Method for evaluating the dynamic stiffness of machine tools

Abstract

This technology enables the evaluation of the dynamic rigidity of machine tools using the tools and holders actually used to process workpieces. [Solution] A method for evaluating the dynamic rigidity of a machine tool that processes a workpiece using a tool during the process, comprising: a first step of measuring the vibration of the tool and the processing reaction force that the tool receives from the workpiece while processing the workpiece under predetermined driving conditions; and a second step of measuring the vibration of the tool caused by driving the tool under the same conditions as the predetermined driving conditions, independently of the processing of the workpiece, and evaluating the dynamic rigidity during the process of processing the workpiece by determining the difference between a first value calculated from the processing reaction force and vibration measured in the first step and a second value calculated from the vibration measured in the second step.

Description

Technical Field

[0001] The present invention relates to a method for evaluating dynamic stiffness of a machine tool, and particularly to a method for evaluating dynamic stiffness of a machine tool using vibration of a tool during machining. Background Art

[0002] Conventionally, as stiffness evaluation methods for machine tools, dynamic stiffness tests by hammering have been performed in addition to static stiffness tests. Specifically, when evaluating the dynamic stiffness of the spindle of a machine tool such as a machining center, a method is known that comprises exciting a tool or the like attached to the spindle with an impulse hammer, measuring vibration with an acceleration pickup, and analyzing the obtained vibration using an FFT (Fast Fourier Transform) analyzer or the like.

[0003] As another conventional evaluation method, for example, in the method for measuring dynamic stiffness of a spindle for a machine tool described in Patent Document 1, an electromagnet and a non-contact displacement sensor are used. While the spindle is rotated, a measurement target such as the spindle of the machine tool spindle or a tool attached to the spindle is excited in a non-contact state by the magnetic attraction force of the electromagnet, and the displacement of the excited measurement target is measured in a non-contact state by the non-contact displacement sensor. Prior Art Documents Patent Documents

[0004] Patent Document 1 Japanese Unexamined Patent Publication No. 11-19850 Summary of the Invention Problems to be Solved by the Invention

[0005] However, even with dynamic stiffness tests using hammering, as well as static stiffness tests as described above, evaluations based on test results on, for example, the spindle of a machine tool or a workpiece, differ from the actual trends in tool behavior during cutting. Furthermore, dynamic stiffness evaluation using hammering is a skilled task requiring knowledge of vibration measurement and FFT analyzers, as well as hammering techniques, so it cannot be said to be an evaluation method that anyone can easily perform.

[0006] Furthermore, in the method for measuring dynamic stiffness described in Patent Document 1, an excitation force is forcibly applied from the outside, such as the spindle and tools attached to the spindle, by the magnetic attraction force of an electromagnet. Dynamic stiffness is then calculated from the relationship between the applied excitation force and the displacement generated in the spindle, etc. Therefore, the obtained value of dynamic stiffness includes the value of the centrifugal force generated in the spindle, etc., by the excitation force when the spindle rotates as an error, which presents a problem in that it is not possible to accurately evaluate the dynamic stiffness of the spindle, etc.

[0007] Therefore, the object of the present invention is to provide a technology that enables the evaluation of the dynamic rigidity of a machine tool using the tools and holders actually used when machining a workpiece. [Means for solving the problem]

[0008] The inventors of this invention have diligently researched methods for accurately evaluating the dynamic stiffness of the spindle and other components of machine tools. As a result, they have devised a method for evaluating dynamic stiffness from the vibration of the tool during machining and the machining reaction force that the tool receives from the workpiece. They have found that this method allows for the evaluation of dynamic stiffness during cutting operations, for example, and also enables testing of the dynamic characteristics of the tools and holders actually used.

[0009] In other words, in order to achieve the above objective, the present invention provides a method for evaluating the dynamic rigidity of a machine tool that processes a workpiece with a tool during the process, comprising: a first step of measuring the vibration of the tool and the processing reaction force that the tool receives from the workpiece while processing the workpiece under predetermined driving conditions; and a second step of measuring the vibration of the tool caused by driving the tool under the same conditions as the predetermined driving conditions, independently of the processing of the workpiece, and the dynamic rigidity during the process of processing the workpiece is evaluated by determining the difference between a first value calculated from the processing reaction force and vibration measured in the first step and a second value calculated from the vibration measured in the second step.

[0010] Furthermore, in the first and second steps, it is desirable to measure the vibration of the tool in at least two axial directions, the X direction and the Y direction.

