JP7853564B2 - machine tool - Google Patents

Description

This invention relates to a machine tool that performs vibratory cutting of a workpiece held in a spindle.

As a machine tool, the NC (numerical control) automatic lathe, which has a spindle that grips the workpiece, is well known. If the chips generated from the workpiece rotating with the spindle become long, they can wrap around the cutting tool, potentially affecting continuous machining of the workpiece. Therefore, vibratory cutting is performed to break up the chips by feeding the tool in an alternating motion of moving it forward toward the workpiece and moving it backward toward the workpiece. Chips are also called metal shavings.
If the frequency of the vibratory cutting is close to the natural frequency of the tool post or other components, resonance may occur, which can prevent the chips from being properly broken up.

The correction device for an NC machine tool described in Patent Document 1 calculates a correction amount to reduce or increase the feed rate of each feed mechanism and/or the rotational speed of the spindle when the machining vibration frequency is within a predetermined range relative to the natural frequency. This correction amount is calculated by multiplying the current commanded feed rate for the feed mechanism and/or the commanded rotational speed for the spindle by a predetermined constant.

Japanese Patent Publication No. 2003-108206

However, if the feed rate of the feed mechanism is reduced by multiplying the commanded feed rate by a constant, or if the spindle rotation speed is reduced by multiplying the commanded rotation speed by a constant, the machining time will increase. Conversely, if the feed rate of the feed mechanism is increased by multiplying the commanded feed rate by a constant, or if the spindle rotation speed is increased by multiplying the commanded rotation speed by a constant, the machining time will decrease, but the load on the tool and spindle motor will increase.
Furthermore, the aforementioned problems exist not only in lathes but also in various other machine tools such as machining centers.

This invention discloses a machine tool capable of performing vibration cutting while suppressing the impact on processing time and avoiding resonance.

The machine tool of the present invention is
A rotary drive unit that rotates the spindle that grips the workpiece,
A tool for cutting the workpiece and a feed drive unit for moving at least one of the drive targets of the spindle,
The system includes a control unit that controls the feed movement of the driven object such that when the workpiece is being cut, the tool moves forward along the cutting direction toward the workpiece and moves backward in the opposite direction to the forward movement, accompanied by vibration.
The control unit,
Whether parameter (A) included in a command that includes the parameter (A) relating to the period of the vibration is prohibited or permitted is determined based on information (NF) regarding the natural frequency.
If it is determined that parameter (A) is prohibited, the vibration is controlled such that the vibration includes a portion of the vibration with a period according to parameter (AB) which is permitted with a longer period than parameter (A), and a portion of the vibration with a period according to parameter (AC) which is permitted with a shorter period than parameter (A).

According to the present invention, it is possible to provide a machine tool that can perform vibration cutting while suppressing the impact on processing time and avoiding resonance.

This diagram schematically shows an example of a machine tool configuration. This is a block diagram schematically showing an example of the configuration of an electrical circuit in a machine tool. This diagram schematically shows an example of the tool position relative to the spindle rotation angle when the chip length coefficient A1 is 2. This diagram schematically shows an example of the tool position relative to the spindle phase when the chip length coefficient A1 is 2. This diagram schematically shows an example of the tool position relative to the spindle rotation angle when the chip length coefficient A1 is 3. This diagram schematically illustrates an example of controlling the position of a tool during feed movement based on a vibration feed command. This diagram schematically shows an example of the tool position relative to the spindle rotation angle when the chip length coefficient A1 is 2/3. This diagram schematically shows an example of the tool position relative to the spindle phase when the chip length coefficient A1 is 2/3. This diagram schematically illustrates an example of the structure of an information table. This diagram schematically shows an example of the tool position relative to the spindle rotation angle when the control unit varies the chip length coefficient. This diagram schematically shows an example of the tool position relative to the spindle phase when the control unit varies the chip length coefficient. This figure schematically illustrates an example of a variation pattern in the chip length coefficient. This diagram schematically shows an example of the tool position relative to the spindle phase when the control unit varies the chip length coefficient to a value less than 1. This flowchart schematically illustrates an example of vibration control processing that executes a vibration feed command. This diagram schematically illustrates another example of tool position relative to spindle rotation angle.

The following describes embodiments of the present invention. Of course, the following embodiments are merely illustrative of the present invention, and not all of the features shown in the embodiments are necessarily essential to the solution of the invention.

(1) Overview of the technology included in the present invention:
First, an overview of the technology included in the present invention will be described with reference to the examples shown in Figures 1 to 15. Note that the figures in this application are schematic examples, and the magnification in each direction shown in these figures may differ, and the figures may not be consistent. Of course, the elements of this technology are not limited to the specific examples indicated by the reference numerals.

[Aspect 1]
As illustrated in Figures 1 and 2, a machine tool 1 according to one aspect of this technology comprises a rotary drive unit U1, a feed drive unit U2, and a control unit U3. The rotary drive unit U1 rotates a spindle 11 that grips a workpiece W1. The feed drive unit U2 moves at least one of the drive targets (e.g., tool TO1) of the tool TO1 that cuts the workpiece W1 and the spindle 11. As illustrated in Figure 3, the control unit U3 controls the feed movement of the drive target so that when cutting the workpiece W1, the tool TO1 is accompanied by vibrations including a forward movement M1 toward the workpiece W1 along the cutting direction (e.g., feed axis F1) and a backward movement M2 in the opposite direction to the forward movement M1. As illustrated in Figure 14, the control unit U3 determines whether a parameter (A) included in a command (e.g., a vibration feed command) that includes a parameter (A) relating to the period of the vibration is prohibited or permitted, based on information (NF) relating to the natural frequency. If the control unit U3 determines that parameter (A) is prohibited, it controls the vibration so that, as illustrated in Figure 10, the vibration includes a portion of the vibration with a period according to parameter (AB) which is permitted with a longer period than parameter (A), and a portion of the vibration with a period according to parameter (AC) which is permitted with a shorter period than parameter (A).

When parameter (A) related to the vibration period is prohibited, the cutting vibration will consist of a portion with a period according to parameter (AB), which is permitted with a longer period than parameter (A), and a portion with a period according to parameter (AC), which is permitted with a shorter period than parameter (A). This avoids resonance. Because the cutting vibration includes both longer and shorter periods than the commanded period, it is possible to suppress the increase in machining time compared to when the cutting vibration changes to a longer period throughout, and to suppress the decrease in machining time compared to when the cutting vibration changes to a shorter period throughout. Therefore, the above embodiment can provide a machine tool capable of performing vibration cutting while suppressing the impact on machining time and avoiding resonance.

Here, machine tools include lathes, machining centers, and so on.
The feed drive unit may move the tool along the cutting direction without moving the workpiece, or move the workpiece along the cutting direction without moving the tool, or move both the tool and the workpiece along the cutting direction.
The parameter (A) relating to the period of vibration can be any parameter related to the period, and is not limited to the period itself. Parameter (A) may include the chip length coefficient A1, the period A2, etc.
Information regarding natural frequencies (NF) may be stored in advance in the memory of the machine tool, or it may be retained by a detection unit provided in the machine tool detecting the natural frequencies, or it may be retained by receiving input of natural frequencies from the operator.
If the machine tool has a resonance avoidance mode and a resonance non-avoidance mode, the control unit may, when it determines that parameter (A) is prohibited in the resonance avoidance mode, control the vibration so that the vibration includes a portion with a period according to parameter (AB) and a portion with a period according to parameter (AC), and may always control the vibration according to parameter (A) in the resonance non-avoidance mode.
The above-mentioned supplementary statement also applies in the following embodiments.

[Aspect 2]
The command may include a constant spindle rotation speed (S) per unit time. If the control unit U3 determines that parameter (A) is prohibited, it may leave the constant spindle rotation speed (S) as is and include in the vibration a period according to parameter (AB) and a period according to parameter (AC) without changing the cycle time of the command.
In the above case, since the spindle speed (S) remains constant, the increase in load on the rotary drive unit U1 is suppressed. Therefore, the above configuration allows for vibration cutting that avoids resonance while suppressing an increase in load on the rotary drive unit and preventing changes in machining time.

