1.0 INTRODUCTION

Today is the 21st century and in this century the competition has increased to

many folds. Over the last decade or so there has been considerable demand for cost

effective and high turnaround machine tool for use in industries. The demand for

composites in aerospace and industrial applications has been skyrocketing in the past

decade and the same trend will continue for the next two decades. Conventional

machine tools are often not suitable for machining composites and so are lasers. This

method of abrasive waterjet machining involves entraining abrasive particles carried

in air into a high velocity waterjet. This stream is directed by means of a suitably

designed nozzle on to the work piece to be machined [4]. Metal removal occurs due to

erosion caused by the abrasive particles impacting the work surface at high speed.

AWJs have several inherent merits that are unmatchable by most other machine tools:

Preservation of structural/chemical integrity – No HAZ and minimum surface

hardening and no tearing with minimum fraying.

Fatigue performance enhancement by combining AWJ and low-cost dry-grit

blasting.

Material independence – Even for nanomaterials that are integrated seamlessly

at the molecular level.

No contact tool to wear and break when machining extremely hard and tough

materials

For example- silicon carbide ceramic matrix composites.

Cost-effective with fast turnaround (no tooling or mask needed) for ones and

twos (R&D) and/or for thousands (production) – Complete a part from design

to finish in minutes to several hours, saving manufacturing jobs from

outsourcing.

Minimum limitation in part size – from macro to micro.

Multi-machining mode - roughing, parting, drilling, turning, milling, and

grooving, etc. in a single setup with no need for tool change and part transfer

No contact tool to break when machining extremely hard and tough materials.

Compatible with “Just-In-Time” practice for lean manufacturing

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The method is very inefficient with less than 3% of a waterjet’s energy being

transferred to abrasive particles. The process of entraining abrasive carried in air

becomes increasingly ineffective at jet diameters under 500 micro meters and ceases

to operate at jet diameters of 300 micro meters. As jet diameters less than 100 micro

meters are required for micromachining the current generation of abrasive waterjets

cannot be used to micromachine. Since the introduction of AWJs there has been no

paradigm shift in the way abrasive waterjets are generated but there have been very

substantial improvements in AWJ cutting performance. Improved cutting

performance is the result of incremental developments in ultra high pressure pumps,

cutting heads, software and control systems. Improving cutting performance,

combined with advances in machine tool design and innovative marketing and sales

activities, has resulted in AWJs becoming one of the three major non contact cutting

methods; the others being lasers and wire electric discharge machining (WEDM).

Probably the most important development leading to widespread commercialization

of AWJ based machine tools was the adoption of reacted tungsten carbide for cutting

head focus tubes – a paradigm shift in super hard materials technology by a major

chemical company (Dow Chemical Company), exploited by a nozzle manufacturer

(Boride Products Inc, now part of Kennametal Inc). A twenty times improvement in

focus tube life to 50 to 100 hours transformed the prospects of abrasive waterjets from

a niche market to a main stream machine tool [6].

2.0 PRINCIPLE Waterjets are fast, flexible, reasonably precise, and in the last few years have

become friendly and easy to use. They use the technology of high-pressure water

being forced through a small hole (typically called the “orifice” or “jewel”) to

concentrate an extreme amount of energy in a small area. The restriction of the tiny

orifice creates high pressure and a high-velocity beam, much like putting your finger

over the end of a garden hose. The inlet water for a pure waterjet is pressurized

between 20,000 and 60,000 Pounds per Square Inch (PSI) (1300 to 6200 bar). This is

forced through a tiny hole in the jewel, which is typically 0.007" to 0.020" in diameter

(0.18 to 0.4 mm). This creates a very high-velocity, very thin beam of water (which is

why some people refer to waterjets as "water lasers") travelling as close to the speed

of sound (about 600 mph or 960 km/hr) [3].

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An abrasivejet starts out the same as a pure waterjet. As the thin stream of water

leaves the jewel, however, abrasive is added to the stream and mixed. The high-

velocity water exiting the jewel creates a vacuum which pulls abrasive from the

abrasive line, which then mixes with the water in the mixing tube. The beam of water

accelerates abrasive particles to speeds fast enough to cut through much harder

materials. The cutting action of an abrasivejet is two-fold. The force of the water and

abrasive erodes the material, even if the jet is stationary (which is how the material is

initially pierced). The cutting action is greatly enhanced if the abrasivejet stream is

moved across the material and the ideal speed of movement depends on a variety of

factors, including the material, the shape of the part, the water pressure and the type of

abrasive. Controlling the speed of the abrasivejet nozzle is crucial to efficient and

economical machining. The most commonly used abrasive is garnet because of its

optimum performance of cutting power versus cost and its lack of toxicity. It is also a

good compromise between cutting power and wear on carbide mixing tubes. There

are two types of garnet that are generally used: HPX® and HPA®, which are

produced from crystalline and alluvial deposits, respectively.13 HPX garnet grains

have a unique structure that causes them to fracture along crystal cleavage lines,

producing very sharp edges that enable HPX to outperform its alluvial counterpart.

