Abstract:
In a method and a machine tool for machining metallic workpieces, a cooling device is provided to supply a cryogenic coolant to a first tool and a precooling device is provided to supply the cryogenic coolant to a second tool. The first tool is held in a first chuck and processes a workpiece, with the first tool during processing being cooled via the cryogenic coolant. The second tool is held in a second chuck and will subsequently be used for processing the workpiece. The second tool is supplied via the precooling device with the cryogenic coolant and thus cooled to the operating temperature required for processing. Due to the fact that the second tool is precooled to the required processing temperature, after the change of tools, the processing of the workpiece can be continued immediately. By precooling the second tool during the primary processing time, the total processing time of the workpiece is reduced, providing the machine tool with higher productivity.

Description:
FIELD 
     The invention relates to a method for machining metallic workpieces in which a tool is precooled by connecting it to a source of coolant before it is used to machine a workpiece. 
     BACKGROUND 
     A machine tool is known from DE 601 11 162 T2 (equivalent to EP 1 208 940 B1) comprising a tool carousel for machining metallic workpieces. The tool carousel comprises several chucks, in which one tool each is arranged for processing the workpiece. Cryogenic coolant is guided through the tool carousel to the respective tool presently engaged with the workpiece to be processed. High productivity in processing workpieces is yielded by the effective cooling using said cryogenic coolant. 
     The invention is based on the objective to provide a method allowing higher productivity in machining metallic workpieces. 
     Using the method according to the invention, the productivity in processing workpieces is increased such that during processing of the workpiece by a first tool, a second tool provided for a subsequent processing is precooled by the cryogenic coolant simultaneously during processing. For the cryogenic processing of workpieces the tools first need to be cooled to the required low operating temperature before the machining process can begin. This cooling process for the cryogenic processing of workpieces can only begin in methods and machine tools of prior art when the tool provided for processing is located in the operating position and/or in the chuck provided for processing. For example, the cooling process in methods and machine tools of prior art can only begin when the tool provided for processing is located in the tool spindle connected to a supply line for the cryogenic coolant. 
     Due to the fact that as early as during the processing of the workpiece via a first tool the second tool provided for subsequent processing is being cooled by the cryogenic coolant, the precooling period required for the second tool is moved from the primary processing time into the secondary processing time. Due to this precooling simultaneous to the primary processing time, the workpiece, after being processed by the first tool, can directly be further processed without any additional precooling by the already precooled second tool. This way, no primary processing time is lost, in which the second tool had to be cooled to the lower processing temperature. The cryogenic processing of a workpiece can therefore be continued directly after the change of tools, with the second tool during processing being cooled by the cryogenic coolant. 
     The precooling of a tool provided for subsequent processing occurs for example via a precooling device, which can be coupled to a tool magazine, a tool carousel, or any other chuck. 
     The method ensures a simple and effective precooling of the second tool provided for processing. For example, liquid or gaseous nitrogen, liquid or gaseous oxygen, gaseous hydrogen, gaseous helium, liquid or gaseous argon, gaseous carbon dioxide, and liquid or gaseous natural gas may serve as the cryogenic coolant. Preferably, during processing, the same cryogenic coolant is used, preferably nitrogen, for the precooling and the subsequent cooling of a workpiece. 
     The method ensures a rapid and effective precooling of the tool provided for subsequent processing. Preferably the cryogenic coolant provides the lower temperature even when it hits and/or flows through the workpiece to be precooled. 
     The method ensures that the processing of the workpiece can be continued immediately after the change of tools. The change of tools may occur, for example, by the precooled second tool being changed into the first chuck, which may be embodied as a tool spindle, for example. Alternatively the change of tools can occur by the second chuck with the second tool held there and precooled being moved from a storage position into a processing position. This is the case, for example, in tool carousels, with the different tools being brought by way of rotation of the carousel disk from a storage position into the processing position. The precooling device is particularly embodied such that the tool exchange occurs so fast that the second tool provided for processing essentially does not heat up prior to the change into the processing position. 
     The method ensures an efficient precooling of the tool provided for processing. 
