Cutting coefficient identification system in machine tool and cutting coefficient identification method in machine tool

Abstract

To provide a cutting coefficient identification system in a machine tool and a cutting coefficient identification method in a machine tool which can identify a cutting coefficient with high accuracy using motor torque in the machine tool, without using an additional sensor.SOLUTION: In a cutting coefficient identification system 21, a measuring / cutting force obtaining part 22 obtains average torque that is required in a cutting process by a spindle 4 and a feed shaft of a machine tool to calculate measuring / cutting force. Subsequently, a tool information obtaining part 23 obtains tool information, and a cutting condition obtaining part 24 obtains a cutting condition. An estimated cutting force calculating part 25 calculates estimated cutting force, on the basis of the above information. Subsequently a cutting coefficient identifying part 26 compares the measuring / cutting force with the estimated cutting force to identify a cutting coefficient.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a cutting coefficient identification system for a machine tool, which identifies cutting coefficients used to predict chatter stability limits and the like, and a cutting coefficient identification method for a machine tool. [Background technology]

[0002] Conventionally, one of the parameters that determines the machining capacity of a machine tool is, for example, the stability limit of self-excited chatter. If self-excited chatter occurs during cutting, the machining accuracy and machined surface quality of the product will decrease. Therefore, suppressing self-excited chatter has become an issue. For example, Patent Document 1 discloses that a stability limit diagram is created for predicting the occurrence of self-excited chatter using a transfer function measured by a hammering test and an estimated value of the specific cutting resistance as a cutting coefficient, and a cutting condition that is highly efficient and does not cause self-excited chatter is selected.

[0003] On the other hand, as a method for identifying a cutting coefficient, Non-Patent Document 1 discloses a method for identifying a cutting coefficient by correcting the cutting coefficient so as to reduce the difference between the cutting force measured by a dynamometer, which is a sensor with a high response frequency, and the cutting force estimated by a prediction model using a provisionally determined cutting coefficient. Furthermore, Non-Patent Document 2 discloses that a conventional cutting force prediction model is configured with two cutting coefficients, one representing the cutting force per unit cutting cross-sectional area and the other representing the cutting force per unit cutting edge length, for each of the cutting forces in three orthogonal directions. By replacing these cutting coefficients with a single cutting coefficient that uses the shear angle as a parameter, the cutting coefficient can be identified from the torque of the spindle motor. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2022-21378 [Non-patent literature]

[0005] [Non-Patent Document 1] Tomohiro Akagi, Junichi Kaneko, Kenichiro Horio. Development of a high-speed automatic estimation system for specific cutting force in cutting force prediction. Proceedings of the JSPE Annual Conference 2014A, pp669-670, 2014 [Non-Patent Document 2] Kazuki Kaneko, Isamu Nishida, Ryuta Sato, Keiichi Shirase. Cutting force model for end milling based on oblique cutting theory. Transactions of the Japan Society of Mechanical Engineers, Vol.83, No.856, 2017 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in Patent Document 1, the cutting coefficient, which changes depending on the machining conditions such as the combination of the tool and the workpiece material and the presence or absence of coolant, is obtained by estimation from the tool rake angle, friction coefficient, etc. in order to omit the measurement work. Therefore, there is a problem that it is difficult to predict the occurrence of self-excited chatter with high accuracy. In addition, Non-Patent Document 1 assumes that the cutting force is measured using a dynamometer with a high response frequency, but dynamometers are very expensive. Also, when they are attached to a machine tool as a production facility, for example, there is a possibility that interference between the dynamometer itself and the wiring may occur. Furthermore, there is a possibility that the loop rigidity from the tool to the workpiece may decrease. Therefore, there is a problem that it is difficult to use in the production site. In addition, the cutting coefficient with the shear angle as a parameter disclosed in Non-Patent Document 2 merely replaces only the cutting coefficient expressing the cutting force per unit cutting cross-sectional area among the conventional cutting coefficients. Therefore, there is a problem that the identification accuracy decreases for materials with a large cutting coefficient expressing the cutting force per unit cutting edge length, such as titanium alloys and Ni-based heat-resistant alloys.

[0007] Therefore, the present disclosure has been made in consideration of the above problems, and aims to provide a cutting coefficient identification system and a cutting coefficient identification method for a machine tool, which are capable of identifying the cutting coefficient with high accuracy without using any additional sensors and regardless of the material of the workpiece. [Means for solving the problem]

