Machine tool

Abstract

A machine tool (100) that performs a turning process by rotating a workpiece W comprises: a workpiece rotating device (106) having a workpiece attachment unit (120) for attaching the workpiece and a rotary motor (Mb) for rotating the workpiece attachment unit; a control device (140) for calculating an allowable rotational speed of the workpiece rotating device on the basis of the amount of variation in the torque of the rotary motor when the workpiece rotating device to which the workpiece has been attached is rotated; and a display device (142) for displaying the allowable rotational speed of the workpiece rotating device calculated by the control device.

Description

machine tool

[0001] The present invention relates to a machine tool that calculates the allowable rotation speed of a rotating device based on the amount of change in torque of a rotary motor when the rotating device, to which a workpiece is attached, is rotated.

[0002] When a workpiece is attached to a rotating device, rotated, and turned with a turning tool, vibrations can occur due to an unbalanced load acting on the rotating device. To prevent this, the invention in Patent Document 1 describes a method for correcting torque imbalance in a workpiece, which determines the imbalance from the torque change of a drive motor when the workpiece is rotated, and specifies the mass and position of a counterweight to be added to a balance table, or the cutting mass and cutting position, in order to correct the imbalance.

[0003] Japanese Patent Application Laid-Open No. 2007-061918

[0004] The invention of Patent Document 1 can detect torque imbalance and rotation angle and correct the imbalance, but it cannot know what rotation speed is necessary to perform machining with sufficient machining accuracy when actually performing turning.

[0005] The present invention has as its technical object to solve the problems of the prior art, and to provide a machine tool that notifies the operator of the maximum command rotation speed or allowable rotation speed at which machining errors caused by vibrations due to unbalanced loads do not exceed the allowable range.

[0006] According to the present invention, there is provided a machine tool that rotates a workpiece to perform turning, comprising a workpiece rotation device having a workpiece mounting section for mounting the workpiece and a rotary motor for rotating the workpiece mounting section, a control device that calculates the allowable rotation speed of the workpiece rotation device based on the amount of change in torque of the rotary motor when the workpiece rotation device with a workpiece mounted thereon is rotated, and a display device that displays the allowable rotation speed of the workpiece rotation device calculated by the control device.

[0007] According to the present invention, when a workpiece is attached to a workpiece rotating device, an operator can know, prior to machining, the allowable rotation speed of the workpiece rotating device that will keep machining errors caused by vibrations due to unbalanced loads within an allowable range.

[0008] FIG. 1 is a schematic side view showing an example of a machine tool to which the present invention is applied, showing a rotary table in a horizontal position; FIG. 2 is a schematic side view showing an example of a machine tool to which the present invention is applied, showing a rotary table in a vertical position; FIG. 3 is a diagram for explaining a method of measuring an unbalanced load on a rotary table; FIG. 4 is a diagram showing an example of a screen displaying the unbalanced load measurement results and the maximum command rotation speed displayed on a display device; FIG. 5 is a diagram showing an example of a screen displaying the unbalanced load measurement results and the maximum command rotation speed displayed on a display device; FIG. 6 is a diagram showing an example of a screen displaying the unbalanced load measurement results and the maximum command rotation speed displayed on a display device;

[0009] A preferred embodiment of the present invention will now be described with reference to the accompanying drawings. Figures 1 and 2 are schematic side views of a machine tool as an example of a machine to which the present invention can be applied. Figure 1 shows the rotary table in a horizontal position, and Figure 2 shows the rotary table in a vertical position. In Figures 1 and 2, the machine tool 100 constitutes a horizontal machining center and includes a bed 102 as a base fixed to the factory floor. On the upper surface of the front portion (left side in Figure 1) of the bed 102, a table 106 as a work rotation device for rotating an attached workpiece W is provided so as to be movable in the front-to-rear direction or the Z-axis direction (left-to-right direction in Figure 1).

