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Waterjet 101 |
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Comprehensive Overview of Abrasivejet Technology A brief history, traditional applications and development of systems specifically for machine shops Industrial uses of ultra-high pressure waterjets began in the early 1970s. It was discovered that at pressures between 40,000 and 60,000 psi (276,000 and 414,000 kPa), a jet of water approximately 0.005" (0.1 mm) in diameter could neatly cut everything from cardboard to granola bars. Special production line machines were developed to solve manufacturing problems related to materials that had been previously been cut with knives or mechanical cutters. Examples of early applications include:
Early waterjet systems were expensive and often troublesome to maintain. But alternatives such as knives and mechanical cutters were even more costly and problematic. Consequently, waterjets quickly gained acceptance as a solution for cutting challenging materials. In addition to special-purpose production line systems, a number of waterjet specialty shops were formed for the sole purpose of applying the technology on a contract or job-shop basis. These shops typically cut foam rubber, gasket substance, and other material into particular shapes for manufactured products, custom signs, and other applications. Although waterjets were the ideal solution for cutting troublesome soft materials , they were less successful at cutting engineering materials such as metals and ceramics. Then, in the early 1980s the abrasivejet was born. In addition to standard waterjet components, a special nozzle was developed—not only did it create the waterjet, it introduced a small amount of abrasive powder into the jet by means of aspiration. With the help of abrasives such as garnet, a waterjet could be used on difficult-to-machine material such as titanium, Inconel®, glass and ceramics. Like the early waterjets, the first abrasivejet systems were expensive to operate and maintain, and were used only for special applications, such as titanium wing panels for military aircraft, or special shapes out of glass or ceramics. These systems cut in air, so the operator could actually see the jet, allowing manual adjustment of the feed rate as it maneuvered bends and corners. But these systems were noisy, and often produced a cloud of material cuttings and abrasive powder that covered everything in the shop. This limited early abrasivejet systems to specialized job shops employing highly trained operators. In the early 1990s Dr. John Olsen, a pioneer of the waterjet cutting industry, began to explore the concept of abrasivejet cutting as a practical alternative for traditional machine shops. The goal was to develop an abrasivejet cutting system without all the noise, dust, and complexities that plagued earlier systems and kept them in special facilities. The new system also needed to be simple enough to maintain without extensive training or expertise. Finally, and perhaps most importantly, Dr. Olsen envisioned a computerized control system that eliminated the need for operator expertise and trial-and-error programming. If such a system could enable an unskilled operator to quickly produce an individual part to precise specifications on the first try, it could be used by thousands of small job shops and prototype shops for making short-run and one-off parts. Dr. Olsen teamed up with Dr. Alex Slocum of the Massachusetts Institute of Technology, in order to design the mechanical system. He used cutting test results and a theoretical cutting model originally proposed by researchers at the University of Rhode Island as a guide in developing the unique control system. The result was a PC-based control system coupled to a precision X-Y cutting table on which parts could be cut under water to eliminate excessive noise and dust. This was the first abrasivejet cutting system designed specifically for the short-run and limited-production machine shop market. Pump Intensifier pumps Early ultra-high pressure cutting systems used hydraulic intensifier pumps exclusively. At the time, the intensifier-type pump was the only pump capable of reliably creating pressures of 40,000 to 60,000 psi (276,000 to 414,000 kPa). Figure 1 shows a schematic drawing of a hydraulic intensifier pump. An engine or electric motor typically drives a hydraulic pump which pumps hydraulic fluid at pressures from 1,000 to 4,000 psi (6,900 to 27,600 kPa) into the intensifier cylinder. The hydraulic pressure operates on a relatively large piston to generate a high force on a relatively small-diameter plunger, this plunger pressurizes water to a level that is proportional to the relative cross-sectional areas of the large piston and the small plunger. For example, if 3000 psi (20,700 kPa) hydraulic fluid acts on a large piston with an area 20 square inches (125 cm2), and if that piston pushes on a plunger with an area of only 1 square inch (6.2 cm2), the water that is pushed on by the plunger will be pressurized to 60,000 psi (414,000 kPa) [3000 x 20/1 (20,700 x 125/6.2)]. The intensifier cylinder is a double-acting cylinder in which hydraulic fluid is introduced alternately into one side and then the other. In turn, the hydraulic piston alternately pressurizes the water by way of small-diameter plungers at each end of the intensifier assembly. A series of check valves alternately allows low pressure water into the plunger cylinder as the plunger retracts and then directs the pressurized water into the outlet manifold as the plunger moves into its compression stroke. The back and forth action of the intensifier piston produces a pulsating flow of water at very high pressure. To help make the flow of water more uniform (thus resulting in a smoother cut), the intensifier pump is typically equipped with an "attenuator" cylinder, which acts as a high-pressure surge vessel. The use of this attenuator reduces pressure fluctuation to a few thousand psi per stroke. Crankshaft pumps The centuries-old technology behind crankshaft pumps is based on the use of a mechanical crankshaft to move any number of individual pistons or plungers back and forth in a cylinder. Check valves in each cylinder allow water to enter the cylinder as the plunger retracts and then exit the cylinder into the outlet manifold as the plunger advances into the cylinder.