[0008] In order to achieve the above object, a first configuration of the present disclosure is a cutting coefficient identification system for a machine tool, the cutting coefficient identification system including at least one of a tool spindle having a tool mounted thereon and driven by a tool spindle motor, and a workpiece spindle having a workpiece mounted thereon and driven by a workpiece spindle motor, and a feed axis for relatively moving the tool and the workpiece by a feed axis motor, the cutting coefficient identification system including at least one of an average torque T of the tool spindle motor, the workpiece spindle motor, and the feed axis motor during an arbitrary period A while the tool is cutting the workpiece, a a measured cutting force acquisition unit that acquires tool information including the number of tool teeth corresponding to the measured cutting force, a cutting condition acquisition unit that acquires cutting conditions including the amount of relative movement between the tool and the workpiece during one rotation of the tool spindle or the workpiece spindle and the amount of cutting corresponding to the measured cutting force, and a cutting coefficient K c and the cutting coefficient K, which represents the cutting force per unit cutting edge length. e An estimated cutting force calculation unit calculates an estimated cutting force, which is the average cutting force during one rotation of the tool spindle or work spindle, based on a formula including the number of tool blades, the relative movement amount, and the cutting depth amount, and compares the measured cutting force with the estimated cutting force to calculate a cutting coefficient K c and the cutting coefficient K e and a cutting coefficient identification unit that identifies the cutting coefficient. Another aspect of the first configuration of the present disclosure is the above-mentioned configuration, wherein the cutting coefficient K c and cutting coefficient K eThe present invention is characterized in that it is equipped with a limit cutting condition calculation unit that calculates limit cutting conditions, which are the limit of the machining capacity of the machine tool, including at least one of the rotational speed of the tool spindle or work spindle, the relative movement amount between the tool and the work during one rotation of the tool spindle or work spindle, and the cutting depth, using at least one of the above. In yet another aspect of the first configuration of the present disclosure, in the above configuration, the measured cutting force acquisition unit is configured to acquire an average torque T b is further used to calculate the measured cutting force. In yet another aspect of the first configuration of the present disclosure, in the above configuration, the measured cutting force acquisition unit is configured to acquire an average torque T c is further used to calculate the measured cutting force. Yet another aspect of the first configuration of the present disclosure is characterized in that, in the above configuration, the parameters determining the limit of the machining capacity include at least one of an upper output limit of at least one of the tool spindle motor, the workpiece spindle motor, and the feed spindle motor, a bending stress of the tool, a shear stress, a quality requirement value of the workpiece, and a stability limit of self-excited chatter. In order to achieve the above object, a second configuration of the present disclosure is a cutting coefficient identification method for a machine tool, the cutting coefficient identification method including at least one of a tool spindle having a tool mounted thereon and driven by a tool spindle motor, and a workpiece spindle having a workpiece mounted thereon and driven by a workpiece spindle motor, and a feed axis for relatively moving the tool and the workpiece by a feed axis motor, the method comprising: determining an average torque T of at least one of the tool spindle motor, the workpiece spindle motor, and the feed axis motor during an arbitrary period A while the tool is cutting the workpiece; a a cutting force acquisition step of acquiring the cutting force by obtaining the cutting force, a tool information acquisition step of acquiring tool information including the number of tool teeth corresponding to the measured cutting force, a cutting condition acquisition step of acquiring cutting conditions including the relative movement amount between the tool and the workpiece during one rotation of the tool spindle or the workpiece spindle and the cutting depth amount corresponding to the measured cutting force, and a cutting coefficient K c and the cutting coefficient K, which represents the cutting force per unit cutting edge length. eAn estimated cutting force calculation step of calculating an estimated cutting force, which is an average cutting force during one rotation of the tool spindle or work spindle, based on a formula including the number of tool blades, the relative movement amount, and the cutting depth amount; and a cutting coefficient K c and the cutting coefficient K e and a cutting coefficient identification step of identifying the cutting coefficient. Effect of the Invention

[0009] According to the present disclosure, the cutting coefficient representing the cutting force per unit cutting cross-sectional area and the cutting coefficient representing the cutting force per unit cutting edge length are identified using the torque of a motor installed in a machine tool, and therefore, the cutting coefficient can be identified with high accuracy without using an additional sensor and regardless of the material of the workpiece. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic configuration diagram showing a cutting coefficient identification system for a machine tool. [Diagram 2] 1 is a flowchart showing a method for predicting processing capability. [Diagram 3] 1 shows motor torque histories relating to periods a, b, and c, where (a) is the motor torque history of the main axis, and (b) is the motor torque history of the feed axis. [Figure 4] FIG. 2 is a schematic diagram showing how milling is performed in a machining center, where (a) is a top view and (b) is a side view. [Diagram 5] 1 is a graph showing the relationship between the spindle rotation speed, which is the upper limit of the output of the spindle motor, and the radial depth of cut in milling processing. [Figure 6] 1 is a graph showing the relationship between the spindle rotation speed, which is the upper limit of the output of the feed shaft motor, and the feed amount per one rotation of the spindle in milling processing. [Figure 7] 1 is a graph showing the relationship between the radial cutting depth, which becomes the bending stress of the tool in milling processing, and the feed amount per one rotation of the spindle. [Figure 8]1 is a graph showing the relationship between the feed rate per revolution of the spindle and the axial depth of cut, which is the shear stress of the tool in milling. [Figure 9] 1 is a graph showing the relationship between the radial depth of cut and the axial depth of cut, which are the required values ​​for the surface roughness of a workpiece in milling. [Figure 10] 1 is a graph showing the relationship between the spindle rotation speed and the axial depth of cut, which is the stability limit of self-excited chatter during milling. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram of a machining center, which is an example of a machine tool. The spindle housing 3 of the machining center M is equipped with a spindle 4 as a tool spindle that can be rotated by a spindle motor. A tool 5 is attached to the tip of the spindle 4. The spindle housing 3, which is a movable body, can be moved in the Z-axis direction by a Z-axis motor via a Z-axis ball screw as a feed axis relative to a column 2 attached to a bed 1. A workpiece 7 is fixed on a table 6, which is a movable body equipped in the machining center M. The table 6 can be moved in the X-axis and Y-axis directions that are perpendicular to each other on the bed 1. The table 6 is moved in the X-axis direction by an X-axis motor via an X-axis ball screw as a feed axis. The table 6 is moved in the Y-axis direction by a Y-axis motor via a Y-axis ball screw as a feed axis.

