ABRASIVE JET MACHINING

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 CONTROL 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.