[0010] A column 104 is mounted on the upper surface of the bed 102 at the rear end side (right side in FIG. 1) of the bed 102 so as to be movable in the X-axis direction (a direction perpendicular to the plane of the paper in FIG. 1). A Y-axis slider 108 is mounted on the front surface of the column 104 so as to be movable in the vertical direction or the Y-axis direction. A spindle head 110 is mounted on the Y-axis slider 108. The spindle head 110 supports a spindle 112 so as to be rotatable about a horizontal rotation axis Os. The spindle head 110 has a spindle servo motor Ms that rotates and drives the spindle 112.

[0011] The column 104 is guided in the X-axis direction by an inclined linear guide 130. The inclined linear guide 130 faces forward and upward as a whole, and is inclined so that the rear portion is positioned higher than the front portion. Specifically, the inclined linear guide 130 includes a front rail 134a and a rear rail 134b as X-axis guide rails, and a front block 132a and a rear block 132b are fixed to the underside of the column 104, sliding along the front rail 134a and the rear rail 134b.

[0012] A ball screw (not shown) extending in the X-axis direction and an X-axis servo motor Mx connected to one end of the ball screw are attached to bed 102 as an X-axis feed device that drives column 104 reciprocally along X-axis guide rails 134a and 134b, and a nut (not shown) that engages with the ball screw is attached to column 104. Also attached to bed 102 is an X-axis scale (not shown) that detects the coordinate position of column 104 in the X-axis direction.

[0013] A pair of Y-axis guide rails (not shown) extending in the Y-axis direction are fixed to the front surface of the column 104, and the Y-axis slider 108 has a block that slides along the Y-axis guide rails. The column 104 is provided with a ball screw (not shown) extending in the Y-axis direction and a Y-axis servo motor My connected to one end of the ball screw as a Y-axis feed device that drives the Y-axis slider 108 back and forth along the Y-axis guide rails, and a nut (not shown) that engages with the ball screw is attached to the Y-axis slider 108. In addition, a Y-axis scale (not shown) that detects the coordinate position of the Y-axis slider 108 in the Y-axis direction is attached to the column 104.

[0014] The table 106 is provided on the bed 102 in front of the column 104. The table 106 includes a first table base 114 and a second table base 116. The first table base 114 is provided on the bed 102 and moves on the bed 102 in the Z-axis direction. More specifically, a pair of left and right Z-axis guide rails 122 (only the right rail is shown in FIGS. 1 and 2 ) extending in the Z-axis direction are fixed to the upper surface of the bed 102, and a block (not shown) that slides along the Z-axis guide rails 122 is fixed to the first table base 114.

[0015] The bed 102 is provided with a ball screw (not shown) extending in the Z-axis direction and a Z-axis servo motor Mz connected to one end of the ball screw as a Z-axis feed device that drives the first table base 114 back and forth along the Z-axis guide rail 122, and a nut (not shown) that engages with the ball screw is attached to the first table base 114. The bed 102 is also provided with a Z-axis scale (not shown) that detects the coordinate position of the first table base 114 in the Z-axis direction.

[0016] With regard to directions in the machine tool 100 according to this embodiment, the rotation axis Os of the spindle 112 extends horizontally, and the direction parallel to the rotation axis Os is defined as the Z-axis direction (also referred to as the front-to-rear direction). The direction in which the spindle 112 protrudes along the Z-axis direction (leftward in FIGS. 1 and 2) is the front, and the opposite direction (rightward in FIGS. 1 and 2) is the rear. The horizontal direction perpendicular to the Z-axis direction (the direction perpendicular to the paper surface of FIGS. 1 and 2) is defined as the X-axis direction (left-right direction), and the vertical direction is defined as the Y-axis direction (up-down direction).

[0017] The first table base 114 has an inclined surface 114a that is inclined with respect to the direction of movement of the table 106. Specifically, the inclined surface 114a is inclined at an angle θ=45° from the horizontal plane so as to face backward and upward.

