Abrasive Jet Machining

(AJM)

BY

NISHTHA CHOUKSEY

0201IP121045

INTRODUCTIONAbrasive 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 process

The major differences between are SB and AJM

• smaller diameter abrasives

• a more finely controlled delivery system

Material 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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Machining System

A gas (Nitrogen, CO2 or air) is supplied at 2 – 8 kg/cm2

Oxygen 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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Aluminium 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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Relative motion between nozzle and workpiece – can be manual

Or 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.

Components of Abrasive Jet

Machiningo Abrasive delivery system

o Control system

o Pump

o Nozzle

o Mixing tube

o 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. PUMP Crankshaft & 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.Nozzle

All 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 tube

The 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 .

6.Motion system In 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 systems

Integrated table/gantry systems

Floor-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 AbArasive 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

Water Jet and Abrasive Water Jet

Machining

Components of AWJMCatcher

(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 tolerances

Material to machine

Material thickness

Accuracy of table

Stability of table

Control abrasive jet

Machine aspects Around curves

Inside corner

Feed rate

Acceleration

Nozzle focus

Speed cutting

Pump pressure

Hardness & thickness

Software controlling the motion

Power at the nozzle

Types of abrasive materials

Different 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 .

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 MachiningModelling of material removal

Material removal in AJM takes place due to brittle fracture of thework 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 arecharacterized by the mean grit diameter

(ii) The kinetic energy of the abrasives are fully utilised inremoving material

(iii) Brittle materials are considered to fail due to brittle fractureand the fracture volume is considered to be hemispherical withdiameter equal to chordal length of the indentation

(iv) For ductile material, removal volume is assumed to be equalto 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 Machiningeffect of process parameters on MRR

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.

Advantages o E x t r e m e l y f a s t s e t u p & p r o g r a m m i n g

o N o s t a r t h o l e r e q u i r e d

o T h e r e i s o n l y o n e t o o l

o L o w c a p i t a l c o s t

o L e s s v i b r a t i o n

o N o h e a t g e n e r a t e d i n w o r k p i e c e

o E n v i r o n m e n t a l l y f r i e n d l y

Disadvantages

o Low metal removal rate

o Due to stay cutting accuracy is affected

o Abrasive powder cannot be reused

o Tapper is also a problem

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o The removal rate is slow.

o Stray cutting can’t be avoided (low accuracy of 0.1 mm).

o The tapering effect may occur especially when drilling in metals.

o The abrasive may get impeded in the work surface.

o Suitable dust-collecting systems should be provided.

o Soft materials can’t be machined by the process.

o Silica dust may be a health hazard.

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

Limitations

Conclusion

o The better performance , and the applications

represented above statements confirm that

ABRASIVE JET MACHINING is continue to

expand .

o 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 .