Reduction of thermal distortion in machine tools

Abstract

A machine tool comprises a machine base (80), and a rotary machine drive (32) mounted on the machine base in a non-adjustable location relative to the machine base. The machine base includes a seat portion which supports the rotary machine drive. The seat portion defines a transverse seating face (86) which surrounds the rotary machine drive, and the base portion of the rotary machine drive defines an engagement surface (92) which engages the seating face. The seating face lies on a seating plane which is on or close to the neutral axis (90) of the machine base, and the engagement surface is substantially the only surface of the rotary machine drive in contact with the machine base.

Description

REDUCTION OF THERMAL DISTORTION IN MACHINE TOOLSField of the disclosure

[0001] The present disclosure relates to thermal distortion of machine tools and, more particularly, it concerns reduction of such distortion.Background to the disclosure

[0002] The present applicant has developed a machine tool configuration based on two rotary machine drives arranged in non-adjustable fixed positions on a common machine base, with their rotational reference axes spaced apart and parallel. This configuration has axisymmetric stiffness properties in the primary motion control drives, namely the two rotary machine drives. This results in a more predictable stiffness loop and higher stiffness, which in turn provides higher levels of precision and repeatability. A machine tool of this form is described in WO-A-2009 / 09 64, for example.

[0003] is a perspective schematic representation of a machine tool described in WO-A-2009 / 09 64. It includes a machine base 10. First and second supports 100, 102 are mounted directly on the base for rotation about the axes of rotation of the respective rotary machine drives which are perpendicular to the plane of the machine base. Their rotational motion is indicated by arrows A and B respectively. Pointers 104 and 106 denote reference points associated with each support. Each pointer has a reference axis 108, 1passing through it.

[0004] A mount 112 is carried by the second support 102 and is movable using a linear machine drive. Reference pointer 104 is on the first support, and reference pointer 106 is on mount 112, carried by the second support 102. Ghost representations 100', 102' and 112' of the first support, second support and mount are included into show different orientations thereof following rotation using their respective rotational machine drives and movement of the mount using the linear machine drive.

[0005] The two rotary machine drives and the linear drive are used to control the positions and orientations of the pointer 104 on the first support and the pointer 106 on the mount relative to the machine base.

[0006] In high precision machining operations, thermal effects are becoming dominant sources of machining errors.Summary of the disclosure

[0007] The present disclosure provides a machine tool comprising:

[0008] a machine base having a reference plane; and a first rotary machine drive having a first base portion which carries a first driven portion, wherein the first rotary machine drive is operable to rotate the first driven portion relative to the first base portion about a first rotational reference axis and to control the orientation of the first driven portion relative to the first base portion about the first rotational reference axis, the first base portion mounted on the machine base in a non-adjustable location relative to the machine base, and the first rotational reference axis is perpendicular to the reference plane, wherein the machine base includes a first seat portion which supports the first rotary machine drive, wherein the first seat portion defines a first transverse seating face which surrounds the first rotary machine drive, the first base portion of the first rotary machine drive defines a first engagement surface which engages the first seating face of the machine base, the first seating face lies on a seating plane which is on or close to the neutral axis of the machine base, and the first engagement surface is substantially the only surface of the first rotary machine drive in contact with the machine base.

[0009] In this machine tool configuration, the transfer of heat energy from the rotary machine drive to the machine base is substantially restricted to localised regions of contact between the two. These regions of “thermal loading” are located on or close to the neutral axis of the machine base. As a result, the associated heating effect and expansion of the machine base is centred on or close to the neutral axis and therefore substantially symmetrical with reference to a plane parallel to the reference plane of the machine tool and including the neutral axis. This minimises any distortion or bending of the machine base as a result of this heating effect.

[0010] The term “neutral axis” denotes a line through the machine base connecting points at which no extension or compression occurs when the base is bent along its length. In some implementations, the reference to the first seating face lying on a seating plane which is on or close to the neutral axis of the machine base may mean within 10mm, or in some examples, within 5mm of the neutral axis.

[0011] The contact between the first engagement surface and the first seating face may provide the overwhelmingly dominant path through which heat conducts from the rotary machine drive to the machine base. In some examples, the reference to the first engagement surface being substantially the only surface of the first rotary machine drive in contact with the machine base may mean that 95% or more of the contact area between the first rotary machine drive and the machine base is provided by the first engagement surface.

[0012] The first seating face may be a planar surface. It may lie in a seating plane which extends parallel to the reference plane of the machine base. The seating face may locate the first rotary machine drive relative to the machine base in a direction along the first rotational reference axis. The first seat portion may bear the weight of the first rotary machine drive. The first engagement surface may be the only surface of the first rotary machine drive in contact with the machine base.

[0013] A support may be carried by the first driven portion for supporting a tool or workpiece of the machine tool.

