ELECTROCHEMICAL MACHINING (ECM)

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ELECTROCHEMICAL MACHINING (ECM)


INTRODUCTION:-
ELECTRO CHEMICAL MACHINING (ECM) IS A METHOD OF REMOVING METAL BY AN ELECTROCHEMICAL PROCESS. IT IS NORMALLY USED FOR MASS PRODUCTION AND IS USED FOR WORKING EXTREMELY HARD MATERIALS OR MATERIALS THAT ARE DIFFICULT TO MACHINE USING CONVENTIONAL METHODS. ITS USE IS LIMITED TO ELECTRICALLY CONDUCTIVE MATERIALS; HOWEVER, THIS INCLUDES ALL METALS. ECM CAN CUT SMALL OR ODD-SHAPED ANGLES, INTRICATE CONTOURS OR CAVITIES IN EXTREMELY HARD STEEL AND EXOTIC METALS SUCH AS TITANIUM, HASTELLOY, KOVAR, INCONEL AND CARBIDE.

ECM IS OFTEN CHARACTERIZED AS "REVERSE ELECTROPLATING," AND IS SIMILAR IN CONCEPT TO ELECTRICAL DISCHARGE MACHINING IN THAT A HIGH CURRENT IS PASSED BETWEEN AN ELECTRODE AND THE PART, THROUGH AN ELECTROLYTE MATERIAL REMOVAL PROCESS HAVING A NEGATIVELY CHARGED ELECTRODE (CATHODE), A CONDUCTIVE FLUID (ELECTROLYTE), AND A CONDUCTIVE WORK PIECE (ANODE); HOWEVER, IN ECM THERE IS NO TOOL WEAR. THE ECM CUTTING TOOL IS GUIDED ALONG THE DESIRED PATH VERY CLOSE TO THE WORK BUT IT DOES NOT TOUCH THE PIECE. UNLIKE EDM HOWEVER, NO SPARKS ARE CREATED. VERY HIGH METAL REMOVAL RATES ARE POSSIBLE WITH ECM, ALONG WITH NO THERMAL OR MECHANICAL STRESSES BEING TRANSFERRED TO THE PART, AND MIRROR SURFACE FINISHES ARE POSSIBLE. ELECTROCHEMICAL MACHINING (ECM) ALSO USES ELECTRICAL ENERGY TO REMOVE MATERIAL. AN ELECTROLYTIC CELL IS CREATED IN AN ELECTROLYTE MEDIUM, WITH THE TOOL AS THE CATHODE AND THE WORKPIECE AS THE ANODE. A HIGH-AMPERAGE, LOW-VOLTAGE CURRENT IS USED TO DISSOLVE THE METAL AND TO REMOVE IT FROM THE WORKPIECE, WHICH MUST BE ELECTRICALLY CONDUCTIVE. ECM IS ESSENTIALLY A DEPLATING PROCESS THAT UTILIZES THE PRINCIPLES OF ELECTROLYSIS. THE ECM TOOL IS POSITIONED VERY CLOSE TO THE WORKPIECE AND A LOW VOLTAGE, HIGH AMPERAGE DC CURRENT IS PASSED BETWEEN THE TWO VIA AN ELECTROLYTE. MATERIAL IS REMOVED FROM THE WORKPIECE AND THE FLOWING ELECTROLYTE SOLUTION WASHES THE IONS AWAY. THESE IONS FORM METAL HYDROXIDES WHICH ARE REMOVED FROM THE ELECTROLYTE SOLUTION BY CENTRIFUGAL SEPARATION. BOTH THE ELECTROLYTE AND THE METAL SLUDGE ARE THEN RECYCLED.
UNLIKE TRADITIONAL CUTTING METHODS, WORK PIECE HARDNESS IS NOT A FACTOR, MAKING ECM SUITABLE FOR DIFFICULT-TO-MACHINE MATERIALS. TAKES SUCH FORMS AS ELECTROCHEMICAL GRINDING, ELECTROCHEMICAL HONING AND ELECTROCHEMICAL TURNING. THE PROCESS SCHEMATIC IS SUCH THAT A CATHODE (TOOL) IS ADVANCED INTO AN ANODE (WORK PIECE). THE PRESSURIZED ELECTROLYTE IS INJECTED AT A SET TEMPERATURE TO THE AREA BEING CUT. THE FEED RATE IS THE EXACT SAME RATE AS THE RATE OF LIQUEFACTION OF THE MATERIAL. THE AREA IN-BETWEEN THE TOOL AND THE WORK PIECE VARY WITHIN .003 IN. AND .030 IN.
AS FAR BACK AS 1929, AN EXPERIMENTAL ECM PROCESS WAS DEVELOPED BY W.GUSSEF, ALTHOUGH IT TOOK UNTIL 1959 FOR A COMMERCIAL PROCESS TO BE ESTABLISHED BY THE ANOCUT ENGINEERING COMPANY. MUCH RESEARCH WAS DONE IN THE 1960S AND 1970S, PARTICULARLY IN THE GAS TURBINE INDUSTRY. THE RISE OF EDM IN THE SAME PERIOD LARGELY STOPPED RESEARCH INTO ECM IN THE WEST, ALTHOUGH WORK CONTINUED BEHIND THE IRON CURTAIN. THE ORIGINAL PROBLEMS OF POOR DIMENSIONAL ACCURACY AND ENVIRONMENTALLY POLLUTING WASTE HAVE LARGELY BEEN OVERCOME, ALTHOUGH THE PROCESS REMAINS A NICHE TECHNIQUE. THE CUTTING HEADS ON ALL PHILIPS 'PHILISHAVE' SHAVERS ARE MADE USING ECM.THE ECM PROCESS IS MOST WIDELY USED TO PRODUCE COMPLICATED SHAPES WITH GOOD SURFACE FINISH IN DIFFICULT TO MACHINE MATERIALS, SUCH AS TURBINE BLADES. IT IS ALSO WIDELY AND EFFECTIVELY USED AS A DEBURRING PROCESS.
EQUIPMENTS:-
ECM MACHINES COME IN BOTH VERTICAL AND HORIZONTAL TYPES. DEPENDING ON THE WORK REQUIREMENTS THESE MACHINES ARE BUILT IN MANY DIFFERENT SIZES AS WELL. THE VERTICAL MACHINE IS COMPRISED OF A BASE, COLUMN, TABLE, AND SPINDLE HEAD. THE SPINDLE HEAD HAS A SERVO-MECHANISM THAT AUTOMATICALLY ADVANCES THE TOOL AND CONTROLS THE GAP BETWEEN THE CATHODE (TOOL) AND THE WORKPIECE.

