GB1591087A

GB1591087A - Machine tools

Info

Publication number: GB1591087A
Application number: GB4968376A
Authority: GB
Original assignee: National Research Development Corp UK; National Research Development Corp of India
Current assignee: National Research Development Corp UK; National Research Development Corp of India
Priority date: 1977-11-24
Filing date: 1977-11-24
Publication date: 1981-06-10

Description

(54) IMPROVEMENTS IN OR RELATING TO MACHINE TOOLS (71) We, NATIONAL RESEARCH DEVELOPMENT CORPORATION, a British Corporation established by Statute, of Kingsgate House, 66 74 Victoria Street, London, S.W.1, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to machine tools of the kind in which a carriage travels along a slideway against which it is supported by hydrostatic bearing units, and the accuracy of movement required of the carriage is greater than that which the slideway alone will guarantee. Perhaps the slideway, which must often be of robust construction, has never been finished to the required accuracy; another possibility is that a slideway, especially a horizontal one, that has once been true may have distorted due to settlement of foundations. A system is thus needed to monitor the movement of the carriage and to correct it when necessary.

This invention provides such a monitoring and correction system, is defined by the claims at the end of this specification, and apparatus according to the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic plan view of one apparatus; Figure 2 is a schematic diagram to illustrate the setting and working of the apparatus of Figure 1; Figure 3 is a detailed sectional view of parts of the apparatus of Figure 1; Figure 4 is a diagrammatic illustration of the fluid supply of one of the hydrostatic bearing units in Figure 1; Figure 5 is similar to Figure 4 but shows an alternative form of fluid supply; Figure 6 is a diagrammatic plan view of another apparatus; Figure 7 shows a fibre-optic sensor, and Figure 8 is a schematic view including a circuit diagram for three fibre-optic sensors.

Figure 1 shows a machine tool comprising a carriage 1 supported upon the rails 2 of a slideway 3. The carriage is driven up and down the rails by a leadscrew 4 rotated by a motor 5. The carrier is supported upon the slideway by hydrostatic bearings. Location in the horizontal plane is achieved by bearings 6, which bear against the side walls of the rails 2 and are supplied with fluid by systems including pressure sources 7 and restrictors 8.

In the vertical plane support is provided by three hydrostatic bearing units 9, 12 and 15 each mounted upon the carriage and confronting the upper surface of one of the rails 2. Each unit includes a main load-supporting pad and a connected pilot pad; the main and pilot pads of units 9, 12 and 15 are referenced 10, 11; 13, 14 and 16, 17 respectively. The main and pilot pads of each unit are both connected to a supply system including a pressure source and a restrictor. In Figure r the source 18 and restrictor 19 which feed the unit 15 are shown diagrammatically, but the supplies of units 9 and 12 would be similar. The fluid supplies to the three units 9, 12 and 15 are operated by a control system, comprising a computer 20 which receives an input by way of line 21 from a position transducer associated with motor 5 to indicate the position of carriage 1 along the slideway, and which also contains a programme indicating the errors of the slideway. The programme thus constitutes the datum means of the present invention, and part of the function of the position transducer is to act as the datum sensor For every input that it receives from the transducer, computer 20 adjusts the fluid supply to units 9, 12 and 15 to vary the clearance of main bearings 10, 13 and 16 so as to hold carriage 1 to a datum vertical level regardless of slideway errors.

The horizontal location of the carriage may alternatively be provided by two hydrostatic bearing units similar to items 9, 12 and 15, these two units confronting either rail 2 or rail 3. In such a case the opposed pads confronting the other rail may be of the form shown in Figure 1, or may be supplied at constant pressure or may be supplied by way of adding valves as described in UK Patent No.

1,027,395 from the pilot valves of the horizontal units confronting the first rail.

