JP2017170600A - Method for suppressing chattering in machine tool - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique capable of suppressing chattering caused by components with frequencies several ten times greater than that of rotation.SOLUTION: A PLC 12 of a machine tool 10 comprises: a pseudo-random number generation part 121; DC free modulation part 122; and an override command generation part 123. Chattering can be suppressed by generation of pseudo-random numbers at the pseudo-random number generation part 121, suppression of low frequency components using a DC free encoding method at the DC free modulation part 122, and generation of rotation speed command override at the override command generation part 123.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a chatter suppressing method that suppresses occurrence of chatter due to rotation of a tool of a machine tool or a workpiece.

  In general, a machine tool performs machining by rotating a tool or a workpiece and scraping the workpiece while bringing the tool and the workpiece into contact with each other. At this time, vibration is generated due to various factors. For example, when the tool is an end mill, periodic vibration is generated by intermittent contact. Further, for example, even a tool generates vibration due to anisotropy of a workpiece and a chuck that supports the workpiece. Furthermore, vibration may occur due to unbalance of the tool / workpiece, and depending on the driving method, for example, if gear driving is performed, vibration related to gear meshing occurs.

  In response to these vibrations, the natural frequency of the entire system composed of workpieces, tools, and machines is generated, and depending on the damping level, vibrations begin to amplify, so-called chatter vibration phenomenon occurs. It is known that processing accuracy (particularly surface accuracy) at the time of processing an object deteriorates. Such chatter vibration is broadly divided into forced chatter vibration and self-excited chatter vibration, and it is considered that forced chatter vibration occurs when an excessive external force acts or when the frequency of the external force and the resonance frequency of the vibration system are synchronized. It has been. On the other hand, self-excited chatter vibration includes regenerative chatter vibration (regenerative chatter vibration) and mode-coupled chatter vibration. Regenerative chatter vibration is caused by periodic fluctuation of cutting resistance and periodic fluctuation of cutting thickness. It is thought that the interaction is caused by continuing cutting that strengthens each other (so-called regenerative effect), and mode-coupled chatter vibration is caused when the vibration modes in two directions have close resonance frequencies. It is thought to occur in combination.

  On the other hand, a machine tool may have a function that allows an operator to adjust the rotation speed by applying a coefficient to the rotation speed command of a tool or a workpiece, and is called rotation command override or simply override.

  Conventionally, of the above chatter vibrations, there is a method of avoiding regenerative chatter vibrations by adjusting the number of rotations of the tool or workpiece, and deep cutting can be performed while avoiding chatter for each machine / tool. A method for finding the rotational speed by experiment has also been proposed, and it is useful for handling a tool rotating at high speed.

For example, in the conventional example described in Non-Patent Document 1, when a machine tool and a tool to be used are specified by paying attention to the fact that there is a stable region where chatter does not occur in a range where the spindle speed is extremely high (referred to as a stable pocket). In addition, a technique is disclosed in which a spindle speed at which a stable pocket exists and a possible cutting amount are predicted, machining is started based on the prediction, and a deviation from the prediction that actually occurs is corrected while machining.

Further, in the conventional example described in Patent Document 2, the vibration generated when the rotation of the tool or the work is started is detected, and the vibration detected from the start of the rotation is the threshold value of the vibration when the machine spindle is idle. When it is determined that the vibration exceeds the above, the vibration is analyzed by Fourier series expansion, the frequency due to the playback chatter is calculated, and the playback chatter avoidance frequency is obtained by adding the estimated damping ratio to the frequency due to the playback chatter. A chatter suppressing method for a work machine that adjusts the rotational speed is disclosed.

Improving cutting efficiency so far: Understanding and practical use of stable pockets (Special issue: “Vibration vibration” and suppression technology in machining) Tetsutaro Hoshi Mechanical technology Mechanical technology 61 (5), 22-29, 2013-05 Nikkan Kogyo Publishing Production 1953

JP2014083674

  However, when the adverse conditions of the tool and workpiece overlap, even if the vibration is initially small, if the work is continued with constant cutting and constant feed, the component of the tool (or workpiece) rotation will be several tens of times May cause chatter. For such a phenomenon, the conventional chatter avoidance measures as described above can be applied to the chatter phenomenon caused by a component several tens of times the tool (or workpiece) rotation. If it is doubled, it becomes a difference of several hundred revolutions, so it is not realistic. Therefore, development of a technique for suppressing chatter vibration caused by a component several tens of times higher than that of rotation has been awaited.

