Machine tool and method for computing attachment position of balancer in machine tool

When a rotary table rotates, a main control section of a multi-tasking machine detects vibration of the rotary table on which a workpiece is mounted based on fluctuation of a position droop computed by a servo system. The main control section computes the arrangement position (the eccentricity amount and the eccentricity angle) of the workpiece with respect to the rotary table based on, for example, the detected vibration, the weight of the workpiece, and the rotation speed of the rotary table. The main control section computes the attachment position of a balancer with respect to the rotary table based on the computed arrangement position of the workpiece. Therefore, a suitable attachment position of a vibration suppressing balancer with respect to the rotary table is easily obtained.

RELATED APPLICATION

This application claims priority to Japanese Patent Application 2004-217615, filed on Jul. 26, 2004, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a machine tool equipped with a rotary table, and to a method for computing the attachment position of a balancer with respect to the rotary table.

A typical machine tool equipped with a rotary table is designed such that the rotary table is linearly moved forward and backward, and rotated. A workpiece attached to the rotary table is, for example, cut by a tool provided on a tool post.

Depending on the attaching state of a workpiece or a jig on the rotary table, a rotational imbalance may be caused on the rotary table. If such a rotational imbalance is caused, the rotary table vibrates. When a turning process is performed in this state, the workpiece becomes defective. In some cases, the rotary table might get broken, or the workpiece might be detached from the rotary table.

Therefore, a vibration sensor for detecting vibration of the rotary table may be provided on the rotary table. In this case, when the rotary table causes vibration due to the rotational imbalance, an operator is informed of the situation. However, since informing the operator of the situation does not eliminate the rotational imbalance, the problem caused by the rotational imbalance is not fundamentally solved.

Japanese Laid-Open Patent Publication No. 2002-28858 discloses a machine tool that causes a tool post equipped with a rotary tool to move forward and backward with respect to a workpiece. The machine tool includes a servo system that controls a linear motor that moves the tool post forward and backward with respect to the workpiece, and disturbance predicting means that predicts a disturbance that the rotational imbalance of the rotary tool causes to act on the servo system. The servo system is controlled to compensate for the influence of the disturbance predicted by the disturbance predicting means.

More specifically, based on a current command value output from a speed feedback loop of the servo system, and a position feedback value output from a position detector, which detects the position of the tool post, the disturbance that the rotational imbalance of the rotary tool causes to act on the servo system is predicted. In accordance with the predicted disturbance, a current command value entered to a current feedback loop of the servo system is corrected, compensating for the influence of the disturbance. As a result, the speed fluctuation of a grinding head caused by the rotational imbalance of the rotary tool is suppressed, which improves the machining accuracy of the workpiece.

However, the technique disclosed in the above publication is for suppressing the speed fluctuation of the grinding head, and does not eliminate the rotational imbalance of the rotary tool. Thus, even if the technique disclosed in the above publication is applied to the machine tool equipped with the rotary table, the rotational imbalance of the rotary table is not suppressed, and problems such as damage to the rotary table or detachment of the workpiece are not solved.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to easily achieve a suitable attachment position of a vibration suppressing balancer with respect to a rotary table.

To achieve the foregoing and other objectives and in accordance with the purpose of the invention, a method for computing an attachment position of a balancer with respect to a rotary table of a machine tool is provided. The method includes: detecting vibration of the rotary table when the rotary table on which an object is mounted is rotated; computing the arrangement position of the mounted object with respect to the rotary table based on the detected vibration, the weight of the mounted object, and physical quantity representing the rotation state of the rotary table; and computing the attachment position of the balancer with respect to the rotary table based on the computed arrangement position of the mounted object.

The present invention also provides a machine tool including a rotary table on which an object is mounted, a vibration detection section, an arrangement position computing section, and an attachment position computing section. The vibration detection section detects vibration of the rotary table during rotation of the rotary table on which the object is mounted. The arrangement position computing section computes the arrangement position of the mounted object with respect to the rotary table based on the detected vibration, the weight of the mounted object, and physical quantity representing the rotation state of the rotary table. The attachment position computing section computes the attachment position of a balancer with respect to the rotary table based on the computed arrangement position of the mounted object.

The present invention provides another including a rotary table, a rotary device, a movement device, a control section, and a vibration detection section. The rotary drive device rotates the rotary table. The movement device moves the rotary table along a predetermined moving direction. The control section controls the movement device and configures a servo system, which includes a position feedback loop. The vibration detection section detects vibration of the rotating rotary table based on an input value entered in the position feedback loop.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A multi-tasking machine10includes a bed12as shown inFIG. 1. Linear guide rails14(two in the drawing), which are parallel to each other, extend along an X-axis on the upper surface of the bed12. A workpiece support apparatus20is provided on the linear guide rails14. The workpiece support apparatus20includes a base22. The base22is guided by the linear guide rails14and is movable along a predetermined moving direction, that is, the X-axis.

A nut23(seeFIG. 4) is provided on the lower surface of the base22, and the nut23is screwed to a ball screw16provided on the bed12. The base22moves forward and backward or reciprocates along the X-axis as the ball screw16is selectively rotated forward and reverse by an X-axis drive motor Mx provided on the bed12. The X-axis drive motor Mx functions as a movement device.