Figure 3 illustrates the construction and variable mounting of a pilot bearing 17, comprising a conduit 25 which fluid from source 18 reaches by way of port 26, and which terminates at mouth 27. In use, this outlet lies within a plenum 28 bounded by a ring seal 29 and by the surface of rail 2. Seal 29 wipes the surface of the rail so that little of the fluid escapes between them and almost all of it exhausts through port 30. Conduit 25 is carried by a plunger 31 which is springmounted on diaphragms 33 to slide within a tube 32. An emergency spring 34 allows conduit 25 to yield a little if mouth 27 accidentally grounds against the surface of the rail. The top end 35 of plunger 31 bears against a circular cam 36 mounted on shaft 37 driven by a stepper motor 38 which receives its command from computer 20. A pheto-detector 39 and a disc 40 carried by shaft 37 co-operate to enable motor 38 to be zeroed.

The inter-action between the pilot bearing and stepper motor and other parts of the apparatus is illustrated diagrammatically in Figure 2. Figure 4 shows the essentials of one suitable association of a pilot bearing (17) with its main bearing (16), the latter being of the "sealed" type and comprising a bearing block 50 with a central fluid port 51, and faced with a resilient member 52 which has a matching port 53 and which defines a load-carrying plenum 54 surrounded by sealing walls 55, 56 defining between them a groove 57 through which the main bearing fluid, like that of the pilot bearing, exhausts to the tank. In a system as shown in Figure 4 in which the flow of fluid through both main and pilot bearings will be small, it may be quite satisfactory as shown to connect them in parallel to the same source. Where greater flow is expected at the main bearing an isolation stage must be added in the main bearing supply, as shown for instance at 60 in Figure 5. The main inlet and outlet of this stage are shown at 61 and 62 respectively, and the restriction offered by the stage is controlled by an input 63 from the pilot bearing. If substantial variation of the load upon the carriage is to be expected, the response of the apparatus may further be improved by adding a diaphragm-controlled restrictor 64, as described in UK Patent No.

1,027,395.

In the alternative apparatus shown in Figures 6 to 8, the datum means for the true movement of carriage 1 along rails 2 is provided not by a pre-programmed computer 20 but by a datum mounted on the apparatus itself. A datum plane is provided by two interconnected troughs 70 containing liquid, the still surface of which will follow the curvature of the earth and will thus for most purposes be flat within acceptable limits. Each of the bearing units 9, 12, 15 will be associated with a separate datum sensing device 71 mounted to confront the liquid surface and to give an output indicating any variation of its clearance with that surface. For simplicity only one such device, associated with bearing unit 9, is shown in Figure 6. The output of each sensing device controls the stepper motor of each bearing unit, and operation is essentially as follows. If the carriage comes to a low point on the rail, then the clearance between the sensing head and the liquid level will fall, giving rise to an output which will cause the stepper motor to lower the pilot bearing, thus decreasing the clearance for fluid flow at that bearing and causing the fluid pressure upstream of the bearing to rise.

Using the circuits of either of Figures 4 and 5, this increases the pressure of the fluid supply to the associated main bearing, which in turn tends to increase the clearance at that bearing. As the carriage rises, the clearance between the sensing head and the fluid level rises also towards its normal value, and equilibrium is attained when that clearance is returned to normal, thus indicating that the carriage is again on its true course, and with the main and pilot bearings adjusted in relative vertical height so that the clearance of the pilot bearing sets the fluid pressure at such a level that the associated main bearing maintains the necessary clearance between the carriage and the rails. When the carriage comes to a high spot on the slideway, a converse compensation process takes place.

An optical sensing head is preferred, and Figure 7 shows one such head comprising a "Y"-shaped junction 75 of an input limb of random-bundled optical fibres 76 and a similar output limb 77. As the inset to Figure 7 shows, these two limbs each form one semicircle of the end face 78 of the "leg" of the "Y"-junction. Other groupings of fibres could be used, for instance concentric circles. End face 78 is mounted on the carriage, close to its associated bearing unit, to face the surface of the liquid, and the device is calibrated by plotting the clearance against the proportion of input optical energy in limb 76 that is reflected to limb 77. Typically, as the head moves away from contact with the surface, this proportion first rises quite sharply, then reaches a maximum and starts to fall more gradually. Figure 8 shows in outline how three such optical sensing devices 80, 81 and 82, each of which will be associated with one of the bearing units 9, 12 and 15, may be supplied from a common stabilised light source 83. The output of each device passes to a comparator (84, 85, 86) where it is compared with a signal from line 87 representing the intended clearance at each sensing head. Any discrepancy operates the stepper motor (88) of the associated bearing unit to initiate compensating vertical movement in the manner already described.

