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Abrasive Jet Machining | Machining | Pump

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  International Journal of Modern Engineering Research (IJMER)www.ijmer.com Vol. 3, Issue. 3, May.-June. 2013 pp-1504-1511 ISSN: 2249-6645  www.ijmer.com 1504 | Page  Shripad Chopade 1 , Sagar Kauthalkar  2 , Dr. Pushpendra Kumar Sharma 3   1 (shripad shashikant chopade, scholar student of Mechanical Department, Nri-ist Bhopal, India)2   (sagar pradip kauthalkar, scholar student of Mechanical Department, Nri-ist Bhopal, India)   3(Dr. Pushpendra kumar sharma, Head & Guide of Mechanical Department, Nri-ist Bhopal, India) ABSTACT: Abrasive water jet machine tools are suddenly being a hit in the market since they are quick to program andcould make money on short runs. They are quick to set up, and offer quick turn-around on the machine. They complementexisting tools used for either primary or secondary operations and could make parts quickly out of virtually out of anymaterial. One of the major advantages is that they do not heat the material. All sorts of intricate shapes are easy to make.They turns to be a money making machine. Keywords  Abrasive Delivery System, Control System, Mixing tubes, Pump, Nozzle, Motion System I.   Introduction A machine shop without a water jet is like a carpenter without a hammer ultimately. Sure the carpenter can use the back of his crow bar to hammer in nails, but there is a better way. It is important to understand that abrasive jets are not thesame thing as the water jet although they are nearly the same. Water Jet technology has been around since the early 1970s or so, and abrasive jets extended the concept about ten years later. Both technologies use the principle of pressuring water toextremely high pressure, and allowing the water to escape through opening typically called the orifice or jewel. Water jetsuse the beam of water exiting the orifice to cut soft stuffs like candy bars, but are not effective for cutting harder materials.The inlet water is typically pressurized between 20000 and 60000 Pounds per Square Inch (PSI). This is forced through a tiny wall in the jewel which is typically .007” to .015” diameter (0.18 to0.4 mm ).This creates a very high velocity beam of water. Abrasive jets use the same beam of water to accelerate abrasive particles to speeds fast enough to cut through muchfaster material. II.   Abrasive   Delivery System  A simple fixed abrasive flow rate is all that's needed for smooth, accurate cutting. Modern abrasive feed systems areeliminating 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 thenozzle 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 programand no adjustment or fine-tuning of abrasive flow will ever be needed. The small abrasive hopper located on the Y-axiscarriage typically holds about a 45-minute supply of abrasive and can be refilled with a hand scoop while cutting isunderway. 2.1   Control System   Fundamental limitation of traditional CNC control systems.  Historically, water jet and abrasive jet cutting tables have used traditional CNC control systems employing thefamiliar machine tool G-code. However, there is a rapid movement away from this technology for abrasive jet systems, particularly those for short-run and limited-production machine shop applications. G-code controllers were developed tomove a rigid cutting tool, such as an end mill or mechanical cutter. The feed rate for these tools is generally held constant or varied only in discrete increments for corners and curves. Each time a change in the feed rate is desired programming entrymust be made. A water jet or abrasive jet definitely is not a rigid cutting tool; using a constant feed rate will result in severeundercutting or taper on corners and around curves. Moreover, making discrete step changes in feed rate will also result in anuneven cut where the transition occurs. Changes in the feed rate for corners and curves must be made smoothly andgradually, with the rate of change determined by the type of material being cut, the thickness, the part geometry and a host of nozzle parameters.The control algorithm that computes exactly how the feed rate should vary for a given geometry in a given materialto make a precise part. The algorithm actually determines desired variations in the feed rate every 0.0005 (0.012 mm) alongthe tool path to provide an extremely smooth feed rate profile and a very accurate part. Using G-Code to convert this desiredfeed rate profile into actual control instructions for the servo motors would require a tremendous amount of programmingand controller memory. Instead, the power and memory of the modern PC can be used to compute and store the entire tool path and feed rate profile and then directly drive the servomotors that control the X-Y motion. These results in a more precise part that is considerably easier to create than if G-code programming were used. 2.2 Pump: Intensifier pump Early ultra-high pressure cutting systems used hydraulic intensifier pumps exclusively. At thetime, the intensifier pump was the only pump capable of reliably creating pressures high enough for water jet machining. Anengine or electric motor drives a hydraulic pump which pumps hydraulic fluid at pressures from 1,000 to 4,000 psi (6,900 to27,600 kpa) into the intensifier cylinder. The hydraulic fluid then pushes on a large piston to generate a high force on a   Abrasive Jet Machining  International Journal of Modern Engineering Research (IJMER)www.ijmer.com Vol. 3, Issue. 