Method and machine tool for damping chatter vibrations

Abstract

A chatter vibration damping method for damping chatter vibrations of a machine tool that performs cutting work by rotating a rotating body containing a workpiece (4) or a tool according to a machining program stored in a storage unit (11), comprising: - an inertia detection step (S3) to detect the moment of inertia of the rotating body; - a recording step (S4) to record the value of the detected moment of inertia in the machining program; and - a calculation step (S5) to calculate the variable amplitude and the variable period from the value for the moment of inertia recorded in the machining program and the maximum power consumption of a motor (6) for rotating the rotating body when the speed of the rotating body is varied in order to dampen the chatter vibrations.

Description

BACKGROUNDTechnical field

[0001] The present invention relates to a method for controlling the damping of chatter vibrations generated during machining with a machine tool by changing the speed of the rotating shaft, and to a machine tool. Similar area

[0002] When cutting work is performed by a machine tool, so-called "chatter vibration" is occasionally generated when the tool or workpiece has low strength. The generation of chatter vibration causes problems such as tool breakage or a reduction in the surface accuracy of the workpiece. This chatter vibration is generated due to fluctuations in the cutting thickness of the workpiece and an increase in vibration due to the generation of a phase lag between undulations (vibrations) induced on the machined surface before rotation and vibrations due to instantaneous cutting. As a method for damping chatter vibration, measures in Patent Reference 1 and Patent Reference 2 have been proposed.The measures described in Patent Reference 1 and Patent Reference 2 vary the rotational speed of the rotating shaft to make the power applied to the tool irregular, thus damping chatter vibrations. Patent References 1 and 2 disclose a device that dampens chatter vibrations by setting a variable amplitude and period to vary the rotational speed of the rotating shaft.

[0003] DE 10 2011 084 875 describes suppressing chatter vibration of a rotating shaft in a machine tool by oscillating the rotating shaft's speed, thereby enabling easy adjustment of a fluctuation parameter. A graph showing the fluctuation amplitude of the speed is displayed in a monitoring device of the machine tool. On this graph, a current fluctuation position (first point) is indicated by a black circle, and a motor's performance limit line is also shown for a specific period of time based on a predetermined equation. In a range not exceeding the performance limit line, a new fluctuation position (second point) with a larger fluctuation amplitude and a shorter fluctuation period than the current fluctuation position is calculated and displayed with an arrow A leading from the current fluctuation position to this new point.

[0004] DE 10 2011 085 270 A1 describes a machine tool having a rotary shaft configured to mount a tool or a workpiece thereon, and having a drive unit for rotating the rotary shaft. In this machine tool, a rotational speed of the rotary shaft is regulated to change in such a way that the rotational speed oscillates with a given amplitude and a given time period with respect to a given average rotational speed, so that chatter vibrations can be suppressed. A parameter display control unit is provided, which is configured to have a display device display parameter information regarding predetermined parameters for changing at least one of the average rotational speed, the amplitude, and the time period, so that the parameters can be changed based on the parameter information displayed in the display device (9). CITATION LISTPatent References Patent Reference 1: JP 49-105277 A Patent Reference 2: JP 61-3522 U Patent reference 3: DE 10 2011 084 875 A1 Patent reference 4: DE 10 2011 085 270 A1 SUMMARYTechnical problem

[0005] However, the methods described in Patent Reference 1 and Patent Reference 2 require setting two values ​​for a variable amplitude and a variable period to vary the rotational speed. To dampen chatter vibrations, it is known that the effect is large when the variable period is set short and the amplitude is set at a fixed value or higher. The dampening effect on chatter vibrations is known to be large when the rotating shaft undergoes very rapid acceleration and deceleration. However, if the rotating shaft is repeatedly accelerated and decelerated excessively, the motor's power consumption increases, and there is a possibility of the motor overheating and damage.

[0006] Accordingly, a method for performing the originally intended machining after determining a limit value for the rotational speed from the maximum power consumption of the motor and the moment of inertia of the rotating body is considered. To do this, it is necessary to perform an appropriate process to determine the moment of inertia and have it recorded by an NC device that monitors the machine tool. When the same workpiece is being machined, it is not necessary to update the moment of inertia, but when a different workpiece is being machined, it is necessary to perform the appropriate process to determine the moment of inertia again, or the operator must input the previously determined moment of inertia into the NC device, thus increasing the number of machining steps. If the same workpiece is machined with a different machine tool, the same number of machining steps is required, and this method is not reasonable.

[0007] The present invention has been achieved in view of the above-mentioned problems, and an advantage of the invention is to provide a method for damping chatter vibrations without damaging the motor, in which a limit for the variation of the rotational speed can be easily calculated without determining the moment of inertia of the workpiece each time, and to provide a machine tool. Solution to the problem

[0008] The problem is solved by a method for damping chatter vibrations having the features of patent claim 1 and by a machine tool having the features of patent claim 4.

