Machinery's Handbook, 31st Edition
1350 Electro-Thermal Processes dross. For a given application, there usually is a speed range for dross-free cutting, though there may be no dross-free speed for very thick workpieces. Gouges are made with plasma systems by tilting the torch relative to the workpiece, so the molten material is blown away on the top of the plate. This technique may be used to remove material in preparation for repair or re-work. Plasma Cutting Assist Gas: Plasma cutting gas selection for best cut quality depends on material type and thickness. Oxygen is the best gas for cutting mild steel, because it allows the highest cutting speed for a high-quality cut. Air also can be used to cut mild steel, but nitriding can occur and should be considered if the cut edge is to be welded. Stainless steel can be cut with good speed and bevel angle using air, but the cut edges will be rough and black because of oxidation and may require secondary cleanup operations. Using nitrogen plasma gas on stainless steel reduces oxidation and gives a smooth cut sur- face but still adds dark color, especially with material thicker than 0.25 in. (6 mm). Adding a water shield flow when using nitrogen plasma gas can give an edge color that is close to the base material color and improve the taper and rounding of the cut edge. Plasma gases containing hydrogen provide a further improvement in the appearance of the cut edge. F5 plasma gas (95 percent nitrogen, 5 percent hydrogen) can give bright silver cut edges in stainless steel up to 0.375 in. (9.5 mm) thick, while H35 (65 percent argon, 35 percent hydrogen) improves the edge color for thicknesses greater than 0.5 in. (12.7 mm). Nitrogen is the most common shield gas when cutting with hydrogen-containing plasma gases. Aluminum cutting can be done with air, nitrogen, or hydrogen-containing gases. Edge quality is often rougher with the oxygen-containing gases and smoother with the hydrogen gas processes. Some modern systems can mix gases, such as argon, hydrogen and/or nitrogen to improve results on both stainless steel and aluminum. Electron Beam Machining (EBM).— This non-contact machining method employs a beam of high-velocity electrons, usually applied in a vacuum, to make cuts and gouges. As the electrons collide with the workpiece, their kinetic energy produces heat and vaporiza- tion of material. This process is often chosen to perforate thin materials at high speeds. Almost any material up to 0.4 in. (10mm) can be machined with this method, usually with- out burr, and with relatively minor thermal effects. Tapered holes and cuts occur when part thickness exceeds 0.005 in. (0.13 mm). The finish and speed of electron beam hole drilling are superior to most other methods, but the capital equipment cost is high. Electron beams also are used in hardening, diffusion bonding, powder bed fusion, and welding processes. CNC NUMERICAL CONTROL PROGRAMMING Numerical control programming—better known as CNC programming—is an integral part of technology known as Numerical Control (NC), or its modern equivalent—Comput erized Numerical Control (CNC). This technology can be defined as an operation of ma - chine tools by means of specifically coded sequential instructions to be processed by the control system. These instructions are combinations of letters of the English alphabet “A- Z”, all ten digits “0–9”, and a few selected symbols, such as a decimal point “.”, negative sign “-”, parentheses “( )”, forward slash “/”, and several others. The main purpose of a CNC program is to provide detailed instructions necessary to machine a part (also called a workpiece) for a given machine setup—with minimal human interaction. All instructions to the control system must be written in a logical order and in a specified format called the program structure. The resulting CNC Program or a Part Program can be stored on various media for the future and used repeatedly to achieve identical machining results at any time. A completed part program is processed by the CNC unit—commonly called the control system—and individual program instructions are carried out to complete desired actions of the CNC machine. These actions include all required machine activities, such as motion control, spindle speed, cutting feed rates, coolant functions, program flow control, various adjustments, etc. In order to develop a CNC program, a qualified person—typically called a CNC Programmer—is assigned to the task.
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