JP2009291916A - Machine tool - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a machine tool improved in machining quality by taking account of a conventionally unconsidered tool spring. <P>SOLUTION: The machine tool is provided with: a magnetic bearing control means for controlling a plurality of sets of magnetic bearings 13, 14 and 15 for supporting a rotation axis 11 without contact; and a machining control means 30 for controlling a cut amount and feed speed of a grinding wheel 2. The magnetic bearing control means has a machining resistance calculating means 42 for calculating a cutting resistance in real time from control current. The machining control means 30 controls the feed speed to make the cutting resistance constant. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a machine tool such as an internal grinding machine.

  As a machine tool such as a grinding machine, a rotating shaft on which a tool (for example, a grindstone) is mounted, a plurality of sets of magnetic bearings that support the rotating shaft in a non-contact manner, a magnetic bearing control means for controlling the magnetic bearing, and a cutting depth of the tool And a processing control means for controlling the feed rate are known (Patent Document 1).

In such a machine tool, the rotary shaft is supported by radial magnetic bearings arranged at two locations in the axial direction, and so-called bearing springs are formed.
Japanese Patent Laid-Open No. 9-108991

  In machine tools, when a workpiece is machined with a tool, a tool spring is formed between the tools, the spring constant changes before and during machining, and the resonance frequency of the spring-rotary system changes. To do. However, conventionally, such a tool spring is not taken into consideration, and the control constant of the radial magnetic bearing is adjusted and set in the state without the tool spring. The contribution of the tool spring to the bearing spring is relatively large, and the variation range of the resonance frequency accompanying machining is large. As a result, the control constant adjusted and set without the tool spring is not optimal, There is a possibility that the processing quality is lowered such as resonance.

  An object of the present invention is to provide a machine tool with improved machining quality by considering a tool spring that has not been considered in the past.

  A machine tool according to the present invention controls a rotating shaft on which a tool is mounted, a plurality of sets of magnetic bearings that support the rotating shaft in a non-contact manner, magnetic bearing control means for controlling the magnetic bearing, and a cutting depth and feed speed of the tool. In the machine tool provided with the machining control means, the magnetic bearing control means has machining resistance calculation means for calculating the machining resistance in real time from the control current, and the machining control means has a constant machining resistance. Thus, the feed rate is controlled as described above.

  In order to prevent degradation of the machining quality, it is preferable to keep the tool spring constant in consideration of the tool spring when setting the control constant of the radial magnetic bearing, but the tool spring changes from moment to moment. It is difficult to control. On the other hand, since it is estimated that the tool spring depends on the machining resistance (cutting resistance) or the feed speed, “machining the tool spring to be constant” means that the feed speed is set so that the machining resistance is constant. In other words, “control”.

  The magnetic bearing control means monitors the control current and processes each component (normal force Fn, tangential force Ft, and axial force) of machining resistance (cutting resistance, grinding resistance, etc.) acting on the tool (for example, a grindstone). The force Fz) is calculated in real time, and the machining control means controls the feed rate so that the machining resistance becomes constant. In order to control the feed speed so that the machining resistance is constant, for example, a relational expression between the feed speed and each component (Fn, Ft and Fz) of the machining resistance is obtained in advance, and based on this relational expression. What is necessary is just to make it change a feed rate.

  The machine tool includes a spindle device in which a grindstone (or other tool) is attached to the tip of a rotating shaft that is rotationally driven, such as an internal grinding machine. The spindle device includes, for example, a casing, a rotating shaft disposed horizontally or vertically in the casing, one controlled axial magnetic bearing that supports the rotating shaft in a non-contact manner, and two controlled radial magnetic bearings in the front and rear or top and bottom, and One axial displacement sensor for detecting the axial position of the rotating shaft and two radial displacement sensors for detecting the position of the rotating shaft in the radial direction are provided. Here, each magnetic bearing and each displacement sensor are publicly known, and usually each radial magnetic bearing is composed of an X-axis direction magnetic bearing and a Y-axis direction magnetic bearing, and each radial displacement sensor is X It consists of an axial direction displacement sensor and a Y-axis direction displacement sensor.

  According to the machine tool of this invention, the feed rate is controlled so that the machining resistance is constant, so that the tool spring can be approximately constant, and resonance caused by the change of the tool spring does not occur. Therefore, deterioration of processing quality is prevented.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 shows a machine tool according to the invention.