[0012] The machining center M includes an NC device 11 capable of controlling itself. The NC device 11 includes a CPU and a memory connected to the CPU, and realizes operations by using these. The NC device 11 includes a machine operation command unit 12 that controls each part such as the spindle motor and each feed axis motor based on a program input by an operator through input means (not shown) such as a keyboard or a touch panel. The machine operation command unit 12 commands the rotation of the tool 5 and the relative movement between the tool 5 and the workpiece 7, thereby performing cutting processing.

[0013] The machining center M further includes a cutting coefficient identification system 21. The cutting coefficient identification system 21 includes a measured cutting force acquisition unit 22, a tool information acquisition unit 23, a cutting condition acquisition unit 24, an estimated cutting force calculation unit 25, a cutting coefficient identification unit 26, and a limit cutting condition calculation unit 27. Although the cutting coefficient identification system 21 is assumed to be included in the NC device 11, it may be included in a computer separate from the machining center M that is mechanically and electrically connected to the machining center M.

[0014] The measured cutting force acquisition unit 22 acquires the torque of each of the spindle motor and the feed axis motor from the machine operation command unit 12 and calculates the measured cutting force. The tool information acquisition unit 23 acquires tool information including the number of tool edges Z and the tool diameter D from the tool information storage unit 13 provided in the NC device 11.

[0015] The cutting condition acquisition unit 24, based on the program, determines the spindle rotational speed S, the feed per spindle revolution f corresponding to the relative movement amount between the tool 5 and the workpiece 7 during one rotation of the spindle 4 r , the tool axis direction cutting depth d a , and the tool diameter direction cutting depth d r and acquires the cutting conditions including them. The estimated cutting force calculation unit 25 calculates the estimated cutting force based on the tool information acquired by the tool information acquisition unit 23 and the cutting conditions acquired by the cutting condition acquisition unit 24.

[0016] The cutting coefficient identification unit 26 identifies the cutting coefficient by comparing the measured cutting force acquired by the measured cutting force acquisition unit 22 with the estimated cutting force calculated by the estimated cutting force calculation unit 25. The limit cutting condition calculation unit 27, based on the cutting coefficient identified by the cutting coefficient identification unit 26 and the tool information, calculates each combination of the spindle rotational speed S, the feed per spindle revolution f r , the tool axis direction cutting depth d a , and the tool diameter direction cutting depth d r at which the machining ability reaches the limit.

[0017] Next, a method for predicting the machining capacity of the machining center M using the cutting coefficient identification system 21 will be described. First, a method for identifying a cutting coefficient by the cutting coefficient identification system 21 will be described based on the flowchart of FIG. 2. FIG. 2 is a flowchart showing a method for predicting the machining capacity. S1-S7 indicate steps 1-7, respectively. Moreover, S1-S7 are realized by each of the units 22-27 included in the cutting coefficient identification system 21 appropriately executing the above-mentioned operations.

[0018] In step S1, cutting is actually performed on the workpiece 7 using the tool 5, and then the torque history of each axis motor is acquired, as shown in FIGS. 3(a) and 3(b). Then, as S2, the torque T during an arbitrary period a during cutting is obtained from the torque history. a , the torque T during any period c when each axis motor is operating and not cutting c The cutting force in the axial direction of each axis motor is calculated based on the above. S1 and S2 are the cutting force acquisition steps in this disclosure.

[0019] In order to accurately capture the cutting phenomenon that occurs at high speed, it can be said that the accuracy is insufficient only with a specific value in the control band of each axis motor. Therefore, in the cutting force acquisition step, the average value for each arbitrary period that can be obtained with high accuracy is used as the torque value used to calculate the cutting force. For example, the cutting force F that the workpiece 7 receives in the X-axis direction during period a is xm is a value obtained by a machine information acquisition unit (not shown) provided in the cutting coefficient identification system 21 from a machine information storage unit (not shown) provided in the NC device 11 based on the specifications of the ball screw, such as the lead L and the mechanical efficiency η, and the average torque T of the X-axis motor in the periods a and c. ax , T cx Based on the above, the average torque T of the Y-axis motor and the Z-axis motor in the periods a and c is calculated using the same calculation formula as in the formula 1. ay , T az , T cy , T cz From the above, the cutting force F in each axial direction is ym, F zm can be calculated.

[0020]

number

[0021] In addition, during the period a, the cutting force F tm is the average torque T of the spindle motor in periods a and c. as , T cs Based on this, it is calculated using Equation 2.

[0022]

number

[0023] Next, tool information is acquired in S3, and cutting conditions are acquired in S4. S3 is a tool information acquisition step in this disclosure, and S4 is a cutting condition acquisition step in this disclosure.

[0024] Next, in S5, a cutting coefficient is provisionally determined, and an estimated cutting force is calculated based on the provisionally determined cutting coefficient, the tool information acquired in S3, and the cutting conditions acquired in S4. Furthermore, in S6, the cutting coefficient is identified by comparing the measured cutting force with the estimated cutting force. If the difference between the measured cutting force and the estimated cutting force is large, the cutting coefficient is provisionally re-determined and the estimated cutting force is re-calculated. On the other hand, if the difference between the measured cutting force and the estimated cutting force is small, the identification of the cutting coefficient is completed.

[0025] Then, as S7, the spindle speed S and the feed rate f per spindle revolution at which the machining capacity is limited are determined based on the identified cutting coefficient and the tool information. r , tool axial depth of cut d a , and tool radial depth of cut d r Then, the machining capacity of the machining center M is predicted based on the calculated results.

