Cut-off protection system for electrical discharge machining apparatus

A programmable system for controlling machining current magnitude and reducing it in a step function as gap voltage 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. 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.

BACKGROUND OF THE INVENTION 
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. 
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 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. Pat. No. 
3,649,802, issued on Mar. 14, 1972 for "Protective System for Electrical 
Discharge Machining Power Supply Circuit", which patent is of common 
ownership herewith. 
A further arrangement for a digital type EDM pulse generator is shown and 
described in Oliver A. Bell, Jr., U.S. Pat. No. 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. Pat. No. 4,071,729, issued on Jan. 31, 1978, for 
"Adaptive Control System and Method for Electrical Discharge Machining." 
This patent shows an on and off time generator which received 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. Reference is made to our co-pending U.S. Pat. No. 4,320,279 
issued on Mar. 16, 1982 for "Programmable Pulse Generator for Electrical 
Discharge Machining Apparatus". 
SUMMARY 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 
computer control. Each successive off-time in the set is doubled or 
multiplied by four or some other integer to reduce machining current.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows the basic parts of a programmable computer and an associated 
electrical discharge machining apparatus. Gap voltage is sensed by a 
voltage sensing network 10. The gap voltage is passed through an analog to 
digital converter 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 
microprocessor currently manufactured and sold by the Mostek Corporation, 
1215 West Crosby Road, Carrollton, Tex. 75006 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 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 that 
will be further explained in the section "Description of Operation," 
hereinafter. It will further be clarified by inclusion of a program for 
the microprocessor. 
FIG. 3 is a block diagram of the major elements in 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 40. The 
several interfaces and data busses are also shown in the drawing. While 
the described embodiment of the invention includes elements of a 
microprocessor, the invention is not limited to this type of computer. A 
variety of different programmable computers can be used. 
DESCRIPTION OF OPERATION 
The machining pulse off-time and on-time are entered from memory 16 or from 
an operator keyboard. The protection 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 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 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 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 representation 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 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 cut-off 
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 FIG. 
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 0E00 
CUTOFF 
LD C,O SET UP FOR OFFTIME LOAD WHEN LDCT IS 
CALLED 
01018 0203 D9 EXX SWAP REG 
01019 0204 3E00 LD A,O CLEAR 
01020 0206 328120 &gt; 
LD (ELOP),A SAVE IT 
01021 0209 7A LD A,D ALT. DE = ARC AND REF 
01022 020A D9 EXX SWAP BACK 
