GB2085771A - Precision grinder - Google Patents

Abstract

A precision grinding machine has a worktable which is axially slidable along a first path. The worktable is guided by means of pairs of opposing hydrostatic guide bearings on the apparatus bed and is supported by further hydrostatic bearings acting against the weight of the worktable. A tool head is provided which is movable towards and away from a workpiece on the worktable, the direction of movement being normally perpendicular to the worktable axis. The tool head is also guided by means of hydrostatic guide bearings and supported by further hydrostatic support bearings. The apparatus has a head stock whose spindle is rotatably supported on further hydrostatic bearings and whose axial position is maintained by means of hydrostatic thrust bearings acting on a radially outwardly extending flange of a spindle. The thrust bearings are arranged in opposing pairs. The tool holder rotatably supports a grinding wheel and is moved towards and away from a workpiece by means of a single D.C. servo motor controlled by a numerical control unit. The latter can compensate for redressing of the grinding wheel and also controls acceleration, deceleration and speed of movement of the tool head. <IMAGE>

Description

SPECIFICATION Precision grinder The present invention relates to apparatus for machining workpieces.

A typical such apparatus, such as a precision grinding machine, has a worktable and a tool head mounted for sliding movement in respective orthogonal directions on a bed. Lubricating oil or low friction co-efficient plastics materials are normally used to reduce friction between the worktable, tool head and bed and in some cases the worktable and tool head may be supported on the bed using guide rollers.

One of the disadvantages of such grinding machines is that the worktable and tool head guides are subject to considerable wear which effect the precision of the machine.

The present invention seeks to provide an improved apparatus for machining a workpiece.

Accordingly the present invention provides an apparatus for machining a workpiece comprising a worktable movable along a first path; a tool head for holding a tool, the tool head being movable along a second path transversely of said path; and wherein each of said worktable and tool head is mounted by way of hydrostatic bearing means.

One advantage of an apparatus according to the present invention is that the lenght of the worktable and tool head guides can be considerably reduced and precision maintained for considerably longer than with known workpiece machining apparatus.

The present invention is further described hereinafter, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a precision grinding machine according to the present invention; Figure 2 is a plan view, partially in section, of the machine of Figure 1; Figure 3 is a front elevation, partly in section, of the machine of Figure 1; Figure 4 is a part sectional view along the line IV-IV of Figure 3; Figure 5 is a section along the line V-V of Figure 4; Figure 6 is a section along the line VI-VI of Figure 2, in an enlarged scale; Figure 7 is a partial section along the line VII-VII of Figure 6; Figure 8 is a partial section along the line VIII-VIII of Figure 6 in an enlarged scale; and Figure 9 is a schematic front elevation of the machine of Figure 1 showing the machine grinding wheel at a different orientation to that in Figures 1 and 2.

Referring to the drawings, these show a precision grinding machine which has an elongate worktable 11 slidable longitudinally on a bed 10, the table 11 mounting a subsidiary table 12 on which a headstock 13 and tailstock 15 are mounted so as to be slidable towards and away from one another on the su bsidary work table. The two stocks carry respective spindles 14 and 16 between which a workpiece 17 is supported for grinding.

the bed 10 also mounts a grinding wheel head 18 which can be transversed in a direction perpendicular to the longitudinal axis of the worktable 11 to bring a rotatable grinding wheel 19 into contact with theworkpiece 17.

The subsidiary table 12 is mounted on the worktable 11 by means of a vertical pivot 20 which allows the subsidary table to be rotated relative to the worktable 11 by means of a micrometer screw 21 having a graduated scale 22 to enable the mounting axis of the workpiece 17 to be accurately aligned with the path of movement of the worktable 11. As will be appreciated, the pivot range of the subsidiary table is restricted and the subsidiary table 12 is axially secured relative to the worktable 11 by means of two end stops 23 which may conveniently serve as clamps for the subsidiary table 12.Screwthreaded rod 26 is rotatably mounted on the bed 10 by way of two supports 27 below the worktable 11 with its axis substantially parallel thereto, the rod engaging the ball screw secured to the underside of the worktable 11 to enable axial movement of the worktable 11 to be effected by rotation of the rod.

The ball screw is preloaded to take up any play in the engagement of the rod with the ball screw.

One extreme end of the rod 26 carries a pulley 29 which is coupled through a belt 30 to a further pulley 31 of a D.C. servo electric motor 32. The motor is controlled by a numerical control system or gear 33 which controls axial movement of the worktable 11 together with the subsidary table 12 and workpiece 17.