[Appearance 3]
As illustrated in Figures 10 and 12, if the control unit U3 determines that parameter (A) is prohibited, it may control the vibration such that a portion with a period of one or more cycles according to parameter (AB) and a portion with a period of one or more cycles according to parameter (AC) are alternately repeated.
In the above case, during cutting vibration, the vibration is repeated with one or more cycles longer than the command period and one or more cycles shorter than the command period. Therefore, changes in the machining time of vibratory cutting are suppressed, and changes in the number of vibrations during cutting are suppressed. Accordingly, the above embodiment makes it possible to perform vibratory cutting while suppressing changes in machining time and number of vibrations and avoiding resonance.

[Aspect 4]
If the period of the vibration indicated by the parameter (AC) is greater than one rotation of the main shaft 11, the control unit U3 may control the difference in the rotation angle of the main shaft 11 between the first change point C1, in which the forward movement M1 changes to the backward movement M2, and the second change point C2, in which the backward movement M2 changes to the forward movement M1, in one period of the vibration, to 360°, as illustrated in Figure 10, when the period of the vibration indicated by the parameter (AC) is greater than one rotation of the main shaft 11.
As a result, the phases of the spindle 11 coincide at the first change point C1 and the second change point C2, and the chips are efficiently divided. Therefore, this embodiment can provide a suitable example for dividing chips when the vibration period is greater than one rotation of the spindle.

[Aspect 5]
The control unit U3 may, with OD being an odd number of 3 or more, and when the period of the vibration indicated by parameter (A) is 2/OD, control the difference in the rotation angle of the main shaft 11 between the first change point C1, where the forward movement M1 changes to the backward movement M2, and the second change point C2, where the backward movement M2 changes to the forward movement M1, in one period of the vibration, to a rotation angle corresponding to half the period of the vibration, as illustrated in Figure 7.
As a result, the phases of the spindle 11 at the first change point C1 and the second change point C2 are almost identical, and the chips are efficiently divided. Therefore, this embodiment can provide a suitable example for dividing chips when the vibration period is smaller than one rotation of the spindle.

(2) Specific examples of machine tool configurations:
Figure 1 schematically illustrates the configuration of a lathe as an example of a machine tool 1. The machine tool 1 shown in Figure 1 is an NC automatic lathe equipped with an NC (numerical control) device 70 that numerically controls the machining of a workpiece W1. Since a computer 100 is not an essential element in the machine tool 1, an external computer 100 may not be connected to the machine body 2. In this specific example, the NC device 70 is an example of a control unit U3. Note that the control unit U3 may be provided in the computer 100.

The machine tool 1 is an NC machine tool equipped with a headstock 10 into which a spindle 11 having a gripping part 12 is incorporated, a headstock drive unit 14, a tool post 20, a feed drive unit U2 for the tool post 20, an NC device 70, etc. Here, the headstock 10 is a general term for the front headstock 10A and the rear headstock 10B, also called the opposing headstock. The front headstock 10A incorporates a front spindle 11A having a gripping part 12A such as a collet. The rear headstock 10B incorporates a rear spindle 11B having a gripping part 12B such as a collet. The spindle 11 is a general term for the front spindle 11A and the rear spindle 11B, also called the opposing spindle. The gripping part 12 is a general term for the gripping part 12A and the gripping part 12B. The headstock drive unit 14 collectively refers to the front headstock drive unit 14A, which moves the front headstock 10A, and the rear headstock drive unit 14B, which moves the rear headstock 10B. The rotation drive unit U1 of the spindle 11 includes a motor 13A that rotates the front spindle 11A around the spindle centerline AX1, and a motor 13B that rotates the rear spindle 11B around the spindle centerline AX1. Built-in motors integrated into the spindle can be used for motors 13A and 13B. Of course, motors 13A and 13B may also be located outside the spindle 11.

The control axes of the machine tool 1 shown in Figure 1 include the X-axis (indicated as "X"), the Y-axis (indicated as "Y"), and the Z-axis (indicated as "Z"). The Z-axis direction is horizontal, along the spindle centerline AX1, which is the rotation center of the workpiece W1. The X-axis direction is horizontal, perpendicular to the Z-axis. The Y-axis direction is vertical, perpendicular to the Z-axis. Note that the Z-axis and X-axis do not need to be perpendicular as long as they intersect, the Z-axis and Y-axis do not need to be perpendicular as long as they intersect, and the X-axis and Y-axis do not need to be perpendicular as long as they intersect. Furthermore, the drawings referenced in this specification are merely examples to illustrate this technology and do not limit it. Also, the explanation of the positional relationships of each part is merely illustrative. Therefore, reversing left and right, or reversing the direction of rotation, etc., are also included in this technology. Furthermore, the identity of direction and position is not limited to strict agreement, but includes deviations from strict agreement due to errors.

The machine tool 1 shown in Figure 1 is a spindle-moving lathe. The front headstock drive unit 14A moves the front headstock 10A in the Z-axis direction, and the rear headstock drive unit 14B moves the rear headstock 10B in the Z-axis direction. Of course, the machine tool 1 could also be a spindle-fixed lathe where the front headstock 10A does not move, or the front headstock 10A could move in the Z-axis direction while the rear headstock 10B does not move.

The front spindle 11A grips the workpiece W1 with the gripping section 12A so as to be able to release it, and is rotatable together with the workpiece W1 around the spindle centerline AX1. If the workpiece W1 before machining is, for example, a long cylindrical (rod-shaped) material, the workpiece W1 may be supplied to the gripping section 12A from the rear end (left end in Figure 1) of the front spindle 11A. In this case, a guide bush may be arranged on the front side (right side in Figure 1) of the front spindle 11A to support the workpiece W1 so as to be able to slide in the Z-axis direction. If the workpiece W1 before machining is a short material, the workpiece W1 may be supplied to the gripping section 12A from the front end of the front spindle 11A. The motor 13A rotates the front spindle 11A together with the workpiece W1 around the spindle centerline AX1. After front machining, the workpiece W1 is transferred from the front spindle 11A to the rear spindle 11B. The rear spindle 11B, by its gripping portion 12B, releasably grips the workpiece W1 after front machining and is rotatable together with the workpiece W1 around the spindle centerline AX1. The motor 13B rotates the rear spindle 11B together with the workpiece W1 around the spindle centerline AX1. The workpiece W1, after front machining, becomes the final product through back machining.

The tool post 20 is mounted on multiple tools TO1 for machining the workpiece W1 and is movable in the X-axis and Y-axis directions. The X-axis and Y-axis directions are examples of feed axes F1. Of course, the tool post 20 may also move in the Z-axis direction. The tool post 20 may be a turret tool post, a comb-type tool post, etc. The multiple tools TO1 include cutting tools, rotary tools such as rotary drills and end mills, etc. The feed drive unit U2 moves the tool post 20, on which the multiple tools TO1 are mounted, along the feed axis F1. In this specific example, the target of the feed drive unit U2 is the tools TO1, and the feed drive unit U2 moves the tools TO1 along the feed axis F1.
Furthermore, the feed axis F1 may be a virtual axis that interpolates the X and Y axes. If the tool post 20 to which the tool TO1 is attached is also movable in the Z-axis direction, the feed axis F1 may be the Z-axis, or a virtual axis that interpolates the X, Y, and Z axes. Even if the tool post 20 does not move in the Z-axis direction, the headstock 10 to which the spindle 11 is incorporated moves in the Z-axis direction, so that a feed axis F1 that interpolates the three axes can be set with both the tool TO1 and the spindle 11 as the driving targets. In any case, the direction along the feed axis F1 is the cutting direction.

The external computer 100 connected to the NC device 70 includes a processor (CPU - Central Processing Unit) 101, semiconductor memory (ROM - Read Only Memory) 102, semiconductor memory (RAM - Random Access Memory) 103, storage device 104, input device 105, display device 106, audio output device 107, interface (I/F) 108, clock circuit 109, etc. The control program of the computer 100 is stored in the storage device 104, read into the RAM 103 by the CPU 101, and executed by the CPU 101. The storage device 104 can be semiconductor memory such as flash memory, magnetic recording media such as hard disk, etc. The input device 105 can be a pointing device, a keyboard, a touch panel attached to the surface of the display device 106, etc. The I/F 108 is connected to the NC device 70 by wired or wireless connection and receives data from the NC device 70 and transmits data to the NC device 70. The connection between computer 100 and machine tool 1 may be via a network connection such as the Internet or an intranet. Computer 100 includes personal computers, including tablet terminals, and mobile phones such as smartphones.