  This machine tool is an internal grinding machine, and a magnetic bearing spindle (1) with a grindstone (tool) (2) for grinding the inner surface of the workpiece (W), and a workpiece (W) whose inner surface is the surface to be ground And a work holding base (3) for holding and rotating, and a machining control means (30) for controlling the cutting amount and feed speed of the grindstone (2).

  Although not shown, the machining control means (30) is a spindle moving means for moving the magnetic bearing spindle (1) in the axial direction, and moves the work holding base (3) relative to the magnetic bearing spindle (1). And a work holding table moving means for causing the grindstone (2) to perform a cutting operation on the work (W).

  The magnetic bearing spindle (1) is a vertical type in which a vertical axis-shaped rotating shaft (11) rotates inside a vertical cylindrical casing (10). The grindstone (2) is attached to the rotating shaft (11) via the quill (12). In the following description, the control axis (axial control axis) in the vertical axis direction (axial direction) of the rotating shaft (11) is the Z axis, and two horizontal radial directions (radial directions) orthogonal to the Z axis and orthogonal to each other. These control axes (radial control axes) are defined as an X axis and a Y axis.

  The machine body of the magnetic bearing spindle (1) has a set of control-type axial magnetic bearings (13) that support the rotary shaft (11) in a non-contact manner in the axial direction, and the rotary shaft (11) in a non-contact support in the radial direction. Two sets of upper and lower control type radial magnetic bearings (14), (15), one axial displacement sensor (16) for detecting the axial displacement of the rotating shaft (11), and the radial of the rotating shaft (11) Two sets of radial displacement sensors (17) and (18) for detecting displacement in the direction, built-in type electric motor (19) for rotating the rotating shaft (11) at high speed, and the shaft of the rotating shaft (11) Two sets of upper and lower sets that mechanically support the rotating shaft (11) when the rotating shaft (11) is not supported by the magnetic bearings (13), (14), and (15) by restricting the movable range in the radial and radial directions Protective bearings (20) and (21) for touchdown are provided.

  The controller part (magnetic bearing controller) of the magnetic bearing spindle (1) includes a sensor circuit (22), an AD converter (23), a DSP (digital signal processor) (24), a DA converter (25), an electromagnet A drive circuit (26) and an inverter (27) are provided.

  The axial magnetic bearing (13) is a pair of axial electromagnets (13a) (13b) arranged so as to sandwich a flange portion (12a) integrally formed at the lower portion of the rotating shaft (11) from both sides in the Z-axis direction. It has.

  The axial displacement sensor (16) is disposed so as to face the upper end surface of the rotating shaft (11) from the upper side in the Z-axis direction, and outputs a distance signal proportional to the distance (gap) from the lower end surface of the rotating shaft (11). Output.

  Two sets of radial magnetic bearings (14) and (15) are arranged in the vicinity of the upper and lower ends of the rotating shaft (11), and a motor is interposed between the upper radial magnetic bearing (14) and the axial magnetic bearing (13). (19) is arranged. Each radial magnetic bearing (14) (15) includes two pairs of radial electromagnets (14a) (15a) arranged so as to sandwich the rotating shaft (11) from both sides in the X-axis direction and from both sides in the Y-axis direction. ing.

  The upper radial displacement sensor (17) is located near the upper radial magnetic bearing (14), and the lower radial displacement sensor (18) is located near the lower radial magnetic bearing (7). The radial displacement sensors (17) and (18) each output a distance signal proportional to the distance from the outer peripheral surface of the rotating shaft (11).

  The protective bearings (20) and (21) are rolling ball bearings such as angular ball bearings, the outer ring of each protective bearing (20) and (21) is fixed to the casing (10), and the inner ring is fixed around the rotating shaft (11). It is arranged with a gap. The two sets of protective bearings (20) and (21) can both be supported in the radial direction, and at least one set can also be supported in the axial direction. In a state where the magnetic bearings (13), (14), and (15) are not operated due to operation stop or power failure, the rotating shaft (11) is supported by the protective bearings (20) and (21).

  The sensor circuit (22) drives each displacement sensor (16), (17), (18), and based on the output of each displacement sensor (16), (17), (18), the axial direction of the rotating shaft (11). The displacement and the displacement in the X-axis direction and the Y-axis direction in the upper and lower radial displacement sensors (17), (18) are calculated, and the displacement signal as a result of the calculation is converted into the DSP (24 through the AD converter (23). ).

