Advanced Manufacturing Processes: Waterjet

Water jet machining

Index

Water jet machining ......................................................................................................................................... 1

Introduction ................................................................................................................................................. 1

Process parameters ...................................................................................................................................... 3

Outflow regimes ........................................................................................................................................... 9

Kerf shape and cutting quality .................................................................................................................... 11

Optimal selection of parameters and innovative solutions ........................................................................ 15

Introduction

The physical principle of this process is the conversion of the fluid pressure energy into kinetic energy.

Hypotheses are incompressibility of the fluid, no friction with walls and laminar flow. Abrasive waterjet

exploits the presence of particles: in this case, water has the only function of accelerating them. The

temperature of the process must be lower than

80°C. WJ and AWJ are used for a great number of

processes such as cutting, drilling, surface

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cleaning, decoating and forming . The great

advantage of WJ is the possibility of a cold cut:

water is a vector for energy but also a cooling

system, impeding structural alterations in

metallic materials and degradation in polymers;

cutting doesn’t leave burrs and there is not

contact between tool and workpiece. Even if

material removal is possible, forces are small and

the clamping system is of low importance; the

process can start everywhere and achievable

geometries are different, with small tolerances.

The process can be automatic. Alignment is of

high importance: all the components are grinded

for high precision and nylon inserts help to keep

positions stable.

The main components are:

 Water treatment system: its principle is to use softening, deionization, filtering and reverse osmosis

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to depurate water . A good quality assures hundreds of hours of operation without changing the

tool.

1 Water jet forming exploit the impact of a jet to give plastic deformation to the workpiece.

2 The particles suspended in the water pipes would lead to an accelerated wear of the mechanical parts of the intensifier,

valves and the sapphire orifice; moreover, calcium and magnesium salts dissolved in the water may deposit thus

damaging the parts and the hydraulic efficiency of the system; finally the chlorides and sulfates are responsible for the

corrosion of metal parts. The osmosis process is the passage of water from a less concentrated solution to a more

concentrated solution through a semi-permeable membrane (i.e. able to stop only the particles above a certain size).

To purify the water it is necessary to get a reverse stream to extract fluid from a solution yet full of impurities. To do

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 Pumping system: a hydraulic fluid is pumped into the intensifier, whereas depurated water is

transported into the system.

 High-pressure water transport system and connection, made of both rigid and flexible pipes.

 Cutting head: it contains the primary nozzle or orifice that transforms the pressure energy into kinetic

energy. It is really important that the jet has coherence in order to keep the energy on a small area:

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a focuser is here introduced . In case of AWJ, the mixing chamber is present, otherwise particles

would be rejected by the water flux. To assure wear,

corrosion and high pressure resistance, the primary nozzle is

made of sapphire, the mixing chamber and the focuser in

carbides, typically WC or BC (100 h of tool life). Misalignment

between focuser and primary nozzle is cause of asymmetrical

erosion. Generally, the focuser is subjected to erosion in the

upper part in contact with the mixing chamber and to

abrasion in the lower part where particles embedded in the

flux have lower impact angles. A small divergence of pure water

from the primary nozzle may decrease rejection and reflection

of particles entering the focuser. Regular shapes and perfect

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holes are needed to avoid much friction and to guarantee

perfect alignment.

 Abrasive feeding system if AWJ. A rubber canal inside a rigid

chamber composes it. Increase the pressure oil in the rigid

chamber allows the deformation of the canal and the

modulation of abrasive quantity. The particles must have high

hardness, homogeneous dimension and low hygroscopic

behaviour.

 Intensifier: it increases the pressure at low frequency, 2 Hz,