Abrasive Waterjet Technology
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,
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ManufacturingEngineeringMedia.com | November 2013 See us at FABTECH Booth #S901
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