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Experimental Research on Micro-milling of Hard Alloy by Diamond Coated Tools

Cemented carbide has the advantages of high hardness, good wear resistance, corrosion resistance and small thermal expansion coefficient. It is widely used in the manufacture of molds for optical glass forming, metal stretching and wear-resistant and corrosion-resistant parts. Carbide molds not only have a long service life, but also are ten times or even hundreds of times more than steel molds; and the surface quality of the products is very high, and the injection molded glass lenses and other parts can meet the optical surface quality requirements.

        Carbide has poor processing properties and is a typical difficult material to process. Grinding and EDM are two common methods of machining carbide molds. With the advent of superhard tools such as CBN and diamond, direct machining of hard alloys has become possible, and more and more attention has been paid. Foreign scholars have carried out more research. B.Bulla et al. analyzed the influence of machining parameters on the surface profile of cemented carbide machining in diamond turning. After obtaining the excellent machining parameters, the influence of tool geometry on surface roughness and tool wear was further studied. N.Suzuki et al. conducted a diamond ultrasonic elliptical vibration turning cemented carbide test and found that the surface quality of ultrasonic elliptical vibrating turning is better than that of ordinary turning, and the tool wear is smaller. The micro prism with optical surface quality is also processed through experiments. Carbide molds such as spherical lenses.

        Manufacturing, complex, long-life cemented carbide molds are an important indicator of the level of national mold manufacturing. The micro-milling technology has the advantages of high processing efficiency, wide range of processing materials, three-dimensional complex shape processing, high surface quality, etc. It is very suitable for processing hard alloy micro-molds and small parts, and has broad application prospects. In this paper, the diamond-coated tool is used to carry out the experimental research on fine-milling cemented carbide, and the cutting force, surface quality and tool wear during machining are analyzed.

        1 Test equipment and solutions

        The self-built high-fine milling machine (see Figure 1) is designed for the micro-milling of micro-miniature parts. It consists of a marble bed, a feed mechanism, a high-speed air-floating spindle, and a PMAC-based motion control system. Due to the small diameter of the micro-milling cutter, it is not easy to achieve the tool setting. The machine is equipped with a microscope tool setting system, which can also be used to monitor the micro-milling process online.

Figure 1 Micro Milling Machine

        A diamond-coated spiral-edge micro-milling cutter (see Figure 2a) is used. The tool base material is cemented carbide and a diamond film is coated by chemical vapor deposition (CVD). The shank has a diameter of 6 mm, a blade diameter of 1 mm, a blade length of 2 mm, a tool rake angle of 2°, a back angle of 14° and a helix angle of 35°. From the SEM side view of the tool, the radius γ ε of the tool tip is about 11 μm (see Fig. 2b); the radius γβ of the edge of the tool measured from the SEM top view is about 8 μm (see Fig. 2c).

(a) (b)

(c)

Figure 2 Diamond coated micro-milling cutter

        Milling groove machining with different processing parameters using diamond coated tools. The surface of the workpiece was polished before the test, and then fixedly clamped on the Kistler 9256C1 dynamometer with a sampling frequency of 20 kHz. Dry-cutting conditions were used for all tests. The micro-milling test parameters are shown in Table 1. The spindle speed n is fixed at 20000r/min, and the milling depth ap is selected at 2μm and 4μm. The feed fz per tooth range is 0.3-1.5μm. After the test, the workpiece was cleaned using an ultrasonic cleaner, and the surface roughness and microscopic profile of the machined surface were measured along the feed direction using a Mahr surface roughness meter, and the surface morphology and tool wear morphology were observed by electronic scanning electron microscopy.

        2 test results and analysis

        (1) Cutting force

        The milling force signal is an important parameter for monitoring the milling process and can reflect the tool wear state and the quality of the machined surface in real time. During the milling process, the cutting thickness changes continuously, and as the milling cutter rotates from zero to large and then decreases to zero, the milling force signal also appears troughs and peaks. From the milling force signal waveform, the machining process can be observed. Abnormal behavior such as uniform cutting and vibration.

        Figure 3 is a waveform diagram of the measured milling force signal, where Fx is the main cutting force, Fy is the feed force, and Fz is the axial force. It can be seen from the milling force waveform that in the three component forces of the milling process, the amplitude of the axial force Fz is large, much larger than the other two component forces, followed by the main cutting force Fx and the small is the feed force Fy. The reason is analyzed. The milling depth ap in micro-milling is very small, much smaller than the radius γε of the tool nose of the micro-milling cutter. The tool actually participates in cutting only a small part of the bottom of the tool-point arc, which is equivalent to the tool being small. The main yaw angle is cut, resulting in a large axial milling force component.

        Each revolution of the milling cutter, two symmetrical cutting edges will participate in the cutting in turn, showing two peaks in the cycle of the milling force signal. As can be seen from the waveform diagram, the amplitudes of the two peaks are not the same, and the amplitude of the first half of the peak is significantly larger than the second half. This shows that in the actual milling process, the cutting depth of the two cutting edges of the double-tooth milling cutter is different, one blade cuts more material, and the other blade removes less material, resulting in uneven milling. Severe uneven milling can cause fluctuations in milling force and increase vibration during machining, which is not conducive to the stable operation of micro-milling.