Figure 2 schematically illustrates the configuration of the electrical circuit of the machine tool 1. In the machine tool 1 shown in Figure 2, the NC device 70 is connected to an operating unit 80, a rotational drive unit U1 for the spindle 11, a headstock drive unit 14, a feed drive unit U2 for the tool post 20, etc. The rotational drive unit U1 is equipped with a motor 13A and a servo amplifier (not shown) to rotate the front spindle 11A, and a motor 13B and a servo amplifier (not shown) to rotate the rear spindle 11B. The headstock drive unit 14 includes a front headstock drive unit 14A and a rear headstock drive unit 14B. The feed drive unit U2 is equipped with servo amplifiers 31, 32 and servo motors 33, 34. The NC device 70 is equipped with a processor CPU 71, semiconductor memory ROM 72, semiconductor memory RAM 73, a clock circuit 74, an I/F 75, etc. Therefore, the NC device 70 is a type of computer. In Figure 2, the interfaces for the operating unit 80, rotary drive unit U1, headstock drive unit 14, feed drive unit U2, external computer 100, etc., are collectively shown as I/F 75.

ROM 72 contains the control program PR1 for interpreting and executing the machining program PR2, natural frequency information NF for the tool post 20, etc. Natural frequency information NF is an example of information regarding the natural frequency of the tool post 20, etc. In this specific example, natural frequency information NF is assumed to be a numerical value representing the natural frequency of the tool post 20, etc., and when simply written as "NF," NF is assumed to be the numerical value of the natural frequency itself. The natural frequency of the tool post 20, etc. is measured by a hammering test before the machine tool 1 is shipped and written to ROM 72. ROM 72 may also be a semiconductor memory with rewritable data.

RAM 73 stores the machining program PR2, created by the operator, in a rewritable format. The machining program is also called the NC program.
The CPU 71 uses the RAM 73 as a work area and executes the control program PR1 recorded in the ROM 72 to realize the functions of the NC device 70. Of course, some or all of the functions realized by the control program PR1 may be realized by other means such as an ASIC (Application Specific Integrated Circuit).

The operation unit 80 comprises an input unit 81 and a display unit 82, and functions as the user interface for the NC device 70. The input unit 81 consists of, for example, buttons or a touch panel for receiving operation input from the operator. The display unit 82 consists of, for example, a display that shows the contents of various settings received from the operator's operation input and various information related to the machine tool 1. The operator can store the machining program PR2 in the RAM 73 using the operation unit 80 or the computer 100.

The feed drive unit U2 includes a servo amplifier 31 connected to the NC device 70 and a servo motor 33 connected to the servo amplifier 31, for moving the tool post 20 along the X-axis. Furthermore, the feed drive unit U2 includes a servo amplifier 32 connected to the NC device 70 and a servo motor 34 connected to the servo amplifier 32, for moving the tool post 20 along the Y-axis.

Servo amplifier 31 controls the position and movement speed of the tool post 20 in the X-axis direction according to commands from the NC device 70. Servo amplifier 32 controls the position and movement speed of the tool post 20 in the Y-axis direction according to commands from the NC device 70. Servo motor 33, equipped with an encoder 35, rotates according to commands from servo amplifier 31 and moves the tool post 20 in the X-axis direction via a feed mechanism and guide (not shown). Servo motor 34, equipped with an encoder 36, rotates according to commands from servo amplifier 32 and moves the tool post 20 in the Y-axis direction via a feed mechanism and guide (not shown). A ball screw mechanism or the like can be used for the feed mechanism. A sliding guide, such as a combination of dovetail and dovetail grooves, can be used for the guide.

The NC device 70 issues position commands to servo amplifiers 31 and 32 during the feed movement of the tool post 20 to which the tool TO1 is attached. Servo amplifier 31 receives the X-axis position command from the NC device 70, inputs position feedback based on the output from the encoder 35 of the servo motor 33, corrects the position command based on the position feedback, and outputs a torque command to the servo motor 33. In this way, the NC device 70 controls the position of the tool post 20 during feed movement along the X-axis. It can also be said that the NC device 70 controls the position of the tool TO1 during feed movement along the X-axis. Furthermore, servo amplifier 32 receives the Y-axis position command from the NC device 70, inputs position feedback based on the output from the encoder 36 of the servo motor 34, corrects the position command based on the position feedback, and outputs a torque command to the servo motor 34. In this way, the NC device 70 controls the position of the tool post 20 during feed movement along the Y-axis. It can also be said that the NC device 70 controls the position of the tool TO1 during feed movement along the Y-axis.

Although not shown in the diagram, the headstock drive unit 14 also includes a servo amplifier and a servo motor. The front headstock drive unit 14A moves the front headstock 10A in the Z-axis direction via a feed mechanism and guide (not shown), and the rear headstock drive unit 14B moves the rear headstock 10B in the Z-axis direction via a feed mechanism and guide (not shown).

When the tool TO1 mounted on the tool post 20 cuts the workpiece W1, chips, also known as metal chips, are generated. If the feed drive unit U2 causes the tool TO1 to cut along the feed axis F1 without vibration, relative to the workpiece W1 rotating around the spindle centerline AX1, continuous, long chips are generated. These continuous, long chips can potentially affect the machining of the workpiece W1 by wrapping around the tool TO1. Therefore, as illustrated in Figure 3, the chips are broken up by vibratory cutting, where the tool TO1 is repeatedly moved forward and backward along the feed axis F1 during cutting of the workpiece W1. First, an example of vibratory cutting in the absence of resonance will be explained.

Figure 3 schematically illustrates the tool position with respect to the spindle rotation angle when the chip length coefficient A1, which is an example of a parameter (A) related to the vibration period, is 2. The spindle rotation speed S per unit time is assumed to be constant. The chip length coefficient A1 means the number of rotations of the spindle 11 (front spindle 11A or back spindle 11B) required for one vibration period, and can also be said to be the number of spindle rotations required for one tool miss. Tool miss means that cutting of the workpiece W1 does not occur due to the vibration of the tool TO1. Hereafter, tool miss will be simply referred to as miss. The spindle rotation angle is the rotation angle of the spindle 11, with the rotation angle when the tool TO1 is at its current position P1 being 0°. The tool position is the control position of the tool TO1 in the cutting direction (feed axis F1), with the position when it is at its current position P1 being 0. The dashed line from the current position P1 to the endpoint P2 shows the tool position 201 during normal cutting, not vibration cutting. The solid broken line from the current position P1 to the endpoint P2 indicates the tool position 202 during vibratory cutting. The lower part of Figure 3 shows an enlarged view of the tool position relative to the spindle rotation angle for one vibration period.
The tool position shown in Figure 3 is the position controlled by the NC device 70; therefore, the actual tool position will deviate from the shown position due to delays in the servo system's response, etc. The same applies to the tool positions shown in Figures 4 to 8. Note that the specific numerical values shown in Figure 3, etc., are merely examples.

The vibration shown in Figure 3 represents the alternating forward movement M1 of the tool TO1 toward the workpiece W1 along the cutting direction, and the reverse movement M2 in the opposite direction to the forward movement M1. The NC device 70 controls the feed movement of the tool TO1 to include vibrations, including both the forward and reverse movements M1 and M2, during the cutting of the workpiece W1. The polyline of the tool position relative to the spindle rotation angle includes a first transition point C1 where the movement changes from forward M1 to reverse M2, and a second transition point C2 where the movement changes from reverse M2 to forward M1. In the example shown in Figure 3, the waveform of the tool position relative to the spindle rotation angle is shown as a waveform with a triangular wave vibration superimposed on the normal cutting feed. The spindle rotation angle from the current position P1 to the end point P2 corresponds to the cycle time of the vibration feed command.