  The present invention has been made for the above-described circumstances, and an object thereof is to provide a technique capable of suppressing chatter vibration caused by a component several tens of times as high as the rotation described above.

  As a result of earnest research from various viewpoints regarding the possibility of a technique capable of suppressing chatter vibration caused by a component several tens of times as described above, the present inventors have found that When higher-order vibration components lead to chatter, it is easier to work by actually moving the machine and experimenting with a slight change in the number of rotations than to predict the avoidance number of rotations by calculation. I found out.

  Here, the order refers to a multiple component such as a certain vibration frequency, the 1st component is called the primary component, the 2nd is the 2nd order, the 3rd is the 3rd order, etc. Higher order means a frequency component having a higher order.

  Generally, a method of slightly changing the rotational speed command is taken for chatter in which vibration is amplified when machining is continued for a long time. Therefore, a possible solution is to continuously switch several percent of the spindle rotation command override generated using pseudorandom numbers at regular intervals. Actually, an override command in which several percent is added to 100% is applied.

  For example, a pseudo-random number sequence can be generated by using a linear feedback shift register. If any one bit of this random number sequence is used and this becomes 0, an override command is 100%, and when it becomes 1, a command sequence is generated that becomes 99%.

  However, when trying to apply raw pseudo-random numbers as they are to the spindle override command, a low frequency component appears, such as 0 command continuing for a long time (100% continuous in actual command), and several percent as expected. There was no switching.

  Of course, it goes without saying that two or more bits can be used for the assignment of overrides as an implementation.

Therefore, the chatter suppressing method in the machine tool of the present invention is characterized in that a pseudo random number sequence is generated, and further, a low frequency component is suppressed using a DC-free encoding method or the like, and a rotation speed command override is generated. . That is, instead of using the pseudo-random number sequence as it is, DC free encoding is performed using an 8-16 modulation table or the like to suppress long-period components and then used to generate an override command.

  In an implementation in which two or more bits are allocated, the random number sequence is adjusted so as not to continue by passing a constant low-pass filter.

Further, the machine tool of the present invention comprises at least a rotating shaft, a tool attached to the rotating shaft, a rotation driving means for the rotating shaft, and a numerical control unit for sending a rotation speed command to the rotation driving means, In the method of suppressing chatter vibration in a machine tool that processes a workpiece with the tool by causing the rotating shaft driving means to rotate the rotating shaft by a predetermined number of rotations based on the rotation number command, a pseudo random number sequence generation unit; The pseudo-random number sequence generation unit generates a pseudo-random number sequence and transmits the pseudo-random number sequence to the DC-free modulation unit, and the DC-free modulation unit performs a DC-free encoding method. The low frequency component is suppressed and transmitted to the override command generation unit 123, and the override command generation unit is configured to transmit a rotation speed command on the basis of the signal with the low frequency component suppressed. It generates Baraido, characterized by transmitting the rotation speed command overrides the numerical control unit.

  According to the present invention, it is possible to suppress chatter vibration caused by a component several tens of times higher than the rotation that may occur when the work is continued with constant cutting and constant feed.

It is a graph which shows the example of the signal to which this invention is applied, and its frequency analysis example, (a) shows the example of the signal with which the high-order component was mixed, (b) shows the frequency analysis example, respectively. It is a flowchart which shows the chatter suppressing method as a comparative example of this invention, and is an example which uses a random number sequence as it is. It is an example of the random number sequence which shows the problem of the chatter suppressing method as a comparative example. It is a figure which shows the structure of the control apparatus in the machine tool to which this invention is applied. It is a flowchart which shows the chatter suppressing method of embodiment of this invention, and is an example used by modulating.

  As a result of earnest research from various viewpoints regarding the possibility of a technique capable of suppressing chatter vibration caused by a component several tens of times as described above, the present inventors have found that When higher-order vibration components lead to chatter, it is easier to work by actually moving the machine and experimenting with a slight change in the number of rotations than to predict the avoidance number of rotations by calculation. I found out. FIG. 1 is a graph showing an example of a signal to which the present invention (and a comparative example) is applied and an example of frequency analysis thereof. (A) is an example of a signal mixed with higher-order components, and (b) is its frequency. Analysis examples are shown respectively. Here, as shown in FIGS. 1A and 1B, the order is intended to be a multiple component such as a certain vibration frequency, and the 1st component is called the primary component, the 2nd is the 2nd, the 3rd is the 3rd, etc. “Next” means a frequency component having a higher order.

