Waterjet

Question 1

What are the two types of waterjet?

Answer 1

Abrasive Waterjet (AWJ) and Pure Waterjet (PWJ)

Question 2

How big are the cutting forces in the Waterjet cutting process?

Answer 2

Small, only a few tens of Newton

Question 3

What is the purpose of ‘bridges’ on waterjet?

Answer 3

The purpose is to avoid the cut material to move after the completion of the process. The ‘bridges’ are material remaining that can be broke afterwards by hand.

Question 4

From which thickness and on does the waterjet become competitive against laser for example?

Answer 4

After 20-30 mm thickness and upwards

Question 5

Which thicknesses is the waterjet able to cut?

Answer 5

It can cut 400 mm stainless steel but reducing the feedrate to 1mm/min

Question 6

Is there any thermal affectation on the cut material while using waterjet?

Answer 6

No, the waterjet technology is a cold cutting technology

Question 7

Which kind of materials are the typical working field of waterjet?

Answer 7

Fragile and composite materials

Question 8

Other than straight cutting, is there any other important application for waterjet?

Answer 8

Yes, drilling is another important application for waterjet

Question 9

What is the strategy to drill composite materials using waterjet?

Answer 9

Because of the jet of water, some composite materials suffer from delamination so a ‘normal’ mechanical drilling is used in this case to start the process. For this reason some waterjet machines mount a drilling head as well.

Question 10

What is the strategy to drill glass using waterjet?

Answer 10

To create the hole, it is necessary to reduce the pressure of the waterjet (for example 70 MPa) in order to reduce the thrust force (which is high when the hole is blind). Afterwards, the pressure can be increased to ‘normal’ parameters of around 250 MPa or more.

Question 11

What is the strategy when drilling metals using waterjet?

Answer 11

Metals don’t have issues to overcome when drilling, nonetheless, it is important to consider a deviation of the dimension due to water deflection while the hole remains blind.

Question 12

What are the main components of the waterjet system?

Answer 12

Water treatment system, Water pump system, High pressure transport system and connections, Cutting head, Abrasive feeding system, Handling system, Catcher

Question 13

What are the steps of water treatment for WJ?

Answer 13

Softening and deionization: The water passes on a stage of cationic resins which reduces the concentration of metal ions of calcium and magnesium (hardness) while other types of resins reduce the presence of chlorides and sulfates. Filtering stage: An activated charcoal filter removes suspended solid particles and the residual chlorine. Reverse osmosis (optional)

Question 14

How is the pumping system composed for the waterjet process?

Answer 14

It is composed by two pressure circuits: one of low pressure of around 20 MPa and a high pressure one with pressures up to 400 MPa (typical pressure, some machine reach up to 600 MPa)

Question 15

What is the attenuator?

Answer 15

The attenuator is a reservoir that stores water at high pressure which can be released or stored according to the flow rate needed by the cutting head.

Question 16

How does a double acting pump work?

Answer 16

The double acting pump is able to pressurize water in both directions of the stroke. One of the important characteristics of this kind of pump is its capability to deviate oil flow rate when no water flow is needed, for example when moving the head from the final position of one piece and the start position of the next one.

Question 17

What is the drawback of a double acting pump?

Answer 17

Its efficiency as it only reaches efficiency of around 60 - 70 %

Question 18

What is the weak point of the waterjet pump?

Answer 18

The sealing at the plunge as the pressures reached are of around 400 MPa which represents more or less a 40 Km tall body of water over a single component.

Question 19

Which other types of pumps exist?

Answer 19

Single-acting pumps. In this case the pump only pressurizes the water on one direction of the stroke. This kind of pumps are less reliable and are usually used in groups of pumps arranged in-phase to overcome the pressure drops generated by the movement of the plunge to the start position of displacement. Nonetheless, this allows to get rid of the accumulator.

Question 20

What is the typical intensification factor of a pump system used for waterjet?

Answer 20

Typical values are of around 20

Question 21

What is a direct pump system?

Answer 21

These are crankshaft pumps which don’t require accumulator using proper piston phasing. This kind of pumps are used when high flow rates are needed (of tens of liters per minute), for example, in the case of cleaning systems.

Question 22

What is the typical range of flow rate for a high power pumping system used in waterjet?

Answer 22

Typical ranges are from 3 to 4 liters per minute

Question 23

What is the function of the accumulator in the pumping system?

Answer 23

To reduce the pressure fluctuations caused by the pressure drop when alternating pumps are used.

Question 24

Are the pressure fluctuations decremental of cutting quality?

Answer 24

No, the fluctuations in small pressure drops are not decremental of quality but are contributors for the fatigue of the components involved in the high pressure circuits.

Question 25

What is another cause of the erratic pressure drop in the pressuring system?

Answer 25

The compressibility of water, which at high pressures cannot be considered uncompressible anymore and causes a drop of pressure of up to 15% at pressures of 400 MPa.

Question 26

How to increase the water flow rate with the pumping system?

Answer 26

By increasing the number of pumps used. Multiple pumps can feed systems that use more than 1 cutting head.

Question 27

What are the two types of pressure pipes?