[0018] The first table base 114 has an inclined rotary guide 118 along an inclined surface 114a. The inclined rotary guide 118 faces rearward and upward, with the front portion positioned higher than the rear portion. The inclined rotary guide 118 rotates the second table base 116 about an inclined axis Oc that is perpendicular to the inclined surface 114a (i.e., perpendicular to the inclined rotary guide 118). The rotational feed direction of the second table base 116 is defined as the C-axis direction. The inclined rotary guide 118 can have, for example, a cross-roller bearing.

[0019] The first table base 114 has a built-in C-axis servo motor Mc that rotates the second table base 116 around the tilt axis Oc. The first table base 114 is equipped with a C-axis detection device that detects the coordinate (rotational position) of the second table base 116 in the C-axis direction. The C-axis detection device can be formed by, for example, a rotary encoder 124 attached to the C-axis servo motor Mc.

[0020] The second table base 116 is mounted on an inclined rotary guide 118. The second table base 116 has a swivel surface 116a. The swivel surface 116a is inclined at an angle of 45° with respect to the inclined surface 114a. The swivel surface 116a swivels around the tilt axis Oc as the second table base 116 rotates.

[0021] The second table base 116 has a rotary table 120 that is rotatably mounted along a swivel plane 116a. The rotary table 120 is supported by the second table base 116 so as to be rotatable about a variable axis Ob that is perpendicular to the swivel plane 116a. It will be understood from reference to Figures 1 and 2 that the orientation of the variable axis Ob changes depending on the position of the second table base 116 in the C-axis direction. The rotational feed direction of the rotary table 120 about the variable axis Ob is defined as the B-axis direction.

[0022] The second table base 116 has a built-in B-axis servo motor Mb that rotates a rotary table 120, which serves as a workpiece mounting portion for mounting a workpiece W, about a variable axis Ob. The second table base 116 is equipped with a B-axis detection device that detects the coordinate (rotational position) of the rotary table 120 in the B-axis direction. The B-axis detection device can be formed, for example, by a rotary encoder 126 attached to the B-axis servo motor Mb.

[0023] The rotary table 120 has a workpiece mounting surface 120a on which a workpiece W is mounted. The workpiece W can be mounted on the workpiece mounting surface 120a via a pallet (not shown). The workpiece W may also be mounted directly on the workpiece mounting surface 120a. The workpiece mounting surface 120a is positioned in any orientation depending on the position of the second table base 116 in the C-axis direction movable range. For example, in FIG. 1, the workpiece mounting surface 120a is in a horizontal orientation, and in FIG. 2, the workpiece mounting surface 120a is in a vertical orientation.

[0024] Machine tool 100 may be a machining center further equipped with a tool magazine (not shown) that stores a plurality of tools used in machining, an automatic tool changer (not shown) that changes tools between the tool magazine and spindle 112, and a coolant supply device (not shown) that supplies coolant to the machining area of ​​machine tool 100, and these peripheral devices are housed in a cover (not shown) together with machine tool 100. Machine tool 100 may also include a setup station (not shown) where an operator attaches workpiece W to a pallet, a pallet changer (not shown) that changes pallets between the setup station and table 106, and the like.

[0025] Machine tool 100 further includes a control device 140 that controls machine tool 100. Control device 140 can be configured from a computer and associated software including a CPU (Central Processing Unit), a memory device such as a RAM (Random Access Memory) or a ROM (Read Only Memory), a storage device such as a HDD (Hard Disk Drive) or an SSD (Solid State Drive), an input / output port, and a bidirectional bus that interconnects these.

[0026] The control device 140 can be configured in software as part of a machine control device that controls an NC device that controls the spindle servo motor Ms, X-axis servo motor Mx, Y-axis servo motor My, Z-axis servo motor Mz, B-axis servo motor Mb, and C-axis servo motor Mc, as well as the above-mentioned machine tool's associated equipment such as a tool magazine (not shown), an automatic tool changer (not shown), a pallet changer, and a coolant supply device, as well as an oil / air supply device (not shown) and a compressed air supply device (not shown).

[0027] Machine tool 100 further includes a display device 142. As an example, display device 142 can be formed by a touch panel (not shown) attached to an operation panel (not shown) attached to the above-mentioned cover that surrounds machine tool 100. When display device 142 is formed by a touch panel, display device 142 also forms an input device with which an operator inputs numerical values ​​into control device 140 and selects an operation mode.