There are other abrasives that are more or less aggressive than garnet [2].

3.0 WORKING Intensifier, shown in Fig. 1 is driven by a hydraulic power pack. The heart of the

hydraulic power pack is a positive displacement hydraulic pump. The power packs in

modern commercial systems are often controlled by microcomputers to achieve

programmed rise of pressure etc. The hydraulic power pack delivers the hydraulic oil

to the intensifier at a pressure of ph . By using direction control valve, the intensifier

is driven by the hydraulic unit. The water may be directly supplied to the small

cylinder of the intensifier or it may be supplied through a booster pump, which

typically raises the water pressure to 11 bar before supplying it to the intensifier.

Sometimes water is softened or long chain polymers are added in “additive unit”.

Thus, as the intensifier works, it delivers high pressure water. As the larger piston

changes direction within the intensifier, there would be a drop in the delivery

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pressure. To counter such drops, a thick cylinder is added to the delivery unit to

accommodate water at high pressure. This is called an “accumulator” which acts like

a “fly wheel” of an engine and minimises fluctuation of water pressure. High-pressure

water is then fed through the flexible stainless steel pipes to the cutting head. It is

worth mentioning here that such pipes are to carry water at 4000 bar (400 MPa) with

flexibility incorporated in them with joints but without any leakage. Cutting head

consists of orifice, mixing chamber and focussing tube or insert where water jet is

formed and mixed with abrasive particles to form abrasive water jet.

Fig-1 Intensifier-Schematic

Fig. 2 shows a cutting head or jet former both schematically and photographically.

Typical diameter of the flexible stainless steel pipes is of 6 mm. Water carried

through the pipes is brought to the jet former or cutting head. The potential or

pressure head of the water is converted into velocity head by allowing the high-

pressure water to issue through an orifice of small diameter (0.2 – 0.4 mm). The

velocity of the water jet thus formed can be estimated, assuming no losses as using

Bernoulli’s equation, pw is the water pressure and ρw is the density of water. The

orifices are typically made of sapphire. In WJM this high velocity water jet is used for

the required application where as in AWJM it is directed into the mixing chamber.

The mixing chamber has a typical dimension of inner diameter 6 mm and a length of

10 mm. As the high velocity water is issued from the orifice into the mixing chamber,

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low pressure (vacuum) is created within the mixing chamber. Metered abrasive

particles are introduced into the mixing chamber through a port.

Fig-2 Schematic and photographic view of the cutting head

Fig. 3 schematically shows the mixing process. Mixing means gradual entrainment

of abrasive particles within the water jet and finally the abrasive water jet comes out

of the focussing tube or the nozzle. During mixing process, the abrasive particles are

gradually accelerated due to transfer of momentum from the water phase to abrasive

phase and when the jet finally leaves the focussing tube, both phases, water and

abrasive, are assumed to be at same velocity. The mixing chamber is immediately

followed by the focussing tube or the inserts.

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Fig-3 Schematic view of mixing process

The focussing tube is generally made of tungsten carbide (powder metallurgy

product) having an inner diameter of 0.8 to 1.6 mm and a length of 50 to 80 mm.

Tungsten carbide is used for its abrasive resistance. Abrasive particles during mixing

try to enter the jet, but they are reflected away due to interplay of buoyancy and drag

force. They go on interacting with the jet and the inner walls of the mixing tube, until

they are accelerated using the momentum of the water jet.

4.0 PROCESS PARAMETERS: The variables that influence the rate of metal removal and accuracy of machining

in this process are:

Carrier Gas:

Carrier gas, to be used in abrasive jet machining, must not flare excessively when

discharged from the nozzle into the atmosphere. Further, the gas should not be

nontoxic, cheap, easily available and capable of being dried and cleaned without

difficulty. The gasses that can be used are air, carbon dioxide or nitrogen. Air is most

commonly used owing to easy availability and little cost.