[0014] The first base portion of the first rotary machine drive may define first outer, axially extending surfaces above and below the engagement surface which are adjacent to the machine base, wherein the first outer, axially extending surfaces are radially spaced from the machine base. Thus, a radial clearance may be present between the axially extending surfaces and the adjacent surfaces of the machine base. This serves to avoid direct thermal conduction due to the presence of an air gap. These axially extending surfaces may extend circumferentially around the first rotary machine drive relative to the first rotary reference axis. In some implementations, the first outer, axially extending surfaces are spaced from the machine base by at least 1mm or at least 2mm.

[0015] The first base portion may include a first raised guide portion in order to locate the first base portion laterally relative to the machine base. This guide may be in the form of a shoulder, ridge or lip feature, for example. The guide portion may be continuous or discontinuous. It may be formed of a thermally insulating material, to minimise heat transfer therethrough.

[0016] A relatively small feature may be sufficient to facilitate location of the first base portion relative to the machine base, without materially affecting conduction of heat energy from the first base portion to the machine base. For example, the axial extent of the region of contact between the first raised guide portion the first base portion may be around 5mm or less, and could be around 1 to 2 mm.

[0017] The machine tool may include a second rotary machine drive which comprises a second base portion and a second driven portion, with the second rotary machine drive being operable to rotate the second driven portion relative to the second base portion about a second rotational reference axis to control the orientation of the second driven portion relative to the second base portion about the second rotational reference axis, and the second base portion is mounted on the machine base in a non-adjustable location relative to the machine base, with the second rotational reference axis arranged to be parallel to and spaced laterally from the first rotational reference axis.

[0018] A support may be carried by the second driven portion for supporting a tool or workpiece of the machine tool for engagement with a tool or workpiece carried by the first driven portion during a machining operation.

[0019] Furthermore, the machine base may include a second seat portion which supports the second rotary machine drive, wherein: the second seat portion defines a second transverse seating face which surrounds the second rotary machine drive;

[0020] the second base portion of the second rotary machine drive defines a second engagement surface which engages the second seating face of the machine base; the second seating face lies on the seating plane; and the second engagement surface is substantially the only surface of the second rotary machine drive in contact with the machine base.

[0021] The second base portion of the second rotary machine drive may define second outer, axially extending surfaces above and below the second engagement surface which are adjacent to the machine base, wherein the second outer, axially extending surfaces are radially spaced from the machine base. For example, the second outer, axially extending surfaces may be spaced from the machine base by at least 2mm.

[0022] The second seating face may include a second raised guide portion in order to locate the second base portion laterally relative to the machine base.Brief description of the drawings

[0023] Examples of the present disclosure will now be described with reference to the accompanying schematic drawings, wherein:

[0024] is a simplified representation of a known machine tool configuration;

[0025] is a perspective view of the machine base and two rotary machine drives of a machine tool having the configuration shown in;

[0026] is a cross-sectional perspective view of the machine base of a machine tool similar to that shown inshowing a simulated heat distribution;

[0027] is a cross-sectional perspective view of the machine base ofshowing simulated distortion of the machine base;

[0028] Figure is a cross-sectional perspective view of a machine base and the base portions of two rotary machine drives according to an example of the present disclosure;is an enlarged view of one of the rotary machine drives of Figure ;

[0029] is a cross-sectional perspective view of the machine base of Figure showing a simulated heat distribution;

[0030] is a cross-sectional perspective view of the machine base of Figure showing simulated distortion of the machine base; and

[0031] Figures 9 to 11 are enlarged cross-sectional perspective views of parts of a base portion of a rotary machine drive and the machine base.Detailed description

[0032] shows part of a machine tool having the configuration represented inTwo rotary machine drives 32 and 34 are mounted in a machine base 70 and are the machine tool’s primary drives. The machine base is in the form of a horizontal, generally solid slab which supports the rotary machine drives.

[0033] The rotary machine drives have mutually parallel respective axes of rotation 36 and 38. The machine tool is shown with upper components removed so that the rotary machine drives are more clearly visible. The two rotational machine axes 32 and 34 are mounted at fixed locations the machine base 70, such that the spacing between their axes of rotation is constant. The X and Z reference axes of the machine tool are parallel to a reference plane of the machine tool and the Y axis is perpendicular to the plane.

[0034] Each rotary machine drive has a base portion which includes a mounting structure or “pod” 40, 42 which engages with the machine base. The cross-sectional view ofillustrates these pods together with the machine base 70. Each pod includes a circumferential flange 44, 46. Each flange defines a planar engagement surface 48, 0 on its underside which rests on a respective planar seating or clamping face 2, 4 of the machine base.

[0035] During operation of the machine tool, the rotary drives generate heat energy, causing the temperature of the inner portions 8, 60 of the pods to rise. This leads to heat energy being conducted through each pod into the machine base via the surfaces of the pods which are in contact with the machine base. The surfaces include not only the engagement surfaces 48, 0, but also axially extending circumferential walls 62, 64 which are also in engagement with the machine base.