01023 020B CB3F SRL A DIVIDE BY 2 
01024 020D 47 LD B,A SAVE IT 
01025 020E 3A7B20 &gt; 
LD A,(CUREF) GET BIAS 
01026 0211 80 ADD A,B OFFSET THE ARC NUMBER 
01027 0212 47 LD B,A SAVE THE ORIGINAL VALUE 
01028 0213 210211 &gt; 
LD HL,NORMV SET UP FOR NORMAL CUTTING VOLTAGE 
01029 MTEST HIPON,DRIVE 
WAS HIPOL ON 
0216 3A6B20&gt;+ LD A,(DRIVE) 
0219 CB47 + BIT HIPON,A 
01030 021B 78 LD A,B GET ARC VOLTAGE BACK 
01031 021C CA2202&gt; 
JP Z CU1 Z FLAG SET IF HIPOL OFF 
01032 020F 21FC10&gt; 
LD HL,HIV SET FOR HIGH VOLTAGE 
01033 0222 96 
CU1 SUB (HL) IS ARC&lt;FIRST VALUE 
01034 0223 23 INC HL BUMP POINTER TO ARC COMASON 
01035 0224 F23502&gt; 
JP P,CU2 IF NO OVERFLOW ARC&gt;10 VOLTS 
01036 0227 3E80 LD A,080H RUBBOUT 
01037 0229 328120&gt; 
LD (ELOP),A STUFF IT 
01038 022C 2A0F20&gt; 
LD HL,(CUTAB1) 
GET OFFTIME TIMES 64 
01039 022F 225520&gt; 
LD (CUTOF),HL SAVE IT 
01040 0232 C39202&gt; 
JP LDCT LOAD NEW OFFTIME 
01041 0235 78 
CU2 LD A,B RESTORE VALUE 
01042 0236 96 SUB (HL) IS ARC&lt;SECOND VALUE 
01043 0237 23 INC HL BUMP COME POINTER 
01044 0238 F24902&gt; 
JP P,CU3 NO 
01045 023B 3E80 LD A,080H RUBBOUT 
01046 023D 328120&gt; 
LD (ELOP),A SAVE IT 
01047 0240 2A1120&gt; 
LD HL,(CUTAB2) 
OFF TIME * 32 
01048 0243 225520 &gt; 
LD (CUTOF), HL 
SAVE IT 
01049 0246 C39202 &gt; 
JP LDCT LOAD IT 
01050 0249 78 
CU3 LD A,B RESTORE VALUE 
01051 024A 96 SUB (HL) IS ARC&lt;THIRD VALUE 
01052 024B 23 INC HL BUMP COME POINTER 
01053 024C F25D02&gt; 
JP P,CU4 NO 
01054 024F 3E80 LD A,080H RUBBOUT 
01055 0251 328120&gt; 
LD (ELOP), A SAVE IT 
01056 0254 2A1320&gt; 
LD HL, (CUTAB3) 
OFFTIME * 16 
01057 0257 225520&gt; 
LD (CUTOF),HL SAVE IT 
01058 025A C39202&gt; 
JP LDCT AND LOAD IT 
01059 025D 78 
CU4 LD A,B RESTORE THE VALUE 
01060 025E 96 SUB (HL) IS ARC&lt;FORTH VALUE 
01061 025F 23 INC HL BUMP COME POINTER 
01062 0260 F27102&gt; 
JP P,CU5 NO 
01063 0263 3E80 LD A,080H RUBBOUT 
01064 0265 328120&gt; 
LD (ELOP),. A SHOW IT 
01065 0268 2A1520&gt; 
LD HL, (CUTAB4) 
OFTIME * 8 
01066 026B 225520&gt; 
LD (CUTOF),HL SAVE IT 
01067 026E C39202&gt; 
JPLDCT LOAD IT 
01068 0271 78 
CU5 LD A,B RESTORE THE VALUE 
01069 0272 96 SUB (HL) IS ARC&lt;FIFTH VALUE 
01070 0273 23 INC HL BUMP COME POINTER 
01071 0274 F28002&gt; 
JP P,CU6 NO 
01072 0277 2A1720&gt; 
LD HL, (CUTAB5) 
OFFTIME * 4 
01073 027A 225520&gt; 
LD (CUTOF),HL SAVE IT 
01074 027D C39202&gt; 
JP LDCT LOAD IT 
01075 0280 78 
CU6 LD A,B RESTORE THE VALUE 
01076 0281 96 SUB (HL) IS ARC&lt;SIXTH VALUE 
01077 0282 23 INC HL BUMP COME POINTER 
01078 0283 F28C02&gt; 
JP P,CU7 NO 
01079 0286 2A1920&gt; 
LD HL,(CUTAB6) 
OFFTIME * 2 
01080 0289 225520&gt; 
LD (CUTOF),HL SAVE IT 
01081 028C 2A0320&gt; 
CU7 LD HL, (OFTIM) 
GET ORIGINAL OFF TIME 
01082 028F 225520&gt; 
LD (CUTOF),HL SAVE IT 
__________________________________________________________________________ 
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 multivibrator 18. The first time that we 
get an A to D conversion with a number that is 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 
manner rather than a linear manner. This mode of operation is shown in 
FIG. 2. 
Thus the pre-computed off-times are substituted one after the other 
seriatim in order of progressively larger values in place of the regular 
off-time until return to normal gap voltage level. 
It will therefore be seen, that we have provided a new and improved cut-off 
protection system for electrical discharge machining.