# Patent

## Title
Machine tool

## Abstract

To provide a machine tool in which a workpiece can be delivered to/from a first and a second spindle headstock without any increase in front/back direction dimension of a machine main body. A second spindle headstock 4 is capable of moving to a position for workpiece delivery to/from a first spindle headstock 3 so as to be close to or to abut on the first spindle headstock 3. When the second spindle headstock 4 is positioned at the workpiece delivery position, a tool post 5 is positioned right under the first spindle headstock 3 so that its cutting point “c” is positioned vertically under an axis “a” of the first and second spindle headstocks 3, 4.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a machine tool including: a first and a second spindle headstock disposed on a bed; and a tool post disposed between the first and second spindle headstocks.
2. Description of the Related Art
An example of a machine tool of this type is structured such that, when seen from a front side of the machine, a first spindle headstock is fixedly disposed on a left end portion of a bed, a second spindle headstock is disposed on a right side to be movable in a Z-axis direction, and an upper tool post and a lower tool post are disposed between the first and second spindle headstocks to be movable in the Z-axis direction and an X-axis direction (see, for example, FIG. 4 of Patent document 1). In this machine tool, a workpiece having been subjected to predetermined machining by the first spindle headstock is transferred and attached to the second spindle headstock by a workpiece delivery device, and is subjected to the other machining by the second spindle headstock.
[Patent document 1] Japanese Patent Application Laid-open No. 2001-87901
Incidentally, the aforesaid conventional machine tool requires the workpiece delivery device, which poses a problem that cost of the machine accordingly increases.
In order to eliminate the need for the workpiece delivery device, it is necessary for the second spindle headstock to move up to a workpiece delivery position of the first spindle headstock for direct delivery of the workpiece. In this case, a tool post is disposed at a position where it does not interfere with the second spindle headstock, for example, on a front side when seen from the front side of the machine. However, disposing the tool post at the position not causing the interference gives rise to a problem that the front/back direction dimension of the machine main body accordingly increases.
SUMMARY OF THE INVENTION
The present invention was made in consideration of the above circumstances and it is an object thereof to provide a machine tool in which a workpiece can be directly delivered to/from a first and a second spindle headstock without any increase in a front/back direction dimension of a machine main body.
In an invention, a machine tool includes: a bed; a first spindle headstock disposed on the bed; a second spindle headstock disposed to be coaxial with the first spindle headstock and to be movable in an axis (Z-axis) direction; and a tool post disposed between the first spindle headstock and the second spindle headstock to be movable in the axis direction, wherein the second spindle headstock is movable up to a position for workpiece delivery to/from the first spindle headstock, and wherein the tool post has: a turret to which a plurality of tools are attached; and a supporting member by which the turret is supported to be movable in an up/down (X-axis) direction, and when the second spindle headstock is positioned at the workpiece delivery position, the turret is positioned between the first and second spindle headstocks under the axis and a cutting point of the tool post is positioned vertically under the axis.
In the invention, when the second spindle headstock is positioned at the position for workpiece delivery to/from the first spindle headstock, the turret of the tool post is positioned under the axis of the first and second spindle headstocks, and the cutting point of the tool post is positioned vertically under the axis. Therefore, the second spindle headstock can move to the delivery position without interfering with the tool post, which allows the direct delivery of the workpiece to/from the first and second spindle headstocks and can thus eliminate a need for the conventional workpiece delivery device, so that cost can be reduced.
Further, the turret is positioned between the first and second spindle headstocks under the axis, so that it is possible to reduce the front/back direction dimension of the machine main body compared with a case where the tool post is positioned on the front side when seen from the front side of the machine, which enables downsizing.
In a preferable embodiment of the invention, the first spindle headstock is in a tunnel shape, having a pair of leg parts and a spindle case part disposed between upper ends of the leg parts to support a first spindle, and the supporting member of the tool post is movable into/out of the tunnel of the first spindle headstock.
