Machine tool control device

Abstract

A machine tool control device for performing oscillation cutting for cutting a workpiece while causing the workpiece and a tool to perform an oscillation operation relative to each other, the machine tool control device comprising: an initial oscillation condition setting unit that sets an initial oscillation condition for the oscillation operation; a load acquisition unit that acquires load information of a drive shaft that causes the oscillation operation; an air cutting determination unit that determines whether air cutting is being performed from the load information acquired by the load acquisition unit; an oscillation condition resetting unit that resets the oscillation condition on the basis of the result of the air cutting determination unit; an oscillation command calculation unit that calculates an oscillation command for commanding the oscillation operation on the basis of the oscillation condition set by the oscillation condition initial setting unit or the oscillation condition resetting unit; and an execution control unit that controls the drive shaft on the basis of the oscillation command.

Description

Control device for machine tool

[0001] The present disclosure relates to a control device for a machine tool.

[0002] Conventionally, oscillating cutting that chips into small pieces by relatively oscillating a workpiece and a tool to periodically perform air cutting is known. In order to perform oscillating cutting, it is necessary to appropriately set the relative movement between the workpiece and the tool. Specifically, periodic air cutting can be achieved by appropriately setting the frequency and amplitude of oscillation. For example, Patent Document 1 discloses a technique for calculating the optimal value of the oscillation frequency from the acceleration / deceleration command of a machining program.

[0003] Japanese Unexamined Patent Publication No. 2018-181103

[0004] Appropriate oscillation conditions may vary depending on the tool, workpiece, machining method, machining shape and other factors. With the technique of Patent Document 1, there is a possibility that optimal oscillating cutting cannot be performed due to the influence of unconsidered conditions. Generally, oscillation conditions that increase air cut are set so that air cutting occurs even if actual cutting deviates from what was expected. In order to determine such oscillation conditions, it was necessary to cover conditions such as tools, workpieces, machining methods, machining shapes, etc., and conduct trial and error involving actual operation. This is practically difficult, and oscillation conditions are determined after consideration within a feasible range. Therefore, it is difficult to perform optimal oscillating cutting for all machining conditions, and even taking this situation into account, there is a problem that man-hours are spent to determine oscillation conditions. For this reason, there is a need for technology that can support appropriate oscillation condition determination and reduce man-hours.

[0005] A control device for a machine tool according to one aspect of the present disclosure is a control device for a machine tool that performs oscillating cutting, in which the workpiece is cut while the workpiece and the tool are oscillating relative to each other, and comprises: an oscillating condition initial setting unit that sets initial conditions for the oscillating motion; a load acquisition unit that acquires load information of the drive shaft that performs the oscillating motion; an air cut determination unit that determines whether or not air cut is occurring based on the load information acquired by the load acquisition unit; an oscillating condition resetting unit that resets the oscillating conditions based on the result of the air cut determination unit; an oscillating command calculation unit that calculates an oscillating command to instruct the oscillating motion based on the oscillating conditions set by the oscillating condition initial setting unit or the oscillating condition resetting unit; and an execution control unit that controls the drive shaft based on the oscillating command.

[0006] This is a block diagram illustrating the configuration of a control device for a machine tool according to the first embodiment of this disclosure. This is a schematic diagram illustrating the machining controlled by the control device of the machine tool shown in Figure 1. This is a graph illustrating the theoretical torque waveform during oscillating cutting. This is a graph illustrating the frequency analysis of the torque command. This is a Bode plot illustrating the frequency response characteristics of the machine tool.

