GB2256606A - High-precision positional control. - Google Patents

Abstract

A positional adjustment device comprises an elongate member 60 which is supported at one end thereof by a fixed portion 52 of the device and which extends through a pressure chamber 74 in the device. The device further comprises means for applying a selectively controllable pressure to a hydraulic fluid in the said pressure chamber 74, thereby to compress the said elongate member laterally so as to bring about an increase in the length thereof so that there is produced at the other end of the elongate member a movement, relative to the said fixed portion 52 of the device, that can be applied to a body (45 Fig. 2) whose position is to be adjusted. Such a device can be incorporated to a toolpost in a grinding/turning machine to control the height of a tool above a support surface of the machine on which the toolpost is mounted. <IMAGE>

Description

2 2 75:5 t, b -I- HIGH-PRECISION POSITIONAL CONTROL The present invention

relates to high-precision positional control, for example in precision grinding/turning machines such as may be used for ductile-regime grinding of glass and other brittle materials to produce complex components having highly smooth surfaces suitable for optical applications.

Figure 1 of the accompanying drawings shows a diagrammatic perspective view of a precision grinding/turning machine which comprises a bed 1, known as a T-bed, which supports a first carriage (referred to alternatively as a workslide) 2 that is movable relative to the bed 1 along a first horizontal axis (the X-axis) of the machine. The T-bed 1 also supports a second carriage (toolslide) 3 which is movable relative to the bed 1 along a second horizontal axis (the Z-axis) perpendicular to the X-axis.

The workslide 2 is supported on the bed 1, at opposite sides of the toolslide 3 in the manner of a bridge spanning the toolslide 3. The workslide 2 supports a rotatable workspindle 4 which projects through a front face of the workslide 2 and whose axis of rotation is parallel to the said Z-axis. A workhead 5 is mounted on the forward end of the workspindle 4 and serves to retain a workpiece to be ground/turned when the machine is in use.

The toolslide 3 may comprise a rotary table 6, sometimes known as a Baxis rotary table, supported on a rotary table spindle 7 of the machine. The central axis of the spindle 7, about which it rotates, is parallel to a third axis (Y- axis) which extends vertically.

The rotary table 6 carries on its upper main face a toolpost 8 which at its upper end supports a grinding/turning tool 9 having a cutting edge made, for example, of diamond.

The workslide 2 and toolslide 3 have respective linear drive mechanisms 10 and 11, for example friction drive mechanisms, for displacing them along their respective axes of movement. Rotary drive means 12 and 13 are also provided for rotating the workspindle 4 and rotary table spindle 7.

Displacement monitoring means, denoted schematically at 14 and 16, are provided for measuring such linear displacement of the workslide 2 and toolslide 3 along their respective axes of movement. Such monitoring means may advantageously be based upon laser interferometry.

An angular position encoder 15, associated with the rotary table 6, produces data indicative of the angular position of the tool 9.

A tool setting station (not shown) is carried by the workslide 2, adjacent to the workhead 5. Respective horizontal and vertical air bearing probes (not shown) projecting from a corner portion of the tool setting station serve to facilitate presetting of the position of the tool 9 prior to a machining operation.

Computer numeric control means 17 are provided for controlling operation of the machine.

The machine as a whole is preferably enclosed in a containment 19 (shown dotted in Fig. 1) supporting an oil shower thermal control system, in order to protect the machine components from undesirable environmental effects.

In use of the grinding/turning machine of Fig. 1, the workpiece to be machined is mounted on the front face of the workhead 5, and a tool 9 is mounted on the toolpost 8. Before machining can begin, the position of the tool must be preset, such that the cutting point of the tool (i.e. the point on the tool which contacts the workpiece) lies in a working disposition along the central axis of the rotary table spindle 7 and at the same height as the central axis of the workspindle 4. To facilitate such presetting, the workslide 2 is driven along the X-axis to a position such that the air bearing probes can move into contact with the cutting edge of the tool 9. The position of the tool may then be adjusted in situ with reference to measurement values provided by the air bearing probes, until the cutting edge of the tool is in the desired position.