A disk-like rotary table30is provided on the base22to be rotatable about a C-axis, which is parallel to a Z-axis. As shown inFIG. 4, the base22is provided with a rotary drive device, which is a workpiece main spindle motor Mwsin the first embodiment, for rotating the rotary table30. The upper surface of the rotary table30forms a workpiece mounting surface32on which the workpiece W is mounted. As shown inFIGS. 4 and 13, jigs34are mounted on the workpiece mounting surface32. Each jig34reciprocates along one of pairs of guide grooves35, which extend substantially radially from a rotational center (C-axis) of the rotary table30, and is fixable to the workpiece mounting surface32with a fixing device, which is not shown. The jigs34removably hold the workpiece W on the workpiece mounting surface32. The workpiece W and the jigs34correspond to mounted objects mounted on the rotary table30.

As shown inFIGS. 13,14(a), and14(b), balancer attachment grooves36, which extend radially from the rotational center (C-axis) of the rotary table30, are formed on the workpiece mounting surface32. The attachment grooves36each have different arrangement angle α (seeFIG. 13) with respect to a reference orientation position Px (reference angular position) defined on the rotary table30in advance. The reference orientation position Px is a specific angular position in the circumferential direction on the rotary table30serving as a reference. Each attachment groove36functions as a balancer attachment portion. The attachment grooves36are formed on the workpiece mounting surface32, and are also open on the circumferential surface of the rotary table30. Each attachment groove36has a reversed T-shaped cross-section as shown inFIGS. 14(a) and14(b). The lower portion (bottom portion) of each attachment groove36is wide and the upper portion (opening side) is narrow.

A head portion37aof a bolt37, which serves as an attachment member, is selectively inserted in one of the attachment grooves36through the opening on the circumferential surface of the rotary table30. The bolt37is slidable along the longitudinal direction of the attachment groove36. The head portion37aof the bolt37is engaged with a step36A formed between the wide portion and the narrow portion of the attachment groove36. The bolt37cannot be taken out from the opening on the workpiece mounting surface32. When the bolt37is inserted in one of the attachment grooves36, the distal end of the bolt37projects from the workpiece mounting surface32and is inserted in a through hole42formed in the balancer40. Through holes42(two inFIG. 13) are formed in the balancer40. The bolt37is selectively inserted in one of the through holes42. In a state where the balancer40is fitted to the bolt37inserted in the attachment groove36, the distal end of the bolt37projects from the balancer40. A nut44is removably screwed to the distal end of the bolt37.

As shown inFIG. 1, a portal column50is provided above the bed12astride the linear guide rails14. Guide members, which are linear guide rails54(two inFIG. 1) in first embodiment, are located on the front surface of the column50extending along the Z-axis, which is the vertical direction. The linear guide rails54are parallel to each other. A saddle52is provided on the linear guide rails54to be movable along the Z-axis. A pair of nuts are provided on the surface of the saddle52facing the column50. The nuts are respectively screwed to a pair of ball screws (not shown) provided on the column50. When a pair of Z-axis drive motors Mz located on the top surface of the column50rotates the corresponding ball screws forward and reverse, the saddle52reciprocates in the Z-axis direction.

Linear guide rails58(two inFIG. 1), which are parallel to each other, extend along a Y-axis on the surface of the saddle52opposite to the column50. A head supporting device60is provided on the linear guide rails58. The head supporting device60is guided by the linear guide rails58and is movable along the Y-axis direction. Nuts, which are not shown, are located on the surface of the head supporting device60facing the saddle52. The nuts are screwed to a ball screw59provided on the saddle52. When the ball screw59is rotated forward and reverse by a Y-axis drive motor My provided on the saddle52, the head supporting device60reciprocates along the Y-axis direction.

A tool main spindle head70is pivotally located at the lower portion of the head supporting device60with the rotational center at a B-axis, which is parallel to the Y-axis. That is, the head supporting device60is provided with a B-axis drive motor Mb (seeFIG. 2), and the B-axis drive motor Mb rotates the tool main spindle head70about the B-axis. The tool main spindle head70is equipped with a built-in tool main spindle motor MTS(seeFIG. 2), and the tool main spindle motor MTSrotates a spindle, which is not shown. A machining tool for turning is attachable to the tool main spindle head70. The tool main spindle head70is selectively locked at a certain angle with a lock mechanism, which is not shown, with the machining tool attached to the tool main spindle head70.

The multi-tasking machine10is provided with a CNC control device, which is a machining control device100in first embodiment, as shown inFIG. 2. As shown inFIG. 2, the machining control device100includes a main control section110, which is a CPU. The main control section110is connected to a machining program memory120, a system program memory130, a buffer memory140, a machining control section150, an operation panel160, which includes a keyboard and the like, and a display170, which is a liquid crystal display, through a bus line105. The main control section110corresponds to a position control section that controls the position of the rotary table30in the moving direction (X-axis direction in first embodiment).