[0011] Furthermore, it is preferable that the tool is a rotary tool and is used for lateral cutting of the workpiece. [Effects of the Invention]

[0012] According to the present invention, for example, in cutting processes, the dynamic characteristics during cutting can be evaluated, and the dynamic characteristics can be evaluated using the actual tools and holders used. Therefore, the dynamic stiffness of the spindle of a machine tool can be evaluated more accurately. [Brief explanation of the drawing]

[0013] [Figure 1]Figures (A) and (B) are schematic diagrams showing the main parts of a machine tool (machining center) to which the evaluation method of the embodiment of the present invention is applied. Figure (A) is a schematic diagram showing, from the side, a tool holder attached to the tip of the spindle, an end mill as a tool (hereinafter sometimes referred to as either the tool or the end mill), a workpiece to be machined, a jig plate and two gap sensors in the X and Y directions attached thereto, and a cutting force meter provided on the bottom surface of the workpiece via a vise. Figure (B) is a schematic diagram showing, from the top surface of the workpiece, the movement of the end mill relative to the workpiece during side cutting using the end mill to which the evaluation method of this embodiment is applied, i.e., the four cutting directions. [Figure 2] Figure 1(B) shows a graph illustrating the relationship between the displacement of the end mill in the X-axis direction, measured by the gap sensor X (see Figure 1(A)), and time during side cutting in the X-direction, one of the four cutting directions. The graph shows the relationship between the measured value due to tool vibration during machining (Milling) and the vibration value due to rotational runout of the tool before machining (No load), i.e., unrelated to machining. It shows sinusoidal data obtained from the calculated values ​​before machining (No load) and during machining (Milling) (the data is sinusoidal because the end mill is rotating). In the figure, the difference (dx) between the upper data during machining (Milling) and the lower data before machining (No load) corresponds to the amount of displacement due to the machining reaction force that the tool receives from the workpiece during machining. [Figure 3] (A) is a graph showing the relationship between the displacement amount (dx) in the X-axis direction due to the machining reaction force received by the tool from the workpiece during machining and time in the X-direction side cutting shown in Figure 1(B), and shows the change in the displacement amount in the X-axis direction over time. (B) is a graph showing the relationship between the displacement amount (dy) in the Y-axis direction due to the machining reaction force received by the tool from the workpiece during machining, measured by the Y-axis direction gap sensor Y (see Figure 1(A)) in the same way as the X-axis direction in Figure 2, and time in the X-direction side cutting shown in Figure 1(B), and shows the change in the displacement amount in the Y-axis direction over time. [Figure 4] This graph shows the distribution in the XY plane of the displacement amounts (dx) and (dy) obtained in Figures 3(A) and 3(B), respectively, during lateral cutting in the X-direction shown in Figure 1(B). [Figure 5]Figure 1(B) shows the time variation of the cutting force during side cutting in the X-direction. (A) is a graph showing the relationship between Fx(N) and time, and (B) is a graph showing the relationship between Fy(N) and time. [Figure 6] This graph shows the variance in the XY plane of the cutting forces Fx(N) and Fy(N) obtained in Figures 5(A) and 5(B), respectively, during lateral cutting in the X-direction shown in Figure 1(B). [Figure 7] Figure 1(B) is a graph showing the trajectory of one rotation of the tool (end mill) when the displacement (dx) is plotted on the horizontal axis and the displacement (dy) is plotted on the vertical axis during side cutting in the X-direction. It was found that the displacement (dx) on the horizontal axis is in the range of -0.0008 mm to -0.002 mm, and the displacement (dy) on the vertical axis is in the range of -0.0006 mm to -0.0015 mm, with the tool trajectory distributed within the range of θmin = 23.8° and θmax = 46.3°. [Figure 8] Figure 1(B) shows a graph