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Cutting with laser and water
The laser beam is focused in a nozzle while
passing through a pressurised waterchamber (see Fig. 1). The water jet emitted
from the nozzle guides the laser beam by
means of total internal reflection at thetransition zone between water and air, in a
manner similar to conventional glass fibres.The water jet can thus be referred to as a
fluid optical wave-guide of variable length.
The water jet is essentially transparent forthe laser beam. However, if the laser beam
encounters a body, which absorbs it, the
surface of the material is heated to such
an extent that plasma is created. The
plasma separates the water jet and thematerial and it efficiently couples the
energy to the work piece.
mm/min.
Best results with CBN are achieved with a
nozzle of 75 microns in diameter. Thisparameter is of importance, since it
determines the kerf width; knowing that
CBN is still a precious material, thenarrower the kerf is, the smaller the loss
in material.
LMJ Versus ConventionalManufacturing Processes
All of the above could not be achievedwith a conventional laser. The LMJ has
indeed indisputable advantages over this
older technology. The main difference lies
in the light guiding and cooling effects of
the water jet. Because of the cylindrical shape of the jet, kerf walls are parallel,
and there is no conical shape effect (see
fig. 3). The running water of the jet also
enables continuous cooling - the LMJ is
sometimes referred to as "cold lasercutting"- which ensures the absence of
burrs, the deposition of slag, and material
changes due to heating. Finally, the edges
obtained with this new laser process arealmost perfect, making any further
grinding steps unnecessary.
The LMJ has nothing to envy the diamond
wheel technology, except the fact that it
is the most common process. For example,large amounts of liquid coolant are
required when using diamond wheels. Of
1 Submitted to Finer Points
The water jet-guided laser (LMJ) technology was developed ten years
ago at the Swiss Federal Institute of Technology Lausanne. It was first
dedicated to medical applications but fast turned out to be efficient in
different domains such as the semiconductor, electronics, automotive
and more recently the tooling industry. A Swiss company now produces
this hybrid technology that uses laser power and water, which has
indeed shown outstanding results for the processing of tools inserts out
of standard CBN discs.
Laetitia Mayor ([email protected]) &Rafal Romanowicz ([email protected])
New CBN Machining Possibilities
with the Water Jet-Guided Laser
Fig. 1: Laser-Microjet Principle.
The plasma only remains as long as the
laser beam is activated. Because a pulsedlaser is used, the continuous water jet is
able to immediately re-cool the cut,
resulting in only a very slight depth of
thermal penetration.
Generally speaking, the main advantage of
the system is its excellent cutting quality,
due to:
- the absence of any thermal stress, and
contamination burrs due to constantcooling of the water jet.
- the absence of mechanical constraints on
the pieces to be cut, the water jet pressure
being kept low (50 to 500 bars with a jet
diameter varying from 50 to 100 microns).
Cutting CBN discs with the LMJ
Cubic boron nitride (CBN) inserts usually
have to be cut out of 4 inches discs withthicknesses varying from 1 to 4 millimetres.
All usual geometries of triangle, rectangle
and lozenge inserts are easily processed
with the LMJ since the system allows omni-
directional cutting. In fact, just about anyshape can be cut with the LMJ, and high
quality drilling is possible (see fig. 4).
Very low tolerances can be achieved since
the machine has an absolute precision of
+/- 3 microns for a processing area of 300
x 300 mm and a maximum axis velocity of 1000 mm/sec. The typical cutting speedfor a thickness of 3.25 mm is about 7
Fig. 2: CBN inserts cut with the LMJ without
any post-processing.
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course, the LMJ uses de-ionised water, but
in small quantities (maximum 50 ml/min).
Furthermore, due to the hardness of CBN,
cutting with diamond wheels results inhigh tool wear, resulting a slowed process
with inconstant results. With the LMJ,
once the nozzle is aligned, cutting can be
performed for hundreds of hours withoutreplacement. LMJ running costs are
therefore lower than those of the diamond
wheel method.
Electro-discharge machining (EDM) issometimes used for ultra-hard materials,
but it is only usable in the case of
electrically conductive materials, which is
not the case for pure CBN. The presence of
electrically conductive material is notrequired for the LMJ process. In addition,
EDM is a very slow process. Achievable
speed with the LMJ is much higher.
Cutting Other Hard Materials with LMJThe processing capabilities of the LMJ are
not limited to cubic boron nitride inserts.
Other hard materials such as synthetic
diamond and silicon nitride can be cut
Fig. 3: On the right, the conical shape effect that
appears in conventional laser cutting. On the left,
the laser beam is guided in the water jet and kerf
walls are parallel.
efficiently as well.
In fact, any material
can be processed by
the LMJ when its
a b s o r p t i o n
coefficient at thelaser's wavelength is
sufficient. The LMJ
was first developed
using the infrared
w a v e l e n g t h ,
because it was the
most interesting in
terms of power (up
to 300 W).
The absorption of
the infrared laser
radiation in CBNand Si3Ni4 is
not really the case for transparent
materials such as synthetic diamonds.
Nevertheless, good cutting results can be
obtained when using a pulse infrared
laser, enabling the processing of low
absorption materials.
In summary, the water jet guided laser
has unsurpassed results compared to other
standard methods. The high cutting
speed, kerf wall quality, and especially the
ability of the system to cut any shaped
insert are the main reasons that render
the LMJ very interesting for the tooling
industry. With these capabilities, new CBN
products in terms of material and shape,
or new services such as custom-shaped
tools available in 24 hours, have now
become realistic.
Fig. 4: hole drilled in CBN by LMJ (hole diameter: 2,6 mm)
Fig. 5: CBN edge cut with LMJ without any
post-processing; magnified 70 times.
Fig. 6: Si3Ni4 edge cut with LMJ magnified 70 times.
Submitted for
the June edition of
FINER POINTS
Journal of the
Industrial Diamond
Association of America
2
Table 1: Characteristics of the water jet.
The liquid used is de-ionized, filtered water.
Water pressure 20-500 bars
Water jet speed up to 300 m/s (at 500 bars)
Water jet diameter 50, 60,75 or 100 microns
Water flow rate 20-50 ml/min