     The method allows precooling in a simple fashion. The precooling device comprises for example a thermally isolated reservoir for providing the cryogenic coolant and a corresponding cooling condenser for cooling the cryogenic coolant as well as a precoolant-feeding line to supply the cryogenic coolant from the reservoir to the second chuck. The precoolant supply line is preferably embodied in a thermally isolated fashion. Preferably the precoolant device is coupled to a cryogenic cooling device to cool the tools during processing such that for precooling and for cooling during processing, the cryogenic coolant is taken from the same reservoir. 
     The method ensures in a simple fashion that during the change of tools no cryogenic coolant escapes the precooling device in an uncontrolled fashion. This way, a long operating life of the machine tool is ensured because cryogenic coolant only flows out of the precooling device when a tool to be precooled is coupled thereto. 
     The method ensures a simple and efficient cooling of the tool provided for processing. The influx of cryogenic coolant is only released by the precooling device when the tool to be precooled is coupled to the precooling device. 
     The invention is further based on the objective to further develop a machine tool of the generic type such that an increase of productivity is yielded in the machining of metallic workpieces. 
     The advantages of the machine tool according to the invention are the same as the above-described advantages of the method according to the invention. The machine tool according to the invention comprises a cooling device for the cryogenic cooling of the first tool, which is presently engaging the workpiece to be processed. Furthermore, the machine tool comprises a precooling device, serving to precool a second tool provided in a second chuck and intended for the subsequent processing of the workpiece. The cryogenic cooling device and the cryogenic precooling device are addressed by a control device such that during processing of the workpiece by a first tool the second tool provided for the subsequent processing of the workpiece is precooled and brought to the lower processing temperature required. This way, in the above-described manner, the precooling time for the second tool is changed from the primary processing time to the secondary processing time. After changing the second tool into the position intended for processing the workpiece the processing of the workpiece can continue immediately. This way, the primary processing time and/or cycle time for the tool processing is reduced and the productivity of the machine tool is increased. 
     The machine tool ensures a simple precooling of the tool provided for processing. Preferably the reservoir and the corresponding cooling condenser are both part of the precooling device as well as a part of the cooling device, so that the cooling of the tool processing the workpiece and the precooling of the tool provided for processing can occur with the same cryogenic cooling medium. For this purpose, a precoolant-supply line leads from the reservoir to the second chuck and a coolant-supply line from the reservoir to the first chuck. 
     The machine tool ensures effective precooling of the tool provided for processing. The supply of the cryogenic coolant to the tool to be pre-cooled is only released when the tool is mechanically coupled to the coupling unit. The release of the cryogenic coolant can occur either mechanically or electromechanically. 
     The machine tool ensures in a simple fashion the coupling and decoupling of the precooling device to and/or from the second chuck. For this purpose, either the precooling device can be arranged fixed at a basic frame of the machine tool and the second chuck can be displaceable or vice versa. 
     The machine tool ensures the precooling of a multitude of different tools in a simple fashion. The first chuck may be embodied as a common tool spindle, for example, which performs a tool exchange in the pick-up method and here places the first tool into the tool magazine and accepts the precooled second tool from the second chuck. Alternatively, a common tool changer may be used. 
     The machine tool ensures an efficient precooling and simultaneously an efficient cooling during processing. The reservoir is both a part of the precooling device as well as a part of the cooling device so that only one reservoir is required in order to guide the cryogenic coolant via the precoolant-supply line to the second chuck and via the coolant supply line to the first chuck. 
