Still a relatively young technology, abrasive waterjet has evolved to the point where it offers substantial benefit to some manufacturers of surgical instruments and a broad variety of other medical applications. With the ability to hold positioning accuracy as tight as ±0.001" and ongoing technological advances in micronozzles and the ability to cut angles, an increasing number of medical manufacturers are adopting abrasive waterjet machines to boost the productivity of their operations.
Contrary to common belief among non-users, water does not actually perform the cutting on abrasive waterjet machines. Rather, it acts as a delivery vehicle for miniscule abrasive particles known as garnet, which are accelerated to supersonic speeds to provide highly productive cutting via erosion distantly akin to a grinding process.
All abrasive waterjets share the same basic components. A pump sends water to the nozzle, where it passes through a small-diameter jewel orifice to form a narrow jet. This then passes through a small mixing tube where the Venturi effect creates a slight vacuum that pulls garnet and air into the jet of water. Typically, a hopper feeds garnet to the system. Imbued with abrasive particles, the jet exits the nozzle and cuts whatever material has been loaded onto the machine.
Benefits for Medical Manufacturers
Abrasive waterjet’s chief benefit comes in the form of increased productivity. Compared to traditional metal-cutting processes, it offers substantially higher cutting speeds with less time-consuming setups. The process is straight-forward. An operator simply uploads a drawing of the part and specifies the material being cut and required tolerances. Onboard software automatically calculates the tool paths, then creates and executes the corresponding cutting data. The speed and ease with which a part can be set up and cut provides an excellent alternative for prototyping applications as well as larger runs of components.
The ability to cut a broad range of materials also proves highly beneficial to many medical manufacturers. Other than letting the controller know of the material change, there are no other programming or equipment steps required to accommodate cutting metals, composites, ceramics, and other materials. Depending on the material and its thickness, the software controller automatically adjusts the speed of the X- and Y-axes to maximize productivity while maintaining the needed quality.
Abrasive waterjet also holds key advantages compared to other cutting processes. Both laser and electrical discharge machining (EDM) technologies create heat-affected zones in the workpiece being cut. This can cause surface hardening and alter the chemical properties of the material. As a cold cutting process, abrasive waterjet offers immunity from these effects. At the same time, it offers an environmentally superior alternative over photo-chemical etching, which usually produces toxic and/or hazardous waste. Lastly, abrasive waterjet exerts a very small amount of force on the workpiece being cut, especially when compared to traditional metalcutting or stamping equipment. This allows the cutting of delicate features that would otherwise prove impossible or too costly to produce.
In its early days, abrasive waterjet often could not achieve the tolerances needed for many medical applications. This no longer holds true for parts requiring down to 0.002" tolerances. Even for higher precision work, the technology can provide substantial benefit by quickly cutting near-net parts that are then finished with an ancillary process. The same applies to part geometries beyond the relatively flat dimensions that abrasive waterjet can produce, although the recent development of new bevel-cutting nozzle accessories has expanded the geometries that the technology can produce outright.
Micro-Meso Abrasive Waterjet Machining
Emerging innovations in the field will prove to be of immense interest to manufacturers producing surgical instruments or other components that require extreme levels of accuracy and precision. In particular, the development of micronozzles holds the promise of allowing the productivity and cost benefits of abrasive waterjet to apply to a wider range of medical applications. OMAX Corporation (Kent, WA) was awarded a Phase II grant (Grant #1058278) in the National Science Foundation’s (NSF) Small Business Innovation Research (SBIR) program to further development of micronozzles for abrasive waterjet. This follows successful completion of Phase I and Phase I-B research (Grant #0944239) conducted by the company in partnership with industrial and academic collaborators, including the Precision Engineering Research Group (PERG) at MIT.
Through Phase I and Phase I-B research, OMAX demonstrated the feasibility of performing micro-meso abrasive waterjet machining, which is defined as producing features down to 0.002" in size. Involving more than mere positioning accuracy and the ability to hold the necessary tolerances, micro-meso machining requires creation of a cutting stream with a beam diameter of just 0.002"–0.004". Traditionally, the minimum beam diameter that could be achieved was around 0.012", allowing the cutting of features from 0.012"–0.015" in size.
Two significant challenges have limited the minimum size of abrasive waterjet nozzles that can be created. The first involves non-uniform abrasive feeding. To produce such a narrow stream and remain effective, an extremely fine garnet must be used. Unfortunately, the extremely small size of such garnet particles can lead to a clumping effect, impeding the ability to provide the smooth flow of materials required by the cutting process. Clumps can occur in feed tubes, causing an intermittent stream that then results in the skipping of cuts or damage to the material being worked with.
The second hurdle stems from the fact that with sufficiently small nozzle orifices and mixing tubes, a water column will remain within the tubes. The lower speed at the front of the abrasive waterjet stream then impinges on the surface of the water column, resulting in a backsplash that wets residual abrasive remaining in the mixing chamber or feed port entrance. Over the course of operation, wet abrasive accumulates in these areas and will eventually create clogs that halt operation of the machine.
In the past, manufacturers have experimented with vacuum assist and water flushing to eliminate the above issues. Unfortunately, these have resulted in very complex solutions that involve bulky nozzle designs with multiple ports, along with extensive addition or modification of connecting tubes, ejectors, water pumps, control software, and other devices. Such complicated systems would negate some, if not all, of the cost and time benefits of the technology.
By taking a different approach, the research undertaken by OMAX has resulted in several patent-pending, proprietary processes that minimize nozzle clogging and abrasive flow interruption without vacuum assist or water flushing. The solution involves a miniature nozzle design that has proven successful with an orifice diameter of 0.007" and mixing tube diameter of 0.015". Research has shown that this design will be able to be miniaturized even further, with some prototypes featuring orifice and mixing tube diameters as small as 0.003" and 0.008", respectively. Continued R&D under the NSF SBIR Phase II grant will further miniaturize diameters of the orifices and mixing tubes. At that level, the technology will provide micro-meso machining without use of accessories that would complicate the abrasive waterjet cutting process.
Today, many medical shops use abrasive waterjet, both alone and with complementary cutting technologies, for a wide variety of applications. From surgical instruments and medical devices, to orthotics and prosthetics, the technology’s footprint can be found throughout the industry. As further refinements in the precision and capabilities of abrasive waterjet emerge, its impact on productivity will continue to benefit an increasing percentage of medical manufacturers.
This article was written by Dr. Peter Liu, Senior Scientist for OMAX® Corporation, Kent, WA. For more information, Click Here .