representing the cutting force per rotation of the tool (end mill) when the cutting force Fx(N) is plotted on the horizontal axis and the cutting force Fy(N) is plotted on the vertical axis during side cutting in the X-direction. It was found that the cutting force Fx(N) on the horizontal axis ranges from -19(N) to -36(N), and the cutting force Fy(N) on the vertical axis ranges from -17(N) to -30(N), with the cutting angles being distributed within the range of θmin = 37.2° and θmax = 44.7°. [Figure 9] Figure 1(B) is a graph showing the trajectory of one rotation of the tool (end mill) for each of the four cutting directions. The horizontal axis represents displacement (dx), and the vertical axis represents displacement (dy). The graph shows the trajectory of one rotation of the tool (end mill) for each cutting direction in Figure 1(B): X+ direction (1st quadrant), X- direction (3rd quadrant), Y+ direction (2nd quadrant), and Y- direction (4th quadrant). The distribution of trajectories within each of the 1st, 2nd, 3rd, and 4th quadrants was observed. [Figure 10]Figure 1(B) is a graph showing the variation in cutting force per rotation of the tool (end mill) for each of the four cutting directions. The horizontal axis represents the cutting force Fx (N) and the vertical axis represents the cutting force Fy (N). The graph shows the variation in cutting force per rotation of the tool (end mill) for each cutting direction in Figure 1(B): X+ direction (1st quadrant), X- direction (3rd quadrant), Y+ direction (2nd quadrant), and Y- direction (4th quadrant). [Figure 11] Figure 1(B) shows a graph representing the change in stiffness over time during side cutting in the X-direction. (A) is a graph showing the relationship between the stiffness value (kx [N / μm]) in the X-axis direction and time during the above cutting process, and shows the change in stiffness value (kx [N / μm]) in the X-axis direction over time. (B) is a graph showing the relationship between the stiffness value (ky [N / μm]) in the Y-axis direction and time during the above cutting process, similar to the relationship in the X-axis direction, and shows the change in stiffness value (ky [N / μm]) in the Y-axis direction over time. [Figure 12] (A) is a graph showing the distribution of stiffness (instantaneous stiffness) values ​​per rotation of the tool during the above cutting process, with kx [N / μm] on the horizontal axis and ky [N / μm] on the vertical axis, in the side cutting in the X+ direction of Figure 1(B). (B) is a graph showing the distribution of stiffness (instantaneous stiffness) values ​​per rotation of the tool during the above cutting process, with kx [N / μm] on the horizontal axis and ky [N / μm] on the vertical axis, in the side cutting in the X- direction of Figure 1(B). [Figure 13] (C) is a graph showing the distribution of stiffness (instantaneous stiffness) values ​​per tool rotation during side cutting in the Y+ direction as shown in Figure 1(B), with kx [N / μm] on the horizontal axis and ky [N / μm] on the vertical axis. (D) is a graph showing the distribution of stiffness (instantaneous stiffness) values ​​per tool rotation during side cutting in the Y- direction as shown in Figure 1(B), with kx [N / μm] on the horizontal axis and ky [N / μm] on the vertical axis. [Figure 14] Figure 1(B) is a graph showing the movement trajectory of the tool (end mill) center when performing lateral cutting in the X-direction, with Position X [mm] on the horizontal axis and Displacement [μm] on the vertical axis. [Figure 15]In the side cutting in the X-direction of Fig. 1(B), this is a graph representing analytical values of surface roughness when the horizontal axis represents Position X [mm] and the vertical axis represents Displacement [μm], which is calculated theoretically from the tool center path and tool diameter. [Figure 16] In the side cutting in the X-direction of Fig. 1(B), this is a graph representing measured values of surface roughness when the horizontal axis represents Position X [mm] and the vertical axis represents Roughness [μm], and the measured values of surface roughness are measured by a laser microscope. [Figure 17] In the side cutting in the X-direction of Fig. 1(B), this is a photograph showing the surface measured by a laser microscope after side cutting. Mode for Carrying Out the Invention