In the above-described embodiment, the first spindle headstock is formed in the tunnel shape, having the pair of leg parts and the spindle case part disposed between the upper ends of the both leg parts, and the supporting member supporting the turret of the tool post is capable of moving into/out of the tunnel. Therefore, when the supporting member is housed in the first spindle headstock, the turret can be positioned right under the spindle of the first spindle headstock, so that the second spindle headstock is allowed to move to the delivery position for the delivery of the workpiece without interfering with the tool post.
In another preferable embodiment of the invention, the second spindle headstock is supported to be movable in the axis direction by a pair of spindle guide rails disposed on the bed, the supporting member of the tool post is mounted on the spindle guide rails, a ball screw for driving the tool post is disposed adjacently to one of the spindle guide rails, and a ball screw for driving the second spindle headstock is disposed adjacently to the other spindle guide rail.
In the above-described embodiment, the tool post is mounted on the spindle guide rails supporting the second spindle headstock. Therefore, one pair of the spindle guide rail can serve both for the second spindle headstock and for the tool post, which can reduce the number of parts and cost.
Further, the ball screw for driving the tool post is disposed adjacently to the one spindle guide rail and the ball screw for driving the second spindle headstock is disposed adjacently to the other spindle guide rail. Therefore, a vacant space near the guide rails can be utilized for arranging the ball screws that independently drive the reciprocal movements of the tool post and the second spindle headstock.
In still another preferable embodiment of the invention, a trough in a groove shape whose upper side is open is formed between the both spindle guide rails of the bed.
In the above-described embodiment, since the trough is formed between the both spindle guide rails of the bed, a vacant space made available under the guide rails can be effectively utilized for collecting chips.
In still another preferable embodiment of the invention, the first spindle headstock is thermally symmetrical with respect to a plane of symmetry which is a plane including an axis of the spindle case part and being parallel to the X axis.
“Thermally symmetrical” here in the present invention means a state in which both sides of the plane of symmetry are equal in thermal expansion amount when the temperature rises due to heat generation or the like of the machine. Concretely, a cross section taken along a plane orthogonal to the axis has a substantially laterally symmetrical shape.
In the above-described embodiment, the first spindle headstock is formed to be thermally symmetrical. Therefore, when the temperature of the first spindle headstock rises due to the heat generation or the like caused by the rotation of a spindle, both sides of the plane of symmetry are substantially equal in thermal expansion amount, so that the first spindle headstock expands so as to move vertically upward, which can reduce an adverse influence that the thermal expansion gives to machining accuracy.
In still another preferable embodiment of the invention, a vertical line going through the axis of the first and second spindle headstocks goes through a substantial center of an interval between the spindle guide rails.
In the above-described embodiment, the vertical line going through the cutting point is positioned at a substantial center of the interval between the spindle guide rails supporting the second spindle headstock. Therefore, a reaction force of a cutting force is transmitted uniformly via the tool post to the left and right spindle guide rails, and further to the bed, which can also reduce the adverse influence to machining accuracy.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a composite lathe according to one embodiment of the present invention;
FIG. 2 is a side view of the composite lathe;
FIG. 3 is a perspective view of a bed of the composite lathe;
FIG. 4 is a perspective view of the bed;
FIG. 5 is a front view of a supporting mechanism for a third spindle of the composite lathe;
FIG. 6 is a side view of a first spindle headstock and a tool post disposed on the bed;
FIG. 7(a), FIG. 7(b), and FIG. 7(c) are views showing operations of the first spindle headstock, a second spindle headstock, and the tool post;
FIG. 8 is a perspective view of the first spindle headstock;
FIG. 9 is a perspective view of the first spindle headstock;
FIG. 10 is a perspective view of the first spindle headstock;
FIG. 11(a) and FIG. 11(b) are views of the first spindle headstock;
FIG. 12 is a perspective view of a cooling oil jacket of the first spindle headstock;
FIG. 13 is a perspective view of the tool post;
FIG. 14 is a side view of the tool post;
FIG. 15 is a front view of the tool post;
FIG. 16 is a perspective view of a supporting member of the tool post;
FIG. 17 is a perspective view showing an X-axis stroke of the third spindle; and
FIG. 18 is a side view showing a Y-axis stroke of the third spindle.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
An embodiment of the present invention will be hereinafter described based on the attached drawings.
