Post on 25-Aug-2018
November 2013 | ManufacturingEngineeringMedia.com 59
Shrinking an abrasive waterjet machine down
to work at micro sizes is no small task.
Adapting abrasive waterjets for micromachin-
ing requires greatly reducing the size of the
waterjet nozzles and mixing tubes that carry
smaller garnet abrasives through the water-
jet’s high-pressure cutting tool delivery system.
Abrasive waterjet machining excels as a versatile
alternative cutting process capable of cutting virtually any
material, from exotic alloys and titanium to stainless, ce-
ramics, glass, rubber and plastics. A cool process, abrasive
waterjet machining has no heat-affected zone (HAZ), un-
like laser or wire EDM processes, and it offers a substantial
speed advantage over EDM.
The Finecut abrasive waterjet cuts titanium, generating sparks as
new surfaces of the erosion fragment under exothermal reaction
with air. The system uses fine-grained precision powder abrasives
to produce very fine surfaces in the range of 1 µm in Ra value.
Scaling Down Waterjets to the Micro Level
New technical advances are moving abrasive waterjet technology squarely into the micromachining realm
Patrick WaurzyniakSenior Editor
Abrasive Waterjet Technology
Photo courtesy Finepart Sweden AB
In the last few years, some key advancements in downsiz-
ing abrasive waterjet technology have been developed by Pe-
ter Liu, senior scientist, OMAX Corp. (Kent, WA), whose work
under a National Science Foundation (NSF) Small Business
Innovation Research (SBIR) grant culminated in August with
OMAX’s release of its new MicroMax JetMachining Center.
This machine is primarily aimed at cutting very thin metals
used in medical, aerospace and other industries. It features a
high-precision 0.1-µm linear optical encoder system, a highly
rigid structure, and patent-pending processes for feeding fine
abrasive at a constant flow rate. OMAX’s 7/15 Mini MaxJet5i
nozzles reach position repeatability of better than ±0.0001"
(±2.5 µm) and positioning accuracy of ±0.0006" (±15 µm).
While OMAX isn’t the first company with micro abrasive wa-
terjet systems, it may be refining the technology to another level.
Other abrasive waterjet micromachining systems on the market
in recent years include systems from Finepart Sweden AB (Bol-
lebygd, Sweden) and Micro Waterjet LLC (Huntersville, NC).
Micro Waterjets Gaining Wider Acceptance
Among the barriers to wider use of micro waterjets is
changing the mindset of some machine shop owners. “The
main obstacle may be in the mindset of precision workshop
owners that have recently tried state-of-the-art standard water-
jet systems,” said Christian Öjmertz, CEO of Finepart Sweden,
developer of the Finecut micro waterjet machining systems
introduced in 2009. “The fact that the level of tolerance of the
waterjet process now can be 10 times higher than what was
available only a few years ago can be difficult to digest.
“To be able to obtain fine tolerances with micro abrasive
waterjets you need to keep process parameters very stable,”
Öjmertz said. “Water pressure variations should be kept at a
minimum and abrasives are precision-fed [only 20–30 g/min
is used for a 200–300-µm nozzle size]. Abrasive media must
be of very fine quality, and we test abrasive for approval to use
in the Finecut process. The abrasive media should be free
from fine dust as it binds moisture that can obstruct the flow.”
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60 ManufacturingEngineeringMedia.com | November 2013
Abrasive Waterjet Technology
See us at FABTECH Booth #S3526
Fittings and nozzle shape are important to achieve fine
tolerances, Öjmertz said. “An ovality in the nozzle bore of
0.01 mm will cause a reduced capacity to maintain toler-
ances within ±0.01 mm,” Öjmertz said. Micro abrasive
waterjets require a special machine design to obtain the fine
tolerances, he added, noting that Finecut machines are built
with linear drive motors to prevent backlash problems and
provide excellent dynamics.
With the Finecut micro systems, tolerances to ±0.01 mm
can obtained, depending on material and part geometry,
Öjmertz said. “The micro AWJ [abrasive waterjet] utilizes a
fine-grained precision powder abrasive and produces very fine
surfaces in the range of 1-µm in Ra value. The surface rough-
ness depends on the type of material being cut and in general
harder materials will exhibit finer surfaces.”