TOOLS & GEOMETRY PRODUCED:-

BOTH EXTERNAL AND INTERNAL GEOMETRIES CAN BE MACHINED WITH AN ELECTROCHEMICAL MACHINE. COPPER IS OFTEN USED AS THE ELECTRODE MATERIAL. BRASS, GRAPHITE, AND COPPER-TUNGSTEN ARE ALSO OFTEN USED BECAUSE OF THE ABILITY TO BE EASILY MACHINED, THEY ARE CONDUCTIVE MATERIALS, AND THEY WILL NOT CORRODE.

PRINCIPAL & METAL REMOVAL ANAYLSIS:-

THE PHYSICS - AN ELECTRODE AND WORK PIECE (CONDUCTOR) ARE PLACED IN AN ELECTROLYTE, AND A POTENTIAL VOLTAGE IS APPLIED. ON THE ANODE (+VE) SIDE THE METAL MOLECULES IONIZE (LOSE ELECTRONS) BREAK FREE OF THE WORK PIECE, AND TRAVEL THROUGH THE ELECTROLYTE TO THE ELECTRODE (A CATHODE; HAS A -VE CHARGE; A SURPLUS OF ELECTRONS).

NOTE: IN EDM AN ARC WAS USED TO HEAT METAL, HERE THE METAL DISSOLVES CHEMICALLY.


· VARIATION IN THE CURRENT DENSITY WILL RESULT IN WORK TAKING THE ELECTRODES SHAPE.