FIG. 1 to FIG. 18 are views to illustrate a composite lathe according to one embodiment of the present invention. FIG. 1 and FIG. 2 are a perspective view and a right side view of the composite lathe respectively, FIG. 3 and FIG. 4 are perspective views of a bed on which a first spindle headstock and a tool post are mounted, FIG. 5 is a front view of a supporting mechanism supporting a third spindle, FIG. 6 is a right side view of the first spindle headstock and the tool post, FIG. 7(a) to FIG. 7(c) are front views showing operations of the first spindle headstock, a second spindle headstock, and the tool post, FIG. 8 to FIGS. 11(a), (b) are views of the first spindle headstock, FIG. 12 is a perspective view of a cooling oil jacket of the first spindle headstock, FIG. 13 to FIG. 16 are views of the tool post, and FIG. 17 and FIG. 18 are views showing X-axis and Y-axis direction strokes of the third spindle. Note that front/back and left/right mentioned in this embodiment mean front/back and left/right when the machine is seen from a front side, unless otherwise noted.
In the drawings, 1 denotes the composite lathe. When seen from the front side of the machine shown in FIG. 1, the composite lathe 1 includes: a bed 2; a first spindle headstock 3 disposed on a left end portion on the bed 2; a second spindle headstock 4 disposed on the right side of the first spindle headstock 3 to be coaxial with the first spindle headstock 3 and to be movable in a Z-axis (left/right) direction; a tool post 5 disposed between the first and second spindle headstocks 3, 4 to be movable in an X-axis (up/down) direction and in the Z-axis direction; and a third spindle 6 disposed on the bed 2 to be movable in the X-axis direction, a Y-axis (front/back) direction, and the Z-axis direction. On a back left end portion on the bed 2, provided is a tool changer 7 automatically changing a preceding-process tool attached to the third spindle 6 to a subsequent-process tool.
As shown in FIG. 3, the bed 2 is constituted of a front bed part 2a and a back bed part 2b which are integrally molded. A first and a second mounting surface 2a′, 2a′′ are formed on the front bed part 2a and third mounting surfaces 2b′ are formed on the back bed part 2b, all these surfaces being horizontal along the Z-axis direction and the Y-axis direction.
Compared with the second mounting surface 2a′′ of the front bed part 2a, the first mounting surface 2a′ is positioned higher than the second mounting surface 2a′′, and the third mounting surfaces 2b′ of the back bed part 2b are positioned lower than the first mounting surface 2a′ and higher than the second mounting surface 2a′′, so that they are formed in a stepped manner.
The first spindle headstock 3 is mounted on the first mounting surface 2a′. On the second mounting surface 2a′′, the second spindle headstock 4 and the tool post 5 are mounted to be movable in the Z-axis direction. Further, on the third mounting surfaces 2b′, a supporting mechanism 10 movably supporting the third spindle 6 is mounted.
The supporting mechanism 10 includes: a column 11 in a rectangular frame shape which is fixed on the third mounting surfaces 2b′ of the back bed part 2b to extend vertically upward; a saddle 12 in a rectangular frame shape which is supported on a front face of the column 11 to be movable in the Z-axis direction; a cross slide 13 supported on a front face of the saddle 12 to be movable in the X-axis direction; and a ram 14 supported by the cross slide 13 to be movable in the Y-axis direction and supporting the third spindle 6.
The column 11 is structured such that left and right support posts 11c, 11d and upper and lower beam parts 11a, 11b, which couple upper and lower ends of the left and right support posts 11c, 11d, are integrally molded. The column 11 is firmly fixed on the third mounting surfaces 2b′ of the back bed part 2b. 
Similarly to the column 11, the saddle 12 is structured such that left and right support posts 12a, 12b and upper and lower beam parts 12c, 12d, which couple upper and lower ends of the left and right support posts 12a, 12b, are integrally molded. The saddle 12 is supported to be movable in the Z-axis direction by a pair of upper and lower Z-axis guide rails 15, 15 which are disposed in parallel to the Z axis on front faces of the upper and lower beam parts 11a, 11b of the column 11. Z-axis ball screws 16, 16 are screwed to nut parts 12e, 12e formed in the upper and lower beam parts 12c, 12d of the saddle 12. The saddle 12 is driven to reciprocate in the Z-axis direction when the Z-axis ball screws 16, 16 are rotary driven by servo motors 16a, 16a. Note that the Z-axis ball screws 16, 16 are disposed in parallel to the Z axis on the front faces of the upper and lower beam parts 11a, 11b of the column 11.