The main control section110is also connected to an X-axis control section200, a Y-axis control section210, a Z-axis control section220, a B-axis control section230, and a workpiece main spindle control section240through the bus line105. Only one set of the Z-axis control section220, a drive circuit222, and the Z-axis drive motor Mz is shown inFIG. 2. However, two sets of the Z-axis control section220, the drive circuit222, and the Z-axis drive motor Mz actually exist corresponding to the two Z-axis drive motors Mz shown inFIG. 1. Each of the control sections200,210,220,230,240drives the corresponding one of the motors Mx, My, Mz, Mb, Mws in accordance with a command from the main control section110through the corresponding one of the drive circuits202,212,222,232,242.

Position detectors, which are rotary encoders204,214,224,234,244are each attached to the corresponding one of the motors Mx, My, Mz, Mb, Mws. Each rotary encoder outputs a pulse signal corresponding to the rotation amount of the associated motor to the corresponding one of the control sections200,210,220,230,240. The pulse signal is used to generate a position feedback signal or a speed feedback signal for the corresponding motor. The rotary encoder204corresponds to a position detector, which detects the position of the rotary table30in the moving direction, that is, in the X-axis direction.

When stopping the rotary table30, the main control section110outputs a control signal for stopping the rotary table30at a predetermined stop rotational phase position to the workpiece main spindle control section240in accordance with a system program stored in the system program memory130. The workpiece main spindle control section240controls the workpiece main spindle motor Mws such that the rotary table30stops at the stop rotational phase position based on the control signal. In the first embodiment, when the rotary table30is stopped at the stop rotational phase position, the reference orientation position Px defined on the rotary table30is parallel to the X-axis and is aligned with a line that passes through the rotational center (C-axis) of the rotary table30.

The X-axis control section200, which is the characteristic configuration of the present invention, will now be described. As shown inFIG. 3, the X-axis control section200includes a speed detection section203, a position control section205, a speed control section206, a current control section207, and a latch portion208. The speed detection section203produces a speed feedback signal from an output pulse (a position feedback signal) of the rotary encoder204. The position control section205produces a speed command in accordance with a position command from the main control section110and an output pulse (a position feedback signal) from the rotary encoder204. The position command from the main control section110represents the target position of the rotary table30in the X-axis direction. The output pulse from the rotary encoder204represents the actual position of the rotary table30in the X-axis direction. The difference between the position command and the position feedback signal is referred to as a position droop, which is computed using a subtractor205a.As described above, the X-axis control section200configures a servo system that includes a position feedback loop. The position command and the position feedback signal correspond to input values entered to the position feedback loop.

The speed control section206produces a current command such that the difference between the speed command and the speed feedback signal becomes zero. As described above, the X-axis control section200includes a speed feedback loop. The current control section207produces a voltage command such that the difference between the current command and the current value detected by a current detection section209becomes zero. The current detection section209, which is not shown inFIG. 2, detects the level of current (current value) that flows from the drive circuit202to the X-axis drive motor Mx. As described above, the X-axis control section200includes a current feedback loop. The latch portion208latches the position droop computed at any given time, and outputs to the main control section110.

The Y-axis control section210, the Z-axis control section220, the B-axis control section230, and the workpiece main spindle control section240shown inFIG. 2differ from the X-axis control section200in that they do not include the latch portion208. Therefore, the detailed explanations are omitted.

The drive circuit202shown inFIG. 2includes an inverter circuit, which generates voltage that is actually applied to the X-axis drive motor Mx in accordance with the voltage command. Since the drive circuits212,222,232,242have the same configuration as the drive circuit202, the detailed explanations are omitted.

As shown inFIG. 2, the main control section110is connected to a tool main spindle control section250via the bus line105. The tool main spindle control section250receives a spindle rotation command from the main control section110and outputs a spindle speed signal to a drive circuit252. Based on the spindle speed signal, the drive circuit252rotates the tool main spindle motor MTSat a rotation speed that corresponds to the spindle rotation command, thereby rotating a cutting tool with the spindle.

A method for detecting vibration caused during rotation of the rotary table30of the multi-tasking machine10configured as described above will now be described.

FIG. 5is a flowchart showing the procedure for detecting vibration of the rotary table30.FIG. 4shows the X-axis drive motor Mx, the workpiece main spindle motor Mws, and components related to the control thereof. The vibration detecting procedure shown inFIG. 5is executed, for example, before executing various machining programs stored in the machining program memory120, by the main control section110in accordance with a vibration detecting program stored in the system program memory130. At first, the workpiece W is held on the rotary table30with the jigs34, and the balancer40is not attached to the rotary table30. In addition, the base22is located at the initial position (original position before starting machining).

As shown inFIG. 5, at step S10, the X-axis control section200controls the X-axis drive motor Mx in accordance with a position command regarding the X-axis direction sent from the main control section110to move the base22from the initial position along the X-axis direction. The X-axis control section200then performs a position feedback control based on the output pulse from the rotary encoder204, and determines whether the base22(the rotary table30) has moved to the position corresponding to the position command. When the base22moves to the position corresponding to the position command, the X-axis control section200stops the base22, and controls the X-axis drive motor Mx such that the base22is held at that position.