[0014] A dynamic stiffness evaluation method for a machine tool according to a first embodiment of the present invention will be described. Figs. 1(A) and 1(B) are schematic diagrams showing main parts of a machine tool (machining center) to which the evaluation method according to an embodiment of the present invention is applied. Fig. 1(A) is a schematic side view showing a tool holder attached to the tip of a spindle, an end mill serving as a tool (hereinafter may be referred to as either a tool or an end mill), a workpiece as a processing target, a jig plate and two gap sensors in X and Y directions attached thereto, and a cutting dynamometer provided via a vise on the bottom side of the workpiece. Fig. 1(B) is a schematic top view of the workpiece showing the movement of the end mill relative to the workpiece in side cutting by the end mill to which the evaluation method of the present embodiment is applied, that is, four cutting directions. In the machine tool (machining center) 100, a jig plate 106 is attached to the lower end of the spindle 102 so as to surround the tool holder 104, and a gap sensor X and a gap sensor Y for detecting displacements of the end mill 108 as a tool held by the tool holder 104 in the X direction and Y direction respectively are attached to the jig plate 106. Further, a cutting dynamometer 112 is provided on the bottom side of the workpiece W via a vise 110.

[0015] As described above, the machine tool (machining center) 100 of this embodiment has a cutting force meter 112 that can measure the force applied to the tool (end mill) 108 during machining (machining reaction force received from the workpiece) in the X-axis direction and the Y-axis direction, respectively, and gap sensors X and Y that can measure the amount of displacement caused by the vibration (shaking) of the tool (end mill) 108 during machining in the X-axis direction and the Y-axis direction, respectively. It should be noted that the gap sensors X and Y can also measure the amount of displacement caused by the tool (end mill) 108 free-spinning and vibrating (shaking) by, for example, operating the machine tool (machining center) 100 before machining (unrelated to machining) and rotating the spindle 102, respectively, in the X-axis direction and the Y-axis direction. With the above configuration, the force applied to the tool (end mill) 108 during machining (machining reaction force received from the workpiece) is measured in the X-axis direction and the Y-axis direction by the cutting force meter 112, and the amount of displacement caused by the vibration (vibration) of the tool (end mill) 108 due to the force applied to the tool (end mill) 108 (machining reaction force received from the workpiece) is measured in the X-axis direction and the Y-axis direction by the gap sensor X and the gap sensor Y, respectively. From the relationship between this force (Fx, Fy) and displacement (dx, dy), the stiffness values ​​(kx, ky) in the X-axis direction and the Y-axis direction can be calculated. In this embodiment of the machine tool (machining center) 100, the above-mentioned machining reaction force was measured in the X-axis direction and the Y-axis direction by the cutting force meter 112, but for the X-axis direction and the Y-axis direction, for example, the load applied to the servo motor that rotates the ball screw may be measured as a current value, and the above-mentioned machining reaction force may be measured using that load (the same applies hereafter).

[0016] Furthermore, the dynamic rigidity evaluation method of the machine tool in this embodiment is applied when a workpiece W is side-cut using a tool (end mill) 108 with a machine tool (machining center) 100. That is, as shown in Figure 1(B), when the four sides of the workpiece W are side-cut using the machine tool (machining center) 100 while rotating the end mill 108 while scanning them, the rigidity value at that instant during machining is determined by measuring the force (machining reaction force) applied to the tool (end mill) 108 and the displacement of the tool (end mill) 108 during machining. Here, "instantaneous stiffness value" refers to the stiffness value per revolution of the tool (end mill) 108. For example, in this embodiment where an end mill (regardless of the number of teeth, similar analysis is possible even for single-blade boring) is used for side cutting of a workpiece, this instantaneous stiffness value includes components such as vibration damping from the moment one tooth hits the workpiece until the next tooth hits it. Therefore, it is possible to detect and evaluate characteristics that are closer to the actual dynamic characteristics (stiffness value per revolution of the tool) during workpiece machining. In other words, conventionally, it has not been possible to evaluate the behavior of the tool when it is actually in contact with the workpiece during machining, i.e., to evaluate the dynamic stiffness. However, the inventors have devised a method to evaluate the dynamic stiffness of a machine tool from side cutting in a cutting process. Specifically, a jig was attached below the spindle faceplate, and gap sensors were installed on it in the X-axis and Y-axis directions of the tool, respectively, to measure (detect) the runout (displacement) of the tool in the X-axis and Y-axis directions during machining. The force applied to the tool was measured by measuring the force transmitted through the workpiece using a cutting force meter. Specifically, the force applied to the workpiece was detected in the X-axis and Y-axis directions during machining using a cutting force meter. The machine tool (machining center) 100 to which this evaluation method is applied, although not shown in the figures, has a mechanism for moving the spindle up and down, and is a machine tool with 5-axis control, including a Z-axis due to the up and down movement of the head, and A-axis and C-axis due to the rotation of the table, in addition to the X-Y axes mentioned above.

[0017] Figure 2 is a graph showing the relationship between the displacement of the end mill in the X-axis direction, measured by the gap sensor X (see Figure 1(A)), and time during side cutting in the X-direction, one of the four cutting directions in Figure 1(B). The graph shows the relationship between the measured value due to tool vibration during machining (Milling) and the vibration value due to rotational runout of the tool before machining (No load), i.e., unrelated to machining. It shows sinusoidal data for both before machining (No load) and during machining (Milling) (the data is sinusoidal because the end mill is rotating). In the figure, the difference (dx) between the upper data during machining (Milling) and the lower data before machining (No load) corresponds to the amount of displacement due to the machining reaction force that the tool receives from the workpiece during machining. As shown in Figure 2, the value measured by the gap sensor X before machining (No load) is the lower line, and during machining (Milling), deformation occurs, the tool bends and runs around, so the line rises to the upper side. Furthermore, the measured values ​​due to the vibration of the tool (end mill) 108 (see Figure 1) are sinusoidal because the end mill 108 is rotating. In Figure 2, the difference (dx) between the upper data during machining (Milling) and the lower data before machining (No load) corresponds to the amount of displacement of the tool (end mill) 108 due to the machining reaction force received from the workpiece W during machining. In this way, by comparing the data before machining (No load) and during machining (Milling), it is possible to determine how much the tool deforms during machining (Milling) compared to before machining (No load). By organizing the data in this way, the runout of the tool during machining can be detected.