Using AWJ abrasive waterjet machining for micro parts
offers some advantages over other alternative machining
processes, noted Steve Parette, managing director, Micro Wa-
terjet LLC. “When compared to laser and wire EDM, you have
no heat-affected zones [HAZ],” Parette said, “and there is a
wider range of material compatibility with abrasive waterjet.
• Precise metal and plastic marking capabilities
• Generous 32" x 20" work area
• Advanced job control for increased throughput
• Power confi gurations up to 50 watts
• Joystick control for easy operation
• High-quality, USA-Made Equipment
e p i l o g l a s e r . c o m / m e • 8 8 8 - 4 3 7 - 4 5 6 4
62 ManufacturingEngineeringMedia.com | November 2013
Abrasive Waterjet Technology
With the OMAX MicroMax abrasive waterjet, users can cut
parts or part features smaller than 400 microns across a
wide range of materials including exotic metals, titanium,
advanced composites, polymer thermoplastics and glass.
Pho
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ourt
esy
OM
AX
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See us at FABTECH Booth #N1282
Micromachining requires special tooling depending on the
application. Abrasive waterjet uses no special tooling.
“We concentrate on thin materials, usually 2 mm and
thinner,” he said. “The finest finish achieved is N6, or 32 mi-
croinches; currently parts are being cut
in production with tolerances ±0.0005"
[±0.010 mm].”
Micro or Not?
With its NanoJet abrasive waterjet,
Flow International offers a specialized
small-footprint system for semicon-
ductor singulation. It features a Paser
ECL abrasive cutting head, a patented
vacuum assist feature, integrated vision
positioning system, and an X-Y-Z cutting
envelope of 32 × 13 × 3.5" (820 × 480
× 90 mm). Linear servomotors help
enable path accuracy to ±0.001" (0.025
mm) and repeatability to ±0.001".
“The first need for thin-kerf cutting
with abrasive waterjet was about 10 years
ago, in the semiconductor industry,” not-
ed Mohamed Hashish, Flow International
senior vice president, Technology. “At
the time, the state of the art of waterjet
diameter was in the 20 thousandths of
an inch and above. Features on microSD
cards needed to be smaller than that.
“Different people define microma-
chining differently, and in the waterjet
industry it’s addressed very loosely, so
in a way, coming from a micro level is
probably inaccurate,” said Hashish. “In
my opinion, we’re not talking about mi-
cro jets. We’re talking about jets that are
in a few microns, and we are in a few
tens of microns, so we are in an order of
magnitude higher than microns.”
Moving Microjets Forward
During the past three years, Liu’s
research at OMAX under the NSF’s
$550,000 SBIR II grant has concentrated
on reducing the nozzle size among other
technical hurdles for abrasive waterjet micromachining applica-
tions. “We decided to get the nozzle size as small as possible,
especially the mixing tube, which governs the kerf width of the
part,” Liu said. “And it turns out that even though you want to go
November 2013 | ManufacturingEngineeringMedia.com 63
down as small as possible, there are limitations, for example, one
of which is the capability of making a mixing tube that small.”
Working with development partner Kennametal Corp.
(Latrobe, PA), supplier of abrasive waterjet nozzles made of
composite carbide, Liu was able to significantly shrink the
size of the nozzles. Kennametal, which exclusively licenses
the ROCTEC (Rapid Omnidirectional Compaction) process for
developing a tungsten carbide-based material used in mix-
ing tubes, is a major supplier of abrasive waterjet nozzles to
waterjet machine tool builders. “I’ve been working with them
all along,” Liu said. “The best they can do is something around
6–8 thousandths of an inch [0.15–0.20 mm] in the ID of the
mixing tube, in order to get good quality, or circular, holes
through the length of it, and the material’s one of their best that
allows minimizing the wear for abrasive waterjet applications.”
With Kennametal, Liu worked on downsizing the nozzles
and mixing tubes, eventually developing a 5/10 nozzle ver-
sion—with a 0.005" (0.13-mm) orifice and 0.010" (0.25-mm)
mixing tube—that is currently being beta tested. “We wanted to
see how small we can go,” he said. “Now the obstacle is, from
a pure fluid mechanics point of view, how small of a mixing
tube can you squeeze the waterjet through? When you have a
large mixing tube, with a large diameter, the flow or the fluid
mechanics is the so-called gravity flow. But when you get down
to a very small one, then the capillary effect on it becomes
important—you actually increase the resistance through the
mixing tube. The surface tension becomes important, instead of
gravity, so the process is dominated by the capillary effect.”
