Abstract:
A piece-threading device on a numerically controlled lathe comprises a supporting assembly having an inner rotatively driven body which, on a side thereof facing a workpiece to be threaded, comprises a plurality of threading tools, the rotatively driven body having a built-in electrical rotor ( 9 ) to which an electric stator ( 8 ) also built-in in the supporting assembly is operatively coupled, the stator ( 8 ) having a plurality of circumferential channels ( 6 ) for circulating therethrough a refrigerating and lubricating fluid.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a piece-threading device to be used on a numerically controlled lathe. 
     As is known, in the medical field are conventionally used screws, stems or other mechanical pieces to be screwed into the bone structure of a patient. 
     At present, the best method used in the prior art for making threaded elements to be used in the medical or dental field, is a specifically designed cutting process, the so-called “swivel” threading process or, in a word-wide manner, the “Thread Whirling Process”. 
     Prior methods allow to make threaded pieces with a variable pitch thread, a very long pitch thread, highly buttressed threads, bevel threads or variably buttressed threads. 
     It should be apparent that the selection of the thread will depend on the specific bone structure, the screw must be screwed into. 
     The prior machining method allowing to make different types of threads on a lathe, also allows to machine, without any technical problems, very hard metals, such as stainless steel, titanium or the like. 
     From the prior art it is also known that the surface quality of a thread depends on clearances which are present in the kinematic chain driving the tools used for performing the above mentioned thread whirling process. 
     The prior method for making the above mentioned threads, however, is not a continuous cutting method, but an interrupted cutting operation, like a milling machining operation, and this type of machining greatly stresses the driving means; moreover, because of unavoidable clearances, the surfaces of the resulting threads are inevitably affected by machining defects and rags. 
     Usually, in prior devices, the threading tools are driven by a driving motor through a series of gears which, because of their nature, necessarily require a minimum clearance, to provide a proper mutual meshing, and this clearance, even if it has a minimum value, negatively affects the precision and quality of the resulting thread surface. 
     To further limit the above undesired clearance, prior deriving gears have been recently replaced by a belt transmission, designed to allow the motor to rotatively drive the machining tool. 
     This approach, however, has the drawback that a resilient element in the form of a belt must be used in order to transmit the rotary motion from the driving motor to the machining tool. The use of a belt generates inevitable undesired oscillations in the rotary movement of the tool that must be subjected to continuous control. Moreover, the driving belt must be subjected to periodic controls for its inevitable wear and to repeatedly accomplish the tensioning thereof. 
     Thus, both the above mentioned gear assembly and driving belt, as conventionally used for rotatively driving prior threading tools, cause an undesired increase of the overall size of the device. 
     Moreover, it has been also found that in prior threading device, also called “turbothreaders” designed for making a swivel thread, it would be absolutely necessary to rotatively drive the head/spindle assembly for performing the thread helix; moreover, for making some types of threads, it would be indispensable to tilt the threading head with respect to the horizontal plane and, moreover, to provide a tool holder spindle having a conical inner configuration, in order not to impact against the workpiece during the threading operations. 
     SUMMARY OF THE INVENTION 
     Accordingly, the aim of the present invention is to overcome the above mentioned drawbacks of the prior art, and improve the cutting threading method so as to provide variable pitch threads free of metal rags and with thread surfaces having a minimum roughness, while allowing to omit prior gear cascade arrangements or belt drives to rotatively operate the threading tool. 
     According to the present invention, the above mentioned objects are achieved by a threading device for threading pieces designed to be used in the medical field, said device comprising a supporting assembly having an inner rotatively driven body, which, on a side thereof facing the workpiece to be threaded, comprises a plurality of threading tools, said rotatively driven body housing an electric rotor to which an electric stator built-in in said supporting assembly is operatively coupled. 
     Advantageously, said stator has a stator outer circumference including a plurality of channels for circulating therethrough a refrigerating fluid. 
     Moreover, the electric stator is encompassed by a sleeve having a plurality of circumferential channels, therethrough a refrigerating and lubricating fluid flows. Said refrigerating and lubricating fluid passes through the inside of the supporting assembly and exit the latter through an opening. 
     Advantageously, the related interface includes electric cables to power supply the electric motor and the ducts required for pressurizing the device. 
     Between the device and the machine tool a single interface is provided. 
     Advantageously, a speed sensor for controlling the electric motor, and being coupled to the rotary body is further provided. 
     The electric motor driving the rotary body may be advantageously controlled by control means of a MRAS type (Model Reference Adaptive System), thereby allowing the speed or revolution sensor to be omitted. 
