Abrasive Water Jet Machining

7/14/2019 Abrasive Water Jet Machining

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Abrasive Water Jet Machining


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Introduction

Abrasive water jet (AWJ) cutting is a non-

conventional machining process in which,

abrasive grains ,entrained in a high speed

water jet ,collide with the work piece and

erode it.

A water jet is used to accelerate the

abrasive grains and to assist the material

removal process.

WJM is suitable for cutting plastics, rubber

insulation, and most textiles whereas

,harder materials such as glass, ceramics,

concrete and tough composites can be cut

by adding abrasives to the water jet.


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AWJM uses a pressure of 4.2 bar to accelerate a large volume of a water (70

percent) and abrasive (30 percent) mixture up to a velocity of 30 m/s.


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Working Principle

In AWJM, the water jet stream accelerates abrasive particles, not the

water, to cause the material removal.

The velocity of the water jet is up to 900m/s.

It is obtained by a high pressure water pump with a typical pressure value

of 400MPa.

The pressurized water is forced through an orifice made of sapphire.

The velocity of the water jet thus formed can be estimated, assuming no

losses as

vwj= (2pw

/ w)1/2 using Bernoullis equation.

Where, pwand w

are pressure and density of water respectively.

The important parameters of the abrasives are the material structure and

hardness, grain shape, size, and distribution.


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Types of AWJM Systems

There are two different systems of abrasive water jet. One is called

entrained abrasive water jet system and another one called suspended

abrasive water jet system.

Entrained abrasive water jet is much more popular and widely used in the

industry.

Entrained water jet, is basically composed of three phases, abrasive

+water + air.

Suspended water jet is composed of abrasive and water.

In suspension AWJM, preformed mixture of water and abrasive particles is

pumped to a sufficiently high pressure and stored in pressure vessel.

Then the premixed high-pressure water and abrasive is allowed to

discharge from a nozzle to form abrasive water jet.


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After the pure water jet is created, abrasives are added using either the

injection or suspension methods as shown.

Injection jet Suspension jet


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There are three different types of suspension AWJ formed by direct,

indirect and Bypass pumping method.


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Setup of AWJM


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The Basic Machining System of AWJM

Water delivery

Abrasive hopper and feeder

Intensifier

Filters

Mixing chamber

Cutting nozzles Catcher


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AWJM SYSTEM


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Intensifier

Intensifier is a positive displacement hydraulic pump.

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.

As the larger piston changes direction within the intensifier, there would

be a drop in the delivery 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 ofwater pressure


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Hydraulic power packdelivers hydraulic oil at a pressure ph to Intensifier.

Thus, pressure amplification at the small cylinder takes place as follows:


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Abrasive Hopper

Waterjet ADS (Abrasive Delivery

Systems) Hoppers used with

waterjet tables, precisely

monitor the availability of

sufficient abrasive closely during

the entire waterjet cutting

process.

Abrasive is stored in the abrasive

storage vessel and pneumatically

transferred to the abrasive

metering assembly.


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Mixing Chamber

Mixing means gradual entrainment ofabrasive 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 dueto 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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The abrasives 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. During mixing process momentum loss

occurs as the abrasives collide with

the water jet and at the inner wall of

the focussing tube multiple times

before being entrained.

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


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Abrasive Jet Nozzle

Purpose of the abrasive jet nozzle is to provide efficient mixing of

the abrasives and the water jet and to form the high-velocity abrasive

water-jet combination.

There is a difference between a pure water jet nozzle and anabrasive jet nozzle. With the abrasive jet nozzle, an opening in the

side of the nozzle allows for the introduction of the abrasive to the

high-pressure water stream. The two are mixed in a mixing tube and

then exit the nozzle. With a pure water jet nozzle, there is no opening

and no mixing tube and the high-pressure water is directed to thematerial after it exits the jewel.


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Nozzle Material- sapphire, tungsten carbide, boron carbide, ruby or diamond

orifice.

The nozzles are subject to wear and their cost must be taken intoconsideration for job costing. The orifice of a mixing tube will typicallyincrease in diameter by about 0.001" per hour of cutting because of theerosion of the abrasive flowing through the nozzle. If this wear occurs in aneven fashion, up to 120 hours of cutting can be realized with a nozzle.Controls allow for compensating for this wear by allowing the operator tochange the tool offset.

If the wear of the nozzle is not symmetrical, the water jet stream will alsonot be round. This will result in wider tolerances when cutting in a certaindirection.

The tolerance requirements of a job will dictate the actual number of hoursa user can expect from a nozzle. The nozzle can, of course, be set aside andused later for looser tolerance jobs.

Orifice on right is

off-center


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Types Of Nozzle

Two major design concepts are currently used for the design of abrasive jet

nozzles.