[0018] Figure 3(A) is a graph showing the relationship between the displacement in the X-axis direction (dx) due to the machining reaction force received by the tool from the workpiece during machining and time in the X-direction side cutting shown in Figure 1(B), and shows the time change of the displacement in the X-axis direction measured by gap sensor X (see Figure 1(A)). Similarly, Figure 3(B) is a graph showing the relationship between the displacement in the Y-axis direction (dy) due to the machining reaction force received by the tool from the workpiece during machining, measured similarly by gap sensor Y (see Figure 1(A)) in the X-direction side cutting shown in Figure 1(B), and time, and shows the time change of the displacement in the Y-axis direction. As shown in Figure 3(A), the displacement (dx) shows a substantially the same waveform periodically between 0 and 0.5 s, but its value remains within the range of approximately -0.0008 mm to -0.002 mm. On the other hand, as shown in Figure 3(B), the displacement (dy) similarly shows a roughly the same waveform periodically between 0 and 0.5 s, but its value remains within the range of approximately -0.0006 mm to -0.0017 mm. In other words, comparing the displacement (dx) in the X-axis direction and the displacement (dy) in the Y-axis direction, (dx) > (dy), meaning that the displacement of the tool (end mill) 108 during machining is larger in the X-axis direction, and the amplitude of the runout is also larger. Thus, in the side cutting in the X-direction shown in Figure 1(B), it was confirmed that the vibration (runout) of the tool (end mill) 108 during machining is greater in the X-axis direction than in the Y-axis direction.

[0019] Figure 4 is a graph showing the dispersion in the XY plane of the displacement amounts (dx) and (dy) obtained in Figures 3(A) and 3(B), respectively, during side cutting in the X-direction as shown in Figure 1(B). From Figure 4, it can be seen that the displacement due to vibration (runout) of the tool (end mill) 108 during machining is greater in the X-axis direction than in the Y-axis direction, but the displacement at each instant while the tool (end mill) 108 is rotating is dispersed (distributed) as shown in Figure 4.

[0020] Figure 5 is a graph showing the time change of the cutting force during machining, measured by the cutting force meter 112 described above, in the side cutting in the X-direction shown in Figure 1(B). (A) is a graph showing the relationship between the cutting force Fx(N) in the X-axis direction and time, and (B) is a graph showing the relationship between the cutting force Fy(N) in the Y-axis direction and time. Note that the cutting force referred to here is synonymous with the machining reaction force that the tool (end mill) 108 receives from the workpiece W during the machining described above, if we consider the tool (end mill) 108 as the center. As shown in Figure 5(A), the cutting force Fx(N) in the X-axis direction shows a substantially the same waveform periodically between 0 and 0.5 s, but its value remains in the range of approximately -20(N) to -36(N). On the other hand, as shown in Figure 5(B), the cutting force Fy(N) in the Y-axis direction similarly shows a substantially the same waveform periodically between 0 and 0.5 s, but its value remains in the range of approximately -20(N) to -30(N). In other words, comparing the cutting force (machining reaction force) Fx(N) and Fy(N), Fx(N) > Fy(N), meaning that in the side cutting in the X-direction shown in Figure 1(B), the cutting force (machining reaction force) during machining is greater in the X-axis direction than in the Y-axis direction, and the amplitude of the cutting force (machining reaction force) is also larger. Thus, in the side cutting in the X-direction shown in Figure 1(B), it was confirmed that the cutting force (machining reaction force received from the workpiece) of the tool (end mill) 108 during machining is greater in the X-axis direction than in the Y-axis direction.

[0021] Figure 6 is a graph showing the distribution of the cutting forces Fx(N) and Fy(N) obtained in Figures 5(A) and 5(B), respectively, in the X-direction side cutting shown in Figure 1(B). From Figure 6, it can be seen that the cutting force (machining reaction force received from the workpiece) of the tool (end mill) 108 during machining is greater in the X-axis direction Fx(N) than in the Y-axis direction Fy(N). However, it can be seen that the cutting force (machining reaction force) at each instant while the tool (end mill) 108 is rotating is distributed in the overall shape shown in Figure 6.

