Coupling for clamping a pallet

Described is a coupling, provided at the spindle head of a machine tool, for clamping an especially plate-shaped work-holding fixture, for example a pallet, in a specified, repeatable and precise position relative to a spatial reference system, wherein the spindle head exhibits a collet, which can be moved axially and radially with respect to its face. To increase the accuracy when machining a workpiece, there is a plate, which is to be connected to the face and is provided with spatial references on the front side, and which has a central passage opening to receive a tension rod, connected to the backside of the work-holding fixture, and a bolt, whose front end can be coupled with the tension rod and whose rear end can be connected to the collet.

SPECIFICATION
 The invention relates to a coupling, provided at the spindle head of a
 machine tool, for clamping an especially plate-shaped work-holding
 fixture, for example a pallet, in a specified, repeatable and precise
 position relative to a spatial reference system, wherein the spindle head
 exhibits a collet, which can be moved axially and radially with respect to
 its face.
 The collet usually serves to grasp a workpiece, which is to be machined by
 an automatic lathe, a milling machine or the like. Said workpiece is
 tightened by the collet, for example in an internal cone, provided at the
 spindle head, for the purpose of positioning. However, the internal cone
 does not constitute a sufficiently accurate reference for the purpose of
 numerically controlling an automatic lathe so that sufficiently accurate,
 uniform and automatic machining of a large number of workpieces to be
 machined in succession is difficult.
 Therefore, the invention is based on the problem of increasing the accuracy
 of machining the workpiece in a machine tool, as described above.
 For the aforementioned coupling, according to the invention, there is also
 a plate, which is to be connected to the face and is provided with spatial
 references on the front side. Said plate has a central passage opening to
 receive a tension rod, connected to the backside of the work-holding
 fixture, and a bolt, whose rear end can be connected to the collet and
 whose front end can be coupled with the tension rod. The coupling of the
 invention leads to significantly higher accuracy of the automatic,
 numerically controlled machining of the workpiece, is not limited to use
 with a specific clamping system and finally allows the user the simple
 transition to known couplings, for example, according to the ISO standard,
 through removal of the plate and the bolt in the case that there are still
 machining problems to be solved whose accuracy allows work with couplings
 according to the ISO standard.
 In a preferred embodiment of the invention the bolt can be guided axially
 in the plate. Furthermore, it is recommended to provide at the front end
 of the bolt a ball lock, which interacts with the profile on the tension
 rod. For the connection of the rear end of the bolt with the collet it is
 recommended that either the end is screwed together with the collet or the
 end is coupled with the collet by means of a ball rim, engaging with a
 bolt profile. A good guide of the bolt during the collet movements is
 obtained, when a hollow cone, into whose axial passage borehole the bolt
 is guided, is fastened advantageously on the rear side of the plate.
 Furthermore, it is recommended that to increase the tension force there is
 at least one axially acting spring, which is braced against the plate and
 which rests against the bolt.
 Other preferred embodiments of the invention are disclosed in the dependent
 claims.
 The invention is explained in detail in the following with reference to the
 embodiment illustrated in the attached drawings. This embodiment uses an
 x, y, z orthogonal system as the spatial reference system.
 FIG. 1 is a diagrammatic axial view of the coupling of the invention with a
 clamped work-holding fixture and with the alternative possibility of a
 coupling between bolt and collet. Said possibility is indicated by the
 dash-dotted line.
 FIG. 2 is a drawing similar to FIG. 1 with the work-holding fixture
 detached from the spindle head.

The spindle head 1 of an automatic milling machine, grinding machine,
 lathe, or the like, has a central axis of rotation 8, a free face 5, which
 extends vertically to the axis 8, and a central internal cone 9, which
 opens in the direction of the face 5. In the spindle head 1 an axially
 moveable collet 3 is coupled on the rear side with a control mechanism,
 which is also not illustrated. When a tool change is about to occur, said
 control mechanism allows the collet 3 to advance along the axis 8 in the
 direction of the face 5 for the purpose of releasing a hitherto used tool
 and to grasp a new tool; and, after the grasping procedure has been
 completed, said control mechanism allows said collet to move back again
 into the illustrated position. In so doing, the front end of the collet 3
 and the shape of the internal cone 9 can be designed according to the ISO
 standards.
