Metal Cutting - Built-up Edge Formation; Cutting in Microstructure
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C 1247 en The cutting process can be influenced and changed through the formation of built-up edges. Under practical conditions, such as here on the lathe, this remarkable process of forming a built-up edge, which takes place at the actual point of impingement of workpiece and tool, cannot be distinguished.
With the help a special experimental technique, the metal-cutting operation can be prepared in such a way that microscopic observation and filming are possible. The cutting operation is performed as an orthogonal
cutting at the definite optical plane of a quartzglass plate. It is thus possible to visualise the individual phases of material structure deformation in the area of the chip root and the processes in the zones of tool contact.The following shot shows the penetration of
a rounded tool cutting wedge into the surface layer of a polished and etched workpiece.High pressure, friction, adhesive and welding processes in the zones of tool contact, as well as - Deformations and strain hardening of cut material contribute to the formation of a built-up edge.The frame width of these cinemicrographs represents about 0.4 mm. From the flowing material a built-up edge arises; its shape, size and stability depending on the cutting conditions. If the balance of forces at the built-up edge is deranged, it detaches itself from the cutting edge and runs off with the workpiece. Immediately afterwards, the formation of a new built-up edge is resumed. The formation process. A stable phase with a sharp wedge at the built-up edge and forward dislocation of shearing process, beginning instability - sliding off - and impressing into the surface layer of the workpiece. Renewed formation of a built-up edge. At a rake angle of 0 grad the chip is deflected more forcefully, and the area of deformation round the cutting edge is smaller. The built-up edge grows and juts out over the rake face until it becomes unstable - and runs off. Renewed formation process. The foremost area slides off - followed by the formation of a new built-up edge. The cutting speed proves to be the decisive limiting factor in built-up edge formation. Speed acts via the temperature and the deformation velocity upon the deformation behaviour and stability of the cut material. In cutting carbon steel C 45 at a speed of 2.5 m per min., only a small built-up edge becomes affixed to the cutting edge. Up to this speed sharply defined camera shots are possible. At higher cutting speeds, such as here at 10 m per min., motion unsharpness is inevitable. In this case, a very stable and acutely shaped built-up edge has joined the cutting wedge of the tool. It promotes chip formation while at the same time protecting the cutting edge. At the apex of this built-up edge, small particles of material settle and run off irregularly. With increasing cutting speed - here at 20 m per min. - the built-up edge also increases in size. It grows in the cutting direction and takes on a slender appearance. Apart from the momentary accumulations of minor particles at the apex, this built-up edge of about 70 micrometer in height is extremely stable. At a cutting speed of 30 m per min., the built-up edge reaches considerable proportions. The wedge, which becomes increasingly pointed towards the apex, now grows out beyond the cutting edge, with the result that a space is formed between flank and cutting edge, considerably increasing the thickness of the uncut chip. At the apex of the built-up edge, a wider area of changing material accumulations is located about a stable nucleus. At a cutting speed of 60 m per min., the increasing influence of the cutting temperature causes an intensive, changing flow of cut material in the area of the built-up edge. In conformity with the changing shape of the built-up edge, the chip formation is also irregular. The cutting temperature also influences the strength behaviour of the cut material. Like a pasty mass the material is piled up at the cutting edge, compressed, pushed along with the flow of the cut material, and finally torn off. The parts of the built-up edge that slip away run off with the chip as well as with the cut surface of the workpiece.
The carburizing steel 16 manganese chromium 5 tends to smear, and therefore it forms more rounded built-up edges. At a speed of 1.25 m per min., the built-up edge is still relatively small. In shape it exhibits a negative first flank. At 2.5 m per min., a larger, stable built-up edge is formed. This built-up edge is characterised by its relatively large wedge angle in conjunction with a negative clearance angle. On its flank, small particles of cut material settle irregularly, are dislodged again and run off with the workpiece. At 5 m per minute cutting speed, the size of the built-up edge increases still further. It has a more slender shape and its apical area changes with the momentary accumulations of material. The rounded apex projects in front of the tool cutting wedge, resulting in a space being formed between the flank and the cut surface. The ball-bearing steel 100 chromium 6 with annealed fine-grained structure can readily be cut. At this very low cutting speed of 0.01 m per min., the stability of the small built-up edge is preserved even under changing loads. At a cutting speed of 20 m per min., the built-up edge takes on a remarkable appearance. Round the stable, almost rectangular wedge of the built-up edge, the cut material flows with exceptional steadiness in the transition area between built-up edge and chip and workpiece. The cut surface of the workpiece also has an accordingly smooth appearance.The directions taken by the flowing material are indicative of the directions of the principle shear stresses.
|Titel||Metal Cutting - Built-up Edge Formation; Cutting in Microstructure|
|Alternativer Titel||Zerspanen metallischer Werkstoffe - Aufbauschneidenbildung; Schnittvorgang im Feingefüge|
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|IWF-Filmdaten||Film, 16 mm, LT, 115 m ; SW, 10 1/2 min|
|Abstract||Microcinematographic shots. Slow-motion pictures of an original frequency up to 7000 f/s. Built-up edge formation during chip build-up in the polished and etched structure of steel C 45, steel 100 Cr 6, and steel 16 MnCr 5. Form, size, and stability of built-up edges as a result of deformation, strainhardening and cutting temperature during changes of the cutting geometry and cutting speed.|
built-up edge formation
steel / fine etching