[0007] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. Figure 1 is a block diagram showing the configuration of a control device 1 for a machine tool according to the first embodiment of the present disclosure. The control device 1 for the machine tool controls a machine tool that performs oscillating cutting, in which the workpiece W is cut while the workpiece W and the tool T are oscillating relative to each other. In the description of this embodiment, we will assume that oscillating cutting is performed in which the workpiece W is rotated by the spindle and the tool is moved while oscillating by the feed axis, as shown in Figure 2, but it may also be applied to oscillating cutting in which a multi-tooth rotary tool is moved in the feed direction while oscillating. In the oscillating cutting shown in Figure 2, it is assumed that the tool T passes through an area where the material of the workpiece W has already been removed, as the trajectory of the tool T, shown by the dashed line, intersects with the trajectory of the workpiece W one relative rotation prior, thereby cutting the chips by creating an air cut that does not cut the workpiece W.

[0008] The machine tool control device 1 comprises an oscillation condition initial setting unit 11, a load acquisition unit 12, an air cut determination unit 13, an oscillation condition resetting unit 14, an oscillation command calculation unit 15, a feed command generation unit 16, a feedback adder 17, an integrator 18, an oscillation adder 19, and an execution control unit 20. The machine tool control device 1 according to this embodiment is configured, for example, by a computer having a CPU, memory, etc.

[0009] The oscillation condition initial setting unit 11 sets the oscillation conditions for the oscillation operation, that is, the oscillation frequency and oscillation amplitude, so that air cut can be performed for each oscillation period. The oscillation condition initial setting unit 11 may be configured to automatically set the initial oscillation conditions based on the rotational speed of the spindle that causes relative rotation between the workpiece W and the tool T, and the feed rate that moves the tool T in a direction parallel to the spindle except for the oscillation operation, or it may be configured for the user to input the rotational speed of the spindle and the feed rate of the tool T. The initial setting value of the oscillation frequency may be an odd multiple of half the spindle rotation speed, based on well known technology, and the initial setting value of the oscillation amplitude may be a value slightly larger than the feed amount of the tool T per oscillation period.

[0010] The load acquisition unit 12 acquires load information of the drive shaft (feed shaft) that performs the oscillating motion. The load acquisition unit 12 may be configured to acquire torque commands for the drive shaft from the execution control unit 20 as load information. Alternatively, the load acquisition unit 12 may be configured to acquire detected values ​​from a load sensor provided on the tool T or the feed shaft as load information.

[0011] The air cut determination unit 13 determines whether or not air cut is being performed based on the load information acquired by the load acquisition unit 12. Specifically, the air cut determination unit 13 can be configured to estimate the cutting load from the load information, identify the air cut phase, which is the spindle phase at which air cut should be performed based on the oscillation command, and determine that air cut is being performed if the cutting load is below a preset no-load threshold during the air cut phase. As shown in Figure 3, in oscillating cutting, a combined torque (dotted line) is applied to the tool T, which is the sum of the cutting load torque (solid line) and the acceleration / deceleration torque (dashed line). The cutting load torque is sinusoidal in shape corresponding to the oscillation speed when air cut is not performed, but becomes approximately zero and deviates from a sine wave while air cut is performed. The acceleration / deceleration torque is sinusoidal in phase with the cutting load torque. The acceleration / deceleration torque can be estimated from the acceleration, the inertia of the drive shaft, and the torque constant and drive current of the motor M that drives the drive shaft. Therefore, the cutting load torque can be calculated by subtracting the acceleration / deceleration torque from the combined torque. The no-load threshold is set to a value near zero, taking into account errors and other factors. Furthermore, the air cut determination unit 13 may be configured to determine that air cut is occurring if the cutting load remains below the no-load threshold for a certain period of time or longer, in order to eliminate noise.

[0012] As an alternative, the air cut determination unit 13 may be configured to determine that air cut is occurring when the waveform of the cutting load deviates from a sine wave by a certain value or more, or when there are multiple maximum or minimum points per cycle of the oscillation command. The combined torque is basically a sinusoidal waveform with the cutting load torque as the dominant force, but only while air cut is occurring does it become a sinusoidal waveform with the cutting load torque as the dominant force and in opposite phase. When air cut occurs, two maximum points and two minimum points appear during one cycle of oscillation, so the presence or absence of air cut can be determined by the number of maximum or minimum points. Furthermore, the air cut determination unit 13 may be configured to determine that air cut is occurring when there is a mathematical discontinuity point (a point that cannot be differentiated) in the cutting load.