In Figure 3, the normal cutting feed rate F is the feed rate of tool TO1 when performing normal cutting that is not vibratory cutting, and is the feed rate of tool TO1 when it is not vibrating. The unit of the normal cutting feed rate F is, for example, mm/rev, which represents millimeters per revolution of the spindle. The chip length coefficient A1 is the number of rotations of the spindle 11 required for one period of vibration of tool TO1, and can be said to be the vibration period expressed in terms of the rotations of the spindle 11. The unit of the chip length coefficient A1 is, for example, rev. The chip length coefficient A1 is a positive value that is at least excluding 1 rev. The advance amount D is the distance over which the position of tool TO1 changes per period of vibration, and indicates the relative endpoint position (position of the first change point C1) of each advance movement M1. The unit of the advance amount D is, for example, mm. The retraction amount E1 is the distance of the retraction movement M2 of tool TO1 during one period of vibration, and indicates the relative endpoint position (position of the second change point C2) of each retraction movement M2. The unit of the retraction amount E1 is, for example, mm. The distance tool TO1 moves during forward movement during one period of vibration is D + E1. In this specific example, if A1 > 1, during one period of vibration, tool TO1 is first controlled to move forward by distance (D + E1) / 2 (M1), then controlled to move backward by retraction amount E1 (M2), and finally controlled to move forward by distance (D + E1) / 2 (M1).

In this specific example, the settings for "normal cutting feed rate F," "chip length coefficient A1," and "retraction amount E1" eliminate the need for trial-and-error adjustments of parameters such as the feed rate Fd for forward movement and the feed rate B for retraction movement.

Figure 3 shows an example of setting the first change point C1 and the second change point C2 in one period of vibration when A1 > 1, specifically when A1 = 2. Figure 4 schematically illustrates the tool position relative to the spindle phase when A1 = 2. For clarity, the tool position on even-numbered rotations of the spindle is shown with a dashed line in Figure 4.
To reduce the load on mechanisms such as the feed mechanism and guides, it is preferable to minimize the speed Fd,B, forward movement D, and backward movement E1 as much as possible. Free-running is most efficient when the peak (first change point C1) and trough (second change point C2) of the movement path of the tool TO1 coincide in the phase of the spindle 11. To make the peak and trough coincide, for example, the peak can be set at a spindle rotation angle of -180° from the midpoint (A1/2) spindle rotation angle in one period of vibration, and the trough can be set at a spindle rotation angle of +180°. If A1 = 2, setting the peak at a spindle rotation angle of (2/2) × 360 - 180 = 180° and the trough at a spindle rotation angle of (2/2) × 360 + 180 = 540° will result in the spindle phases of the peak and trough coinciding, as shown in Figure 4. The difference in spindle rotation angles between the peaks and valleys is 360°, and the retraction amount E1 is greater than 0. Therefore, the valley on the next even-numbered rotation (second change point C2) is slightly further back than the peak on the odd-numbered rotation (first change point C1). This causes the chips to break up. Furthermore, since the change in tool position during forward movement is constant, the chips are broken up efficiently.

Figure 5 schematically illustrates the tool position relative to the spindle rotation angle when A1 = 3. When A1 = 3, if the peak (first change point C1) is set at a spindle rotation angle of (3/2) × 360 - 180 = 360° and the trough (second change point C2) is set at a spindle rotation angle of (3/2) × 360 + 180 = 720°, the spindle phases of the peak and trough coincide. This allows for efficient chip breaking.
The "chip length coefficient A1" is not limited to an integer if it is greater than 1. The same can be done when A1 > 3, 2 < A1 < 3, or 1 < A1 < 2, allowing for the setting of peaks and valleys. However, when 1 < A1 < 2, the feed rate Fd for forward movement may become excessive, so it is preferable that A1 be 2 or greater. Furthermore, it is preferable that A1 be 2 or greater, as the chip length coefficient A1 may be varied to suppress resonance.

Although not shown in the diagram, it is also possible to set the trough (second change point C2) at a spindle rotation angle of -180° from the midpoint (A1/2) of the vibration cycle, and the peak (first change point C1) at a spindle rotation angle of +180°.
Based on the above, when A1 > 1, the NC device 70 controls the difference in spindle rotation angle between the first change point C1, where the movement changes from forward motion M1 to backward motion M2 in one cycle of vibration, and the second change point C2, where the movement changes from backward motion M2 to forward motion M1 in one cycle of vibration, to 360°.

Furthermore, if A1 > 2, it is possible to set a trough or peak at a spindle rotation angle of -360° and a peak or trough at a spindle rotation angle of +360° from the midpoint (A1/2) spindle rotation angle in one cycle of vibration. If A1 > 3, it is also possible to set a trough or peak at a spindle rotation angle of -540° and a peak or trough at a spindle rotation angle of +540° from the midpoint (A1/2) spindle rotation angle in one cycle of vibration. To reduce the number of spindle rotations required for chip breaking and to break the chips into finer pieces, it is most efficient to set a peak or trough at a spindle rotation angle of -180° and a trough or trough at a spindle rotation angle of +180° from the midpoint (A1/2) spindle rotation angle in one cycle of vibration.

In order for the NC device 70 to move the tool TO1 in the cutting direction (feed axis F1) without changing the feed rate from the commanded cutting speed, it is sufficient to control the total amount of tool TO1 movement per spindle revolution so that it is the same as the normal cutting feed rate F, which is the amount of movement during normal cutting. Since the chip length coefficient A1 is the number of spindle rotations 11 required for one period of vibration of the tool TO1, the amount of tool TO1 movement along the cutting direction per period of vibration is A1 × F. As shown in Figures 3 and 5, the tool TO1 is controlled in the following order during one period of vibration: forward movement M1 of a distance (D + E1) / 2, backward movement M2 of a distance E1, and forward movement M1 of a distance (D + E1) / 2. Therefore,
A1×F={(D+E1)/2}×2-E1
The above equation holds true. From the above equation, the amount of forward movement D is expressed by the following equation.
D=A1×F...(1)

The feed rate Fd of the forward movement of tool TO1 is expressed by the following formula.
Fd={(D+E1)/2}/{(A1-1)/2}
=(D+E1)/(A1-1)
=(A1×F+E1)/(A1-1)…(2)
The feed rate B of the tool TO1 during its retraction is expressed by the following formula:
B = E1/1
= E1 … (3)

Based on the above, when A1 > 1, the NC device 70 receives input of "normal cutting feed rate F", "chip length coefficient A1", and "retraction amount E1" for the cutting direction (feed axis F1), and can determine the forward amount D and speeds Fd, B according to the above equations (1), (2), and (3). Once the forward amount D and speeds Fd, B are determined, the NC device 70 can control the position of the tool TO1 during feed movement based on the forward amount D and speeds Fd, B for the cutting direction. Therefore, a vibration feed command specifying "normal cutting feed rate F", "chip length coefficient A1", and "retraction amount E1" can be considered as a command in the machining program PR2. Here, we assume that this vibration feed command has the format "G*** X(U)_Y(V)_Z(W)_A**_F**_E**". The "***" after G indicates the vibration feed command number, "X(U)_Y(V)_Z(W)" indicates the position of the endpoint P2, the "**" after A indicates the value of the chip length coefficient A1, the "**" after F indicates the value of the normal cutting feed rate F, and the "**" after E indicates the value of the retraction amount E1.

Figure 6 schematically illustrates how the NC device 70 controls the position of the tool TO1 during feed movement based on the forward movement amount D and speeds Fd and B obtained from the vibration feed command.
The NC device 70 sets multiple positions P3 in the cutting direction (feed axis F1) from the current position P1 to the endpoint P2 by repeatedly performing forward movement M1 and backward movement M2, based on the amount of forward movement D and speeds Fd and B, and sequentially issues position commands to servo amplifier 31 or servo amplifier 32 to move the tool TO1 to position P3. In Figure 6, each position P3 is shown as a white circle. The set positions P3 are not limited to change points (first change point C1 and second change point C2) or the endpoint P2, but may also include positions in between the forward movement M1 and the backward movement M2. As the aforementioned position commands are repeated, the position of the tool TO1 during feed movement is controlled to a position based on the amount of forward movement D and speeds Fd and B.

