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Saturday, 3 December 2011

MACHINING PROCESS

ABRASIVE JET MACHINING
INTRODUCTION:-
IT IS A PROCESS OF MATERIAL REMOVAL THROUGH THE ACTION OF A FOCUSED STREAM OF FLUID WITH ABRASIVE PARTICLES. IT IS ESPECIALLY USED FOR MACHINING SUPER ALLOYS, CERAMICS; GLASS AND REFRACTORY MATERIAL. IT IS ONE OF THE NON-POLLUTING METHODS AVAILABLE FOR MACHINING HARD MATERIALS TO PRECISION DETAILS. IN ABRASIVE WATER JET MACHINING THE ABRASIVE PARTICLES UTILIZES WATER JETS TO ACCELERATE OR DECELERATE THEIR SPEEDS DEPENDING UPON THE TYPE OF MATERIAL TO BE MACHINED. THIS MACHINING PROCESS CARRIES AN ADDITIONAL ADVANTAGE FOR IT CANNOT BE REACTIVE WITH ANY OF THE JOB MATERIAL AND FOR IT'S VERSATILITY THAT ANY MATERIAL, HOW MUCH HARDER IT IS CAN BE MACHINED. THE BEST OF ALL NO TOOL CHANGES ARE REQUIRED, FIXTURES TO HOLD THE WORK PIECE ARE MINIMAL IN AJM.IT CAN BE USED TO VIRTUALLY MACHINE ANY PARTS WITH ANY INTRICACIES OF SHARP CORNERS OR TIGHT RADIUS AT CORNERS ETC.

THE MATERIAL WHICH ARE BEING MACHINED BY THIS PROCESS DO NOT EXPERIENCE ANY HARDENING DUE TO PROCESS BECAUSE HEAT GENERATED IS VERY LESS. ALSO, SINCE MAJOR CUTTING FORCES ARE DIRECTED IN DOWNWARD DIRECTION IT CAN BE USED TO MACHINE MATERIALS WITH VERY SMALL WALL THICKNESS. THE DEPTH OF CUT OR THICKNESS OF PART TO BE MACHINED IS A FUNCTION OF SPEED AND BEST MATERIAL MACHINING IS OBTAINED FOR THICKNESS LESS THAN 1INCHES.




METAL REMOVAL MECHANISM:-
ABRASIVE JET MACHINING UTILIZES THE PRESSURE OF FLUID STREAM TO REMOVE MATERIAL FROM THE SURFACE OF THE JOB. WHEN USING AIR AS A MEDIUM THE MIXTURE OF AIR AND ABRASIVES ARE ALLOWED TO IMPINGE ON THE WORK SURFACE AT ABOUT 200 TO 400M/S THROUGH THE NOZZLE AND WORK MATERIAL IS ERODED BY THE HIGH VELOCITY ABRASIVE PARTICLES. THE INSIDE DIAMETERS OF THE NOZZLE ARE ABOUT 0.04MM AND STANDOFF DISTANCE IS KEPT ABOUT 0.7 TO1.0MM. THE PROCESS CAN BE EASILY CONTROLLED TO VARY THE METAL REMOVAL RATE WHICH DEPENDS ON FLOW RATE AND SIZE OF ABRASIVE PARTICLES .THE CUTTING ACTION IS COOLED BECAUSE THE CARRIER GAS SERVES AS A COOLANT.

GAS USED ARE NITROGEN OR CARBON DIOXIDE OR EVEN AIR WHICH ARE SUPPLIED UNDER PRESSURE(2-8 KG/CM^2) FILTERED THROUGH REGULATOR IS PASSED TO A MIXING CHAMBER(CONTAINING ABRASIVE PARTICLES) VIBRATING AT 50C/S.FROM THE MIXING CHAMBER, THE GAS ALONG WITH THE ENTRAINED ABRASIVES PARTICLES OF SIZE 10-50 MICRO METER PASSES ON TO NOZZLE HAVING ITS TIP TUNGSTEN CARBIDE AND DIAMETER OF AROUND 0.45MM,WITH A VELOCITY OF 150 TO 300 M/S.THE AIR CONSUMPTION IS OF ORDER OF 0.6M^3/HR.THE NOZZLE TIP DISTANCE IS OF ORDER OF 0.81MM.THE ABRASIVE POWDER FEED RATE IS CONTROLLED BY THE AMPLITUDE OF VIBRATION OF MIXING CHAMBER. THE RELATIVE MOTION BETWEEN THE NOZZLE AND THE WORK PIECE IS OBTAINED BY THE PROGRAMMABLE TORCH OR BY CAMS AND PANTOGRAPHS TO CONTROL THE SIZE AND SHAPE OF THE CUT. DUST REMOVAL EQUIPMENT IS INCORPORATED TO PROTECT THE ENVIRONMENT. THE MATERIAL REMOVAL RATE, GEOMETRY OF CUT, SURFACE ROUGHNESS, AND NOZZLE WEAR RATE ARE INFLUENCED BY THE SIZE AND DISTANCE OF NOZZLE, COMPOSITION, STRENGTH, SIZE AND SHAPE OF ABRASIVE FLOW RATE; AND COMPOSITION, PRESSURE AND VELOCITY OF CARRIER GAS.
THE ABRASIVE PARTICLES SHOULD HAVE IRREGULAR SHAPE AND CONSIST OF SHORT EDGES RATHER THAN HAVING ROUNDED SHAPES. ABRASIVES GENERALLY USED ARE ALUMINUM OXIDE, SILICON CARBIDE, SODIUM BICARBONATE, DOLOMITE, GLASS BEADS; THEIR SELECTION AND THEIR GRAIN SIZE DEPENDING ON THE MACHINING OPERATION.
THE MATERIAL REMOVAL RATE IS MAINLY DEPENDENT ON THE FLOW RATE AND SIZE OF THE ABRASIVE PARTICLES .HIGH GRAIN SIZE WILL ALWAYS PRODUCE MORE METAL REMOVAL. AT A PARTICULAR PRESSURE METAL REMOVAL RATE INCREASES WITH THE ABRASIVE FLOW RATE BUT AFTER REACHINING A OPTIMUM VALUE, THE MATERIAL REMOVAL RATE DECREASES WITH INCREASE IN ABRASIVE FLOW RATE .THIS IS BECAUSE MASS FLOW RATE OF THE GAS DECREASES WITH INCREASE OF ABRASIVE FLOW RATE AND MIXING RATIO INCREASES CAUSING A DECREASE IN MATERIAL REMOVAL RATE BECAUSE OF LESS ENERGY AVAILABLE FOR EROSION. THE ABRASIVE PARTICLES ARE GENERALLY NOT USED AGAIN AND AGAIN.
THE MATERIAL REMOVAL RATE FIRST INCREASES WITH THE INCREASES OF TIP DISTANCE FROM WORK UP TO A CERTAIN LIMIT AFTER WHICH IT REMAINS UNCHANGED FOR A CERTAIN TIP DISTANCE AND THEN FALLS GRADUALLY. IN THIS PROCESS THE LIMITATIONS ARE THAT THE MATERIAL REMOVAL RATE IS LOW, STRAY CUTTING CAN'T BE AVOIDED, TAPERING EFFECT MAY BE FOUND BECAUSE OF UNAVOIDABLE FLARING OF THE ABRASIVE JETS ,ABRASIVES MAY GET EMBEDDED I THE WORK SURFACE, AND SUITABLE DUST COLLECTING SYSTEM HAS TO BE PROVIDED.

