Published in: Canadian Industrial Machinery MagazineOctober 2013
Many factors influence the precision and efficiency of a waterjet cutting system. The type of CNC machine used, the materials being cut, and the speed and pressure of the abrasive water all play a role in the cleanliness of the cut. However, the waterjet cutting head is perhaps the single most important component in a waterjet cutting system, and so understanding the different components and their roles in the cut will help you ensure you are getting the most out of any CNC waterjet machine. It is important to note that each component of the waterjet cutting head must be accurately machined and assembled to ensure a consistent water stream and wear. Using quality components and replacing parts in a timely manner will enable your CNC waterjet cutting system to provide you with quality cuts for years. The following components affect cut quality:
High-pressure Water. Normal tap water is enhanced to pressures reaching 50,000 to 90,000 PSI through the use of a specially designed intensifier. A properly sized intensifier, in conjunction with a jewel orifice, ensures that the pressure is maintained at the cutting head during the cutting operation. Jewel Orifice. The jewel orifice, a critical component in the cutting head, focuses the water stream. Typically, this orifice is made of diamond, ruby, or sapphire with an opening ranging in size from about 0.005 to 0.016 inch diameter. Diamond is more than four times harder than ruby or sapphire, and it is fast becoming the industry standard for jewel orifices. Some parts require long, timeintensive cuts, and replacing a jewel orifice midway can ruin the cut. Switching to a longer-lasting diamond orifice helps to prevent the need to stop the cut prematurely for orifice replacement. A diamond orifice also produces a consistent stream, resulting in more even wear inside the cutting head.
Abrasive. Most applications require an abrasive to be added to the water stream to increase the cutting power, rather than pure water alone. Two factors affect the cut in regards to abrasives: the type of abrasive used and the speed at which the abrasive is added to the water stream. The most popular type of abrasive is made from garnet, a hard, brittle gemstone. A coarser abrasive will provide a faster cutting speed, but will wear out the mixing tube faster. A softer abrasive will cut slower, but will wear out the cutting head components slower as well. Ultimately, the ideal abrasive to use will depend on the materials you wish to cut. The abrasive is metered from a minihopper through a plastic tube down to the cutting head and is sucked into the waterjet stream in the mixing chamber. As more abrasive is added, cutting speed will increase until a saturation point is reached. However, if too much abrasive is added, the mixing tube will clog. A CNC abrasive metering system precisely controls the amount of abrasive that is allowed to flow to the cutting head. Because abrasive usage can represent as much as 67 percent of a waterjet’s operating costs, monitoring the flow of abrasive not only ensures longer life for consumables, it also reduces the amount of abrasive unnecessarily wasted.
Mixing/Focusing Tube. The mixing tube, as the name suggests, is the area where the abrasive mixes with the higher-pressure water before striking the material. The mixing tube also is the component that most frequently needs to be replaced in a waterjet cutting head. The opening (orifice) of a mixing tube typically increases in diameter by about 0.001 in. per day of cutting as the abrasive erodes the nozzle. If the wear is even, a nozzle can be used up to 120 hours. However, if the wear of the nozzle is not symmetrical, the waterjet stream will not be round, and the nozzle may need to be replaced sooner. The length of the tube itself also plays a role in cut quality. A longer abrasive mixing tube produces a more coherent waterjet stream. A mixing tube length of between 3 and 4 in. is optimum.
Mixing Tube Orifice. Although the tube orifice is part of the mixing tube, its opening diameter requires separate consideration. The smaller the opening, the more coherent the stream, but the trade-off is a slower cut than with a larger opening. Smaller orifices also wear faster and can clog more easily if the wrong amount or type of abrasive is used. However, larger orifices do not result in the same amount of precision as a smaller orifice; the desired outcome of the cut must be taken into consideration. When choosing a mixing tube and orifice, it is helpful to use a feed rate calculator. By changing the parameters, you can calculate cost per inch. Keep in mind, however, that these estimates are on straight lines only, and the shape of your cut will play a role as well.
Guard. A splash guard can be added to a cutting head to reduce the amount of water that is deflected during a cut. Although not an essential component of the cutting head, it can also protect the nozzle from accidental bumps, decreasing the frequency of nozzle replacement.
Finally, the distance between the cutting head and the material also will affect your cut. Typically, the optimum height for abrasive waterjet cuttingis between 0.040 and 0.060 in. Distances greater than that will result in a rounding of the cut. A higher nozzle also will result in a wider kerf (width of the cut) angle. Automatic height control is the most reliable and accurate way to maintain proper nozzle distance. Many CNC waterjet cutting machines include a system that calculates optimum nozzle height and cutting speed to obtain cut consistency.
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