High-speed spindle with forced mechanical vibratory assistance

Abstract

A spindle for a machine tool, including a housing, a shaft for driving a cutting tool, rotatably mounted inside the housing with a possibility of axial movement relative to the housing, a single ball, axially interposed between a bearing ring fixed relative to the housing and a bearing ring movable with the shaft, one of these rolling rings defining an inclined bearing surface that is not perpendicular to the axis of rotation of the shaft, so that the rotation of the ball generates an axial oscillation of the shaft.

Claims

1. A high-speed spindle for a machine tool, having a housing, a shaft for driving a cutting tool, mounted rotatably inside the housing so as to be able to move axially relative to the housing, a single ball, interposed axially between a rolling bearing outer ring that is fixed relative to the housing and a rolling bearing outer ring that is movable with the shaft, one of these rolling bearing outer rings defining an inclined rolling bearing surface that is not perpendicular to an axis of rotation of the shaft, such that the rotation of the ball brings about an axial oscillation of the shaft, the spindle being capable of operating at a rotational speed of the shaft greater than 10 000 rpm wherein the ball is partially fitted in a radially open annular groove formed in the shaft.

2. The spindle as claimed in claim 1, wherein the fixed rolling bearing outer ring is the one that defines the inclined rolling bearing surface.

3. The spindle as claimed in claim 1, wherein the inclined rolling bearing surface is planar.

4. The spindle as claimed in claim 1, wherein the ball is made of ceramic.

5. The spindle as claimed in claim 1, wherein the ball is situated at a rear part of the spindle opposite to a front part of the spindle, said front part of the spindle forming a working extremity of the spindle.

6. The spindle as claimed in claim 1, comprising two sets, each set comprising two ball bearings, one set of said sets being mounted on a front part of the spindle, said front part of the spindle forming a working extremity of the spindle, the other set of said sets being mounted at a rear part of the spindle, said rear part being opposite to the front part of the spindle.

7. The spindle as claimed in claim 1, comprising at least one flat spring ensuring an axial preloading of the shaft toward a rear part of the spindle, said rear part being opposite to a front part of the spindle, said front part of the spindle forming a working extremity of the spindle.

8. The spindle as claimed in claim 1, wherein the rolling bearing outer rings are held by strips of annular shape, said strips having, on an outer circumference, fixing tabs that are fixed relative to the housing and, between these fixing tabs, tabs for retaining the rolling bearing outer rings, each strip comprising a portions extending between the fixing tabs and the tabs for retaining the rolling bearing outer rings said portions of the strips being dimensioned to allowing the rolling bearing outer rings to move axially during the axial oscillations of the shaft.

9. The spindle as claimed in claim 8, wherein the rolling bearing outer rings are mounted on bearings supports that are prevented from rotating relative to the strips by pins passing through the strips, the bearings supports having bearing sectors forming a protrusion on their end edge, the bearing sectors resting against strip sectors of the strips in the region of the tabs in order to retain the rolling bearing outer rings, the strips being in contact with an outer rings of the rolling bearing outer rings via the retaining tabs.

10. The spindle as claimed in claim 9, comprising flat springs, said flat springs pressing the strips against the outer rings of the rolling bearing outer rings.

11. The spindle as claimed in claim 8, comprising a main tubular spacer, said tubular spacer being fixed relative to the housing, the spindle comprising fixed positioning rings one of said fixed positioning rings being disposed on a first side of the main tubular spacer, an other of said fixed positioning rings being disposed on a second side of the main tubular spacer, the second side of the main tubular spacer being opposite to the first side of the main tubular spacer, the fixing tabs of the strips being gripped between the main tubular spacer and the positioning rings.

12. The spindle as claimed in claim 10, comprising end rings, wherein for each bearing support of said bearings supports, one of said end rings being disposed on a first side of said bearing support, an other of said end rings being disposed on a second side of said bearing support the second side of the bearing support being opposite to the first side of the bearing supports, said pins being fitted on said end rings, the flat springs pressing against one end of said end rings, an other end of said end rings bearing against the tabs for retaining the strips.

13. The spindle as claimed in claim 8, wherein the rolling bearings and are angular contact rolling bearings.

14. The spindle as claimed in claim 1, comprising a peripheral rolling bearing ring coaxial with the shaft, said peripheral rolling bearing outer ring being configured to resist to centrifugal forces of the single ball.

