Thermal deformation in machine tools

Abstract

A machine tool includes two parallel and spaced rotating machine shafts (32, 534) and at least one linear deformation sensor (78). The sensor is arranged to detect a change in distance between a first pair of reference points located on a first reference line on a machine base and to detect a change in distance between a second pair of reference points located on a second reference line on the machine base. The axis of rotation (36, 38) of the machine shaft lies in a longitudinal reference plane. The first reference line and the second reference line are transverse with respect to the first rotation axis and the second rotation axis, and are spaced apart in the direction of the first rotation axis and the second rotation axis, and the first pair of reference points and the second pair of reference points are located on the same side of the longitudinal reference plane and are spaced apart from the same side of the longitudinal reference plane.

Description

Technical Field

[0001] The present disclosure relates to thermal deformation of machine tools, and more particularly, to the measurement and compensation of such deformation. Background Art

[0002] In high-precision machining operations, thermal effects are becoming the main source of machining errors.

[0003] In a typical machine tool that includes multiple linear machine axes mounted on a machine bed, it can be difficult to accurately measure and compensate for deformation caused by localized heating of a machine area.

[0004] By way of example, Figure 1 The simulation results of a known grinding machine 2 are shown. The workpiece 4 is rotatably mounted between the headstock 6 and the tailstock 8. The grinding wheel 10 is mounted on a drive spindle 12. The drive spindle is supported on a linear guide 14 to facilitate linear movement of the drive spindle in a direction parallel to the longitudinal axis 16 of the workpiece. Figure 1 , a grinding wheel, a driving spindle is shown in two different positions 12 and 12' along its guideway (with the spindle carrying the grinding wheel 10' at position 12'), towards opposite ends of its range of travel along the guideway.

[0005] Figure 1 The temperature of different areas of the machine tool is shown in a graph, where the darkness of the shading increases with the temperature. Figure 1 The deformation of the machine tool shape due to local heating and expansion is exaggerated.

[0006] Figure 1 The temperature distribution shown represents a scenario where the grinding wheel at position 10' has machined a workpiece and is inputting heat into the area identified by oval 18. At the other end of the drive spindle's travel range, the machine bed has cooled after being heated when previously grinding in the area marked by oval 20 using the grinding wheel at position 10.

[0007] The continuous heating and cooling of different areas of a conventional machine tool bed during machining operations results in significant machine deformations in different directions, which can result in tens of microns of unwanted motion between the tool and the workpiece. The effects of these deformations are complex and vary with location in the machine, and these deformations may involve both bending and twisting of the machine bed. Typically, such machines use multiple linear machine axes extending in orthogonal directions and may be stacked one on top of another. These configurations further increase the asymmetry of the machine and the complexity and extent of thermal deformation.

[0008] These factors make it difficult to predict, measure and compensate for thermal deformation of the machine bed in these machines. Attempts have been made to compensate for thermal deformation in conventional machine tools by mounting temperature sensors on the machine bed in order to measure temperature differences and then adjust for the resulting errors in the position of the tool and workpiece. However, such methods have been found to be too difficult to implement with sufficient accuracy in practice. Summary of the invention

[0009] The present disclosure provides a machine tool for machining a workpiece, the machine tool comprising:

[0010] Machine base;

[0011] a first support provided by a first rotating machine axis rigidly mounted on the base in a position immovably fixed relative to the base and comprising a first drive operable to rotate the first support about a first rotation axis;

[0012] a second support provided by a second rotating machine shaft rigidly mounted on the base in a position immovably fixed relative to the base and including a second drive operable to rotate the second support about a second rotational axis, wherein the second rotational axis of the second rotating machine shaft is parallel to and laterally spaced from the first rotational axis of the first rotating machine shaft, and the first rotational axis and the second rotational axis lie in a longitudinal reference plane; and

[0013] at least one linear deformation sensor arranged to detect a change in the distance between a first pair of reference points on the machine base located on a first reference line and to detect a change in the distance between a second pair of reference points on the machine base located on a second reference line,

[0014] Wherein the first reference line and the second reference line are transverse relative to the first rotation axis and the second rotation axis and are spaced apart along the direction of the first rotation axis and the second rotation axis, and the first pair of reference points and the second pair of reference points are located on the same side of the longitudinal reference plane and are spaced apart from the same side of the longitudinal reference plane.