[0022] Figure 7 is a graph showing the trajectory of one rotation of the tool (end mill) when the displacement (dx) is plotted on the horizontal axis and the displacement (dy) is plotted on the vertical axis during side cutting in the X-direction as shown in Figure 1(B). It was found that the displacement (dx) on the horizontal axis is in the range of -0.0008 mm to -0.002 mm, and the displacement (dy) on the vertical axis is in the range of -0.0006 mm to -0.0015 mm, with the tool trajectory distributed within the range of θmin = 23.8° and θmax = 46.3°. Furthermore, Figure 8 is a graph showing the cutting force per rotation of the tool (end mill) when the cutting force Fx(N) is plotted on the horizontal axis and the cutting force Fy(N) is plotted on the vertical axis during side cutting in the X-direction as shown in Figure 1(B). It was found that the cutting force Fx(N) on the horizontal axis is distributed in the range of approximately -20(N) to -36(N), and the cutting force Fy(N) on the vertical axis is distributed in the range of approximately -20(N) to -30(N), with θmin = 37.2° and θmax = 44.7°.

[0023] Figure 9 is a graph showing the trajectory of one rotation of the tool (end mill) for each of the four cutting directions in Figure 1(B). Specifically, it is a graph showing the trajectory of one rotation of the tool (end mill) for each cutting direction in Figure 1(B) for the X+ direction (first quadrant), X- direction (third quadrant), Y+ direction (second quadrant), and Y- direction (fourth quadrant), with displacement (dx) on the horizontal axis and displacement (dy) on the vertical axis. The distribution of trajectories within each of the first, second, third, and fourth quadrants was observed.

[0024] Figure 10 is a graph showing the variation in cutting force per rotation of the tool (end mill) for each of the four cutting directions in Figure 1(B). Specifically, it is a graph showing the variation in cutting force per rotation of the tool (end mill) for each cutting direction in Figure 1(B): X+ direction (1st quadrant), X- direction (3rd quadrant), Y+ direction (2nd quadrant), and Y- direction (4th quadrant), with cutting force Fx(N) on the horizontal axis and cutting force Fy(N) on the vertical axis. The distribution of the variation in cutting force within each of the 1st, 2nd, 3rd, and 4th quadrants was observed.

[0025] Now, since the stiffness value can be calculated by measuring the displacement in response to force, we decided to calculate the stiffness value per revolution of the tool using the above-mentioned measurements. Figure 11 is a graph showing the change in stiffness value over time during side cutting in the X-direction shown in Figure 1(B). Specifically, Figure 11(A) is a graph showing the relationship between the stiffness value (kx [N / μm]) in the X-axis direction and time during the above cutting process, and shows the change in stiffness value (kx [N / μm]) in the X-axis direction over time. Figure 11(B) is a graph showing the relationship between the stiffness value (ky [N / μm]) during the above cutting process, obtained in the Y-axis direction similarly to the X-axis direction, and time, and shows the change in stiffness value (ky [N / μm]) in the Y-axis direction over time. As shown in Figure 11(A), the stiffness value (kx [N / μm]) during machining shows approximately the same waveform periodically between 0 and 0.5 s, but its value remains in the range of approximately 16 [N / μm] to 27 [N / μm]. On the other hand, as shown in Figure 11(B), the stiffness value (ky [N / μm]) during machining similarly shows approximately the same waveform periodically between 0 and 0.5 s, but its value ranges from approximately 16 [N / μm] to 37 [N / μm]. In other words, comparing the stiffness values ​​(kx [N / μm]) and (ky [N / μm]) during machining, ky > kx, meaning that the stiffness value of the tool (end mill) 108 during machining is greater in the Y-axis direction than in the X-axis direction, and the amplitude (range of change) of the stiffness value is also larger. Thus, in the side cutting in the X-direction shown in Figure 1(B), the stiffness value of the tool (end mill) 108 during machining is, on average, greater in the Y-axis direction than in the X-axis direction, but the change in the stiffness value during machining is more drastic in the Y-axis direction. Thus, it was found that the stiffness value of the tool (end mill) 108 during machining differs between the X-axis and Y-axis directions. The fact that the rigidity value of the tool (end mill) 108 during machining differs (anisotropy) in the X-axis and Y-axis directions is understood to be partly due to the difference in the overall (casting) configuration of the machine tool, including the column, bed, etc., in addition to the head including the spindle (the overall mass of the machine tool differs between the X-axis and Y-axis directions).

