Mechanical Erosion Processes Mechanical Erosion Processes Machinery's Handbook, 31st Edition
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Many conductive and non-conductive materials can be machined or cut using mechani- cal erosion processes that exert relatively low force on the workpiece or part and do not introduce thermal effects. Currently, the most prevalent of these processes are water jet machining (WJM) and abrasive water jet machining (AWJM). Other mechanical erosion processes include ultrasonic machining (USM), abrasive jet machining (AJM), abrasive flow machining (AFM), and magnetic abrasive finishing (MAF). Water Jet Machining (WJM).— Conductive, non-conductive, reflective, thin, and very thick materials can be cut using water jet machining technology, also known as hydrody- namic machining, which includes both abrasive water jet machining and pure water jet machining. In both processes, a pump or intensifier pressurizes water and drives it through a small orifice to form a supersonic jet that exerts force on the workpiece. A typical system pressure is 60 ksi (414 MPa); some systems are capable of up to 90 ksi (620 MPa); and with some pump and orifice combinations, jet velocity can be as high as three times the speed of sound, or 2952 fps (900 m/s). The jet orifice normally is made of jewel, often sapphire or ruby; as the orifice can crack or chip from use, diamond may be used to extend tool life. One limitation of water jet cutting is its inability to cut into hollow sections without special precautions. Upon cutting through the first wall of the section, the jet will spread rapidly and either just gouge or cut a wide path through the far wall. A sacrificial part can be placed inside the hollow to absorb the jet force, but if the inserted part does not com- pletely fill the space, some blasting damage will occur inside the hollow due to ricochet. Water Jet Cut Quality: Water jet systems can create relatively smooth cuts that require no secondary finishing. Cut quality typically decreases with decreasing speed. Subjective cut quality designations typically used in industry range from Q1 to Q5 and refer to surface qual- ity. Q1 cuts, suitable for rapid separations or roughing, have a heavily striated surface, with curved striations, due to stream lag induced by head travel. Q2 is appropriate for general- purpose through cuts, often producing heavy striations toward the bottom. Q3 cuts are com- monly used for general-purpose clean cuts, while Q4 cuts produce a finer surface finish. Q5 cuts have the best surface finish, often targeting a roughness average (Ra) of 80 μ in. (2 μ m). Additional cut characteristics, such as accuracy, kerf width, taper (bevel), entrance rounding, and exit burr, are managed through process and maintenance. (Refer to Table 1 and Table 2 for some typical performance characteristics of precision water jet systems.) Obtaining a straight cut edge may require optimizing process parameters or employing a tilting head to compensate. Head component wear affects accuracy and is of particular concern in abrasive water jet processes. Kerf width, or the width of material removed in cutting, is a function of jet diameter, standoff distance, cutting speed, and taper. With increased standoff distance, kerf width and taper increase. Standoff distance between the nozzle and workpiece typically ranges from 0.063 in. (1.6 mm) to 0.125 in. (3.2 mm). Excessive rounding of the entrance often occurs at standoff distances beyond this range. Taper angle grows with increasing cutting speed; it is possible to produce a cut with neg- ligible taper, but the process is likely to be slow. High speeds also will produce a tapered cut that is wider at the entrance than it is at the exit. Taper tends to increase in workpieces less than 0.125 in. (3.2 mm) thick, so stacking parts may be used to decrease taper. Very slow speeds or very soft workpieces can result in a reverse tapered cut that is nar- rower at the entrance than it is at the exit; minimizing standoff distance can reduce this taper. When the workpiece is very thick, cuts may exhibit barrel taper, where the kerf is widest halfway through the cut, and narrowest at both entrance and exit. When cutting through ductile materials, the presence of a small burr at the cut exit is likely, but can be minimized through speed and standoff reduction. Abrasive Water Jet Machining (AWJM): This process is appropriate for cutting hard and/or thick materials, including certain metals, ceramics, and stone. Metal workpieces less than 2 in. (50.8 mm) thick are well suited for abrasive water jet machining, and even
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