Residual stresses induced by machining operations can be assessed directly using, for instance, the X-ray diffraction method to measure the distance between planes of atoms, or indirectly, employing strain gauges or optical, electronic or mechanical displacement transducers to determine the deformation induced when the stresses are relieved. The residual stresses induced on a component are the result of a combination of mechanical and thermal effects. In general, the mechanical action (burnishing) leads to plastic deformation and promotes compressive residual stresses. The thermal effect (temperature rise due to friction and plastic strain) however, may promote tensile or compressive residual stresses depending on the maximum temperature reached at the workpiece and corresponding microstructural changes that take place. For instance, the transformation from austenite to martensite during rapid cooling (by the bulk material, cutting fluid or air) involves a volume expansion caused by the change from a face-centred cubic structure to a more open tetragonal structure, thus resulting in compressive stresses in the surface layers. The layers beneath the surface, however, reach lower temperatures and cool at slower rates, therefore, their contraction is restrained by the higher strength of the layers above. Consequently, tensile residual stresses may be induced below the machined surface. The resulting residual stress depends on the magnitude of the mechanical and thermal effects, nevertheless phase transformation induced by cutting can be neglected as a cause of residual stresses on hardened steels.
(Machining of Hard Materials, J. Paulo Davim; Page:129-130)
Microstructural Alterations (31 Mart 2011 07:09)
Microstructural changes in steels may be either mechanically or thermally induced. In the case of hardenable steels, if the workpiece temperature exceeds the austenization temperature during machining (due to friction and plastic strain), austenite will be formed and, after quenched by the cold bulk material or by the cutting fluid, a brittle, highly stressed and crack-prone martensite layer (usually called a white layer) is formed at the surface. However, if the workpiece temperature exceeds the tempering temperature only, then overtempering will take place, leading to the softening of the affected layers (identified as a dark layer after etching). The formation of the white layer is a thermal process involving phase transformation and, probably plastic deformation, which has not been fully understood yet. Turning tests on hardened AISI H13 tool steel (54–56 HRC) with PCBN inserts indicated that the hardness and depth of the white layer decreased as cutting speed was elevated owing to a slight reduction in the workpiece temperature. In
contrast, neither a white layer nor a heat affected zone were observed after high-speed milling AISI H13 hot-work die steel hardened to 47–49 HRC with TiAlN coated carbide ball-nose endmills. This difference can be explained by the fact that in the former work cutting speeds of 100, 400 and 700 m/min were tested, the thickest white layer being observed at the lowest cutting speed. For cutting speeds of 400 and 700 m/min, the depth of the white layer decreased dramatically. In the case of the latter work, cutting speeds of 200 and 300 m/min were used, which seem to be above the critical value required for the formation of the white layer.
(Machining of Hard Materials, J. Paulo Davim; Page:124-125)
Stepper Motors (31 Mart 2011 15:22)
The essential property of the stepping motor is its ability to translate switched excitation changes into precisely defined increments of rotor position (‘steps’). Stepping motors are categorised as doubly salient machines, which means that they have teeth of magnetically permeable material on both the stationary part (the ‘stator’) and the rotating part (the ‘rotor’). Magnetic flux crosses the small airgap between teeth on the two parts of the motor. According to the type of motor, the source of flux may be a permanent-magnet or a current-carrying winding or a combination of the two. However, the effect is the same: the teeth experience equal and opposite forces, which attempt to pull them together and minimise the airgap between them. As the diagram shows, the major component of these forces, the normal force (n), is attempting to close the airgap, but for electric motors the more useful force component is the smaller tangential force (t ), which is attempting to move the teeth sideways with respect to each other. As soon as the flux passing between the teeth is removed, or diverted to other sets of teeth, the forces of attraction decrease to zero.
(Stepping Motors a Guide to Theory and Practice, 4th edition, Paul Acarnley; Page:1)
3D Blow Molding (31 Mart 2011 15:57)
3D blow molding was introduced to
• Reduce flash
• Lower clamping forces
• Improve wall thickness distribution
• Reduce finishing work on the outside surface
3D blow molded parisons are made by moving the tool as the parison is pushed out of the die or accumulator. This lays the parison into the 3D mold. Alternatively, a robot arm can be used to position the parison. This process has gained its name from the three dimensional parison manipulation. In the first step, the parison is laid across the bottom of the tool; in the subsequent two steps, the bottom half of the tool moves downward as the parison is laid across its surface. After the parison is completely across the bottom mold, the top half closes to clamp the parison just prior to inflation.
(Extrusion: The Definitive Processing Guide and Handbook, Harold F. Giles, Jr., John R.Wagner, Jr.; Eldridge M. Mount, III; Page: 509)
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