 To make the spindle head 1 suitable for clamping a workpiece into an exact
 position with respect to an x, y, z orthogonal system without extensive
 reconstruction of said spindle head, there is, first of all, a slender,
 essentially cylindrical bolt 30 with an outer diameter that is virtually
 equal to that of the collet 3. The rear end 31 of the bolt 30 that tapers
 radially is provided with an external thread and is screwed into an
 internal thread, which is tapped into the collet 3. In this manner the
 bolt 30 constitutes an axial extension of the collet 3 in the hollow
 enclosing the internal cone 9 and hence participates in all movements of
 the collet 3.
 On the front end 33 the bolt 30 carries a ball lock, comprising several
 balls, which are distributed uniformly over its circumference and of which
 two are labelled 32 and 34. An axial groove 38 at the front end 33
 provides the bolt 30 with a ring wall, in whose radial, passage boreholes
 the balls of the ball lock are caught so as to be able to move radially to
 a limited degree, as will be explained below. The center of the floor of
 the groove 38 exhibits an axial bolt channel 66, which is closed by means
 of a stopper 36 with head 37, which rises over the floor and which will
 also be explained below.
 Furthermore, a plate 10 is bolted to the free face 5 with screws 21, bears
 on its front side x-y references 12, 14, 16, 18 and at least one z
 reference 17 and exhibits a central passage opening 15, whose center line
 lies on the axis 8 and whose rear end has a radial expansion, forming a
 radial annular shoulder 19. A hollow cone 20 is connected with screws 26
 to the rear side of the plate 10. The cone's central opening 22 faces the
 axis 8 and encloses the bolt 30 so as to guide it axially. The conical
 external contour of the hollow cone 20 is adapted to the rise of the
 internal cone 9 so that, when the plate 10 is fastened to the spindle head
 1, the hollow cone 20 is securely seated in the internal cone 9.
 The opening 22 exhibits in the vicinity of the plate 10 a radial expansion
 24 with a ramp, which faces radially outwardly and in which the balls of
 the ball lock can escape radially, when the bolt 30 advances far enough in
 the direction of the plate 10, and then no longer protrude into the
 interior of the groove 38 (FIG. 2). In the retracted position of the bolt
 according to FIG. 1, the interior wall of the opening 22 displaces the
 balls so far radially inwardly that their spherical caps protrude into the
 interior of the groove 38 and can form there an effective coupling with a
 tension rod 4.
 There is also a ring 11, which is fastened with elements 13 to the outer
 periphery of the plate 10 and protrudes axially beyond the front side, in
 particular beyond the x-y references 12, 14, 16, 18.
 The tension rod 4 is fastened in the middle on the rear side of a
 work-holding fixture 2 by means that are not explained here in detail and
 protrudes axially beyond said fixture. The work-holding fixture 2 can be a
 pallet, on whose front side one or several workpieces (not illustrated)
 can be held. The essentially cylindrical tension rod 4 fits so as to move
 axially into the passage borehole 15 of the plate 10. In the vicinity of
 its free end the tension rod 4 exhibits an annular, radial depression 42,
 adapted to the shape of the said spherical caps of the balls; and the free
 face of the said tension rod has a central groove 44.
 To clamp the pallet 2 to the spindle head 1, the collet 3 is moved to the
 front by the said control mechanism so that the face 35 of the bolt 30
 moves in front of the annular shoulder 19 (FIG. 2). Now the pallet 2 is
 moved in such a manner in the direction of the front side of the plate 10
 that the tension rod 4 dips with its front end through the passage opening
 15 into the groove 38, and the floor of the groove 44 finally rests
 against the head 37. In so doing, the rear side of the pallet 2 pushes
 into the chamber, enclosed by the ring 11 and front side of the plate 10.