Figure 1: Crankshaft pump Crankshaft pumps are inherently more efficient than intensifier pumps because they do not require a power-robbing hydraulic system. In addition, crankshaft pumps with three or more cylinders can be designed to provide a very uniform pressure output without needing to use an attenuator system. Crankshaft pumps were not generally used in ultra-high pressure applications until fairly recently. This was because the typical crankshaft pump operated at more strokes per minute than an intensifier pump and caused unacceptably short life of seals and check valves. Improvements in seal designs and materials, combined with the wide availability and reduced cost of ceramic valve components, made it possible to operate a crankshaft pump in the 40,000 to 50,000 psi (280,000 to 345,000 kPa) range with excellent reliability. This represented a major breakthrough in the use of such pumps for abrasivejet cutting. Today, crankshaft pumps can operate reliably up to 55,000 psi (379,000 kPa). Experience has shown that an abrasivejet does not really need the full 60,000 psi (414,000 kPa) capability of an intensifier pump. In an abrasivejet, the abrasive material does the actual cutting while the water merely acts as a medium to carry it past the material being cut. This greatly diminishes the benefits of using ultra-high pressure. Indeed many abrasivejet operators with 60,000 psi (414,000 kPa) intensifier pumps have learned that they get smoother cuts and more reliability if they operate their abrasivejets in the 40,000 to 50,000 psi (276,000 to 345,000 kPa) range. Now that crankshaft pumps produce pressures in that range, an increasing number of abrasivejet systems are being sold with the more efficient and easily maintained crankshaft-type pumps. Nozzles All abrasivejet systems use the same basic two-stage nozzle design shown in Figure 2. First, water passes through a small-diameter jewel orifice to form a narrow jet. The waterjet then passes through a small chamber where a Venturi effect creates a slight vacuum that pulls abrasive material and air into this area through a feed tube. The abrasive particles are accelerated by the moving stream of water and together they pass into a long, hollow cylindrical ceramic mixing tube. The resulting mix of abrasive and water exits the mixing tube as a coherent stream and cuts the material. It's critical that the jewel orifice and the mixing tube be precisely aligned to ensure that the water jet passes directly down the center of the mixing tube. Otherwise the quality of the abrasivejet will be diffused, the quality of the cuts it produces will be poor, and the life of the mixing tube will be short. In the past, most nozzle designs required the operator to adjust the alignment of the jewel and mixing tube during operation. Modern nozzle designs rely on precisely machined components to align the jewel and mixing tube during assembly, thereby eliminating the need for operator adjustments.
Figure 2: Typical abrasivejet nozzle The typical orifice diameter for an abrasivejet nozzle is 0.010" to 0.014" (0.25 mm to 0.35 mm). The orifice jewel may be ruby, sapphire or diamond, with sapphire being the most common. Diamond is recognized to last longer than the other two, but most operators find that it is not worth the additional cost. A typical high-quality jewel assembly consisting of a sapphire orifice and a precision stainless steel mount with integral abrasive feed chamber costs about $50. A similar assembly using a diamond orifice would cost several hundred dollars and does not provide a reasonable payback. Ruby and sapphire are very similar in their life expectancy, neither having a distinct advantage over the other. In theory, a jewel orifice should operate reliably until dissolved solids and minerals in the water build up next to the water passage. The jewel does not really fail, but it no longer produces a straight, smooth stream of water because of scale build-up. In reality, however, many jewels fail when struck by dirt or abrasive particles that have managed to get upstream of the jet during nozzle changes or overhauls. This causes the jewel to crack or pit, substantially altering water flow through the jewel. Once water flow through the jewel is disturbed, the cut quality will be poor and the mixing tube life will be shortened dramatically. A cracked $50 jewel assembly can quickly ruin a $150 ceramic mixing tube. Many operators change the jewel orifice as a matter of course whenever they overhaul a nozzle. The majority of mixing tubes used with abrasivejet cutting systems are manufactured by Boride Corporation, a division of Kenametal, using a proprietary process originally developed by Dow Corning Corporation. The OMAX mixing tube has a 0.030" (0.75 mm) inside diameter and is 4" (10 cm) long. It features a smaller bore and longer length than the more-common 0.040" (0.35 mm) diameter by 3" (7.5 cm) long tube used by most abrasivejet system manufacturers. Tests indicate that