[0028] Control device 140 outputs current command values ​​to servo amplifier 144 in accordance with the NC program read and interpreted by the NC device. Servo amplifier 144 supplies current to each of spindle servo motor Ms, X-axis servo motor Mx, Y-axis servo motor My, Z-axis servo motor Mz, B-axis servo motor Mb, and C-axis servo motor Mc in accordance with the current command values ​​from control device 140. In this way, machine tool 100 machines workpiece W in accordance with the NC program.

[0029] The operation of this embodiment will be described below. Machine tool 100 has a rotary tool (not shown) attached to the tip of spindle 112, and while rotating spindle 112, moves table 106 and spindle 112 relatively in the three orthogonal axial directions of X, Y, and Z and in the rotational directions of B and C axes, thereby milling a workpiece W attached to rotary table 120 with the rotary tool. Machine tool 100 also has a cutting tool 128 attached to the tip of spindle 112, and while rotating rotary table 120 when rotary table 120 is in a horizontal position, moves table 106 and spindle 112 relatively in the three orthogonal axial directions of X, Y, and Z, thereby performing turning.

[0030] When turning the workpiece W, the rotary table 120 must rotate at a higher speed than when milling with a rotary tool. The rotation speed of the rotary table 120 during turning can be, for example, 100 to 800 rpm. A relatively heavy workpiece W may be attached to the rotary table 120, and the workpiece W may have a variety of shapes, particularly asymmetrical shapes. As a result, an unbalanced load acts on the bearings that support the rotary table 120 for rotation on the second table base 116, which can cause vibrations in the rotary table 120 when it rotates. Vibration of the rotary table 120 not only reduces the machining accuracy of the turning process, but can also damage tools and machines, making machining impossible in some cases.

[0031] Therefore, in the present invention, as described below, before machining, the workpiece W is attached to the rotary table 120, the workpiece attachment surface 120a is positioned in a vertical or inclined position, and the amount of change in torque of the B-axis servo motor Mb when the rotary table 120 is rotated is calculated, and the allowable rotational speed of the rotary table 120 is calculated based on the amount of change in torque.

[0032] 3 shows the change in torque of the B-axis servo motor Mb when the rotary table 120 is rotated. The torque of the B-axis servo motor Mb can be detected by detecting the current value output from the servo amplifier 144 to the B-axis servo motor Mb. Furthermore, the rotational position of the rotary table 120 in the B-axis direction or the rotational position of the B-axis servo motor Mb can be detected by the rotary encoder 124 provided on the B-axis servo motor Mb. In this way, the change in torque of the B-axis servo motor Mb can be obtained based on the current value supplied to the B-axis servo motor Mb and the rotational position of the B-axis servo motor Mb at that time.

[0033] FIG. 3 shows an example of torque changes in the B-axis servo motor Mb. The torque of the B-axis servo motor Mb is positive when rotated forward and negative when rotated reversely. When the workpiece mounting surface 120a is positioned vertically or inclined and rotated, the torque of the B-axis servo motor Mb changes in one cycle per rotation of the rotary table due to an unbalanced load. In FIG. 3, the torque change of the B-axis servo motor Mb is approximated by a sine wave curve. The vertical axis of the graph in FIG. 3 represents the torque (kgm) of the B-axis servo motor, and in this embodiment, the torque is measured with the B-axis servo motor rotated in the reverse direction, so it is a negative value.

[0034] In FIG. 3 , curve SC1 represents an example of the torque change of the B-axis servo motor Mb when the workpiece W is not attached to the rotary table 120. Curve SC2 represents an example of the torque change of the B-axis servo motor Mb when the workpiece W is attached to the rotary table 120. Curves SC1 and SC2 are obtained by fitting the model function T = α sin(θ - δ) + Ta using a curve fitting method such as the least squares method, where T is the measured torque value of the B-axis servo motor Mb and θ is the angular position relative to the B-axis origin around the tilt axis Ob. Here, α is the torque amplitude, δ is the phase shift relative to the B-axis origin, and Ta is the average value of the torque change of the B-axis servo motor Mb. In the example of FIG. 3 , when the workpiece W is attached to the rotary table 120 and rotated, the deviation from Ta on curve SC2 representing the torque change of the B-axis servo motor Mb is maximum (Dmax) when the angular position relative to the B-axis origin is θ = 120°.