     The machine tool ensures efficient precooling and cooling of the respective tool. The supply lines preferably show a specific thermal conductivity at 0° C. of maximally 0.40 W/(mK), particularly no more than 0.30 W/(mK), and particularly no more than 0.20 W/(mK). By a thermally isolated embodiment any undesired heating of the cryogenic coolant is prevented over the path from the first and/or the second chuck. This way, the respective tool can be precooled and/or cooled extremely effectively. Preferably the supply lines are embodied entirely in a thermally insulated fashion, i.e. over their entire length. Furthermore, the supply lines are preferably embodied in a vacuum-isolated fashion. For this purpose they comprise an inner tube and an outer tube surrounding it, connected to each other and limiting an evacuated isolated space. In the vacuum-isolated embodiment the supply lines preferably show a specific thermal conductivity at 0° C. of no more than 0.01 W/(mK). In a vacuum-isolated embodiment preferably at least one of the tubes can be changed in its length so that a compensation to different longitudinal changes of the inner and the outer tube is possible. When the supply lines conduct the cryogenic coolant the respectively inner tube essentially assumes its temperature, while the respectively outer tube, based on the isolation medium arranged between the two tubes, cools considerably less. The inner tube changes its length therefore to a considerably greater extent than the outer tube. In order to avoid any damage to the supply lines at least one of the tubes is capable of changing its length. Preferably the outer tube comprises a pleated metallic bellows for a thermal compensation of length. 
     Additional features, advantages, and details of the invention are discernible from the following description of an exemplary embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a machine tool with a tool magazine and a preliminary cooling device cooperating therewith. 
         FIG. 2  is a top view of the machine tool in  FIG. 1 . 
         FIG. 3  is a schematic illustration of the precooling device cooperating with the tool magazine. 
         FIG. 4  is a front view of the tool magazine. 
         FIG. 5  is a cross-section of a coupling unit of the precooling device coupled to a tool to be pre-cooled. 
         FIG. 6  is an enlarged illustration of the coupling unit of  FIG. 5 . 
         FIG. 7  is a cross-section of the coupling unit of the precooling device decoupled from the precooled tool. 
         FIG. 8  is an enlarged illustration of the coupling unit in  FIG. 7 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a machine tool  1  comprising a base frame  2  with a machine bed  3  and a stand  4  arranged thereon. The machine bed  3  extends essentially in a horizontal x- and a horizontal z-direction. The stand  4  is fastened at its end at the machine bed  3  and essentially extends in the x-direction and a vertical y-direction. The base frame  2  is fastened on a base plate  5 . The x-, y-, and z-directions each extend perpendicular in reference to each other and form a Cartesian coordinate system. 
     An x-sled  6  is arranged at the stand  4 , which can be linearly displaced on the corresponding x-guidance rails  7  via an x-drive motor  8  parallel to the x-direction. A y-sled  9  is arranged in turn at the x-sled  6 , which can be linearly displaced on the y-guidance rails  10  via a y-drive motor  11  parallel to the y-direction. A tool spindle  12  is fastened at the y-sled  9 , comprising a first chuck  13  embodied as a clamping set, which can be rotationally driven via a drive motor  14  about a rotary axis  15  extending in the z-direction. A first tool  16  is held in the first chuck  13 , serving for the cutting processing of a metallic workpiece  17 . 
     The machine tool  1  has a processing space  18  in front of the tool spindle  12  in which a workpiece support  19  for the workpiece  17  to be processed is arranged on the machine bed  3  as shown in  FIG. 2 . The workpiece support  19  is supported on z-guidance rails  20  and is linearly displaceable parallel to the z-direction via a z-drive motor  21 . The workpiece support  19  is embodied as a rotary plate and is rotated by a drive motor  22  about a rotary axis  23  extending parallel to the y-direction. For example, workpiece pallets with workpieces  17  clamped thereon may be fastened on the rotary plate  19 . 
     The machine tool  1  comprises a tool magazine  24  as shown in  FIGS. 3 and 4  fastened at the stand  4 . The tool magazine  24  is embodied as a disk magazine. Accordingly the tool magazine  24  comprises a magazine disk  25  with a plurality of chucks  26  arranged thereon. For the subsequent processing of the workpiece  17  different tools  27  are arranged in the chucks  26 . The magazine disk  25  is supported for rotation at a magazine sled  28  and can be rotationally driven via a drive motor  29  about a rotary axis  30  relative to the magazine sled  28 . The magazine sled  28  is arranged displaceable, parallel to the z-direction, at a magazine support  31 , which in turn is fastened at a stand  4 . For this purpose, z-guidance rails  32  are arranged at the magazine support  31 , on which the magazine sleds  28  can be linearly displaced via a drive motor  33  parallel to the z-direction. The magazine disk  25  is partially surrounded by a magazine housing  34 . 