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Types Of Abrasive:

The choice of abrasive depends upon the type of machining operations, for

example, roughing, finishing etc., work material and cost. The abrasive should have a

sharp and irregular shape and be fine enough to remain suspended in carrier gas and

should also have excellent flow characteristics.

Grain Size:

The rate of metal removal depends on the size of the abrasive grain. Finer grains

are less irregular in shape, and hence, possess lesser cutting ability. Moreover, finer

grains tend to stick together and choke the nozzle. The most favourable grain size

ranges from 10-50 µ.

Jet Velocity:

The kinetic energy of the abrasive jet is utilized for the metal removal by erosion.

Finnie and Sheldon have shown that for erosion to occur, the jet must impinge the

work surface with a certain minimum velocity. Figures 4 and 5 shows the effect of

nozzle pressure on the rate of metal removal.

Fig-4 Pressure vs. MRR Fig-5 Pressure vs. MRR(Grain Size- 40µ, 24µ, 10µ) (Abrasive- AL 2O3

Work material- cemented carbideGrain Size- 40µ)

Mean Number Of Abrasive Grains Per Unit Volume Of Carrier Gas:

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An idea about mean number of abrasive grains per unit volume of the carrier gas

can be obtained from the mixing ratio M. A large value of M should result in higher

rates of metal removal but a large abrasive flow rate has been found to adversely

influence jet velocity, and may sometimes even clog the nozzle. Thus, for the given

conditions, there is an optimum mixing ratio that leads to a maximum metal removal

rate.

Work Material:

AJM is recommended for the processing of brittle materials, such as glass,

ceramics, refractories, etc. Most of the ductile materials are practically unmachinable

by AJM.

Stand Off Distance:

A large SOD results in the flaring up of the jet which leads to poor accuracy. Fig 6

shows the relationship between the SOD and the rate of material removal. Small metal

removal rates at a low SOD is due to a reduction in nozzle pressure with decreasing

distance, whereas a drop in material removal rate at large SOD is due to a reduction in

the jet velocity with increasing distance.

Fig-6 SOD vs. MRRAbrasive- AL2O3, Grain Size- 40µ, Work Material- Glass,

Pressure- 0.03)

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Nozzle Design:

The nozzle has to withstand the erosive action of abrasive particles, and hence,

must be made of materials that can provide high resistance to wear. The common

materials for nozzle are sapphire and tungsten carbide. The nozzle should be so

designed that the pressure loss due to bends, friction, etc. is as little as possible.

Shape Of Cut:

The accuracy of machining is also dependent upon the shape of cut. It may not be

possible to machine components with sharp corners because of stray cutting in this

process [7].

5.0 NOZZLE WEAR IN AWJM: As shown in Fig. 7 the focusing nozzle, which is the most critical part in AWJ

cutting systems, is subjected to two modes of wear:

Impact erosion beginning at the entry cone down to approximately one third

of the nozzle length;

Sliding erosion in the downstream area, where particles travel parallel to the

wall and the wear mode shifts from shallow impact to abrasion.

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Fig-7 Interaction of a particle containing waterjet with AJW nozzle As the bore diameter of the nozzle increases owing to wear of the material, the

coherence of the jet beam decreases, which ultimately leads to failure of the nozzle:

The widening jet beam increases the kerf width, i.e. the width of the cut, and

decreases the cutting efficiency. The material of a brittle work piece is removed

during AWJ cutting owing to a network of cracks created by the direct impact of

erodent particles and by adjacent impacting particles. The crack network model Fig.

8, assumes a vertical impact of erodent particles, which fragment during the impact.

During impact two types of cracks are produced:

Median and radial cracks normal to the surface,

Lateral cracks which are parallel to the surface.

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Fig-8 Brittle material removal by impact of a waterjet containing abrasive particles

The interaction of lateral and radial cracks is considered to result in material

removal, i.e. the spallation of tiny chips off the surface. Boron carbide displayed the

highest calculated wear resistance as compared with hard metals, alumina-based

ceramics, silicon nitride based ceramics and some grades of silicon carbide ceramics.

However, in erosion experiments boron carbide showed best resistance only at low

impact angles ( <20º), followed by a “brittle response” regime of erosion wear, in

which wear rates peak at 90º erodent impact angle (see Fig. 9). The technical

superiority of boron carbide as a blast nozzle material is well established; therefore its

poor erosion resistance at high impact angles does not compromise its effectiveness in

this particular application.