Even if the rotary table30is located at a position corresponding to the position command in the X-axis direction, the X-axis control section200continues computing the position droop, which is the difference between the position feedback signal based on the output pulse from the rotary encoder204and the position command. If the rotary table30is located at a position corresponding to the position command and the rotary table30is not vibrating, the position droop is zero.

Subsequently, at step S20, the main control section110outputs a speed command to the workpiece main spindle control section240, and starts rotating the rotary table30. The speed command is output to the workpiece main spindle control section240such that the rotary table30rotates at a predetermined target rotation speed. The workpiece main spindle control section240performs the speed feedback control on the workpiece main spindle motor Mwsbased on the speed command and the output pulse from the rotary encoder244.

After outputting the speed command, the main control section110performs the first droop monitoring process of step S30until the rotary table30reaches the target rotation speed. The first droop monitoring process performed during accelerated rotation of the rotary table30will now be described.

If rotational imbalance is caused when the rotary table30is stopped at the predetermined position in the X-axis direction, vibration occurs that corresponds to the rotation speed of the rotary table30and the degree of imbalance. That is, the level of vibration fluctuates in accordance with the level of the centrifugal force acting on the rotary table30. The vibration appears as the fluctuation of the position droop in the X-axis direction via the ball screw16. The centrifugal force acting on the rotary table30correlates to the amplitude of vibration generated on the rotary table30, and the centrifugal force correlates to the position droop in the X-axis direction. In other words, the amplitude of vibration correlates to the position droop in the X-axis direction. Therefore, vibration of the rotary table30is detected by monitoring the fluctuation amount of the position droop in the X-axis direction.

FIG. 9(a) is a graph showing the relationship between the rotation speed of the rotary table30and the amplitude of vibration of the rotary table30in the X-axis direction.FIG. 9(a) shows the measurement results of the amplitude of vibration with respect to the rotation speed of the rotary table30according to five examples A to E in which the eccentricity amount of the workpiece W in the radial direction from the rotational center of the rotary table30is varied from zero to 4 mm by 1 mm increments.FIG. 9(b) is a graph showing the relationship between the rotation speed of the rotary table30and the position droop of the rotary table30in the X-axis direction.FIG. 9(b) shows the measurement results of the position droop with respect to the rotation speed of the rotary table30according to five examples A to E that are the same as those inFIG. 9(a).

FIG. 10(a) is a graph showing the relationship between the centrifugal force acting on the rotary table30and the amplitude of vibration of the rotary table30in the X-axis direction. The graph is obtained from the measurement result shown in theFIG. 9(a).FIG. 10(b) is a graph showing the relationship between the centrifugal force acting on the rotary table30and the position droop of the rotary table30in the X-axis direction. The graph is obtained from the measurement result shown inFIG. 9(b). The centrifugal force acting on the rotary table30is computed using the following equation.
Centrifugal Force [kN]=(π2·M·R·N2)/(9×108)

In the equation, M represents the weight [kg] of the workpiece W, R represents the eccentricity amount [mm] of the workpiece W from the rotational center (C-axis) of the rotary table30, and N represents the rotation speed [min−] of the rotary table30.

Based onFIGS. 10(a) and10(b), the relationship between the position droop and the amplitude of vibration is represented by a graph ofFIG. 11. As shown inFIG. 11, since the position droop has a close relationship to the amplitude of vibration, monitoring the position droop permits accurately estimating the level of the amplitude of vibration. Therefore, in the first droop monitoring process of step S30inFIG. 5, the level of vibration of the rotary table30in the X-axis direction is detected by monitoring the fluctuation of the position droop in the X-axis direction.

When the position droop in the X-axis direction is represented by DX, the value of the position droop DX alternately changes between a positive value and a negative value due to the vibration of the rotary table30. Therefore, at step S30ofFIG. 5, the main control section110determines whether the absolute value (|DX|) of the position droop entered from the latch portion208at a given time is less than or equal to a predetermined first threshold value γ1. The first threshold value γ1is set to a value greater than a second threshold value γ2, which will be described below. That is, rotation of the rotary table30is accelerated until the rotary table30reaches the target rotation speed. In this state, as shown inFIG. 8, the vibration of the rotary table30is greater as compared to a case where the rotary table30is rotated constantly at the target rotation speed. Therefore, in the first droop monitoring process performed during the accelerated rotation of the rotary table30, the first threshold value γ1is set to a relatively large value to increase the range of a permissible amplitude. InFIG. 8, a first detection range represents the time period during which the fluctuation of the position droop is judged using the first threshold value γ1.

At step S30, if the absolute value (|DX|) of the position droop is less than or equal to the first threshold value γ1, the main control section110proceeds to step S40, and if the absolute value (|DX|) of the position droop exceeds the first threshold value γ1, the main control section110proceeds to step S70.

At step S40, the main control section110determines whether the rotation speed of the rotary table30has reached the target rotation speed. The rotation speed of the rotary table30is computed based on the output pulse from the rotary encoder244. If the rotation speed of the rotary table30has not reached the target rotation speed, the main control section110returns to step S30. If the rotation speed of the rotary table30has reached the target rotation speed, the main control section110commands the workpiece main spindle control section240to maintain the rotation speed of the rotary table30at the target rotation speed, and then proceeds to step S50.

