Abstract:
A lathe includes a frame for supporting rotatable chucking equipment adapted to hold a work piece. The frame further supports at least one lathe tool for machining the work piece. The chucking equipment and the lathe tool are displaceable relative to each other in at least a radial direction. The lathe includes a housing attached to the frame, with the housing extending about a longitudinal axis and having a cylindrical inner space extending from one end of the housing. The inner space has a longitudinal axis parallel to, though offset from, the longitudinal axis of the housing. An angularly displaceable shaft is arranged in the inner space and carries the chucking equipment. A rotatable outer casing peripherally encloses the cylindrical elongate housing. The rotatable outer casing carries the lathe tool. Rotation of the rotatable outer casing causes the lathe tool to effect a radial displacement relative to the chucking equipment.

Description:
This application is a continuation of International Application No. PCT/SE99/01087, filed on Jun. 17, 1999, which designates the United States. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a lathe according to the preamble of claim  1  and to a method for turning a work piece on a lathe. 
     BACKGROUND OF THE INVENTION 
     A traditional lathe generally comprises a bed carrying a head-stock and a tall-stock between which a work piece is supported. The work piece is held in chucking equipment attached to the headstock and is rotated by a motor driving the chucking equipment. Such a lathe further comprises a carriage arranged for displacement along the bed and carrying a slide rest by which a machine tool is held and traversed. To obtain high precision and accuracy during machining, the constituent components of the lathe must be rigid and stable. This generally implies that the lathe is bulky and heavy. 
     Lathes are also known in which the work piece is stationary and the machine tool is rotated about the work piece. Examples of such lathes are described in EP-B-0 278 898, DE-A-26 42 965 and CH-260918. Common to each of these lathes is the need for a relatively complex system for obtaining radial displacement of the machine tool relative the work piece. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a lathe which offers high machining precision, though which is less complex and less bulky than traditional lathes. 
     This object is achieved by means of the lathe according to claim  1 . 
     It is a further object of the invention to provide a method for turning a work piece on a lathe, which method implies high machining precision, but which can be executed on a relatively simple lathe. 
     This object is achieved by the method according to claim  13 . 
     Preferred embodiments of the lathe and the method according to the present invention are detailed in the respective dependent claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in greater detail in the following by way of example only and with reference to embodiments shown in the attached drawings, in which: 
     FIG. 1 shows in a schematic longitudinal sectional view of a first embodiment of a lathe according to the invention; 
     FIG. 2 is a schematic end view of the lathe shown in FIG. 1, and 
     FIG. 3 is a schematic longitudinal sectional view of a second embodiment of a lathe according to the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the drawings, reference numeral  10  generally denotes a lathe according to the present invention. The lathe  10  incorporates a frame  12  having a base section  14  and a support section  16  extending generally perpendicular to the base section  14 . The support section  16  presents an annular housing  18  in which a stationary cylindrical elongate housing  20  is centrally located by means of being rigidly affixed in a recess  22  in the frame  12 . 
     The cylindrical elongate housing  20  extends about a first longitudinal axis  24 . The cylindrical elongate housing  20  is provided with a longitudinal cylindrical inner space  26  extending from a first end  28  of the housing  20 , the first end of the housing being that end of the housing remote from the support section  16  of the frame  12 . The cylindrical inner space  26  extends along a second longitudinal axis  30  which is parallel to, though offset from, the first longitudinal axis  24  of the cylindrical elongate housing  20 . In the drawings, the cylindrical inner space  26  extends the entire length of the elongate housing  20 , though it will be apparent from the following description that the invention can also be practised using an elongate housing having an offset cylindrical inner space which extends only part through the housing from the first end. 
     The cylindrical inner space  26  is adapted to support a shaft  32  such that the shaft  32  may execute angular displacement within the inner space. To attain angular displacement, the lathe  10  comprises first displacement means  34 , preferably in the form of an electric torque motor, which advantageously can be accommodated within an internal recess  36  in the elongate housing  20 . Thus, the stator of the motor can be carried by the elongate housing while the rotor is carried by the shaft  32 . The internal recess  36  is preferably formed by providing a bore of greater diameter than the cylindrical space  26  in the end of the elongate housing remote from the first end  28 . 
     At its end adjacent the first end  28  of the elongate housing  20 , the shaft  32  carries chucking equipment  38  centered about the longitudinal axis  30  of the inner space  26  and thereby the shaft  32 . The chucking equipment  38  is adapted to hold a work piece  40  which is to be machined. The chucking equipment  38  may be of any conventional type which is suitable for clamping the work piece, such as centric chucking, micro-centric chucking or magnetic chucking. In the drawings, the shaft  32  has been illustrated having a longitudinally extending bore  39  extending the entire length of the shaft. Such an embodiment is useful when the work piece is in the form of a rod of material or the like. Naturally, if work pieces of relatively short length are to be machined, the invention may also be practised using a solid shaft to carry the chucking equipment  38  or a shaft having a longitudinally extending bore which extends only part way along the shaft. 
     In applications in which the work piece constitutes an end portion of a rod of material, the lathe  10  may be provided with material feeding means  41  cooperating with the rod of material to thereby advance the rod when machining of one work piece has terminated and machining of a subsequent work piece is to commence. The material feeding means  41  may be any known means which is suitable for indexing and/or advancing the rod of material a predetermined distance. 
