Description

The present invention relates to a centring device for centring a bar-shaped body, such as a rod or tube, in a cylindrical hollow bore, such as the hollow bore of the hollow shaft of a machining machine.

In the case of lathes, the bar-shaped material to be machined is often fed via a so- called passage to the chuck jaw so that it can be machined at the other side of the chuck jaw. Said passage is often formed by the hollow drive shaft of the lathe. The axial length of the passage can easily be 400 mm to 1500 mm. During machining, the bar-shaped material to be machined, which extends through the passage, will rotate on the lathe, frequently at high speed. This means that the bar-shaped material extending through the passage must be centred around the axis of rotation. If this is not done, or is not done properly, the bar-shaped mateπal will tend to chatter during machining on the lathe, which can lead to damage to the machining machine. Furthermore, in the case of lathes which come up to speed very fast, the bar-shaped material to be machined will, if it is not properly centred, be subjected to high centrifugal acceleration, which can lead to very substantial damage to the machining machine and to very dangerous situations.

In order to prevent the abovementioned problems, centring devices for centring a bar- shaped body in a cylindrical hollow bore of the hollow shaft of a machining machine have been proposed.

For instance, German "Offenlegungsschrift" 2 528 130 discloses a centring device which consists of a bush which can, accurately fitting, be placed in the bore of the hollow shaft and at one end is retained by a retaining nut which engages on the external screw thread of the housing of the hollow shaft. Said holder is provided in two positions with a guide ring which has an internal diameter which must have a play of 0.5 mm with respect to the bar-shaped body to be centred. Said known device must be manufactured with high accuracy and nevertheless does not produce good centring of the bar-shaped body. Furthermore, said centring device must be fitted at that end of the drive shaft which is remote from the chuck jaw and tightened by means of the retaining nut which can be screwed onto that part of the housing of the

hollow shaft located at said end. Consequently, said known centring device cannot be positioned in an arbitrary position in the passage formed by the hollow shaft of the lathe.

A centring device according to the preamble of Claim 1 is disclosed in US-A

2 398 278. This publication discloses an adjustable stop with a centring clamping element for clamping in a bore. The stop and a conical body which forms part of the clamping element are fixed relative to one another. The bar-shaped stop is provided at one end with a slot in which a screwdriver can engage. Expansion segments are provided on the conical body, which expansion segments produce the clamping in the bore on tightening a nut.

When tightening the nut, a screwdriver has to be placed in the slot provided in the stop to prevent the centring clamping device also rotating when the nut is tightened. This has the disadvantage that the slot has to be provided in an accessible position.

DE-A 1 925 067 discloses an accessory for fitting in a tube which has to be machined on a workbench. Said accessory consists of a mandrel which has three slots, distributed over the circumference, in which rollers are arranged which are able to roll in the circumferential direction of the mandrel in the slots so as to be able to effect clamping of the mandrel in the tube on rotation of the mandrel with respect to the tube. In view of the shaping of the slot, said clamping is achieved both when the mandrel is rotated in the anticlockwise direction and when the mandrel is rotated in the clockwise direction. This makes removal of the mandrel from the tube appreciably more difficult. The aim of the present invention is to provide an improved centring device which can be positioned in an arbitrary position in a cylindrical hollow bore.

According to the invention, this aim is achieved with a centring device of the type indicated in the preamble, in that the centring device comprises a holder which is hollow in the longitudinal direction, is slidable in the hollow shaft and at one end interacts with a clamping bush via a threaded connection, in that at least one clamping element is arranged on the holder, which clamping element is able to exert a radial clamping force on the inside wall of the hollow shaft under the influence of an axial force exerted on said clamping element on tightening the clamping bush, and in that the holder is further provided with at least two, preferably three or more, pre¬ clamping systems, distributed uniformly around the circumference of the holder, each pre-clamping system comprising a groove formed in the outer surface of the holder

and extending in the circumferential direction, which groove is provided with a rolling body which is rollable in the circumferential direction through the respective groove, and each groove having a depth, viewed in the radial direction, which increases from a relatively shallow depth to a relatively deep depth in the tightening direction of the clamping bush, such that on tightening the clamping bush the rolling bodies move towards the shallow section of the grooves and engage with the inner wall of the hollow shaft to prevent rotation of the holder relative to the hollow shaft during tightening of the clamping bush, whilst on rotation of the centring device in the unscrewing direction of the clamping bush, said rolling bodies seek out the relatively deeper points in the grooves to allow said rotation.