[0015] The inventors have recognised that this form of machine construction tends to deform in a less complex and more easily measured manner than a machine tool which uses a plurality of linear axes mounted on a machine base to move the tool and workpiece relative to each other. This is because the above-described machine construction employs driven rotary machine axes which rotate about axes which are in non-adjustably fixed positions relative to the machine bed. In a preferred embodiment, the two rotary machine axes may be the only driven machine axes mounted directly on the machine bed. It has been determined that the at least one deformation sensor constructed as described above may alone be sufficient to detect the major deformations which occur in such a machine construction during machining operations. The at least one sensor is capable of giving a measurement of these deformations which can then be effectively compensated for.

[0016] In a preferred example, the machine tool comprises:

[0017] The at least one linear deformation sensor is arranged to detect a change in the distance between a third pair of reference points on the machine base located on a third reference line, and to detect a change in the distance between a fourth pair of reference points on the machine base located on a fourth reference line,

[0018] Wherein the third reference line and the fourth reference line are transverse relative to the first rotation axis and the second rotation axis and are spaced apart along the direction of the first rotation axis and the second rotation axis, and the third pair of reference points and the fourth pair of reference points are spaced apart from the opposite side of the longitudinal reference plane from the first pair of reference points and the second pair of reference points.

[0019] Providing pairs of reference points on opposite sides of the machine base in this way facilitates measuring deformation with greater accuracy and allows detection of different components of the deformation. The pairs may individually enable detection of displacements of the rotational axes relative to each other about all three rotational degrees of freedom.

[0020] The machine tool may include:

[0021] A controller for controlling the first and second actuators and arranged to receive a signal from the at least one linear deformation sensor and to adjust control signals sent to the first and second actuators in response to the signal from the at least one linear deformation sensor during a machining operation so as to compensate for deformation of the machine base.

[0022] Thus, deformation compensation can be integrated into the controller of the machine so that positioning error correction can be incorporated into the control signals sent to the drive of the machine, which control signals govern the position of the components carried by each rotating machine axis and contacted by the drive of the machine (for example, such components may be tools such as grinding wheels and workpieces).

[0023] The reference line may be perpendicular to the first rotation axis and the second rotation axis. Preferably, the reference lines are parallel to each other. The reference line may extend parallel to the longitudinal reference plane of the machine tool.

[0024] The at least one linear deformation sensor may be mounted on the machine base. Alternatively, the position of the reference point on the machine base may be detected using at least one sensor mounted elsewhere on the machine tool or using at least one separate device that may be remote from the machine tool.

[0025] The linear deformation sensor is a device arranged to detect a change in the distance between a pair of points. The device may make a measurement by which, or generate a signal from which, such a change in distance may be calculated. For example, the sensor may comprise an electronic position sensor for sensing the position of a point in space or an electronic displacement sensor for sensing the displacement of one point relative to another point.

[0026] In some examples, the linear deformation sensor may include a light source, such as a laser light source. The position of the reference point in space may be determined by detecting light directly reflected from the reference point. For example, such an arrangement may be implemented using the RLD10 plane mirror interferometer sold by Renishaw.

[0027] The deformation sensor can be used in combination with a trackable device, which can include a reflector or a light sensor. The deformation sensor can be configured to determine the position of the trackable device mounted at a selected point on the machine axis and record the position. Laser tracker systems are sold, for example, by Hexagon Manufacturing Intelligence and Faro.

[0028] The deformation sensor may include a contact probe and may be capable of detecting the position of a reference point in space when the contact probe contacts a reference point on a machine base. The sensor may include an articulated arm on which the contact probe is mounted. Examples of suitable measuring arms are sold by Nikon, Trimos, Faro, and Hexagon Manufacturing Intelligence.

[0029] In a further preferred example, the first rotation axis and the second rotation axis are respectively located in a first reference plane and a second reference plane, the first reference plane and the second reference plane are parallel to each other and perpendicular to the longitudinal reference plane, and one reference point in each pair of reference points is located in one of the first reference plane and the second reference plane, and the other reference point in each pair of reference points is located in the other of the first reference plane and the second reference plane. Therefore, the position of the reference point is aligned with the rotation axis, and thus facilitates accurate measurement of its displacement.

[0030] Each linear deformation sensor may comprise: a rod having one end fixed relative to one of the respective pair of reference points; and a transducer arranged to generate an output signal in response to movement of the other end of the rod relative to the other of the respective pair of reference points. Alternatively, the or each linear deformation sensor may take other forms, such as a linear encoder, a laser-based measurement system, or a Bragg grating-based deformation detection technique (where deformation of the grating changes the wavelength of light reflected by the grating).