The cross slide 13 is formed in a rectangular thick plate shape and is supported to be movable in the X-axis direction by a pair of left and right X-axis guide rails 17, 17 which are disposed in parallel to the X axis on the front faces of the left and right support posts 12a, 12b of the saddle 12. X-axis ball screws 18, 18 are screwed to nut parts formed on left and right sides of the cross slide 13. The cross slide 13 is driven to reciprocate in the X-axis direction when the X-axis ball screws 18, 18 are rotary driven by servo motors 18a, 18a. Note that the X-axis ball screws 18, 18 are disposed in parallel to the X axis on the front faces of the left and right support posts 12a, 12a of the saddle 12.
The ram 14 is inserted in a ram guide hole 13a formed in the cross slide 13 and is supported to be movable in the Y-axis direction by guide plates (not shown) which are arranged on an inner circumferential surface of the ram guide hole 13a, being a predetermined angle apart from one another. One Y-axis ball screw 21 disposed in a lower portion of a widthwise center of the cross slide 13 is screwed to a nut part of the ram 14. The ram 14 is driven to reciprocate in the Y-axis direction when the Y-axis ball screw 21 is rotary driven by a servo motor.
The third spindle 6 is rotatably inserted in the ram 14. At a tip of the third spindle 6, a tool spindle 6a is disposed, with its axis directed perpendicularly to the Y axis. A tool is attached to a tip of the tool spindle 6a and the tool spindle 6a is rotary driven by a driving motor 6b. Further, the third spindle 6 can be rotary indexed around the Y axis (B axis) for positioning by a built-in rotary indexing mechanism (not shown).
The front bed part 2a includes: a first mounting part 2d having the first mounting surface 2a′ formed on an upper surface thereof; and a second mounting part 2e having the second mounting surface 2a′′ which is formed so as to be positioned lower than the first mounting surface 2a′. 
The first mounting part 2d has a recession 2f continuing from the second mounting surface 2a′′, and a bottom of the recession 2f is flush with the second mounting surface 2a′′. Therefore, the first mounting surface 2a′ is in a U-shape when seen from above.
Further, a center trough 2c in a groove form whose upper side is open is formed in a front/back-direction center portion of the second mounting surface 2a′′ of the front bed part 2a. At a right wall of the front bed part 2a, the center trough 2c is open to the outside. In the center trough 2c, a chip conveyer (not shown) conveying chips to the outside through the opening is disposed.
On the second mounting surface 2a′′ of the front bed part 2a, a pair of front and back second spindle guide rails 24, 24, by which the second spindle headstock 4 is supported to be movable in the Z-axis direction, are disposed in parallel to the Z axis. The second spindle guide rails 24 both extend to the inside of the recession 2f of the first mounting part 2d. On an inner side of and adjacently to the front-side guide rail 24 out of the both second spindle guide rails 24, one second spindle ball screw 25 is disposed to drive the Z-axis direction reciprocal movement of the second spindle headstock 4. Further, on an inner side and adjacently to the opposite guide rail 24, one tool post ball screw 32 is disposed to drive the Z-axis direction reciprocal movement of the tool post 5.
The second spindle headstock 4 includes: a movable base 4a mounted on the second spindle guide rails 24, 24 to be slidable in the Z-axis direction; and a spindle headstock main body 4b fixed on the movable base 4a and having a cylindrical part. A second spindle 26 facing the first spindle headstock 3 is rotatably inserted in the spindle headstock main body 4b. The second spindle 26 includes a chuck 26a holding a workpiece and is rotary driven by a built-in driving motor.