At step S50, the main control section110performs a second detection droop monitoring process. The second droop monitoring process is a process for monitoring the fluctuation of the position droop output from the latch portion208of the X-axis control section200when the rotary table30is rotated constantly at the target rotation speed. If the absolute value (|DX|) of the position droop is less than or equal to the second threshold value γ2, which is smaller than the first threshold value γ1, the main control section110proceeds to step S60, and if the absolute value (|DX|) of the position droop exceeds the second threshold value γ2, the main control section110proceeds to step S70.

At step S60, the main control section110determines whether the number of rotations of the rotary table30after reaching the target rotation speed has reached a predetermined determination number of times. The determination number of times may be, for example, few rotations. InFIG. 8, a second detection range represents the time period during which the fluctuation of the position droop is judged using the second threshold value γ2. The second detection range corresponds to time required for the number of rotations of the rotary table30to reach the determination number of times.

The main control section110includes a pulse counter, which is not shown, for counting output pulses from the rotary encoder244. As shown inFIG. 7(a), the pulse counter counts the output pulses of the rotary encoder244entered during 60/N. When the count value reaches a predetermined number h, the pulse counter resets the count value to zero and resumes counting. N represents the rotation speed [min−1] of the rotary table30. Every time the pulse counter counts the output pulses up to the predetermined number h, a rotation counter, which is not shown, of the main control section110increments the count value that represents the number of rotations of the rotary table30by one. If the count value of the rotation counter does not reach the determination number of times, the main control section110determines that the decision outcome of step S60ofFIG. 5is negative, and returns to step S30. If the count value of the rotation counter has reached the determination number of times, the main control section110determines that the decision outcome of step S60is positive, and ends the vibration detecting procedure.

InFIG. 5, if it is determined that the decision outcome of step S60is negative, the main control section110returns to step S30. However, the main control section110may return to step S50.

If the absolute value (|DX|) of the position droop exceeds the first threshold value γ1at step S30, or the absolute value (|DX|) of the position droop exceeds the second threshold value γ2at step S50, the main control section110proceeds to step S70. At step S70, the main control section110outputs a stop control signal to the workpiece main spindle control section240to stop the rotary table30at the stop rotational phase position. At the subsequent step S80, the main control section110outputs an alarm signal to the display170to inform the operator of an abnormality. As a result, the workpiece main spindle control section240stops the workpiece main spindle motor Mws, and stops the rotary table30at the stop rotational phase position. The display170simultaneously displays an alarm indicating, for example, that the rotation of the rotary table30has stopped or vibration has occurred. The stop control signal and the alarm signal correspond to signals indicating the abnormality of the rotary table30.

At the subsequent step S90, the main control section110performs, for example, a procedure for computing the balancer attachment position. The computation procedure includes computation of the amount of the eccentricity R, computation of the eccentricity angle θ, and computation of the balancer attachment angle (θ+π). As shown inFIG. 6, the eccentricity amount R is an amount of radial displacement of the workpiece W from the rotational center of rotary table30. The eccentricity angle θ is the displacement angle of the workpiece W from the reference orientation position Px in the circumferential direction. The balancer attachment angle (θ+π) is the angle of attachment position of the balancer40on the rotary table30from the reference orientation position Px in the circumferential direction. The eccentricity amount R and the eccentricity angle θ represent the arrangement position of the workpiece W with respect to the rotary table30.

(Computation of Eccentricity Amount R)

The computation of the eccentricity amount R will now be described. In the following equations, the values are represented by the following symbols (seeFIG. 6).

N: rotation speed [min−1] of the rotary table30

ω: angular speed [rad/s] of the rotary table30

M: weight [kg] of the workpiece W

Fx: centrifugal force acting on the rotary table30in the X-axis direction

Dx: position droop in the X-axis direction

t: time

The weight M of the workpiece W is entered via the operation panel160in advance and stored in the buffer memory140. The mass M of the workpiece W is read from the buffer memory140when performing the system program.

The centrifugal force Fx is obtained using the following equations (1) and (2).

The maximum value Fxmax of the centrifugal force Fx is represented by the function of the maximum value Dxmax of the position droop Dx, more specifically, the polynominal expression of the maximum value Dxmax of the position droop Dx as shown in the following equation (3).FIG. 12is a graph showing the relationship between the maximum value Dxmax of the position droop Dx and the maximum value Fxmax of the centrifugal force Fx according to the multi-tasking machine10of the first embodiment.FIG. 12differs fromFIG. 10(b) in that the horizontal axis is assumed to be the position droop Dx and the vertical axis is assumed to be the centrifugal force Fx. The curved line shown in the graph ofFIG. 12represents the function of the equation (3). The function of the equation (3) is defined based on data obtained through experiments in advance and is stored in the system program memory130.
Fxmax=f(Dxmax)   (3)

The following equation (4) is obtained from the above equations (1) to (3). The main control section110computes the eccentricity amount R using the equation (4).

The main control section110computes the eccentricity angle θ using the following equation (5).