It will be appreciated that the datum need not be of the liquid-level type but could be of other kinds, for instance optical (e.g. laser or collimated light beam) or mechanical (e.g.

straight edge or shaped cam). An advantage of a gravity-sensitive system like a liquid level is that the problems of accommodating it to the rest of the accurate structure of the apparatus, which will itself have been constructed and installed to tolerances and measurements that themselves involved gravity, may be less than if the datum is itself gravity-independent.

WHAT WE CLAIM IS: 1. A machine tool including a carriage, an approximately true slideway, datum means and a datum sensor, in which the carriage is supported from the slideway by a plurality of fluid bearing units each comprising a main load-supporting hydrostatic bearing and a connected pilot hydrostatic bearing, in which the relative setting of the main and pilot bearings of at least one bearing unit is adjustable in response to an output of the datum sensor and in which, when the carriage is at an inaccurate part of the slideway, such adjustment in response to such output is adapted to vary the main bearing fluid supply so as to change the equilibrium position of that bearing unit to one in which a new clearance at the main bearing supports the carriage in its proper position and thus compensates for the slideway inaccuracy.

2. A machine tool according to Claim 1 in which the datum means comprises an indicator of the position of the carriage along the length of the slideway and a store of information as to the inaccuracy of the slideway along its length, and in which the datum sensor is responsive to the indicator and to the store to adjust the relative setting of main and pilot bearings of at least one bearing unit and thus to vary the clearance at the main bearing of that unit.

3. A machine tool according to Claim 2 in which at least one pilot bearing is mounted on the carriage in a manner permitting it to travel in a direction substantially at right angles to the slideway surface.

4. A machine tool according to Claim 3 in which at least one pilot bearing is mounted on the end of a movable plunger itself mounted on the carriage, the position of the plunger being controlled by abutment against a rotary cam driven by a motor and in which the input to the motor is derived from the outputs of the indicator and the store.

5. A machine tool according to Claim 1 in which the slideway is approximately horizontal, in which the datum means comprises the level of a trough of liquid extending along the length of the slideway, and in which the datum sensor is of optical type.

6. A machine tool according to Claim 5 in which the optical sensor comprises a fibreoptic device.

7. A machine tool according to Claim 1 in which at least some of the hydrostatic bearings are of the sealed type.

8. A machine tool according to Claim 1 in which at least some of the hydrostatic bearings are of the unsealed type.

9. A machine tool according to Claim 1 substantially as described with reference to Figures 1-4 or to Figure 5 of the accompanying drawings.

10. A machine tool according to Claim 5, substantially as described with reference to Figure 6 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. intended clearance at each sensing head. Any discrepancy operates the stepper motor (88) of the associated bearing unit to initiate compensating vertical movement in the manner already described. It will be appreciated that the datum need not be of the liquid-level type but could be of other kinds, for instance optical (e.g. laser or collimated light beam) or mechanical (e.g. straight edge or shaped cam). An advantage of a gravity-sensitive system like a liquid level is that the problems of accommodating it to the rest of the accurate structure of the apparatus, which will itself have been constructed and installed to tolerances and measurements that themselves involved gravity, may be less than if the datum is itself gravity-independent. WHAT WE CLAIM IS:

1. A machine tool including a carriage, an approximately true slideway, datum means and a datum sensor, in which the carriage is supported from the slideway by a plurality of fluid bearing units each comprising a main load-supporting hydrostatic bearing and a connected pilot hydrostatic bearing, in which the relative setting of the main and pilot bearings of at least one bearing unit is adjustable in response to an output of the datum sensor and in which, when the carriage is at an inaccurate part of the slideway, such adjustment in response to such output is adapted to vary the main bearing fluid supply so as to change the equilibrium position of that bearing unit to one in which a new clearance at the main bearing supports the carriage in its proper position and thus compensates for the slideway inaccuracy.