[0035] The unbalanced load is measured automatically by executing an unbalanced load measurement program in the control device 140. When the unbalanced load measurement program is executed, a measurement screen 200 such as that shown in Fig. 4 is displayed on the display device 142. The measurement screen 200 includes a first machining condition display area 202 that displays the precision machining conditions, a second machining condition display area 204 that displays the rough machining conditions, an unbalance display area 206 that displays the magnitude (kgm) and direction (deg) of the unbalanced load torque, a counterweight display area 208 that displays the position and mass of the counterweight to be attached, a graphic area 210 that visually displays the position of the unbalanced load and the counterweight attachment position, and a start button 212.

[0036] After confirming that the workpiece W is attached to the rotary table 120, the operator taps the start button 212, which causes the control device 140 to move the first table base 114 to a predetermined Z position (measurement position) and rotate the second table base 116 in the C-axis direction around the tilt axis Oc. When the rotary table 120 moves to the vertical position shown in FIG. 2, the control device 140 stops the rotation of the rotary table 120. This causes the workpiece W attached to the rotary table 120 to be set in a direction that will fall due to gravity.

[0037] When the turntable 120 reaches the vertical position, which is the gravity drop position, the control device 140 then rotates the turntable 120 at a predetermined measurement rotation speed, for example 10 rpm, and reads the position of the turntable 120 in the B-axis direction, i.e., the rotational position of the turntable 120 around the variable axis Ob, from a signal from the rotary encoder 126 of the B-axis servo motor Mb. The control device 140 also accesses the servo amplifier 144 to read the current value supplied from the servo amplifier 144 to the B-axis servo motor Mb, and by correlating these values, calculates the angular position θmax around the variable axis Ob that gives the maximum unbalanced load Dmax.

[0038] When the measurement is completed, the unbalanced load value α and angular position θmax determined by the calculation are displayed in the unbalance display area 206 (see FIG. 5). In the example shown in FIG. 5, the unbalance display area 206 displays the unbalanced load value α=3.5 kgm and the angular position θmax (unbalance direction)=235°.

[0039] The control device 140 also calculates the maximum command rotation speed or allowable rotation speed as a precision machining condition, and the maximum command rotation speed or allowable rotation speed as a rough machining condition. As shown below, the control device 140 stores in advance machine-specific threshold values ​​A(i) based on actual measurement values ​​of vibration amplitude allowable as a precision machining condition, in association with a plurality of rotation speeds, and determines the maximum command rotation speed based on the above calculation results so that the value of α×A does not exceed A(i). Here, α is the amplitude of the B-axis motor torque, and A is a machine-specific coefficient that represents the susceptibility to vibration calculated based on the machine rigidity. 800 rpm: A(9) 700 rpm: A(8) 600 rpm: A(7) 500 rpm: A(6) 400 rpm: A(5) 300 rpm: A(4) 200 rpm: A(3) 100 rpm: A(2) 0 rpm: A(1)

[0040] Similarly, the control device 140 stores in advance machine-specific threshold values ​​B(i) based on actual measured values ​​of vibration amplitudes allowed as rough machining conditions, in association with a plurality of rotational speeds, and determines the maximum command rotational speed based on the above calculation results so that the value of α×A does not exceed B(i). 800 rpm: B(9) 700 rpm: B(8) 600 rpm: B(7) 500 rpm: B(6) 400 rpm: B(5) 300 rpm: B(4) 200 rpm: B(3) 100 rpm: B(2) 0 rpm: B(1)