PROCESS VARIABLES:-
  1. THE MAIN ASPECT OF THIS MACHINING IS CORNERED ON MIXING TUBE LIFE AND ORIFICE LIFE THOUGH THE ORIFICE LIFE ARE FAR GREATER THAN MIXING TUBE LIFE BUT THEY HAVE A TYPICAL LIFE DEPENDING UPON MATERIAL OF WHICH THEY ARE MADE. THE MATERIALS KNOWN TO ME ARE DIAMOND, RUBY AND SAPPHIRE. THE MIXING TUBE GENERALLY GETS WORN DOWN IN LESS THAN HALF THE TIME AS REQUIRED BY ORIFICE.
  2. ALTHOUGH VERY TIGHT TOLERANCES ARE ACHIEVABLE BY THIS PROCESS BUT THEY ARE DEPENDENT ON VARIOUS OTHER PARAMETERS OF MACHINING THAT RANGE FROM FEED RATE TO MATERIAL THICKNESS TO OPERATOR EXPERIENCE.
  3. JOB MATERIAL- THE HARDER THE MATERIAL THE LESS WOULD BE THE TAPER WHEN ABRASIVE JET PASSES AT THE BOTTOM OF THE WORK PIECE, SINCE ITS DISPERSION WOULD BE LESS AS COMPARED TO SOFTER MATERIAL.
  4. FEED RATE-THE FEED RATE ALSO CONTRIBUTES TO THE TOLERANCE WHICH COULD BE ACHIEVED. A SLIGHT CHANGE IN FEED COULD BRING ABOUT CHANGES IN THE JET PROFILE AND THUS TOLERANCE THAT COULD BE ACHIEVED.
  5. NO INITIALIZATION OF HOLES IS REQUIRED FOR STARTING OF OPERATION AS THAT IS REQUIRED BY EDM.
  6. THERE IS ALMOST ZERO TOOL SETUP TIME INVOLVED AND ALMOST NEGLIBLE PROGRAMMING IS REQUIRED FOR TOOL MOTION.
  7. TOLERANCE IS ALSO DEPENDENT ON THE MATERIAL THICKNESS BECAUSE THOSE ONLY CONTROLS THE BEHAVIOR OF THE JETS AS IT EXITS OUT THE BOTTOM. THIS CAN CAUSE TAPERING AROUND CURVES.
  8. THE JET LAG BETWEEN THE POINTS WHERE IT FIRST ENTERS AND WHERE IT EXITS ALSO HAS CONSIDERABLE AFFECT ON THE TOLERANCE OF THE PART MACHINED.
  9. MATERIAL UTILIZATION IS VERY HIGH BECAUSE NO MATERIAL IS WASTED IN MACHINING OF CORNERS AND INTRICATE SHAPES. LIMITATIONS TO THE PRECISION BY AJM ARE GENERALLY CONTROLLED BY TAPER, LEAD IN AND LEAD OUTS, JET LAG AND KERFS.
ADVANTAGES: -
THE ADVANTAGES OF THIS PROCESS ARE THAT IT CAN BE USED TO CUT INTRICATE HOLE SHAPES IN HARD AND HARD AND BRITTLE MATERIALS; EVEN FRAGILE AND SENSITIVE MATERIALS CAN BE CUT WITHOUT DAMAGE ,AND THE INITIAL COST IS LOW.
DISADVANTAGES:-
ITS DISADVANTAGES ARE THAT IT REMOVES MATERIAL AT VERY LOW RATE, STRAY CUTTING CAN OCCUR RESULTING IN POOR ACCURACY, AND SOFT MATERIALS CAN'T BE MACHINED BY THIS PROCESS.



WATER JET CUTTING


INTRODUCTION:-
MOST WATER JET CUTTING THEORIES EXPLAIN WATER JET CUTTING AS A FORM OF MICRO EROSION AS DESCRIBED HERE. WATER JET CUTTING WORKS BY FORCING A LARGE VOLUME OF WATER THROUGH A SMALL ORIFICE IN THE NOZZLE. THE CONSTANT VOLUME OF WATER TRAVELING THROUGH A REDUCED CROSS SECTIONAL AREA CAUSES THE PARTICLES TO RAPIDLY ACCELERATE. THIS ACCELERATED STREAM LEAVING THE NOZZLE IMPACTS THE MATERIAL TO BE CUT. THE EXTREME PRESSURE OF THE ACCELERATED WATER PARTICLES CONTACTS A SMALL AREA OF THE WORK PIECE. IN THIS SMALL AREA THE WORK PIECE DEVELOPS SMALL CRACKS DUE TO STREAM IMPACT. THE WATER JET WASHES AWAY THE MATERIAL THAT "ERODES" FROM THE SURFACE OF THE WORK PIECE. THE CRACK CAUSED BY THE WATER JET IMPACT IS NOW EXPOSED TO THE WATER JET. THE EXTREME PRESSURE AND IMPACT OF PARTICLES IN THE FOLLOWING STREAM CAUSE THE SMALL CRACK TO PROPAGATE UNTIL THE MATERIAL IS CUT THROUGH.