· THE ELECTRODE IS FED WITH A CONSTANT VELOCITY, AND THE ELECTROLYTE IS FED THROUGH THE TOOL. THE TOOL IS DESIGNED TO ELIMINATE DEPOSITION OF THE IONIZED METAL ON THE ELECTRODE.


· SUPPLY V = 8 TO 20V, I = >1000A.

· ELECTRODE GAP IS TYPICALLY 0.1 TO 0.2 MM.

· MRR IS ABOUT 1600MM3/MIN. PER 1000A, OR 3KWHR FOR 16000 MM3 (NOT VERY EFFICIENT, 30 TIMES MORE THAN STANDARD MACHINING TECHNIQUES).

· MRR IS INDEPENDENT OF MATERIAL HARDNESS.

· GOOD FOR LOW MACHINABILITY, OR COMPLICATED SHAPES.

· VERY LITTLE TOOL WEAR,

· FORCES ARE LARGE WITH THIS METHOD BECAUSE OF FLUID PUMPING FORCES.
· FARADAY'S LAWS STATE THAT,

· THE BASIC PRINCIPLE IS SHOWN BELOW


· THE CHEMICAL REACTION BETWEEN AN ELECTRODE AND THE ELECTROLYTE LEADS TO ELECTRONS BEING ADDED, OR REMOVED FROM THE ELECTRODE METAL. THIS ADDITION/SUBTRACTION LEADS TO A VOLTAGE POTENTIAL.


· TO MAKE A BATTERY.


· TO DO ELECTROLYSIS.




· E.G.



· ACTUAL RATES MAY VARY FROM THEORY AS OTHER FACTORS COME INTO EFFECT.



· THE TABLE BELOW SHOWS VARIOUS MATERIALS AND RELEVANT PROPERTIES,



E.G.

· WHILE THE CURRENT REQUIRED IS RELATED TO THE METAL REMOVED, THE VOLTAGE REQUIRED DEPENDS UPON,
1. - ELECTRODE POTENTIAL.
2. - THE CURRENT FLOW IN AND ABOUT THE ELECTRODES WILL DISTURB THE NORMAL DISTRIBUTION OF VOLTAGE. EXTRA POTENTIAL IS REQUIRED TO OVERCOME THE EFFECTS.
3. - ION COLLECT NEAR ELECTRODES AND IMPEDE ION TRANSFER FROM THE ELECTRODE TO THE ELECTROLYTE, ALSO ADDING A POTENTIAL.
4. - SOME SOLID FILM FORMS ON THE SURFACE OF THE ELECTRODE, ALSO INCREASING RESISTANCE.
5. - ELECTROLYTE RESISTANCE,


· THE FEED OF THE ELECTRODES HAS THE FOLLOWING EFFECTS



· THE ECM PROCESS WILL ERODE MATERIAL IN A RADIAL DIRECTION, SO CARE MUST BE MADE IN TOOLING DESIGN.


· AS CURRENT FLOWS THROUGH THE ELECTROLYTE, IT IS HEATED, AND CONDUCTIVITY DECREASES.

· SURFACE FINISH IS AFFECTED BY,
1. - SELECTIVE DISSOLUTION
2. - SPORADIC BREAKDOWN OF THE ANODIC FILM
3. - FLOW SEPARATION AND FORMATION OF EDDIES
4. - EVOLUTION OF HYDROGEN
· SUMMARY OF ECM CHARACTERISTICS,
1. - MECHANICS OF MATERIAL REMOVAL - ELECTROLYSIS
2. - MEDIUM - CONDUCTING ELECTROLYTE
3. - TOOL MATERIAL - CU, BRASS, STEEL
4. - MATERIAL/TOOL WEAR - INFINITE
5. - GAP 50 TO 300 ΜM
6. - MAXIMUM MRR 15*103 MM3/MIN
7. - SPECIFIC POWER CONSUMPTION 7W/MM3/MIN
8. - CRITICAL PARAMETERS - VOLTAGE, CURRENT, FEED RATE, ELECTROLYTE, ELECTROLYTE CONDUCTIVITY
9. - MATERIALS APPLICATION - ALL CONDUCTING METALS AND ALLOYS
10. - SHAPE APPLICATION - BLIND COMPLEX CAVITIES, CURVED SURFACES, THROUGH CUTTING, LARGE THROUGH CAVITIES.
11. - LIMITATIONS - HIGH SPECIFIC ENERGY CONSUMPTION (ABOUT 150 TIMES THAT REQUIRED FOR CONVENTIONAL PROCESSES), NOT APPLICABLE WITH ELECTRICALLY NON-CONDUCTING MATERIALS AND JOBS WITH VERY SMALL DIMENSIONS, EXPENSIVE MACHINES.
12. - SURFACE FINISHES DOWN TO 25 ΜIN.

