16-06-2014, 12:09 PM
NONTRADITIONAL MACHINING
NONTRADITIONAL MACHINING.pdf (Size: 1.06 MB / Downloads: 237)
INTRODUCTION
Machining processes that involve chip formation have a number of inherent limitations which limit their application in industry. Large amounts of energy are expended to produce unwanted chips which must be removed and discarded. Much of the machining energy ends up as undesirable heat that often produces problems of distortion and surface cracking. Cutting forces require that the workpiece be held which can also lead distortion. Unwanted distortion, residual stress, and burrs caused by the machining process often require further processing. Finally, some geometries are too delicate to machine while others are too complex. Figure 28.1 shows some geometries which are difficult to machine by conventional methods.
ELECTROCHEMICAL MACHINING[/b]
Electrochemical machining, commonly designated ECM, removes material by anodic solution with a rapidly flowing electrolyte. It is basically a de-plating process in which
2
tool is the cathode and the workpiece is the anode; both must be electrically conductive. The electrolyte, which can be pumped rapidly through or around the tool, sweeps away any heat and waste product (sludge) given off during the reaction. The sludge is captured and removed from the electrolyte through filtration. The shape of the cavity is defined by the tool which is advanced by means of a servomechanism that controls the gap between the electrodes (i.e., the interelectrode gap) to a range from 0.076 to 0.76 mm. (0.25 mm typical).
MECHANICAL NTM PROCESSES ULTRASONIC MACHINING
Ultrasonic machining (USM), sometimes called ultrasonic impact grinding, employs ultrasonically vibrating tool to impel the abrasives in a slurry at high velocity against workpiece. The tool is fed into the part as it vibrates along an axis parallel to the tool feed at an amplitude on the order of several
3
thousandths of an inch and a frequency of 20 kHz. As the tool is fed into the workpiece, a negative of the tool is machined into the work piece. The cutting action is performed by the abrasives in the slurry which is continuously flooded under the tool. The slurry is loaded up to 60% by weight with abrasive particles. Lighter abrasive loadings are used to facilitate the flow of the slurry for deep drilling (to 5mm deep). Boron carbide, aluminum oxide, and silicon carbide are the most common used abrasives in grit sizes ranging from 400 to 2000. The amplitude of the vibration should be set approximately to the size of the grit. The process can use shaped tools cut virtually any material but is most effective on materials with hardness greater than Rc 40 including brittle and non conductive materials such as glass. Figure 28-15 shows a simple schematic of this process.
WATERJET CUTTING
Waterjet cutting (WJC), also known as water jet machining or hydrodynamic machining, uses a high-velocity fluid jet impinging on the workpiece to perform a slitting operation (Figure 28-16). Water is ejected from a nozzle orifice at high pressure (up to 60,000 psi). The jet is typically 0.076 to 0.5 mm in diameter and exits the orifice at velocities up to 900 m/sec. Key process parameters include water pressure, orifice diameter, water flow rate, and working distance (distance between the workpiece and the nozzle). Nozzle materials include synthetic sapphire due to its machinability and resistance to wear. Tool life on the order of several hundred hours is typical. Mechanisms for
4
tool failure include chipping from contaminants or constriction due to mineral deposits. This emphasizes the need for high levels of filtration prior to pressure intensification.
THERMAL NTM PROCESSES Electrical Discharge Machining [EDM
EDM processes remove metal by discharging electric current from a pulsating DC power supply across a thin interelectrode gap between the tool and the workpiece. See Figure 28-21 for a schematic.
5
The gap is filled by a dielectric fluid which becomes locally ionized at the point where the interelectrode gap is the narrowest, generally, where a high point on the workpiece comes close to a high point on the tool.
Wire EDM
Wire EDM, shown in Figure 28-25, involves the use of a continuously moving conductive wire as the tool electrode. The tensioned wire of copper, brass, tungsten, or molybdenum is used only once, travelling from a take-off spool to a take-up spool while being "guided" to produce a straight narrow kerf in plates up to 75 mm thick. The wire diameter ranges from 0.05 to 0.25 mm with positioning accuracy up to ± 0.005 mm in machines with NC. The dielectric is usually deionized water because of its low viscosity. This process is widely used for the manufacture of punches, dies, and stripper plates, with modern machines capable of routinely cutting die relief, intricate openings, tight radius contours, and corners.
Advantages and Disadvantages of EDM
EDM is applicable to all materials that are fairly good electrical conductors, including metals, alloys and most carbides. The hard ness, toughness, or brittleness of the material imposes no limitations. EDM provides a relatively simple method for making holes and pockets of any desired cross section in materials that are too hard or too brittle to be machined by most other methods. The process leaves no burrs on the edges. About 80 to 90% of the EDM work performed in the world is in the manufacture of tool and die sets for injection molding, forging, stamping, and extrusions. The absence of almost all mechanical forces makes it possible to EDM fragile or delicate parts without distortion. EDM has been used in microma-chining to make feature sizes as small as 0.01 mm.
PARTICLE BEAM MACHINING
As a metals-processing tool, the electron beam is used mainly for welding, to some extent for surface hardening, and occasionally for cutting (mainly drilling). Electron beam machining (EBM) is a thermal process that uses a beam of high-energy electrons focused on the workpiece to melt and vaporize metal. This process shown in Figure 28-26 is performed in a
9
vacuum chamber (10-5 torr), The electron beam is produced in the electron gun (also under vaccum) by thermionic emission. In its simplest form, a filament (tungsten) is heated to temperatures in excess of2ooo°C where a stream (beam) of electrons (more than 1 billion per second) is emitted from the tip of the filament.
LASER BEAM MACHINING
Laser beam machining (LBM) uses an intensely focused, coherent stream of light (a laser) to vaporize or chemically ablate materials. A schematic of the LBM process is shown in Figure 28-27. Lasers are also used for joining (welding, brazing, soldering), heat treating materials. Power density and interaction time are the basic parameters in laser processing as shown in Figure 28-28. Drilling requires higher power densities and shorter interaction times compared to most other
applications.