15. The spindle as claimed in claim 1, wherein a ratio between a diameter of the ball and a diameter of a trajectory of the contact point of the ball along the inclined rolling bearing surface is between and .

16. A machining method, in which the shaft of a spindle as defined in claim 1, is driven at a rotational speed of at least 10 000 rpm.

17. A machining method in which an vibration frequency of the shaft of the spindle as claimed in claim 1 is between 0.4 and 0.6 axial oscillations per revolution.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention may be understood better from reading the following description of nonlimiting implementation examples thereof and from examining the appended drawing, in which:

(2) FIG. 1 schematically shows a perspective view of an example of a spindle according to the invention,

(3) FIG. 2 is a longitudinal section through the spindle in FIG. 1,

(4) FIG. 3 shows the rear of the spindle in more detail,

(5) FIG. 4 shows the front of the spindle in more detail,

(6) FIG. 5 shows a perspective view of an elastic strip on its own,

(7) FIG. 6 shows a perspective view of a bearing end ring on its own,

(8) FIG. 7 shows a perspective view of a bearing on its own,

(9) FIG. 8 shows the ring having an inclined rolling bearing surface in axial section, and

(10) FIG. 9 shows an embodiment variant of the spindle in longitudinal section.

DETAILED DESCRIPTION

(11) The spindle 1 according to the invention, shown in particular in FIGS. 1 to 4, has a housing 10, preferably metallic, with the overall shape of a cylinder of revolution about a longitudinal axis X.

(12) The casing 10 is mounted in a guiding and advancing mechanism (not shown), known per se, of the machine tool. A support 11 fixed to the housing allows said mechanism to move the spindle 1 axially by the distance necessary to produce the drilled hole.

(13) The spindle 1 has a shaft 20 which is intended to carry at the front a tool such as a drill bit (not shown) and which is coupled at the rear to a pulley 21 for driving it in rotation. The drill bit has for example a diameter less than or equal to 2.5 mm.

(14) The rotational speed of the shaft 20 is for example between 10 000 and 20 000 revolutions per minute.

(15) The invention is not limited to a particular tool, or to the creation of the drilled hole. It may in particular prove useful to carry out machining operations such as milling operations, counterboring operations, etc.

(16) The shaft 20 is guided in rotation about the axis X relative to the housing 10 by sets of front rolling bearings 30 and rear rolling bearings 40.

(17) The set of front rolling bearings 30 has two angular contact ball bearings 31, the angle of contact of which is for example 15, each having an inner ring 32, in contact with the shaft 20, balls 33, an outer ring 34 and flanges 35. The rolling bearings 31 bear against one another and are fitted in a front bearing 50.

(18) The set of rear rolling bearings 40 is embodied in a similar way, with two angular contact rolling bearings 41, the angle of contact of which is for example 15, each having an inner ring 42, in contact with the shaft 20, balls 43, an outer ring 44 and flanges 45. The rolling bearings 41 bear against one another and are fitted in a rear bearing 51.

(19) The inner ring 42 of the rearmost rolling bearing comes to bear axially against a shoulder 23 of the shaft 20, as can be seen in particular in FIG. 3.

(20) A tubular inner spacer 24 is mounted on the shaft 20 between the sets of front rolling bearings 30 and rear rolling bearings 40, and comes to bear at its ends against the inner rings 32 and 42 of the corresponding rolling bearings.

(21) A blocking ring 70 is fixed to the shaft 20 at the front and immobilizes the inner rings 32 of the rolling bearings 31, the inner spacer 24 and the inner rings 42 of the rolling bearings 41 and causes them to axially preload the shaft.

(22) The ring 70 is fixed to the shaft in the example illustrated with the aid of three cone-point set screws 71 which make it possible to correct an out-of-roundness, if necessary.

(23) An O-ring seal 72 is accommodated in a groove 73 in the shaft 20 and presses against the blocking ring 70.

(24) The housing 10 is closed at the front by a front nut 90, screwed therein, and at the rear by a rear closure part 95, which can be held in various ways on the housing 10, for example with the aid of a nut %, as illustrated in FIG. 2.

(25) The front nut 90 has a forwardly directed collar 190, which forms a chicane 192 with a rearwardly directed collar 191 of the blocking ring 70.

