Quality Waterjet Newsletter – 04/26/2005

Pure Waterjet Cutting with a Coherent Stream

On many advertisement photos of waterjet cutting, the jet often appears to be a glass-rod-like stream, coherent and static. This kind of picture is taken with a stream at very low velocity and such a stream is not suitable for cutting (good for pictures though). In reality, stream in waterjet cutting usually breaks up into droplets because of the high velocity. The stream also tends to spread out to be a larger diameter as it travels through air and before it reaches the material to be cut. Such a stream is not desirable in most cutting applications. It wastes more materials (such as in fish fillet cutting and other food processing jobs). It brings unwanted moisture into the products it cuts (such as in paper product cutting). It creates undesirable ripples on the cut surface (such as in foam cutting, cake cutting, etc.). It also reduces the cutting efficiency because of the energy wasted on removing wider kerfs.

There are a few things that can be done to make the stream tighter to minimize the undesirable effects of the spreading stream. If things are done right, you may see a glass-rod-like stream within a short distance from the exit of the orifice. The length of such a glass-rod-like stream shows the coherency of the jet and can be used to measure the effects of the controlling factors. Among the several controlling factors, orifice edge geometry is the most critical one. A sharp edge orifice (usually made of sapphire or ruby) produces the tightest stream (the longest “glass-rod”). However, because the sharp edge is taking a high level of stress, it is vulnerable to the impacts of impurities in the water and tends to be more easily chipped. A chipped orifice will destroy the integrity of the stream and usually becomes unusable. A rounded edge can reduce the stress and is thus more robust, but the length of the “glass-rod” will be shorter. This suits well for cutting thin materials. Otherwise a sharp edge diamond orifice can be used, which is more robust at a higher cost. The geometry and the space immediately above the orifice is also important. Geometry with a streamline design and a larger volume will produce a longer “glass-rod”. For some applications that demand a long and tight stream, using a longer inlet tube often does the trick. However, keep it in mind that a larger upstream volume takes a longer time to discharge and therefore a longer dwell time is needed at the end of cutting before the nozzle traverses to next location. Minimizing the distance between nozzle and the material to be cut is also a good thing to do. A higher pressure and a larger orifice present more challenges. If nothing else works or an extra long “glass-rod-like” stream is needed, some additives can be added into water to enhance the stream coherency. “Super-Waterâ” from Berkeley Chemical Research, Inc. is an example and it has showed strong effect in enhancing stream coherency. This may be another topic for the future.

Unusual Cut

A friend of Mike Gracey, Mr. Andy Conn of Conn Consulting related a story that he calls "My oddest ever use of waterjetting Technology". A tannery in Newark, New Jersey which specializes in making belts, boots, etc. from the skin of sharks was having problems with their supply of skins. The native fisherman in Central and South America did not always follow the rules of properly salting the skins before shipping to the USA. Andy said, "the smell of spoiled shark is something you don't want to even think about". The tannery hoped to enlist the help of US fisherman along the Gulf coast but the skins had to be removed by hand. The idea arose to strip the skins from these prehistoric creatures using automated high pressure water jets, so Andy and his crew headed to Newark towing a 10,000 psi water blaster with a hand held gun. The water jet found the the interface between skin and the meat even with the hand held gun in very cold weather. During the lunch break, the crew warmed up and so did the shark smell. Andy said the office cat got very friendly. To use US fisherman, the tannery factored in the revenue from selling the shark meat to cat food manufacturers to help defray the cost of the new equipment. The project was discontinued when US restrictions on interstate shipping of cat food precluded using the shark meat because of mercury and other heavy metals. It seems large sharks entrap mercury in their organs so the Gulf Coast sources could not be used for cat food.