the OMAX tube delivers a more precise cut with less taper than the other tubes. The tradeoff for this precision is that an OMAX mixing tube will typically last 40 to 100 hours, compared to the 60 to 120 hour life of a "standard" tube. OMAX now offers a 0.040" (0.35 mm) diameter by 4" (10 cm) long mixing tube for customers who are willing to trade off precision and taper for longer mixing tube life. Mixing tubes are expensive, typically around $140 to $250 regardless of brand. Many ceramics and tooling companies have tried to enter the market with mixing tubes at prices as low as $50, but none of these low-cost tubes have demonstrated an acceptable life span. As the market for machine shop abrasivejet cutting systems continue to grow, viable low-cost mixing tubes will probably emerge. It is interesting to note that discussions with many operators of non-OMAX abrasivejet systems reveal that the largest reason for mixing tube replacement is not that the mixing tube is worn out; more often, the brittle mixing tube was damaged when the nozzle accidentally came in contact with a piece of scrap or raised material on the cutting table. For this reason, the OMAX nozzle assembly is equipped with a protective nozzle guard and the nozzle can be run into an immovable object on the cutting table at full traverse speed without damaging the mixing tube. The extra thickness of the OMAX mixing tubes provide additional protection, even if the cutting is performed without the mixing tube protector. The venturi chamber between the jewel orifice and the top of the mixing tube is an area that is subject to wear. This wear is caused by the erosive action of the abrasive stream as it enters the side of the chamber and is entrained by the waterjet. Some nozzles (most notably the Ingersoll-Rand nozzle) provide a carbide liner to minimize this wear. Others simply recommend periodic replacement of the entire nozzle body. The OMAX MaxJet 4 nozzle incorporates the venturi chamber into the disposable jewel orifice mount. This means that it is replaced every time the mixing tube and jewel mount are replaced. The nozzle body itself is not subject to abrasive flow, so its life is quite long. Precise alignment of the jewel orifice and the mixing tube is critical to mixing tube life. This is particularly true for the relatively small diameter 0.030" (0.75 mm) mixing tube used by the standard OMAX precision nozzle. The jewel orifice is mounted to a precisely-machined stainless steel holder which fits into a precisely-machined bore in the nozzle body. The mixing tube, in turn, fits into a precisely-machined tapered section in the lower part of the jewel holder to assure proper alignment of the mixing tube and jewel holder. If dirt, particles of abrasive, or any other contaminants prevent the jewel holder from mating precisely with the nozzle body or mixing tube, then the mixing tube life will be shortened substantially. Regardless of the brand of nozzle, nozzles must be kept scrupulously clean when being overhauled. For maximum mixing tube life, a small ultrasonic cleaner should be used to insure that the nozzle body is free of all contaminants. The nozzle should be overhauled in the cleanest space available, not just anywhere in the shop. In addition to cleanliness, it is critical that good water quality be maintained to insure the longest possible mixing tube life. If water entering the mixing tube has high mineral content, scale deposits will build up on the orifice jewel and affect the jet quality and mixing tube life. OMAX offers standard particle filtration and optional water conditioning as needed to insure best water quality. OMAX will also test your water before installing your equipment to help determine the need for a water purification system. Abrasive Delivery Systems Throughout the years, abrasivejet systems featured a variety of complex abrasive metering systems. This complexity originated from the belief that the abrasive flow rate needed to be adjusted for different material types and thicknesses. Nowadays, the preferred approach is to use a simple fixed ratio of abrasive flow to water flow for the full range of materials to be cut, and to change only the nozzle cutting speed. Nozzle speed is easily altered by the X-Y table control system. This means that a simple fixed abrasive flow rate is all that's needed for smooth, accurate cutting. Modern abrasive feed systems are eliminating the trouble-prone vibratory feeders and solids metering valves of earlier systems and using a simple fixed-diameter orifice to meter the abrasive flow from the bottom of a small feed hopper located immediately adjacent to the nozzle on the Y-axis carriage. An orifice metering system is extremely reliable and extremely repeatable. Once the flow of abrasive through the orifice is measured during machine set-up, the value can be entered into the control computer program and no adjustment or fine tuning of abrasive flow will ever be needed. *Courtesy of OMAX Corporation. |