Figure 3 Micro-milling force signal waveform

        During the test, the milling force under different micro-milling parameters was recorded. The peak value of the corresponding milling force was taken as the test result when the large cutting thickness was in the tool rotation cycle. Figure 4 shows the measurement results of the X, Y and Z three-way component forces. At the same milling depth, the milling force increases as the feed per tooth fz increases. The main cutting force Fx and the feed force Fy rise relatively slowly. At the milling depths ap=2μm and 4μm, the main cutting force Fx rises from 0.44N and 0.92N to 1.34N and 2.05N, respectively; the feed force Fy is from 0.38N and 0.81N rises to 1.07N and 1.49N; the axial force Fz increases by a large margin, rising from 1.21N and 2.45N to 3.43N and 6.87N, respectively. Similarly, an increase in milling depth can also result in an increase in milling force. The axial force Fz of the three-way component is sensitive to the milling parameters. The reason is that the feed per tooth fz in micro-milling is smaller than the radius γβ of the edge of the micro-milling cutter, which makes the bottom flank of the micro-milling cutter The contact area of the workpiece is relatively large, and the frictional force on the flank face accounts for a large proportion of the milling force.

Figure 4 Milling force with processing parameters

        (2) Surface quality

        Cemented carbide is a hard and brittle material. In traditional cutting, the hard and brittle material is usually removed in the form of brittle fracture, which causes brittle fracture defects on the machined surface and affects the quality of the machined surface. Studies have shown that when the processing parameters are controlled so that the cutting thickness during processing is less than a certain critical value, the brittle material can also be plastically deformed, and the smooth and ductile surface is called ductile cutting. Fig. 5 is a surface topography and profile curve of micro-milled cemented carbide at ap = 2 μm and fz = 1.2 μm. It can be seen from the figure that the shape of the machined surface is mainly reproduced by the geometry of the tool, and a clear texture of the knife mark is distributed. The feed tool mark of the tooth can be observed from the contour curve, and there is almost no brittle fracture defect. The actual cutting thickness in micro-milling is very small, and ductile cutting of cemented carbide can be achieved. The cemented carbide material is removed by plastic deformation to obtain a good surface quality.

(a) Surface topography

(b) contour curve

Figure 5 Machining surface topography and contours

        Figure 6 is a graph showing the surface roughness Ra of the fine-milled cemented carbide with the processing parameters. It can be seen from the figure that the surface roughness Ra of the obtained cemented carbide is small due to the ductile cutting in the micro-milling. The surface roughness Ra gradually increases with the increase of the feed amount per tooth ap and the milling depth fz, but the influence of the feed amount per tooth on the surface roughness is greater than the influence of the milling depth. When ap = 2 μm and fz = 0.3 μm, the small surface roughness was 0.073 μm; at ap = 4 μm and fz = 1.5 μm, the surface roughness was increased to a large value of 0.151 μm.

Figure 6 Surface roughness curve with processing parameters

        (3) Tool wear

        Figure 7 shows the tool wear profile of a diamond-coated micro-milling cutter after machining a hard alloy for a distance. The diamond-coated micro-milling cutter has uneven wear on the two teeth, one tooth is severely worn, and one tooth is slightly worn, which further verifies the uneven milling phenomenon in the milling force signal waveform.

        Due to the high hardness and wear resistance of the cemented carbide, the teeth with a large amount of cutting material are severely worn, the cutting edge becomes dull, and the radius of the cutting edge of the cutting edge becomes large (see Fig. 7b). The main feature of the non-gradual wear process of the blade edge chipping is that it can clearly see that a large part of the material is missing from the entire tool tip collapse, resulting in a large area of the diamond coating falling off and exposing the tool base material. The teeth with less material removed were slightly worn and no chipping was observed. The friction marks on the surface of the tool indicated that the diamond coating was a slow wear process and the tip was still sharp (see Figure 7c). There are many reasons for the uneven milling phenomenon, the symmetry error in the manufacture of the double-tooth milling cutter itself, the clamping error of the tool and the jumping of the spindle itself.

(a) (b)

(c)

Figure 7 Tool wear profile

Figure 8 Effect of tool wear on surface roughness

        Figure 8 shows the surface roughness as a function of the micro-milling path. As can be seen from the figure, the surface roughness Ra gradually increases as the milling path increases. When the milling distance reaches 700mm, the surface roughness increases greatly. When the milling distance exceeds 700mm, the surface roughness increases slowly. After milling 1000mm length, the surface roughness Ra reaches 0.224μm. After the tool wears, not only the milling force increases, but also the extrusion and friction of the workpiece become more serious, which increases the possibility of brittle fracture of the cemented carbide material, produces brittle fracture defects on the machined surface, and deteriorates the surface quality of the machined surface. The surface roughness increases.

        summary

        (1) Since the milling depth is much smaller than the radius of the tool nose arc, only the bottom of the tool nose arc is actually involved in the cutting, resulting in a large axial component. Uneven milling occurs in the micro-milling of diamond-coated double-tooth milling cutters, and the milling force increases as the feed per tooth and the depth of milling increase.

        (2) The actual cutting thickness in micro-milling is small, and ductile cutting of cemented carbide can be achieved, and a good surface quality can be obtained. The surface roughness Ra gradually increases as the feed per tooth and the depth of milling increase.

        (3) Uneven milling phenomenon causes uneven wear of the two blades and severe wear of the bearing blade. The surface roughness gradually increases as the milling path increases.

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