· THIS TECHNIQUE HAS BEEN COMBINED WITH A METAL GRINDING WHEEL IN A PROCESS CALLED ELECTROLYTIC DRILLING. THE WHEEL DOES NOT TOUCH THE WORK, AND GIVES A SURFACE FINISH FROM 8 TO 20 ΜIN.


ADVANTAGES:-
  1. THE COMPONENTS ARE NOT SUBJECT TO EITHER THERMAL OR MECHANICAL STRESS.
  2. THERE IS NO TOOL WEAR DURING ELECTROCHEMICAL MACHINING.
  3. NON-RIGID AND OPEN WORK PIECES CAN BE MACHINED EASILY AS THERE IS NO CONTACT BETWEEN THE TOOL AND WORK PIECE.
  4. COMPLEX GEOMETRICAL SHAPES CAN BE MACHINED REPEATEDLY AND ACCURATELY
  5. ELECTROCHEMICAL MACHINING IS A TIME SAVING PROCESS WHEN COMPARED WITH CONVENTIONAL MACHINING
  6. DURING DRILLING, DEEP HOLES CAN BE MADE OR SEVERAL HOLES AT ONCE.
  7. ECM DEBURRING CAN DEBUR DIFFICULT TO ACCESS AREAS OF PARTS.
  8. FRAGILE PARTS WHICH CANNOT TAKE MORE LOADS AND ALSO BRITTLE MATERIAL WHICH TEND TO DEVELOP CRACKS DURING MACHINING CAN BE MACHINED EASILY THROUGH ELECTROCHEMICAL MACHINING
  9. SURFACE FINISHES OF 25 µ IN. CAN BE ACHIEVED DURING ELECTROCHEMICAL MACHINING
ELECTROCHEMICAL GRINDING (ECG):-
ELECTROCHEMICAL GRINDING COMBINES ELECTRICAL AND CHEMICAL ENERGY FOR METAL REMOVAL WITH AN EDM FINISH. IT IS A NON-ABRASIVE PROCESS AND, THEREFORE, PRODUCES PRECISE CUTS THAT ARE FREE OF HEAT, STRESS, BURRS AND MECHANICAL DISTORTIONS. IT IS A VARIATION ON ELECTROCHEMICAL MACHINING THAT USES A CONDUCTIVE, ROTATING ABRASIVE WHEEL. THE CHEMICAL SOLUTION IS FORCED BETWEEN THE WHEEL AND THE WORK PIECE. THE SHAPE OF THE WHEEL DETERMINES THE FINAL SHAPE.

ELECTROCHEMICAL DEBURRING IS ANOTHER VARIATION ON ELECTROCHEMICAL MACHINING DESIGNED TO REMOVE BURRS AND IMPART SMALL RADII TO CORNERS. THE PROCESS NORMALLY USES A SPECIALLY SHAPED ELECTRODE TO CAREFULLY CONTROL THE PROCESS TO A SPECIFIC AREA. THE PROCESS WILL WORK ON MATERIAL REGARDLESS OF HARDNESS. IN THE DEBURRING PROCESS, THE ECM USES TECHNIQUES AS DESCRIBED ABOVE TO REMOVE PIECES OF METAL THAT ARE LEFT OVER FROM THE MACHINING PROCESS, AND TO DULL OUT SHARP EDGES. THIS PROCESS IS VERY FAST AND MUCH MORE CONVENIENT THAN THE CONVENTIONAL METHOD OF DEBURRING BY HAND OR NONTRADITIONAL MACHINING PROCESSES. IT WILL TEND TO LEAVE BETTER SURFACE FINISHING, AND NO METAL DEFORMATION WILL OCCUR BECAUSE THE TOOL PIECE DOESN’T ACTUALLY TOUCH THE METAL.

