Magneforming (24 Mart 2011 12:45)
Magneforming is also called electromagnetic forming(EMF) is an assembly technique that is widely used to both join and shape metals and other materials with precision and rapidity, and without the heat effects and tool marks associated with other techniques. Also known as magnetic pulse forming, the EMF process uses the direct application of a pressure created in an intense, transient magnetic field. Without mechanical contact, a metal workpiece is formed by the passage of a pulse of electric current through a forming coil.
The parameters that determine the applicability of the EMF process are:
· Forming can be accomplished through a nonmetallic coating or container because the magnetic field passes through electrical nonconductors
· Most of the forming takes place after the pressure impulse has ended, in contrast to most metal-forming processes. The metal is rapidly accelerated, gaining a large amount of kinetic energy by moving only a short distance during the impulse. This kinetic energy subsequently does the actual work of forming
· The metals that are most efficiently formed by EMF are those with relatively high electrical conductivity, such as copper, aluminum, low-carbon steel, brass, and molybdenum. Metals with lower conductivity, such as stainless steel, can be formed by using either very high energy or an intermediate, highly conductive "driver" · The ratio of the masses of pieces used in assembly operations may be much more significant than their relative mechanical strength or elastic properties. Because EMF does not use static forces, relatively light structures can be used to support the dies
· No torque is applied to the workpiece in swaging and expanding operations, in contrast to spinning and rolling. Because the magnetic field behaves much like a compressed gas, it exerts a uniform pressure that is relatively independent of variations in spacing between the workpiece and the forming coil
· No lubricant is required because the contact between the magnetic field and the workpiece is frictionless
· The peak pressure is limited (by the strength of the forming-coil material) to much lower values than are commonly encountered in shearing, punching, and upsetting operations. However, the pressure that can be applied by the magnetic pulse can be very high compared to the average pressure in mechanical forming
· The process, being purely electromagnetic, is not limited to repetition rate by the mechanical inertia of moving parts. The timing of the magnetic impulse can be synchronized with microsecond precision, and machines can be made to function at repetition rates of hundreds of operations per minute. The strength of the magnetic impulse can be controlled electrically with high precision The major application of EMF is the single-step assembly of metal parts to each other or to other components, although it is also used to shape metal parts. Within the transportation industry, for example, one automotive producer assembles aluminum driveshafts without welding to save a significant amount of weight in light trucks and vans to meet requirements for reduced energy consumption. Using the EMF process allows the joining of an impact-extruded aluminum yoke to a seamless tube without creating the heat-affected zone associated with welding.
(Forming and Forging,ASM Handbook Volume 14, Joseph R. Davis; Page:1420,1421)
Trochoidal Milling (24 Mart 2011 20:22)
A trochoidal toolpath is defined as the combination of a uniform circular motion with a uniform linear motion, i.e., toolpath is a kinematics curve so-called trochoid. Light engagement conditions and high-speed milling are applied, in addition to large axial depth of cut. In this way a large radial width of cut is avoided. Slots wider than the cutting diameter of the tool can be machined, all with the same endmilling tool, usually an integral one. Since a small radial depth of cut is used, cutters with close pitch can be applied, leading to higher feed speed and cutting speed than with ordinary slot-milling applications. A main drawback is that toolpath length is much higher compared to standard toolpaths such as zigzag because large tool movements are without engagement into the material. Moreover, in the case of sculptured surfaces, overlarge steps are produced on the surface, making very difficult the following semi-finishing operation. Therefore it is recommended for slotted shapes but not for free-form machining. Currently all commercial computer-aided manufacturing (CAM) packages allow easy programming of this method.
(Machining of Hard Materials, J. Paulo Davim; Page:81-82)
Hard Broaching (24 Mart 2011 20:28)
In the field of metal cutting for mass production of parts with complex profiles, the broaching operation is the most economical method if high production rates combined with great consistency of machined parts are required. The advantages of broaching are based on its technical principle which includes a multi-toothed tool with cutting edges one after the other and graduated in depth of chip thickness. The profile of a part can be broached in single stroke. Internal broaching is started from a pre-machined hole, while external broaching is to machine a surface profile. Broaching is possible in both directions horizontal and vertical. Cutting motion can be linear or helical.
Two different methods of hard broaching are feasible with this tool configuration:
• hard broaching without defined stock removal: the parts are finish-broached before hardening and hard broaching means only clearing the heat distortion;
• hard broaching with defined stock removal (0.1–0.2 mm diametrical), which requires a corresponding finish of the pre-broaching tool considering the expected heat distortion.
(Machining of Hard Materials, J. Paulo Davim; Page:19)
Plunge Milling (24 Mart 2011 20:34)
This is a high-performance roughing technique in which a milling tool is moved multiple times in succession in the direction of its tool axis or of its tool vector into the material area that is to be removed, forming plunge-milling bores. The bores are superposed to eliminate the material of a pocket or zone. This technique is also referred to as milling in the Z-axis; it is more efficient than conventional endmilling for pocketing and slotting difficult-to-machine materials and applications with long overhangs. The machining parameters depend on the insert size, the tool overhang and the tool diameter. When a tool overhang of ∅ 6 mm is used, the usual step between two bores must be lower than 0.75 ∅. The radial depth of cut is 1 mm less than the radial length of the insert edge. If overhang increases the step must be reduced.
The advantages of the plunge-milling technique are:
• reduction by half in the time needed to remove large volumes of material;
• reduced part distortion;
• lower radial stress on the milling machine, meaning spindles with worn bearings can be used to plunge mill;
• long reach, which is useful for milling deep pockets or deep side walls.
Plunge milling is recommended for jobs such as roughing cavities in moulds and dies. It is recommended for aerospace applications, especially in titanium and nickel alloys. Inserts specifically for plunging are available for roughing and semi-finishing, but inserts suitable for high-feed milling can be also used for this technique.
(Machining of Hard Materials, J. Paulo Davim; Page:78-79)
No comments:
Post a Comment