The axial movement of the worktable 11 is guided by two parallel guides 34 and 35 extending longitudinally and integral with the bed 10, which cooperate with corresponding rectangular cross-section guides 36 and 37 formed on the underside of the worktable 11. Each guide 34,35 is provided with a respective hydrostatic bearing, the bearings being located on the guides equidistant from the point of action of the grinding wheel. Each bearing comprises a chamber 41,42 in the bed guide 34,35, which chamber opens towards the cooperating guide 36, 37 of the worktable 11. Hydraulic fluid, such as oil, is fed to the chambers through feed lines 45 from a gearbox or reservoir 46 by means of a pump 47. The oil is filtered through suitable filters and is supplied at a pressure of typically 40 atmospheres.Each chamber 41,42 has a respective throttle 43144 located in the chamber and includes a throttle piston 40 which is screw-engaged in the extreme end of the chamber 41,42 remote from the opposing guide 36,37. This enables the position of the piston 40 in the chamber 41,42 to be adjusted thus varying the cross-sectional area of that portion of the chamber coupling the associated feed line 45 to its opening adjacent the cooperating guide 36,37 on the worktable 11. The force applied to be cooperating guides 36,37 of the worktable 11 by the pressurised oil at each of the hydrostatic bearing locations can therefore be adjusted to ensure balance of the worktable 11 and compensate for any transverse forces applied to the worktable 11 by the coupling between the ball screw 24 and the rod 26.The guides 34,35 of feed bed 10 are normally longer than the cooperating guides 36, 37.

The worktable 11 is vertically supported on the guides 34,35, by two sets of four hydrostatic bearings, one set being formed in each of the guides 36 and 37 of the worktable 11 with the chambers of each bearing opening into a horizontal undersurface of the worktable 11. The bearings in each set are naturally aligned parallel to the worktable axis.

Each bearing has a corresponding throttle mechanism 51,52, the chambers being fed through feed lines 50. These hydrostatic bearings do not oppose one another as do the first mentioned hydrostatic bearings since the weight of the worktable 11 is sufficient to provide a balance with the force generated by the pressurised oil. It is not necessary to ensure that that disposition of the hydrostatic bearings vertically supporting the worktable 11 is symetrical about the point of action of the grinding wheel 19. The number of these bearings ensures that the grinding wheel operates on the workpiece at a location between two pairs of the vertically and horizontally supporting bearings.

Any oil leaking from the bearings is collected in a cavity 55 formed in the bed 10 beneath the worktable 11 and conveyed towards the reservoir or gearbox.

The grinding wheel 19 is secured on a spindle 51 of the wheel head 18, the spindle being driven by a D.C. servo motor 53 via an endless belt 52. Tension in the belt is adjusted by means of a hand control mechanism 108. A guard 54 covers the upper portion of the grinding wheel 19. The wheel head 18 is supported on the bed 10 by means of a base 105 to which a support rack 66 is secured by means of bolts 112. The support rack 66 is formed with two upstanding parallel rails having horizontal flanges 63,64 extending inwardly towards one another.

These flanges extend transversely of the axis of the spindle 51 and engage in co-operating grooves formed by horizontal outwardly projecting flanges 58, 59 of an inverted "T" cross member 57 secured to the horizontal underside of the wheel head 18. The flanges 63,64, 58, 57 and the co-operating underside of the wheel head 18 serve as a guide for movement of the wheel head and thus thegrinding wheel 19 in a horizontal path at right angles to the spindle axis.

Further sense of hydrostatic bearings are provided in the wheel head guides to ensure low friction or frictionless movement of the wheel head and thus the grinding wheel 19 towards and away from the workpiece. The bearings are formed similar to the bearings previously discussed. To pairs of opposed bearings are provided in the vertical walls of the cross member 57 opposing the flanges 63 and 64 with each bearing of each pair having a respective chamber 69,70 opening in the associated wall.

These bearings provide horizontal guidance for the wheel head 18 and are fed via respective feed lines, the bearing chambers being provided with throttle mechanisms 72 and 73. Vertical guidance of the wheel head 18 is effected by two sets of hydrostatic bearings, the sets being provided at selective locations longitudially of the cross member 57. Each set comprises two pairs of opposed bearings located symetrically about the cross member axis with each pair comprising an upper bearing in the undersurface of the wheel head 18 and a lower bearing formed in the respective flange 58,59 of the cross member 57. The upper bearings comprise respective chambers 74,75 provided with throttle mechanisms 78,79 and fed via feed lines 77. The lower opposing bearings have respective chambers 83,82 with co-operating throttle mechanisms and are fed via associated feed lines 84,85.The hydrostatic bearings provide low resistance to movement of the wheel head 19 and serve to compensate for reaction forces acting on the head 18 through the grinding wheel 19. Oil leakage from the bearings is collected in a cavity 87 formed in the base of the support rack 66 and returned to the reservoir or gearbox 46.

