Metal Cutting - Chip Formation; Cutting in Microstructure
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C 1246 en The basic process of machining is chip formation. Under practical conditions, such as here on a lathe, details of chip formation taking place at the actual point of impingement between workpiece and tool cannot be observed.
With the help of 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. As the following shot shows, while cutting, the wedge
of a tool penetrates into the surface layer of a workpiece. It displaces the cutting material in the direction of work feed, causing it to run off as a chip across the face to the left. The ferritic-pearlitic material structure has been rendered visible by polishing and etching. The frame width of these cinemicrographs represents about 0.4 mm. If the shear zone runs continuously without any fractures arising, the shearing process gives rise to a flow chip with a correspondingly consistent structure. Flow chips are formed while cutting ductile materials such as this carbon steel C 45. As the ductility of a given material depends on the state of stress pertaining, small shearing fissures occur on the surface. The degree of deformation of cut material in the shear zone is governed entirely by the rake angle. A rake of 45 Grad results in a relatively slender, knife-like cutting wedge which only causes small shearing deformations during chip formation. The flow chip is accordingly consistent in structure, with no distinctive deformations. The smooth upper surface of this chip of about 120 aim thickness is only occasionally interrupted by minute shearing fissures.
On a surface with a rake angle of 0 Grad, the chip flow is diverted at right-angles so that the compression and shearing processes produce a distinct deformation structure at the chip-forming zone. The shearing fissures on the upper surface of the chip give to comma-like spikes and a correspondingly roughened appearance.
At a negative rake angle of minus 20 Grad, the load on the cut material is exceptionally high. In addition to the primary shearing deformation, the friction on the chip surface together with the high compression stresses give rise to secondary shearing on the lower side of the chip. This results in a clearly delineated, bright flowing layer. The irregular shearing fissures on the upper surface give the chip a jagged appearance. An insignificant increase in the thickness of the uncut chip under otherwise constant cutting conditions results in this material in a changeover to a continuous chip. In periodic sequence, individual elements of a continuous chip are formed by static friction and upsetting of the cut material on the one hand, and - as a consequence of the increasing cutting force - by shearing and gliding on the other. A new upsetting and adhering phase - shearing and gliding phase - and again the transition to upsetting and adhesion - as well as shearing and gliding of the chip along the cut surface.
A rounded edge represents an area of transition from face to flank, in which the rake angle varies to negative values. In this area the cut material is subject to high compression stresses, so that a flowing layer is formed at the cut surface. In the case of a scraping tool, that is, one with a very narrow uncut chip thickness, the chip is formed at the rounded edge directly in front of the flank in the region of greater negative rake. The cut material is compressed here and slides over the rounded edge as wedge-shaped parts of a continuous chip without touching the cutting surface again. Following this example of the formation of continuous chips with minimum uncut chip thickness again the formation
of larger elements of a continuous chip with free-cutting steel 9S20. Final phase of upsetting and commencement of shearing a new upsetting process beginning at the same time. The upset material now becomes detached from the face and, starting at the cutting edge the shearing process begins which is accomplished further to the right. A wedge-shaped material accumulation with superposition of additional layers of deformed material. Shearing off starting from the cutting edge, and renewed upsetting.
When cutting the aluminium alloy AlCuMgPb, chip formation varies between the flow chip and the continuous chip type. The influence of the changing crystalline orientation of this coarse-grained structure makes itself felt. Chip formation changes at grain boundaries. And with it the deformation structure changes, as shown by the light and dark shading. At a grain boundary, alternating continuous chip shapes give way to a flow chip with a clearly defined shear plane. At the next grain boundary, the chip thickness increases. In this crystal structure, a flow chip with uniform structure is again formed.
Tear chips arise while cutting brittle material such as cast iron GG 20. In this case, separate irregularly shaped chips are torn out of the cut material with negligible deformation. By preference, the areas of detachment are the graphite flakes embedded in the cast iron, which are visible in the polished but un-etched structure. Because of the wedge effect of the cutting wedge the work material is stressed even beyond the cutting edge to such an extent that the cast structure is partially loosened. In places individual structural elements are also broken out completely. This gives rise to the surface finish typical of cast iron. In this shot the frame width represents about 0,8 mm.
Resuming the previous frame width of about 0.4 mm, the behaviour of the cast structure stressed by the cutting edge is more clearly perceptible. While the pearlitic structural constituents exhibit onsets of plastic deformation, the embedded graphite flakes are squeezed out as a soft mass and thereby act as a lubricant. The areas of detachment of the individual chip elements are as irregular and discontinuous as the chip elements themselves with regard to direction and shape. The areas of detachment extend partly into the workpiece and irregularly back to the surface depending on the direction and location of the graphite flakes.
At a rounded edge, brittle materials are also subject to plastic deformations on account of a high state of compressive stress. As a consequence of the secondary shearing at the flank, material flows in the surface layer of the workpiece. The surface is thereby smoothed over with a pearlitic flowing layer, which causes the grey cast iron to lose its typical finish and characteristic properties.
With increasing uncut chip thickness, the wedge effect of the cutting wedge is intensified. The tearing out of individual pieces of chip is also simultaneously increased, resulting in the typical cast iron chip formation pattern.
Proof <Graphische Technik>
|Titel||Metal Cutting - Chip Formation; Cutting in Microstructure|
|Alternativer Titel||Zerspanen metallischer Werkstoffe - Spanentstehung; Schnittvorgang im Feingefüge|
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|IWF-Filmdaten||Film, 16 mm, LT, 145 m ; SW, 13 1/2 min|
|Abstract||Microcinematographic shots. Chip formation (flow chips, shear chips, tear chips) in polished and etched structures of steel C 45, steel 9 S 20, aluminium alloy AlCuMgPb and cast iron GG 20 during the cutting process with cutting tools of differing cutting wedges (rake angle and rounded edge). Shear deformation, shear plane, flow zone.|
aluminium alloy / fine etching / chip build-up
built-up edge formation
cast iron / fine etching
light metal / fine etching
steel / fine etching
chip formation / flow chips / metal
chip formation / tear chips / metal
chip formation / shear chips / metal