[0036] The rotary machine drives may be the only sources of heat energy inputted into the

[0037] machine base during a machining operation. In machine tools of the form shown in, the machine base may not come into contact with coolant fluids used during machining, which are instead captured by a tray overlying the machine base. The machine base may therefore be essentially thermally isolated from heat generated by machining processes.

[0038] In the machine tool configurations of Figures 2 and 3, the pods are located towards the upper surface of the machine base. This means that the rotary machine drives are provided closer to the working region of the machine tool. This in turn reduces the degree of compliance likely to be exhibited by the machine components that are located between the machine base and the tool and workpiece. As a result, it can be seen that the surfaces of the pods inwhich are in contact with the machine base are entirely located against upper portions of the machine base.

[0039] Figures 3 illustrates the results of a computer simulation of the distribution of heat energy during operation of the machine tool. The temperature at each location is shown in grayscale, ranging from the darkest shade for the hottest regions to the lightest shade for the coolest regions. Heat energy from the rotary machine drives is conducted into machine base via the contact surfaces 48, 0, 62 and 64. The flow of heat energy from the pod 40 into and through the machine base is indicated by arrows 66, 68. Due to the location of the contact surfaces towards the upper surface of the machine base, the upper portion of the machine base is heated to a greater extent than the lower portion, as can be seen in the central region 6 of the machine base in, as it is darker in its upper region relative to its lower region. This causes expansion of the machine base at different rates in different regions. The effect of this is illustrated by the computer simulation results shown inThe distortion of the machine base is exaggerated infor the purposes of illustration. It shows that the greater heating of the upper portions of the machine base causes it to bend along its length as indicated by the arrows 72, 74.

[0040] The simulation result inis shaded so as to highlight the regions of greatest displacement due to expansion of the machine base in dark shades. This illustrates that the expansion is unevenly distributed horizontally. In particular, the contour lines 76, 78 marking the lower edges of the dark regions towards the tops of each of the pods 40, 42 are angled relative to the upper rims 77, 79 of the pods, indicating that the pods are tilted out of alignment, such that the associated rotational reference axes are no longer parallel.

[0041] Whilst the distribution of heat within the machine base may be expected to become uniform over time, these gradual changes in the temperature distribution cause further changes in the shape of the machine base. These changes to the geometry of the machine base will lead to ongoing and variable machining inaccuracy.

[0042] Figure shows a cross-sectional view of a machine base 80 in combination with two rotary machine drive pods 82 and 84, according to an example of the present disclosure. The machine base provides a pair of planar seating faces 86 and 88 for engaging with respective pods. The seating faces lie on a seating plane which is on or close to the neutral axis 90 of the machine base. The seating plane is parallel with the reference plane of the machine base. Each seating face surrounds the respective pod, that is, it lies in the seating plane and extends entirely around the pod.

[0043] In comparison to the configuration shown in, it can be seen that the pods have been modified to locate the associated rotary machine drives at substantially the same height relative to the machine base by locating the two planar engagement surfaces 92, 94 closer to the lower ends of the respective pods.

[0044] In addition, the circumferential faces of the pods which are opposed to adjacent faces of the machine base are spaced radially from them, such that the seating faces 86, 88 of the machine base and the engagement surfaces 92, 94 of the pods are the only surfaces of these components in contact with each other. It can be seen in Figures and 6 that the machine base and pods are dimensioned such that the upper circumferential faces 96, 98 of the pods, extending upwardly from the engagement surfaces 92, 94 and the lower circumferential faces 100, 102 of the pods (below the engagement surfaces) are radially inwardly spaced from the adjacent surfaces of the machine base. A spacing of around 2mm may provide a sufficient gap. Usually, this gap will be filled with air, with around 2mm providing a sufficient thermal barrier.

[0045] Accordingly, in the configuration shown in Figures and 6, the conduction of heat energy from the pods to the machine base is localised to contact surfaces lying on or close to the neutral axis of the machine base. As illustrated by the temperature simulation shown in, this results in the heat energy distribution within the

[0046] machine bed being substantially symmetrical about a horizontal plane (that is, the seating plane) including the machine neutral axis 90. The arrows 1and 112 indicate the equal flow of heat energy into the upper and lower portions of the machine base. As the machine base is heated symmetrically with respect to the horizontal plane including the neutral axis, any material distortion of the machine base resulting from heat energy received from the rotary machine drives is avoided. This creates a machine bed configuration that is inherently thermally symmetric about both horizontal and vertical planes of symmetry. As a result, the machine base is essentially “athermalised” and intrinsically highly thermally stable.

