Turret tool post and machine tool equipped with turret tool post

A turret tool post includes a turret turnably supported by a post body, turret turning part having a turret turning shaft and the like to turn the turret, a tool attachment part turnably holding a rotatable tool and attached to the turret, and tool-attachment-part turning part turning the tool attachment part to turn the rotatable tool. The tool-attachment-part turning part and the turret turning part are provided independently and arranged to face each other. The tool-attachment-part turning part includes a tool turning motor installed inside the turret and an attachment-part transmission shaft connected to the tool turning motor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase under 35 U.S.C. § 371 of International Application No. PCT/JP2015/060046, filed Mar. 31, 2015, designating the U.S., and published in Japanese as WO 2015/152200 A1 on Oct. 8, 2015, which claims priority to Japanese Patent Application No. 2014-073261, filed Mar. 31, 2014, the entire contents of which are incorporated herein by reference.

BACKGROUND

This disclosure relates to a turret tool post and a machine tool equipped with the turret tool post.

DESCRIPTION OF THE RELATED ART

A turret tool post that is provided at an automatic lathe and includes a turret turnably supported by a post body has been known. With the turret tool post, a tool such as a bite and an end mill is turnably attached to the turret with a tool attachment part.

The above turret tool post rotates a rotatable tool attached to the tool attachment part and turns the rotatable tool relative to the turret so as to process a workpiece (see Patent Literature 1 and Patent Literature 2).

Patent Literature 1: JP3129696 U

Patent Literature 2: JP2013-226611 A

Patent Literature 1 discloses a tool post including tool-attachment-part turning means for turning the tool (tool attachment part). The tool-attachment turning means is, however, configured with a plurality of gears complicatedly connected to each other. Patent Literature 2 discloses a tool post including a turret turning drive shaft that has a cylindrical shape and transmits a turn force to the turret, a rotatable-tool drive shaft attached to the tool attachment part (tool unit), and a turning drive shaft that transmits a turn force to the tool unit to turn the rotation shaft of the tool. However, the rotatable-tool drive shaft and the turning drive shaft are attached inside the turret turning drive shaft. Namely, the structure of the turret tool post is complicated.

SUMMARY

An object of the present disclosure is, therefore, to provide a turret tool post and a machine tool post equipped with the turret tool post that can turn a tool supported by a tool attachment part relative to a turret with a simple structure.

In order to achieve the above objective, a turret tool post according to an embodiment of the present disclosure includes a post body, a turret that is turnably supported on the post body, turret turning part that is configured to turn the turret, a tool attachment part that turnably holds a tool and is attached to the turret, and tool-attachment-part turning part that is configured to turn the tool attachment part to turn the tool. The machine tool selects the tool by turning the turret and performs processing on a material by turning the tool using the tool attachment part. The tool-attachment-part turning part and the turret turning part are provided independently and arranged to oppose each other.

A machine tool according to another embodiment of the present disclosure includes the above-mentioned turret tool post.

DETAILED DESCRIPTION

Hereinafter, the present disclosure will be described with reference to illustrated embodiments.

FIG. 1is a schematic perspective view illustrating a vicinity of a main spindle of an automatic lathe, which is one example of a machine tool equipped with a turret tool post according to an embodiment.FIG. 2is a schematic cross-sectional view illustrating an internal structure of the turret tool post according to an embodiment.

A main spindle2of an automatic lathe1is rotatably mounted on a main spindle headstock3. A distal end part of the main spindle2detachably grips a workpiece W with a main spindle chuck (not illustrated). A turret tool post10is disposed in a vicinity of the main spindle2. Note that in this embodiment, a direction along a main-spindle axis line C is defined as a Z-axis direction, a horizontal direction orthogonal to the Z-axis direction is defined as an X-axis direction, and an up-and-down direction orthogonal to both the Z-axis direction and the X-axis direction is defined as a Y-axis direction.

The turret tool post10includes a post body11and a turret12having a substantially polygonal shape. The turret12is supported by the post body11in an indexable and turnable manner. Various tools14are detachably attached to a plurality of turret faces13on the circumferential surface of the turret12, respectively. Using the turret tool post10as a support, each of the tools14is turnably supported by the turret tool post10with the turret12.