     Further characteristics of the invention will become apparent from the sub-claims and the accompanying disclosure and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter according to the present invention will be disclosed and illustrated in a more detailed manner hereinafter, with reference to a preferred embodiment given only by way of an example. In the drawings: 
         FIG. 1  is a schematic partially cross-sectioned view of the novel threading head including a novel driving motor; 
         FIG. 2  is a cross-sectional view of the threading head; and 
         FIG. 3  is a further schematic partial cross-sectioned view showing a portion of the threading head with the driving motor stator and rotor coupled to the rotary driving assembly of the threading device. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in  FIG. 1 , the threading device according to the present invention, which has been generally indicated by the reference number  1 , comprises a supporting assembly or body  2 , having a throughgoing opening  3  for supplying a refrigerating and lubricating fluid  4 . 
     The supporting body or assembly  2  is mounted on a slide of a machine tool, not herein shown. 
     The refrigerating and lubricating fluid passes through channels, not specifically shown, of the supporting assembly  2 , as it will be disclosed in a more detailed manner hereinafter, said fluid  4  exiting the supporting assembly  2  through an outlet channel  5 . 
     Before exiting said supporting assembly or body  2 , the refrigerating fluid  4  passes through a plurality of circumferential channels  6  of an annular body  7  encompassing the stator  8  operating in cooperation with a rotor  9  integral with a further body  10  which, at a region  11  thereof, supports a plurality of machining tools (not shown in  FIG. 1 ) for machining a bar, schematically indicated by  12 .  FIG. 2 , is a cross-sectional view showing the body  10  of the device  1 , integral with said electric rotor  9 , which is rotatively driven through said electric stator  8 , said electric rotor  9  and stator  8  forming together an electric motor. 
     To said electric stator  8  is operatively coupled said tubular sleeve body  7 , including said plurality of circumferential channels  6 , to which refrigerating fluid for removing the generated heat is supplied. 
       FIG. 2  shows moreover in a very clear manner the arrangement of the threading tools  11  for threading, as desired, the free end portion of the bar  12 . 
     The refrigerating fluid  4 , exiting the opening or outlet  5  of the supporting assembly  2 , as is shown in  FIG. 1 , is not only used as a refrigerating fluid proper passing through the channels  6  of the annular body  7 ; in fact said refrigerating fluid  4 , upon exiting the supporting assembly  2  through the channel  5 , will moreover freely descend, to be also used as a refrigerating fluid for the machining tools  11  forming threads  13  on the free end portion of the bar  12  which threaded bar will be finally cut to any desired length. 
     After the threading operation, the fluid  4  will be filtered and fed again to the cooling channels  6 . 
     As is further shown in  FIG. 2 , the rotary body  10  is very accurately supported on said supporting assembly  2  by precision bearings  14 . 
     Thus, the provision of said precision bearings  14  as well as of the electric rotor  9  in a single body with the rotary body  10  and a toroidal stator  8  fixedly mounted within the supporting assembly  2 , and of the cooling channels  6  encompassing the stator  8  body, allows the machining tools  11  to be directly driven with a very high precision, without using, as in the prior art, a composite driving assembly including a series of gears, a trapezoidal drive belt or the like. 
     Moreover, the device  1  comprises an interface  20  to properly drive and control the motor  8 ,  9 , which interface  20  is advantageously operatively coupled to a machine Numeric Control Device (not shown). 
     Said interface, moreover, is so pressurized as to prevent machining chips from depositing in the apparatus and wearing rotary parts of the latter. 
     Thus, said single interface providing both a passage for the electric motor power supply cables and a pressurized operating environment, allows to make a compact and operatively flexible device, which can be easily and quickly supported by a slide of any desired single or multiple spindle machine tool without the need of modifying the latter. 
       FIG. 3  shows a schematic cross-sectional view of the portion  10  of the supporting assembly  2 , said portion  10  supporting not shown threading tools and comprising said electric rotor  9  forming, with said electric stator  8 , a synchronous brushless electric motor  8 ,  9  (for example of a type offered and produced by the Company Phase Motion Control of Genoa).  FIG. 3  also clearly shows the circumferential channels  6  formed in the sleeve  7  therethrough said refrigerating fluid  4  supplied, for example, through a channel  3  of a central unit (not shown) flows, said fluid  4  being so temperature controlled as to properly cool said stator  8 ; moreover said fluid upon exiting the device  1  through the channel  5  ( FIG. 1 ) will be also conveyed to the machining tools  11  to also properly lubricate and cool the latter, and remove any machining chips from the resulting thread  13  at the free end portion of the bar  12 . 
     In  FIG. 2 , the reference number  21  shows the fixed portion and the reference number  22  the rotary portion of a speed sensor for sensing the rotary speed of the body  10 . 
     According to a modified embodiment of the invention, to further improve the compactness and operating flexibility of the apparatus, it would be also possible to omit the speed sensor, and detect said rotary speed by reading out the operating current of the motor thereby properly controlling said motor by a sensorless arrangement, as controlled by a MRAS or Model Reference Adaptive System. 
     Thus a turbothreading device of very simple and compact construction will be obtained.