TYPE OF WATER JET MACHINING:-

WATER JETS: WATER JET CUTTING USES ONLY A PRESSURIZED STREAM OF WATER TO CUT THROUGH MATERIAL. THIS TYPE OF CUTTING IS LIMITED TO MATERIAL WITH NATURALLY OCCURRING SMALL CRACKS OR SOFTER MATERIAL.
ABRASIVE WATER JETS: SLURRY: AN ABRASIVE WATER JET SLURRY SYSTEM MIXES THE ABRASIVE WITH THE WATER JET EARLY IN THE SYSTEM. THIS IS DONE BEFORE THE WATER IS PRESSURIZED. SLURRY SYSTEMS THEN ACCELERATE THE ABRASIVE PARTICLES WITH THE WATER THROUGHOUT THE SYSTEM. THIS CAUSES MORE WEAR ON THE INTERNAL PARTS THAN AN ENTRAINMENT SYSTEM. AN ABRASIVE WATER JET ENTRAINMENT SYSTEM MIXES THE ABRASIVE WITH THE WATER JET IN A MIXING CHAMBER JUST AFTER THE NOZZLE. IN MOST SYSTEMS BEING BUILT TODAY, A VENTURI EFFECT IS UTILIZED TO PULL THE ABRASIVE INTO THE WATER JET. THE ABRASIVE PARTICLES ARE ACCELERATED INTO THE STREAM AND THEN WITH THE STREAM OUT THE ORIFICE.
ABRASIVE WATER SUSPENSION JETS: AN ABRASIVE WATER SUSPENSION JET SYSTEM ACCELERATES A SUSPENSION OF ABRASIVE IN WATER THROUGH THE SYSTEM. THIS TYPE OF SYSTEM APPEARS TO BE MORE EFFICIENT THAN PREVIOUSLY DEVELOPED ABRASIVE WATER JET SYSTEMS.
METAL REMOVAL MECHANISM:-