When seen in the Z-axis direction, the first spindle headstock 3 includes: left and right leg parts 3a, 3a in a wide flat plate shape positioned on the left and right; a cylindrical spindle case part 3b coupling upper ends of the leg parts 3a; and a coupling part 3c coupling the spindle case part 3b and the left and right leg parts 3a, 3a on an opposite side of the second spindle headstock 4, and these components of the first spindle headstock 3 are integrally molded. Therefore, a tunnel part 3d surrounded by the leg parts 3a, the spindle case part 3b, and the coupling part 3c is formed in the first spindle headstock 3.
When seen from above, the coupling part 3c forms a V-shape continuing from the left and right leg parts 3a to extend to the opposite side of the second spindle headstock 4, and when seen from the side, it forms a triangular shape continuing from the spindle case part 3b to widen toward an obliquely downward direction. A flange part 3e protruding outward is integrally formed along the whole periphery of lower ends of the left and right leg parts 3a, 3a and the coupling part 3c. The flange part 3e is fixedly bolted to the first mounting surface 2a′. 
The first spindle headstock 3 is formed to be thermally symmetrical. Here, “thermally symmetrical” means a state in which a left portion and a right portion across a plane of symmetry are substantially equal in thermal expansion amount and the whole structure expands along this plane of symmetry. Concretely, when a cross section taken along a plane including the X axis and the Y axis is seen in the Z-axis direction (see FIG. 11(a)), a plane including a vertical line “b” going through an axis “a” of the spindle case part 3b is a plane of symmetry, and the left and right leg parts 3a, 3a, the coupling part 3c, and the spindle case part 3b are formed to be almost laterally symmetrical with respect to the plane of symmetry. With this structure, the first spindle headstock 3 thermally expands only in the direction of the vertical line “b”.
On the other hand, when a cross section of the first spindle headstock 3 taken along a plane including the X axis and the Z axis is seen in the Y-axis direction, the cross section is not symmetrical (see FIG. 11(b)) in this embodiment. Therefore, strictly speaking, the aforesaid thermal symmetry is difficult to realize. Therefore, this embodiment adopts a structure such that a cooling oil jacket 40 is formed in the first spindle headstock 3 as shown in FIG. 12 to more surely realize the aforesaid thermal symmetry by cooling by the cooling oil jacket 40.
Specifically, in the cooling structure of this embodiment, the leg parts 3a, 3a and the coupling part 3c are more surely cooled by a large amount of cooling oil than the spindle case part 3b so that the aforesaid axis “a” moves only upward along the vertical line “b” due to thermal expansion. Concretely, the cooling oil jacket 40 has: a spindle cooling oil passage 40a in a coiled shape formed on an outer circumferential portion of the spindle case part 3b; a pair of upper and lower upper leg cooling oil passages 40b, 40b formed along an upper portion of each of the left and right leg parts 3a, 3a; lower leg cooling oil passages 40c, 40c formed along lower portions of the left and right leg parts 3a, 3a; and a coupling part cooling oil passage 40d formed along a lower portion of the coupling part 3c. Compared with the spindle cooling oil passage 40a, the other cooling oil passages 40b to 40d are larger in diameter. This structure makes it possible to more surely cool the leg parts 3a, 3a which may possibly hinder the realization of the aforesaid thermal symmetry, so that as a whole, the first spindle headstock 3 can expand only in the upward direction along the vertical line “b”.
The cooling oil is supplied from a supply port provided in the middle of the upper cooling oil passage 40b disposed on a back side in FIG. 12. The cooling oil moves in the right direction in the drawing in the upper cooling oil passage 40b, moves in the left direction in the drawing in the cooling oil passage 40b immediately under via an external pipe, goes down, moves in 40c on the back side in the drawing and 40d, moves in 40c on the front side in the drawing, goes up, moves in the right direction in the lower-side 40b disposed on the front side in the drawing, moves in the left direction in 40b immediately above via an external pipe, moves spirally in 40a, and is discharged from an outlet formed in 40a. 
A first spindle 28 is rotatably inserted in the spindle case part 3b to be coaxial with the second spindle 26. The first spindle 28 includes a chuck 28a holding a workpiece and is rotary driven by a built-in driving motor.
A presetter 38 is further attached to the first spindle headstock 3, as shown in FIG. 8 The presetter 38 is intended for measuring a blade edge position of a tool before machining and it performs the measurement both for a tool of a turret 31 and for a tool of the third spindle 6, as will be described later.