CHEMICAL MACHINING:-
CHEMICAL MACHINING AIDES IN THE MANUFACTURE OF LIGHT GAUGE METAL PARTS. THE PHOTO ETCHING PROCESS (ALSO CALLED CHEMICAL ETCHING AND CHEMICAL MILLING) ALLOWS PEOPLE TO PRODUCE INTRICATE METAL COMPONENTS WITH CLOSE TOLERANCES THAT ARE IMPOSSIBLE TO DUPLICATE BY OTHER PRODUCTION METHODS. IT IS ALSO KNOWN AS CHEMICAL MILLING.

APPLICATIONS -
CHEMICAL MACHINING IS UTILIZED IN THE MANUFACTURING OF ENCODERS, MASKS, FILTERS, LEAD FRAMES, FLAT SPRINGS, STRAIN GAUGES, LAMINATIONS, CHIP CARRIERS, STEP COVERS, FUEL CELL PLATES, HEAT SINKS, SHUTTER BLADES, ELECTRON GRIDS, FLUIDIC CIRCUIT PLATES, RETICLES, DRIVE BANDS, HAPTICS, AND SHIMS.

ELECTRICAL DISCHARGE MACHINING (EDM)


INTRODUCTION:-
ELECTRICAL DISCHARGE MACHINING (OR EDM) IS A MACHINING METHOD PRIMARILY USED FOR HARD METALS OR THOSE THAT WOULD BE IMPOSSIBLE TO MACHINE WITH TRADITIONAL TECHNIQUES. ONE CRITICAL LIMITATION, HOWEVER, IS THAT EDM ONLY WORKS WITH MATERIALS THAT ARE ELECTRICALLY CONDUCTIVE. EDM CAN CUT SMALL OR ODD-SHAPED ANGLES, INTRICATE CONTOURS OR CAVITIES IN PRE-HARDENED STEEL WITHOUT THE NEED FOR HEAT TREATMENT TO SOFTEN AND RE-HARDEN THEM AS WELL AS EXOTIC METALS SUCH AS TITANIUM, HASTELLOY, KOVAR, AND INCONEL.

SOMETIMES REFERRED TO AS SPARK MACHINING OR SPARK ERODING, EDM IS A NON-TRADITIONAL METHOD OF REMOVING MATERIAL BY A SERIES OF RAPIDLY RECURRING ELECTRIC ARCING DISCHARGES BETWEEN AN ELECTRODE (THE CUTTING TOOL) AND THE WORKPIECE, IN THE PRESENCE OF AN ENERGETIC ELECTRIC FIELD. THE EDM CUTTING TOOL IS GUIDED ALONG THE DESIRED PATH VERY CLOSE TO THE WORK BUT IT DOES NOT TOUCH THE PIECE. CONSECUTIVE SPARKS PRODUCE A SERIES OF MICRO-CRATERS ON THE WORK PIECE AND REMOVE MATERIAL ALONG THE CUTTING PATH BY MELTING AND VAPORIZATION. THE PARTICLES ARE WASHED AWAY BY THE CONTINUOUSLY FLUSHING DIELECTRIC FLUID. IT IS ALSO IMPORTANT TO NOTE THAT A SIMILAR MICRO-CRATER IS FORMED ON THE SURFACE OF THE ELECTRODE, THE DEBRIS FROM WHICH MUST ALSO BE FLUSHED AWAY. THESE MICRO-CRATERS RESULT IN THE GRADUAL EROSION OF THE ELECTRODE, MANY TIMES NECESSITATING SEVERAL DIFFERENT ELECTRODES OF VARYING TOLERANCES TO BE USED, OR, IN THE CASE OF WIRE EDM MACHINING, CONSTANT REPLACEMENT OF THE WIRE BY FEEDING FROM A SPOOL.ACCORDING TO THE MANUFACTURING REFERENCE GUIDE BY ROBET H. TODD, DELL K. ALLEN, AND LEO ALTING, EDM GRINDING IS A MASS-REDUCING PROCESS THAT USES A ROTATING CONDUCTIVE WHEEL TO REMOVE ELECTRICALLY CONDUCTIVE MATERIAL BY MEANS OF CONTROLLED, REPETITIVE SPARK DISCHARGES. A DIELECTRIC FLUID IS USED TO FLUSH AWAY THE CHIPS, REGULATE THE DISCHARGE, AND COOL THE WHEEL AND THE WORKPIECE.