In the first embodiment, the rotational phase position of the rotary table30when the count value of the pulse counter is reset to zero is defined as the stop rotational phase position.FIG. 7(b) shows the fluctuation of the position droop Dx. Δt in the equation (5) corresponds to a time period from when the position droop Dx has reached the peak value as shown inFIG. 7(b) to when the count value of the pulse counter ofFIG. 7(a) is reset to zero. In other words, Δt corresponds to a time period from when the position droop Dx has reached the maximum value Dxmax until the rotary table30is rotated to the stop rotational phase position. Therefore, when the rotary table30is rotated to the stop rotational phase position simultaneously as the position droop Dx reached the maximum value Dxmax, Δt becomes zero. In this case, in accordance with the equation (5), the eccentricity angle θ is also zero. That is, when Δt is zero, the workpiece W is attached to the reference orientation position Px on the rotary table30.

The main control section110computes the balancer attachment angle (θ+π) in the following manner.FIG. 15(a) shows a state where the rotary table30is stopped at the stop rotational phase position. Although not shown inFIG. 1, a partition500and a machine door510are arranged along the path of the X-axis direction of the rotary table30as shown inFIG. 15(a). The operator can selectively attach and  remove the workpiece W and the balancer40to and from the rotary table30by opening the machine door510. An area formed by opening the machine door510serves as an operation area Ar for the operator to perform operation (seeFIG. 15(b)).

In the first embodiment, when the rotary table30has a rotational imbalance, the workpiece main spindle motor Mwsis controlled such that suitable attachment region (the attachment groove36) of the balancer40on the rotary table30faces or comes adjacent to the machine door510(operation area Ar). More specifically, the main control section110computes the balancer attachment angle (θ+π) based on the computed eccentricity angle θ. The main control section110then computes the rotational angle (π/2−θ) of the rotary table30required for the attachment region on the rotary table30corresponding to the balancer attachment angle (θ+π) to face or come adjacent to the machine door510. Subsequently, based on the rotational angle (π/2−θ), the main control section110controls the workpiece main spindle motor Mws. As a result, the attachment region (the attachment groove36) corresponding to the balancer attachment angle (θ+π) faces or comes adjacent to the machine door510(seeFIG. 15(b)).

In the first embodiment, the rotational imbalance of the rotary table30is suppressed using a single balancer40. In this case, the attachment region of the balancer40is selected in the following manner.FIG. 16shows an example of the positions of the balancer attachment regions (twelve in the drawing), that is, the balancer attachment positions Pn(n=0 to 11) provided on the rotary table30. The arrangement angle of each balancer attachment position Pnwith respect to the reference orientation position Px is represented by αn(n=0 to 11). The arrangement angles α0to α11are stored as fixed values in the system program memory130in advance.FIG. 17(a) shows a state where the workpiece W and the balancer40are balanced on rotary table30.FIG. 17(a) shows an ideal balancer attachment angle (θ+π) and the arrangement angle αmof the balancer attachment position Pmclosest to the ideal attachment angle (θ+π) The balancer attachment position Pmis a position to which the balancer40should actually be attached among balancer attachment positions P0to P1.FIG. 17(b) shows the difference β between the ideal balancer attachment angle (θ+π) and the arrangement angle αm.

First, the main control section110computes the weight mm[kg] of the balancer40to be attached using the following equation (6).

In the equation, rmrepresents the distance [m] between the rotational center of the rotary table30and the balancer attachment position Pm, and is stored in the system program memory130in advance. As described above, R represents the eccentricity amount [m] of the workpiece W, and M represents the weight [kg] of the workpiece W.

The main control section110computes the differenceβ between each of the arrangement angles αn(n=0 to 11) and the ideal balancer attachment angle (θ+π) using the following equation (8). The main control section110then selects the arrangement angle αnat which the absolute value of the difference β becomes minimum as the arrangement angle αmclosest to the ideal balancer attachment angle (θ+π), and selects the attachment position Pmhaving the arrangement angle αmas a position to which the balancer40should actually be attached.

The centrifugal force FAgenerated by the rotational imbalance of the rotary table30in a state where the balancer40is attached to the attachment position Pmis obtained using the following equation (7).

The main control section110determines whether the difference β between the ideal attachment angle (θ+π) of the balancer40and the arrangement angle αmclosest to the ideal attachment angle (θ+π) satisfies the following balance requirement.
−π/3<β<π/3

If the difference β satisfies the balance requirement, the centrifugal force FAbecomes less than the centrifugal force MRω2before attaching the balancer40, which suppresses the vibration of the rotary table30. If the difference β does not satisfy the balance requirement, the rotational imbalance of the rotary table30increases. That is, since the centrifugal force before attaching the balancer40is MRω2, vibration of the rotary table30is suppressed if the centrifugal force FAafter attaching the balancer40obtained using the equation (7) becomes less than MRω2. To make the centrifugal force FAbecome less than MRω2, |2 sin(β/2)|<1 must be true in the equation (7). |2 sin(β/2)|<1 can be transformed to |sin(β/2)|<½. To satisfy |sin(β/2)|<½, −π/6<β/2<π/6 must be true, that is, −π/3<β<π/3 must be true.