The tool post 5 has the turret 31 and a supporting member 30 by which the turret 31 is supported to be movable in the X-axis direction. The turret 31 includes: a turret head 31 a to whose outer circumferential portion a large number of tools T are attached, being a predetermined angle apart from one another; and a rotary indexing mechanism 31b supporting the turret head 31a rotary indexably. The rotary indexing mechanism 31b rotary-indexes a desired tool T at a predetermined machining position to clamp it at the machining position. An indexing motor 31c of the rotary indexing mechanism 31b is disposed on a back face of the turret 31 when seen from the right side in the Z-axis direction.
The supporting member 30 is a molded product in a rectangular frame shape and has: left and right leg parts 30a, 30a positioned on the left and right when seen in the Z-axis direction; an upper beam part 30b integrally coupling upper ends of the both leg parts 30a, 30a; and a lower beam part 30c integrally coupling lower ends thereof. Therefore, a portion inside the frame of the supporting member 30 is a rectangular space.
The respective leg parts 30a have: vertical wall parts 30d, 30d in a wide flat plate shape; and supporting parts 30e, 30e formed in lower portions of outer walls of the respective vertical wall parts 30d and extending in the Z-axis direction. When seen in the Y-axis direction, left ends 30f of the respective supporting parts 30e are in line with left end faces of the vertical wall parts 30d, while right ends 30f′′ thereof extend up to the vicinity of a center portion of the turret head 31a across right end faces of the vertical wall parts 30d. 
In the supporting member 30, linear guides 30g disposed on bottom faces of longitudinal both ends of the respective supporting parts 30e, 30e are supported by the aforesaid front and back second spindle guide rails 24, 24, so that the supporting member 30 is movable in the Z-axis direction. Further, a nut member 30h is disposed in a lower portion of the right supporting part 30e, and the tool post ball screw 32 provided on the inner side of the back-side second spindle guide rail 24 is screwed to the nut member 30h. The supporting member 30 reciprocates in the Z-axis direction when the tool post ball screw 32 is rotated by a servo motor 32a. 
When seen from a right side in the Z-axis direction, on front faces (faces on the front side) of the left and right leg parts 30a, 30a of the supporting member 30, provided are turret guide rails 33, 33 extending in the X-axis direction. Linear guides 33a, 33a are supported by the turret guide rails 33, 33 to be slidable in the X-axis direction, and the linear guides 33a are fixed on the back face of the turret 31. With this structure, the turret 31 is supported by the supporting member 30 to be movable in the X-axis direction.
Further, one turret ball screw 34 is disposed in parallel to the X axis in the supporting member 30 so as to extend between the upper and lower beam parts 30b, 30c. A nut member 34a is screwed to the turret ball screw 34, and the nut member 34a is integrally provided on the back face of the turret 31.
Further, in the supporting member 30, an extension part 30f is formed in the lower beam part 30c to extend in a backward direction from the leg part 30a, and a servo motor 35 is disposed on the extension part 30f. The servo motor 35 is coupled to the turret ball screw 34 via a timing belt 36. The turret 31 reciprocates in the X-axis direction when the ball screw 34 is rotary driven by the servo motor 35.
Here, when seen in the Z-axis direction, as shown in FIG. 16, the turret ball screw 34 and the servo motor 35 are disposed to be deviated toward the right side from the center line of the supporting member 30. Therefore, a left portion of the rectangular space of the supporting member 30 is a space where pipes, wiring, and so on can be arranged. Note that the indexing motor (not shown) of the rotary indexing mechanism 31b is positioned in a substantially center portion in the rectangular space of the supporting member 30 and moves in accordance with the X-axis direction movement of the turret 31.
The shape and dimension of the supporting member 30 are set so that it can move into/out of the tunnel part 3d of the first spindle headstock 3. In a state in which the supporting member 30 is housed in the tunnel part 3d, the turret 31 is positioned right under the first spindle 28. To be in more detail, a cutting point by a tool positioned at the machining position out of the tools T attached to the turret 31 is constantly positioned on the vertical line “b” going through the aforesaid axis “a”.