The computer numeric control means 17 is supplied with control data defining a required profile of the workpiece. During machining this control data is translated by the control means into the necessary drive signals for the linear drive mechanisms of the workslide 2 and toolslide 3, and for the rotary drive means of the workspindle 4 and rotary table spindle 7, so as to cause the required profile to be generated.

As discussed above, the toolpost 8 is required to support the tool 9 in its working disposition with respect to the workpiece during machining.

Conventional toolpost designs generally include mechanisms (for example screw-driven slides) for adjusting the position of the tool in respective directions along the X, Y and Z axes. In certain circumstances, for example where very fine adjustment of the tool height is required, adjustments provided by such a mechanism may be too coarse.

In addition, in the most vigorous machining applications the nature of such adjustment mechanisms inhibits the achievement of high overall stiffness in the toolpost structure.

Furthermore, the overall positional performance of such a toolpost is limited by the precision with which the individual axis adjustments can be performed, and it is found in practice that an adjustment performed along one axis (for example the Y-axis) may cause unintentional changes along the other two axes (in this case the X and Z axes) which must then be corrected.

Also, conventional toolpost designs can suffer from thermal effects, caused by hands-on adjustment, and from errors due to stress relaxation arising from high clamping forces.

According to a first aspect of the present invention there is provided a positional adjustment device operable, according to the Poisson principle (i.e. the principle whereby material which is compressed in one direction expands in an orthogonal direction), to adjust the position of a first body relative to a second body.

According to a second aspect of the present invention there is provided a positional adjustment device comprising an elongate member which is supported at one end thereof by a fixed portion of the device and which extends through a pressure chamber in the device, and further comprising means for applying a selectively controllable pressure to a hydraulic fluid in the said pressure chamber, thereby to compress the said elongate member laterally so as to bring about an increase in the length thereof so that there is produced at the other end of the elongate member a movement, relative to the said fixed portion of the device, that can be applied to a body whose position is to be adjusted.

Such a device can be incorporated in a toolpost, carrying a grinding/turning tool, for adjusting the height of the tool (in the Y direction) with respect to a rotary table carrying the toolpost. The range of height adjustment may be quite small, of the order of micrometers or tens of micrometers, for example up to 20 Pm.

Reference will now be made by way of example to the accompanying drawings, in which:

Fig. 1 shows a diagrammatic perspective view of a grinding/turning machine; Fig. 2 shows a perspective view of a toolpost embodying the present invention, for use in the Fig. 1 machine; Fig. 3 shows a plan view of the toolpost of Fig. 2; Fig. 4 shows a rear elevational view of the toolpost of Fig. 2; Fig. 5 shows a longitudinal sectional view of the toolpost of Fig. 2, as seen along line V-V in Fig. 3; Fig. 6 shows a longitudinal sectional view of a part of the Fig. 2 toolpost, as seen along a line VI-W in Fig. 7; and Fig. 7 shows an axial sectional view of part of the Fig. 2 toolpost, as seen along a line VII-VII in Fig. 5.

Fig. 2 shows a perspective view of a toolpost embodying the present invention. The toolpost 20 is fixed to an upper main face of a rotary table 21 at a peripheral region thereof.

The toolpost 20 has a partly hollowed metallic housing 22 with a flared base 23. Above the base 23 the housing 22 has seven faces 24 to 27, the two front faces 24 and the two side faces 25 having respective recesses, formed in the housing 22, in which removable cover plates 28 are fitted.

At its upper end, the housing 22 is formed with a recess 30 (see Fig. 4) for receiving a tool mounting block 31. A clamp 32, connected releasable to the housing 30 by bolts 33, is located alongside the tool mounting block 31 in the recess 30. The recess 30, tool mounting block 31 and clamp 32 together form a dovetail slide arrangement by means of which the tool mounting block 31 is slidable along the recess in the forward/ rearward direction when the clamp 32 is released, and is retained firmly by the housing 22 by the wedge action of the clamp 32 when the clamp 32 is bolted down by the bolts 33.

A dead stop abutment 34 at the rear end of the recess 30 prevents the block 31 from being moved beyond the rear face 27 of the housing 22.