[0009] The chatter vibration damping method of the present invention is a chatter vibration damping method that dampens chatter vibration of a machine tool that performs cutting work by rotating a rotating body including a workpiece or a tool according to a machining program stored in a storage unit, and comprises: a detecting step that detects the moment of inertia of the rotating body; a recording step that records the value of the detected moment of inertia in the machining program; and a calculating step that calculates the variable amplitude and the variable period from the value of the moment of inertia recorded in the machining program and the maximum power consumption of a motor for rotating the rotating body when the speed of the rotating body is varied to dampen chatter vibration.

[0010] According to a preferred embodiment, a value obtained by subtracting the previously stored moment of inertia of the rotating part of the machine tool from the detected moment of inertia of the rotating body is recorded in the machining program as the value of the moment of inertia. According to another preferred embodiment, when the machining program instructs the workpiece to be machined in a plurality of operations, the moment of inertia of the rotating body is detected for each operation step in the detection step; and the value of the moment of inertia of the rotating body detected for each operation step is stored in the machining program for each operation step in the recording step.

[0011] The machine tool according to another aspect of this invention is a machine tool for performing cutting work by rotating a rotating body including a workpiece or a tool according to a machining program stored in a storage unit. The machining program includes an inertia moment detection unit for detecting the inertia moment of the rotating body; a recording unit for recording the detected inertia moment in the machining program when the inertia moment is detected by the inertia moment detection unit; and a calculation unit for calculating the variable amplitude and the variable period from the inertia moment recorded in the machining program and the maximum power consumption of a motor for rotating the rotating body when the rotational speed of the rotating body is varied to damp chatter vibrations. Effects of the invention

[0012] According to the present invention, since the moment of inertia once determined is recorded in the machining program, it is not necessary to perform the same operation to determine the moment of inertia when machining the same workpiece, and a limit for the variation of the rotational speed can be easily calculated to dampen chatter vibrations without damaging the motor. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Preferred embodiment(s) of the present invention will be described in detail with reference to the following figures, wherein: Fig. 1 is a schematic structural view of an NC lathe according to an embodiment of the present invention; Fig. 2 is an explanatory view showing an example of the fluctuations in the rotational speed of the rotating shaft; Fig. 3 is a flow chart of the chatter vibration damping process; and Fig. 4 is an explanatory view showing part of the editing program. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0014] Embodiments of the present invention will be described below with reference to the accompanying drawings. Fig. 1 is a schematic structural view of an NC lathe according to an embodiment of the present invention. A headstock 7 axially and rotatably supports a main shaft 1, which holds a workpiece 4 via a chuck 2 and a lock 3. A rotating body composed of the main shaft 1, a rotor 6b, the chuck 2, the lock 3, and the workpiece 4 is rotated by a motor 6 including a stator 6a fixed to the headstock 7 and the rotor 6b fixed to the main shaft 1. An encoder 5 for detecting the rotational speed of the main shaft 1 is fixed to the headstock 7. A rotary shaft controller 8 is connected to the motor 6 and the encoder 5, and an NC device 9 instructs the rotational speed to the rotary shaft controller 8.The rotary shaft controller 8 maintains monitoring of the rotational speed of the main shaft 1, which is determined by the encoder 5, and adjusts the input current applied to the motor 6 for rotating the rotating body to the rotational speed commanded by the NC device.

[0015] The NC device 9 is connected to a storage unit 11 for storing a machining program and the like, and to an input / output unit 10 for output to a screen or the like and for input via a keyboard or the like. The NC device 9 performs cutting work by moving a tool (not shown) toward the rotary shaft and / or in the radial direction toward the workpiece 4 while the main shaft rotates according to the machining program stored in the storage unit 11. In addition, the NC device 9 receives the rotational speed, a variable amplitude, and a variable period for the main shaft 1 as input from the input / output unit 10. In this way, by means of the NC device 9 and the rotary shaft controller 8, the rotational speed of the main shaft 1 can be varied to the specified variable amplitude and variable period, as shown in Fig. 2 is evident.

[0016] The moment of inertia can be determined from the electric current supplied to the stator 6a and the change in the rotational speed of the rotating body. For example, when a speed command is issued from the NC device 9 to the rotary shaft controller 8 to rotate the rotating body at a certain speed, the rotating body gradually accelerates from a stopped state to the commanded speed. At this time, the moment of inertia J and the rotational acceleration α of the rotating body are expressed by Expression 1 in terms of the torque T of the motor 6. T=J×α

[0017] The torque T can be expressed by expression 2 in terms of the input current I and the torque constant k, which depends on the motor. T=k×I

[0018] Here, the rotational acceleration α can be calculated from the amount of change in the rotational speed detected by the encoder 5, and the input current I provided by the rotary shaft controller 8 is well known. Thus, the moment of inertia J can be calculated using Expression 1 and Expression 2. This calculation is performed by the NC device 9, and the determined moment of inertia is recorded in the machining program of the NC device 9. The encoder 5 and the NC device 9, which calculates the moment of inertia, serve as detection units for detecting the moment of inertia of the rotating body.