FIG. 2 is a flowchart showing a chatter suppressing method as a comparative example of the present invention, in which a random number sequence is used as it is. As described above, for chatter in which vibration is amplified when machining is continued for a long time, a method of slightly changing the rotational speed command is used. Therefore, in this comparative example, several percent of the spindle rotation command override generated using pseudorandom numbers at regular intervals is continuously switched. Actually, an override command in which several percent is added to 100% is applied. For example, a pseudo-random number sequence can be generated by using a linear feedback shift register. When one bit of this random number sequence is used and this becomes 0, the override command is 100%, and when it becomes 1, a command sequence is generated that becomes 99%.

That is, as shown in FIG. 2, when the flow of the chatter suppressing method according to this comparative example is started (S201), a pseudo-random number is generated at regular intervals, and the previous random number is updated (S202). For example, a random number sequence indicated by 200R in FIG. 2 is generated and updated to this random number sequence 200R. Then, the instruction bit (here, BITNo. 0 is the instruction bit) is confirmed in the updated random number sequence 200R (S203). Subsequently, it is determined whether the instruction bit is 1 (1 or 0) (S204). If it is 1 (Yes in S204), a 100% override command is generated (S205). The command is displayed (S206) and the process returns to S202 and updated to the next random number. On the other hand, if the instruction bit is not 1 (No in S204), a 99% override command is generated (S207), this command is displayed (S206), and the process returns to S202 to update to the next random number.

  However, as described above, if the raw pseudo-random number 200R is continuously applied to the spindle override command as it is, a low-frequency component appears such that 0 command continues for a long time (100% continues in actual command), A few percent of the changes were not made as expected.

FIG. 3 is an example of a random number sequence showing the problem of the chatter suppressing method as a comparative example. As shown in FIG. 3, in this example, when Fibonacci LFSR (bit number 16) is used for pseudo-random number generation and the least significant bit is designated as an instruction bit, 0 may be contiguous for a maximum of 15 times. Even if 1 comes after that, the continuation of 0 may be resumed. Therefore, theoretically, such a situation is also conceivable. Therefore, it is useful to suppress the long period component and use it for generating the override command by applying some other device.

  Therefore, the chatter suppressing method according to the embodiment of the present invention does not use a pseudo random number sequence as it is, but uses DC-free encoding using an 8-16 modulation table or the like to suppress a long-period component and then apply it. Used to generate override command.

  FIG. 4 is a diagram showing a configuration of a control device in a machine tool to which the present invention is applied. As shown in FIG. 4, a machine tool (processing control device) 10 to which the present invention is applied includes a programmable controller (PLC) 12, an interface unit 14, and various switches / sensors 16. Various input / output signals from various switches / sensors 16 are transmitted to and from the PLC 12 via the interface unit 14. Further, the machining control device 10 includes a numerical controller (CNC) 18, an input device (keyboard switch) 20 for an operator to input commands such as numerical values to the CNC 18, and inputs (controls) set by the commands. And a display 22 for displaying the status. Here, the numerical controller (CNC) 18 further includes a tool axis controller 181 and a plurality of axis controllers 182, 183, 184,. A spindle motor 181 a is connected to the tool axis controller 181, and the tool axis controller 181 rotates the spindle motor 181 a at a predetermined rotation speed and speed based on a command such as a numerical value of the CNC 18. Accordingly, the work is processed by rotating the tool attached to a spindle (not shown). The axis controllers 182, 183, 184,... Correspond to the X axis, Y axis, Z axis, etc., respectively, if the present invention is applied to a machine tool with four or more axes. The motors are connected to respective motors (not shown).