The centring device according to the invention can be slid in the hollow shaft and the bar-shaped body to be centred can be accommodated in the longitudinal direction of the hollow holder, if appropriate with the aid of an adapter matched to the bar-shaped body. The hollow holder can be slid through the hollow shaft until it is in the correct position, after which the clamping bush can be tightened, by means of a tool to be inserted in the hollow shaft, to bring the clamping element into engagement with the inside wall of the hollow shaft. Clamping elements which can be considered in this context are one or more splines, which run up over an upward sloping surface in the axial direction on tightening the clamping bush and thus exert a clamping force acting in the radial direction on the inside wall of the hollow shaft. With this arrangement, the prc-clamping systems ensure that, during tightening of the clamping bush, the holder and the at least one clamping element do not turn with respect to the hollow shaft which, in fact, would impede tightening of the clamping bush or make this more difficult. Specifically, the rolling bodies arranged in the grooves will roll towards the shallow side of the grooves on slight turning of the holder and as a consequence of this will engage with the inside wall of the hollow shaft. If the centring device has to be moved or removed from the hollow shaft, the rolling bodies will not exert any clamping force on the inside wall of the hollow shaft, so that, after releasing the clamping effect under the influence of the at least one clamping element by unscrewing the clamping bush sufficiently, the centring device is able freely to translate in the hollow shaft and is able freely to rotate in the unscrewing direction of the clamping bush.

The receiving opening of the holder, which is hollow in the longitudinal direction, must be adapted depending on the cross-sectional shape and dimensions of the bar-shaped

body to be centred. This can be effected in a simple manner by means of an accessory. According to the invention, such an accessory can advantageously and easily be fitted in the holder when the holder has a retaining ring provided with the pre-clamping systems located at the other end from the clamping bush, which retaining ring can be fixed to the holder by means of a threaded connection, and when the centring device also has an accessory which can be accommodated in the hollow holder and which can be retained in the holder by means of the retaining ring in an interchangeable manner. With this arrangement the screw thread for the threaded connection of the ring to the holder will be tapped in the same direction as the screw thread for the threaded connection between the holder and the clamping bush. By fitting the pre-clamping systems on the external circumferential surface of the retaining ring, it is also possible with this arrangement to replace this part of the centring device as a separate component if the grooves have become worn or the rolling bodies have been damaged.

According to the invention, very good clamping action between the inside wall of the hollow shaft and the centring device can be achieved when the clamping clement is an expansion sleeve fitted on the holder, which expansion sleeve is provided with a multiplicity of incised slits which run in the longitudinal direction and alternately are open in opposing directions towards an axial end of the expansion sleeve, one end of said sleeve being chamfered such that it widens towards the outside and the other end being chamfered such that it narrows towards the inside, and when the holder is provided with two circumferential surfaces which essentially slope correspondingly, such that on tightening the clamping bush the expansion sleeve is forced with its chamfered ends over the sloping circumferential surfaces.

In order further to prevent rotation of the centring device with respect to the cylindrical hollow bore during tightening or loosening of the clamping bush, it is advantageous, according to the invention, when an axial bearing, such as an axial ball bearing, is fitted between the clamping bush and the clamping clement. With this arrangement, that end of the clamping bush which faces the holder will preferably be provided with a pressure ring and the axial bearing will have been arranged between the pressure ring and the clamping bush. The purpose of the axial bearing with this arrangement is to reduce, and if possible completely to prevent, forces acting in the circumferential direction, which act on the at least one clamping clement, which forces could lead to high friction between the clamping clement and the clamping bush on

tightening. Such friction would lead to wear of the clamping bush and/or the at least one clamping element. Furthermore, co-rotation of the entire centring device when the clamping bush is tightened or loosened is further prevented by this means. This is because, on tightening, from a certain point in time the at least one clamping element will take over from the pre-clamping systems to provide clamping. Furthermore, because of their construction, the pre-clamping systems will not exert a clamping effect on unscrewing.

The present invention will be explained in more detail below with reference to an illustrative embodiment shown in the drawing. In the drawing:

Fig. 1 shows a longitudinal section, partially in elevation, of a centring device according to the invention;

Fig. 2 shows a view of the centring device according to Fig. 1; and

Fig. 3 shows a cross-sectional view along II — II in Fig. 2.

Fig. 1 shows a centring device 1 having, as most important components, the holder 2, the clamping element 6 fitted on the holder 2, the clamping bush 3, the accessory 7 and the retaining ring 8 for retaining the accessory.

