The formation of chips and the relationship between the cutting force and the cutting speed are greatly influenced by the workpiece material and its heat treatment state. In addition to the thermal and mechanical properties of the material, the microstructure and chemical composition of the material determine whether or not to use a segmented cutting operation during high-speed cutting.
Fig. 1 Whether cutting force can be reduced during HSC machining depends on the material quality of the workpiece. Therefore, in order to achieve smaller cutting forces and smaller shape deviations, it is not necessary to use very high cutting speeds
Figure 2 features of high-speed cutting
High-speed cutting (HSC) is characterized by high cutting speeds and high cutting rates compared to conventional cutting processes. As the cutting speed continues to increase, for many materials, it is necessary to observe the drop in cutting force (see Figure 2). For a single group of materials, the cutting speed range can be determined empirically. A mathematical hypothesis describing the relationship between cutting force drop and cutting speed can be used to calculate the minimum limit cutting speed νHSC that can indicate high speed cutting conditions. Accordingly, the cutting force consists of a constant component associated with the cutting speed and an exponentially decreasing component as the cutting speed increases. If the dynamic component of the cutting force drops to 14% of its original value, the limit cutting speed is reached by definition (see Figure 3).
Figure 3 Calculating the cutting speed VHSC from the cutting force curve
In addition to the fact that the cutting force decreases as the cutting speed increases, it is also necessary to pay attention to the fact that when machining certain materials, the chips will change from continuous banded chips to fan-shaped chips. From the perspective of literature, there are currently two different explanations for the formation of fan-shaped chips:
(1) As the cutting speed increases, the speed at which the shape of the workpiece changes and the heat converted from heat in the effective machining area also increase. The first explanation is based on the assumption that due to the imbalance between heat generation and heat conduction, heat blockage (heat mode) occurs on the shear plane, which ultimately leads to thermal failure of the material. This phenomenon, known as the thermal insulation effect, is particularly evident on certain materials with poor temperature conductivity.
(2) The second explanation comes from hard machining, which is derived from the principle of high stress caused by the cyclic cracking of the shear surface in the cutting process. At the cutting edge, the material deforms plastically, producing banded chips that bind together the individual fan-shaped swarf.
A test was conducted at Bremen University to study the effects of mechanical, thermal and structural properties of the material on the chip formation characteristics, and the relationship between cutting forces and cutting speeds at high speeds when performing longitudinal cylindrical cutting. . Different material properties can be obtained by selecting different materials and by targeted heat treatment. Figure 3 shows the thermal energy (temperature conductivity) and mechanical (tensile strength at room temperature) characteristics of the tested material that have an important influence on chip formation. According to this mode of mechanical and thermal energy interpretation of fan-shaped chips, it can be assumed that materials with strong temperature conductivity but weak tensile strength are more likely to produce continuous banded chips (area above the diagonal line in Fig. 4); Materials with poor temperature conductivity but high tensile strength are more likely to generate fan-shaped chips (area below the diagonal line in Figure 4).
Figure 4 Arrangement of materials used according to their tensile strength and temperature conductivity
In order to express the characteristics of the chip type, the concept Gs of the degree of fanning is quoted, which can be used to evaluate the chips appearing through microscopic pictures and based on the ratio of the minimum chip thickness to the maximum chip thickness (Figure 5). Micrographs are made of chips produced at a cutting speed greater than the limit cutting speed νHSC. For a continuous banded chip, the fan-shaped degree index is equal to 0; if it is a completely fan-shaped chip, the fan-shaped degree index is 1.
Figure 5 Calculation of the degree of sectorization Gs
Conditioning affects chip formation
From a wide range of test materials, Figure 6 shows examples of different chip types that occur during high-speed cutting based on chip micrographs of 42CrMo4 and pure copper. To make the results comparable, CBN tools with negative cutting angles were used in both cases. Obviously the effect of the quenched and tempered state of 42CrMo4. At a hardness of 33 HRC, virtually no chip scalloping occurred. The lower Gs=0.22 should be attributed to the fact that the upper side of the chip was torn. When the hardness reaches 54HRC, the degree of fanning will reach Gs=0.45. Chip micrographs are still unable to show what the reason for the chip shape is: either thermal cutting (caused by heat) or cyclic cracking (mechanical). If there is little change in the temperature conductivity due to the high intensity of tempering, then the reason why the scalloped chips are generated is that the cracks are continuously generated at the front end of the shear region.
Cutting copper generated chips
Copper has the characteristics of good toughness and strong thermal conductivity. Therefore, this material does not generate scalloped chips. This property was also verified by the test results that the cutting speed reached νc = 5500 m/min. The micrograph (Fig. 6c) shows a continuous band of chips resulting from continuous deformation.
Fig. 6 Scalloping degree of chips of quenched and tempered 42CrMo4 and copper (c) with hardness of 33HRC (a) and 54HRC (b) in high-speed profile longitudinal turning test νC > νHSC, f = 0.1mm, Î±Ï = 0.1 mm, dry cooling ; Material: cutting material CBN, geometry PCLN-R161H12; Tool geometry: chamfer 95o, cutting angle -7 o, free angle 7 o, tip radius of 0.8 mm.
Other materials (such as aluminum alloy AlZnMgCu1.5 and free-cutting brass CuZn39Pb3) also indicate that they will affect the microstructure formation and the chemical formation of the cut. For aluminum alloys, tests were performed on different stretch conditions (maximum hardening, underage aging). Scalloped chips can only be observed in under-aged conditions. This can be attributed to an interactive separation that prevents the displacement of the shear region during deformation. In brass, perhaps due to the chip breaking action of lead, the formation of scalloped chips was caused.
A large portion of the material will decrease in cutting force as the cutting speed increases (see the table below). However, the test results show that this phenomenon is not due to the mechanism of chip formation alone, but the main reason is that the increase of the shear angle means that the cutting ratio decreases. For most steels, this is the case when fan-shaped chips are generated due to thermal mechanical properties during HSC machining.
In the tested pure aluminum, pure copper and pure iron materials, the cutting force was significantly reduced due to the increased cutting speed. In the range of cutting speeds tested, this material produced only continuous ribbon chips. This is mainly because these materials have very good temperature conductivity (aluminum and copper) and low tensile strength (aluminum/copper/ferric). The dependence of the observed cutting forces on the cutting speed is not significant in the formation of scalloped material due to chemical composition or microstructure.
Scalloping of chips will increase the cutting edge load
In the high-speed machining process, the fan-shaped variation of the chips can be observed according to the different materials used and their heat treatment conditions. In addition to the mechanical and thermal properties of the material, the chemistry and microstructure of the material also play an important role. In the design of high-speed cutting tools, special attention should be paid to this knowledge. This is because the scalloping of the chips will cause periodic loading of the cutting edges, resulting in shortened tool life. Whether the cutting force is reduced is also related to the material used. Therefore, in the actual work of industrialization, do not use high-speed cutting to achieve lower machining forces and smaller form deviations if it is not necessary.
The reduction in cutting force as the cutting speed increases is currently used in a DFG-sponsored program to reduce the form error when machining small 100Cr6 workpieces. In particular, the influence of deformation due to the generation and elimination of stress in the cutting must also be considered.