ADVANTAGES

SOME OF THE ADVANTAGES OF EDM INCLUDE MACHINING OF:
  • COMPLEX SHAPES THAT WOULD OTHERWISE BE DIFFICULT TO PRODUCE WITH CONVENTIONAL CUTTING TOOLS
  • EXTREMELY HARD MATERIAL TO VERY CLOSE TOLERANCES
  • VERY SMALL WORK PIECES WHERE CONVENTIONAL CUTTING TOOLS MAY DAMAGE THE PART FROM EXCESS CUTTING TOOL PRESSURE.

DISADVANTAGES

SOME OF THE DISADVANTAGES OF EDM INCLUDE:
  • THE INABILITY TO MACHINE NON-CONDUCTIVE MATERIALS.
  • THE SLOW RATE OF MATERIAL REMOVAL.
  • THE ADDITIONAL TIME AND COST USED FOR CREATING ELECTRODES FOR RAM / SINKER EDM.
  • REPRODUCING SHARP CORNERS ON THE WORKPIECE IS DIFFICULT DUE TO ELECTRODE WEAR.

APPLICATION:-

THE EDM PROCESS IS MOST WIDELY USED BY THE MOULD-MAKING TOOL AND DIE INDUSTRIES, BUT IS BECOMING A COMMON METHOD OF MAKING PROTOTYPE AND PRODUCTION PARTS, ESPECIALLY IN THE AEROSPACE, AUTOMOBILE AND ELECTRONICS INDUSTRIES IN WHICH PRODUCTION QUANTITIES ARE RELATIVELY LOW. IN SINKER EDM, A GRAPHITE OR PURE COPPER ELECTRODE IS MACHINED INTO THE DESIRED (NEGATIVE) SHAPE AND FED INTO THE WORK PIECE ON THE END OF A VERTICAL RAM.

COINAGE DIE MAKING
FOR THE CREATION OF DIES FOR PRODUCING JEWELRY AND BADGES BY THE COINAGE (STAMPING) PROCESS, THE POSITIVE MASTER MAY BE MADE FROM STERLING SILVER, SINCE (WITH APPROPRIATE MACHINE SETTINGS) THE MASTER IS NOT SIGNIFICANTLY ERODED AND IS USED ONLY ONCE. THE RESULTANT NEGATIVE DIE IS THEN HARDENED AND USED IN A DROP HAMMER TO PRODUCE STAMPED FLATS FROM CUTOUT SHEET BLANKS OF BRONZE, SILVER, OR LOW PROOF GOLD ALLOY. FOR BADGES THESE FLATS MAY BE FURTHER SHAPED TO A CURVED SURFACE BY ANOTHER DIE. THIS TYPE OF EDM IS USUALLY PERFORMED SUBMERGED IN AN OIL-BASED DIELECTRIC. THE FINISHED OBJECT MAY BE FURTHER REFINED BY HARD (GLASS) OR SOFT (PAINT) ENAMELING AND/OR ELECTROPLATED WITH PURE GOLD OR NICKEL. SOFTER MATERIALS SUCH AS SILVER MAY BE HAND ENGRAVED AS A REFINEMENT.