The second spindle headstock 4 is movable to a delivery position where it can directly receive a workpiece attached to the first spindle 28 of the first spindle headstock 3. Concretely, the second spindle headstock 4 is movable in the Z-axis direction up to a position so that front faces of the chucks of the first and second spindles 28, 26 abuttingly face each other (see FIG. 7(c)).
When the first and second spindles 28, 26 abut on each other, the tool post 5 can be positioned right under the first spindle headstock 3 so that the cutting point “c” is positioned vertically under the axis “a” of the first and second spindle headstocks 3, 4. Therefore, a workpiece cutting direction of the tool of the turret 31 is in line with the vertical line “b”. Concretely, when seen from the right side in the Z-axis direction, as shown in FIG. 6, the cutting point “c” and a rotation center “d” of the turret 31 are collinear on the vertical line “b” going through the axis “a” of the first and second spindle headstocks 3, 4. Further, the vertical line “b” goes through the center of an interval between the second spindle guide rails 24, 24.
Further, a tool gripper of the third spindle 6 is constantly positioned within an area surrounded by the aforesaid pair of upper and lower Z-axis guide rails 15, 15 and pair of left and right X-axis guide rails 17, 17. A movement area of a cutting point of the third spindle 6 overlaps a movement area of the cutting point “c” of the tool post 5 and further includes an area under the cutting point “c” when seen in the Z-axis direction.
Here, when seen from above, an XY supporting plane A (see FIG. 15) parallel to the X axis and the Y axis by which the turret 31 of the tool post 5 is supported to be movable in the X-axis direction is orthogonal to an XZ supporting plane B (see FIG. 2) parallel to the X axis and the Z axis by which the third spindle 6 is supported to be movable in the X-axis direction. To be in more detail, the XY supporting plane A means a plane including both sliding surfaces of the left and right turret guide rails 33, 33 of the supporting member 30. Further, the XZ supporting plane B means a plane including both sliding surfaces of the left and right X-axis guide rails 17, 17 of the saddle 12.
Further, as shown in FIG. 2, an X-axis direction movement line (the aforesaid vertical line “b”) of the turret 31 in the tool post 5 is arranged a predetermined distance “t” apart in the Y-axis direction from the XZ supporting plane B of the third spindle 6. Further, as shown in FIG. 15, in a state where the third spindle 6 is machining the shortest workpiece held on the first spindle headstock 3, an X-axis direction movement line “f” of the third spindle 6 is arranged a predetermined distance t′ apart from the XY supporting plane A of the turret 31.
In the composite lathe of this embodiment, while the workpiece is rotated by the first spindle headstock 3 or the second spindle headstock 4, the tool post cuts in the workpiece to perform cutting work and the third spindle 6 performs machining such as grinding and boring.
According to the composite lathe of this embodiment, when the chuck of the second spindle headstock 4 is positioned at the workpiece delivery position where it abuts on the chuck of the first spindle headstock 3, the tool post 5 is disposed right under the first spindle headstock 3 so that the cutting point “c” of the tool post 5 is positioned vertically under the axis “a” of the first and second spindle headstocks 3, 4. Therefore, the first and second spindle headstocks 3, 4 can deliver the workpiece directly to/from each other without interfering with the tool post 5, so that the conventional workpiece delivery device need not be provided, which enables cost reduction.
Further, since the tool post 5 is disposed right under the first spindle headstock 3, it is possible to reduce the front/back direction dimension of the machine main body compared with a case where the tool post 5 is positioned on the front side when seen from the front side of the machine, which enables downsizing of the machine main body.
In this embodiment, the first spindle headstock 3 has the tunnel part 3d formed by the pair of leg parts 3a, 3a and the spindle case part 3b coupling the upper ends of the both leg parts 3a, and the supporting member 20 supporting the turret 31 of the tool post 5 is capable of moving into/out of the tunnel part 3d. Therefore, when the supporting member 30 is housed in the first spindle headstock 3, the turret 31 can be positioned right under the first spindle 28 of the first spindle headstock 3, which can reduce the front/back direction dimension and a widthwise dimension of the machine main body to realize downsizing.