Therefore, by specifying only the "normal cutting feed rate F," "chip length coefficient A1," and "retraction amount E1" in the machining program PR2, the operator can perform vibration cutting in the same machining time as normal cutting. Here, when the "chip length coefficient A1" increases, the chip becomes longer while the amplitude decreases. The optimal values for the "chip length coefficient A1" and "retraction amount E1" depend on the tracking ability of the servo system that moves the tool TO1, and are determined by the spindle rotation speed per unit time and the feed rate of the tool TO1. Therefore, as illustrated in Figure 9, by preparing guideline values for combinations of "chip length coefficient A1" and "retraction amount E1" corresponding to the "spindle rotation speed per unit time S" and "normal cutting feed rate F" as an information table TA1, the operator can easily specify the "chip length coefficient A1" and "retraction amount E1." As shown in Figure 9, the information table TA1 associates multiple combinations of A1 and E1 with each combination of S and F. If we denote the identification number for the combination of A1 and E1 as j, Figure 9 shows that, for example, for the combination S=S1 and F=F1, multiple combinations represented by A1=a1j and E1=e1j are associated. The information table TA1 shown in Figure 9 can also be described as an information table for outputting multiple recommended combinations of "chip length coefficient A1" and "recess amount E1" in response to inputs of "spindle rotation speed per unit time S" and "normal cutting feed rate F". Of course, the number of combinations of A1 and E1 is finite.

Therefore, once the "spindle speed per unit time S" and the "normal cutting feed rate F" are determined, it becomes possible to select a combination of the "chip length coefficient A1" and the "retraction amount E1" from the information table TA1. Furthermore, by pre-storing the information table TA1 in the RAM 73 of the NC device 70, the NC device 70 can suggest recommended values for the "chip length coefficient A1" and "retraction amount E1" based on the "spindle speed per unit time S" and the "normal cutting feed rate F".

Figure 7 schematically illustrates the tool position with respect to the spindle rotation angle when the chip length coefficient A1, i.e., when the chip length coefficient A1 is 2/3. In this specific example, when 0 < A1 < 1, in one period of vibration, the tool TO1 is controlled to move forward by distance (D + E1) M1 in the first half and to move backward by retraction amount E1 M2 in the second half. Figure 8 schematically illustrates the tool position with respect to the spindle phase when A1 = 2/3.
When 0 < A1 < 1, in order to efficiently achieve a misfire, the "chip length coefficient A1" is restricted to A1 = 2/3, 2/5, 2/7, ... such that the denominator is an odd number of 3 or more and the numerator is 2. In order to make the peak (first change point C1) and trough (second change point C2) in the phase of the spindle 11 coincide, for example, the peak can be set at the midpoint (A1/2) spindle rotation angle in one period of vibration, and the trough can be set at the end (A1) spindle rotation angle in one period of vibration. When A1 = 2/3, if the peak is set at a spindle rotation angle of (2/3)/2 × 360 = 120° and the trough is set at a spindle rotation angle of (2/3) × 360 = 240°, the spindle phases of the peak and trough will coincide as shown in Figure 8. The spindle phases at which the peak and trough coincide are 120°, 240°, and 360°.

If A1 = 2/5, then setting the spindle rotation angle to (2/5)/2 × 360 = 72° for the peak and (2/5) × 360 = 144° for the trough will result in the spindle phases of the peaks and troughs coinciding. The spindle phases where the peaks and troughs coincide are 72°, 144°, 216°, 288°, and 360°.
The "chip length coefficient A1" may be 2/7 or less. However, if A1 < 2/3, the feed rate of the tool TO1 and the rotational speed of the spindle 11 per unit time may have to be considerably lower in terms of the servo system's ability to follow the control, so A1 is preferably 2/3.

Although not shown in the diagram, it is also possible to set the trough at the midpoint (A1/2) of the spindle rotation angle in one period of vibration and the peak at the final (A1) point of the spindle rotation angle in one period of vibration.
Based on the above, the NC device 70 controls the difference in spindle rotation angle between the first change point C1, where the movement changes from forward motion M1 to backward motion M2, and the second change point C2, where the movement changes from backward motion M2 to forward motion M1, to {(A1/2) × 360}° when the denominator of the "chip length coefficient A1" is an odd number of 3 or more and the numerator of the "chip length coefficient A1" is 2 in one period of vibration.

In order for the NC device 70 to move the tool TO1 in the cutting direction (feed axis F1) without changing the feed rate from the commanded cutting speed, it is sufficient to control the total amount of tool TO1 movement per spindle revolution to be the same as the normal cutting feed rate F, which is the amount of movement during normal cutting. As described above, the amount of tool TO1 movement along the cutting direction per cycle of vibration is A1 × F. As shown in Figures 7 and 8, the tool TO1 is controlled to move forward by distance (D + E1) M1 and backward by retraction by amount E1 M2 in order during one cycle of vibration. Therefore,
A1×F=(D+E1)-E1
The above equation holds true. From the above equation, the amount of forward movement D is expressed by the following equation.
D=A1×F…(4)

The feed rate Fd of the forward movement of tool TO1 is expressed by the following formula.
Fd=(D+E1)/(A1/2)
=2(D+E1)/A1
=2(A1×F+E1)/A1…(5)
The feed rate B of the tool TO1 during its retraction is expressed by the following formula:
B=E1/(A1/2)
=2E1/A1...(6)

Based on the above, when A1 < 1, the NC device 70 receives inputs of "normal cutting feed rate F," "chip length coefficient A1," and "retraction amount E1" for the cutting direction (feed axis F1), and can determine the forward amount D and speeds Fd and B according to the above equations (4), (5), and (6). Once the forward amount D and speeds Fd and B are determined, the NC device 70 can control the position of the tool TO1 during feed movement in the cutting direction based on the forward amount D and speeds Fd and B. As shown in Figure 6, the NC device 70 sets multiple positions P3 from the current position P1 to the endpoint P2 by repeatedly performing forward movement M1 and retraction movement M2 in the cutting direction, based on the forward amount D and speeds Fd and B, and sequentially issues position commands to the servo amplifier 31 or servo amplifier 32 to move the tool TO1 to position P3. By repeating the above position commands, the position of the tool TO1 during feed movement is controlled to the position based on the forward amount D and speeds Fd and B.

Incidentally, when the frequency of the vibratory cutting (let's call it f) is close to the natural frequency of the tool post, a large vibration called resonance occurs. As a result, even though the machine is set to produce a lot of misfires, the chips may not be broken up.
The frequency f of vibratory cutting is calculated using the following formula, with respect to the spindle rotation speed S (rev/min) per unit time and the chip length coefficient A1 (rev).
f=S/A1/60 (Hz)...(7)

If the frequency f of the vibratory cutting is near the natural frequency of the tool post, for example, multiplying the spindle rotation speed S per unit time by a certain constant will cause the frequency f of the vibratory cutting to deviate from the vicinity of the natural frequency. However, lowering the commanded "spindle rotation speed S per unit time" increases the machining time. Increasing the commanded "spindle rotation speed S per unit time" shortens the machining time, but increases the load on the tool TO1 and the rotary drive unit U1. Of course, raising and lowering the spindle rotation speed S per unit time during vibratory cutting places a particularly large load on the rotary drive unit U1.
Multiplying the chip length coefficient A1 by a constant value can shift the frequency f of the vibratory cutting away from the natural frequency. However, increasing the chip length coefficient A1 lengthens the vibration period, slowing down the forward movement M1 and resulting in longer machining time. Decreasing the chip length coefficient A1 shortens the machining time, but increases the load on the tool TO1 and the rotary drive unit U1.

Therefore, in this specific example, when the vibration frequency corresponding to the commanded chip length coefficient A1 is near the natural frequency of the tool post, the vibration is designed to include a period according to a chip length coefficient AB that is larger than A1, and a period according to a chip length coefficient AC that is smaller than A1. This allows for vibration cutting that avoids resonance while suppressing the impact on machining time. Furthermore, since the NC device 70 executing the control program PR1 automatically predicts the occurrence of resonance and switches the chip length coefficient from A1 to AB and AC, the operator does not need to create a special machining program for vibration cutting that avoids resonance.