[0041] In the above example, the control device 140 displays A(i) and B(i) for 0 rpm, 100 rpm, 200 rpm, 300 rpm, 400 rpm, 500 rpm, 600 rpm, 700 rpm, and 800 rpm. However, the control device 140 may select a larger or smaller number of rotational speeds. Alternatively, the control device 140 may interpolate A(i) or B(i) values ​​for a rotational speed between the plurality of rotational speeds, for example, 750 rpm, which is between 700 rpm and 800 rpm as shown in FIG. 6, from A(i) or B(i) values ​​for 700 rpm and 800 rpm. As an example, in FIG. 5, a maximum command rotational speed of 500 rpm as a precision machining condition is displayed in the first machining condition display area 202, and a maximum command rotational speed of 750 rpm as a rough machining condition is displayed in the second machining condition display area 204.

[0042] At the same time, the control device 140 displays 55° as the direction in which to attach the counterweight in the counterweight display area 208, and displays an icon 214 indicating the direction of imbalance and an icon 216 indicating the direction in which to attach the counterweight in the graphic area 210. The direction in which to attach the counterweight is the diagonal direction φ of the angular position θmax that gives the maximum unbalanced load Dmax, and is calculated as φ = θmax + 180°.

[0043] Because the workpiece W is attached to the rotary table 120, the position where the counterweight should be attached cannot be freely determined. Therefore, the counterweight display area 208 and the graphic area 210 indicate the direction in which the counterweight should be attached. Based on the measurement results of the unbalanced load, the operator selects an appropriate position on the workpiece attachment surface 120a of the rotary table 120 along the φ direction where the counterweight can be attached, and enters the distance r of that position from the center of rotation of the rotary table 120 into an input dialog box 218 in the counterweight display area 208 (see FIG. 6 ). This input operation can be performed, for example, using a keyboard disposed on the operation panel. Alternatively, when the operator taps the input dialog box 218 on the touch panel constituting the display device 142, a numeric keypad (not shown) may be displayed on the touch panel, and the operator may tap the numeric keypad to enter information.

[0044] As an example, Fig. 6 shows that the operator has input 200 mm into the input dialog box 218. If the mass of the counterweight is M, then the mass M of the counterweight can be calculated from M = α / r. In Fig. 6, as a result of the operator inputting 200 mm (r = 0.2 m), the control device 140 displays 17.5 kg in the counterweight display area 208 as the mass M of the counterweight to be attached to the rotary table 120 based on M = α / r.

[0045] At this time, if the desired rotation speed of the rotary table 120 is lower than the maximum command rotation speed displayed in the first machining condition display area 202 or the maximum command rotation speed displayed in the second machining condition display area 204, the desired rotation speed can be input to the control device 140 to perform precision turning or rough turning. The precision turning or rough turning is performed in accordance with the machining program. If the desired rotation speed of the rotary table 120 is higher than the maximum command rotation speed, the operator can attach a counterweight to the rotary table 120 and perform the unbalanced load measurement process and the maximum command rotation speed calculation process again.

[0046] When the operator attaches a counterweight with a mass M of 17.5 kg to the workpiece mounting surface 120 a of the rotary table 120 at a position φ=55° and r=0.2 m and taps the start button 212, the control device 140 again executes the above-described unbalanced load measurement process in accordance with the unbalanced load measurement program. Referring to FIG. 7 , the unbalanced load measurement results with the counterweight attached are shown. In FIG. 7 , the unbalanced load display area 206 displays the unbalanced load value α=0.2 kgm and the angular position θmax (unbalance direction)=235°. A maximum command rotation speed of 800 rpm, which is a precision machining condition, is displayed in the first machining condition display area 202, and a maximum command rotation speed of 800 rpm, which is a rough machining condition, is displayed in the second machining condition display area 204.

[0047] If the desired rotation speed of the rotary table 120 is lower than the maximum command rotation speed displayed in the first machining condition display area 202 or the maximum command rotation speed displayed in the second machining condition display area 204, the operator inputs the desired rotation speed into the control device 140 to perform precision turning or rough turning. The precision turning or rough turning is performed in accordance with the machining program.