 In this respect profiles are envisaged that allow the pallet 2 to be
 fitted in only one of, for example, four specified directions of rotation
 with respect to the axis 8.
 From this position of the pallet 2 in preparation for the clamping
 according to FIG. 2, the clamping takes place by retracting the collet 3.
 In so doing the balls 32, 34 ( and the other balls) of the ball lock are
 pushed radially inwardly by means of the ramp of the expansion 24 so that
 their spherical caps engage with the depression 42 and thus couple the
 tension rod 4 to the bolt 30. Thereupon, the tension rod 4 and thus the
 pallet 2 follow the rearward motion of the bolt 30. The retracting
 movement and thus the clamping of the pallet 2 are completed when one (of
 several) z counter-references, formed on the rear side of the pallet,
 comes to rest against the z reference 17 located on the front side of the
 plate 10. Then the x-y references 12, 14, 16, 18 engage with the x-y
 counter-references provided on the rear side of the pallet.
 To increase the tension force there are compression springs (not
 illustrated), which are uniformly distributed around the passage opening
 15 and each of which is braced against the annular shoulder 19 and rests
 against the face 35.
 Furthermore, compressed air channels, which communicate with each other,
 pass through the plate 10, the hollow cone 20 and the bolt 30; said
 compressed air channels can be attached to an externally controlled
 compressed air source by means of a radial channel 62, which opens into
 the outer periphery of the plate 10 or the ring 11. In many cases the
 compressed air channels are provided with compressed air by the machine,
 to which the spindle head 1 is fastened, by means of an axial channel 70,
 which passes through the middle of the collet and is in alignment with the
 bolt channel 66. In so doing the radial channel 62 is closed on the outer
 periphery by means of a stopper 61. The compressed air channels serve to
 keep the z-reference 17 free of foreign particles.
 To this end the radial channel 62 in the plate 10 opens by means of a cross
 intersecting channel 63 with respect to the z reference 17. At its
 internal end the radial channel 62 opens into a ring channel 64, which is
 formed between the rear side of the plate 10 and the front side of the
 hollow cone 20 and which in turn is connected with the bolt channel 66 by
 means of another cross intersecting channel 65 in the hollow cone 20 and a
 radial borehole 67 in the bolt 30.
 In another alternative embodiment, shown by the dash-dotted line in the
 figures, the bolt 30 can be coupled with the collet 3 by means of a ball
 rim with balls 6 and 7 present at the end of the collet 3. It can
 compensate for any difference between the existing travel of the collet 3
 and the small travel that is needed only for the bolt 30. To this end, the
 front end of the collet 3 exhibits several axially protruding fingers 51,
 53, which are uniformly spaced in the circumferential direction and in
 which balls 6 and 7 are caught so as to be able to move radially to a
 limited degree. Between a thickened rear end 52 of the bolt 30 and a
 radial stop shoulder 58 of the bolt 30 there is a radially tapered and
 axially extending bolt segment 55, along which the balls 6, 7 can run
 axially. The inner wall 59 of the collet guide in the spindle head 1
 forces the spherical caps of the balls 6, 7 against the bolt segment 55 so
 that the bolt is carried along by the collet 3 in its rearward motion. The
 forward travel of the bolt is caused by the fingers 51, 53 resting against
 the stop shoulder 58 of the bolt 30, which can be moved so far forward to
 overcome the said difference that the bolt 30 remains unaffected by the
 initial part of the collet forward and backward travel.
 The uncoupling of the bolt 30 from the collet 3 is rendered possible by a
 radial expansion 57 of the inner wall 59 of the collet guide, into which
 the balls 6, 7 escape radially and thus can release the bolt 30.