     For the cryogenic cooling of the tools  16 ,  27 , the machine tool  1  includes a cooling device  35  as well as a corresponding precooling device  36 . The cooling device  35  and the precooling device  36  comprise a common reservoir  37  in order to provide a cryogenic coolant  38 , embodied in a thermally isolated fashion. A cooling condenser  39  is provided to cool the cryogenic coolant  38 . Using a first pump  40  the cryogenic coolant  38  can be fed via a coolant supply line  41  from the reservoir  37  to the first chuck  13  and/or the tool  16  held therein. A first shut-off valve  42  is arranged in the coolant supply line  41  to interrupt the supply of the cryogenic coolant  38  to the chuck  13 . The reservoir  37 , the cooling condenser  39 , the pump  40 , the coolant supply line  41 , and the shut-off valve  42  form the cooling device  35 . 
     In order to precool one of the secondary tools  27  the cryogenic coolant  38  can be supplied via a second pump  43  from the reservoir  37  through the precoolant-supply line  44  to one of the secondary chucks  26  of the tool magazine  24 . In the precoolant-supply line  44 , in order to shut off the supply of cryogenic coolant  38 , a second shut-off valve  45  is arranged. The reservoir  37 , the cooling condenser  39 , the pump  43 , the precoolant-supply line  44 , the shut-off valve  45 , and a coupling unit  46 , described in the following, form the precooling device  36 . The supply lines  41 ,  44  are embodied in a vacuum-insulated fashion to avoid any undesired heating of the cryogenic coolant  38 . 
     As shown in  FIGS. 5 ,  7 , and  8 , the coupling unit  46  is arranged with one end at a precoolant-supply line  44  and embodied such that the supply of the cryogenic coolant  38  is released to the respectively secondary tool  27  when the tool  27  is mechanically coupled to the coupling unit  46  and the supply of the cryogenic coolant  38  to the respective tool  27  is interrupted when the tool  27  is mechanically decoupled from the coupling unit  46 . For a mechanical coupling and/or decoupling of the respective tool  27  to/from the coupling unit  46  the chucks  26  of the tool magazine  24  are linearly displaceable in reference to the coupling unit  46  via the drive motor  33 . 
     As shown in  FIG. 6 , the coupling unit  46  is fastened at the magazine housing  34  and embodied as a spring-loaded slider. For this purpose, the coupling unit  46  comprises an inner slider  47  and an outer slider  48 , which are embodied telescopically in reference to each other and are pre-stressed by a spring element  49 . The sliders  47 ,  48  and the spring element  49  are arranged in an allocated housing  50 . The sliders  47 ,  48  limit an inner chamber  51 , in which the precoolant-supply line  44  is guided. The housing  50  shows a penetrating opening  52  on the side facing the tool  27 , through which a coupling line  53  is guided from the inner space  51  towards the outside. The coupling line  53  is closed at the face of the end arranged in the inner space  51 , however at its end it comprises entry openings  60  over its circumference. The inner slider  47  comprises a releasing section  54  and a closing section  55  for opening and/or closing these entry openings  60  at a side facing the coupling line  53 . 
     The coupling unit  46  is embodied such that it can be inserted into a fastening recess  56  of the tool  27  and the coupling line  53  can be connected to a coolant duct  57  extending in the tool  27 . For this purpose, a sealing insert  58  is arranged at the coolant duct  57 , by which the outer slider  48  can be operated when the tool  27  is coupled. 
     In order to control the machine tool  1 , it is provided with a control device  59  embodied such that during processing of the workpiece  17  via a first tool  16  the second tool  27  provided for the subsequent processing of the workpiece  17  is precooled via the cryogenic coolant  38 . 
