIRST VA 01034 0223 23 INC HL BUMP POINTR TO AR 01035 0224 F23502 > JP P f CU2 IF NO OVERFLOW AR 01036 0227 3E80 LD A,080H . RUBBOUT 01037 0229 328120 > , LD (ELOP),A STUFF IT 01038 022C 2AOF20> ' LD HL,(CUTAB1) GET OFFTIME TIMES 01039 022F 225520 > LD (CUTOF),HL SAVE IT 01040 0232 C39202 > JP LDCT LOAD NEW OFFTIME 01041 0235 78 CU2 LD A,B RESTORE VALUE 01042 0236 96 SUB (HL) IS ARC < SECOND V 01043 0237 23 INC HL BUMP COMPARE POIN 01044 0238 F24902> JP P.CU3 NO 01045 023B 3E80 LD A,080H RUBBOUT 01046 023D 328120 > LD (EL0P),A ' SAVE IT 01047 0240 2A1120 > LD HL,(CUTAB2) OFFTIME * 32 It will be seen that so long as the gap voltage output from A/D converter 12 is greater than the first reference number in the table, we will continue to leave the regular off-time loaded in the off-time counter portion of the pulse generator or multi¬ vibrator 18. The first time that we get an A to D conversion with a number that τ s below the reference number, the pointer in the cut-off table will start searching down the table to see how low it is. As this gap voltage goes lower, the off-time that will be loaded in the off time counter of the pulse generator will be higher. Thus, the magnitude of gap current will be reduced in a gradual stepped rather than a linear manner, It will therefore be seen, that we have provided a new and improved cut-off protection system for electrical discharge machining. 'BUREA

Claims: CLAIMS 1. A system for gap short circuit protection for electrical d scha ge machining in an apparatus having a pulse generator in- - eluding an on time and an off ' time control means, comprising: means for sensing gap voltage and providing a representation thereof; means for predetermining an off time value and providing a ' digital representation thereof; means for successively multiplying the last mentioned repre¬ sentation by a constant integer and storing a plurality of such products as precomputed off-time in an ordered table^ and means operably connected to said sensing means and responsive to drop in gap voltage for successively substituting said pre¬ computed off-time from said table in place of said predetermineci off-time value. 2. The combina.tion as set forth in claim 1 wherein said predetermined off time value is entered from a computer memory. 3. The * combination as set forth in claim 1 wherein said predetermined off time value is entered from an operator keyboard. 4. The combination as set forth in claim 1 wherein said constant integer is two. 5. The combination .as se "forth in claim 1 wherein said constant integer is four. 6. The combination as set forth in claim 1 wherein said on and off time control means comprises an on time and an off time counter. 7. The combination as set forth in claim 6 wherein said off time value is entered in said off time counter in digital form. , as 8. The combination/set forth in claim 6 wherein said means for sensing gap voltage is coupled to an analog to digital con¬ verter for placing it in digital form. 9. A gap short circuit protection system for electrical discharge machining apparatus having an off time and an on time control means, comprising: means for sensing gap voltage and providing an output signal responsive to its level; means for presetting the off time of said control means; means for predetermining a set of progressively larger values for off-time; and means for successively substituting said values of said set responsive to drop in magnitude of said output signal. 10. A gap short circuit protection system for electrical discharge machining apparatus ' aving a precisely settable machining power pulse off time and on time, comprising: means for sensing gap voltage and providing an output signal corresponding to its level ; means for presetting the off time and on time; means for predeterminimg and storing a set of progressively larger values for off-time; and means for successively substituting said values of said set in order of their increase for said preset off time responsive to drop in magnitude of said output signal. 11. The combination as set forth in claim 10 wherein said off time and said set are entered in a register of a programmable computer for operation. IJTJREAZ OMPI 12. The combination as set forth in claim 10 wherein said means for presetting the off time and on time comprises a counter 13. The combination as set forth in claim 10 wherein said • progressively larger values of said set are computed by succes- ' sive common factor multiplication. 14. The combination as set forth in claim 13 wherein said common factor comprises the integer two. BUkl_/: } PA-PI

Inventors: BELL O, CHANCE D, GILLELAND R
Applicants: COLT IND OPERATING CORP
Application year: 1980
Main classification: B23P1
IPC codes: B23H1