[0047] shows the results of a simulation of how heating during machining affects the shape of the machine base 80.is shaded so as to highlight the regions of greatest displacement due to expansion in the darkest shades. This illustrates that the expansion is evenly distributed horizontally. In particular, the contour lines 114, 116 marking the edges of the dark regions towards the tops of each of the pods 82, 84 are generally parallel with the upper rims 113, 1 of the pods, indicating that the pods remain aligned, such that the associated rotational reference axes are still parallel.

[0048] The thermal stability of a machine tool having a configuration as described herein may be monitored using techniques described in the applicant’s co-pending UK patent application No: 2212420.0 (publication no. GB2621878), which can be used to fine tune control signals to send to the machine tool in response to any detected distortion.

[0049] The machine base may be formed from cast iron or a thermally matched combination of polymer concrete and steel, for example.

[0050] Figures 9 to 11 illustrate ways in which each pod may be laterally located relative to the machine base 80. In the example depicted in, a small shoulder profile 118 is provided at the intersection between the seating face 88 and the adjoining cylindrical face 1of the machine base. In the implementation shown in, a raised annular ridge 122 is formed in the cylindrical face 1of the machine base, with the ridge spaced a short distance from the seating face 88. In a further implementation shown in, a raised annular ridge 124 is provided on the cylindrical face 126 of the machine base so as to engage the lower circumferential surface 102 of the pod 84. It is desirable to minimise the contact area between the alignment feature and the pod, and position the feature close to the seating plane, in order to minimise any heat transfer by conduction which is asymmetrical relative to the seating plane coincident with the neutral axis. The axial extent of the raised feature may for example only be around mm or less, and 1 to 2mm may be sufficient.

[0051] It will be appreciated that references herein to perpendicular or parallel relative orientations and the like are to be interpreted as defining perpendicular or parallel relationships between components within practical tolerances.

Claims

[Rectified under Rule 91, 24.02.2026]1. A machine tool comprising:a machine base having a reference plane; anda first rotary machine drive having a first base portion which carries a first driven portion, wherein the first rotary machine drive is operable to rotate the first driven portion relative to the first base portion about a first rotational reference axis and to control the orientation of the first driven portion relative to the first base portion about the first rotational reference axis, the first base portion is mounted onthe machine base in a non-adjustable location relative to the machine base, and the first rotational reference axis is perpendicular to the reference plane, wherein the machine base includes a first seat portion which supports the first rotary machine drive, wherein the first seat portion defines a first transverse seating face which surrounds the first rotary machine drive,the first base portion of the first rotary machine drive defines a first engagement surface which engages the first seating face of the machine base, the first seating face lies on a seating plane which is on or close to the neutralaxis of the machine base, and the first engagement surface is substantially the only surface of the first rotary machine drive in contact with the machine base.[Rectified under Rule 91, 24.02.2026]A machine tool of claim 1, wherein the first base portion of the first rotary machine drive defines first outer, axially extending surfaces above and below the engagement surface which are adjacent to the machine base, wherein the first outer, axially extending surfaces are radially spaced from the machine base.[Rectified under Rule 91, 24.02.2026]A machine tool of claim 1, wherein the first outer, axially extending surfaces are spaced from the machine base by at least 2mm.[Rectified under Rule 91, 24.02.2026]A machine tool of any preceding claim, wherein the first base portion includes a first raised guide portion in order to locate the first base portion laterally relative to the machine base.[Rectified under Rule 91, 24.02.2026]A machine tool of any preceding claim including a second rotary machine drive which comprises a second base portion and a second driven portion, with the second rotary machine drive being operable to rotate the second driven portion relative to the second base portion about a second rotational reference axis to controlthe orientation of the second driven portion relative to the second base portion about the second rotational reference axis, and the second base portion is mounted on the machine base in a non-adjustable location relative to the machine base, with the second rotational reference axis arranged to be parallel to and spaced laterally from the first rotational reference axis.[Rectified under Rule 91, 24.02.2026]A machine tool of claim 5, wherein the machine base includes a second seat portion which supports the second rotary machine drive, wherein:the second seat portion defines a second transverse seating face whichsurrounds the second rotary machine drive;the second base portion of the second rotary machine drive defines a second engagement surface which engages the second seating face of the machine base; the second seating face lies on the seating plane; and the second engagement surface is substantially the only surface of the secondrotary machine drive in contact with the machine base.A machine tool of claim 6, wherein the second base portion of the second rotary machine drive defines second outer, axially extending surfaces above and below the second engagement surface which are adjacent to the machine base, wherein the second outer, axially extending surfaces are radially spaced from themachine base.A machine tool of claim 7, wherein the second outer, axially extending surfaces are spaced from the machine base by at least 2mm.[Rectified under Rule 91, 24.02.2026]A machine tool of any of claims to 8, wherein the second seating face includes a second raised guide portion in order to locate the second base portion laterally relative to the machine base.

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