     In order to process the workpiece  17  it is clamped in a conventional fashion on the workpiece support  19  and the first tool  16  is clamped in a first chuck  13  of the tool spindle  12 . The workpiece  17  is processed in a common fashion by the tool  16 , by the workpiece support  19  and/or the tool spindle  12  being linearly displaced in the x-, y-, and/or z direction. The first tool  16  is cooled during processing by the cryogenic coolant  38 . For this purpose, the cryogenic coolant  38  is conveyed by a pump  40  from the reservoir  37  via the coolant supply line  41  to the tool spindle  12 . The coolant supply line  41  is coupled to the coolant duct  57  of the tool  16  so that the tool  16  during processing receives and is cooled by the cryogenic coolant  38 . The cryogenic coolant  38  is selected from the group consisting of nitrogen, oxygen, hydrogen, helium, argon, carbon dioxide, natural gas and mixtures thereof. Preferably, nitrogen is the cryogenic coolant  38 . 
     When exiting the tool  16 , the cryogenic coolant  38  preferably shows a temperature of less than −60° C., particularly less than −120° C., particularly less than −150° C., and particularly less than −180° C. 
     When processing the workpiece  17  the second tool  27  provided for the subsequent processing of the workpiece  17  is arranged in the tool magazine  24  at a precooling position and connected via the coupling unit  46  to the precooling device  36 . This is illustrated in  FIGS. 5 and 6 . The cryogenic coolant  38  is supplied via the pump  43  from the reservoir  37  through the precoolant-supply line  44  to the coupling unit  46 , which due to the mechanic coupling to the tool  27  releases the supply with cryogenic coolant  38 . For this purpose, the sealing insert  58  contacts the outer slider  48  such that the inlet openings  60  are released to the coupling line  53  and thus also the supply of the cryogenic coolant  38  to the tool  27 . The cryogenic coolant  38  therefore flows through the tool  27  and cools it to the operating temperature required for processing. This precooling process occurs simultaneous to the processing of the workpiece  17 . When cooling the tool  27  the cryogenic coolant  38  shows a temperature of less than −60° C., particularly less than −120° C., particularly less than −150° C., and particularly less than −180° C. 
     When the processing of the workpiece  17  via the tool  16  is concluded, the tools are changed. For this purpose, the magazine disk  25  with the chucks  26  and the tools  27  accepted therein are first displaced parallel in reference to the z-direction via the drive motors  33  such that the coupling unit  46  is decoupled from the precooled tool  27 . 
     The outer slider  48  is displaced by the pre-stressed spring element  49  such that the closing section  55  is arranged in the area of the entry openings  60  of the coupling line  53  so that the supply of the cryogenic coolant  38  to the coupling line  53  is interrupted. Subsequently the magazine disk  25  is rotated about the rotary axis  30  such that an empty chuck  26  is arranged at the tool exchange place. Subsequently the tool spindle  12  is moved to the tool magazine  24  and places the tool  16  into the empty chuck  26 . 
     Subsequently the magazine disk  25  is moved about the rotary axis  30  such that the pre-cooled tool  27  is arranged at the tool changing site and here is accepted by the free tool spindle  12 . By the tool spindle  12  moving, the second tool  27  is moved into a processing position to the workpiece  17  to be processed further. At the beginning of the processing of the workpieces  17 , the second tool  27  shows a temperature of maximally −60° C., particularly maximally −120° C., especially maximally −150° C., and preferably maximally −180° C. If the second tool  27  after decoupling from the precooling device  36  has excessively heated it can already be cooled via the cooling device  35  prior to processing the workpiece  17  in the tool spindle  12 . 
     When processing the workpiece  17  via the second tool  27  it is cooled in the above-described manner via the cryogenic coolant  38 . 
     During the processing of the workpiece  17  with the tool  27  the magazine disk  25  is rotated about the rotary axis  30  such that a tool  27  provided for the subsequent processing of the workpiece  17  is once more arranged at the pre-cooling site. By a linear displacement of the magazine disk  25  the tool  27  located at the pre-cooling site is mechanically coupled with the coupling unit  46 . Here, the gasket set  58  of the tool  27  operates the outer slider  48  against the spring force of the spring element  49  such that the release section  54  is arranged in the area of the entry openings  60  of the coupling line  53  and releases the supply of the cryogenic coolant  38  to the tool  27 . The cryogenic coolant  38  flows through the coupling line  53  and the cooling duct  57  and cools the tool  27  provided for the subsequent processing to the processing temperature required.