In this embodiment, since the tool post 5 is movably supported by the second spindle guide rails 24, 24 supporting the second spindle headstock 4, one pair of the second spindle guide rails 24 can serve both for the second spindle headstock 4 and for the tool post 5, which can reduce the number of parts and cost.
Further, the tool post ball screw 32 for driving the tool post 5 is disposed on the inner side of the back-side second spindle guide rail 24 and the second spindle ball screw 25 for driving the second spindle headstock 4 is disposed on the inner side of the front-side second spindle guide rail 24. Therefore, the ball screws that independently drive the reciprocal movements of the tool post 5 and the second spindle headstock 4 can be easily arranged by utilizing a vacant space near the guide rails.
In this embodiment, since the center trough 2c is formed between the both second spindle guide rails 24, 24 of the bed 2, a vacant space made available in the bed 2 can be effectively utilized for collecting chips.
In this embodiment, the first spindle headstock 3 is formed to be thermally symmetrical. Therefore, when the temperature of the first spindle headstock 3 rises due to the heat generation or the like from the first spindle 28, the left and right sides of the plane of symmetry are substantially equal in thermal expansion amount, so that the first spindle headstock 3 thermally expands vertically upward, which can reduce an adverse influence that the thermal expansion gives to machining accuracy.
In this embodiment, the vertical line “b” going through the cutting point “c” of the tool post 5 is positioned at the center of the interval between the both second spindle guide rails 24, 24. Therefore, a reaction force of a cutting force is transmitted uniformly via the tool post 5 to the left and right second spindle guide rails 24, 24, and further to the bed 2, which can also reduce the adverse influence to machining accuracy. Moreover, the first and second spindle headstocks 3, 4 and the tool post 5 are well-balanced in terms of gravity center and weight.
The above embodiment has described, as an example, the composite lathe including not only the first and second spindles 28, 26 but also the third spindle 6, but the present invention is also applicable to a lathe including only the first and second spindles (including a tail stock).
The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof.
The present embodiments are therefore to be considered in all respects as illustrative and no restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

## Claims

1. A machine tool comprising:
a bed;
a first spindle headstock disposed on said bed, said first spindle headstock having a tunnel shape having a pair of leg parts and a spindle case part disposed between upper ends of the leg parts;
a second spindle headstock disposed to be coaxial with said first spindle headstock along a Z-axis and to be movable in the Z-axis direction;
and a tool post disposed between said first spindle headstock and said second spindle headstock to be movable in the Z-axis direction,
wherein said second spindle headstock is movable to a position for workpiece delivery to and from said first spindle headstock, and
wherein said tool post has: a turret to which a plurality of tools are attached; and a supporting member by which the turret is supported to be movable in an up and down X-axis direction, and when said second spindle headstock is positioned at the workpiece delivery position, the turret is positioned between the first and second spindle headstocks under the Z-axis and a cutting point of the tool post is positioned vertically under the Z-axis, and wherein the supporting member of the tool post is movable into and out of the tunnel of the first spindle headstock.

2. The machine tool according to claim 1, wherein said spindle case part of said first spindle headstock supports a first spindle.

3. The machine tool according to claim 2, wherein said first spindle headstock is thermally symmetrical with respect to a plane of symmetry which is a plane including an axis of the spindle case part and said plane being parallel to the X axis.

4. The machine tool according to claim 1, wherein said second spindle headstock is supported to be movable in the Z-axis direction by a pair of spindle guide rails disposed on said bed, the supporting member of said tool post is mounted on the spindle guide rails, a ball screw for driving said tool post is disposed adjacently to one of the spindle guide rails, and a ball screw for driving said second spindle headstock is disposed adjacently to the other spindle guide rail.

5. The machine tool according to claim 4, wherein a trough in a groove shape whose upper side is open is formed between the both spindle guide rails of said bed.

6. The machine tool according to claim 4, wherein a vertical line going through the Z-axis of said first and second spindle headstocks goes through a substantial center of an interval between the spindle guide rails.

7. The machine tool according to claim 1, wherein said first spindle headstock is thermally symmetrical with respect to a plane of symmetry which is a plane including an axis of the spindle case part and said plane being parallel to the X axis.