With an indexing turn of the turret12, each of tools14turns and a desired tool14is selectively positioned at a processing position corresponding to a position of the workpiece W. Here, the post body11is configured to be controlled by a non-illustrated moving mechanism to move in the X, Y, and Z-axes directions. In accordance with the movement of the turret tool post10, the workpiece W is processed by the tool14selected by the indexing turn of the turret12.

As illustrated inFIG. 2, a cylindrical fixed shaft22is fixed inside the post body11along the Z-axis direction. A cylindrical tool-rotation drive shaft21is inserted into and rotatably supported by the fixed shaft22with bearings24a,24b. Further, a turret turning shaft23is rotatably fitted onto the fixed shaft22. A pipe20is inserted into the tool-rotation drive shaft21. Here, the pipe20is fixed to the post body11.

A pulley26is attached to one distal end side (i.e., the right side onFIG. 2) of the tool-rotation drive shaft21, and a pulley29is attached to a motor shaft28aof a tool rotating motor28. The pulley26and the pulley29are connected via a belt27. With this, a rotation force of the tool rotating motor28is transmitted to the tool-rotation drive shaft21through the pulley29, the belt27, and the pulley26. A bevel gear30is provided at the other distal end side (i.e., the left side onFIG. 2) of the tool-rotation drive shaft21.

A turret turning gear31is integrally attached to one distal end side (i.e., right side onFIG. 2) of the turret turning shaft23. A drive force of a turret turning motor (schematically illustrated inFIG. 6) is transmitted to the turret turning gear31. The turret12is integrally fixed to the other distal end side (i.e., the left side onFIG. 2) of the turret turning shaft23. The turret12is turnably supported by the post body11with the turret turning shaft23.

A sliding shaft32is fitted onto the turret turning shaft23with bearings25a,25bso as to be slidable in the Z-axis direction. The bearings25a,25ballow the sliding shaft32to slide in the Z-axis direction. At one distal end side of the sliding shaft32, a piston34is formed and inserted into a cylinder chamber33that is formed in the post body11. Further, a coupling element35, which configures a coupling mechanism, is integrally fixed at the one distal end side of the sliding shaft32. Coupling elements36and37, which face the coupling element35, are integrally fixed to the post body11and the turret turning shaft23respectively. Here, the coupling element35, the coupling element36, and the coupling element37configure the coupling mechanism.

When the sliding shaft32is moved to the turret head side (i.e., the left side ofFIG. 2) by operating the piston34, the coupling element35is engaged with the coupling elements36,37. The coupling element36fixed to the post body11is then engaged with the coupling element37fixed to the turret turning shaft23through the coupling element35. Accordingly, the turn of the turret turning shaft23is stopped. When the sliding shaft32is slid to disengage the coupling element35from the coupling elements36,37, the coupling element36and the coupling element37are separated. As a result, the turret turning shaft23is allowed to turn.

By allowing the turret turning shaft23to turn and turning the turret turning shaft23, the turret12is turned. The turn of the turret turning shaft23is then stopped at a predetermined turning angle position to achieve the indexing turn of the turret12. With the indexing turn of the turret12, a predetermined turret face13of the turret12is selected so as to select a desired tool14.

A support part38is integrally fixed at a distal end of the fixed shaft22. The support part38is positioned inside a hollow part12aof the turret12. The support part38is integrally fixed to the post body11with the fixed shaft22. A tool-rotation transmission shaft39is rotatably supported by the support part38. A bevel gear40is attached to the tool-rotation transmission shaft39. The bevel gear30is engaged with the bevel gear40. The drive force is thereby transmitted to the tool-rotation transmission shaft39from the tool rotation shaft21.

At the support part38, a tool turning motor41is fixed with a bracket42so as to be positioned inside the hollow part12a. Note that electric lines connected to the tool turning motor41are led to the turret tool post10through the hollow pipe20and are connected to a controller (not illustrated).

As illustrated inFIGS. 2 and 3, the turret face13has a fixed part13aformed in a cylindrical shape. A rotatable tool device T is detachably fixed to the fixed part13awith a tool attachment part15. Here, the rotatable tool device T holds a rotatable tool14asuch as a drill and an end mill as the tool14, and the tool attachment part15is configured with an outer holder15aand an inner holder15b. The tool attachment part15is attached to the turret face13by detachable fixing the outer holder15ato an inner circumferential surface of the fixed part13a.