3, May.-June. 2013 pp-1504-1511 ISSN: 2249-6645  www.ijmer.com 1505 | Page 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.  Crankshaft Pump The centuries-old technology behind crankshaft pumps is based on the use of a mechanical crankshaft tomove 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 intothe cylinder. 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 veryuniform pressure output without needing to use an attenuator system. FIG: 1 Crankshaft pump Crankshaft pumps were not generally used in ultra-high pressure applications until fairly recently. This was because thetypical 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 345,000kpa) range with excellent reliability. This represented a major breakthrough in the use of such pumps for abrasive jet cutting.Experience has shown that an abrasive jet does not really need the full 60,000 psi (414,000 kpa) capability of an intensifier  pump. In an abrasive jet, the abrasive material does the actual cutting while the water merely acts as a medium to carry it past the material being cut.  FIG: 2 Typical 20/30 horsepower crankshafts driven triplex pump. This greatly diminishes the benefits of using ultra-high pressure. Indeed many abrasive jet operators with 60,000 psi(414,000 kPa) intensifier pumps have learned that they get smoother cuts and more reliability if they operate their abrasive jets 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 abrasive jet systems are being sold with the more efficient and easily maintained crankshaft-type pumps. 2.3Nozzles All types of abrasive jet systems use the same basic two-stage nozzle as shown in the FIG. First, water passesthrough a small-diameter jewel orifice to form a narrow jet. The water jet then passes through a small chamber where aVenturi 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 mixingtube. The resulting mix of abrasive and water exits the mixing tube as a coherent stream and cuts the material. It's critical  International Journal of Modern Engineering Research (IJMER)www.ijmer.com Vol. 3, Issue. 3, May.-June. 2013 pp-1504-1511 ISSN: 2249-6645  www.ijmer.com 1506 | Page 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 abrasive jet will be diffused, the quality of the cuts it produces will be poor, andthe life of the mixing tube will be short. FIG: 3. Typical abrasive jet nozzle The typical orifice diameter for an abrasive jet nozzle is 0.010 to 0.014 (0.25 mm to 0.35 mm). The orifice jewelmay be ruby, sapphire or diamond, with sapphire being the most common.The venturi chamber between the jewel orifice and the top of the mixing tube is an area that is subject to wear. Thiswear is caused by the erosive action of the abrasive stream as it enters the side of the chamber and is entrained by the water  jet. Some nozzles provide a carbide liner to minimize this wear. Precise alignment of the jewel orifice and the mixing tube iscritical to mixing tube life. This is particularly true for the relatively small diameter 0.030 (0.75 mm). Mixing Tube  The mixing tube is where the abrasive mixes with the high-pressure water.The mixing tube should be replaced when tolerances drop below acceptable levels. For maximum accuracy, replace themixing tube more frequently. The size of the kerf and cutting performance are the best indicators of mixing tube wear. FIG: 4 Mixing Tube   2.4   Motion System: X-Y TablesIn order to make precision parts, an abrasive jet system must have a precision X-Y table and motion control system. Tablesfall into three general categories:i) Floor-mounted gantry systems with separate cutting tablesii) Integrated table/gantry systems  International Journal of Modern Engineering Research (IJMER)www.ijmer.com Vol. 3, Issue. 3, May.-June. 2013 pp-1504-1511 ISSN: 2249-6645  www.ijmer.com 1507 | Page iii) Floor-mounted cantilever systems with separate cutting tablesEach type of system has its benefits and drawbacks. 1. Floor-mounted gantry with separate cutting table A floor-mounted gantry with a separate cutting table is the most common approach used by water jet systemmanufacturers. A framework that supports the X-Y motion system is secured directly to the floor and straddles a separatecutting table and catcher tank. The nozzle(s) is mounted to a carriage which moves along a gantry beam that straddles thetable. The gantry beam is supported on each end by a guide system and is moved by ball screws, rack and pinion assembliesor drive belts located at each end. The parallel drive mechanisms are either operated by two electronically-coupled drivemotors or by a single motor driving a mechanically-coupled drive system. 2. Integrated table/gantry system The integrated table/gantry system is very similar to the traditional gantry system previously described, except thatthe guides for the gantry beam are integrated into the cutting table. Because of this the X-Y motion system and the materialsupport table are part of the same overall structure and unwanted relative motion between them is eliminated. In this type of system, the floor is not a vital part of the system structure. This system is typically more accurate than the more traditionalseparate gantry and table. 3. Floor-mounted cantilever system with separate cutting table
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