[0031] The present disclosure also provides a method for compensating for deformation of the machine base of a machine tool of the type to which the present disclosure relates, the method comprising the following steps:

[0032] (a) receiving a signal from the at least one linear deformation sensor; and

[0033] (b) adjusting control signals sent to the first actuator and the second actuator during a machining operation in response to the signal from the at least one linear deformation sensor to compensate for deformation of the machine base.

[0034] The present disclosure also provides a method for detecting deformation of a machine base of a machine tool for machining a workpiece, the machine tool comprising:

[0035] Machine base;

[0036] a first support provided by a first rotating machine axis rigidly mounted on the base in a position immovably fixed relative to the base and comprising a first drive operable to rotate the first support about a first rotation axis; and

[0037] a second support provided by a second rotating machine shaft rigidly mounted on the base in a position immovably fixed relative to the base and comprising a second drive operable to rotate the second support about a second rotational axis, wherein the second rotational axis of the second rotating machine shaft is parallel to and laterally spaced from the first rotational axis of the first rotating machine shaft, and the first rotational axis and the second rotational axis lie in a longitudinal reference plane,

[0038] The method comprises the following steps:

[0039] detecting a change in a distance between a first pair of reference points on a base of the machine located on a first reference line using at least one linear deformation sensor; and

[0040] detecting a change in the distance between a second pair of reference points on the machine base located on a second reference line using the at least one linear deformation sensor,

[0041] The first reference line and the second reference line are transverse relative to the first rotation axis and the second rotation axis and are spaced apart along the direction of the first rotation axis and the second rotation axis, and the first pair of reference points and the second pair of reference points are located on the same side of the longitudinal reference plane and are spaced apart from the same side of the longitudinal reference plane.

[0042] The at least one electronic linear deformation sensor may then determine or generate an output signal which may be used by an electronic processor to determine changes in the distances between pairs of reference points. These changes may then be used by another (or the) electronic processor to calculate the deformation of the machine base and enable compensation of the calculated deformation. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The present disclosure describes the existing machine tool structure and examples of the present disclosure with reference to the attached schematic diagrams, wherein:

[0044] Figure 1 are images illustrating thermal deformation of a conventional linear axis based machine tool;

[0045] Figure 2 and Figure 3 is a perspective view of a dual rotary axis machine tool;

[0046] Figure 4 is an image illustrating the thermal deformation of the machine bed of a dual rotary axis machine tool.

[0047] Figure 5 is a perspective view of a dual rotary axis machine tool including a pair of linear deformation sensors. DETAILED DESCRIPTION

[0048] Figure 2 and Figure 3 There is shown an example of a machine tool 30 to which the present disclosure relates. A machine tool of this type is disclosed in GB-A-2456843.

[0049] The primary driven shafts of the machine are two rotating machine shafts 32 and 34 having respective rotation axes 36 and 38 which are parallel to each other. Figure 2 In FIG, the machine tool is shown with the upper parts removed so that the rotating machine axes are more clearly visible. The two rotating machine axes 32 and 34 are mounted in fixed positions on the machine base 40 so that the spacing between their rotating axes is fixed.

[0050] exist Figure 3In the example shown, additional components have been mounted on the rotating machine shaft. These components are usually temperature controlled using hydrostatic oil, motor cooling water and grinding coolant. The first rotating machine shaft 32 provides a first support 42, Figure 3 In the embodiment of the present invention, a turntable 44 is mounted to the first support. The turntable carries a plurality of tools and measuring instruments for selectively engaging with the workpiece. The second rotation axis 34 provides a second support 46, Figure 3 In the embodiment of the present invention, a driven linear machine axis 48 is mounted on the second support. The linear machine axis in turn carries a workpiece drive spindle 50. The drive spindle comprises a chuck 52 for receiving a workpiece to be machined.

[0051] Figure 2 and Figure 3 The machine tool includes a control arrangement which is operable to control the orientation of the first support 42 of the first rotary machine axis 32, the orientation of the second support 46 of the second rotary machine axis 34 (to manage the orientation of the workpiece drive spindle 50 relative to the second rotary axis 38) and the position of the drive spindle 50 along the linear machine axis 48 so as to determine the positioning and orientation of the first support and the workpiece drive spindle relative to each other.