  The DSP (24) controls the electromagnets (13a) (13b) (14a) (15a) of the magnetic bearings (13) (14) (15) based on the displacement signal input from the AD converter (20). The current value is obtained by PID control, and an excitation current signal obtained by adding the control current value to a constant steady current value is output to the electromagnet drive circuit (26) via the DA converter (25). Then, the drive circuit (26) is connected to the electromagnets (13a) (13b) (14a) (14) of the magnetic bearings (13) (14) (15) corresponding to the excitation current based on the excitation current signal from the DSP (24). 15a), whereby the rotary shaft (11) is brought into non-contact with a predetermined target position. The DSP (24) also outputs a rotation speed command signal for the motor (19) to the inverter (27), and the inverter (27) controls the rotation speed of the motor (19) based on this signal. As a result, the rotating shaft (11) is rotated at high speed by the motor (19) while being supported in a non-contact manner at the target position by the magnetic bearings (13), (14), and (15).

  In the magnetic bearing spindle (1), as shown in FIG. 2, the rotary shaft (11) is supported by radial magnetic bearings (14) and (15) arranged in two axial directions, so-called bearing springs are formed. Has been. When the workpiece (W) is machined with the tool (grinding stone (2)), a tool spring is formed between the grinding stone (2) and the workpiece (W). For this reason, the spring constant changes before and during processing, the resonance frequency of the spring-rotation system changes, and there is a possibility that the processing quality is reduced due to resonance. In order to prevent deterioration of the processing quality, it is preferable to keep this constant in consideration of the tool spring.

  In view of this, the DSP (24) is provided with grinding resistance (machining resistance) calculation means (42) in addition to various conventional control units. The grinding resistance calculation means (42) monitors each control current of the magnetic bearings (13), (14) and (15), and each component of the grinding resistance acting on the grinding wheel (2) (normal force Fn, tangential direction) The force Ft and the axial force Fz) are calculated in real time and output to the machining control means (30).

  FIG. 3 shows the configuration of the control means (41) for keeping the tool spring constant by using the grinding resistance obtained by the grinding resistance calculation means (42).

  In this figure, during processing, a spring-rotation system (31) including a bearing spring (32) and a tool spring (33) is formed. The bearing spring (32) is controlled by a control constant (34) for PID control. The tool spring (33) is defined and depends on the grinding resistance (35) and the feed rate (36). The control means (41) of the spring-rotation system (31) includes a grinding resistance calculation means (42) provided in the DSP (24) of the magnetic bearing control device and a tool provided in the machining control means (30). Spring characteristic storage means (43) and feed speed changing means (44).

  The tool spring characteristic storage means (43) stores the relationship between the feed speed and the grinding resistance related to the tool spring characteristics, and the feed speed changing means (44) stores the relationship between the feed speed and the grinding resistance. Based on this, the feed speed, that is, the axial and radial movement speed of the magnetic bearing spindle (1) is controlled.

  As described above, the grinding resistance (Fn, Ft and Fz) is calculated in real time in the grinding resistance calculation means (42), and the machining control means (30) adjusts the feed speed so that this grinding resistance is constant. Control. This control is feedback control, whereby the grinding resistance is kept substantially constant as schematically shown in the graph of FIG. Therefore, the tool spring (33) is approximately constant, and the change in the resonance frequency of the spring-rotation system (31) is suppressed.

  In order to control the feed speed so that the machining resistance becomes constant, for example, a relational expression between the feed speed and each component (Fn, Ft, and Fz) of the grinding resistance is obtained in advance for the workpiece (W) to be machined. This relational expression may be stored in the tool spring characteristic storage means (43).

  In the above description, the magnetic bearing spindle (1) has been described for an internal grinding machine, but it can also be applied to various machine tools and other devices other than grinding. In the above embodiment, the rotation axis (1) is shown as a vertical axis, but the rotation axis may be a horizontal axis.

FIG. 1 is a schematic configuration diagram of a main part showing an embodiment of a machine tool according to the present invention. FIG. 2 is a diagram schematically showing a spring-rotation system during processing. FIG. 3 is a block diagram showing the main part of the control means of the machine tool according to the present invention.

Explanation of symbols

(2) Whetstone (tool)
(11) Rotating shaft
(13) (14) (15) Magnetic bearing
(30) Machining control means
(42) Grinding resistance calculation means (machining resistance calculation means)

Claims (1)

A rotary shaft on which a tool is mounted, a plurality of sets of magnetic bearings that support the rotary shaft in a non-contact manner, a magnetic bearing control unit that controls the magnetic bearing, and a machining control unit that controls the cutting depth and feed rate of the tool Machine tools
The magnetic bearing control means has a machining resistance calculation means for calculating the machining resistance from the control current in real time, and the machining control means controls the feed rate so that the machining resistance becomes constant. machine.

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