Abrasive JET Machining

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Abrasive Jet Machining (AJM)BYNISHTHA CHOUKSEY0201IP121045

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INTRODUCTION

Abrasive jet machining 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 .

Introduction(cotd.)A stream of abrasive grains (Al2O3 or SiC) is carried by high pressure gas or air (compressed).Impinges on the work surface at very high velocity through a nozzle of 0.3 to 0.5 mm diameter. Sand Blasting (SB) - a similar processThe major differences between are SB and AJM smaller diameter abrasives a more finely controlled delivery systemMaterial removal by mechanical abrasion action of the high velocity abrasive particles.Best suited for hole drilling in superhard materials. Typically used to cut, clean, peen, deburr, deflash and etch glass, ceramics and other hard materials.

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4Machining System

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A gas (Nitrogen, CO2 or air) is supplied at 2 8 kg/cm2Oxygen should never be used. (because, it causes violent chemical action with the workpiece chips or abrasive particles).Gas passes through a mixing chamber after filtration and regulation.In the mixing chamber, abrasive particles (10 40 m) are present and vibrated at 50 Hz.Amplitude of vibration to control the feed rate of abrasives.(Gas + abrasives) - passed through a 0.45 mm diameter tungsten carbide nozzle at a speed of 150 300 m/s.The nozzle is directed over the area to be machined. Machining System Contd.

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6Aluminium oxide (Al2O3) and silicon carbide (SiC) powders are used for heavy cleaning, cutting and debarring.Magnesium carbonate is recommended for use in light cleaning and etching.Sodium bicorbonate fine cleaning and cutting of soft materials.Commercial grade powders are not suitable because their sizes are not well classified. Also, they may contain silica which can cause a health hazard.Abrasive powders are not reused. because, contaminations and worn grits will reduce the machining rate (MRR).The nozzle stand off distance is 0.81 mm.Machining System Contd.

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7Relative motion between nozzle and workpiece can be manualOr automatically controlled using cam drives, tracer mechanisms or using computer controlled according to the cut geometry required.Masks of copper, glass or rubber can be used to concentrate the jet stream of abrasives to a confined area on the workpiece.Intricate and precise shapes can be produced using masks with corresponding contours.Dust removal or collecting equipment must be incorporated to protect the environment.Machining System Contd.

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Components of Abrasive Jet MachiningAbrasive delivery system

Control system

Pump

Nozzle

Mixing tube

Motion system

1. Abrasive Delivery SystemAuto abrasive delivery system has the capability of storing abrasive & delivery the abrasive to the bucket . Its works auto programming system by help of once measuring record & no adjustment or fine tuning system . High sensitive sensor gives extremely reliable & repeatable .

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2. Control systemThe 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 .

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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 .

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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 .

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NOZZLE

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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 .

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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

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Working ProcessHigh 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 .

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Water Jet and AbArasive Water Jet MachiningWJM - 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

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Water Jet and Abrasive Water Jet Machining

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Components of AWJMCatcher

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

(a) water basin

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Components of AWJM

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Components of AWJM

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Fig . Schematic diagram of AJM

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Abrasive Jet Machining

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AJM features There are main features of AJM

Obtainable tolerances

Material to machine

Material thickness

Accuracy of table

Stability of table

Control abrasive jet

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Machine aspectsAround curves

Inside corner

Feed rate

Acceleration

Nozzle focus

Speed cutting

Pump pressure

Hardness & thickness

Software controlling the motion

Power at the nozzle

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Types of abrasive materialsDifferent types of abrasive are used in abrasive jet machining like garnet , aluminium 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 .

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ModellingPhotographic view of kerf (cross section)

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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

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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 characterized 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.

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Process Parameters and Machining Characteristics

Abrasive : Material Al2O3 / SiC / glass beads Shape irregular / spherical Size 10 ~ 50 m 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

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effect of process parameters on MRR

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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.

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Advantages Extremely fast setup & programming

No start hole required

There is only one tool

Low capital cost

Less vibration

No heat generated in work piece

Environmentally friendly

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Disadvantages Low metal removal rate

Due to stay cutting accuracy is affected

Abrasive powder cannot be reused

Tapper is also a problem

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35The removal rate is slow.

Stray cutting cant be avoided (low accuracy of 0.1 mm).

The tapering effect may occur especially when drilling in metals.

The abrasive may get impeded in the work surface.

Suitable dust-collecting systems should be provided.

Soft materials cant be machined by the process.

Silica dust may be a health hazard.

Ordinary shop air should be filtered to remove moisture and oil.

Limitations

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ConclusionThe better performance , and the applications represented above statements confirm that ABRASIVE JET MACHINING is continue to expand .

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

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Abrasive JET Machining