[0052] It has been determined that in machine tools having two main rotating machine axes mounted in fixed positions relative to the machine base, the heating of the machine bed during machining operations is mainly caused by the heat radiated from the drive motors of the rotating machine axes. This occurs at fixed positions in the machine bed and tends to be symmetrical, which in turn results in relatively simple deformation of the machine bed. This is consistent with the Figure 1 This contrasts with the deformation of conventional linear axis based machine tools of the form shown, where heating may occur in an asymmetrical manner at different locations, resulting in complex deformations.

[0053] Figure 4 The results of a simulation of the effect of heating a machine bed 60 of a machine tool to which the present disclosure relates are shown. Darker shading indicates higher temperatures. The machine bed includes a central vertical plate 62. The bed also defines two cylindrical openings 64 and 66 on opposite sides of the plate 62 for receiving respective rotating machine axes, wherein their axes of rotation are parallel in the assembled machine. It can be seen that the effect of heating by the motors of the two rotating machine axes is largely symmetrical about a position midway between their axes of rotation.

[0054] It is recognized that these are the main deformations occurring in such machine tools and that in practice they can be reliably measured and compensated for.

[0055] Figure 5There is shown a machine bed 70 of a machine tool of the type to which the present disclosure relates. Two rotating machine axes 32 and 34 are mounted in the machine bed. The machine bed is in the form of a horizontal, generally solid plate which supports the rotating machine axes.

[0056] Four rods or spars formed of a low thermal expansion material such as Invar are mounted on the machine bed. Two spars 72 are mounted on one side of the bed with the other two spars mounted on the opposite side. Each spar is clamped in a fixed position on the machine bed at one end 74 while the opposite end 76 is supported but free to move axially relative to the machine bed. This movement is measured using a corresponding transducer 78 mounted to the bed.

[0057] A pair of spars 72 on each side of the machine bed extend parallel to each other in a direction perpendicular to the rotational axes 36 and 38 of the rotating machine axis. The spars extend parallel to a longitudinal reference plane of the machine bed, which includes the rotational axis. Each pair of spars is spaced apart in the direction of the first rotational axis and the second rotational axis. It is preferred to make this spacing as large as possible in order to increase the difference between the deformations detected by each spar, thereby obtaining a more accurate deformation measurement result.

[0058] By monitoring the four distances measured by the deformation sensors, the resulting changes in the bending of the machine bed and the separation of the rotating machine axes can be determined.

[0059] The ends of the spars are preferably aligned along their length with the positions of the axes of rotation of the two rotating machine axes. Thus, the movement of their ends closely corresponds to the movement of the corresponding axes due to any deformation of the machine bed.

[0060] The transducer 78 generates a signal which can then be used to determine deformation of the machine bed which can then be compensated for. For example, the control signals sent to the two rotating machine axes and other driven components governing the relative position of the workpiece and tool can be adjusted in response to the signal from the transducer to counteract the effects of deformation of the machine bed.

[0061] For example, each transducer may be in the form of a linear variable differential transformer (LVDT) or a capacitance or eddy current based motion sensor.

[0062] It will be appreciated that deformation of the machine bed may result in displacement of each of the rotational axes having rotational components about three mutually perpendicular reference axes. Figure 5 As shown, these components are rolling motion about a horizontal longitudinal axis "X" passing through both of the rotation axes 36 and 38, pitching motion about a horizontal axis "Z" perpendicular to the "X" axis, and yaw motion about a vertical axis "Y". Two linear deformation sensors located on one side of the machine bed (such as in Figure 5The spar 72 visible in the example above) enables the pitch motion of the rotational axes to be detected. The deployment of four linear deformation sensors, a pair of which are located on opposite sides of the machine bed, facilitates the detection of all three components of displacement of each rotational axis. This can be used to calculate the final position error between the tool and the workpiece. The machine controller then adjusts the control signals sent to the driven machine axes in order to compensate for these errors.

Claims

1. A machine tool for machining a workpiece, the machine tool comprising: Machine base; a first support provided by a first rotating machine axis rigidly mounted on the base in a position immovably fixed relative to the base and comprising a first drive operable to rotate the first support about a first axis of rotation; a second support provided by a second rotating machine shaft rigidly mounted on the base in a position immovably fixed relative to the base and comprising a second drive operable to rotate the second support about a second rotational axis, wherein the second rotational axis of the second rotating machine shaft is parallel to and laterally spaced from the first rotational axis of the first rotating machine shaft, and the first rotational axis and the second rotational axis lie in a longitudinal reference plane; and at least one linear deformation sensor arranged to detect a change in distance between a first pair of reference points on the machine base located on a first reference line and to detect a change in distance between a second pair of reference points on the machine base located on a second reference line, Wherein the first reference line and the second reference line are transverse to the first rotation axis and the second rotation axis and are spaced apart along the direction of the first rotation axis and the second rotation axis, and the first pair of reference points and the second pair of reference points are located on the same side of the longitudinal reference plane and are spaced apart from the same side of the longitudinal reference plane.