As described above, the main control section110outputs a selection signal, which is a control signal, to the workpiece main spindle control section240based on the rotational angle (π/2−θ) such that the attachment position Pmhaving the arrangement angle αmclosest to the ideal balancer attachment angle (θ+π), in other words, the attachment groove36on the rotary table30to which the balancer40should actually be attached faces or comes adjacent to the machine door510. The workpiece main spindle motor Mws rotates the rotary table30from the stop rotational phase position, and stops the rotary table30in a state where the attachment position Pmfaces or is adjacent to the machine door510. The workpiece main spindle control section240and the workpiece main spindle motor Mws function as a rotation control section.FIG. 15(b) shows a case where the attachment position Pmhaving the arrangement angle αmclosest to the ideal balancer attachment angle (θ+π) faces the machine door510. To facilitate illustration,FIG. 15(b) shows a case where β=0 in.

At step S100ofFIG. 5, the main control section110outputs a display signal representing the correction information of the rotational imbalance of the rotary table30to the display170based on the computation result at step S90. If the difference β satisfies the balance requirement, the correction information includes, the weight m1(m1=mm) of the balancer40to be used, the distance r1(r1=rm) between the rotational center of the rotary table30to the attachment position of the balancer40, and the arrangement angle θ1(θ1=αm) of the attachment groove36(attachment region) to which the balancer40should be attached. As shown inFIG. 18, the display170displays the correction information on a display screen172based on the display signal. The main control section110outputs a control signal to the display170such that the arrangement of the balancer40with respect to the rotary table30is displayed on the display screen172. After displaying the correction information on the display170, the main control section110ends the procedure ofFIG. 5.

The operator prepares the balancer40having the weight m1displayed on the display screen172. The attachment groove36to which the balancer40should be attached is arranged in the vicinity of or facing the machine door510. Therefore, at the operation area Ar, the operator inserts the bolt37to the attachment groove36, and attaches the balancer40to a portion of the bolt37protruding from the attachment groove36. After moving the balancer40along the attachment groove36to obtain the distance r1displayed on the display screen172, the operator tightens the nut44to the bolt37to secure the balancer40to the rotary table30.

If the difference β does not satisfy the balance requirement, the main control section110causes the display170to display that even if the balancer is attached, the rotational imbalance cannot be corrected, and the workpiece W must be reattached.

According to the multi-tasking machine10configured as described above, the main control section110, which functions as a vibration detection section, detects vibration of the rotary table30during rotation of the rotary table30. Based on the detected vibration, the weight M of the workpiece, and the physical quantity (the rotation speed N and the angular speed ω) representing the rotation state of the rotary table30, the main control section110, which functions as an arrangement position computing section, computes the arrangement position (the eccentricity amount R and the eccentricity angle θ) of the workpiece W with respect to the rotary table30. The main control section, which functions as an attachment position computing section, computes the attachment position (the balancer attachment angle (θ+π)) of the balancer40with respect to the rotary table30based on the computed eccentricity angle θ. Therefore, the attachment position of the balancer40with respect to the rotary table30is easily grasped. Since the main control section110, which functions as a selecting section, selects one of the attachment grooves36that has the arrangement angle αmclosest to the balancer attachment angle (θ+π), the vibration of the rotary table30is easily suppressed by attaching the balancer40to the selected one of the attachment grooves36.

In the first embodiment, the main control section110, which functions as a weight computing section, computes the weightmof the balancer40to be used based on, for example, the eccentricity amount R. Furthermore, the weight mmis displayed on the display170. Therefore, the operator easily grasps the weight of the balancer40to be used. Since the arrangement angle αmof the attachment groove36to which the balancer40should be attached is displayed on the display170, the operator easily grasps the attachment groove36to which the balancer40is to be attached. Therefore, the operator can easily and properly attach the balancer40to the attachment groove36following the information displayed on the display170.

In addition, the multi-tasking machine10of the first embodiment has the operation area Ar for the operator at part of the surrounding area of the rotary table30. When the rotary table30is stopped, the attachment groove36closest to the ideal balancer attachment angle (θ+π) is arranged opposite to the operation area Ar. Therefore, the operator can easily attach the balancer40to the attachment groove36located opposite to the operation area Ar.

A second embodiment of the present invention will now be described with reference toFIGS. 19 and 20centered on the difference from the first embodiment.

The second embodiment differs from the first embodiment in the processes of steps S90and S100ofFIG. 5, that is, the procedure for correcting the rotational imbalance of the rotary table30.

In the first embodiment, the single balancer40is attached to the rotary table30to correct the rotational imbalance of the rotary table30. In this case, the vibration of the rotary table30is suppressed to be within the permissible value, but the vibration cannot be made zero in theory. Furthermore, if the difference β between the ideal attachment angle (θ+π) of the balancer40and the arrangement angle αmclosest to the ideal attachment angle (θ+π) does not satisfy the predetermined balance requirement (−π/3<β<π/3), the vibration cannot be suppressed.

