24-12-2012, 06:58 PM
Abrasive Jet Machining
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INTRODUCTION
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. They are quick to set up, and offer quick turn-around on the machine. They complement 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 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 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. Both technology use the principle of pressuring water to extremely high pressure, and allowing the water to escape through opening typically called the orifice or jewel. Water jets use 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 vary high velocity beam of water. Abrasive jets use the same beam of water to accelerate abrasive particles to speeds fast enough to cut through much faster material.
Abrasive Delivery System
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.
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.
Control System
Fundamental limitation of traditional CNC control systems.
Historically, water jet and abrasive jet cutting tables have used traditional CNC control systems employing the familiar 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 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 discrete increments for corners and curves. Each time a change in the feed rate is desired programming entry must be made. A
water jet or abrasive jet definitely is not a rigid cutting tool; using a constant feed rate will result in severe undercutting or taper on corners and around curves. Moreover, making discrete step changes in feed rate will also result in an uneven cut where the transition occurs. Changes in the feed rate for corners and curves must be made smoothly and gradually, with the rate of change determined by the type of material being cut, the thickness, the part geometry and a host of nozzle parameters.
Nozzles
All abrasive jet systems use the same basic two-stage nozzle as shown in the FIG. First, water passes through a small-diameter jewel orifice to form a narrow jet. The water jet 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.
Integratedtable/gantrysystem
The integrated table/gantry system is very similar to the traditional gantry system previously described, except that the guides for the gantry beam are integrated into the cutting table. Because of this the X-Y motion system and the material support 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 traditional separate gantry and table.