2. The machine tool according to claim 1, wherein: The at least one linear deformation sensor is arranged to detect a change in the distance between a third pair of reference points on the machine base located on a third reference line, and to detect a change in the distance between a fourth pair of reference points on the machine base located on a fourth reference line, wherein the third reference line and the fourth reference line are transverse relative to the first rotation axis and the second rotation axis and are spaced apart along the direction of the first rotation axis and the second rotation axis, and the third pair of reference points and the fourth pair of reference points are spaced apart from the opposite side of the longitudinal reference plane from the first pair of reference points and the second pair of reference points.

3. A method for detecting deformation of a machine base of a machine tool for machining a workpiece, the machine tool comprising: Machine base; a first support provided by a first rotating machine axis rigidly mounted on the base in a position immovably fixed relative to the base and comprising a first drive operable to rotate the first support about a first axis of rotation; and a second support provided by a second rotating machine shaft rigidly mounted on the base in a position immovably fixed relative to the base and comprising a second drive operable to rotate the second support about a second rotational axis, wherein the second rotational axis of the second rotating machine shaft is parallel to and laterally spaced from the first rotational axis of the first rotating machine shaft, and the first rotational axis and the second rotational axis are located in a longitudinal reference plane, The method comprises the following steps: detecting a change in a distance between a first pair of reference points on the machine base located on a first reference line using at least one linear deformation sensor; as well as detecting, using the at least one linear deformation sensor, a change in the distance between a second pair of reference points on the machine base located on a second reference line, The first reference line and the second reference line are transverse to the first rotation axis and the second rotation axis and are spaced apart along the direction of the first rotation axis and the second rotation axis, and the first pair of reference points and the second pair of reference points are located on the same side of the longitudinal reference plane and are spaced apart from the same side of the longitudinal reference plane.

4. The method according to claim 3, comprising the steps of: detecting, using the at least one linear deformation sensor, a change in a distance between a third pair of reference points on the machine base that are located on a third reference line; as well as detecting, using the at least one linear deformation sensor, a change in a distance between a fourth pair of reference points on the machine base and located on a fourth reference line, wherein the third reference line and the fourth reference line are transverse relative to the first rotation axis and the second rotation axis and are spaced apart along the direction of the first rotation axis and the second rotation axis, and the third pair of reference points and the fourth pair of reference points are spaced apart from the opposite side of the longitudinal reference plane from the first pair of reference points and the second pair of reference points.

5. A machine tool according to claim 1 or claim 2 or a method according to claim 3 or claim 4, wherein the machine tool comprises: a controller for controlling the first and second actuators and arranged to receive signals from the at least one linear deformation sensor and to adjust control signals sent to the first and second actuators in response to the signals from the at least one linear deformation sensor during a machining operation so as to compensate for deformation of the machine base.

6. A machine tool or method according to any preceding claim, wherein the reference line is perpendicular to the first and second axes of rotation.

7. A machine tool or method according to any preceding claim, wherein the reference lines are parallel to each other.

8. A machine tool or method according to any preceding claim, wherein the at least one linear deformation sensor is mounted on the machine base.

9. A machine tool or method according to any preceding claim, wherein the first rotational axis and the second rotational axis are located in a first reference plane and a second reference plane respectively, the first reference plane and the second reference plane are parallel to each other and perpendicular to the longitudinal reference plane, and one reference point in each pair of reference points is located in one of the first reference plane and the second reference plane, and the other reference point in each pair of reference points is located in the other of the first reference plane and the second reference plane.

10. A machine tool or method according to any preceding claim, wherein each linear deformation sensor comprises: a rod having one end fixed relative to one of the corresponding pair of reference points; and a transducer arranged to generate an output signal in response to movement of the other end of the rod relative to the other of the corresponding pair of reference points.

11. A method for compensating for deformation of a machine base of a machine tool according to any one of claims 1, 2 or 5 to 10, the method comprising the steps of: (a) receiving a signal from the at least one linear deformation sensor; as well as (b) adjusting control signals sent to the first and second actuators during a machining operation in response to the signal from the at least one linear deformation sensor to compensate for deformation of the machine base.

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