Contrastingly, in the second embodiment, two balancers40are attached to the attachment positions Pm, Pm+1having the arrangement angles αm, αm+1, that satisfy the following equation (9).
αm<(θ+π)≦αm+1(9)

Assuming that the distances between the rotational center (C-axis) of the rotary table30and the attachment positions Pm, Pm+1are rm, rm+1, and the weight of the balancers40corresponding to the attachment positions Pm, Pm+1are mm, mm+1, the following equation (10) is satisfied in the X-axis direction and the following equation (11) is satisfied in the Y-axis direction due to the balance of moment as shown inFIG. 19. The values of the distances rmand rm+1are stored in the system program memory130in advance.
MRcos θ+mmrmcos αm+mm+1rm+1cos αm+1=0  (10)
i MR sin θ+mmrmsin αm+mm+1rm+1sin αm+1=0  (11)

The following equations (12) and (13) are obtained from the equations (10) and (11).

In the second embodiment, at step S90ofFIG. 5, the main control section110selects two attachment positions Pm, Pm+1having the arrangement angles αm, αm+1that are adjacent to each other with the balancer attachment angle (θ+π) in between. More specifically, the main control section110computes, in the same manner as the first embodiment, the difference β between each of the arrangement angles αn(N=0 to 11) and the ideal balancer attachment angle (θ+π). The main control section110then selects, as the arrangement angle αm, one of the arrangement angles αnsmaller than the balancer attachment angle (θ+π) that causes the absolute value of the difference β to be the minimum, and as the arrangement angle αm+1, one of the arrangement angles αngreater than the balancer attachment angle (θ+π) that causes the absolute value of the difference β to be the minimum. The main control section110then selects the attachment positions Pm, Pm+1corresponding to the arrangement angles αm, αm+1.

The main control section110computes the weights mm, mm+1of the balancers40corresponding to the attachment positions Pm, Pm+1using the above equations (12) and (13). In the second embodiment also, in the same manner as the first embodiment, the main control section110performs computation of the eccentricity amount R, computation of the eccentricity angle θ, and computation of the balancer attachment angle (θ+π).

In the second embodiment, at step S100ofFIG. 5, the main control section110outputs a display signal representing the correction information of the rotational imbalance of the rotary table30to the display170based on the computation results at step S90. The correction information includes the weights m1(m1=mm), m2(m2=mm+1) of the two balancers40to be used, the distances r1(r1=rm), r2(r2=rm+1) from the rotational center of the rotary table30to the attachment positions of the balancers40, and the arrangement angles θ1(θ1=αm), θ2(θ2=αm+1) of the two attachment grooves36(attachment regions) to which the balancers40should be attached. As shown inFIG. 20, the display170displays the correction information on the display screen172based on the display signal. The main control section110also outputs a control signal to the display170such that the arrangement of the two balancers40on the rotary table30is displayed on the display screen172.

The operator prepares the two balancers40having the weights mm, mm+1displayed on display screen172. The two attachment grooves36to which the balancers40should be attached are arranged in the vicinity of or facing the machine door510. Therefore, at the operation area Ar, the operator inserts the bolt37to each of the attachment grooves36and attaches each balancer40to a portion of the bolt37protruding from the corresponding attachment groove36. After moving the balancers40along the attachment grooves36to obtain the distances r1, r2displayed on the display screen172, the operator tightens the nut44to each bolt37to secure each balancer40to the rotary table30.

According to the second embodiment, the main control section110selects one of the attachment grooves36having the arrangement angle αm+1that is greater than and is closest to the balancer attachment angle (θ+π), and one of the attachment grooves36having the arrangement angle αmthat is smaller than and is closest to the balancer attachment angle (θ+π). The main control section110displays the selection results on the display170. Therefore, the operator can easily and properly attach the balancers40to the two attachment grooves36in accordance with the information displayed on the display170. As a result, the vibration of the rotary table30is eliminated substantially.

The present invention is not restricted to the illustrated embodiments but may be embodied in the following modifications.

In each of the above embodiments, the present invention need not be applied to the multi-tasking machine, but may be embodied in various types of machine tools equipped with the rotary table. The rotary table30is not limited to one that moves in a uniaxial direction, but-may be one that moves in a biaxial direction of X-axis and Y-axis.

At step S60ofFIG. 5, the main control section110determines whether the number of rotations of the rotary table30has reached the predetermined determination number of times. However, instead of this, the main control section110may determine whether a determination time corresponding to the determination number of times has elapsed. In this case, a timer for measuring time is used. The determination time is computed using, for example, the following expression.
Determination Time [ms]=(determination number of times)×60000/(target rotation speed [min−1] of rotary table)

The balancer attachment portion provided on the rotary table30is not limited to the attachment groove36, but may be provided in any form as long as the balancer40can be attached.

In each of the above embodiments, the balancer may be attached to the rotary table30using a balancer automatic attachment apparatus (not shown). The balancer automatic attachment apparatus is provided, for example, at the operation area Ar. In the first embodiment, for example, the main control section110outputs, to the balancer automatic attachment apparatus, the correction information including the weight m1of the balancer40to be used, the distance r1from the rotational center of the rotary table30to the attachment position of the balancer40, and the arrangement angle θ1of attachment groove36to which the balancer40should be attached. The balancer automatic attachment apparatus selects and attaches the balancer40based on the entered correction information. That is, the balancer automatic attachment apparatus selects the balancer40having the instructed weight m1and attaches the selected balancer40to the attachment groove36located at the instructed arrangement angle θ at the instructed distance r1.