[0026] Hereinafter, a milling process will be described by taking as an example the milling process performed by relatively moving the rotated tool 5 and the workpiece 7 in the X-axis direction as shown in Fig. 4. The relative positions of the tool 5 and the workpiece 7 in the Y-axis direction and the Z-axis direction are determined in advance so that milling can be performed. The calculation of the estimated cutting force in S5 is performed by applying the instantaneous cutting force model to an end mill that has no helix angle. The cutting force acting on the workpiece 7 from one cutting edge of the tool 5 at a certain tool rotation angle θ is called the tool tangential cutting force F ts , tool radial cutting force F rs , and the tool axial cutting force F as The cutting force in the tool tangential direction F is defined as three orthogonal components. ts , tool radial cutting force F rs , and the tool axial cutting force F as can be calculated using Equations 3-5, respectively.

[0027]

number

[0028] Here, the tool tangential cutting coefficient K tc , tool radial cutting coefficient K rc , and the tool axial cutting coefficient K ac is the cutting force per unit cutting cross-sectional area. Also, g is a unit step function that distinguishes whether the cutting edge of the tool 5 is involved in cutting or not. g is 1 during cutting and 0 during non-cutting.

[0029] On the other hand, the tool tangential cutting coefficient K te , tool radial cutting coefficient K re , and the tool axial cutting coefficient K ae is the cutting force per unit cutting edge length. Also, the cutting edge length l is the axial depth of cut d a Therefore, the cutting cross-sectional area A can be calculated using Equation 6.

[0030]

number

[0031] Here, h is the cutting thickness. Also, f z is the feed rate per tooth of the tool 5.

[0032] The cutting is φ st ≦φ≦φ ex The tool is cut at the radial depth of cut d r In the case of up-cutting, φ st and φ ex can be calculated using Equation 7. On the other hand, in the case of down cutting, φ st and φ ex can be calculated using Equation 8.

[0033]

number

[0034]

number

[0035] The cutting force calculated by Equation 3-5 can be expressed as the cutting force component F in the X-axis direction by using Equation 9-11. xs , cutting force component in the Y-axis direction F ys , and the cutting force component F in the Z-axis direction zs can be converted to

[0036]

number

[0037] As described above, in the past, a sensor with a high response frequency such as a dynamometer was used to obtain the change in cutting force per rotation of the spindle 4, and the cutting force component F in the X-axis direction for each rotation angle of the tool 5 was calculated. xs and the cutting force component F in the Y-axis direction ys and the cutting force component F in the Z-axis direction zs The tool tangential cutting coefficient K per unit cutting cross-sectional area is set so that the difference between tc , tool radial cutting coefficient K rc, and the tool axial cutting coefficient K ac , and the tool tangential cutting coefficient K per unit cutting edge length te , tool radial cutting coefficient K re , and the tool axial cutting coefficient K ae However, with a low response frequency, for example, the cutting force calculated based on the torque obtained from each axis motor of the machining center M, it was not possible to identify the cutting coefficient with high accuracy.

[0038] Therefore, the applicant decided to use the average torque value obtained from each axis motor for a predetermined period to calculate the cutting force in each axis direction as described above. In addition, the applicant also found Equation 12-15 to calculate the average value for the instantaneous cutting force model.

[0039]

number

[0040] Here, the torsion angle of the tool 5 is the average cutting force component F in the spindle rotation direction. ts ￣(Indicates an overline indicating the average value, same below), Average cutting force component F in the x-axis direction xs , Average cutting force component in the y-axis direction F ys ￣ and the average cutting force component F in the z-axis direction zs Therefore, even if the tool 5 is an end mill with a helix angle, the average cutting force component F in the spindle rotation direction and each feed axis direction for the instantaneous cutting force model can be calculated by using Equation 12-15. ts ￣, F xs ￣, F ys ￣ and F zs In this embodiment, the average cutting force components F in the spindle rotation direction and each feed axis direction calculated in this way can be calculated. ts ￣, F xs ￣, F ys ￣ and F zs ￣ is treated as the estimated cutting force.

[0041] An example of identifying the cutting coefficient using the acquired cutting force in the case shown in Figure 3 is shown below. Here, when acquiring the cutting force using the feed axis, the torque of the X-axis motor, which is the moving axis, is used because the error tends to be large for the stationary axis. In Figure 3, the cutting force predicted by the instantaneous cutting force model is superimposed on the actual torque data when not cutting.

[0042] For the machining, it was assumed that an end mill with a tool diameter D of 20 mm and a tool blade number Z of 4 was used as the tool 5. Also, the tool axial depth of cut d a = 5 mm, the radial depth of cut as a percentage of the tool diameter d r = 25%. The workpiece 7 was assumed to be S45C carbon steel. The machining was assumed to be up-cut. In the following, the cutting depth in the tool radial direction d r is expressed as a percentage of the tool diameter.

[0043] When identifying the cutting coefficient from the cutting force obtained from the spindle 4 and X-axis motors, the unknowns are the tool tangential cutting coefficient K per unit cutting cross-sectional area. tc , and tool radial cutting coefficient K rc , and the tool tangential cutting coefficient K per unit cutting edge length te , and tool radial cutting coefficient K re In contrast, the cutting force that can be obtained under one cutting condition is the cutting force F tm , and the cutting force F acting on the workpiece 7 in the X-axis direction xm Therefore, although it may be possible to successfully identify the cutting coefficients by devising a way to give the initial values ​​of the cutting coefficients, it is desirable to obtain the cutting forces under two or more cutting conditions.

