ABRASIVE JET MACHINING

Melbin K Mathew

What is abrasive jet machining ?It is the material removal process where the material is removed by high velocity stream of air/gas or water and abrasive mixture .

An abrasive is small, hard particle having sharp edges and an irregular shape .

High velocity jet is aimed at a surface under controller condition .

components of abrasive jet

machiningAbrasive delivery system Control system Pump Nozzle Mixing tube Motion system

1.abrasive delivery system

Auto abrasive delivery system has the capability of storing abrasive & delivery the abrasive to the bucket . It’s works auto programming system by help of once measuring record & no adjustment or fine tuning system . High sensitive sensor gives extremely reliable & repeatable .

2.Control system

The control algorithm that computes exactly how the feed rate should vary for a given geometry in a given material to make a precise part .

The algorithm actually determines desired variation in the feed rate & the tool path to provide an extremely smooth feed rate .

3.pumpCrankshaft & intensifier pump are mainly use in the abrasive jet machine .

The intensifier pump was the only pump capable of reliably creating pressures high .Crankshaft pumps are more efficient than intensifier pumps because they do not require a power robbing

hydraulic system ultra high pressure & more stroke per minute .

4.nozzleAll abrasive jet systems use the same basic two stage nozzle . First , water passes through a small diameter jewel orifice to form a narrow jet .

The abrasive particles are accelerated by the moving stream of water & they pass into a long hollow cylindrical ceramic mixing tube.Generally two type of nozzle use , right angle head & straight head .

NOZZLE

5. Mixing tubeThe mixing tube is where the abrasive

mixes with the high pressure water .The mixing tube should be replaced when tolerances drop below acceptable levels .For maximum accuracy , replace the mixing tube more frequently .

Water Jet and Abrasive Water Jet Machining

WJM - Pure WJM - with stabilizer

AWJM – entrained – three phase – abrasive, water and air AWJM – suspended – two phase – abrasive and water o Direct pumping o Indirect pumping o Bypass pumping

6.Motion systemIn order to make precision parts , an abrasive

jet system must have a precision x-y table and

motion control system .Tables fall into three general categories .

Floor-mounted gantry systemsIntegrated table/gantry systemsFloor-mounted cantilever systems

working process

High pressure water starts at the pump , and is delivered through special high pressure plumbing to the nozzle .

At the nozzle , abrasive is introduced & as the abrasive/water mixture exits , cutting is performed .

Once the jet has exited the nozzle , the energy is dissipated into the catch tank , which is usually full of water & debris from previous cut .

Water Jet and Abrasive Water Jet Machining

Components of AWJM

Catcher

(c) catcher plates (TiB2) (b) steel/WC/ceramic balls(a) water basin

Components of AWJM

Components of AWJM

fig . schematic diagram of ajm

Abrasive Jet Machining  

ajm featuresThere are main features of AJM

Obtainable tolerancesMaterial to machineMaterial thicknessAccuracy of tableStability of tableControl abrasive jet

Machine aspectsAround curvesInside cornerFeed rateAcceleration Nozzle focus Speed cuttingPump pressureHardness & thicknessSoftware controlling the motionPower at the nozzle

Types of abrasive materials

Different types of abrasive are used in abrasive jet machining like garnet , aluminum oxide , olivine , silica sand , silicon carbide ,etc .

Virtually any material can be cut by using abrasive jet machining method , i.e harder

materials like titanium to steel.Abrasive particles must be hard ,high toughness,

irregular in shape & edges should be sharp .

Advantages

Extremely fast setup & programming

No start hole requiredThere is only one toolLow capital costLess vibrationNo heat generated in work piece

Environmentally friendly

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• Extremely fast set-up and programming

• Very little fixturing for most parts

• Machine virtually any 2D shape on any material

• Very low side forces during the machining

• Almost no heat generated on the part

• Machine thick plates

Advantages of AWJM

Disadvantages Low metal removal rateDue to stay cutting accuracy is affectedAbrasive powder cannot be reusedTapper is also a problem

Modelling

Photographic view of kerf (cross section)

General Experimental conditions

Orifice – Sapphires – 0.1 to 0.3 mm

Focussing Tube – WC – 0.8 to 2.4 mm

Pressure – 2500 to 4000 bar

Abrasive – garnet and olivine - #125 to #60

Abrasive flow - 0.1 to 1.0 Kg/min

Stand off distance – 1 to 2 mm

Machine Impact Angle – 60o to 900

Traverse Speed – 100 mm/min to 5 m/min

Depth of Cut – 1 mm to 250 mm

Abrasive Jet Machining  

Modelling of material removal Material removal in AJM takes place due to brittle fracture of the work material due to impact of high velocity abrasive particles.

Modelling has been done with the following assumptions: (i) Abrasives are spherical in shape and rigid. The particles are characterised by the mean grit diameter (ii) The kinetic energy of the abrasives are fully utilised in removing material (iii) Brittle materials are considered to fail due to brittle fracture and the fracture volume is considered to be hemispherical with diameter equal to chordal length of the indentation (iv) For ductile material, removal volume is assumed to be equal to the indentation volume due to particulate impact.

Process Parameters and Machining Characteristics

Abrasive : Material – Al2O3 / SiC / glass beads Shape – irregular / spherical Size – 10 ~ 50 μm Mass flow rate – 2 ~ 20 gm/min

Carrier gas : Composition – Air, CO2, N2 Density – Air ~ 1.3 kg/m3 Velocity – 500 ~ 700 m/s Pressure – 2 ~ 10 bar Flow rate – 5 ~ 30 lpm

Abrasive Jet : Velocity – 100 ~ 300 m/s Mixing ratio – mass flow ratio of abrasive to gas Stand-off distance – 0.5 ~ 5 mm Impingement Angle – 600 ~ 900 Nozzle : Material – WC Diameter – (Internal) 0.2 ~ 0.8 mm Life – 10 ~ 300 hours

Abrasive Jet Machining  

effect of process parameters on MRR

Abrasive Jet Machining In AJM, generally, the abrasive particles of around 50 μm grit size would impinge on the work material at velocity of 200 m/s from a nozzle of I.D. of 0.5 mm with a stand off distance of around 2 mm. The kinetic energy of the abrasive particles would be sufficient to provide material removal due to brittle fracture of the work piece or even micro cutting by the abrasives.

Abrasive Jet Machining In Abrasive Jet Machining (AJM), abrasive particles are made to impinge on the work material at a high velocity. The high velocity abrasive particles remove the material by micro-cutting action as well as brittle fracture of the work material.

conclusion

The better performance , and the applications represented above statements confirm that

ABRASIVE JET MACHINING is continue to expand .

The new software’s used to minimize time and investments , there by making it possible for more manufacturers of precision parts to install AJM centers .