On its upper face the tool mounting block 31 has a groove 35 which narrows in cross-section at its forward end. As best seen in Fig. 5, a tool clamp 40 having a rear end portion 41 of T-shaped cross section is fitted within the narrow portion of the groove 35 and secured to the tool mounting block 31 by a shoulder screw 42 and two bolts 43. A packer plate 44 is interposed between the tool clamp 40 and tool mounting block 31.

A tool 45, for example a diamond tool, is clamped between the tool clamp 40 and the packer 44 by the two bolts 43.

Referring now to the longitudinal sectional view of Fig. 5 the housing 22 comprises three integral portions: an upper housing portion 50, a middle housing portion 51, and a lower housing portion 52. The middle housing portion 51 does not extend forwardly (towards the left hand side in Fig. 5) as far as the upper and lower housing portions 50 and 52 so that, together with the cover panels 28 on the two front faces 24 of the housing 22, a triangular space 53 (see also Fig. 7) is defined within the housing 22.

At its upper end the space 53 is continuous with a horizontal slot 54 which extends in the rearward direction of the housing between the upper housing portion 50 and the middle housing portion 51. The slot 54 extends fully across the width of the housing 22 between the respective cover panels 28 on the two side faces 25.

A first circular bore 55 extends from a point midway along the rear edge of the slot 54 through the upper end of the middle housing portion 51 to the rear face 27 of the housing. A second circular bore 56 extends from the lower end of the space 53 through the lower end of the middle housing portion 51 to the rear face 27 of the housing. A rectangular slot 57 (Figure 7) also extends from an opening 58 in the left-hand rear face 26 of the housing 22 through the middle housing portion 51 into the chamber 53. A sealing plate 59 is fitted in the opening 58.

Extending centrally and longitudinally through the space 53 is a solid metal cylinder (elongate member) or pusher bar 60. The pusher bar 60 extends at its lower end into a circular hole 61 provided in the lower housing portion 52 and is secured therein by a clamping sleeve arrangement generally denoted at 62 and comprising two interengaging wedge-action sleeves. At its upper end the pusher bar has a threaded longitudinal bore 63. A bolt 65, having a head 66 fitted within a recess 67 in the upper housing portion 50, passes into the bore 63 to secure the pusher bar 60 to the upper housing portion 50.

A washer 68 is positioned between the bolt head 66 and the bottom of the recess 67. The bolt 65 is tightened such that the slot 54 is partially closed up, the bolt 65 being thereby pretensioned.

Also within the space 53 a hollow substantially cylindrical body 70, referred to herein as the pusher body, is provided, as shown in section in Fig. 6. The pusher bar 60 extends longitudinally through the pusher body 70 and is tight therewith.

Referring now to Figs. 6 and 7, the pusher body 70 is made up of a main outer portion 71, having ribs 72 formed on the outer surface thereof, and an inner sleeve 73 braised at both ends to the outer portion 71. Between its two ends the inner diameter of the outer portion 71 is stepped to provide a thin annular pressure chamber 74 between the inner sleeve 73 and the outer portion 71.

As best seen in Fig. 7, the outer portion 71 has a neck 75 through which a passageway 76 extends, the inner end of passageway 76 communicating at a circumferential region 77 thereof with the annular chamber 74. The passageway 76 does not penetrate the inner sleeve 73.

The neck 75 of the pusher body 70 also has an inlet tube 77 extending at right angles to the passageway 76 through the outer portion 71 of the pusher body between the passageway 76 and the outside. The inlet tube 77 narrows in cross-section at its inner end which is open to the passageway 76. The outer end of the inlet tube 77 is threaded and a grub screw 78 is fitted therein. The grub screw 78 serves to retain a steel ball within the inner end of the inlet tube 77.

The neck 75 of the pusher body 70 is connected to a rectangular housing 80, known as the plunger housing, which is located in the rectangular slot 57 of the housing 22.

The plunger housing 80 includes a plunger 84 which extends at its outer end through the sealing plate 59 and has a threaded portion 85 within the housing 80 which engages with a correspondingly-threaded bore 86 of the housing 80. At its inner end 87, the plunger penetrates into a passageway 88 which is continuous with the passageway 76 of the pusher body 70. A needle roller 89 extends within the passageway 88.