[0013] Alternatively, the air cut determination unit 13 may be configured to perform a frequency analysis of the torque command to the drive shaft and determine that air cut is occurring if the resulting torque deviates by a certain value or more from the sinusoidal torque estimate that would be expected if air cut were not performed, or if frequency components other than the oscillation frequency are detected at a certain value or more. In other words, the air cut determination unit 13 records the time-dependent change data of the torque command, performs a Fourier transform on the time-dependent change data of the torque command, and determines that air cut is occurring if the frequency components different from the oscillation frequency are at or above a value recognized as noise. Since the torque command will be a value that matches the composite torque described above, as shown in Figure 4, the frequency analysis result of the torque command when air cut does not occur will consist only of the oscillation frequency component excluding noise, but the frequency analysis result of the torque command when air cut occurs will clearly include frequency components different from the oscillation frequency. For this reason, it is possible to confirm whether or not air cut is occurring by frequency analysis of the torque command. Furthermore, the air cut determination unit 13 may be configured to determine that air cut is occurring when the waveform of the torque command has multiple maximum points or multiple minimum points per cycle of the oscillation command.

[0014] As a further alternative, the air cut determination unit 13 may be configured to pass the torque command through a band-stop filter with the oscillation frequency as its center frequency, and to determine that air cut is occurring if the output of the band-stop filter changes by a certain value or more. By passing the waveform data of the torque command through a band-stop filter, frequency components close to the oscillation frequency can be removed, so it is possible to check whether the waveform of the torque command contains frequency components caused by air cut that are different from the oscillation frequency.

[0015] The oscillation condition resetting unit 14 resets the oscillation conditions based on the result of the air cut determination unit. The oscillation condition resetting unit 14 may repeatedly reset the oscillation conditions to search for the limit condition under which air cut is possible, that is, the minimum value of at least one of the oscillation amplitude and oscillation frequency. Increasing the air cut increases the amount of cutting in the feed direction, thus increasing the cutting load. Therefore, by performing cutting under the limit condition under which air cut is possible, the cutting load can be suppressed to the necessary minimum.

[0016] As a specific example, the oscillation condition resetting unit 14 may be configured to increase the oscillation amplitude when the air cut determination unit determines that air cut has not occurred. The minimum oscillation amplitude at which air cut can occur can be estimated relatively accurately from the number of tool teeth, oscillation frequency, spindle speed, and feed rate. Therefore, by gradually increasing the oscillation amplitude from this estimated value and searching for the minimum oscillation amplitude at which air cut actually occurs, an appropriate oscillation amplitude can be set relatively efficiently. In particular, when the oscillation operation is performed with a single-frequency sine wave, if the number of tool teeth is n and the frequency multiplier is I, the minimum oscillation amplitude multiplier K at which air cut occurs can be derived by the following equation (1). Note that the search range for oscillation amplitude may be limited based on, for example, the spindle speed, feed rate, etc.

[0017]

[0018] The oscillation condition resetting unit 14 may be configured to change the oscillation frequency and sequentially increase the oscillation amplitude from the minimum value to search for conditions under which air cut occurs if it is not determined that air cut occurs within the search range of the oscillation amplitude. The search range of the oscillation frequency by the oscillation condition resetting unit 14 can also be set in advance. The machine tool may have frequency response characteristics as shown in Figure 5. In the example in Figure 5, the search range may be a frequency range of f1 to f2. In Figure 5, the range of frequencies less than f1 is excluded from the search range because the response is poor due to high friction. The range of frequencies greater than f2 and less than or equal to f3 is excluded from the search range because the response to high frequencies is poor and resonance may occur at frequencies around f3. The oscillation condition resetting unit 14 may be set to have multiple frequency bands as its search range. Since the load increases with higher oscillation frequencies, it is preferable that the search for air cut by the oscillation condition resetting unit 14 starts from a low oscillation frequency and continues the search by increasing the oscillation frequency if air cut does not occur by changing the oscillation amplitude.