Said centring device fits in its entirety in a slidable manner in a cylindrical hollow bore 12, indicated by dash-and-dot lines, of the hollow shaft of a lathe, which is not shown in more detail. The cylindrical hollow bore 12 forms the so-called passage, which in practice frequently has a length of 400 mm to 1500 mm. Longer and shorter passage lengths arc also found in practice.

In the illustrative embodiment shown, the accessory 7 is an adapter ring, the external diameter of which essentially corresponds to the internal diameter of the holder 2 without play and the internal diameter of which preferably likewise corresponds, without play, to that of the rod 13 shown diagrammatically by dash-and-dot lines, which is to be machined on the lathe. The rod 13 will preferably be slidable through the adapter 7. If a rod or tube of a different diameter or different cross-sectional shape has to be machined, an adapter ring 7 matched to said different diameter or cross-sectional shape will be fitted in the centring device 1. This replacement of the adapter ring 7 proceeds as follows. The retaining ring 8, which has been screwed onto the holder 2 by means of a threaded connection 17, is unscrewed, the adapter ring 7 is removed, a new adapter ring 7 is slid into the holder 2 and the retaining ring 8 is screwed back onto the holder 2 until the adapter ring 7 is retained between the stop 18, formed on the holder 2, and the retaining ring 8.

The adapter ring 7 can also be made closed at an axial end. This can, for example,

be the case if a bar-shaped body 13 which extends through the hollow shaft of a lathe and is to be machined does not extend through the complete hollow shaft and has to be supported at that end thereof which is located inside the hollow shaft. It is also conceivable for the accessory 7 to be an essentially solid cylindrical body which is provided with one or more long pins arranged in the axial direction of the centring device, centrally around the axis of rotation of the hollow shaft 12. If use is made of one pin, said pin will then extend along the axis of rotation of the hollow shaft 12. Such pins can be used, for example, as stop elements for a relatively short bar-shaped body which is to be machined on the lathe and which extends only over a relatively short section into the hollow shaft 12.

The centring device 1 according to the invention functions as follows. The centring device, provided with a suitably chosen accessory 7, is slid into the cylindrical bore 12 of the hollow drive shaft of a workbench until said device is in the correct position in the hollow shaft. By means of a tool, which can be inserted in the hollow shaft and is provided with coupling means for engaging on the clamping bush 3, the clamping bush 3 is then tightened, as a result of which the latter moves in the axial direction towards the holder 2. As a result pressure is applied to the clamping element 6, which will be described in more detail, in the axial direction, so that said element expands in the radial direction. During this operation, the clamping element 6 comes into clamping contact with the inside wall 12 of the hollow shaft.

To enable the clamping bush 3 to be tightened with the aid of a tool which can be inserted in the hollow shaft, the clamping bush 3 is, in the illustrative embodiment shown, provided with two diametrically opposite passages 15 in which two transverse pins arranged on a (long) rod are able to engage. An engagement construction of this type for causing a tool to engage on a sleeve located in a passage is known per se.

In order to prevent the entire centring device from rotating relative to the cylindrical hollow bore 12 during tightening of the clamping bush 3, three pre-clamping systems 9 are fitted in the retaining ring 8. Said pre-clamping systems are arranged with mutual angular spacings of 120° in order to be able to achieve pre-clamping acting uniformly over the circumference of the centring device. The prc-clamping systems 9 each consist of a groove 10 formed in the outer circumferential surface of the retaining ring 8 with a rolling body arranged in said groove. The rolling bodies used can be, for example, cylinders. In the illustrative embodiment shown, balls 11 are used as rolling bodies. The grooves of the pre-

