Abrasive Jet machining is a kind of blasting process and one of the most hopeful micro machining process for hard and brittle materials like glasses and ceramics. Metal is removed due to the erosion caused by impact of high speed abrasive jet. Abrasive Jet machine has precision X-Y axis rigidly mounted to the cutting table and it has programmable Z axis with nozzle assembly. According to the proper design calculations we can select different components of Abrasive Jet Machine.
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In this project, I develop the model using different CAD packages like AUTO CAD and SOLID WORKS. Moreover I also explain the effect of several AJM operating parameters like standoff distance, size and type of abrasives on metal removal rate of AJM. I explain how we can maximize the efficiency of AJM by controlling their parameters. In this model care has been taken that use less fabricated component rather than directly procuring them because sometimes we find less accuracy in fabricated component so performance of machine can be low.
Introduction of Abrasive Jet Machining
Material removal processes is a part of manufacturing process and it can be divided into two major groups. One is conventional machining processes and another is Non- conventional machining processes. Different types of Conventional machining processes are Milling, Shaping, Drilling, Slotting, Boring, etc. And the examples of Non Conventional Processes are Abrasive Jet Machining (AJM), Electro-Discharge Machining (EDM), Laser Beam Machining (lBM), Abrasive Water Jet Machining (AWJM) etc..
Abrasive jet machining also called, Abrasive Micro blasting, it is a material removing process and material is removed by a focused steam of abrasive particles carried by high pressure gas or air. A fine abrasive particle strikes on work surface through nozzle and the material is removed by erosion by the high velocity abrasive particle.
AJM is used mainly to cut materials that are very sensitive to heat damage and thin section of hard materials that chip easily and it is also used to cut intricate shapes and holes which are more difficult to produce by other processes.
Equipments of Abrasive Jet Machine
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A layout of Abrasive Jet Machining is shown in fig. Abrasive particles are fed from the hoper into the mixing chamber. High pressure air or gas (such as Carbon Di-oxide or Nitrogen) used as carrier gas is supplied to the mixing chamber which containing abrasive powder and vibrating at 50 c/s. with the help of pressure regulator we can control the gas flow and pressure. The high pressure air/gas entrains the abrasive particles and this mixture comes out from small nozzle at high velocity. This stream of abrasive particles strikes on the surface of work-piece and cut the material.
The abrasive stream is directed to the work piece locations by nozzle, which may be manually positioned or mounted in some designed fixture for automatic operation.
For automatic operation, cam drives, tracer mechanism or pantographs used to automatically position either nozzle or work piece.
Abrasive Jet Machining Mechanism:
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In this process kinetic energy of abrasive jet is strike on the surface of work piece then it converts into pressure energy. If pressure energy is more than the yield strength or tensile strength of work piece material then material gets cut.
List of Components of AJM:
Abrasive Delivery System ( Including Hopper)
Pressure Regulator
Air filter valve
Mixer
Nozzle
Suitable arrangement to hold the work piece
Elements of Abrasive Jet Machine
There are three main elements of AJM:
Nozzle
Abrasive
Carrier gas
The above mentioned three elements of AJM influence:
Material removal rate and accuracy of machining
Nozzle wear rate
Surface roughness
Geometry of cut
Variables in Abrasive Jet Machine
The process can be easily controlled to vary the metal removal rate which Depends on following variables.
Nozzle design ( mean Shape of nozzle such as circular, square)
Distance between the nozzle and work piece
Inclination to the work surface
Type of abrasive
Size and shape of abrasive
Flow rate of abrasive
Type of carrier gases
Velocity of abrasive jet
Work piece material
Nozzle design: Nozzle must be resistant under different conditions like pressure and kinetic energy of jet and common materials used for nozzle are tungsten carbide and synthetic sapphire. Pressure losses at the end of nozzle should be low.
Distance between nozzle and work piece: Standoff distance affect on material removal rate of AJM. Material removal rate is increase if we increase standoff distance up to certain limit and then fall gradually. Small standoff distance gives better penetration of abrasive particle into the work piece and large standoff distance might cause flaring. Successfully experiments show that effective standoff distance is 0.8mm.