Movement of the wheel head is effected by a D.C.

electric servo motor 88, conveniently a permanent magnet motor, secured to the wheel head 18 and drives a sprocket wheel 92, rotatably mounted on an appendix 94 of the wheel head 18, through a pinion 89 and transmission gear 91. The hub 93 carries a ball screw 96 which engages a screw threaded rod 97 which is secured to the support rack 66 by way of two end mountings 98. The rod 97 has its axis parallel with that of the support rack 66 and cross member 57. When the motor 88 is energised rotation of the sprocket wheel 92 carries it along the screwthreaded rod 97 thus moving the wheelhead 18 towards or away from the workpiece, depending on the direction of rotation of the sprocket wheel 92.

The servo motor 88 has a speedometer dynamo 99 and an encoder 101 coupled to numerical control or gear 33, the latter controlling acceleration, deceleration, speed and position of the servo motor 88. The numerical control 33 is able to compute a program and readjust the operating position of the grinding wheel 19 following periodicregrinding of the wheel by a regrinding device 102 to compensate for the consequent reduction in the wheel diameter. The numerical control 33 also controls both rapid approach and retraction movement of the wheel head 18 and grinding wheel 19 and also power strokes of the wheel 19. The servo motor 88 is preferably a low power, low inertia motor which is capable of being accelerated from 0 to 3,000 rpm in a time of 30 milliseconds.

The numerical control 33 controls the motor speed and acceleration to provide a rapid initial adjustment ofthewheel position and starting point for the wheel power stroke for various diameters of workpiece.

The provision of hydrostatic bearings allows rapid and precise adjustment of the position of the grinding wheel.

The support rack 66 is pivotally mounted on the base 105 by means of a pivot 106 to enable pivoting of the wheel head and thus the grinding wheel 19 about a vertical axis through an angle of substantially 300. Although the wheel head may be pivoted to any anuglar position within this range two present pivot positions are provided respectively to enable frontal grinding of a cylindrical workpiece and simultaneous grinding of a workpiece diameter and shoulder. A pin 108 having a substantial frustoconical upper end region is vertically supported for axial movement in a block 109 fixed to the bed 10 beneath a portion of the support rack 66 of the wheel head 18. The pin 108 has a lower toothed portion which engages with a pinion 110 on a manually rotatable spindle 111. The pin 108 is axially displaced in the block 109 by rotation of the spindle 11.The opposing under surface of the support rack 66 has two recesses 107 which can be brought into vertical alignment with the pin 108 by pivoting of the wheel head and thus the support rack 66 into a respective one of two angular positions. When the support rack 66 is in the selected position the spindle 111 is rotated to drive the pin 108 into the aligned recess 107 and lock the wheel head in position. The two preferred angular positions of the wheel respectively at 900 and 600 to the workpiece axis. Additional recesses 107 may be provided at suitable angular positions.

The workpiece holding a head 13 is secured in position on the worktable 12 by means of a block 115 which is slidably located in a guide 118 in the underside of the head 13. The block 115 has a screw-threaded bore in which a screw-threaded rod 117 engages the rod being accesible from outside the workpiece holding head 13. An upstanding portion of the worktable 12 extends between the block 115 and a depending flange 113 of the workpiece holding head 13 and carries respective flanges 114and 116 opposing the flange 113 and block 115. Rotation of the rod 117 to draw the block 115 towards the flange 116 clamps the upstanding portion of the workpiece 12 between the flange 113 and the block 115 to secure the workpiece holding head in position on the worktable 12.The flanges 113, 114 and 116 extend parallel with the longitudinal axis of the worktable 12.

The spindle 14 comprises a cylindrical sleeve 121 seated in a cylindrical bore 122 in the hed stock 13 and is adapted to receive, for example, a workpiece centre point or chuck for holding one end of the workpiece. The spindle is rotatably driven by a D.C.

electric servo motor 131 through motor and spindle pulleys 129 and 124 interconnected by way of an endless belt 128, the pulley 124 being keyed to the sleeve 121 by means of a key 123. The pulley 124 is supported in a bearing seat 127 of the head stock by means of suitable bearings. The sleeve 121 is supported in the bore 122 by two sets of four hydrostatic bearings, each set comprising four bearings spaced at 900 angles about the bore 122 with the two sets being angularly spaced relative to one another in the bore 122. The hydrostatic bearings are constructed similar to those previously described having respective chambers 134, 135 provided in the bore 122 and fed from a fluid line 137. Each bearing has a respective throttle mechanism 138, 139.

Oil leaking from the hydrostatic bearings is collected in a cavity 141 of the head stock 13 and returned to reservoir gearbox 46.