[0044] Therefore, in this study, the cutting force in an arbitrary period b, in which the cutting conditions are different from those in period a, is also used to identify the cutting coefficient. In period a, the feed rate f r = 0.4 mm / rev, during period b, the feed rate per spindle revolution is f r= 0.8 mm / rev. The X-axis ball screw is assumed to have a lead L of 16 mm and a mechanical efficiency η of 95%. The cutting forces and cutting coefficients for period b are calculated by the average torque T bs , T bx , T by , and T bz It is calculated in the same way as in the case of period a.

[0045] Under the above conditions, the average cutting force F that the cutting edge of the tool 5 receives in the rotation direction of the spindle 4 during period a obtained from the spindle motor tma , and the average value F of the cutting force that the cutting edge of the tool 5 receives in the direction of rotation of the spindle 4 during the period b tmb and the average value F of the tool tangential component of the cutting force estimated for the cutting conditions in period a using the provisionally determined cutting coefficient. tsa , and the average value F of the tool tangential component of the cutting force estimated for the cutting conditions in period b using the provisionally determined cutting coefficient tsb The difference between the evaluation value E t is calculated using Equation 16. The calculated evaluation value E t Based on this, the tool tangential cutting coefficient K per unit cutting cross-sectional area tc , and the tool tangential cutting coefficient K per unit cutting edge length te Identify.

[0046]

number

[0047] The initial value of the provisionally determined cutting coefficient is the tool tangential cutting coefficient K per unit cutting cross-sectional area. tc = 1000MPa, tool tangential cutting coefficient K per unit cutting edge length te When = 0N / mm, each cutting force and evaluation value E t The results are shown in Table 1.

[0048] [Table 1]

[0049] Provisional determination of cutting coefficient and evaluation value E t Repeat the calculation of E t Each cutting force and evaluation value E when it is judged that it is sufficiently small t The results are shown in Table 2. Therefore, the cutting coefficient is the tool tangential cutting coefficient K per unit cutting cross-sectional area. tc = 1998MPa, tool tangential cutting coefficient K per unit cutting edge length te = 10N / mm.

[0050] [Table 2]

[0051] Next, the evaluation value E of the difference between the average cutting force obtained from the X-axis motor and the average cutting force component in the X-axis direction estimated using the provisionally determined cutting coefficient is calculated. x is calculated in the same manner as in Equation 16. The calculated evaluation value E x Based on this, the tool radial cutting coefficient K per unit cutting cross-sectional area rc , and the tool radial cutting coefficient K per unit cutting edge length re The cutting coefficient K tc , and the tool tangential cutting coefficient K per unit cutting edge length te For , use the value already identified above.

[0052] The initial value of the provisionally determined cutting coefficient is the tool radial cutting coefficient K per unit cutting cross-sectional area. rc = 1000MPa, tool radial cutting coefficient K per unit cutting edge length re When = 0N / mm, each cutting force and evaluation value E x The results are shown in Table 3.

[0053] [Table 3]

[0054] Provisional determination of cutting coefficient and evaluation value E x Repeat the calculation of Ex Each cutting force and evaluation value E when it is judged that it is sufficiently small x The results are shown in Table 4. Therefore, the cutting coefficient is the tool radial cutting coefficient K per unit cutting cross-sectional area. rc = 575MPa, tool radial cutting coefficient K per unit cutting edge length re =22N / mm.

[0055] [Table 4]

[0056] In this way, the tool tangential cutting coefficient K per unit cutting cross-sectional area is calculated using the torque obtained from each axis motor installed in the machining center M. tc , and the tool radial cutting coefficient K per unit cutting cross-sectional area rc , and the tool tangential cutting coefficient K per unit cutting edge length te , and the tool radial cutting coefficient K per unit cutting edge length re Since each of these is identified, the cutting coefficient can be identified with high accuracy without using an additional sensor and regardless of the material of the workpiece 7.

[0057] Next, the identified cutting coefficient K rc , and the tool radial cutting coefficient K per unit cutting edge length re Using this, the cutting conditions that are the limit of the machining capacity of the machining center M are calculated. For machining, a carbide end mill with a tool diameter D = 16 mm, number of teeth Z = 4, and a helix angle of 0 deg was assumed as the tool 5. Also, it was assumed that the tool 5, which was held by protruding into the holder with l = 80 mm, was moved in the X-axis direction to perform up-cutting on S45C carbon steel as the workpiece 7. Here, the cutting coefficient was the tool tangential cutting coefficient K per unit cutting cross-sectional area. tc = 2000MPa, tool tangential cutting coefficient K per unit cutting edge length te = 10N / mm, tool radial cutting coefficient K per unit cutting cross-sectional area rc= 600MPa, tool radial cutting coefficient K per unit cutting edge length re = 20N / mm was assumed.

[0058] By using the cutting coefficient, the relationship between each cutting condition and the change in cutting force per spindle revolution is calculated using Equation 3-11, and the cutting condition that is the limit of machining capacity is calculated. For example, the spindle motor output P during machining s is the average cutting force in the tool tangential direction F tave It can be calculated using Equation 17.

[0059]

number

[0060] In Fig. 5, the conditions are: tool axial depth of cut d a = 20 mm, feed rate per spindle revolution f r = 0.8 mm / rev, and assuming mechanical efficiency η = 95%, spindle speed S, tool radial cutting depth d r , spindle motor output during machining P s Here, the radial cutting depth d r is the ratio to the diameter of the tool 5. In Fig. 5, conditions requiring high spindle motor power are shown in dark colors, and conditions requiring low power are shown in light colors. Also, the upper limit of the spindle motor power is assumed to be 10 kW. In this case, the black solid line connecting the points where the spindle motor power is 10 kW indicates the cutting conditions at the limit of the machining capacity.