Types of abrasive: Aluminum oxide is preferable abrasive in majority of application and silicon carbide abrasive also use in certain cases. Abrasive size also prominent factor in AJM. Best results have been obtained with a abrasive size in the range of 10-50 µm.
Abrasive Flow Rate: Maximum material removal rate is achieved with a flow rate at 8-18 g/m. Increase the flow rate above the normal range for a particular nozzle design results in lower stream velocity thus decrease the material removal rate. It is advisable to use acceptable flow rate to conserve abrasives and increase the nozzle life.
Grain size: Abrasives with smaller grain size have high density which may cause chocking problem of nozzle. General grain size use in AJM is 10µ to 50µ. Fine grains are used for polishing and deburring. Coarse grain size cut faster and cut deeper also.
Work piece material: Abrasive jet Machine is generally used for machining brittle material like ceramics and glass and effect of MRR is depends on hardness of material if hardness is high then MRR is low and vice versa.
Advantages of Abrasive Jet Machine:
Low capital cost
Low power consumption
Ability to cut intricate shapes in high hardness and toughness material
Ability to cut fragile and heat sensitive material without damage because no heat generated and overall forces are low
The part produced is free of burs
It is flexible process from design point of view
Tooling time is substantially reduced
Both faces of work piece can be machined simultaneously.
Disadvantages of Abrasive Jet Machine
It is slow process since metal removal rate is very slow.
Larger floor space is required
Skilled operators are needed
Sharp corner cannot be produced
High manufacturing cost
Material thickness that can be machined is limited
Abrasive grains can stick into soft material so cleaning is required
High wear rate of nozzle
The process tends to pollute the environment
Applications of Abrasive Jet Machine
Removing flash and parting lines form the injection molding machine
Deburring and polishing plastic components like nylon and Teflon
Frosting interior surface of glass tubes
Etching marking on glass cylinder
High quality surface can be easily produced
Glue, paints can be removed effectively form paintings.
Different Components of Abrasive Jet Machining set up
Abrasive Delivery System
CNC Control System
Pump
Nozzle
Different Motion System of Nozzle
Catcher plate
Abrasive Delivery System:
Modern abrasive delivery feed system do not need a vibratory feeders and other metering valves.
For smooth and precision cutting operation we need to have fixed and continuous abrasive flow rate.
Now days we use orifice metering system metering system which is very reliable and extremely repeatable.
During machining set up, once we measured the abrasive flow rate through orifice we can enter that value in control computer programmed and then no adjustment or further tuning of abrasive flow is ever be required.
Bulk abrasive delivery system is mounted with the wheel and it is also easy to move and this kind of system is use compressed air to transport garnet from the bulk abrasive hopper to small hopper attached with the machine.
Features: can hold approximately 550lbs abrasive
Can be located 20 feet away from the Abrasive Jet machine.
Height and Width are of following: 1320mm and 960mm
Can handle minimum pressure 80 to 95 psi.
How it works:
Abrasive particles are stored into the hopper and abrasive container has one inlet and one outlet for air passage.
The flow rate of abrasive is controlled by rotational speed of motor.
Due to rotation of cam, abrasive container gets vibration.
The abrasive particle is delivered by rotation of cam and motor arrangement.
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Control System for Abrasive Water jet Machine:
Most of abrasive water jet machine equipped with the CNC control system.
G code programming is installed in CNC machine.
But the limitations of the G code Controller is that feed rate is held at constant speed and feed rate varies only for some regions like curves and corners.
Another limitation of G code controller is that is used for rigid cutting tools.
Abrasive water jet machine is not a kind of rigid cutting tool.
Every time change the feed rate for curves and corner results in uneven cut.
So by considering various parameters like parameters of nozzle and intricate shape of part and variable thickness of the work piece and materials going to cut help to determine the feed rate so that operation can be done smoothly.
For that we have to prepare the control algorithm which compute the desired feed rate based on the above parameters.
By using G code we can convert this feed rate mechanism into a actual programming.
This programming help to drive the servomotors and that controls the X – Y motion and this way we can make part with more precision.