SMALL HOLE DRILLING EDM
SMALL HOLE DRILLING EDM IS USED TO MAKE A THROUGH HOLE IN A WORKPIECE IN THROUGH WHICH TO THREAD THE WIRE IN WIRE-CUT EDM MACHINING. THE SMALL HOLE DRILLING HEAD IS MOUNTED ON WIRE-CUT MACHINE AND ALLOWS LARGE HARDENED PLATES TO HAVE FINISHED PARTS ERODED FROM THEM AS NEEDED AND WITHOUT PRE-DRILLING. THERE ARE ALSO STAND-ALONE SMALL HOLE DRILLING EDM MACHINES WITH AN XY AXIS ALSO KNOWN AS A SUPER DRILL OR HOLE POPPER THAT CAN MACHINE BLIND OR THROUGH HOLES. EDM DRILLS BORE HOLES WITH A LONG BRASS OR COPPER TUBE ELECTRODE THAT ROTATES IN A CHUCK WITH A CONSTANT FLOW OF DISTILLED OR DEIONIZED WATER FLOWING THROUGH THE ELECTRODE AS A FLUSHING AGENT AND DIELECTRIC. THE ELECTRODE TUBES OPERATE LIKE THE WIRE IN WIRE-CUT EDM MACHINES, HAVING A SPARK GAP AND WEAR RATE. SOME SMALL-HOLE DRILLING EDMS ARE ABLE TO DRILL THROUGH 100 MM OF SOFT OR THROUGH HARDENED STEEL IN LESS THAN 10 SECONDS, AVERAGING 50% TO 80% WEAR RATE. HOLES OF 0.3 MM TO 6.1 MM CAN BE ACHIEVED IN THIS DRILLING OPERATION. BRASS ELECTRODES ARE EASIER TO MACHINE BUT ARE NOT RECOMMENDED FOR WIRE-CUT OPERATIONS DUE TO ERODED BRASS PARTICLES CAUSING "BRASS ON BRASS" WIRE BREAKAGE, THEREFORE COPPER IS RECOMMENDED.

CAVITY TYPE EDM (SINKER EDM)

CAVITY TYPE EDM IS ALSO KNOWN AS SINKER EDM. SINKER EDM CONSISTS OF AN ELECTRODE AND WORK PIECE THAT IS SUBMERGED IN AN INSULATING LIQUID SUCH AS OIL. THE ELECTRODE AND WORK PIECE ARE CONNECTED TO A SUITABLE POWER SUPPLY. THE POWER SUPPLY GENERATES AN ELECTRICAL POTENTIAL BETWEEN THE TWO PARTS. AS THE ELECTRODE APPROACHES THE WORK PIECE, DIELECTRIC BREAKDOWN OCCURS IN THE FLUID, AND A SMALL SPARK JUMPS. THE RESULTING HEAT AND CAVITATIONS VAPORIZE THE BASE MATERIAL, AND TO SOME EXTENT, THE ELECTRODE. THESE SPARKS STRIKE ONE AT A TIME IN HUGE NUMBERS AT SEEMINGLY RANDOM LOCATIONS ACROSS THE ELECTRODE. AS THE BASE METAL IS ERODED, AND THE SPARK GAP SUBSEQUENTLY INCREASED, THE ELECTRODE IS LOWERED AUTOMATICALLY BY THE MACHINE SO THAT THE PROCESS CAN CONTINUE UNINTERRUPTED. SEVERAL HUNDRED THOUSAND SPARKS OCCUR PER SECOND IN THIS PROCESS, WITH THE ACTUAL DUTY CYCLE BEING CAREFULLY CONTROLLED BY THE SETUP PARAMETERS. THE TYPICAL PART GEOMETRY IS TO CUT SMALL OR ODD SHAPED ANGLES. VERTICAL, ORBITAL, VECTORIAL, DIRECTIONAL, HELICAL, CONICAL, ROTATIONAL, SPIN AND INDEXING MACHINING CYCLES ARE ALSO USED.