Inside the tool attachment part15, a turn-force transmission mechanism50to transmit a turn force to the inner holder15band a rotation-force transmission mechanism52to transmit a rotation force to the rotatable tool14aare provided. An attachment-part transmission shaft90is provided at a distal end side of a motor shaft43of the tool turning motor41. Between the attachment-part transmission shaft90and the turn-force transmission mechanism50, a first clutch51is provided. The first clutch51connects the attachment-part transmission shaft90and the turn-force transmission mechanism50only when the rotatable tool14ais selected by the indexing turn of the turret12. Here, the attachment-part transmission shaft90may be fixed to the motor shaft43by, for example, a screw or a pressure welding.

Between the other distal end side (the opposite side to the bevel gear40) of the tool-rotation transmission shaft39and the rotation-force transmission mechanism52, a second clutch53is provided. The second clutch53connects the tool-rotation transmission shaft39and the rotation-force transmission mechanism52only when the rotatable tool14ais selected by the indexing turn of the turret12.

The rotation-force transmission mechanism52includes a rotation-force transmission shaft61. The rotation-force transmission shaft61is rotatably inserted into and supported by the inner holder15bwith bearings60a,60b. The inner holder15bis rotatably supported by the outer holder15awith bearings85e,85f.At the distal end part of the rotation-force transmission shaft61, a bevel gear64ais provided.

The second clutch53includes a groove70formed at the distal end of the tool-rotation transmission shaft39and a tenon-shaped projection71formed at the distal end of the rotation-force transmission shaft61. The tenon-shaped projection71is engaged with (fitted to) the groove70when the rotatable tool14ais selected by turning the turret12.

When the tenon-shaped projection71is engaged with the groove70, the second clutch53becomes in an engaged state such that the rotation force transmitted to the tool-rotation transmission shaft39is transmitted to the rotation-force transmission shaft61.

The turn-force transmission mechanism50includes a first turn-force transmission shaft81having a gear80a, a second turn-force transmission shaft82integrally formed with a gear80b, a gear80cattached to the second turn-force transmission shaft82, and a cylindrical shaft83formed with a gear80d. The gear80aand gear80bare engaged, and the gear80cand80dare engaged.

The first turn-force transmission shaft81is rotatably supported by the outer holder15awith bearings85a,85b. The second turn-force transmission shaft82is rotatably supported by the outer holder15awith bearings85c,85d. The cylindrical shaft83is integrally fixed to the inner holder15bwith a connecting member86. Here, the bearings60a,60bare provided between the outer circumferential surface of the rotation-force transmission shaft61and the inner circumferential surface of the inner holder15b. A device body15cof the rotatable tool device T is detachably and integrally fixed to a distal end side of the inner holder15bwith, for example, a bolt.

The rotatable tool device T includes a first rotation-force transmission shaft62rotatably supported by the device body15cwith bearings60c,60dand a second rotation-force transmission shaft63rotatably supported by the device body15cwith bearings60e,60f.The rotatable tool14ais detachably installed on the second rotation-force transmission shaft63with a chuck mechanism66.

The first rotation-force transmission shaft62and the second rotation-force transmission shaft63are associated with each other through gears65a,65b, which are engaged with each other through other gears (not illustrated). The first rotation-force transmission shaft62has a bevel gear64bat a distal end part thereof. When the device body15cis fixed to the inner holder15b, the bevel gear64aand the bevel gear64bare engaged to connect the rotation-force transmission shaft61and the first rotation-force transmission shaft62. The rotation force is transmitted to the rotation-force transmission shaft61from the tool-rotation transmission shaft39through the second clutch53. The rotation force is then transmitted from the rotation-force transmission shaft61to the rotatable tool14athrough the bevel gears64a,64b, the first rotation-force transmission shaft62, the gears65a,65b, and the second rotation-force transmission shaft63.

The first clutch51includes a recessed groove91and a tenon-shaped projection92. The recessed groove91is formed at a distal end of the attachment-part transmission shaft90, which is provided on the post body11side of the turret tool post10. The tenon-shaped projection92is formed at a distal end of the first turn-force transmission shaft81of the turn-force transmission mechanism50in the tool attachment part15attached to the turret12(turret face13), which is disposed on the rotatable tool14aside. With this configuration, when the turret12turns and the rotatable tool14ais selected by the indexing turn, the tenon-shaped projection92is engaged with (fitted to) the groove91. The attachment-part transmission shaft90and the tool-rotation transmission shaft39are arranged to be substantially in parallel to each other.