[0019] The chatter vibration damping method used for the NC lathe of this embodiment will be explained with reference to the flowchart in Fig. 3. In step S1, a machining program for a workpiece is read from the storage unit 11 into the NC device 9. In step S2, it is judged whether the moment of inertia J is recorded in the machining program as a whole. If it is not recorded, the flow proceeds to step S3, and if it is recorded, the flow proceeds to step S5. In step S3, the moment of inertia of the rotating body is measured by a known method. In addition to the method described above, for example, a method that calculates the moment of inertia from the input current applied to the motor 6 when the rotating body is accelerated or decelerated and the change in the rotational speed at that time has been proposed.

[0020] In step S4, the NC device 9, which serves as a recording means, records the moment of inertia determined in step S3 in the machining program within the NC device 9. An example of the recorded machining program is shown in Fig. 4. If the moment of inertia determined in step S3 is, for example, 0.5 kg × m 2, the numerical value "0.5" is subsequently recorded in the variable name INA of the moment of inertia at the head of the machining program. In step S5, the NC device 9, which serves as the calculation means, determines from Expression 3 a limited variable amplitude and a variable period when the speed is varied from the moment of inertia recorded in the program and the maximum power consumption of the motor 6 previously stored in the storage unit 11. In Expression 3, Q is a speed-variable amplitude [%], R is a speed-variable period [sec], S is the speed of the main shaft [min -1 ] and P is the maximum power consumption of the motor. Q=4500×P×R / (π2×S2×J)

[0021] Since the relationship between the variable amplitude and the variable period obtained here is a simple expression, it can be represented as a curve or a representative value can be displayed as a numerical value on the input / output unit in step S6.

[0022] In step S4, the moment of inertia was recorded in the machining program within the NC device 9. By saving the machining program, the machining program in which the moment of inertia was recorded is stored in the storage unit 11. From this point on, the machining program in which the moment of inertia was recorded can be read.

[0023] Since the moment of inertia can be determined once using the above-mentioned method and recorded in the machining program, it is not necessary to repeat the same process of determining the moment of inertia when machining the same workpiece, and a limit for the speed variation can be easily calculated. Therefore, chatter vibrations can be dampened without damaging the motor.

[0024] In step S4, the moment of inertia of the rotating body was recorded, but the moment of inertia obtained by subtracting the moment of inertia of a rotating part of the machine tool can be recorded. That is, the main shaft 1 belonging to the NC lathe and the moment of inertia of the rotor 6b are previously stored in the storage unit 11, and the stored value can be subtracted from the measured moment of inertia of the rotating body to record the result. Thus, when using another NC lathe for machining, the moment of inertia recorded in the machining program can be simply added to the moment of inertia of the main shaft belonging to the NC lathe and the moment of inertia of the rotor to determine the moment of inertia of the rotating body.Therefore, when moving to the location of another NC lathe and using the workpiece and machining program as a group, the process of determining the moment of inertia can be omitted and the chatter vibrations can be damped reasonably and universally.

[0025] The method for measuring the moment of inertia in the initial state before machining was described above, but it is also possible to measure the moment of inertia in each machining step and record the result in the machining program. As the workpiece is reduced through machining, the moment of inertia decreases. Therefore, the moment of inertia is measured in each machining step, such as a rough machining step for an outer shape and a final machining step for a hole, and recorded at the program stop between the machining steps. In this way, the moment of inertia appropriate for each machining step can be obtained. The obtained value can be used to calculate the variable amplitude and variable period optimal for the rotational speed in each machining step.

[0026] In the above, an example of the NC lathe for performing cutting work when the workpiece rotates was described, but the present invention can also be applied to other machine tools such as a machining center that performs the cutting work by rotating the tool.

Claims

[1] A chatter vibration damping method for damping chatter vibrations of a machine tool that performs cutting work by rotating a rotating body containing a workpiece (4) or a tool according to a machining program stored in a storage unit (11), comprising: - an inertia detection step (S3) to detect the moment of inertia of the rotating body; - a recording step (S4) to record the value of the detected moment of inertia in the machining program; and - a calculation step (S5) to calculate the variable amplitude and the variable period from the value for the moment of inertia recorded in the machining program and the maximum power consumption of a motor (6) for rotating the rotating body when the speed of the rotating body is varied in order to dampen the chatter vibrations. [2] A chatter vibration damping method according to claim 1, wherein a value obtained when the previously stored moment of inertia of the rotating part of the machine tool is subtracted from the detected moment of inertia of the rotating body is recorded in the machining program as the value of the moment of inertia. [3] A method for damping chatter vibrations according to claim 1, wherein: - when the machining program instructs the workpiece (4) to be machined in a plurality of work steps, - the moment of inertia of the rotating body is recorded for the respective work step in the recording step; and - the value of the moment of inertia of the rotating body recorded for the respective steps is stored in the recording step (S4) for the respective steps in the machining program. [4] Machine tool for performing cutting work by rotating a rotating body containing a workpiece (4) or a tool according to a machining program stored in a storage unit (11), comprising: - an inertia detection unit to detect the moment of inertia of the rotating body; - a recording unit for recording the detected moment of inertia in the machining program when the moment of inertia has been detected by the moment of inertia detection unit; and - a calculation unit to calculate the variable amplitude and the variable period from the moment of inertia recorded in the machining program and the maximum power consumption of a motor to rotate the rotating body when the speed of the rotating body is varied to dampen the chatter vibrations.

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