Pump:
Two type of pumps used in the Abrasive water jet system. One is Crankshaft pump which was used in old technology buy nowadays for high pressure cutting technology intensifier pump is used.
In this hydraulic power pack system is a type of positive displacement pump.
This system drives the intensifier.
Hydraulic power pack system is controlled by microcomputers to set up the pressure etc…
Electric motor is used to drive the hydraulic pump and as the hydraulic fluid push the large cylinder which generate the high force on small cylinder and this small cylinder pressurize the water at the level which is proportional to the cross section area of larger cylinder to the small cylinder.
Nozzle:
High pressure water flows into the nozzle and cutting head of nozzle which comprise of an orifice and mixing chamber and an insert where water jet mixed with the abrasive and form an abrasive water jet.
Various motion of the nozzle is controlled by motion system across the material.
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How water jet Forms:
Typical diameter of the nozzle is 6mm. High pressure water is then fed into the pipe which can carry the water at 400 Mpa and consisting of different joints without any leakage between them.
When water passes through the orifice flow of water convert onto narrow jet and orifice diameter is usually about 0.2 to 0.4mm.
When water enters into the cutting head section pressure head of water is converted into velocity head.
Velocity of water can be expressed as Vw = 2Pw / ρw.
Where Pw is the pressure of water and ρw is density of the water.
The orifice usually made of sapphire and life of the orifice is about 100-150 hours.
From orifice narrow water jet passes through the small section vacuum is created in mixing chamber and that pulls abrasive particles and air into the mixing chamber.
Abrasive particles are feed through the different techniques like Vibratory bowl or Belt feeder.
Mixing Chamber:
Mixing tube is the section where abrasive particles mixed with the water jet.
Focusing tube generally made of tungsten carbide because of its good abrasive resistance characteristics.
Focusing tube has inner diameter typical about 0.8 to 1.6mm and length of 50 t0 80 mm.
To get the maximum accuracy we have to replace the mixing tube frequently. We can replace it when tolerance goes below the acceptable level.
Numerical modeling of Mixing process can be described as of follow:
If we consider some energy losses at the water jet formation at the end of orifice then water jet velocity described as of following way:
Vw = Φ √ (2Pw/ ρw)
Where Φ is Velocity coefficient of the orifice.
Volume flow rate of water can be expressed as following:
Qw = Ψ x Area of orifice x Vw
= Ψ x ðœ«/4 do² x Φ √ (2Pw/ ρw)
= Cd x 𜫠/4 do² x √ (2Pw/ ρw)
Where Ψ = co efficient of Vena Contracta
Cd = Discharge co efficient of orifice.
So total power of the water jet can be expressed by
Pw = Qw x Pw
= = Cd x 𜫠/4 do² x √ (2Pw³/ ρw)
Due to the loss occur in the mixing process both momentum and energy is not conserved. But assumed that no looses occur in momentum at initially.
So Σ (á¹ x v) before = Σ (á¹ x v)After
(á¹airvair + á¹wvw + á¹ava)before = ( á¹aivairr + á¹wvw + á¹ava)After
As we know that air has very low density so momentum of air, before and after are neglected.
Consider one more assumption and is that after mixing abrasive particle has same velocity of water and also neglect the momentum of abrasive then above equation can be expressed by
á¹wvw = ( á¹w + á¹a) vaw
So vaw = [(1/(1 + R) ] x vw
Where R is the loading factor = á¹a/ á¹w
During mixing process momentum losses occur due to the friction between the abrasive particle and inner wall of focusing tube and collision between the abrasive particle and water jet so for time being consider momentum losses and then equation is described by
V aw = η x [(1/(1 + R) ] x vw
Where η is the momentum loss factor.
Motion System:
Motion system can be designed in many ways according to the motion of nozzle around a work piece.
This mechanism provides excellent operating conditions for the operator.
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Materials are loaded with the help of overhead crane or fork lift truck to the tank.
This mechanism is useful for 5 axis tables and also provides pretty much space to the vertical tilted axis.
This structure has restriction to the movement of nozzle that means movement of nozzle has limitation to twist and long distance between the table and work piece result in error due to the deflection of machine.