IN WIRE ELECTRICAL DISCHARGE MACHINING (WEDM), OR WIRE-CUT EDM, A THIN SINGLE-STRAND METAL WIRE, USUALLY BRASS, IS FED THROUGH THE WORKPIECE, TYPICALLY OCCURRING SUBMERGED IN A TANK OF DIELECTRIC FLUID. THIS PROCESS IS USED TO CUT PLATES AS THICK AS 300MM AND TO MAKE PUNCHES, TOOLS,AND DIES FROM HARD METALS THAT ARE TOO DIFFICULT TO MACHINE WITH OTHER METHODS. THE WIRE, WHICH IS CONSTANTLY FED FROM A SPOOL, IS HELD BETWEEN UPPER AND LOWER DIAMOND GUIDES. THE GUIDES MOVE IN THE XY PLANE, USUALLY BEING CNC CONTROLLED AND ON ALMOST ALL MODERN MACHINES THE UPPER GUIDE CAN ALSO MOVE INDEPENDENTLY IN THE ZUV AXIS, GIVING RISE TO THE ABILITY TO CUT TAPERED AND TRANSITIONING SHAPES (CIRCLE ON THE BOTTOM SQUARE AT THE TOP FOR EXAMPLE) AND CAN CONTROL AXIS MOVEMENTS IN XYUVIJKL–. THIS GIVES THE WIRE-CUT EDM THE ABILITY TO BE PROGRAMMED TO CUT VERY INTRICATE AND DELICATE SHAPES. THE WIRE IS CONTROLLED BY UPPER AND LOWER DIAMOND GUIDES THAT ARE USUALLY ACCURATE TO 0.004 MM, AND CAN HAVE A CUTTING PATH OR KERF AS SMALL AS 0.12 MM USING Ø 0.1 MM WIRE, THOUGH THE AVERAGE CUTTING KERF THAT ACHIEVES THE BEST ECONOMIC COST AND MACHINING TIME IS 0.335 MM USING Ø 0.25 BRASS WIRE. THE REASON THAT THE CUTTING WIDTH IS GREATER THAN THE WIDTH OF THE WIRE IS BECAUSE SPARKING ALSO OCCURS FROM THE SIDES OF THE WIRE TO THE WORK PIECE, CAUSING EROSION. THIS "OVERCUT" IS NECESSARY, PREDICTABLE, AND EASILY COMPENSATED FOR. SPOOLS OF WIRE ARE TYPICALLY VERY LONG. FOR EXAMPLE, AN 8 KG SPOOL OF 0.25 MM WIRE IS JUST OVER 19 KILOMETERS LONG. TODAY, THE SMALLEST WIRE DIAMETER IS 20 MICROMETRES AND THE GEOMETRY PRECISION IS NOT FAR FROM +/- 1 MICROMETRE. THE WIRE-CUT PROCESS USES WATER AS ITS DIELECTRIC WITH THE WATER'S RESISTIVITY AND OTHER ELECTRICAL PROPERTIES CAREFULLY CONTROLLED BY FILTERS AND DE-IONIZER UNITS. THE WATER ALSO SERVES THE VERY CRITICAL PURPOSE OF FLUSHING THE CUT DEBRIS AWAY FROM THE CUTTING ZONE. FLUSHING IS AN IMPORTANT DETERMINING FACTOR IN THE MAXIMUM FEED RATE AVAILABLE IN A GIVEN MATERIAL THICKNESS, AND POOR FLUSHING SITUATIONS NECESSITATE THE REDUCTION OF THE FEED RATE.

ALONG WITH TIGHTER TOLERANCES MULTIAXIS EDM WIRE-CUTTING MACHINING CENTER HAVE MANY ADDED FEATURES SUCH AS: MULTIHEADS FOR CUTTING TWO PARTS AT THE SAME TIME, CONTROLS FOR PREVENTING WIRE BREAKAGE, AUTOMATIC SELF-THREADING FEATURES IN CASE OF WIRE BREAKAGE, AND PROGRAMMABLE MACHINING STRATEGIES TO OPTIMIZE THE OPERATION.
WIRE-CUTTING EDM IS COMMONLY USED WHEN LOW RESIDUAL STRESSES ARE DESIRED. WIRE EDM HAS NO ADDED RESIDUAL STRESS BECAUSE IT HAS NO CUTTING FORCES. THERE IS LITTLE CHANGE IN THE MECHANICAL PROPERTIES OF A MATERIAL IN WIRE-CUTTING EDM DUE TO THESE LOW RESIDUAL STRESSES.

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