When the tenon-shaped projection92is engaged with the groove91, the first clutch51becomes in the engaged state. Accordingly, the turn force transmitted from the motor shaft43to the attachment-part transmission shaft90is transmitted to the first turn-force transmission shaft81through the first clutch51, and then transmitted to the inner holder15bthrough the first turn-force transmission shaft81, the gears80a,80b, the second turn-force transmission shaft82, the gears80c,80d, and the cylindrical shaft83. The inner holder15bis turned by the transmitted turn force. That is, by turning the tool attachment part15to turn the inner holder15b, the rotatable tool device T is integrally turned with the inner holder15b, and therefore the rotatable tool device T is integrally turned with the rotatable tool14a.

As described above, tool-attachment-part turning part is configured with the tool turning motor41provided at the hollow part12aof the turret12, the attachment-part transmission shaft90functioning as a drive-force transmission part, and the like; and turns the tool attachment part15. Turret turning part is configured with the turret turning shaft23to which a driving force is input through the turret turning gear31, the sliding shaft32, the coupling mechanism (coupling elements35,36,37), and the like. The tool-attachment-part turning part is arranged to oppose the turret turning part along the Z-axis direction.

Further, rotation driving part of the rotatable tool14ais configured with the tool-rotation drive shaft21, the bevel gear30, the bevel gear40, the tool-rotation transmission shaft39, and the like. The rotation force is transmitted to the rotation driving part from the tool rotating motor28through the pulley26, the pulley29, and the belt27. The rotation driving part is disposed on the turret turning part side by inserting the tool-rotation drive shaft21into the turret turning shaft23.

The turret tool post10according to the embodiment is configured as described above. When the rotatable tool14asupported by the tool attachment part15with the rotatable tool device T is turned to the indexing position and then selected, the second clutch53is engaged such that the rotation force generated by the tool rotating motor28is transmitted from the rotation driving part to the rotatable tool14athrough the rotation-force transmission mechanism52. Accordingly, the rotatable tool14ais rotated by the transmitted rotation force and performs cutting process or the like on the workpiece W.

Since the first clutch51is engaged, the rotatable tool14aturns on the turret face13together with the rotatable tool device T using the turn-force transmission mechanism50by rotating the tool turning motor41. As a result, it becomes possible to perform processing on the workpiece W under a state in which the rotatable tool14ais inclined by a predetermined angle relative to the outer circumferential surface of or edge of the workpiece W.

As described above, in this embodiment, the tool-attachment-part turning part is provided on the hollow part12aside of the turret12so as to oppose the turret turning part provided on the post body11side. Accordingly, it becomes unnecessary to provide a shaft to turn the tool attachment part15, a transmission part to transmit the driving force, or the like on the post body11side. Furthermore, it becomes possible to locate the tool-attachment-part turning part close to the tool attachment part15. As a result, it becomes possible to simplify the configuration of the tool-attachment-part turning part.

Note that the tool attachment part15and the rotatable tool device T may be configured integrally.

FIG. 4is a schematic cross-sectional view illustrating a vicinity of a turret held by a turret tool post according to an embodiment. The turret tool post10aof this embodiment is substantially identical to the earlier embodiment, except for the configurations of a tool turning motor100and a drive-force transmission part101of tool-attachment-part turning part. Hence, the detailed description and illustration of the tool attachment part15, the rotatable tool device T, and the like are omitted.

In the embodiment, the tool turning motor100is disposed on an end face12bof the turret12. The tool turning motor100is fixed to a fixed plate102. The fixed plate102has a circular shape and is projected from a hole12cformed on the end face12b. The fixed plate102is fixed to a support part38with the bracket42. The fixed plate102is substantially in contact with the hole12cso as to allow the turret12to turn and also prevents, for example, a chip from entering inside the hollow part12a.