[0048] According to this embodiment, when the workpiece W is attached to the rotary table 120, the operator can know the direction in which the unbalanced load will occur, the magnitude of the unbalanced load torque, the maximum command rotational speed under the precision machining conditions and the rough machining conditions, and the direction in which the counterweight should be attached and the mass, and further can know the direction in which the unbalanced load will occur when the counterweight is actually attached, the magnitude of the unbalanced load torque, the maximum command rotational speed under the precision machining conditions and the rough machining conditions, and the direction in which the counterweight should be attached and the mass. By knowing the maximum command rotational speed under the precision machining conditions and the rough machining conditions, the operator can easily decide whether to continue machining as is or to correct the unbalanced load.

[0049] In the above-described embodiment, the unbalanced load is measured with the rotary table 120 positioned at a vertical position where the effect of the unbalanced load is greatest. However, the position of the rotary table 120 during unbalanced load measurement is not limited to this. The machine tool 100 can also perform turning by rotating the rotary table 120 with the rotary table 120 positioned at any tilt position between the horizontal and vertical positions. Therefore, the unbalanced load measurement process and the calculation of the maximum command rotational speed may be performed with the second table base 116 rotated to position the rotary table 120 at a desired tilt position. In other words, the unbalanced load measurement process and the calculation of the maximum command rotational speed can be performed by positioning the rotary table 120 at the tilt angle that will be used during actual turning.

[0050] Furthermore, if the rotation speed of the rotary table 120 input by the operator to the control device 140 is higher than the maximum command rotation speed, an alarm may be sounded or a warning light (not shown) may be turned on.

[0051] 100 Machine tool 102 Bed 104 Column 106 Table 108 Y-axis slider 110 Spindle head 112 Spindle 114 First table base 114a Inclined surface 116 Second table base 116a Swivel surface 120 Rotary table 120a Workpiece mounting surface 122 Z-axis guide rail 124 Rotary encoder 126 Rotary encoder 128 Tool bit 132a Front block 132b Rear block 134a X-axis guide rail 134b X-axis guide rail 140 Control device 142 Display device 144 Servo amplifier 200 Measurement screen 202 First machining condition display area 204 Second machining condition display area 206 Unbalance display area 208 Counterweight display area 210 Graphics area 212 Start button 214 Icon 216 Input dialog box Mb B-axis servo motor Mc C-axis servo motor Ms Main axis servo motor Mx X-axis servo motor My Y-axis servo motor Mz Z-axis servo motor

Claims

1. A machine tool which performs turning by rotating a workpiece, comprising: a workpiece rotating device having a workpiece mounting section for mounting a workpiece and a rotary motor for rotating the workpiece mounting section; a control device which calculates an allowable rotation speed of the workpiece rotating device based on an amount of change in torque of the rotary motor when the workpiece rotating device with a workpiece mounted is rotated; and a display device which displays the allowable rotation speed of the workpiece rotating device calculated by the control device.

2. A machine tool as described in claim 1, wherein the control device calculates an allowable rotational speed for precision machining and an allowable rotational speed for rough machining, and displays the allowable rotational speed for precision machining and the allowable rotational speed for rough machining on a display device.

3. A machine tool as described in claim 1, wherein the work mounting portion comprises a rotating table having a work mounting surface on which the work is mounted, the rotating table being capable of being positioned in a plurality of orientations between a horizontal position where the work mounting surface is horizontal, and a vertical position where the work mounting surface is vertical, and wherein the rotating table is positioned in an inclined position or a vertical position to measure changes in the torque of the rotary motor.

4. A machine tool according to claim 3, wherein said rotary table is positioned at an inclination angle between said horizontal position and said vertical position when actually performing turning, and a change in torque of said rotary motor is measured.

5. A machine tool as described in claim 1, wherein the work rotation device comprises a first table base attached to the bed of the machine tool and a second table base attached to the first table base, the work mounting portion comprises a rotating table having a work mounting surface for mounting the work, the rotating table is rotatably mounted on the second table base, and the second table base is supported by the first table base so as to be rotatable about an inclination axis.

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