[0061] Also, for example, the feed shaft motor output P f is the average cutting force in the tool feed direction F xave It can be calculated using Equation 18.

[0062]

number

[0063] Next, in Fig. 6, the conditions area = 20mm, tool radial cutting depth d r = 70%, and mechanical efficiency η = 95%. r , and the feed axis motor output P during processing f The relationship between the feed motor output and the cutting capacity is shown. Conditions requiring a large amount of power from the feed motor are shown in dark colors, while conditions requiring a small amount of power are shown in light colors. The upper limit of the feed motor output is assumed to be 1kW. In this case, the solid black line connecting the points where the feed motor output is 1kW indicates the cutting conditions at the limit of machining capacity.

[0064] The bending stress σ acting on the tool 5 during machining is expressed as the cutting force F x and the cutting force F perpendicular to the tool feed direction and the tool axis direction. y The maximum value of the resultant force R max Using (Equation 19), it can be calculated using Equation 20.

[0065]

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[0066] Here, I represents the second moment of area of ​​the tool 5.

[0067] Furthermore, in Fig. 7, the conditions are a = 20 mm, spindle speed S = 3000 min ―1 Assuming that the tool radial cutting depth d r , Feed per spindle revolution f r The relationship between the area moment of inertia I of the tool 5 and the bending stress σ acting on the tool 5 during machining is shown. The area moment of inertia I of the tool 5 was calculated assuming that the tool 5 is a solid round bar. Conditions under which the bending stress σ acting on the tool 5 is large are shown in dark colors, and conditions under which the bending stress σ is small are shown in light colors. The allowable bending stress σ of the tool 5 is 1000 N / mm 2 In this case, the bending stress σ of the tool 5 is assumed to be 1000 N / mm 2 The black solid line connecting the points where this occurs indicates the cutting conditions that are the limit of machining capability.

[0068] In addition, the shear stress τ acting on the tool 5 during machining is the maximum value of the tool tangential cutting force F tmax It can be calculated using Equation 21.

[0069]

number

[0070] Here, I p indicates the second polar moment of area of ​​the tool 5.

[0071] Next, in Fig. 8, the conditions are r = 70%, spindle speed S = 3000 min ―1 Assuming that, the feed rate per spindle revolution is f r , tool axial depth of cut d a , and the shear stress τ acting on the tool 5 during machining. The polar moment of area I p was calculated assuming the tool 5 to be a solid round bar. Conditions where the shear stress τ acting on the tool 5 is large are indicated in dark colors, and conditions where the shear stress τ is small are indicated in light colors. In addition, the allowable shear stress τ of the tool 5 is set to 300 N / mm 2 In this case, the shear stress τ of the tool 5 is 300 N / mm 2 The black solid line connecting the points where this occurs indicates the cutting conditions that are the limit of machining capability.

[0072] Required quality values ​​for the workpiece 7 include dimensional accuracy, shape accuracy, surface roughness, etc. These are affected by the deformation of the system including the tool 5 and workpiece 7 due to the cutting force, so it is necessary to calculate the cutting conditions that will be the limit of the machining capacity, taking into consideration the static and dynamic rigidity of the system including the tool 5 and workpiece 7. For example, machining with an end mill as the tool 5 is an intermittent cutting process, so forced vibrations cause relative displacement between the tool 5 and workpiece 7. However, the magnitude of this displacement must be smaller than the required surface roughness. Therefore, the amount of relative displacement is calculated as the surface roughness R z If we consider this, the surface roughness R z is the cutting force F perpendicular to the tool feed direction and the tool axis direction.y is Fourier transformed, the product is taken for each frequency with respect to the relative compliance between the tool 5 and the workpiece 7, and the product is then inverse Fourier transformed to obtain the formula.

[0073] Here, the equivalent mass m of the tool 5 is assumed to be 0.05 kg, the equivalent damping coefficient c to be 47 N·s / m, and the equivalent rigidity k to be 11 MN / m. Furthermore, the workpiece 7 is assumed to be a rigid body, and the cutting conditions are the spindle speed S of 3000 min ―1 , Feed per spindle revolution f r = 0.8mm / rev, assuming cutting depth d in the tool radial direction r , tool axial depth of cut d a , and surface roughness R z The relationship between surface roughness and surface roughness is shown in Fig. 9. Conditions with large surface roughness Rz are shown in dark colors, and z The conditions with small surface roughness R are shown in bright colors. z Assume that the required value of is 25μm. In this case, the surface roughness R z The black solid line connecting the points where the cutting ability is limited is 25 μm.

[0074] The cutting condition that is the stable limit of self-excited chatter, which reduces the machining accuracy and quality of the machined surface of the product, is the cutting coefficient K, which represents the cutting force per unit cutting cross-sectional area. c For example, the calculation can be performed by using the method for obtaining a chatter stability limit diagram as shown in Eiji Shamoto, "Technical Commentary: Mechanism of Chatter Vibration Generation and Suppression in Cutting," Electrical Steel Manufacturing, Vol. 82, No. 2, 2011, pp. 143-155.