(26) An inner ring 195 is mounted on the shaft 20 at the rear, and has a forwardly directed collar 196, which forms a chicane 198 with a collar 197 of the closure part 95.

(27) The chicanes 192 and 198 form a contactless sealing system at the front and the rear of the spindle 1 while providing a clearance allowing the rotation and translational movement without friction between the facing rotating and fixed parts.

(28) An O-ring seal 199 is accommodated in a groove 27 in the shaft 20 and presses against the facing surface of the inner ring 195.

(29) A series of spacers are disposed in the housing 10 in contact with its inner surface, being immobilized between the front nut 90 and the rear closure part 95, namely, from the front to the rear; a ring forming a front spacer 91, a ring forming a front bearing spacer 92, a main tubular spacer 93 and a ring forming a rear bearing spacer 94.

(30) Four stacks 100, 101, 102 and 103 of elastic strips 110 are interposed axially between the spacers 91 and 92, between the spacers 92 and 93, between the spacers 93 and 94, and between the spacers 94 and 95, respectively.

(31) Each stack 100, 101, 102 or 103 has, in the example in question, at least two strips 110, for example five, one of which is shown on its own in FIG. 5.

(32) Each strip 110 has an annular overall shape and has fixing tabs 111, distributed regularly at its periphery, of which there are three in the example in question, which are directed radially toward the outside and which bear with their radially outer edge against the inner surface of the housing 10. The height of the fixing tabs 11l is slightly greater than the thickness of the bearing spacers 92 and 94.

(33) The circular-arc portions 112 connecting the fixing tabs 111 carry, half-way along their length, other tabs 113, which are directed radially toward the inside. These tabs 113 each have a radial slot 114 which opens, at one end, onto the radially inner free edge of the tabs 113 and, at the opposite end, into a circular hole 115 formed in the circular-arc portion 112.

(34) The front bearing 50 is disposed between two bearing end rings 121 and 122. Pins 130 are fitted in drilled holes 140 and 141 corresponding to these end rings 121 and 122 and the front bearing 50, in order to keep the rings 121 and 122 in a predetermined angular orientation with respect to the front bearing 50.

(35) These pins 130 pass through the strips 110 by virtue of the holes 115. The slots 114 make it easier to fit the pins 130. Thus, the stacks 100 and 101 are kept angularly in a predefined position with respect to the bearing 50 and to the end rings 121 and 122.

(36) The bearing 50 and the rings 121 and 122 have, on their facing faces, protruding sectors 143, as can be seen in FIGS. 6 and 7, the angular extent of which corresponds substantially to that of the of the tabs 113, and which enclose the latter between one another.

(37) The rear bearing 51 is similarly disposed between bearing end rings 120 and 121, and pins 130 angularly immobilize the strips 110 disposed therebetween, as in the case of the front bearing 50.

(38) The tabs 113 of the strips 110 come axially into contact with the outer rings 32 and 42 of the rolling bearings 31 and 41.

(39) This assembly allows a certain freedom of movement in the axial direction of the sets of rolling bearings 30 and 40, while keeping them centered as a result of the stiffness of the strips 110 in the radial direction, as will be described in detail below.

(40) The main spacer 93 is formed with a shoulder 171 at each of its ends, set back from an end portion 172 surrounding the corresponding end ring 121 or 122.

(41) A flat spring 170 is mounted inside each end portion 172 and is interposed axially against the shoulder 171 and this end ring 121 or 122.

(42) At the front, two superposed flat springs 170 are mounted around the blocking ring 70 and are interposed axially between the front nut 90 and the end ring 121, as can be seen in FIG. 2.

(43) The closure part 95 is formed with a shoulder 176 and an end portion 177 in front of the latter, which extends around the adjacent end ring 122.

(44) A flat spring 170 is mounted inside the end portion 177 and is interposed axially between the closure part 95 and the adjacent end ring 122.

(45) The flat springs 170 grip the elastic strips around the front and rear rolling bearings via the end rings 121 and 122, by way of the tabs 113 bearing on the outer rings of the rolling bearings.

(46) The presence of an additional flat spring 170 at the front, between the nut 90 and the adjacent end ring 121, creates permanent elastic loading of the shaft 20 toward the rear in order to press the ball 200 onto the rings 201 and 202.

(47) According to the invention, the spindle 1 has a mechanism for generating axial oscillations of the shaft 20 while it rotates.