[0019] The oscillation command calculation unit 15 calculates an oscillation command that commands oscillation operation based on the oscillation conditions set by the oscillation condition initial setting unit 11 or the oscillation condition resetting unit 14. The oscillation command calculation unit 15 may be configured to set the value of the oscillation command to a sinusoidal value that changes with the oscillation period and oscillation amplitude.

[0020] The feed command generation unit 16 generates a position command that displaces at a set feed rate.

[0021] The feedback adder 17 calculates the difference between the feed command generated by the feed command generation unit 16 and the position feedback based on the position detection by the encoder of the feed shaft motor M, i.e., the position deviation.

[0022] The integrator 18 calculates the cumulative value of the position deviation. Specifically, the integrator 18 calculates the cumulative value of the position deviation by accumulating the position deviations calculated by the feedback adder 17.

[0023] The oscillation adder 19 generates a position command that specifies the position of the feed axis by adding an oscillation command to the position deviation.

[0024] The execution control unit 20 controls the motor M of the feed axis based on a position command that reflects the oscillation command calculated by the oscillation command calculation unit 15. The execution control unit 20 may be configured to include a position control unit 21 that calculates a speed command that specifies the speed of the motor based on the position command, and a speed / current control unit 22 that calculates a torque command that specifies the torque of the motor based on the speed command.

[0025] A control device for a machine tool having the above configuration can perform oscillating cutting by executing an oscillating command calculated by the oscillating command calculation unit 15 based on the oscillating conditions, determining whether or not air cut has occurred using the air cut determination unit 13, and if air cut has not occurred, resetting the oscillating conditions using the oscillating condition reset unit. Through trial and error, it is possible to perform oscillating cutting that allows for appropriate air cut.

[0026] The following additional information is disclosed regarding the above embodiments and modifications. (Addendum 1) The control device (1) for a machine tool is a control device (1) for a machine tool that performs oscillating cutting, in which the workpiece is cut while the workpiece and the tool are oscillating relative to each other, and comprises: an oscillating condition initial setting unit (11) that sets initial conditions for the oscillating motion; a load acquisition unit (12) that acquires load information of the drive shaft that performs the oscillating motion; an air cut determination unit (13) that determines whether or not air cut is being performed from the load information acquired by the load acquisition unit (12); an oscillating condition resetting unit (14) that resets the oscillating conditions based on the result of the air cut determination unit (13); an oscillating command calculation unit (15) that calculates an oscillating command to instruct the oscillating motion based on the oscillating conditions set by the oscillating condition initial setting unit (11) or the oscillating condition resetting unit (14); and an execution control unit (20) that controls the drive shaft based on the oscillating command.

[0027] (Note 2) In the control device (1) of the machine tool described in Note 1, the air cut determination unit (13) may estimate the cutting load from the load information and identify the air cut phase, which is the spindle phase at which air cut should be performed based on the oscillation command, and determine that air cut is being performed if the cutting load at the air cut phase is below a preset no-load threshold.

[0028] (Note 3) In the control device (1) of the machine tool described in Note 1, the air cut determination unit (13) may estimate the cutting load from the load information and determine that air cut is occurring if the waveform of the cutting load has multiple maximum points or multiple minimum points per cycle of the oscillation command.

[0029] (Note 4) In the control device (1) of the machine tool described in Note 1, the load acquisition unit (12) acquires a torque command for the drive shaft as load information, and the air cut determination unit (13) performs frequency analysis on the torque command and determines that air cut is occurring when frequency components other than the frequency of the oscillating motion are detected to be above a certain value.

[0030] (Note 5) In the control device (1) of the machine tool described in Note 1, the load acquisition unit (12) acquires a torque command for the drive shaft as load information, and the air cut determination unit (13) passes the torque command through a band stop filter whose center frequency is the frequency of the oscillation operation, and determines that air cut is occurring when the output of the band stop filter changes by a certain value or more.