In this type of mechanism, Nozzle is mounted to the Y axis carriage.
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Y axis is moving beam and is mounted with the X axis.
Back beam is supported at the two ends.
This mechanism is suitable for very large machine.
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In above mechanism, nozzle is very close to the plane of work piece which minimize static and dynamic errors while operation is going on.
This system is not suitable for manually material loading and this provide material loading is equal to the access of the tank.
Cost estimation ( Bill of material)
Item
Cost per single item
No of items required
Total cost
Linear motion guide ways hsr30cm
$385
3
$1155
Precision Ball screw
$188
3
$564
Supporting Unit
$144
3
$432
Frl unit air filter
$48
1
$48
Diamond nozzle for long service life
$900
1
$900
Abrasive hopper with 90lb capacity
$132
1
$132
Other accessories
$250
$250
Total
$3481
Material Removal rate Mechanism and Modeling in Abrasive Jet Machining
As we discussed earlier, material removal rate takes place in abrasive jet machining because of the erosion of the work material and erosion take place due to the high impact of jet velocity strikes on the work piece.
Following assumption has been considered in mechanism of Material removal rate.
Shapes of the abrasives are spherical and abrasive particles are rigid.
Kinetic energy of the abrasive particles is fully used in removing material of the work piece.
Erosion takes place in brittle material due to the brittle fracture and the volume of fracture is considered as hemispherical with the diameter which is equal to length of the indentation.
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From the geometry of the erosion of material
DE² = DF² + EF²
EF² = r² = DE² – DF²
r² = (dg/2)² – (dg/2 – H) ²
= -H² + dg H ≅ dg H
r =
Volume of the erosion of material is equal to the volume of hemispherical crater and is given by:
TB = 2/3
= 2/3 x (dg H) ^3/2
As we know kinetic energy equation is given by ½ mv² so the kinetic energy of
Abrasive particle grit is given by
K.Eg = ½ mg V²
= /12 x dg ρg V²
Where V = Velocity of Abrasive Particle
ρg = Density of Abrasive Grit
dg = Diameter of Single Abrasive Grit
mg = Mass of Abrasive Grit
When Abrasive particle strikes on Work piece material, work piece will be subjected to maximum forces which cause the erosion of material which I indicated in figure by symbol H.
Therefore work done is given by
W = ½ x F H
Impact force F is give by F = Indentation Area x hardness of the material
W = ½ x r² H S
And Work done is equal to Kinetic energy because we assumed that kinetic energy is fully utilized for material removal so
W = Kinetic energy K.E
½ x r² H = = /12 x dg ρg V²
So H = (dg ρg V²)/ (6r²S)
= dg V (ρg/6S) ^1/2
Now material removal rate of brittle material can be expressed as of following:
MrrB = TB x No. of strikes by abrasive grits per second N
= TB x (ma/ /6 dg ρg)
= (6TB ma/ dg ρg )
Now Put the value of TB = 2/3 x (dg H) ^3/2
= [ 6 x (2/3 x (dg H) ^3/2) x ma] / ( dg ρg )
= (4 ma ρg) x (H/dg) ^3/2
As we know that H = dg V (ρg/6S) ^1/2
So MrrB = (4ma/ ρg) x (dgV/dg) ^3/2) x (ρg/6H) ^3/4
= (4maV) ^3/2 / (6^3/4 x ρg^1/4 x S ^3/4)
= (maV^3/2) / (ρg^1/4 x S^3/4)
Fabrication and Assembly of Abrasive Jet Machine
Nozzle:
A standard length of cylindrical rod typically made of Tungsten Carbide or Synthetic Sapphire or MS was cut into the proper length based on the requirements by using power hack saw.
By turning process in Lathe machine we can reduced the diameter of cylindrical rod and bring up to 15mm.
On same lathe machine we can make tip by Taper turning process by tapering one end of cylindrical rod.
Then make a blind hole of 20mm on planner face of rod by using 12mm drill bit in drilling machine.
Due to the tool tip angel, a shape of 118 degree formed at the end of blind hole.