Single-jet side feed nozzle:-This design is based on a central water jet with abrasives fed into the mixing

chamber from the side.

This configuration is easily machined and can be made quite small, which is an

advantage when cutting in tight locations.

But this concept does not provide for optimal mixing efficiency and usually

experiences rapid wear of the exit section.

The major advantage with this system is this that, it incorporates a central,

conventional water jet, the abrasive flow can be stopped and the system will

function as a conventional WJM system.


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Single-jet side feed nozzle


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Multile jet


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Catcher

Once the abrasive jet has been used for machining, they may have

sufficiently high level of energy depending on the type of application.

Such high-energy abrasive water jet needs to be contained before they can

damage any part of the machine or operators.

Catcher is used to absorb the residual energy of the AWJ and dissipate the

same.


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There are three different types of catcherwater basin type, submerged

steel balls and TiB2 plate type.


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Working Parameters in AWJM

The general domain parameters of machining system is given below:

OrificeSapphires0.1 to 0.3 mm

Focussing TubeWC0.8 to 2.4 mm Pressure2500 to 4000 bar

Abrasive flow rate - 0.1 to 1.0 kg/min

Stand off distance1 to 2 mm

Machine Impact Angle60o to 900

Traverse Speed100 mm/min to 5 m/min

Depth of Cut1 mm to 250 mm


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In pure WJM- commercially pure water (tap water) is used for

machining purpose.

In AWJM- water along with abrasives and stabilisers is used.

ABRASIVES USED:-

Garnetis by far the most commonly used abrasive. It is environmentally clean,

contains no free silica, and combines good cutting ability with reasonable wear.Garnet has three basic structural components. They are

Almandine (Fe3, Al2 (SiO4)3), Pyrope (Mg3Al2(SiO4)3) and Spessartite

(Mn3Al2(SiO4)3). The hardness of garnet abrasive particles of Almandine, Pyrope

and Spessartite are 7-7.5 Mohs, 7.5 Mohs and 7-7.5 Mohs respectively. In industries

80 mesh garnet is a popular abrasive.Silicon Carbide (SiC) is known for its very high hardness and abrasion resistance. It

is primarily used for grinding nonferrous materials such as brass, copper, bronze and

Aluminum.

Aluminum Oxide (Al2O3) is another popular abrasive used in AWJM. It is used for

high-speed penetration in tough materials without excessive shedding or fracturingof the grains.

Abrasives Used In AWJM


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Due to its high Mohshardness, aluminum oxide has been rarely used, for

cutting of very hard materials. Because of its high hardness aluminum

oxide rapidly wears out the nozzle and is expensive to operate.

Other less commonly used abrasives are olivine sand, silica sand and slagby-products.Most shops use the abrasive once and then it is disposed as

land fill waste.

Stabilisers:-

The reason for using stabilisers is that at high velocity ,water jet that is

discharged from the orifice, tends to entrain atmospheric air and flares

out decreasing its cutting ability.

Hence, quite often stabilisers (long chain polymers) that hinder the

fragmentation of water jet are added to the water.

The stabilisers used are Xanthan , Zycoprint ,Superwater etc.


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In AWJ machining of ductile materials, material is mainly removed by low

angle impact of abrasive particles.

Further at higher angle of impact, the material removal involves plastic

failure of the material at the sight of impact.

In AWJ machining of brittle materials, material would be removed due to

crack initiation and propagation because of brittle failure of the material.

In abrasive water jet machining, the material removal rate may be

assumed to be proportional to the power of the water jet.

Material Removal in AWJM


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The ability for AWJM to penetrate very thick material may be due to re-

entrainment of abrasive particles in the jet after the initial impacts at the

top of the cut.

The cutting action of an abrasive jet 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 abrasive jet 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 abrasive jet nozzle is crucial to efficient and

economical machining.


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Material Removal Rate

Where u - Constant that depends on the work material

cd - Discharge coefficient of the orifice

do - diameter of the orifice pw - Pressure of water

w- Density of water


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Process Parameters


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Water jet pressure

Pc is the minimum critical

pressure required to cut the

material.

A minimum critical

pressure Pc exits because of

the minimum abrasive

particle velocity required to

cut specific materials.

The value of Pc for mildsteel is between 20.7 and

27.5 Mpa.


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Abrasive Flow Rate And Particle Size

Abrasive flow rate versus depth of cut is a linear relationship up to a

point

Above a critical flow rate, the cutting efficiency decreases.

This is because of the fact that, as the abrasive flow rate increases

( with a fixed water flow rate), particle velocity begins to decrease

faster than the rate at which the number of abrasive particleimpacts increase.

The most common abrasive particle sizes used for AWJM range

from 100 to 150 grit

An optimum abrasive particle size also exists for each particularnozzle mixing chamber configuration.