Figure 10 schematically illustrates the tool position relative to the spindle rotation angle when the vibration frequency corresponding to the commanded chip length coefficient A1 is near the natural frequency of the tool post, etc. The spindle rotation angle from the current position P1 to the end point P2 corresponds to the cycle time of the vibration feed command.
First, the NC device 70 determines, based on the natural frequency information NF, whether the chip length coefficient A1 included in the vibration feed command is prohibited or permitted. If the frequency corresponding to the chip length coefficient A1 is near NF, the NC device 70 determines that the chip length coefficient A1 is prohibited. In this case, the NC device 70 controls the cutting vibration so that the cutting vibration includes a portion of the period according to the chip length coefficient AB, which is permitted with a longer period than the original chip length coefficient A1, and a portion of the period according to the chip length coefficient AC, which is permitted with a shorter period than the original chip length coefficient A1.

As described above, the vibration frequency f corresponding to the chip length coefficient A1 is expressed as f = S/A1/60, where S is the spindle rotation speed per unit time. To determine whether the vibration frequency corresponding to the chip length coefficient A1 is near NF, for example, we can determine whether |f - NF| ≤ Tf, where Tf is a positive threshold for the absolute value of the difference between f and NF. The chip length coefficients AB and AC can be set as AB = A1 + a and AC = A1 - a, for example, within the range where |(S/AB/60) - NF| > Tf and |(S/AC/60) - NF| > Tf, with a positive correction value a. As a result, the chip length coefficient AB = A1 + a is permitted for a longer period than the chip length coefficient A1, and the chip length coefficient AC = A1 - a is permitted for a shorter period than the chip length coefficient A1. Figure 10 shows an example where the chip length coefficient AB = A1 + a and the chip length coefficient AC = A1 - a for one period are alternately repeated. The vibration of the tool TO1 moving from the current position P1 to the endpoint P2 alternately includes a period according to the chip length coefficient AB for one period and a period according to the chip length coefficient AC for one period. The correction value a can be in a range greater than S/2Tf/60, which corresponds to 2 × Tf, for example, A1/10 or less. For example, if A1 = 2 and a = A1/20 = 0.1, the chip length coefficient AB = 2.1 and the chip length coefficient AC = 1.9 for one period are alternately repeated. If A1 = 2 and a = A1/40 = 0.05, the chip length coefficient AB = 2.05 and the chip length coefficient AC = 1.95 for one period are alternately repeated.
As a result of the alternating vibrations according to the chip length coefficients AB and AC, the cycle time shown in Figure 10 remains the same as the cycle time shown in Figure 3, and the number of cutting vibrations also remains unchanged.

When AC > 1, for vibrations according to chip length coefficients AB and AC, the peaks (first change point C1) and troughs (second change point C2) of the movement path can be set in the same way as for vibrations according to chip length coefficient A1. For example, for vibrations according to chip length coefficient AB, the peak can be set at a spindle rotation angle of -180° from the midpoint (AB/2) spindle rotation angle in one period of vibration, and the trough can be set at a spindle rotation angle of +180°. The difference in spindle rotation angles between the peak and the trough is 360°. The lower part of Figure 10 shows an example of setting the peaks and troughs when A1 = 2 and a = 0.1. In this case, the midpoint (AB/2) spindle rotation angle is (2.1/2) × 360 = 378°, the spindle rotation angle at the peak is 378 - 180 = 198°, and the spindle rotation angle at the trough is 378 + 180 = 558°. The spindle phase corresponding to the spindle rotation angle at the valley is 558 - 360 = 198°. For vibrations according to the chip length coefficient AC, the peak can be set at a spindle rotation angle of -180° from the midpoint (AC/2) spindle rotation angle in one period of vibration, and the valley at a spindle rotation angle of +180°. The difference between the spindle rotation angles at the peak and the valley is 360°. When A1 = 2 and a = 0.1, the midpoint (AC/2) spindle rotation angle is (1.9/2) × 360 = 342°, the spindle rotation angle at the peak is 342 - 180 = 162°, and the spindle rotation angle at the valley is 342 + 180 = 522°. Starting from a spindle rotation angle of 756° for one period when AB = 2.1, the spindle rotation angle at the peak during a period when AC = 1.9 is 756 + 162 = 918°, and the spindle phase corresponding to this spindle rotation angle is 198°.

Here, the advance amount D according to the chip length coefficients AB and AC is expressed by the following formula.
D=AB×F…(8)
D=AC×F…(9)
The feed rate Fd for forward motion, according to the chip length coefficients AB and AC, is expressed by the following formula.
Fd=(AB×F+E1)/(AB-1)…(10)
Fd=(AC×F+E1)/(AC-1)…(11)
The feed rate B for the retraction motion, according to the chip length coefficients AB and AC, is expressed by the following formulas.
B = E1 …(12)

Figure 11 schematically illustrates the tool position relative to the spindle phase when AB = 2.1 and AC = 1.9. For clarity, the tool position on even-numbered rotations of the spindle is shown with a dashed line in Figure 11.
When AC > 1, as shown in Figure 11, the peaks (first change point C1) and troughs (second change point C2) of the tool TO1's movement path coincide in the phase of the spindle 11. When AB = 2.1 and AC = 1.9, the spindle phases of both the peaks and troughs are 198°. This causes the chip to break apart. Since the retraction amount E1 is constant for each period, it can be said that the chip length coefficient can be changed without changing the conditions for chip breaking. Furthermore, since the advance amount D fluctuates while the retraction amount E1 remains constant for each period, the period of cutting vibration fluctuates, and resonance is suppressed.

Therefore, when the chip length coefficient A1 is prohibited, the NC device 70 maintains a constant spindle rotation speed S per unit time and includes the portion of the cutting vibration according to the chip length coefficient AB and the portion according to the chip length coefficient AC, without changing the cycle time of the vibration feed command.

As illustrated in Figure 12, various patterns are possible for the variation in the chip length coefficient depending on the vibration period. The chip length coefficient for each vibration cycle is set to be permitted based on the natural frequency information NF.
Pattern PA1 means that the vibration starts with a chip length coefficient AB = A1 + a, and alternates between one period of vibration according to the chip length coefficient AB = A1 + a and one period of vibration according to the chip length coefficient AC = A1 - a. In this case, the NC device 70 controls the cutting vibration so that the period of one cycle according to the chip length coefficient AB = A1 + a and the period of one cycle according to the chip length coefficient AC = A1 - a alternate. Alternatively, instead of pattern PA1, a variation pattern may be used that starts with vibration according to the chip length coefficient AC = A1 - a and alternately repeats the chip length coefficients AC and AB.

Pattern PA2 means that two periods of vibration according to the chip length coefficient AB = A1 + a and two periods of vibration according to the chip length coefficient AC = A1 - a are alternately repeated. In this case, the NC device 70 controls the cutting vibration so that the portion with two periods according to the chip length coefficient AB = A1 + a and the portion with two periods according to the chip length coefficient AC = A1 - a are alternately repeated. Pattern PA2 is preferable for machine tools 1 in which pattern PA2 has a greater effect in suppressing resonance than pattern PA1.
Alternatively, instead of pattern PA2, a variation pattern may be adopted in which the vibration starts with a vibration according to the chip length coefficient AC = A1 - a and alternately repeats the chip length coefficients AC and AB twice each. Furthermore, a variation pattern may be adopted in which periods H1 and H2 are both numbers greater than 1, and the vibration H1 period according to the chip length coefficient AB = A1 + a and the vibration H2 period according to the chip length coefficient AC = A1 - a alternately repeat. In this case, the NC device 70 controls the cutting vibration so that the period of H1 according to the chip length coefficient AB = A1 + a and the period of H2 according to the chip length coefficient AC = A1 - a alternately repeat. While it is preferable that period H2 is the same as period H1, it may be different. Furthermore, while it is preferable that periods H1 and H2 be integers such as 2, 3, 4, ..., they may also be real numbers that are not integers.