MOST WATER JET CUTTING THEORIES EXPLAIN WATER JET CUTTING AS A FORM OF MICRO EROSION AS DESCRIBED HERE. WATER JET CUTTING WORKS BY FORCING A LARGE VOLUME OF WATER THROUGH A SMALL ORIFICE IN THE NOZZLE. THE CONSTANT VOLUME OF WATER TRAVELING THROUGH A REDUCED CROSS SECTIONAL AREA CAUSES THE PARTICLES TO RAPIDLY ACCELERATE. THIS ACCELERATED STREAM LEAVING THE NOZZLE IMPACTS THE MATERIAL TO BE CUT. THE EXTREME PRESSURE OF THE ACCELERATED WATER PARTICLES CONTACTS A SMALL AREA OF THE WORK PIECE. IN THIS SMALL AREA THE WORK PIECE DEVELOPS SMALL CRACKS DUE TO STREAM IMPACT. THE WATER JET WASHES AWAY THE MATERIAL THAT "ERODES" FROM THE SURFACE OF THE WORK PIECE. THE CRACK CAUSED BY THE WATER JET IMPACT IS NOW EXPOSED TO THE WATER JET. THE EXTREME PRESSURE AND IMPACT OF PARTICLES IN THE FOLLOWING STREAM CAUSE THE SMALL CRACK TO PROPAGATE UNTIL THE MATERIAL IS CUT THROUGH.
AFFECTS OF EQUIPMENT VARIATION:-
NOZZLE OPENING SHAPE:
  • THIS ALLOWS THE USER TO CREATE SLIGHTLY DIFFERENT SHAPED CUT.
  • IT CAN ALSO HELP ACCELERATE THE ABRASIVES IN THE MIXING TUBE.
  • FINER OPENINGS ALLOW MORE PRECISE CUTTING AND ACCELERATED WATER JETS.
SIZE AND SHAPE OF THE ORIFICE:
  • THIS ALLOWS THE USER TO BETTER CONTROL THE MIXTURE OF THE ABRASIVE AND WATER.
MULTIPLE NOZZLES:
  • CAN BETTER CONTROL HOLE SHAPE AND DEPTH OF CUT.
  • DECREASES THE PRESSURE IF MULTIPLE NOZZLES ARE PUT ON ONE PRESSURE INTENSIFIER.
  • SMOOTH ROUNDED FINISH TO GLASS (AND OTHER MATERIALS) THEREFORE THE EDGE DOES NOT HAVE TO BE RE-GROUND.
PRESSURE INTENSIFIERS:
  • THE SIZE OF THE PRESSURE INTENSIFIER VARIES APPROXIMATELY FROM 20,000PSI TO 50,000PSI.
  • THIS ALLOWS THE USER TO GET THE PRESSURE REQUIRED WITHOUT HAVING TO PAY A HIGH INITIAL COST.
  • ALLOWS VARIATION IN CUTTING. HIGHER PRESSURE FOR HARDER MATERIALS.
ADVANTAGES:-
WATER JET CUTTING HAS MANY APPLICATIONS, AND THERE ARE MANY REASONS WHY WATER JET CUTTING IS PREFERABLE OVER OTHER CUTTING METHODS. LISTED BELOW ARE SEVERAL ADVANTAGES, ALONG WITH A BRIEF EXPLANATION.
· IN WATER JET CUTTING, THERE IS NO HEAT GENERATED. THIS IS ESPECIALLY USEFUL FOR CUTTING TOOL STEEL AND OTHER METALS WHERE EXCESSIVE HEAT MAY CHANGE THE PROPERTIES OF THE MATERIAL.
· UNLIKE MACHINING OR GRINDING, WATER JET CUTTING DOES NOT PRODUCE ANY DUST OR PARTICLES THAT ARE HARMFUL IF INHALED.
· THE KERFS WIDTH IN WATER JET CUTTING IS VERY SMALL, AND VERY LITTLE MATERIAL IS WASTED.
· WATER JET CUTTING CAN BE EASILY USED TO PRODUCE PROTOTYPE PARTS VERY EFFICIENTLY. AN OPERATOR CAN PROGRAM THE DIMENSIONS OF THE PART INTO THE CONTROL STATION, AND THE WATER JET WILL CUT THE PART OUT EXACTLY AS PROGRAMMED. THIS IS MUCH FASTER AND CHEAPER THAN DRAWING DETAILED PRINTS OF A PART AND THEN HAVING A MACHINIST CUT THE PART OUT.
· WATER JET CUTTING CAN BE EASILY AUTOMATED FOR PRODUCTION USE.
· WATER JET CUTTING DOES NOT LEAVE A BURR OR A ROUGH EDGE, AND ELIMINATES OTHER MACHINING OPERATIONS SUCH AS FINISH SANDING AND GRINDING.
· WATER JETS ARE MUCH LIGHTER THAN EQUIVALENT LASER CUTTERS, AND WHEN MOUNTED ON AN AUTOMATED ROBOT. THIS REDUCES THE PROBLEMS OF ACCELERATING AND DECELERATING THE ROBOT HEAD, AS WELL AS TAKING LESS ENERGY.
DISADVANTAGES:-
WATER JET CUTTING IS A VERY USEFUL MACHINING PROCESS THAT CAN BE READILY SUBSTITUTED FOR MANY OTHER CUTTING METHODS; HOWEVER, IT HAS SOME LIMITATIONS TO WHAT IT CAN CUT. LISTED BELOW ARE THESE LIMITATIONS, AND A BRIEF DESCRIPTION OF EACH.
· ONE OF THE MAIN DISADVANTAGES OF WATER JET CUTTING IS THAT A LIMITED NUMBER OF MATERIALS CAN BE CUT ECONOMICALLY. WHILE IT IS POSSIBLE TO CUT TOOL STEELS, AND OTHER HARD MATERIALS, THE CUTTING RATE HAS TO BE GREATLY REDUCED, AND THE TIME TO CUT A PART CAN BE VERY LONG. BECAUSE OF THIS, WATER JET CUTTING CAN BE VERY COSTLY AND OUTWEIGH THE ADVANTAGES.
· ANOTHER DISADVANTAGE IS THAT VERY THICK PARTS CAN NOT BE CUT WITH WATER JET CUTTING AND STILL HOLD DIMENSIONAL ACCURACY. IF THE PART IS TOO THICK, THE JET MAY DISSIPATE SOME, AND CAUSE IT TO CUT ON A DIAGONAL, OR TO HAVE A WIDER CUT AT THE BOTTOM OF THE PART THAN THE TOP. IT CAN ALSO CAUSE A RUFF WAVE PATTERN ON THE CUT SURFACE.
· TAPER IS ALSO A PROBLEM WITH WATER JET CUTTING IN VERY THICK MATERIALS. TAPER IS WHEN THE JET EXITS THE PART AT A DIFFERENT ANGLE THAN IT ENTERS THE PART, AND CAN CAUSE DIMENSIONAL INACCURACY. DECREASING THE SPEED OF THE HEAD MAY REDUCE THIS, ALTHOUGH IT CAN STILL BE A PROBLEM.
APPLICATION:-
DUE TO THE UNIQUENESS OF WATER JET CUTTING, THERE ARE MANY APPLICATIONS WHERE IT IS MORE USEFUL AND ECONOMICAL THAN STANDARD MACHINING PROCESSES. IN THIS SECTION, SOME OF THE MAJOR APPLICATIONS AND USES FOR WATER JET CUTTING WILL BE DISCUSSED, AND THE REASONS WHY THIS METHOD WORKS BETTER.
FIRST OF ALL, WATER JET CUTTING IS USED MOSTLY TO CUT LOWER STRENGTH MATERIALS SUCH AS WOOD, PLASTICS, AND ALUMINUM. WHEN ABRASIVES ARE ADDED, STRONGER MATERIALS SUCH AS STEEL, AND EVEN SOME TOOL STEELS CAN BE CUT, ALTHOUGH THE APPLICATIONS ARE SOMEWHAT LIMITED. LISTED BELOW ARE DIFFERENT APPLICATIONS, AND REASONS WHY WATER JET CUTTING IS USED FOR EACH ONE.
PRINTED CIRCUIT BOARDS: FOR CIRCUIT BOARDS, WATER JET CUTTING IS MOSTLY USED TO CUT OUT SMALLER BOARDS FROM A LARGE PIECE OF STOCK. THIS IS A DESIRED METHOD, SINCE IT HAS A VERY SMALL KEF, OR CUTTING WIDTH, AND DOES NOT WASTE A LOT OF MATERIAL. BECAUSE THE STREAM IS SO CONCENTRATED, IT CAN ALSO CUT VERY CLOSE TO THE GIVEN TOLERANCES FOR PARTS MOUNTED ON THE CIRCUIT BOARD WITHOUT DAMAGING THEM. ANOTHER BENEFIT IS THAT WATER JET CUTTING DOES NOT PRODUCE THE VIBRATIONS AND FORCES ON THE BOARD THAT A SAW WOULD, AND THUS COMPONENTS WOULD BE LESS LIKELY TO BE DAMAGED.
WIRE STRIPPING: WIRE STRIPPING IS ANOTHER APPLICATION THAT CAN BE USED EFFECTIVELY IN WATER JET CUTTING. IF NO ABRASIVES ARE USED, THE STREAM IS POWERFUL ENOUGH TO REMOVE ANY INSULATION FROM WIRES, WITHOUT DAMAGING THE WIRES THEMSELVES. IT IS ALSO MUCH FASTER AND EFFICIENT THAN USING HUMAN POWER TO STRIP WIRES.
FOOD PREPARATION: THE CUTTING OF CERTAIN FOODS SUCH AS BREAD CAN ALSO BE EASILY DONE WITH WATER JET CUTTING. SINCE THE WATER JET EXERTS SUCH A SMALL FORCE ON THE FOOD, IT DOES NOT CRUSH IT, AND WITH A SMALL KERFS WIDTH, VERY LITTLE IS WASTED.
TOOL STEEL: FOR ABRASIVE WATER JET CUTTING, TOOL STEELS ARE ONE APPLICATION, ALTHOUGH A LIMITED ONE. IT CAN BE VERY USEFUL THOUGH BECAUSE TOOL STEEL IS GENERALLY VERY DIFFICULT TO CUT WITH CONVENTIONAL MACHINING METHODS, AND MAY CAUSE AN UNWANTED BYPRODUCT: HEAT. ABRASIVE WATER JETS, HOWEVER, DO NOT PRODUCE HEAT THAT COULD ALTER THE STRUCTURE OF THE MATERIAL BEING CUT, AND THUS THE STRENGTH OF THE TOOL IS RETAINED.
WOOD CUTTING: WOODWORKING IS ANOTHER APPLICATION THAT ABRASIVE WATER JET MACHINING CAN BE USED FOR. SINCE WOOD IS A SOFTER MATERIAL COMPARED TO STEEL, ALMOST ALL WOOD CAN BE CUT, AND THE ABRASIVE PARTICLES SAND THE SURFACE, LEAVING A SMOOTH FINISH THAT DOESN’T REQUIRE SANDING.

