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Scaling Down Waterjets to the Micro Level

 

New technical advances are moving abrasive waterjet technology squarely into the into the micromachining realm

 

By Patrick Waurzyniak
Senior Editor
 

Shrinking an abrasive waterjet machine down to work at micro sizes is no small task. Adapting abrasive waterjets for micromachining requires greatly reducing the size of the waterjet nozzles and mixing tubes that carry smaller garnet abrasives through the waterjet’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, ceramics, glass, rubber and plastics. A cool process, abrasive waterjet machining has no heat-affected zone (HAZ), unlike laser or wire EDM processes, and it offers a substantial speed advantage over EDM.

In the last few years, some key advancements in downsizing abrasive waterjet technology have been developed by Peter 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).

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.

While OMAX isn’t the first company with micro abrasive waterjet 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 (Bollebygd, 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 waterjet 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.”

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 tolerances 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 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.

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 roughness 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 Waterjet 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. 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 microinches; 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 semiconductor 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,” noted 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 micromachining 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 micro 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 applications. “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 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 mixing 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 version—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, which features a 0.007" (0.18-mm) orifice and a 0.015" (0.38-mm) mixing tube combination for quickly and accurately cutting 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 working on the program so that we can accurately 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 development of micro abrasive-waterjet technology for automated machining features between 50 to 100 µm, according to the SBIR grant description, which cites the biggest challenge being development of nozzles with beam diameters less than 100 µm. Several issues must be resolved due to the complexity of the supersonic, three-phase, and microfluidic 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 titanium 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 formation.” 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 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

 
This article was first published in the November 2013 edition of Manufacturing Engineering magazine.  Click here for PDF


Published Date : 11/1/2013

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