The axial position of the sleeve 121 is maintained by means of a radial flange 142 on the sleeve 121, the flange engaging in an anular groove formed by two opposing flanges 143 and 144 in the head stock 13. The opposing surfaces of the flanges 143 and 144 each carries four hydrostatic bearings spaced at 900 angular intervals with each bearing on one flange being opposite a bearing on the other flange. The bearings are constructed in the manner previously described with each bearing having a respective oil chamber 146, 147 opening into the associated flange surface opposing that of the sleeve flange 142 and supplied with fluid by a feed line 148. Each bearing has a respective throttle mechanism 149. Oil leaking from the bearings is collected in a cavity 149 and conveyed to the reservoir or gearbox 46.The location of the bearings in opposition to one another ensures that the axial thrusts on the spindle 14 are balanced and that any axiall thrust component applied to the workpiece by way of the grinding wheel 19 is compensated for.

The machine is provided with a security device which inhibits operation of the machine until the hydraulic fluid supplied from the reservoir or gearbox 46 attains a pre-determined operating pressure.

Individual adjustment of the throttle mechanisms of the bearings enables accurate alignment of the various parts of the machine and compensation for possible differences in hydraulic fluid pressure in the fluid system. However, the provision of a common fluid circuit facilitates the levelling of fluid pressures in the various bearings.

While particular numbers of hydrostatic bearings are referred to in the description it will be obvious to those skilled in the art that numbers of bearings may be varied. In addition two or more hydraulicfluid circuits may be provided instead of a single circuit.

Finally, although the present invention has been particularly described with reference to a grinding machine it will be appreciated that the invention is applicable to any form of machine tool of this nature.

Claims (17)

1. An apparatus for machining a workpiece comprising a worktable movable along a first path; a tool head for holding a tool, the tool head being movable along a second path transversely of said first path; and wherein each of said worktable and tool head is mounted by way of hydrostatic bearing means.

2. An apparatus as claimed in claim 1 wherein said hydrostatic bearing means comprises at least two hydrostatic guide bearings for guiding movement of each of said worktable and said tool head along the respective paths.

3. An apparatus as claimed in claim 2 wherein each hydrostatic guide bearing has at least one associated hydrostatic support bearing for supporting the worktable and the tool head for movement along the respective paths.

4. An apparatus as claimed in claim 2 or 3 wherein each hydrostatic bearing comprises a chamber for receiving pressurised hydraulic fluid, a hydraulic fluid feed line opening into the chamber and a variable throttle mechanism for throttling the outlet of the feed line into the chamber.

5. An apparatus as claimed in claim 4 wherein the feed lines are connected to a common reservoir or hydroelectric gearbox and wherein the apparatus further comprises means for collecting hydraulic fluid leaking from the bearing and returning this to the reservoir or gearbox.

6. An apparatus as claimed in claim 3, 4 or Sin which at least one pair of guide or support bearings are arranged to act in opposition to one another.

7. An apparatus as claimed in any of claims 2 to 6 wherein two sets of hydrostatic bearings for guiding movement of the worktable are provided at respective locations on each side of the point of action of the tool head on a workpiece and equidistant therefrom.

8. An apparatus as claimed in any of claims 2 to 7 wherein the hydrostatic bearings for supporting the worktable on the apparatus bed are disposed on the table and act in the same direction.

9. An apparatus as claimed in any of claims 2 to 8 wherein two sets of hydrostatic bearings are provided for the tool head, each set having pairs of support bearings and guide bearings and wherein the bearings of each pair are arranged to act in opposing directions.

10. An apparatus as claimed in any of claims 1 to 9 further comprising a head stock whose spindle is rotatably supported by means of two sets of hydrostatic bearings, each set comprising four bearings angularly spaced relative to one another at 90 about the spindle.

11. An apparatus as claimed in claim 10 wherein opposing hydrostatic thrust bearings are provided for restraining axial movement of the spindle.

12. An apparatus as claimed in claim 10 or 11 wherein the spindle is keyed to a co-axial pulley which is rotatably mounted on the head stock and coupled to an electric motor so as to be drivable thereby.

13. An apparatus as claimed in any of claims 1 to 12 further comprising a direct current low-inertia servo motor operable to displae the wheel head along its path through an approach stroke and a power stroke.

14. An apparatus as claimed in claim 13 further comprising a numerical control unit for controlling the motor and operable to provide at least one of the following controls: compensation for redressing of the tool; movement of the tool head into an operating position; and adjustment of the operating position of the tool holder for different diameter portions of a workpiece.

15. An apparatus as claimed in claim 13 or 14 wherein the motor is associated with a tachometer and an encoder by means of which the numerical control is operable to control acceleration, deceleration, speed and retro-action of the motor.

16. An apparatus as claimed in any of claims 1 to 15 wherein the tool head is pivotally mounted relative to the apparatus bed for enabling adjust ment of the angle of attack of the tool head.

17. An apparatus for machining a workpiece substantially as hereinbefore described with reference to the accompanying drawings.