The drive-force transmission part101includes a bevel gear104attached to a motor shaft103of the tool turning motor100, a bevel gear105engaged with the bevel gear104, and an attachment-part transmission shaft106which is attached with the bevel gear105. The attachment-part transmission shaft106is rotatably supported by a supporter44with a bearing107and is arranged to be substantially in parallel to a tool-rotation transmission shaft39. Here, the supporter44is integrally provided on the support part38side. The drive-force transmission part101is connected to the motor shaft103of the tool turning motor100through the bevel gears104,105. A groove91is formed at one distal end side of the attachment-part transmission shaft106. With this groove91and a tenon-shaped projection92that is formed at a distal end of a first turn-force transmission shaft81of a turn-force transmission mechanism50as similar to Embodiment 1, a first clutch51is realized. The first clutch51becomes in an engaged state when the groove91and the tenon-shaped projection92are engaged.

The tool attachment part15is attached to the turret face13. When the rotatable tool14aheld by the tool attachment part15with the rotatable tool device T is selected, a tenon-shaped projection71of a second clutch53is engaged with a groove70. Accordingly, the second clutch53becomes in an engaged state so as to connect the tool-rotation transmission shaft39and a tool-rotation transmission shaft61of a rotation-force transmission mechanism52. Further, the tenon-shaped projection92is engaged with the groove91. Accordingly, the first clutch51becomes in the engaged state so as to connect the first turn-force transmission shaft81and the attachment-part transmission shaft106.

Note that the detailed structures of the turn-force transmission mechanism50and the rotation-force transmission mechanism52are substantially identical to those of the embodiment illustrated inFIGS. 2 and 3, and are thereby omitted in this embodiment illustrated inFIG. 4.

As described above, the tool-attachment-part turning part is configured with the tool turning motor100, the drive-force transmission part101, and the like provided inside the hollow part12aand on the end face12bof the turret12. Turret turning part is configured with a turret turning shaft23to which a driving force is input through a turret turning gear31, a sliding shaft32, a coupling mechanism (coupling elements35,36,37), and the like (seeFIG. 2). The tool-attachment-part turning part is arranged to oppose the turret turning part along the Z-axis direction.

The turret tool post10aaccording to the embodiment is configured as described above. When the rotatable tool14asupported to the tool attachment part15with the rotatable tool device T is selectively turned to the indexing position, a rotation force F1is transmitted to the rotatable tool14afrom the tool-rotation transmission shaft39side through the second clutch53and the rotation-force transmission mechanism52. Accordingly, the rotatable tool14ais rotated by the transmitted rotation force and performs cutting process or the like on the workpiece W.

When the tool turning motor100is rotated, the tool attachment part15is turned by a turn force F2transmitted through the motor shaft103, the bevel gear104, the bevel gear105, the attachment-part transmission shaft106, the first clutch51, and the turn-force transmission mechanism50. Accordingly, the rotatable tool14aturns on the turret face13together with the rotatable tool device T. As a result, it becomes possible to perform processing on the workpiece W under a state in which the rotatable tool14ais inclined by a predetermined angle relative to the outer circumferential surface of or end face of the workpiece W.

As described above, in this embodiment, the tool-attachment-part turning part is provided inside the hollow part12aand on the end face12bof the turret12so as to oppose the turret turning part provided on the post body11side. Accordingly, it becomes unnecessary to provide a shaft to turn the tool attachment part15, a transmission part to transmit the driving force, or the like on the post body11side. Furthermore, it becomes possible to locate the tool-attachment-part turning part close to the tool attachment part15. As a result, it becomes possible to simplify the configuration of the tool-attachment-part turning part.

FIG. 5is a schematic cross-sectional view illustrating a vicinity of a turret held by a turret tool post according to an embodiment. The turret tool post10bof this embodiment is substantially identical to that of the earlier embodiments, except for the configurations of a tool turning motor110and a drive-force transmission part111of tool-attachment-part turning part. Hence, the detailed description and illustration of the tool attachment part15, the rotatable tool device T, and the like are omitted.

In the embodiment, the tool turning motor110is disposed outside of an end face12bof the turret12. The tool turning motor110is attached to a hollow cover member120. The cover member120is connected to a distal end side of a supporting member121that is fixed on an outer surface of a post body11at a base end side thereof. The tool turning motor110(i.e., the cover member120) is supported by and fixed to the post body11with the supporting member121.