[0075] Here, the equivalent mass m of the tool 5 is assumed to be 0.05 kg, the equivalent damping coefficient c to be 47 N·s / m, and the equivalent rigidity k to be 11 MN / m. Furthermore, the workpiece 7 is assumed to be a rigid body, and the cutting conditions are the cutting depth in the tool radial direction d r =70%, feed rate per spindle revolution f r = 0.8mm / rev, the spindle speed S and the tool axial depth of cut d at the stable limit cutting condition for self-excited chatter aThe relationship is shown in Fig. 10. In other words, the graph shown in Fig. 10 is a chatter stability limit diagram when the above conditions are assumed.

[0076] In this way, the torque T obtained from each axis motor installed in the machining center M is a ,T b ,T c Using the cutting coefficients identified using the above, it is possible to calculate the cutting conditions that are the limits of machining capability in various aspects.

[0077] The cutting coefficient identification system 21 for the machine tool having the above configuration is a machining center M equipped with a spindle 4 on which a tool 5 is attached and which is driven by a tool spindle motor, and a ball screw which is a feed axis that moves the tool 5 and the workpiece 7 relatively by a feed axis motor. The cutting coefficient identification system 21 for the machine tool having the above configuration is equipped with a spindle 4 on which a tool 5 is attached and which is driven by a tool spindle motor, and an average torque T a Measure the cutting force F by obtaining m A measured cutting force acquisition unit 22 that calculates the measured cutting force F m A tool information acquisition unit 23 that acquires tool information including the number of tool blades Z corresponding to the cutting force F m The feed amount f per one rotation of the spindle corresponds to the relative movement amount between the tool 5 and the workpiece 7 during one rotation of the spindle 4, r , tool axial depth of cut d a , and tool radial depth of cut d r a cutting condition acquisition unit 24 for acquiring cutting conditions including the cutting coefficient K c , cutting coefficient K, which represents the cutting force per unit cutting edge length e , number of tool teeth Z, feed rate per spindle revolution f r , tool axial depth of cut d a , and tool radial depth of cut d r Based on the formula including the above, the estimated cutting force F s ￣ and a measured cutting force F m and the estimated cutting force F s By comparing with ￣, the cutting coefficient K c , and the cutting coefficient K eand a cutting coefficient identifying unit 26 that identifies the cutting coefficient.

[0078] In addition, the cutting coefficient K c and cutting coefficient K e The rotation speed S of the spindle 4, which is the limit of the machining capacity of the machine tool, and the feed amount f per revolution of the spindle, which corresponds to the relative movement amount between the tool 5 and the workpiece 7 during one revolution of the spindle 4, are used to determine the limit of the machining capacity of the machine tool. r , tool axial depth of cut d a , and tool radial depth of cut d r The present invention is provided with a limit cutting condition calculation unit 27 that calculates limit cutting conditions including at least one of the above.

[0079] In addition, the measured cutting force acquisition unit 22 acquires an average torque T b is further used to calculate the measured cutting force.

[0080] In addition, the measured cutting force acquisition unit 22 acquires an average torque T c is further used to calculate the measured cutting force.

[0081] In addition, the parameters that determine the limit of the machining capacity are the upper limit of the output of at least one of the tool spindle motor, the work spindle motor, and the feed axis motor, the bending stress σ of the tool 5, the shear stress τ, and the surface roughness R which is the quality requirement value of the workpiece. z , and at least one of the stability limits of self-excited chatter.

[0082] Therefore, the torque T obtained from each axis motor installed in the machining center M is a , T b , T c Using this, the cutting coefficient K per unit cutting cross-sectional area is calculated. c , and the cutting coefficient K per unit cutting edge length e In order to identify each of these, the cutting coefficient K c , and the cutting coefficient K e can be identified with high accuracy. In addition, the torque T obtained from each axis motor installed in the machining center M a , T b , T c Using the cutting coefficients identified using the above, it is possible to calculate the cutting conditions that are the limits of machining capability in various aspects.

[0083] The configurations of the cutting coefficient identification system for a machine tool and the cutting coefficient identification method for a machine tool disclosed herein are not limited to the aspects described in the above embodiments, and can be appropriately modified as necessary without departing from the spirit of the invention. For example, in this embodiment, the measured cutting force is obtained from the torque history of the actual cutting after the cutting process is completed, but the measured cutting force may be obtained during the cutting process. Further, in this embodiment, the machine information acquisition unit acquires the ball screw specifications from the NC device, but the machine information acquisition unit may acquire the ball screw specifications from a source other than the NC device, or the machine information may be input directly by providing a separate input means. Further, in this embodiment, the tool information acquisition unit acquires the tool information from the NC device, but the tool information may be acquired from a source other than the NC device, or may be directly inputted by providing a separate input means. Further, in this embodiment, the cutting conditions acquisition unit acquires the cutting conditions based on a program, but the cutting conditions may be input directly by providing a separate input means. In addition, in this embodiment, the cutting coefficient is identified using the torque required for cutting obtained by taking the difference between the motor torque when not cutting and the motor torque when cutting, but it is also possible to use disturbance observer technology to obtain the torque required for cutting without using the motor torque when not cutting, and identify the cutting coefficient. In addition, in this embodiment, the cutting force was obtained under two conditions with different feed amounts per spindle rotation, but the cutting force may be obtained under conditions with different parameters such as the tool axial depth of cut and the tool axial depth of cut, and the feed amount per spindle rotation in this case may be the same. In this embodiment, the tool tangential cutting coefficient K tc and the tool tangential cutting coefficient K per unit cutting edge length te, and the tool radial cutting coefficient K per unit cutting cross-sectional area rc and the tool radial cutting coefficient K per unit cutting edge length re We identified them one by one, but the evaluation value E t and E x It is also possible to aggregate and identify all of them at the same time. In addition, in this embodiment, the identification of the cutting coefficient and the prediction of the machining capacity are performed consecutively, but the identified cutting coefficient may be linked to tool information, etc., and stored in a database, and the stored cutting coefficient may be called up from the database on another occasion to predict the machining capacity. Furthermore, in this embodiment, the cutting coefficient is identified and the machining capacity is predicted from information related to the tool spindle. However, instead of the tool spindle, the cutting coefficient may be identified and the machining capacity may be predicted from information related to a work spindle that can be rotatable by a work spindle motor, or the cutting coefficient may be identified and the machining capacity may be predicted from information related to both the tool spindle and the work spindle. [Explanation of symbols]