Pattern PA3 means repeating four vibration periods, each containing, in order, one vibration period according to the chip length coefficient AB = A1 + a, one vibration period according to the chip length coefficient AC = A1 - 0.5a, one vibration period according to the chip length coefficient AB = A1 + a, and one vibration period according to the chip length coefficient AC = A1 - 1.5a. In this case, the NC device 70 controls the cutting vibration so that the period containing, in order, a portion of one period according to the chip length coefficient AB = A1 + a, a portion of one period according to the chip length coefficient AC = A1 - 0.5a, a portion of one period according to the chip length coefficient AB = A1 + a, and a portion of one period according to the chip length coefficient AC = A1 - 1.5a is repeated. In this way, the NC device 70 may further vary the chip length coefficient AC. In this case as well, the NC device 70 controls the vibration so that a period of one cycle according to the chip length coefficient AB and a period of one cycle according to the chip length coefficient AC are repeated alternately. For machine tools 1 in which pattern PA3 has a greater effect in suppressing resonance than patterns PA1 and PA2, it is preferable to adopt pattern PA3.
Furthermore, the NC device 70 may vary the chip length coefficient AB instead of AC, or it may vary both the chip length coefficients AB and AC.

In accordance with the variation patterns PA1 to PA3 described above, if the NC device 70 determines that the chip length coefficient A1 is prohibited, it controls the cutting vibration so that a portion with a period of one or more cycles according to the chip length coefficient AB and a portion with a period of one or more cycles according to the chip length coefficient AC alternate.
Furthermore, if the NC device 70 fails to achieve sufficient idling by varying the chip length coefficient, it may improve the reliability of idling by increasing the retraction amount E1 from the start.

If the chip length coefficient A1 is less than 1, then A1 = 2/OD, where OD is an odd number greater than or equal to 3. In this case, the chip length coefficients AB and AC are set to values that deviate from 2/OD, and for example, within the range where |(S/AB/60) - NF| > Tf and |(S/AC/60) - NF| > Tf, a positive correction value a can be set to AB = A1 + a and AC = A1 - a. As a result, the chip length coefficient AB = A1 + a becomes a chip length coefficient permitted for a longer period than the chip length coefficient A1, and the chip length coefficient AC = A1 - a becomes a chip length coefficient permitted for a shorter period than the chip length coefficient A1. For vibrations according to the chip length coefficients AB and AC when AC < 1, for example, the peaks (first change point C1) and troughs (second change point C2) of the movement path can be set in the same way as for vibrations according to the chip length coefficient A1. For example, for vibrations following a chip length coefficient AB, the peaks should be set at a spindle rotation angle of (AB/2) × 360°, and the troughs at a spindle rotation angle of AB × 360°. The difference in spindle rotation angles between the peaks and troughs is a rotation angle equivalent to half the vibration period (AB/2) × 360°. For vibrations following a chip length coefficient AC, the peaks should be set at a spindle rotation angle of (AC/2) × 360°, and the troughs at a spindle rotation angle of AC × 360°. The difference in spindle rotation angles between the peaks and troughs is a rotation angle equivalent to half the vibration period (AC/2) × 360°.

Here, the advance amount D according to the chip length coefficients AB and AC is expressed by the following formula.
D=AB×F…(13)
D=AC×F…(14)
The feed rate Fd for forward motion, according to the chip length coefficients AB and AC, is expressed by the following formula.
Fd=2(AB×F+E1)/AB…(15)
Fd=2(AC×F+E1)/AC…(16)
The feed rate B for the retraction motion, according to the chip length coefficients AB and AC, is expressed by the following formula.
B=2E1/AB...(17)
B=2E1/AC...(18)

Figure 13 schematically illustrates the tool position relative to the spindle phase when A1 = 2/3. In Figure 13, the tool position according to the original chip length coefficient A1 is shown by a dashed line. Cutting vibrations begin with vibrations according to the chip length coefficient AC = A1 - a, and one period of vibration according to the chip length coefficient AC = A1 - a and one period of vibration according to the chip length coefficient AB = A1 + a alternate.
When the correction amount a is 0.021, which corresponds to 5°, the spindle rotation angle of the valley in the period of chip length coefficient AC is 235°, and the spindle rotation angle of the peak in the period of chip length coefficient AC is 235/2 = 117.5°. In the period of chip length coefficient AB, the spindle rotation angle of the valley is 245°, and the spindle rotation angle of the peak in the period of chip length coefficient AC is 245/2 = 122.5°. Starting from the spindle rotation angle of 235° for one period of chip length coefficient AC, the spindle rotation angle of the valley in the period of chip length coefficient AB is 235 + 245 = 480°, and the spindle phase corresponding to this spindle rotation angle is 120°. Therefore, the spindle phase of 120° in the period of chip length coefficient AB is different from the spindle phase of 117.5° in the period of chip length coefficient AC. However, when using the original chip length coefficient A1, if the retraction amount E1 is set to ensure sufficient free spin, free spin will be achieved even if the spindle phases of the crests and valleys are slightly misaligned. Of course, if the free spin becomes insufficient due to the variation in the chip length coefficient, the NC device 70 may improve the reliability of free spin by increasing the retraction amount E1 from the beginning.

Even when A1 < 1, the NC device 70, following various fluctuation patterns as shown in Figure 12, determines that the chip length coefficient A1 is prohibited and controls the cutting vibration so that a portion with a period of one or more cycles according to the chip length coefficient AB and a portion with a period of one or more cycles according to the chip length coefficient AC alternate. In this process, the NC device 70 maintains a constant spindle rotation speed S per unit time and includes the portions with periods according to the chip length coefficient AB and the chip length coefficient AC in the cutting vibration without changing the cycle time of the vibration feed command.

Figure 14 schematically illustrates the vibration control process that executes the vibration feed command. The vibration control process is performed by the NC device 70. Note that if the chip length coefficient A1 is greater than 1, the chip length coefficient AC is set to be greater than 1.
When the NC device 70 reads a vibration feed command from the machining program PR2 with a constant spindle rotation speed S per unit time set, it starts vibration control processing. The "spindle rotation speed S per unit time" included in the spindle rotation speed setting command that was read last before the vibration feed command becomes the spindle rotation speed S set in the vibration feed command. The vibration feed command has "normal cutting feed rate F", "chip length coefficient A1", and "retraction amount E1" as specified parameters.

First, the NC device 70 determines whether the chip length coefficient A1 included in the vibration feed command is prohibited or permitted based on the natural frequency information NF (step S102). The natural frequency information NF is read from the ROM 72. As described above, the vibration frequency f corresponding to the chip length coefficient A1 is S/A1/60. For example, if Tf is a positive threshold for the absolute value of the difference between f and NF, the NC device 70 only needs to determine whether |f - NF| ≤ Tf.

If |f - NF| > Tf, the chip length coefficient A1 is permitted, and the NC device 70 proceeds to step S104. If A1 > 1, the NC device 70 calculates the forward movement D according to equation (1) above, calculates the feed rate Fd for the forward movement of tool TO1 according to equation (2) above, and determines the feed rate B for the reverse movement of tool TO1 according to equation (3) above. If A1 < 1, the NC device 70 calculates the forward movement D according to equation (4) above, calculates the feed rate Fd for the forward movement of tool TO1 according to equation (5) above, and calculates the feed rate B for the reverse movement of tool TO1 according to equation (6) above. Then, the NC device 70 proceeds to step S110 and controls the position of tool TO1 during feed movement based on the forward movement D and speeds Fd and B, as shown in Figure 6.

If |f - NF| ≤ Tf, the chip length coefficient A1 is prohibited, and the NC device 70 proceeds to step S106. If A1 > 1, the NC device 70 sets the chip length coefficient (AB or AC) for each vibration according to the variation pattern shown in Figure 12. For example, when pattern PA1 is used, the NC device 70 sets the chip length coefficient AB for odd-numbered period vibrations to A1 + a, and the chip length coefficient AC for even-numbered period vibrations to A1 - a.
Next, the NC device 70 calculates the parameters (D, Fd, B) for each vibration (step S108).

If A1 > 1, i.e., AC > 1, the NC device 70 calculates the forward movement D of each vibration according to equations (8) and (9) above, calculates the feed rate Fd for the forward movement of each vibration according to equations (10) and (11) above, and determines the feed rate B for the backward movement of each vibration according to equation (12) above. If A1 < 1, i.e., AB < 1, the NC device 70 calculates the forward movement D according to equations (13) and (14) above, calculates the feed rate Fd for the forward movement of each vibration according to equations (15) and (16) above, and calculates the feed rate B for the backward movement of each vibration according to equations (17) and (18) above. Then, the NC device 70 proceeds to step S110 and controls the position of the tool TO1 during feed movement based on the forward movement D and speeds Fd and B, as shown in Figure 6.