Answer 27

Rigid: made out of stainless steel with typical inner diameter of 3 mm subjected to autofrettage. Flexible: made out of rubber and cords (steel cords). These are delicate and less reliable but allow tighter turns in the pumping lines.

Question 28

What is autofrettage in pressure pipes?

Answer 28

It is a process where the tube is subjected to higher pressures to deform the material in the plastic zone and create residual stresses only in the inner part of the tube. This residual stresses will compensate for the high pressures. In this way, the tubes are capable of withstand pressures close to the yield limit.

Question 29

What are the main features of the pure waterjet cutting head?

Answer 29

Pressure range: 200 - 300 MPa, Diameter: 0.08 - 0.2 mm, Jet velocity: around 700 m/s

Question 30

What are the main sections / parts of the pure waterjet cutting head?

Answer 30

Retaining nut, Capillarity, High pressure tube, Orifice, Orifice tube

Question 31

How can we obtain more coherent jets?

Answer 31

Using lower diameters, sharp orifice entrance and lower pressure. Smaller orifices produce a longer coherent length as they assure a lower Reynolds number.

Question 32

Considering the Bernoulli equation, what is the formula to calculate jet velocity and what are the hypothesis to get to this formulation?

Answer 32

The hypothesis are: Perfect fluid, Incompressible fluid, No exchanges of energy, Heavy fluid, Validity along each fluid particles trajectory, Kinematical properties depend only on spatial coordinates. The formula is v= √(2p/ρ)

Question 33

What are typical values of waterjet velocity?

Answer 33

Around 0.5 km/s or higher (well above Mach 1 with respect to sound speed in air)

Question 34

Which materials are commonly used for the orifice stone or insert?

Answer 34

Saphire or diamond which usually break without gradual wear as these are very brittle materials.

Question 35

In a typical waterjet company, the intensifier usually runs at its maximum pressure. Explain how that affects the cutting capacity and process speed

Answer 35

High jet power: By setting the pump to its maximum pressure, the jet power is maximized, increasing the cutting capacity and process speed. The jet power density depends mainly on pressure

Question 36

What are the main components of an abrasive waterjet cutting head?

Answer 36

Mixing chamber, orifice, abrasive port, focusing tube

Question 37

What are the typical technical specifications of an abrasive waterjet cutting head?

Answer 37

Orifice diameter: 0.2 - 0.33 mm Focuser diameter: 1 mm Working pressure up to 400 MPa Abrasive size: 170 μm (micrometers)

Question 38

How is the abrasive mixed with the waterjet?

Answer 38

It is mixed using the Venturi effect. The waterjet creates a vacuum from the abrasive chamber to the mixing chamber that moves the abrasive to the waterjet.

Question 39

What is the typical combination of orifice and focusing tube diameter for abrasive waterjet?

Answer 39

Orifice: 0.3 mm Focusing tube diameter: 1 mm

Question 40

What is the function of the focusing tube?

Answer 40

Promote the momentum transfer from waterjet to abrasive particles and create a homogeneous and collimated outflowing hydro-abrasive jet.

Question 41

What is the common working life of an abrasive tube?

Answer 41

Around 100 h

Question 42

What is the common material and dimensions of the focusing tube?

Answer 42

Material: Tungsten or boron carbide Length: 2, 3, 4 inches - 50, 76 and 100 mm Internal diameter: 0.76, 1 and 1.2 mm

Question 43

What produces the typical double barrel internal shape of a worn out focusing tube?

Answer 43

The abrasive particles statistically hit the walls of the focusing tube a couple of times before aligning to the axis of the waterjet creating this double barrel shape.

Question 44

In which section does the focusing tube present commonly the stronger wear?

Answer 44

At the entrance due to the reason explained in the previous question.

Question 45

How is the abrasive flow rate adjusted in the abrasive feeding system?

Answer 45

Usually selecting a proper calibrated hole or using feedforward control. Closed loop or feedback control is used only in micro waterjet cutting applications.

Question 46

What is the most common abrasive used in AWJ?

Answer 46

Garnet, made mainly out of silicon, aluminum and iron oxides.

Question 47

Which are the main requirements for abrasives in AWJ?

Answer 47

High hardness, homogeneous dimension and low hygroscopicity.

Question 48

Which are the most used granulometry in AWJ?

Answer 48

#80 and #120

Question 49

What is the #80 used for?

Answer 49

Mainly for high power cutting applications. Lower numbers indicate bigger diameters of the abrasive particles which gives them more mass, inertia and kinetic energy so these can cut thicker materials.

Question 50

What is the #120 used for?

Answer 50

For cutting applications where better surface roughness is needed (lower roughness).

Question 51

Are the small particles preferred in AWJ?

Answer 51

No, very small particles (powder) are not wanted as they tend to clog in the hose or in the mixing chamber reducing the jet power.

Question 52

What is the function of the catcher?

Answer 52

Dissipation of the jet residual energy (up to 75% of the initial energy), avoid jet back reflection, noise reduction (especially in submerged AWJ cutting), catch and flush the machining scraps.

Question 53

What is the common handling system for waterjet?

Answer 53

5 axis systems or 3 axis systems.