[0026] Figure 12(A) is a graph showing the distribution of stiffness (instantaneous stiffness) values ​​per revolution of the tool during the side cutting process in the X+ direction of Figure 1(B), with kx [N / μm] on the horizontal axis and ky [N / μm] on the vertical axis. Figure 12(B) is a graph showing the distribution of stiffness (instantaneous stiffness) values ​​per revolution of the tool during the side cutting process in the X- direction of Figure 1(B), with kx [N / μm] on the horizontal axis and ky [N / μm] on the vertical axis. In other words, each black dot plotted in Figures 12(A) and (B) represents the (instantaneous) stiffness value during one revolution of the tool. As can be seen by comparing the dashed ellipse in Figure 12(B) with the dashed ellipse in Figure 12(A), the (instantaneous) stiffness value per tool rotation is distributed over a wider range during side cutting in the X- direction in Figure 1(B) than during side cutting in the X+ direction in Figure 1(B), and the change in the (instantaneous) stiffness value per tool rotation is slightly larger.

[0027] Figure 13(C) is a graph showing the distribution of stiffness (instantaneous stiffness) values ​​in the XY plane during side cutting in the Y+ direction as shown in Figure 1(B), with kx [N / μm] on the horizontal axis and ky [N / μm] on the vertical axis. Figure 13(D) is a graph showing the distribution of stiffness (instantaneous stiffness) values ​​in the XY plane during side cutting in the Y- direction as shown in Figure 1(B), with kx [N / μm] on the horizontal axis and ky [N / μm] on the vertical axis. In other words, each black dot plotted in Figures 13(C) and (D) represents the stiffness (instantaneous stiffness) value for one rotation of the tool. As can be seen by comparing the area within the dashed ellipse in Figure 13(D) with the area within the dashed ellipse in Figure 13(C), the (instantaneous) stiffness value per tool rotation is concentrated in a slightly narrower range during side cutting in the Y- direction in Figure 1(B) as shown in Figure 13(D), while the (instantaneous) stiffness value per tool rotation is dispersed over a wider range during side cutting in the Y+ direction in Figure 1(B) as shown in Figure 13(C), indicating that the change in the (instantaneous) stiffness value per tool rotation during machining is slightly larger. Furthermore, as can be seen by comparing the area within the dashed ellipse in Figures 12(A) and (B) with the area within the dashed ellipse in Figures 13(C) and (D), even when comparing with the same cutting direction, the distribution of instantaneous stiffness data in the X-axis and Y-axis directions shows different levels of variation. This difference in variation can be considered to indicate anisotropy of machine stiffness during cutting. In the explanation of Figures 12 and 13, the instantaneous stiffness value refers to the stiffness value per rotation of the tool. For example, in an embodiment where an end mill (not a ball end mill, but a tool in which three or four blades on the side rotate to cut the workpiece from the side) is used for cutting the workpiece from the side, this instantaneous stiffness value includes components such as vibration damping from the time one blade hits the workpiece until the next blade hits it. Therefore, it is possible to detect and evaluate characteristics that are closer to the actual dynamic characteristics (stiffness value per rotation of the tool) during workpiece machining.

[0028] Figure 14 is a graph showing the movement trajectory of the tool (end mill) center in the X-direction side cutting shown in Figure 1(B), with Position X [mm] on the horizontal axis and Displacement [μm] on the vertical axis. As shown in Figure 14, the displacement of the tool (end mill) center shows a periodically similar waveform from 0 to -0.5 mm, but the displacement varied in the range of approximately -2.3 μm to -5.3 μm.

[0029] Figure 15 is a graph showing the analytical values ​​of surface roughness when Position X [mm] is plotted on the horizontal axis and Displacement [μm] on the vertical axis during side cutting in the X-direction as shown in Figure 1(B). This graph represents the machined surface calculated from the tool runout of the tool (end mill) theoretically calculated from the tool center trajectory and tool (end mill) diameter obtained in Figure 14. As shown in Figure 15, the surface roughness (displacement from the reference surface) shows approximately the same waveform periodically as the position of the tool (end mill) center changes from 0 to -0.5 mm, but this displacement (displacement from the reference surface) changes periodically in the range of approximately -2.3 μm to -2.7 μm.

[0030] Figure 16 is a graph showing the measured surface roughness in the X-direction lateral cutting shown in Figure 1(B), with Position X [mm] on the horizontal axis and Roughness [μm] on the vertical axis. This surface roughness measurement was taken using a laser microscope. As shown in Figure 16, the laser microscope measurement of surface roughness changed approximately periodically within the range of approximately +0.6 μm to -0.4 μm as the tool (end mill) center position progressed from 0 to -0.5 mm.