PLASMA ARC MACHINING

INTRODUCTION:-
PLASMA CUTTING IS A PROCESS THAT IS USED TO CUT STEEL AND OTHER METALS (OR SOMETIMES OTHER MATERIALS) USING A PLASMA TORCH. IN THIS PROCESS, AN INERT GAS (IN SOME UNITS, COMPRESSED AIR) IS BLOWN AT HIGH SPEED OUT OF A NOZZLE; AT THE SAME TIME AN ELECTRICAL ARC IS FORMED THROUGH THAT GAS FROM THE NOZZLE TO THE SURFACE BEING CUT, TURNING SOME OF THAT GAS TO PLASMA. THE PLASMA IS SUFFICIENTLY HOT TO MELT THE METAL BEING CUT AND MOVES SUFFICIENTLY FAST TO BLOW MOLTEN METAL AWAY FROM THE CUT. PLASMA CAN ALSO BE USED FOR PLASMA ARC WELDING AND OTHER APPLICATIONS.
THE HF CONTACT TYPE TYPICALLY FOUND IN BUDGET MACHINES USES A HIGH-FREQUENCY, HIGH-VOLTAGE SPARK TO IONIZE THE AIR THROUGH THE TORCH HEAD AND INITIATE AN ARC. THE ARC CAN ONLY BE FORMED IF THE TORCH IS IN CONTACT WITH THE JOB MATERIAL. HF CONTACT TYPE MACHINES ARE NOT SUITABLE FOR APPLICATIONS INVOLVING CNC CUTTING.
THE PILOT ARC TYPE USES A TWO CYCLE APPROACH TO PRODUCING PLASMA. FIRST, A HIGH-VOLTAGE, LOW CURRENT CIRCUIT IS USED TO INITIALIZE A VERY SMALL HIGH-INTENSITY SPARK WITHIN THE TORCH BODY, THEREBY GENERATING A SMALL POCKET OF PLASMA GAS. THIS IS REFERRED TO AS THE PILOT ARC. THE PILOT ARC HAS A RETURN ELECTRICAL PATH BUILT INTO THE TORCH HEAD. THE PILOT ARC WILL MAINTAIN ITSELF UNTIL IT IS BROUGHT INTO PROXIMITY OF THE WORK PIECE WHERE IT IGNITES THE MAIN PLASMA CUTTING ARC. PLASMA ARCS ARE EXTREMELY HOT AND ARE IN THE RANGE OF 15,000 DEGREES CELSIUS.


PLASMA IS AN EFFECTIVE MEANS OF CUTTING THIN AND THICK MATERIALS ALIKE. HAND-HELD TORCHES CAN USUALLY CUT UP TO 2 IN (48 MM) THICK STEEL PLATE, AND STRONGER COMPUTER-CONTROLLED TORCHES CAN PIERCE AND CUT STEEL UP TO 12 INCHES (300 MM) THICK. FORMERLY, PLASMA CUTTERS COULD ONLY WORK ON CONDUCTIVE MATERIALS; HOWEVER, NEW TECHNOLOGIES ALLOW THE PLASMA IGNITION ARC TO BE ENCLOSED WITHIN THE NOZZLE, THUS ALLOWING THE CUTTER TO BE USED FOR NON-CONDUCTIVE WORK PIECES. PLASMA ARC WELDING (PAW) IS AN ARC WELDING PROCESS SIMILAR TO GAS TUNGSTEN ARC WELDING (GTAW). THE ELECTRIC ARC IS FORMED BETWEEN AN ELECTRODE (WHICH IS USUALLY BUT NOT ALWAYS MADE OF SINTERED TUNGSTEN) AND THE WORK PIECE. THE KEY DIFFERENCE FROM GTAW IS THAT IN PAW, BY POSITIONING THE ELECTRODE WITHIN THE BODY OF THE TORCH, THE PLASMA ARC CAN BE SEPARATED FROM THE SHIELDING GAS ENVELOPE. THE PLASMA IS THEN FORCED THROUGH A FINE-BORE COPPER NOZZLE WHICH CONSTRICTS THE ARC AND THE PLASMA EXITS THE ORIFICE AT HIGH VELOCITIES (APPROACHING THE SPEED OF SOUND) AND A TEMPERATURE APPROACHING 20,000 °C. PLASMA ARC WELDING IS ADVANCEMENT OVER THE GTAW PROCESS. THIS PROCESS USES A NON-CONSUMABLE TUNGSTEN ELECTRODE AND AN ARC CONSTRICTED THROUGH A FINE-BORE COPPER NOZZLE. PAW CAN BE USED TO JOIN ALL METALS THAT ARE WELD ABLE WITH GTAW (I.E., MOST COMMERCIAL METALS AND ALLOYS). SEVERAL BASIC PAW PROCESS VARIATIONS ARE POSSIBLE BY VARYING THE CURRENT, PLASMA GAS FLOW RATE, AND THE ORIFICE DIAMETER, INCLUDING:
  • MICRO-PLASMA (< 15 AMPERES)
  • MELT-IN MODE (15–400 AMPERES)
  • KEYHOLE MODE (>100 AMPERES)
  • PLASMA ARC WELDING HAS A GREATER ENERGY CONCENTRATION AS COMPARED TO GTAW.
  • A DEEP, NARROW PENETRATION IS ACHIEVABLE; REDUCING DISTORTION AND ALLOWING SQUARE-BUTT JOINTS IN MATERIAL UP TO ½” (12 MM) THICK.
  • GREATER ARC STABILITY ALLOWS A MUCH LONGER ARC LENGTH (STAND-OFF), AND MUCH GREATER TOLERANCE TO ARC LENGTH CHANGES.
  • SINCE PLASMA CUTTERS PRODUCE A VERY HOT AND MUCH LOCALIZED "CONE" TO CUT WITH, THEY ARE EXTREMELY USEFUL FOR CUTTING SHEET METAL IN CURVED OR ANGLED SHAPES.