clamping systems extend in the circumferential direction of the centring device and have a depth, viewed in the radial direction, which varies in the circumferential direction of the centring device. If the tightening direction of the clamping bush 3 is shown by the arrow R, the deepest point 25 of each groove is then located at that end of the respective groove which faces the direction of rotation and the shallowest point 26 of each groove is located at that end of the respective groove which faces counter to the direction of rotation. The depth of each groove, viewed in the radial direction, thus increases in the tightening direction of the clamping bush from a relatively shallow depth to a relatively deep depth. With this arrangement, the relatively shallow depth is less than the diameter of the ball 11. In the example shown, the relatively deep depth is equal to or greater than the diameter of the rolling body. Assuming that the external diameter of the retaining ring 8 essentially corresponds to the internal diameter of the cylindrical hollow bore 12, this means that the balls 11 will not engage, or at least will not all simultaneously engage, on the cylindrical hollow bore 12 when said balls are in the deep point 25 of the grooves. The balls 11 will be in this position when the retaining ring 8 rotates together with the centring device 1 in the direction opposite to that according to arrow R in Fig. 3. When the retaining ring 8 rotates in the direction indicated by arrow R, the balls 11 will then roll towards the shallow end of the groove 10 and engage with the inside wall 12 of the cylindrical hollow bore. In this way co-rotation of the entire centring device 1 on tightening in the direction R of the clamping bush 3 is prevented. As soon as the clamping element 6 starts to build up a clamping force between the centring device 1 and the inside wall 12 of the cylindrical hollow bore, the clamping force produced by the clamping element will start to predominate over the clamping force produced by the pre-clamping systems 9. From this point in time, the effect of the clamping action of the balls will cease. It will be clear that in principle diverse types of clamping elements are conceivable as clamping element 6. For instance, consideration can be given to a ring made of a compressible material which on axial compression expands in the radial direction. Consideration can also be given to a number of wedge-shaped splines which are arranged distributed over the circumference of the holder 2 and which, on subjection to pressure in the axial direction, move outwards in the radial direction and thus are able to exert a clamping force on the inside wall 12 of the cylindrical hollow bore.

However, the clamping element according to the invention advantageously comprises an expansion sleeve 6 which at one end is chamfered such that it widens towards the

outside - face 20 in Fig. 1 - and at its other end is chamfered such that it narrows towards the inside - face 19 in Fig. 1. The holder 2 is provided with two essentially corresponding sloping peripheral faces 21 and 22. When the expansion sleeve 6 is subjected to pressure in the axial direction its sloping faces 19 and 20 will be pushed along the sloping faces 21 and 22 of the holder so as to expand in the radial direction. With this arrangement, said expansion in the radial direction is particularly promoted because a multiplicity of incised slits 23 and 24 is provided running in the longitudinal direction of the centring device and alternately open in opposing directions towards an axial end of the expansion sleeve. With this arrangement, the incised slits 23 are open towards the right-hand axial end and the incised slits 24 arc open towards the left-hand axial end. The incised slits open into round passages, indicated diagrammatical ly in Fig. 2, which are intended to prevent stresses generated in the expansion sleeve at the location of the incised slits from leading to cracking of the expansion sleeve 6.

The clamping bush 3 is provided at that end thereof which faces the clamping element 6 with a pressure ring 4 which, by means of an axial bearing 5, has a bearing between itself and the clamping bush 3 and is secured in the axial direction on the clamping bush 3 by means of a retaining spring 14. Said pressure ring 4 has an internal diameter which is greater than the external diameter of the screw thread provided for the threaded connection 16 on the end of the holder 2. With this arrangement, the axial bearing 5 ensures that the clamping element 6 is essentially stressed only in the axial direction by the clamping bush 3. Wear as a consequence of a part moving over the clamping element 6 during tightening is thus prevented. Furthermore, torque transfer from the clamping bush 3 to the clamping element 6 is prevented in this way, so that the clamping element 6 is not brought into rotation with respect to the inside wall 12 during tightening or loosening. After all, from a certain point in time the radial clamping force of the clamping element 6 will take over from the clamping force provided by the pre-clamping systems 9. The axial bearing 5 prevents the entire centring device from starting to co-rotate on further tightening or loosening of the clamping bush 3 in the event of very firm clamping of the centring device 1 in the hollow cylindrical bore 12, which co-rotation could make it impossible to unscrew the clamping bush 3, and thus also to remove the centring device from the bore.

It will be clear that the centring device according to the invention is suitable for use in cylindrical bores in general and thus can be used not only for centring in the hollow shaft of a machining machine, such as a lathe. With the centring device according to the

invention it is also readily conceivable that the prc-clamping systems and/or the at least one clamping element 6 are suitable for exerting a clamping force directed radially inwards instead of a clamping force directed radially outwards on the body to be centred. Furthermore, it will be clear that the axial bearing between the prc-clamping bush 3 and the at least one clamping element 6 can also be used in centring devices which arc not provided with one or more pre-clamping systems or which are provided with pre-clamping systems of different construction and that, in that case also, the axial bearing ensures that no wear is produced by frictional engagement with the clamping element caused under the influence of the tightening/loosening torque, and that the ability to remove the centring device from the cylindrical hollow bore is ensured.