Fig-9 Erosion rate as a function of the impact angle

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However, owing to the extreme speeds of abrasive particles, when entrained in a

waterjet (700 m s-l), the bore entry zone of waterjet nozzles is heavily loaded at

impact angles of 15-45º. For very brittle materials such as boron carbide it is well

known, that the critical load for generating lateral cracks (“cracking threshold” P,) is

orders of magnitude lower than for hard metals or “tough ceramics” [1].

6.0 COMPARISON BETWEEN WJM, AWJM AND AJM:

Sr.No. AWJM WJM AJM

1. Abrasive waterjet

machining uses a mixture

of water, abrasive and

sometimes air.

Waterjet machining

uses steam of pure

water to cut work

piece.

Abrasive jet

machining uses a

mixture of air and

abrasive.

2. AWJM can be used to

cut harder materials like

titanium, stainless steel,

ceramic tile etc.

WJM cannot machine

metallic alloy but can

cut polymers, leather

like materials.

AJM can cut almost

all hard and brittle

materials.

3. Flaring problem of

abrasive –water mixture

while coming out of the

orifice is minimum, so no

stabilizer is used.

Flaring problem is

there, so stabilizer is

must.

Flaring problem is

observed.

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4. Nozzle wear is a major

problem in case of

AWJM.

Nozzle wear is not

observed in case of

WJM.

Nozzle wear rate

are influenced by

the size and

distance of nozzle.

5. Finishing is very good as

compared to other

machining methods.

Finishing is better

than other processes

but not as good as

AWJM.

Finishing is not that

good.

6. Used at much higher

pressure greater than

4000 bar.

Used at high pressure

but less than 4000 bar

Lesser pressure as

compared to

AWJM.

7. Disposal of waste is a

major problem in case of

AWJM.

Pure water is used in

this process so no

waste is produced.

Comparatively

greener.

8. Expensive as compared

to other processes.

Relatively cheaper Initial cost is low.

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7.0 ADVANTAGES: No cutter-induced distortion.

Low cutting forces on work piece.

Limited tooling requirements.

Little to no cutting burr.

Small kerf size (.020"-.045") reduces material scrap.

No heat-affected zone.

Localizes structural changes.

No cutter-induced metal contamination.

Eliminates thermal distortion.

Minimal delimitation of edge cut surfaces.

No fraying of edge cut surfaces.

No thermally induced cracking.

No splintering.

No slag or cutting dross.

Precise multiplane cutting of contours, shapes, and bevels of any angle.

Reduced need for secondary finishing.

Dust particles are not produced that can degrade environment.

Used to produce prototype parts efficiently.

Easily automated for production use.

Lighter in weight as compared to other machining processes.

Thick sections can be cut.

Highly precise.

No start hole is required.

Safe operation.

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8.0 APPLICATIONS:

Printed Circuit Boards:

For circuit boards, abrasive waterjet cutting is mostly used to cut out smaller

boards from a large piece of stock (see Fig.-10). This is a desired method, since it has

a very small kerf, or cutting width, and does not waste a lot of material. Another

benefit is that waterjet cutting does not produce the vibrations and forces on the

board.

Fig-10 Cutting of PCB by AWJM

Wire Stripping:

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 waterjet

cutting (see Fig- 11). Since the abrasive waterjet exerts such a small force on the food,

it does not crush it, and with a small kerf width, very little is wasted.

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Fig-11 Cutting of food products by AWJM

Wood Cutting:

Woodworking is another application that abrasive waterjet 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.

Fig- 12 Wood cutting by abrasive waterjet machining

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Tool Steel:

For abrasive waterjet 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 by-product (see

Fig-13).

Fig-13 Cutting of tool steel by AWJM

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9.0 CONCLUSION:

With commercial aerospace and wind energy (turbine blades) being the leading

drivers and microelectronics not far behind, the composites industry has seen strong

double-digit growth. Since composites are not only expensive but also difficult to

machine with established machine tools, there is a considerable need for a cost

effective, fast turnaround and damage free machine tool to meet the anticipated

demand. Abrasive-waterjets have shown to possess inherent merits that were

unmatchable by most other machine tools. For patterns that do not need internal

piercing, AWJs have shown to be a preferred tool for machining composites. Waterjet

technology has inherent technological and manufacturing merits that make it

suitable for machining most materials from macro to micro scales. It has been

established as one of the most versatile precision machining tools and has proven

amenable to micromachining. This technology has emerged as the fastest growing

segment of the overall machine tool industry in the last decade. Efforts are being

made to further downsize μAWJ nozzles for machining features around 100 and 50

μm. This seminar gives a detail study of AWJM.

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