To get the 1mm of Tip of nozzle, we can use the 0.74mm drill bit in Drilling process.
Cam:
Mild steel of 40mm diameter can be used as a raw material of Cam.
Turning process is used to bring down the diameter up to 35mm.
By using power hack saw we can get the required width (Approximate 25mm).
Then both cut faces can be machined in Milling Machine.
Through hole can be made by 16mm Drill Bit in Drilling Machine.
Abrasive Container:
Abrasive container usually made out of the hollow cylinder.
On the both end of container two Cast Iron plates were welded.
On the top plate, two iron pipes fitted through the holes.
There are two pipes in a system, one is inlet which is longer and makes more agitation of abrasive particles and outlet pipe is shorter.
Compressed air goes to the abrasive container through inlet pipe and mixed with the abrasive particles then mixture of Air and Abrasive forward to the nozzle to perform operation
Vibrator Assembly:
Angel section is welded with the base plate by welding process.
Induction motor is placed on the angle section which is parallel to the base plate.
Induction motor is tightening with the nut and bolt with base plate.
Cam is fixed with motor shaft and Abrasive container is connected to the rod and it is freely to rotate around the rod.
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Design data and Technical Specification of Various Components:
X-Y Table
X -Y table is prominent part of the abrasive water jet machine. Various fixturing arrangements have to be provided to hold the work piece while operation is running.
The travel of X-Y table has been decided based on the application.
Table consists of two parts. One is upper plate and another one is lower plate.
Upper table travel in X direction and lower table travel in Y direction.
Based the model of Omax 2626, table can travel 730mm x 660mm.
Y Axis is responsible for movement of work piece in Y direction.
Other components of X-Y are of following.
Linear motion guide way
Recalculating ball screw
Supporting unit for ball screw
Couplings
Nuts and bolts
Other components as required
Linear motion guide way:
General application of linear motion guide is for the linear motion.
Based on the different parameters like travel length, floor space requirement, type of equipment being used, linear motion guide way is selected.
With the help of different rollers which provide rolling elements between the rails and block we can get higher precision linear motion system.
Frictional losses are reduced between the elements by restraint effects between the rails and block and it can withstand high load in any directions.
Assembly of Linear motion Guide way
Source: Hiwin linear motion guide way catalog_G99TE13-0809
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L M Guide way with Dimensions
Structure of linear motion guide way:
As shown in fig, each raw of balls have angle contact 45 degree. Retainer plate has facility to hold the recalculating ball screw which guide linear motion guide way in directions.
End plate is attached with the linear motion blocks which allow the balls to circulate.
Side seal is placed between the row of balls and grease nipple is attached with the end plate.
Linear motion guide way can achieve precise motion by setting up the section height smaller as much as possible based on the requirements and also provide rigidity of the model.
Compare to all others traditional slides, co efficient of friction of linear motion guide way is 1/50 th.
Advantages of linear motion guide way:
Provide high accuracy
Insufficient lubrications in traditional slides give errors in accuracy and in linear motion guide way frictional contact is rolling contact so it has little wear and therefore it has long life.
Due to small frictional resistance a small driving force required to move the load and hence it provides high speed of motion.
Provide Equal loads capacity in all four direction
Best advantage is the interchangeability of the parts.
Ball Screw:
Ball screw assembly for linear motion guide way consists of screw and nuts.
Helical threads or grooves provided between nuts and screws and balls are rotate between them.
Ball return system provides the way to return the balls again that means when ball reach at the end of the nut it return through the return pipe.
Ball nuts in ball screw assembly determine the fatigue failure in the system and ratios of number of threads in ball nut to the number of threads on ball screw decide the wear out.
By considering the table length, calculation of X axis and Y axis ball screw length is determined.
For our table our X axis ball screw length should be 730mm + 200 mm (Breadth for upper table) + 100mm allowances for below cover so it should be 1030mm.
For Y axis table it should be 660mm + 200mm + 100mm so it should be 960mm.
Based on the ball screw length, rail length is determined and which is 50mm larger than ball screw length.