When the process in step S110 is completed, the vibration control process shown in Figure 14 is finished.
In accordance with steps S106 to S110, the NC device 70 controls the cutting vibration so that a portion with a period of one or more cycles according to the chip length coefficient AB, which is permitted with a longer period than the chip length coefficient AB, and a portion with a period of one or more cycles according to the chip length coefficient AC, which is permitted with a shorter period than the chip length coefficient AB, are alternately repeated. Therefore, the processing time of the vibratory cutting is controlled so that it does not change, and the number of cutting vibrations is controlled so that it does not change. In addition, the NC device 70 keeps the spindle rotation speed S per unit time constant and includes the portion with a period according to the chip length coefficient AB and the portion with a period according to the chip length coefficient AC in the cutting vibration so as not to change the cycle time of the vibratory feed command. Since the spindle rotation speed S per unit time remains constant, the increase in load on the rotational drive unit U1 is suppressed. Therefore, this specific example enables vibratory cutting that avoids resonance while suppressing the impact on processing time. Furthermore, since the NC device 70 automatically predicts the occurrence of resonance and switches the chip length coefficient from A1 to AB and AC, a special machining program for vibration cutting that avoids resonance is not required.

Furthermore, if the machine tool 1 has both a resonance avoidance mode and a resonance non-avoidance mode, the NC device 70 may perform the vibration control processing shown in Figure 14 in the resonance avoidance mode, and only perform the processing in steps S104 and S110 in the resonance non-avoidance mode.

(3) Variations:
Various modifications of this invention are conceivable.
For example, the drive target moving along the cutting direction is not limited to the tool TO1, but may also be the spindle 11 that grips the workpiece W1, or both the tool TO1 and the spindle 11. When the drive target is the spindle 11, the NC device 70 should control the feed movement of the spindle 11 so that it vibrates along the cutting direction when cutting the workpiece W1. When the drive target is both the tool TO1 and the spindle 11, the NC device 70 should control the feed movement of both the tool TO1 and the spindle 11 so that it vibrates along the cutting direction when cutting the workpiece W1.
The control unit U3 may be located in the computer 100 (see Figure 1) instead of the machine body 2. Alternatively, both the machine body 2 and the computer 100 can constitute the control unit U3.

The machine tool 1 may be equipped with a detection unit for detecting the natural frequency of the tool post or the like, and may write natural frequency information NF representing the natural frequency detected by the detection unit to the RAM 73 or ROM 72. In the vibration control process shown in Figure 14, the NC device 70 can perform the decision process in step S102 by reading the natural frequency information NF from the RAM 73 or ROM 72.
Furthermore, the machine tool 1 may receive input of the natural frequency of the tool post or the like from the operating unit 80 or the computer 100, and may write natural frequency information NF representing the received natural frequency to the RAM 73 or ROM 72. In the vibration control process shown in Figure 14, the NC device 70 can perform the decision process in step S102 by reading the natural frequency information NF from the RAM 73 or ROM 72.

The natural frequency information (NF) is not limited to a numerical value representing the natural frequency; it may also include information representing the prohibited frequency band (the frequency band containing the natural frequency), etc.

In vibratory cutting, the waveform of the tool position relative to the spindle rotation angle is not limited to a waveform that superimposes triangular wave vibrations onto the cutting feed.

As illustrated in Figure 15, this technology can also be applied to waveforms in which sinusoidal vibrations are superimposed on the normal cutting feed as the waveform of the tool position relative to the spindle rotation angle. Figure 15 schematically shows another example of the tool position relative to the spindle rotation angle. In Figure 15, the component of the normal cutting feed rate F is shown by a dashed line, and the maximum feed rate Fmax is shown by a fine dotted line.
The sine wave obtained by removing the component representing the normal cutting feed rate F from the waveform shown in Figure 15 has a period of A2 and an amplitude of E2. Therefore, period A2 is an example of a parameter related to the period of vibration. The slope of the midpoint C5 from the trough C4 to the peak C3 in the sine wave shown in Figure 15 represents the maximum feed rate Fmax.

Even with the waveform shown in Figure 15, the NC device 70 (or computer 100) can determine, based on the natural frequency information NF, whether period A2 included in the vibration feed command is prohibited or permitted. If the NC device 70 determines that period A2 is prohibited, it can control the cutting vibration so that it includes a portion of the cutting vibration that follows a period longer than period A2 (corresponding to the chip length coefficient AB) and a portion that follows a period shorter than period A2 (corresponding to the chip length coefficient AC). Therefore, a machine tool performing vibration cutting with the waveform shown in Figure 15 can also perform vibration cutting while suppressing the impact on processing time and avoiding resonance.

(4) Conclusion:
As explained above, according to the present invention, it is possible to provide technology for machine tools and the like that can perform vibration cutting while suppressing the impact on processing time and avoiding resonance, in various embodiments. Of course, even a technology consisting only of the constituent elements of the independent claims can obtain the basic functions and effects described above.
Furthermore, configurations obtained by substituting or changing the combinations of each configuration disclosed in the above-mentioned examples, configurations obtained by substituting or changing the combinations of each configuration disclosed in the prior art and the above-mentioned examples, etc., are also possible. The present invention also includes these configurations, etc.

1...Machine tool,
10... Headstock, 11... Main shaft, 12... Gripping section, 13A, 13B... Motor,
14... Headstock drive unit,
20... Knife rest,
31, 32... Servo amplifier, 33, 34... Servo motor, 35, 36... Encoder
70...NC device,
80...Operation unit, 81...Input unit, 82...Display unit,
100... Computers,
201...Tool position during normal cutting, 202...Tool position during vibration cutting,
AX1...Spindle center line,
C1...First change point, C2...Second change point,
F1... Feed axis,
M1...forward movement, M2...backward movement,
NF...Natural frequency information,
P1...Current position, P2...End point, P3...Position
PR1...Control program, PR2...Machining program,
TO1...Tools,
U1... Rotation drive unit, U2... Feed drive unit, U3... Control unit,
W1...Work.

Claims (5)

A rotary drive unit that rotates the spindle that grips the workpiece,
A tool for cutting the workpiece and a feed drive unit for moving at least one of the drive targets of the spindle,
The system includes a control unit that controls the feed movement of the driven object such that when the workpiece is being cut, the tool moves forward along the cutting direction toward the workpiece and moves backward in the opposite direction to the forward movement, accompanied by vibration.
The control unit,
Based on information regarding natural frequencies (NF), it is determined whether parameter (A) included in a command that includes the parameter (A) relating to the period of the vibration is prohibited or permitted.
A machine tool that, upon determining that parameter (A) is prohibited, controls the vibration such that the vibration includes a portion of the vibration with a period according to parameter (AB) which is permitted with a longer period than parameter (A), and a portion of the vibration with a period according to parameter (AC) which is permitted with a shorter period than parameter (A). The command specifies a constant spindle rotation speed (S) per unit time.
The machine tool according to claim 1, wherein the control unit, when it determines that parameter (A) is prohibited, keeps the constant spindle speed (S) as is and includes the periodic portion according to parameter (AB) and the periodic portion according to parameter (AC) in the vibration so as not to change the cycle time of the command. The machine tool according to claim 1 or 2, wherein the control unit, upon determining that parameter (A) is prohibited, controls the vibration such that a portion with a period of one or more cycles according to parameter (AB) and a portion with a period of one or more cycles according to parameter (AC) alternately repeat. The machine tool according to claim 1 or 2, wherein, when the period of the vibration indicated by parameter (AC) is greater than one rotation of the spindle, the control unit controls the difference in rotation angle of the spindle between a first change point in the vibration using parameters (AB) and (AC) to 360°, between a first change point in the vibration where the movement changes from forward to backward, and a second change point in the vibration where the movement changes from backward to forward. The control unit, with OD being 3 or more odd numbers, and when the period of the vibration indicated by parameter (A) is 2/OD, controls the difference in the rotation angle of the spindle between the first change point where the movement changes from forward to backward in one period of the vibration and the second change point where the movement changes from backward to forward in one period of the vibration, in the vibration using parameters (AB) and (AC), to a rotation angle corresponding to 1/2 of the vibration period, according to claim 1 or claim 2.