Contains basic details about water jet machining process.

1. Abrasive Water Jet Machining

2. Process Principle
• An abrasive jet starts out the same as a pure
water jet.
• As the thin stream of water leaves the nozzle,
abrasive is added to the stream and mixed.
• The beam of water accelerates abrasive
particles to speeds fast enough to cut through
much harder materials.

3. • The mixing of abrasive particles in water jet is
in such a manner that water jet’s momentum
is transferred to the abrasives.
• The coherent, abrasive water jet that exits the
AWJM nozzle has the ability to cut various
materials, such as metals, glass, ceramics and
composites.

4. Dynamics of an abrasive water jet
• Two material removal mechanisms have been
identified.
• The first mechanism typically occurs at the
uppermost portion of the kerf where the
abrasive impact angle is shallow.
• In this portion of cut, material is removed
primarily through erosion.

5. • Deeper into the kerf where the abrasive impact
angle is larger, deformation wear becomes the
primary material removal mechanism.
• Although the mechanism are not fully
understood, the ability for AWJM to penetrate
very thick material may be due to reentrainment
of abrasive particles in the jet after the initial
impacts at the top of the cut.

6. • 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.

7. Equipment

8. Pumping system
• Identical as WJM

9. Abrasive Feed system
• Purpose: Controlled flow of abrasive particles
to the abrasive jet nozzle.
• AWJM abrasive feed systems deliver a stream
of dry abrasives to the nozzle.
• Drawback with dry abrasive delivery systems is
that the delivery of abrasives over long
distances is difficult.

10. Water jet
• Water jet used for this process is essentially
the same as used for WJM

11. 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.

12. Abrasive Jet Nozzle
• There is a difference between a pure water jet
nozzle and an abrasive 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 the
material after it exits the jewel.

13. Abrasive Jet Nozzle

14. • To minimize abrasive wear, the nozzle is
usually made from either tungsten carbide or
boron carbide.
• Two major design concepts are currently used
for the design of abrasive jet nozzles.

15. Single-jet side feed nozzle

16. • 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.

18. Multiple jet

19. • While cutting the same type of material, one
can choose the nozzle that works best for that
material.

20. Which nozzle is best for which
material?
Water Jet Nozzle
AbrasiveJet Nozzle
Soft rubber
Hardened tool steel
Foam
Titanium
Extremely thin stuff like Foil
Aluminum
Graphite
Carpet
Hard Rubber
Many ceramics
Paper and cardboard
Stone
Carbon Fiber
Soft Gasket material
Inconel®
Composites
Candy bars
mild steel
Copper
Diapers
Stainless Steel
Soft, or thin wood
Plastic
Nylon

21. Limitations of abrasive jet nozzles
• Despite their simple design, abrasive jet nozzles
can be troublesome at times. There are many
designs, but they share the same problems:
• Short life of the mixing tube
The abrasive jet can cut through just about
anything—including itself. This mixing tube is
expensive and wears out in only a few hundred
hours of use. Replacing mixing tubes will be a
large part of your operating cost.

22. • Occasional plugging of mixing tube
Plugging is usually caused by dirt or large
particles in abrasive. This used to be a big
problem with abrasive jet nozzles, but has
been getting better as manufacturers finetune mixing tube designs.

23. • Wear, misalignment, and damage to the
jewel
The jewel needs to be precisely positioned in
the nozzle while water and thousands of
pounds of pressure impacts it.

26. Process Parameters

27. Water jet pressure

28. • 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 mild steel is between 20.7
and 27.5 Mpa.

29. Water Flow Rate

30. • fig. Shows the depth of cut is affected by varying
the water flow rate (increasing the nozzle
diameter) while maintaining the constant
pressure.
• As the flow rate increases, the slope of the curve
decreases because the saturation point is
reached.
• As the nozzle diameter increases and the water
flow rate increases, the rate of increase in the
particle velocity is reduced, thus reducing the
depth of cut.

31. Abrasive flow rate
• 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 particle impacts increase.

32. Abrasive Particle Size
• The most common abrasive particle sizes used
for AWJM range from 100 to 150 grit
• An optimum abrasive particle size also exists
for each particular nozzle mixing chamber
configuration.

33. Abrasive type
• The type of abrasive used is also an important
parameter.
• Garnet, silica and silicon carbide are the most
commonly used abrasives.
• Selection of abrasive type is usually
determined by the hardness of the material
that is being cut.

34. Traverse rate
• When traverse rates are increased the depth
of cut decreases.
• There is also a minimum critical traverse rate
below which further increases in depth of cut
are not obtained.
• If the traverse rate is not maintained at a
relatively uniform velocity, a rough edge will
result because of the nature of the process.

35. Stand-off-distance
• Data generated by some researchers indicate that
depth of cut 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 tolerated without 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.

36. Process Capabilities
• AWJM can be thought of as a combination of
WJM and AJM principles.
• But in terms of capability, AWJM combines the
best of both processes, resulting in a new
process that can cut materials whether they
are hard or soft at high rates and in very thick
sections.
• AWJM can cut materials as thick as 200mm
and still maintain a comparatively narrow kerf.

37. • Kerf width is a function of the material
thickness and usually is between 1.5 and
2.3mm.
• The resulting taper on the cut edge is a
function of the material hardness,
• Where hard materials have the widest kerf at
the top of the cut and
• Soft materials have the widest kerf at the
bottom of the cut.