     The lathe  10  further comprises a rotatable outer casing  42  which peripherally encloses the cylindrical elongate housing  20  along at least a length of the housing extending from the first end  28  of the housing. The rotatable outer casing  42  is preferably in the form of a cylindrical sleeve which is concentric with the cylindrical elongate housing  20 . A first end  44  of the outer casing  42 , corresponding to the end of the outer casing adjacent the first end  28  of the elongate housing  20 , is adapted to carry at least one lathe tool  46 . Advantageously, the outer casing  42  extends along the elongate housing  20  a distance such that a second end  48  of the casing terminates within the annular housing  18  of the frame  12 . In this manner, the frame  12  can support second displacement means  50 , preferably an electric torque motor, which acts on the rotatable outer casing  42  to effect rotation thereof, with the second displacement means being accommodated within the annular housing  18 . To detect and control the rotation of the outer casing  42 , first sensor means  52  may also be supported by the frame  12  within the annular housing 
     As is most clearly apparent from FIG. 2, the illustrated embodiment of the lathe  10  is provided with a first lathe tool  46  and a parting tool  54 . In the illustrated embodiment, the first lathe tool is a profiling tool which is intended to be displaced radially with respect to the work piece  40  to thereby impart a shape to the mantel surface of the work piece corresponding to the profile of the tool  46 . In accordance with the present invention, the radial displacement of the profiling tool is attained by rotation of the rotatable outer casing  42  about the cylindrical elongate housing  20 . Thus, due to the fact that the work piece  40  is constrained to rotate about the second longitudinal axis  30  and the machine tool has a centre of rotation lying on the first longitudinal axis  24 , rotation of the outer casing  42  in one direction will cause the lathe tool to approach the work piece, whilst rotation of the outer casing in the opposite direction will cause the lathe tool to move away from the work piece. In FIG. 2, arrow  56  indicates the direction of rotation to cause the lathe tool  46  to approach the work piece and arrow  58  indicates the direction of rotation to cause the lathe tool  46  to move away from the work piece. 
     From FIG. 2, it will be apparent that the profiling tool  46  and the parting tool  54  are circumferentially spaced by an angle such that as the profiling tool  46  approaches the work piece  40 , the parting tool  54  is moved away from the work piece. Conversely, as the profiling tool moves away from the work piece, the parting tool approaches the work piece. 
     The lathe of FIGS. 1 and 2 may be operated in the following manner. 
     The work piece  40  is clamped in the chucking equipment  38  with the outer casing  42  in an angular position such that neither the profiling tool  46  nor the parting tool  54  interfere with the work piece. The first displacement means  34  is thereafter activated to rotate the shaft  32  and thereby the work piece  40 . The second displacement means  50  is then activated to cause the outer casing  42  to rotate in the direction of arrow  56  in FIG. 2 to cause the profiling tool  46  to come into contact with the work piece  40 . The outer casing continues to be rotated in the direction of arrow  56  until a desired depth of machining of the mantel surface of the work piece has been attained. 
     When machining of the work piece by the profiling tool  46  has been completed, the outer casing  42  is rotated in the direction of arrow  58  until the profiling tool  46  is clear of the work piece. The rod of material is then displaced axially to the left in FIG. 1 by the material feeding means  41 . This presupposes that the chucking equipment  38  at least temporarily releases its grip on the work piece. This can be achieved by not shown control means coordinating the release of the chucking equipment  38  and the operation of the material feeding means  41 . The rod of material is displaced a distance corresponding to the desired length of the work piece which has just been machined. Thereafter, the chucking equipment re-grips the rod of material and the outer casing  42  is rotated in the direction of arrow  58  to thereby brine the parting tool  54  into contact with the rod of material. Continued rotation of the outer casing in the same direction causes the parting tool to sever the machined work piece from the remainder of the rod of material. Machining of a new work piece at the newly exposed end of the rod of material is effected by rotating the outer casing  42  in the direction of arrow  56 . Thereafter, the previously described cycle is repeated. 
     The embodiment of the lathe shown in FIG. 3 is similar to that described above. Thus, the same reference numerals have been used for corresponding parts. The primary difference between the lathes is that the lathe of FIG. 3 is provided with third displacement means  60  supported by the frame and preferably being accommodated within the annular housing  18 . The third displacement means  60  is preferably a linear electric motor acting on the rotatable outer casing  42  to effect axial displacement thereof along the tubular cylindrical housing  20 . To detect and control the axial displacement of the outer casing  42 , second sensor means  62  may also be supported by the frame  12  within the annular housing  18 . 
     The embodiment of the invention shown in FIG. 3 is useful for producing work pieces which require machining over a relatively large surface. Thus, the lathe tool can be a turning tool  64  such as a round-nosed tool which can be displaced over a length of the work piece by means of the third displacement means  60 . The depth of cut can of course be controlled by rotation of the outer casing  42  in a manner described earlier. The method of operation of the lathe of the FIG. 3 embodiment is essentially identical to that already described, though of course with the added degree of freedom obtained by the axial displacement of the outer casing  42 . This implies that the positioning of the line of severing of the workpiece from the rod of material can be determined either by operation of the material feeding means  41  or by axial displacement of the parting tool  54  carried by the outer casino  42 . 
     The invention is not limited to the embodiments described above and shown in the drawings. Instead, all modifications and variations within the scope of the appended claims are to be deemed to be covered. For example, the action of the material feeding means  41  may instead be achieved manually. It is also conceivable that the material feeding means  41  be replaced by a puller at the chucking equipment end of the lathe.