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Abrasive Jet Machining | Machining | Pump

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Abrasive water jet machine tools are suddenly being a hit in the market since they are quick to program and could make money on short runs.
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  Abrasive Jet Machining INTRODUCTION 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. Theycomplement existing tools used for either primary or secondary operations and could make parts quickly out of virtually out of any material. One of the major advantage is that they donot heat the material. All sorts of intricate shapes are easy to make. They turns to be a money making machine.So ultimately a machine shop without a water jet , is like a carpenter with out a hammer. Sure the carpenter canuse the back of his crow bar to hammer in nails, but there is a better way. It is important to understand thatabrasive jets are not the same 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. Bothtechnology use the principle of pressuring water to extremely high pressure, and allowing the water to escapethrough opening typically called the orifice or jewel. Water jets use the beam of water exiting the orifice to cutsoft 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 vary high velocity beam of water. Abrasive jets use the same beam of water to accelerate abrasive particles to speeds fast enough to cutthrough much faster material.COMPONENTS OF ABRASIVE JET MACHINING CENTER The components of AJM centre include : ã Abrasive Delivery System ã Control System ã Pump ã  Nozzle ã Motion System 2.1Abrasive Delivery System  A simple fixed abrasive flow rate is all that's needed for smooth, accurate cutting. Modern abrasive feedsystems are eliminating the trouble-prone vibratory feeders and solids metering valves of earlier systems andusing a simple fixed-diameter orifice to meter the abrasive flow from the bottom of a small feed hopper locatedimmediately adjacent to the nozzle on the Y-axis carriage. An orifice metering system is extremely reliable andextremely repeatable. Once the flow of abrasive through the orifice is measured during machine set-up, thevalue can be entered into the control computer program and no adjustment or fine-tuning of abrasive flow will  ever be needed.The small abrasive hopper located on the Y-axis carriage typically holds about a 45-minute supply of abrasive and can be refilled with a hand scoop while cutting is underway. 2.2   Control SystemFundamental limitation of traditional CNC control systems. ã Historically, water jet and abrasive jet cutting tables have used traditional CNC control systemsemploying the familiar machine tool G-code. However, there is a rapid movement away from thistechnology for abrasive jet systems, particularly those for short-run and limited-production machineshop applications. G-code controllers were developed to move 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 discreteincrements for corners and curves. Each time a change in the feed rate is desired programming entrymust be made. Awater 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 willalso result in an uneven cut where the transition occurs. Changes in the feed rate for corners and curves must bemade smoothly and gradually, with the rate of change determined by the type of material being cut, thethickness, 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 givenmaterial to make a precise part. The algorithm actually determines desired variations in the feed rate every0.0005 (0.012 mm) along the tool path to provide an extremely smooth feed rate profile and a very accurate part. Using G-Code to convert this desired feed rate profile into actual control instructions for the servo motorswould require a tremendous amount of programming and 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 drivethe servomotors that control the X-Y motion. This results in a more precise part that is considerably easier tocreate than if G-code programming were used. 2.3   Pump:Intensifier pumps Early ultra-high pressure cutting systems used hydraulic intensifier pumps exclusively. At the time, theintensifier pump was the only pump capable of reliably creating pressures high enough for water jet machining.An engine or electric motor drives a hydraulic pump which pumps hydraulic fluid at pressures from 1,000 to4,000 psi (6,900 to 27,600 kPa) into the intensifier cylinder. The hydraulic fluid then pushes on a large piston togenerate a high force on a small-diameter plunger. This plunger pressurizes water to a level that is proportionalto the relative cross-sectional areas of the large piston and the small plunger. 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 ineach cylinder allow water to enter the cylinder as the plunger fig:2 (crankshaft pump)retracts and then exit the cylinder into the outlet manifold as the plunger advances into the cylinder.Crankshaft pumps are inherently more efficient than intensifier pumps because they do not require a power-robbinghydraulic 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. Crankshaft pumps were not generallyused in ultra-high pressure applications until fairly recently. This was because the typical crankshaft pumpoperated at more strokes per minute than an intensifier pump and caused unacceptably short life of seals andcheck valves. Improvements in seal designs and materials, combined with the wide availability and reducedcost 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 abrasive jet cutting.Typical 20/30 horsepower crankshafts driven triplex pump.Experience has shown that an abrasive jet does not really need the full 60,000 psi (414,000 kPa) capability of anintensifier 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. 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 thatthey 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 moreefficient and easily maintained crankshaft-type pumps. 2.4   Nozzles All abrasive jet systems use the same basic two-stage nozzle as shown in the FIG. First, water passes through asmall-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. Theabrasive particles are accelerated by the moving stream of water and together they pass into a long, hollowcylindrical ceramic mixing tube. The resulting mix of abrasive and water exits the mixing tube as a coherentstream and cuts the material. It's critical that the jewel orifice and the mixing tube be precisely aligned to ensurethat the water jet passes directly down the center of the mixing tube. Otherwise the quality of the abrasivejetwill be diffused, the quality of the cuts it produces will be poor, and the life of the mixing tube will be short.FIG: Typical abrasivejet 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 jewel may 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.This wear is caused by the erosive action of the abrasive stream as it enters the side of the chamber and isentrained by the waterjet. Some nozzles provide a carbide liner to minimize this wear. Precise alignment of the jewel orifice and the mixing tube is critical to mixing tube life. This is particularly true for the relatively small
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