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, visit http://info.hotims.com/34456-160.