High pressure oil is present in the annular pressure chamber 74 and the passageways 76 and 88, the steel ball 79 and the needle roller 89 and its seal preventing the escape of oil from the pusher body 70.

In use of the toolpost 20, rotation of the plunger 84 brings about longitudinal movement thereof with respect to the plunger housing 80. Such movement in turn causes longitudinal movement of the needle roller 89 which is maintained in firm contact with the inner end 87 of the plunger 84 by the pressure of the oil contained in the passageways 76 and 88.

In the case of inward longitudinal movement of the needle roller 89 the oil in the pusher body 70 is urged inwardly with respect thereto and, being itself virtually incompressible, applies an increased pressure to the inner sleeve 73 of the pusher body and the pusher bar 60.

Accordingly the pusher bar 60 is compressed laterally.

Such lateral compression of the pusher bar 60 causes it to extend longitudinally by an amount n which is proportional to the lateral compression. The longitudinal extension 1 for a given lateral compression is expressed by-the Poisson ratio of the material from which the pusher bar is made. Thus, the extension 1 is dependent upon the material properties of the pusher bar, -g- and also upon the pressure of the oil in the annular pressure chamber 74.

Referring again to Fig. 5, the longitudinal extension 1 of the pusher bar 60 causes upward displacement of the upper housing portion 50 relative to the lower and middle housing portions 51 and 52, the slot 54 facilitating such upward displacement while providing high structural rigidity. The upward displacement of the upper housing portion 50 in turn brings about upward movement of the tool 45 carried by the tool mounting block 31.

When the plunger is rotated in the anti-clockwise direction, such that the plunger 80 and needle roller 89 move in the outward longitudinal direction with respect to the plunger housing, the pressure applied to the pusher bar 60 by the oil in the annular pressure chamber 74 is reduced, so that the pusher bar 60 relaxes and undergoes a longitudinal contraction.

During such contraction, the pretensioning of the bolt 65 securing the pusher bar 60 to the upper housing portion 50 ensures that the upper housing portion 50 moves downwardly, thereby bringing about downward movement of the tool 45 and the tool mounting block 31.

As will be apparent from the above, by adjustment of the plunger 84 it is possible to control the height of the tool 45 above the upper main face of the rotary table 21 conveniently and with high precision.

The toolpost of Fig. 2 does not incorporate mechanisms for adjusting the position of the tool in the X and Z directions. The absence of X and Z adjustment mechanisms is feasible because the X and Z adjustments can be performed during machining under the control of the machine control means which control displacement of the workslide and toolside along the X and Z axes respectively. Accordingly, any error in the tool position in the X and Z directions can be compensated for by the machine control means once an initialising tool setting routine has been performed to determine those errors, as well be explained hereinafter in more detail. The absence of X and Z tool adjustment mechanisms can result in a toolpost structure of enhanced overall stiffness.

In addition, because the height adjustment is performed without the use of a screw-driven slide, the undesirable effects of such a slide (for example errors in the X and Y directions) are avoided, and the overall stiffness of the toolpost is further enhanced, the toolpost structure being dynamically compliant.

It will be readily understood that adjustment of the position of the tool along the other machine axes (X and Z axes, for example) can be performed effectively by providing suitable further pusher bars extending respectively in parallel with those axes.

In a prototype toolpost a pusher bar made of mild steel was employed, having a Poisson ratio of 0.291 at 200C, and the oil pressure was 200,000 psi (1.49GPa). A maximum extension of approximately 20 pm was observed.

The pusher bar may be made of any suitable stiff material, glass being one such material. In addition the cross-section of the pusher bar need not be circular. Other hydraulic fluids than oil can be employed and could occupy the whole of the space 53 rather than just the annular pressure chamber 74.