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Ball screw Assembly
Supporting Units:
Ball screw supported bearing are designed with the high bearing stiffness compare to other conventional bearing.
Two kinds of supports are available fixed end support unit and supported end support unit.
In fixed end support unit can take all thrust by given work and in another support end support unit acts like usual bearing.
Nut bracket is used to bolt the ball screw nut with the work load platform.
Vertical motion Module:
Vertical motion module comprise of all above components like L M guide way, Pipe holder , Ball screw assemble, supporting units, nozzle holder and nozzle etc.
This assembly is used for adjustment of stand of distance between nozzle and work piece.
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Fixture arrangement for work piece in Abrasive Jet Machine
As abrasive jet machine is used for various kind of applications that also result in various fixture arrangement of work piece based on the application.
So not only one application can use for the applications.
Usually Abrasive jet machine requires less fixturing equipment to get start the operation so combination of different components can be used for particular kind of work.
Fixture is used to locate the work piece correctly and for quick loading to reduce the machining cycle time.
To prevent undesirable movement or rotation of work piece while the work piece is under the process.
When work piece is under the processes, high pressure of water and abrasive strikes on the work piece so it can move in sideways or lift up those results in errors.
Most of abrasive jets are used to make flat parts or add some features to the existing parts. So for all kind of flat parts 3 parameters has to considered and which are X position , Y- position and angle of rotation with respect to X-Y point.
Figure shows the fixture arrangement of work piece for flat parts using square and clamps.
As shown in figure parts are lying on the slats and clamping is provided to prevent the side motion of work piece because slats do not provide better support to the work piece.
A square plate is bolted with the tank. As shown in fig. Screws which are close to the work piece is a clamping screws and which are far from the work piece for the square to adjust the different thickness of material. This system is used to prevent the rotating motion of slats.
Comparison of Abrasive jet Machining process with other Processes:
Abrasive Jet machine Vs Laser Beam Machining:
Comparison with respect to the material can be cut: Abrasive water jet can cut varieties of material with good conditions compare to laser beam machining. For refractory materials, laser beam machining has poor applicability whereas abrasive water jet has good applicability for same materials.
Capital cost and abrasive water jet set up is very low compare to Laser machining.
There is no heat produce in Abrasive water jet marching so it minimize the thermal distortion of material also minimize the secondary operation because it provides continuous or uniform smooth surface with little burr on the surface.
As the thickness of the work piece increase, higher precision can be obtained by abrasive jet machine with the same or higher tolerance than laser.
Abrasive water jets are safer compare to laser machining and abrasive jet nozzle is very simple than laser however life of nozzle is about 100-150 hours of operation.
However there are some disadvantages associated with the abrasive water jet process while compare with the laser machining like tool wear is very low and power consumption also very low in laser machining as abrasive water jet has little high compare to laser.
Abrasive water jet machine or Abrasive jet machine Vs. EDM.
EDM has low applicability for the material like super alloys whereas abrasive water jet machining has good applicability of the same materials.
Abrasive water jet machining has facility to make piercing in the parts by itself while EDM process has required pre drilled hole.
There is no heat affected zone so this process eliminates secondary operations.
Good capabilities to make large parts with economical cost.
Rapid software programming which is very simple and less set up with fixturing make this machine considerable to buyers.
It can work better in any kind of materials like conductive or non conductive.
If the thickness of work piece is less than the 25mm or 1 inch, abrasive jet machine works five to eight times faster than the EDM but less precise as well.
Abrasive water jets Vs Plasma
Material removal rate efficiency of plasma machining is very low compare to the abrasive water jet machining.
Plasma machining is not preferable to refractory material while abrasive water jet machining applicable to wide range of materials.
Abrasive water jet provides better edge finish than plasma machining process but plasma machining process is faster than the Abrasive water jet process.
Abrasive water jet Vs. Milling
Milling can be used for rapid production if set up is already done and fully programmed whereas abrasive jet machining has rapid programming feature and if operator is well trained than set up do not required for the part.
Minimal burring problem, less scrap, environmentally friendly, no oil soaked chips are the reasons of abrasive jet machining which replacing milling process in machine shops.
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