PROCESS VARIABLES:-

GASES

AT LEAST TWO SEPARATE (AND POSSIBLY THREE) FLOWS OF GAS ARE USED IN PAW:
  • PLASMA GAS – FLOWS THROUGH THE ORIFICE AND BECOMES IONIZED
  • SHIELDING GAS – FLOWS THROUGH THE OUTER NOZZLE AND SHIELDS THE MOLTEN WELD FROM THE ATMOSPHERE
  • BACK-PURGE AND TRAILING GAS – REQUIRED FOR CERTAIN MATERIALS AND APPLICATIONS.
THESE GASES CAN ALL BE SAME, OR OF DIFFERING COMPOSITION.

MATERIALS

THE PARTS ARE USUALLY OF CONDUCTIVE METALS AND ALLOYS RANGING FROM 3 MM IN THICKNESS TO .25 IN THICKNESS. ANYTHING LESS THAN THE 3 MM MAY FAIL DUE TO LACK OF MATERIAL AND ANYTHING MORE THAN .25 IN. IN THICKNESS MAY EXPERIENCE A FAILURE DUE TO INSTABILITY WITHIN THE WELD.

KEY PROCESS VARIABLES

  • CURRENT TYPE AND POLARITY
  • DCEN FROM A CC SOURCE IS STANDARD
  • AC SQUARE-WAVE IS COMMON ON ALUMINUM AND MAGNESIUM
  • WELDING CURRENT AND PULSING - CURRENT CAN VARY FROM 0.5 A TO 1200 A; CURRENT CAN BE CONSTANT OR PULSED AT FREQUENCIES UP TO 20 KHZ
  • GAS FLOW RATE (THIS CRITICAL VARIABLE MUST BE CAREFULLY CONTROLLED BASED UPON THE CURRENT, ORIFICE DIAMETER AND SHAPE, GAS MIXTURE, AND THE BASE MATERIAL AND THICKNESS.)

ADVANTAGES:-

(I) IT CUTS CARBON STEEL UP TO 10 TIMES FASTER THAN OXY-FUEL CUTTING, WITH EQUAL QUALITY MORE ECONOMICALLY.


(II) IT LEAVES NARROWER KERFS.


(III) PLASMA CUTTING BEING PRIMARILY A MELTING PROCESS CAN CUT ANY METAL.


(IV)ARC PLASMA TORCHES GIVE THE HIGHEST TEMPERATURE AVAILABLE FROM MANY PRACTICABLE SOURCES. THE ENERGY SEEMS TO BE UNLIMITED IN THIS METHOD.


DISADVANTAGES:-

HIGH INITIAL COST OF THE EQUIPMENT.


APPLICATIONS:-
(I) PLASMA CUTTING IS USED TO CUT PARTICULARLY THOSE NONFERROUS AND STAINLESS METALS THAT CANNOT BE CUT BY THE USUAL RAPID OXIDATION INDUCED BY ORDINARY FLAME TORCHES.

(II) PLASMA CUTTING CAN BE USED FOR STACK CUTTING, PLATE BEVELING, AND SHAPE CUTTING AND PIERCING.

(III) WITH SOME MODIFICATIONS, PLASMA ARC CUTTING CAN BE USED UNDER WATER.

(IV) PLASMA ARC CUTTING FINDS APPLICATIONS IN MANY INDUSTRIES SUCH AS SHIPYARD, CHEMICAL, NUCLEAR AND PRESSURE VESSEL.

(V)IT IS USED FOR REMOVING GATES AND RISERS IN FOUNDRY.

(VI) IT CUTS HOT EXTRUSIONS TO DESIRED LENGTH.

(VII) IT IS USED TO CUT ANY DESIRED PIPE CONTOUR.


LASER BEAM MACHINING

INTRODUCTION:-
LASER BEAM MACHINING (LBM) IS A TECHNIQUE USED TO JOIN MULTIPLE PIECES OF METAL THROUGH THE USE OF A LASER. THE BEAM PROVIDES A CONCENTRATED HEAT SOURCE, ALLOWING FOR NARROW, DEEP WELDS AND HIGH WELDING RATES. THE PROCESS IS FREQUENTLY USED IN HIGH VOLUME APPLICATIONS, SUCH AS IN THE AUTOMOTIVE INDUSTRY. GAS LASERS USE HIGH-VOLTAGE, LOW-CURRENT POWER SOURCES TO SUPPLY THE ENERGY NEEDED TO EXCITE THE GAS MIXTURE USED AS A LASING MEDIUM. THESE LASERS CAN OPERATE IN BOTH CONTINUOUS AND PULSED MODE, AND THE WAVELENGTH OF THE LASER BEAM IS 10.6 ΜM. FIBER OPTIC CABLE ABSORBS AND IS DESTROYED BY THIS WAVELENGTH, SO A RIGID LENS AND MIRROR DELIVERY SYSTEM IS USED. POWER OUTPUTS FOR GAS LASERS CAN BE MUCH HIGHER THAN SOLID-STATE LASERS, REACHING 25 KW