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Stand-off-distance

Data generated by some researchers indicate that depth ofcut is approximately linear relative to SOD.

Increasing SOD decreasing the depth of cut.

When mixing is efficient and process parameters are correct,

a deviation in SOD of up to +-12.7mm can be toleratedwithout degradation of the cut quality.

If SODs are increased to a distances of about 80mm, the

process will no longer cut but will efficiently clean and de-

scale surfaces.


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The cut generated by an AWJM is called a

kerf.

Top of the kerf (bt) is wider than the

bottom of the kerf (bb).

btis equal to the diameter of AWJ or

AWJM.

Diameter of AWJ is equal to the diameter

of the focussing tube or the insert if the

stand-off distance (SOD) is around 1 to 5

mm.

Taper angle of the kerf can be reduced by

increasing the cutting ability of the AWJ.

Factors Affecting Precision & Quality Of The Waterjet


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Effect of feedrate upon kerf angle

Kerf angle, or bevel, refers to the dimensional difference between the top and

bottom of the cut cross-section. Cutting too fast will result in a wider kerf width at the top of the cut cross-

section and a narrower kerf width at the bottom of the zone.

In the picture, from left to right, cuts were done at 26 inches per minute (ipm),14 ipm and 9.7 ipm (660 millimeters per minute [mm/min], 355 mm/min and246 mm/min). All other parameters were held constant.


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Drag of waterjet stream and cut quality

Increasing feed-rate results in increased trail back of waterjet stream. This concept is shown below. Rougher edge quality is the result of more

shearing action versus erosion action of abrasive at slower speeds.

Increasing trail back of waterjet steam with increasing speed


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For best cut quality, an optimum distance between the nozzle and thework piece should be maintained.

Typically, between 0.040" and 0.060" (1.0 - 1.5 mm) is the optimum

height for abrasive waterjet cutting. As the distance increases above

0.060", rounding on the top edge of the cut will result.

This occurs because the waterjet stream looses coherence as it travelsthrough open air.

Increased nozzle height will also result in increased kerf angle.

Effect of nozzle height


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It may be observed that the surface quality at the top of the kerf is rathergood compared to the bottom part.

At the bottom there is repeated curved line formation.

At the top of the kerf, the material removal is by low angle impact of theabrasive particle; where as at the bottom of the kerf it is by plastic failure.

Striation formation occurs due to repeated plastic failure.

Thus, in WJM and AWJM the following are the important product qualityparameters.

Striation formation.

Surface finish of the kerf.

Tapering of the kerf.

Burr formation on the exit side of the kerf.

Striation marks


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Advantages

Higher feed rates/faster machining

High speed

High accuracy

Robust construction

No heat generated.

Leaves a satin smooth finish, thus reducing secondary operations.

Unlike machining or grinding, waterjet cutting does not produce any dustor particles that are harmful if inhaled.

The kerf width in waterjet cutting is very small, and very little material iswasted.

Waterjet cutting can be easily automated for production use.

Waterjet cutting does not leave a burr or a rough edge, and eliminatesother machining operations such as finish sanding and grinding.


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Disadvantages

Slow material removal rate.

The process produces a tapered cut.

Thick parts can not be cut with waterjet cutting and still hold dimensionalaccuracy. If the part is too thick, the jet may dissipate some, and cause itto cut on a diagonal, or to have a wider cut at the bottom of the part thanthe top.

Waterjet lag


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Taper is also a problem with waterjet cutting in very thick materials. Taper

is when the jet exits the part at a different angle than it enters the part,

and can cause dimensional inaccuracy. Decreasing the speed of the head

may reduce this, although it can still be a problem.

Stream lag caused inside corner damage tothis 1-in.-thick stainless steel part. The exit

point of the stream lags behind the entrancepoint, causing irregularities on the insidecorners of the part. The thicker the materialis or the faster an operator tries to cut it, the

greater the stream lag and the morepronounced the damage.


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Applications

Materials that are reflective, conductive, heat resisting, or heat sensitive are

ideal candidates for abrasive water jet cutting. As the material thickness

increases AWJ becomes the preferred cutting technique, especially where

accuracy must be maintained.

Edge finishing De-burring

Polishing

Paint removal

Cleaning

Cutting soft materials

Cutting frozen meat

Textile, Leather industry


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Computer Controlled AWJM

Advanced water jet and AWJ machines are now available where the

computer loads a CAD drawing from another system.

The computer determines the starting and end points and the sequence of

operations.

The operator then enters the material type and tool offset data.

The computer determines the feed rate and performs cutting.

Other machining systems operate with a modem and CAD/CAM

capabilities that permits transfer from CATIA, AUTOCAD, IGES, and DXF

formats.

The computer runs a program that determines, in seconds, how to

minimize the waste when cutting blocks or plates.


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Abrasive Water Jet Machining