[0084] Reference Signs List 1 Bed, 2 Column, 3 Spindle housing, 4 Spindle (tool spindle), 5 Tool, 6 Table, 7 Workpiece, 11 NC unit, 12 Machine operation command unit, 13 Tool information storage unit, 21 Cutting coefficient identification system, 22 Measured cutting force acquisition unit, 23 Tool information acquisition unit, 24 Cutting condition acquisition unit, 25 Estimated cutting force calculation unit, 26 Cutting coefficient identification unit, 27 Limit cutting condition calculation unit.

Claims

1. A machine tool including at least one of a tool spindle having a tool mounted thereon and driven by a tool spindle motor, and a work spindle having a work mounted thereon and driven by a work spindle motor, and a feed spindle for relatively moving the tool and the work by a feed spindle motor, Average torque T of at least one of the tool spindle motor, the work spindle motor, and the feed spindle motor during an arbitrary period A while the tool is cutting the workpiece. ａ A measured cutting force acquisition unit that acquires the measured cutting force and calculates the measured cutting force; a tool information acquisition unit that acquires tool information including a number of tool blades corresponding to the measured cutting force; a cutting condition acquisition unit that acquires cutting conditions including a relative movement amount between the tool and the workpiece during one rotation of the tool spindle or the workpiece spindle and a cutting depth amount corresponding to the measured cutting force; Cutting coefficient K, which represents the cutting force per unit cutting cross-sectional area ｃ and the cutting coefficient K, which represents the cutting force per unit cutting edge length. ｅ an estimated cutting force calculation unit that calculates an estimated cutting force, which is an average cutting force during one rotation of the tool spindle or the workpiece spindle, based on a calculation formula including the number of tool blades, the amount of relative movement, and the amount of cut; The measured cutting force is compared with the estimated cutting force to obtain the cutting coefficient K ｃ and the cutting coefficient K ｅ and a cutting coefficient identification unit that identifies a cutting coefficient.

2. The cutting coefficient K ｃ and the cutting coefficient K ｅ a limit cutting condition calculation unit that calculates limit cutting conditions, which are limits of the machining capacity of the machine tool, including at least one of the rotational speed of the tool spindle or the work spindle, the relative movement amount between the tool and the work during one rotation of the tool spindle or the work spindle, and a cutting depth amount, using at least one of the above.

3. The measured cutting force acquisition unit acquires an average torque T during an arbitrary period B during which the tool is cutting the workpiece under cutting conditions different from those during the arbitrary period A. ｂ 3. The cutting coefficient identification system for a machine tool according to claim 1, further comprising: a cutting coefficient identification unit for identifying a cutting force that is greater than or equal to a cutting coefficient of the cutting force;

4. The measured cutting force acquisition unit is configured to acquire an average torque T ｃ 3. The cutting coefficient identification system for a machine tool according to claim 1, further comprising: a cutting coefficient identification unit for identifying a cutting force that is greater than or equal to a cutting coefficient of the cutting force;

5. 3. The cutting coefficient identification system for a machine tool according to claim 2, wherein the parameters determining the limit of the machining capacity include at least one of an upper output limit of at least one of the tool spindle motor, the workpiece spindle motor, and the feed spindle motor, a bending stress, a shear stress of the tool, a quality requirement value of the workpiece, and a stability limit of self-excited chatter.

6. A machine tool including at least one of a tool spindle having a tool mounted thereon and driven by a tool spindle motor, and a work spindle having a work mounted thereon and driven by a work spindle motor, and a feed spindle for relatively moving the tool and the work by a feed spindle motor, Average torque T of at least one of the tool spindle motor, the work spindle motor, and the feed spindle motor during an arbitrary period A while the tool is cutting the workpiece. ａ A cutting force acquisition step of acquiring the cutting force and calculating a measured cutting force; a tool information acquisition step of acquiring tool information including a number of tool blades corresponding to the measured cutting force; a cutting condition acquisition step of acquiring cutting conditions including a relative movement amount between the tool and the workpiece during one rotation of the tool spindle or the workpiece spindle and a cutting depth amount corresponding to the measured cutting force; Cutting coefficient K, which represents the cutting force per unit cutting cross-sectional area ｃ and the cutting coefficient K, which represents the cutting force per unit cutting edge length. ｅ an estimated cutting force calculation step of calculating an estimated cutting force, which is an average cutting force during one rotation of the tool spindle or the workpiece spindle, based on a calculation formula including the number of tool blades, the amount of relative movement, and the amount of cut; The measured cutting force is compared with the estimated cutting force to obtain the cutting coefficient K ｃ and the cutting coefficient K ｅ and a cutting coefficient identifying step of identifying a cutting coefficient for a machine tool.

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