Liu offered a simple example of the process: “If you have
a glass tube with a small diameter and you put in water, you
can see the column of water rise through the tube—that is the
capillary effect. When you look at the resistance of the flow
through the small tube, it is inversely proportional to the fourth
power of the diameter. That means the smaller the tube you
go through is, the higher the resistance—sooner or later, you
just don’t have anything squeezing it through, and it’s probably
60,000 psi.” Increasing the pressure of the abrasive waterjet
micromachining applications becomes difficult given those
circumstances. “It’s the so-called entrainment pressure of
abrasive waterjet,” Liu added. “We are working on ways to
overcome that, but it will take additional research.”
Refining Abrasive Delivery
The smallest production nozzle currently available from
OMAX is the 7/15 Mini MaxJet5i on its new MicroMax machine,
64 ManufacturingEngineeringMedia.com | November 2013
Abrasive Waterjet Technology
See us at FABTECH Booth #S901
which features a 0.007" (0.18-mm) orifice and a 0.015" (0.38-
mm) mixing tube combination for quickly and accurately cut-
ting delicate, complex patterns. The system’s jet stream uses an
extremely fine abrasive with the nozzle, producing a kerf width
as small as 0.015", and the machine also
features advanced pressure controls for
piercing delicate materials.
OMAX’s 5/10 nozzle has been in
beta-testing for two years now, and is
not yet commercially available. “When
you are trying to cut very thin material,
our current cutting model is not quite
accurate enough,” Liu said. “That’s the
reason why we don’t want to release it as
a product yet. At this point, we are work-
ing on the program so that we can accu-
rately describe it so that the customer will
not have to do a lot of tweaking.”
Among the main aims of the SBIR
Phase II project for OMAX was develop-
ment of micro abrasive-waterjet technol-
ogy for automated machining features
between 50 to 100 µm, according to
the SBIR grant description, which cites
the biggest challenge being develop-
ment of nozzles with beam diameters
less than 100 µm. Several issues must
be resolved due to the complexity of the
supersonic, three-phase, and microflu-
idic flow through micro abrasive-waterjet
nozzles in which, as Liu described
above, capillary dominates gravity.
For medical parts, the new OMAX
micro waterjet has shown it cuts titanium
faster than stainless, Liu said, with tita-
nium cutting as much as 34% faster for
skull meshes, spinal implants and other
components. Another key to achieving
high precision with the 5/10 beta nozzle
is the new machine’s stability, Liu added.
“We found that one critical area is that
you must keep your nozzle stable. In other
words, you cannot have any vibration.
When you are cutting a very small part,
any vibration will cause some wavy forma-
tion.” This waviness, similar to chatter on a CNC-machined part,
is reduced with the MicroMax’s rigid, vibration-isolating design.
Reducing the mixing tube size also requires substantially
reducing the size of the abrasives, Liu noted. As the garnet
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abrasives get smaller, they tend to clump together with poor
flow. “You want to have the particle size about one third of the
ID of the mixing tube,” Liu stated. “If you have only two times
smaller, two of the particles can be bridged inside the mixing
tube and cause clogging, but if you have three times smaller,
that would be very difficult to have three particles lined up and
clog your nozzle. It’s both a theoretical and practical limitation.
“The smaller the abrasive, the better the surface finish
you’ll be able to get,” Liu said. “Under certain circumstances,
you want to go even smaller, but now the problem comes in,
when you have large particles like the one we’ve been using
with our production system, those can flow very well under
gravity feed. When you go down to the powder size, they tend
to clump together. I have developed a couple novel processes
[patents-pending] that allow us to avoid that type of problem.”
Flow rate is key to the process, with a constant flow rate
enabling better cuts, Liu said. With the MicroMax’s 5/10 nozzle
and using a fine 320 mesh garnet at the top quality setting in
OMAX’s cutting model, surface roughness should be less than 5
µm. “It depends on what size of garnet you use,” Liu said. “The
finer the garnet you use, the better the surface finish.” ME
66 ManufacturingEngineeringMedia.com | November 2013
Abrasive Waterjet Technology
Finepart Sweden AB Ph: +46 70 6763355
Web site: www.finecut.se
Flow International Corp.Ph: 253-850-3500
Web site: www.flowcorp.com
Micro Waterjet LLCPh: 704-948-1223
Web site: www.microwaterjet.com
OMAX Corp.Ph: 253-872-2300
Web site: www.omax.com
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