OPERATION:-
LIKE ELECTRON BEAM WELDING (EBW), LASER BEAM WELDING HAS HIGH POWER DENSITY (ON THE ORDER OF 1 MEGAWATT/CM² (MW)) RESULTING IN SMALL HEAT-AFFECTED ZONES AND HIGH HEATING AND COOLING RATES. THE SPOT SIZE OF THE LASER CAN VARY BETWEEN 0.2 MM AND 13 MM, THOUGH ONLY SMALLER SIZES ARE USED FOR WELDING. THE DEPTH OF PENETRATION IS PROPORTIONAL TO THE AMOUNT OF POWER SUPPLIED, BUT IS ALSO DEPENDENT ON THE LOCATION OF THE FOCAL POINT: PENETRATION IS MAXIMIZED WHEN THE FOCAL POINT IS SLIGHTLY BELOW THE SURFACE OF THE WORK PIECE.
A CONTINUOUS OR PULSED LASER BEAM MAY BE USED DEPENDING UPON THE APPLICATION. MILLISECONDS LONG PULSES ARE USED TO WELD THIN MATERIALS SUCH AS RAZOR BLADES WHILE CONTINUOUS LASER SYSTEMS ARE EMPLOYED FOR DEEP WELDS.
LBW IS A VERSATILE PROCESS, CAPABLE OF WELDING CARBON STEELS, HSLA STEELS, STAINLESS STEEL, ALUMINUM, AND TITANIUM. DUE TO HIGH COOLING RATES, CRACKING IS A CONCERN WHEN WELDING HIGH-CARBON STEELS. THE WELD QUALITY IS HIGH, SIMILAR TO THAT OF ELECTRON BEAM WELDING. THE SPEED OF WELDING IS PROPORTIONAL TO THE AMOUNT OF POWER SUPPLIED BUT ALSO DEPENDS ON THE TYPE AND THICKNESS OF THE WORK PIECES. THE HIGH POWER CAPABILITY OF GAS LASERS MAKE THEM ESPECIALLY SUITABLE FOR HIGH VOLUME APPLICATIONS. LBW IS PARTICULARLY DOMINANT IN THE AUTOMOTIVE INDUSTRY.
SOME OF THE ADVANTAGES OF LBW IN COMPARISON TO EBW ARE AS FOLLOWS: THE LASER BEAM CAN BE TRANSMITTED THROUGH AIR RATHER THAN REQUIRING A VACUUM, THE PROCESS IS EASILY AUTOMATED WITH ROBOTIC MACHINERY, X-RAYS ARE NOT GENERATED, AND LBW RESULT IN HIGHER QUALITY WELDS.
A DERIVATIVE OF LBW, LASER-HYBRID WELDING, COMBINES THE LASER OF LBW WITH AN ARC WELDING METHOD SUCH AS GAS METAL ARC WELDING. THIS COMBINATION ALLOWS FOR GREATER POSITIONING FLEXIBILITY, SINCE GMAW SUPPLIES MOLTEN METAL TO FILL THE JOINT, AND DUE TO THE USE OF A LASER, INCREASES THE WELDING SPEED OVER WHAT IS NORMALLY POSSIBLE WITH GMAW. WELD QUALITY TENDS TO BE HIGHER AS WELL, SINCE THE POTENTIAL FOR UNDERCUTTING IS REDUCED.

EQUIPMENT:-


ADVANTAGES:-

(I) DUE TO RAPID RATE OF OPERATION, THE (HEAT) DISTORTION TO THE BASE METAL IS MINIMUM.
(II) THE NARROWNESS AND ACCURACY OF THE CUT TOGETHER WITH RELATIVELY SHALLOW HAZ WITH NEGLIGIBLE SURFACE CRACKING ARE MAJOR ADVANTAGES OF THIS PROCESS.
(III) EVEN MOST COMPLICATED PROFILES CAN BE CUT.
(IV ) VERY HARD MATERIAL LIKE SILICON CARBIDE, FRIABLE MATERIAL LIKE GLASS, STICKY MATERIAL LIKE CONFECTIONERY ETC., CAN BE CUT BY LASER.
(V) LASER IS A FASTER PROCESS THAN SAWING OR NIBBLING AND IS VERY EFFICIENT
.

LIMITATIONS:-
THE LIMIT ON THICKNESS OF SECTION (6 MM IN PLAIN CARBON STEEL; FOR OTHER METALS IT IS STILL LESSER) AND THE FACT THAT WORK PIECE MUST BE MOVED RELATIVE TO THE RATHER CUMBERSOME EQUIPMENT RESTRICT THE USE OF GAS-JET LASER CUTTING.

APPLICATIONS OF LASER BEAM CUTTING


(I) LASER CAN CUT IN LOCATIONS IN A (METAL) STRUCTURE VERY DIFFICULT TO REACH BY ANY OTHER METHOD.


(II) LASER CAN CUT BOTH METALS AND NON-METALS, (E.G., PLASTICS, LEATHER, ETC.).
(III) LASER HAS BEEN EMPLOYED FOR MAKING DIE BOARDS IN BLOCK-BOARD WHICH ARE USED IN THE MANUFACTURE OF PACKAGES, CARTONS AND GASKETS.
(IV)ASBESTOS PRODUCTS CUT READILY AND CLEARLY BY THE LASER.

ELECTRON-BEAM MACHINING - EBM

INTRODUCTION:-
IN ELECTRON-BEAM MACHINING (EBM), ELECTRONS ARE ACCELERATED TO A VELOCITY NEARLY THREE-FOURTHS THAT OF LIGHT (~200,000 KM/SEC). THE PROCESS IS PERFORMED IN A VACUUM CHAMBER TO REDUCE THE SCATTERING OF ELECTRONS BY GAS MOLECULES IN THE ATMOSPHERE. THE ELECTRON BEAM IS AIMED USING MAGNETS TO DEFLECT THE STREAM OF ELECTRONS AND IS FOCUSED USING AN ELECTROMAGNETIC LENS. THE STREAM OF ELECTRONS IS DIRECTED AGAINST A PRECISELY LIMITED AREA OF THE WORK PIECE; ON IMPACT, THE KINETIC ENERGY OF THE ELECTRONS IS CONVERTED INTO THERMAL ENERGY THAT MELTS AND VAPORIZES THE MATERIAL TO BE REMOVED, FORMING HOLES OR CUTS.
TYPICAL APPLICATIONS ARE ANNEALING, WELDING, AND METAL REMOVAL. A HOLE IN A SHEET 1.25 MM THICK UP TO 125 MICRO M DIAMETER CAN BE CUT ALMOST INSTANTLY WITH A TAPER OF 2 TO 4 DEGREES. EBM EQUIPMENT IS COMMONLY USED BY THE ELECTRONICS INDUSTRY TO AID IN THE ETCHING OF CIRCUITS IN MICROPROCESSORS.