[0031] (Note 6) In the control device (1) of the machine tool described in Notes 1 to 5, the oscillation condition resetting unit (14) may increase the oscillation amplitude when the air cut determination unit (13) determines that air has not been cut.

[0032] (Note 7) In the control device (1) of the machine tool described in Notes 1 to 6, the oscillation condition resetting unit (14) may repeatedly reset the oscillation conditions in order to search for the extreme conditions under which air cut can be performed.

[0033] Although the present disclosure has been described in detail above, it is not limited to the individual embodiments described above. These embodiments can be added, replaced, modified, partially deleted, etc., in any way that does not depart from the gist of the present disclosure or from the spirit of the present disclosure derived from the claims and their equivalents. Furthermore, these embodiments can be implemented in combination. For example, the order of operations and processes in the embodiments described above are shown as examples only and are not limited thereto. The same applies when numerical values ​​or mathematical formulas are used in the description of the embodiments described above.

[0034] As a specific example, in the embodiment described above, the position command is calculated by adding the feed command and the oscillation command, but the oscillation command calculation unit may be configured to calculate a command value that includes displacement based on the feed speed. Also, depending on the type of motor, the execution control unit may be configured not to calculate speed or torque from the position command.

[0035] 1 Control device 11 Initial setting unit for oscillation conditions 12 Load acquisition unit 13 Air cut determination unit 14 Reset oscillation conditions unit 15 Oscillation command calculation unit 16 Feed command generation unit 17 Feedback adder 18 Accumulator 19 Oscillation adder 20 Execution control unit M Motor T Tool W Workpiece

Claims

1. A control device for a machine tool that performs oscillating cutting, in which the workpiece and the tool are oscillated relative to each other while the workpiece is being cut, comprising: an oscillating condition initial setting unit that sets initial conditions for the oscillating motion; a load acquisition unit that acquires load information of the drive shaft that performs the oscillating motion; an air cut determination unit that determines whether or not air cut is being performed based on the load information acquired by the load acquisition unit; an oscillating condition resetting unit that resets the oscillating conditions based on the result of the air cut determination unit; an oscillating command calculation unit that calculates an oscillating command to instruct the oscillating motion based on the oscillating conditions set by the oscillating condition initial setting unit or the oscillating condition resetting unit; and an execution control unit that controls the drive shaft based on the oscillating command.

2. The control device for a machine tool according to claim 1, wherein the air cut determination unit estimates the cutting load from the load information, identifies an air cut phase which is the spindle phase in which air cut should be performed based on the oscillation command, and determines that air cut is being performed if the cutting load at the air cut phase is less than or equal to a preset no-load threshold.

3. The control device for a machine tool according to claim 1, wherein the air cut determination unit estimates the cutting load from the load information and determines that air cut is occurring when the waveform of the cutting load has multiple maximum points or multiple minimum points per cycle of the oscillation command.

4. The control device for a machine tool according to claim 1, wherein the load acquisition unit acquires a torque command for the drive shaft as load information, and the air cut determination unit performs frequency analysis of the torque command and determines that air cut is occurring when a frequency component other than the frequency of the oscillating motion is detected to be above a certain value.

5. The control device for a machine tool according to claim 1, wherein the load acquisition unit acquires a torque command for the drive shaft as load information, and the air cut determination unit passes the torque command through a band stop filter whose center frequency is the frequency of the oscillation operation, and determines that air cut is occurring when the output of the band stop filter changes by a certain value or more.

6. The control device for a machine tool according to any one of claims 1 to 5, wherein the oscillation condition resetting unit increases the oscillation amplitude when the air cut determination unit determines that air has not been cut.

7. The control device for a machine tool according to any one of claims 1 to 6, wherein the oscillation condition resetting unit repeatedly resets the oscillation conditions in order to search for the extreme conditions under which air cut can be performed.

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