[0031] Figure 17 is a photograph showing the laser microscope measurement surface after side cutting in the X-direction as shown in Figure 1(B). It was found that the surface roughness measured by this laser microscope roughly corresponds to the machined surface determined from the runout of the tool (end mill) shown in Figure 15. As described above, by measuring the runout during machining, and considering the tool's runout and the constant feed rate during machining, it was found that the trajectory obtained by combining the constant feed rate during machining and the variation due to the tool's runout roughly matches the surface roughness data. Thus, since the trajectory obtained using the process of the dynamic stiffness evaluation method of the present invention roughly matches the surface roughness data of the workpiece after machining, the dynamic stiffness evaluation method of the present invention can be used for the analysis (estimation) of surface roughness after machining. In other words, if only the machining conditions are known before machining, machining can be performed with knowledge of the machine's dynamic characteristics, making it possible to estimate the surface roughness after machining, which has a significant practical advantage. Furthermore, while the present invention provides a method for evaluating the dynamic characteristics of a workpiece from the cutting force and displacement during machining, we believe that by using a process in the reverse order of the one described above, it will also be possible to evaluate the dynamic characteristics from the machined surface as described above.

[0032] As described above, if the dynamic stiffness trend of one machine tool is known, it can be seen that the dynamic stiffness trend will differ for each of several machine tools. In other words, since the dynamic stiffness trend unique to the machine tool can be determined, we have found that the evaluation method of the present invention can be used for testing and evaluating the dynamic stiffness of machine tools. Conventionally, even if hammering was performed, the stiffness trend when actually processing the workpiece with a tool would differ from that of the hammering. However, according to the present invention, the dynamic stiffness trend when actually processing with the machine tool can be determined, so it is extremely useful to know the dynamic stiffness trend of each machine during test processing before shipping a large number of machine tools. Furthermore, we have found that the dynamic stiffness evaluation method of the present invention has the potential to be used for analyzing (estimating) the surface roughness after processing. In other words, if only the processing conditions are known before processing, processing can be performed with knowledge of the machine's dynamic characteristics, so the ability to estimate the surface roughness after processing has a great practical advantage. That is, by using the dynamic stiffness evaluation method of the present invention, the quality of the machine tool can be judged by evaluating the dynamic stiffness (instantaneous stiffness value) of the machine tool. Furthermore, this method can not only be used to estimate the surface roughness after machining with the machine tool, but we also believe that by using the reverse process, it will be possible to evaluate the dynamic characteristics from the machined surface. [Industrial applicability]

[0033] The present invention aims to eliminate variations in the dynamic characteristics of each machine tool (product) and standardize them by comparing the dynamic characteristics evaluations of machine tools, and can also be applied to the evaluation of the deterioration of machine tools over time. In the above embodiment, an end mill (cutting tool) was used as a rotary tool, but it can also be applied to cases where a grinding wheel is used as a rotary tool in a grinding machine, for example. Furthermore, it is understood that even with non-rotating tools such as cutting tools, it is possible to evaluate the dynamic characteristics by measuring predetermined axial vibration (displacement) and machining reaction force during machining and before machining (non-machining). [Explanation of Symbols]

[0034] 100 Machine tool (machining center), 102 Spindle, 104 Tool holder, 106 Fixture plate, 108 End mill, X Gap sensor, Y Gap sensor, W Workpiece, 110 Vise, 112 Cutting force meter

Claims

1. A method for evaluating the dynamic rigidity of a machine tool that processes a workpiece using a tool, comprising: a first step of measuring the vibration of the tool and the processing reaction force that the tool receives from the workpiece while processing the workpiece under predetermined driving conditions; and a second step of measuring the vibration of the tool caused by driving the tool under the same conditions as the predetermined driving conditions, independently of the processing of the workpiece, wherein the dynamic rigidity of the machine tool during processing is evaluated by determining the difference between a first value calculated from the processing reaction force and vibration measured in the first step and a second value calculated from the vibration measured in the second step.

2. A method for evaluating the dynamic rigidity of a machine tool according to claim 1, characterized in that, in the first and second steps, the vibration of the tool is measured in at least two axial directions, the X direction and the Y direction.

3. A method for evaluating the dynamic rigidity of a machine tool according to claim 1 or 2, characterized in that the tool is a rotary tool and cuts the workpiece from the side.

4. A method for evaluating the dynamic rigidity of a machine tool, characterized by evaluating the dynamic rigidity from the machined surface of the workpiece after processing by using all or part of the evaluation method described in any one of claims 1 to 3.

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