WORK HOLDING DEVICES

Abstract: A rotatable work holding device 10 operable by fluid pressure has a slip ring 17 for the distribution of fluid pressure to the work holding device and seals 48 between the slip ring and the work holding device. There are pressure operated means for retracting the seals 48 out of contact with the work holding device when it is desired to rotate the work holding device and this brings about more positive retraction of the seals 48.

Description: Work Holding Devices. The invention relates to work holding devices such as chucks. Rotatable work holding devices such as chucks are known which are operated by fluid pressure which is supplied to a stationary slip ring. When the work holding device is itself stationary fluid can be supplied from the slip ring to the work holding device. Seals must be provided between the slip ring and the work holding device. However these seals must not remain in contact with the work holding device once the device has been caused to grip a worktpiece and starts to rotate, otherwise the seals would burn out. They are therefore normally biased out of contact with the work holding device and are arranged so that when fluid pressure is applied to the slip ring for transmission to the work holding device part of the fluid forces the seals against the work holding device. The invention provides a rotatable work holding device operable by fluid pressure, a slip ring for the distribution of fluid pressure to the work holding device, at least one seal between the slip ring and the work holding device, characterised in that there are pressure operated means for retracting the seal out of contact with the work holding device when it is desired to rotate the work holding device. The provision of fluid operated means to reatrct the or each seal brings about more positive retraction of the seal than with known devices. Preferably the retraction means has its own, independent, fluid supply inlet. Preferably the retraction means is also operable to positively apply the or each seal against the work holding -device. By way of example, a specific embodiment of the invention will now be described, with reference to the accompanying drawings, in which: Figure la is the upper portion of a cross-section through an embodiment of chuck according to the invention; Figure lb is the lower portion of the cross-section through the embodiment; and Figures 2 and 3 are cross-sections through part of the chuck at different cimcumferential locations. The chuck illustrated in the Figures comprises a generally cylindrical body 10 which may be mounted on a spindle of a machine tool for rotation at high speed. The mechanism for gripping a workpiece in the chuck does not comprise part of the present invention and will not be described in detail. It is however described in our co-pending Application No. of even date. The present invention is concerned with a stationary slip ring 17 which surrounds the body of the chuck. However in order not to unduly increase the radial dimensions of the chuck the slip ring 17 is recessed radially inwardly as is evident from Figures la and lb. The ring is supported on the chuck body by means of three bearings spaced apart equally in the circumferential direction, one of the bearings being illustrated at 40 in Figure 2. Each bearing comprises a cam follower in the form of a roller 41 which runs on bearings 42 on a spindle 43 fixed to the ring 17. Each wheel 41 engages in a track 44 extending around the periphery of a part 45 of the chuck body. Kecessed into the slip ring 17 are two anular compressed air distribut#on passages 46 and 47. When compressed air is supplied to passage 46, the jews of the chuck are moved in one direction, and when air is supplied to #the other passage 47, the jaws of the chuck are moved in the opposite direction. As can be seen from Figure lb, air can be supplied to passage 46 through sn inlet 16. Air can also be supplied when necessary, through a similar inlet arranged at a different circumferential location, through the passage 47. It is necessary for the passages 46 and 47 to be separated by seals from each other and from the atmosphere Three '0' ring seals 48 are therefore'provided, one between the passages 46 and 47 and one on each side of the passages. The seals are accommodated in recesses in the slip ring 17. When it is desired to apply fluid pressure to the chuck to open or close the jaws, control means (not shown) are operated. An electric pick-up 49 (shown at the foot of Figure 2 is mounted in the slip ring and is sensitive to the passage of holes provided in the body of the chuck. If the pick-up jails to detect the presence of a hole within a given period of time then this indicates that the chuck is stationary and that it is safe to operate the chuck. The control means then applies fluid pressure through an inlet 50 shown in Figure 3, The inlet 50 has three branches 51 which coininunicate with the bottom of the recesses housing the seals 48. Thus fluid pressure forces the seals 48 against the body of the chuck. Fluid pressure can then be applied to the passage 46 or passage 47, as desired, to operate the chuck, for example as described in our co-pending Patent Application No. of even date. Once the chuck has been opened or closed as desired, it it necessary to ensure that the seals 48 are retracted away from the body of the chuck before rotation of the chuck recommences, otherwise the seals would burn out because of frictional heating. retraction of the seals is achieved by applying suction to the inlet 50, which sucks the seals 48 away from the chuck body. The means to apply suction is not critical to the invention and may for example comprise a vacuum pump or a venturi device. When air pressure is applied to open or close the chuck, forces are applied at the radially extending interface between the slip ring 17 and the chuck body, which tends to force the slip ring 17 away from the chuck body in the left-hand direction as viewed in the Figures. The engagement of the wheels 41 in the track 44 may be insufficient to resist this tendency for movement without causing damage to the bearings 42. Three abutment devices are therefore provided, equally spaced apart. around the periphery of the chuck, each abutment device lying between two of the bearings 40. One of the abutment devices 52 is shown in Figure la. The abutment device 52 comprises a metal block 53 which fits within a recess provided in the slip ring and is clamped against the slip ring by a cover plate 54 which is fastened over the recess by bolts (not shown). A pin 55 secured to the cover plate engages in an aperture in the block 53 to prevent radial movement of the block 53. The block 53 has an abutment face 56 which is ground down so that when the chuck is in use it is just clear of the left-hand radially extending face of the track 44. This is to avoid undesirable rubbing contact between the block 53 and the body of the chuck during rotation of the chuck. When air pressure is applied to open and close the chuck in its stationary condition however, the slightest left-hand movement of the slip ring 17 brings the abutment face 56 into contact with the left-hand radial face of the track 44, thus preventing any further movement of the slip ring. The invention is not restricted to the details of the foregoing embodiment. For instance, instead of using the abutment blocks55, studs or other abutment devices could be provided which project axially from the slip ring 17 to abut against the headetock or other part of the machine tool to which the chuck is to be fitted. The axial length of these devices could be adjustable. Although the use of three abutment blocks has been described, a different number may be used, or alternative means maybe provided to ensure that the slip ring 17 gains support either from the machine tool itself or from the body of the chuck. Dor example, the slip ring 17 may be provided with a continuous flange-like abutment ring which engages with part of the chuck body. If desired, the recesses which house the O-ring seals 48 may be inclined slightly to the axial direction, so that the bottom of each recess lies sliehtly nearer to the centre of the chuck than the mouth of the recess. With such an arrangement the resilience of the seals may tend to bias them into the bottom of the recesses, i.e. away from the body of the chuck.

Claims: Claims 1. A rotatable work holding device (10) operable by fluid pressure, a slip ring (17) for the distribution of fluid pressure to the work holding device and at least one seal (48) between the slip ring and the work holding device, characterised in that there are pressure operated means for retracting the seal out of contact with the work holding device when it is desired to rotate the work holding device. 2. A device as claimed in Claim 1, in which the retraction means has its o-çm, independent, fluid supply inlet. 3. h device as claimed in Claim 2., in which the retraction means is operable to positively apply the or each seal (48) against the work holding device.

Inventors: JACKSON, JOSEPH FENWICK, PICKLES, KENNETH BAILEY PHILIP
Applicants: PRATT BURNERD INTERNATIONAL LIMITED
Application year: 1983
Main classification: B23B31
IPC codes: B23B31