42 Tribology of Metal Cutting
work materials, the chip shape is still a controllable parameter. When compression and
bending act together, much less energy has to be supplied to the machining zone and better
working conditions (at least the absence of dust) may be achieved. The tool geometry
plays an important role here. If selected properly, the chip formed has the appearance
of separate, almost rectangular elements and, therefore, is referred to as the regularly
broken chip.
As one might argue, however, a positive rake angle is not very practical in cutting cast
irons and similar brittle work materials due to the possible presence of significant amount
of hard inclusions. In such a case, a normal grade of tungsten carbide, as a tool material,
cannot withstand peak bending loads. As a result, practically all recommendations for
the tool geometry are the same suggesting a high negative rake angle that unavoidably
leads to the second model in Region A (Fig. 1.27). The chip formed consists of irregular-
shaped fragments of work material and dust, therefore, is referred to as the irregularly
broken chip. To overcome this barrier and to shift from the irregularly broken to the
regularly broken chip type, one should use positive rake angle when feasible. Modern
submicrograin carbides possess sufficient fracture toughness to withstand the discussed
inclusions successfully. The same logic is now applicable to high-speed machining of
high-silicon aluminum alloys widely used in the automotive industry. For many years,
polycrystalline diamond (PSD) brazed and indexable cutting inserts were used for this
purpose with negative rake angles. With the recent development of ultramicrograin PCDs
and advanced tool materials, cutting tool companies (for example, Kyocera, SP3, Mapal)
began to offer PCD insert with high positive (up to 10
◦
) rake angles that significantly
improve machining (tool life, machined surface integrity, reduce the cutting force, etc.)
of such alloys. Unfortunately, the recommendations for suitable tool geometries do not
reflect the great advances made in the last 5–10 years in the properties of tool materials
and coatings.
The last model in Region A is a kind of transitional model applicable to technically brittle
materials. As discussed above, some plastic deformation of the layer being removed is
allowed before a fragment of this layer separates from the workpiece.
Region B of the model shown in Fig. 1.27 covers two basic chip structures found in the
machining of most engineering materials. The chip formed according to the first model
in this region is referred to as the continuous fragmentary chip. As discussed, it is charac-
terized by non-uniform strength along its length. The chip fragments and their connectors
can be clearly distinguished on a micrograph of the chip structure. The difference in the
appearance of the chip fragments and connectors increases significantly with the cutting
speed. The chip formed according to the second model in this region is referred to as the
continuous chip. The major characteristics of its structure and properties were discussed
above. Additionally, it is necessary to point out that this chip is characterized by the
maximum (compared to the other chip structures) ratio “chip hardness/hardness of the
original work material” and by excessive length of the tool–chip interface.
Region C of the model shown in Fig. 1.27 represents two basic cases in the cutting
of highly ductile work materials. The first model in this region is basically the same
as the first model of Region B. However, great toughness of highly ductile materials
results in greater chip deformation and, as discussed above, the distance between the two
successive fragments becomes much smaller than that in the cutting of ductile materials.