The chatter suppressing method of the present invention is a method for generating a rotational speed command override after generating a pseudo-random number sequence and further suppressing a low frequency component using a DC-free encoding method or the like. The machining control device 10 is a machine tool that employs such a chatter suppressing method. That is, the PLC 12 of the machine tool (machining control device) 10 includes a pseudo random number sequence generation unit 121, a DC free modulation unit 122, and an override command generation unit 123. The pseudo random number sequence generation unit 121 generates a pseudo random number sequence and transmits the pseudo random number sequence to the DC free modulation unit 122. The DC free modulation unit 122 further suppresses low frequency components using a DC free encoding method or the like, and an override command. The data is transmitted to the generation unit 123. The override command generation unit 123 generates a rotation speed command override based on the signal in which the low frequency component is suppressed, and transmits the rotation speed command override to the numerical controller (CNC) 18. In FIG. 4, the chatter suppressing program may not be in the PLC 12 but may be on the CNC 18 side or another unit. Further, the pseudo random number sequence generation unit 121 and the DC free modulation unit 122 may also be outside the PLC 12. Further, the pseudo random number sequence generation unit 121, the DC free modulation unit 122, the override command generation unit 123, and the like may be separately provided peripheral devices of the machine tool 10.

FIG. 5 is a flowchart showing the chatter suppressing method according to the embodiment of the present invention, and is an example in which the method is used after modulation. That is, as shown in FIGS. 4 and 5, when the flow of the chatter suppressing method of the present embodiment starts in the PLC 12 of the machine tool (machining control device) 10 (S301), the pseudo random number sequence generation unit 121 is set at regular intervals. 8 pseudo random number sequences are generated (S302), and the instruction bits of each of the eight pseudo random number sequences are confirmed (S303). Subsequently, eight designated bit strings composed of these eight instruction bits are generated (S304), and the DC-free modulation unit 122 performs DC-free encoding using the 8-16 modulation table, and the FIFO of a buffer memory (not shown) Store in the area (first-in first-out buffer memory) (S305). Subsequently, 1 bit is obtained from the FIFO buffer (S306), and it is determined whether or not the obtained bit is 1 (1 or 0) (S307). If it is 1 (Yes in S307). A 100% override command is generated (S308), and this command is displayed (S309). Then, it is determined whether or not there is a remainder in the instruction string (S310). If there is a remainder (Yes in S310), the process returns to S306 to acquire the next 1 bit from the FIFO buffer, and the processing after S307 To continue. When there is no remaining (No in S310), the process returns to S302, and the next eight instruction strings are generated and updated. On the other hand, if the acquired bit is not 1 in S307 (No in S307), a 99% override command is generated (S311), this command is displayed (S309) and the process returns to S302 to return to the next random number. Update to

In the embodiment described above, 1 bit is used for the assignment of the override as an instruction bit of the pseudo random number, but it goes without saying that 2 bits or more can be used for the assignment of the override as an implementation. However, in an implementation that uses two or more bits for allocation of overrides, it is effective to adjust the random number sequence so that it does not continue by passing a constant low-pass prevention filter.

  In the embodiment described above, the present invention is applied to a method of generating a pseudo random number sequence and further generating a rotational speed command override after suppressing a low frequency component using a DC-free encoding method. Of course, it is not limited. The present invention may use a modulation method other than the DC-free coding method as long as low frequency components can be suppressed.

10 machine tools (processing control devices), 12 programmable controllers (PLC),
14 interface units, 16 switches / sensors,
18 Numerical controller (CNC), 20 Input device (keyboard switch),
22 display, 181 tool axis controller, 182, 183, 184 axis controller,
181a spindle motor, 121 pseudorandom number generator, 122 DC free modulator,
123 Override command generator

Claims (2)

A chatter suppressing method for a machine tool, characterized in that a pseudo random number sequence is generated, and further, a low frequency component is suppressed using a DC-free encoding technique, and then a rotation speed command override is generated.   At least a rotation shaft, a tool attached to the rotation shaft, a rotation drive unit for the rotation shaft, and a numerical control unit for sending a rotation number command to the rotation drive unit, and based on the rotation number command, the rotation shaft In a method for suppressing chatter vibration in a machine tool that processes a workpiece with the tool by causing the driving means to rotate the rotating shaft at a predetermined number of revolutions, a pseudo-random number generator, a DC free modulator, an override command A generation unit 123, the pseudo-random number sequence generation unit generates a pseudo-random number sequence and transmits the pseudo-random number sequence to the DC-free modulation unit, and the DC-free modulation unit suppresses low frequency components using a DC-free encoding method. The override command generation unit 123 generates a rotation speed command override based on the signal that suppresses the low frequency component, Machine tool, characterized by transmitting the number command override the numerical control unit.

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