At the very small range of extensions concerned thermal effects, causing expansion and contraction of the pusher bar 60 and other toolpost components, cannot be neglected. As mentioned above, the machine as a whole is preferably enclosed in a containment (shown schematically at 19 in Fig. 1) supporting an oil shower thermal control system, for protecting machine components from undesirable environmental effects. However, it may also be advantageousto circulate temperature-controlled fluid through the space 53 containing the pusher body 70. Such fluid may be introduced to the space 53 from the rear face 27 of the housing 22 through the above-mentioned first circular bore 55 and exhausted from the space by the second bore 56, the cover panels 28 on the two front faces 24 of the housing and the sealing plate 59 on the rear face 26 serving to provide a fluid-tight seal preventing escape of fluid from the space 53.

To minimise thermal effects upon the toolpost, it may be desirable to replace the manual adjustment arrangement for the tool height (as shown in the Figures) with a motorised plunger arrangement, as shown schematically at 90 in Fig. 2. Such a motorised arrangement could be controlled by buttons on a remote control panel or preferably by the main machine control system. In the latter case, if suitable sensing means for monitoring the height of the tool are also provided, it is possible to continuously control the tool height during machining to improve accuracy.

Such sensing means could, for example, comprise a capacitance sensing arrangement in which the capacitor plates are mounted on the opposite main faces of the horizontal slot 54 extending between the upper housing portion 50 and the middle housing portion 51, as shown schematically at 91 in Fig. 5. It may in any case be advantageous to provide such a capacitance sensing arrangement to monitor thermal effects or long-term stability of the toolpost components.

As discussed hereinbefore, the toolpost of Figs. 2 to 7 has only one positional adjustment mechanism, which permits the height of the tool (in the Y direction) to be controlled. For this reason, the tool must be preset to a nominal height and length relative to its tool carrier before the tool carrier is introduced to the machine. Such presetting may be performed with the aid of an optical measuring system, and height and length adjustments are made to the tool carrier to bring the tool to a specified position. Once preset in that position, the tool and its carrier are transferred to the toolpost on the machine. A typical tool set routine may then be followed, except that only tool height is adjusted while the tool co-ordinates are measured. This approach is sufficient for a fixed toolpost operation but additional measurements are required when the tool post is mounted on a rotary table such as that shown in Fig. 1.

In such a case, the tool position is desirably preset such that during machining the cutting point of the tool lies along the axis of rotation of the table (i.e. along the central axis of rotary table spindle 7). Such adjustment is conventionally performed by adjustment of the toolpost. However, as explained hereinbefore, the screw-driven slide mechanisms used to perform such adjustments in conventional toolposts can give rise to positional errors.

In addition to such mechanical errors in the adjustments mechanisms, the rotary table spindle and/or toolpost may give rise to rotational errors at tool height, and the tool itself may have a radius error.

To overcome the above problems when employing a tool post as shown in Figures 2 to 7, it is possible to measure all the above-mentioned sources of errors in a single tool setting routine by rotating the rotary table supporting the tool post and taking continuous measurements of the position of the cutting point relative to the fixed point of the tool set station. The information derived in this way is then stored in the memory of the machine control means to permit error compensation to be performed during machining.

Further aspects of the present invention relate to a method and apparatus as described above for performing such error compensation.

According to one such aspect, for example, there is provided a method of compensating for errors in the position of a rotatable tool, wherein the tool is moved to successive angular positions, and at each such position the location of the working point of the tool is measured by probe means brought into contact therewith, position data from the probes being stored and used subsequently to compensate for errors in the tool 5 position.

According to yet another such aspect, for example, there is provided a tool setting apparatus, mounted on a movable workslide of the machine, which apparatus comprises measurement means that can be brought selectively into contact with a rotatable tool of the machine to determine the position thereof at successive angular positions of the tool, and which also comprises storage means for storing data indicative of the tool position at each such angular position.

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Claims (2)

1. CLAIMS: 1. A positional adjustment device comprising an elongate member

   which is supported at one end thereof by a fixed portion of the device and which extends through a pressure chamber in the device, and further comprising means for applying a selectively controllable pressure to a hydraulic fluid in the said pressure chamber, thereby to compress the said elongate member laterally so as to bring about an increase in the length thereof so that there is produced at the other end of the elongate member a movement, relative to the said fixed portion of the device, that can be applied to a body whose position is to be adjusted.
2. 2. A device as claimed in claim 1, substantially as hereinbefore described with reference to Figures 2 to 7 of the accompanying drawings.