(48) This mechanism has a single ball 200 which rolls between a rotating rolling bearing ring 201, which is mounted on the shaft 20 and rotates therewith, and a fixed rolling bearing ring 202, which is carried by the closure part 95.

(49) A peripheral rolling bearing ring 203 is inserted into the closure part 95, after the shoulder 176, and extends around the path followed by the ball 200 while it rotates. This peripheral ring 203 makes it possible to react the centrifugal forces during the rotation of the ball 200.

(50) The rotating ring 201 is held against a shoulder 28 of the shaft 20, which borders an annular groove 29, the concavity of which substantially conforms to the path followed by the ball 200.

(51) The rolling bearing ring 202 has a rear face 230, which is planar and perpendicular to its axis, and a front face 231, which is planar and extends obliquely, as can be seen in FIG. 8, the normal to this face making an angle g with the axis of the rolling bearing ring 202, which is a few degrees, for example around 0.3 in the example in question. The formula for g is: g=Arctan(amplitude/d.sub.path), with amplitude corresponding to the total peak/trough variation of the vibration oscillation, d.sub.path being the diameter of the path of the contact point.

(52) As can be seen, the rolling bearing surface 231 does not have a step.

(53) Thus, during its rotation about the axis X, the ball 200 carries out a periodic and sinusoidal axial movement which is caused by the inclination of the front face 231. The ball 200 is only in contact, during its high-speed rotation, with the fixed rolling bearing ring 202, the rotating rolling bearing ring 201 and the peripheral rolling bearing ring 203. On account of the use of a single ball, the rolling of the latter induces bending stresses on the shaft, but this remains controlled and with an acceptable amplitude on account of the relatively small distance between the ball 200 and the spindle axis 20.

(54) The fact that there are no undulations on the rolling bearing surface 231 but rather a flat surface makes it possible to produce the latter very easily, with a very good surface state.

(55) Preferably, the ball 200 is made of ceramic. Its diameter is preferably greater than or equal to 5 mm, making it possible to reduce the Hertz pressure at the contacts. Its diameter is for example 6 mm.

(56) To mount the spindle 1, all the internal constituent elements can be disposed on the shaft 20, and the assembly can be inserted via the front end of the housing 10, the closure pan 95 already being in position, and then the front screw 90 can be fixed.

(57) The spindle 1 operates as follows.

(58) The shaft 20 is driven in rotation by the pulley 21, for example by a belt.

(59) The ball 200 rolls between the rolling bearing rings 201 and 202 and in doing so moves the shaft 20 forward counter to the preload associated with the presence of an additional flat spring 170 at the front.

(60) The movement of the shaft 20 is possible on account of the presence of the strips 110, the arced portions 112 of which can bend on account of the clearance provided next to them by the presence of the sectors 143. This bending allows the front bearing 50 and rear bearing 51 to move axially so as to follow the oscillations brought about by the movement of the ball 200.

(61) Axial oscillations of the shaft 20 are thus obtained, the frequency of which is given both by the rotational speed of the shaft 20 and by the Willis formula applied to this rolling bearing with three contact points.

(62) The axial travel of the shaft 20 during the oscillating movement is for example between 0.02 mm and 0.15 mm. The shaft of the spindle oscillates with a vibration frequency of between 0.4 and 0.6 oscillations per revolution, for example around 0.5.

(63) The presence of the groove 29 in the shaft 20, in which groove the ball 200 is partially inscribed, reduces the distance 200 from the axis X and therefore the phenomenon of imbalance linked to the use of a single ball 200. Moreover, the distance traveled by the ball 200 and the resultant wear are reduced. Lastly, the bending moment induced by the asymmetric loading with a single ball is reduced.

(64) Of course, the invention is not limited to the example that has just been described.

(65) It is possible for example to close the housing 10 differently at the rear, as illustrated in FIG. 9. In this figure, it is apparent that the rear closure part 95 is retained in the housing by an elastic ring 300 mounted in a corresponding groove in the housing 10, thereby reducing the axial and radial space requirement of the housing.

(66) It is possible to do away with the flat springs, apart from the one used to provide the axial preload on the shaft. The flat spring used to provide the axial preload on the shaft may be disposed at the rear, i.e. at the position in which, in FIG. 2, the flat spring 170 is located between the rear shoulder 171 and the ring 121 adjacent to the rolling bearing 41 radially on the inside of the bearing 51.