One end (at an end face12bside) of the cover member120is fixed to a fixed plate122. The fixed plate122has a circular shape and is projected from a hole12cformed on the end face12b. The fixed plate122is fixed to a support part38with a bracket42. The fixed plate122is substantially in contact with the hole12cso as to allow the turret12to turn and also prevents, for example, a chip from entering inside a hollow part12a.

The drive-force transmission part111includes an attachment-part transmission shaft131connected to a motor shaft130of the tool turning motor110, a gear132attached to the attachment-part transmission shaft131, a gear133attached to a rotation shaft134and engaged with the gear132, a gear135attached to the rotation shaft134, a gear136engaged with the gear135, and an attachment-part transmission shaft106attached to the gear136.

The attachment-part transmission shaft131and the rotation shaft134are rotatably supported inside the cover member120with bearings138a,138b,138c,138d. The bearings138a,138b,138c,138dare provided on the cover member120side. The attachment-part transmission shaft106is rotatably supported by a supporter44with a bearing107and is arranged to be substantially in parallel to a tool-rotation transmission shaft39. Here, the supporter44is integrally provided on the support part38side. The drive-force transmission part111is provided inside the hollow part12aof the turret12and the hollow cover member120and is connected to the motor shaft130of the tool turning motor110. A groove91is formed at a distal end of the attachment-part transmission shaft106. With this groove91and a tenon-shaped projection92that is formed at a distal end of a first turn-force transmission shaft81of a turn-force transmission mechanism50, a first clutch51is realized. The first clutch51becomes in an engaged state when the groove91and the tenon-shaped projection92are engaged.

The tool attachment part15is attached to a turret face13. When the rotatable tool14aheld by the tool attachment part15with the rotatable tool device T is selected, a tenon-shaped projection71of a second clutch53is engaged with a groove70. Accordingly, the second clutch53becomes in an engaged state so as to connect the tool-rotation transmission shaft61of a rotation-force transmission mechanism52and the tool-rotation transmission shaft39. Further, the tenon-shaped projection92is engaged with the groove91. Accordingly, the first clutch51becomes in the engaged state so as to connect the first turn-force transmission shaft81and the attachment-part transmission shaft106.

Note that the detailed structures of the turn-force transmission mechanism50and the rotation-force transmission mechanism52are substantially identical to those illustrated inFIGS. 2 and 3, and are thereby omitted in this embodiment illustrated inFIG. 5.

As described above, the tool-attachment-part turning part is configured with the tool turning motor110, the drive-force transmission part111, and the like provided inside the hollow part12aand outside of the end face12bof the turret12. Turret turning part is configured with a turret turning shaft23to which a driving force is input through a turret turning gear31, a sliding shaft32, a coupling mechanism (coupling elements35,36,37), and the like (seeFIG. 2). The tool-attachment-part turning part is arranged to oppose the turret turning part along the Z-axis direction.

The turret tool post10aaccording to the embodiment is configured as described above. When the rotatable tool14asupported to the tool attachment part15with the rotatable tool device T is selectively turned to the indexing position, a rotation force F1is transmitted to the rotatable tool14afrom the tool-rotation transmission shaft39side through the second clutch53and the rotation-force transmission mechanism52. Accordingly, the rotatable tool14ais rotated by the transmitted rotation force and performs cutting process or the like on the workpiece W.

When the tool turning motor110is rotated, the tool attachment part15is turned by a turn force F2transmitted through the motor shaft130, the attachment-part transmission shaft131, the gears132,133, the rotation shaft134, the gears135,136, the attachment-part transmission shaft106, the first clutch51, and the turn-force transmission mechanism50. Accordingly, the rotatable tool14aturns on the turret face13together with the rotatable tool device T. As a result, it becomes possible to perform processing on the workpiece W under a state in which the rotatable tool14ais inclined by a predetermined angle relative to the outer circumferential surface of or end face of the workpiece W.

As described above, in this embodiment, the tool-attachment-part turning part is provided inside the hollow part12aand outside of the end face12bof the turret12so as to oppose the turret turning part provided on the post body11side. Accordingly, it becomes unnecessary to provide a shaft to turn the tool attachment part15, a transmission part to transmit the driving force, or the like on the post body11side. Furthermore, it becomes possible to locate the tool-attachment-part turning part close to the tool attachment part15. As a result, it becomes possible to simplify the configuration of the tool-attachment-part turning part.

REFERENCE SIGNS LIST