ELECTRON-BEAM MACHINING (EBM) IS A MACHINING PROCESS WHERE HIGH-VELOCITY ELECTRONS ARE DIRECTED TOWARD A WORK PIECE, CREATING HEAT AND VAPORIZING THE MATERIAL. APPLICATIONS OF THIS PROCESS ARE ANNEALING, METAL REMOVAL, AND WELDING (SEE ELECTRON BEAM WELDING). EBM CAN BE USED FOR VERY ACCURATE CUTTING OF A WIDE
VARIETY OF METALS. SURFACE FINISH IS BETTER AND KERFS WIDTH IS NARROWER THAN THOSE FOR OTHER THERMAL CUTTING PROCESSES.
EBM MACHINES UTILIZE VOLTAGES IN THE RANGE OF 50 TO 200 KV TO ACCELERATE ELECTRONS TO 200,000 KM/S. ELECTROMAGNETIC LENSES ARE USED TO DIRECT THE ELECTRON BEAM, BY MEANS OF DEFLECTION, INTO A VACUUM. THE ELECTRONS STRIKE THE TOP LAYER OF THE WORK PIECE, REMOVING MATERIAL, AND THEN BECOME TRAPPED IN SOME LAYER BENEATH THE SURFACE. THE PROCESS IS SIMILAR TO LASER-BEAM MACHINING, BUT BECAUSE EBM REQUIRES A VACUUM, IT IS NOT USED AS FREQUENTLY AS LASER-BEAM MACHINING.

METAL REMOVAL MECHANISM:-

VACUUMS MUST BE USED TO REDUCE CONTAMINATION, AND MINIMIZE ELECTRON COLLISIONS WITH AIR MOLECULES. BECAUSE WORK MUST BE DONE IN A VACUUM, EBM IS BEST SUITED FOR SMALL PARTS. THE INTERACTION OF THE ELECTRON BEAM WITH THE WORK PIECE PRODUCES HAZARDOUS X-RAYS, AND ONLY HIGHLY TRAINED PERSONNEL SHOULD USE EBM EQUIPMENT.
THE BASIC PHYSICS IS AN ELECTRON BEAM IS DIRECTED TOWARDS A WORK PIECE, THE ELECTRON HEAT AND VAPORIZE THE METAL.

· TYPICAL APPLICATIONS ARE,
1. - ANNEALING
2. - WELDING
3. - METAL REMOVAL

· ELECTRONS ACCELERATED WITH VOLTAGES OF APPROX. 150,000V TO CREATE VELOCITIES OVER 200,000 KM/SEC.

· BEAM CAN BE FOCUSED TO 10 TO 200 MICRO M AND A DENSITY OF 6500 GW/MM2

· GOOD FOR NARROW HOLES AND SLOTS.
1. E.G. A HOLE IN A SHEET 1.25 MM THICK UP TO 125 MICRO M DIAMETER CAN BE CUT ALMOST INSTANTLY WITH A TAPER OF 2 TO 4 DEGREES

· THE ELECTRON BEAM IS AIMED USING MAGNETS TO DEFLECT THE STREAM OF ELECTRONS

· A VACUUM IS USED TO MINIMIZE ELECTRON COLLISION WITH AIR MOLECULES.

· BEAM IS FOCUSSED USING AN ELECTROMAGNETIC LENS.





· SOME EXAMPLES OF CUTTING PERFORMANCE ARE GIVEN BELOW,



· TYPICAL ENERGY REQUIREMENTS FOR CUTTING ARE,



· E.G. TO CUT A 150 MICRO M WIDE SLOT IN A 1MM THICK TUNGSTEN SHEET, USING A 5KW POWER SOURCE, DETERMINE THE CUTTING SPEED.

· BASIC MECHANICS,

· E.G.



· THE HEAT RISE CAN BE ESTIMATED USING A ONE DIMENSIONAL HEAT FLOW EQUATION


· WE CAN ESTIMATE THE MELTING TEMPERATURE WITH,


· E.G.



· OTHER EFFECTS OF EBM
1. - PROCESS DONE IN VACUUM, SO IT IS BEST SUITED TO SMALL PARTS, BUT VACUUM ALSO REDUCES CONTAMINATION
2. - VERY HIGH HEAT CONCENTRATION REDUCES PERIPHERAL HEATING OF SURFACE LESS THAT 50 MICRO M FROM THE CUT THE PART IS AT ROOM TEMPERATURE.

· SUMMARY OF EBM CHARACTERISTICS,
1. - MECHANICS OF MATERIAL REMOVAL - MELTING, VAPORIZATION
2. - MEDIUM - VACUUM
3. - TOOL - BEAM OF ELECTRONS MOVING AT VERY HIGH VELOCITY
4. - MAXIMUM MRR = 10 MM3/MIN
5. - SPECIFIC POWER CONSUMPTION = 450W/MM3/MIN
6. - CRITICAL PARAMETERS - ACCELERATING VOLTAGE, BEAM CURRENT, BEAM DIAMETER, WORK SPEED, MELTING TEMPERATURE
7. - MATERIALS APPLICATION - ALL MATERIALS
8. - SHAPE APPLICATION - DRILLING FINE HOLES, CUTTING CONTOURS IN SHEETS, CUTTING NARROW SLOTS
9. - LIMITATIONS - VERY HIGH SPECIFIC ENERGY CONSUMPTION, NECESSITY OF VACUUM, EXPENSIVE MACHINE


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