Abstract:
A shock-absorbing device is intended to be mounted on a spindle headstock of a machine tool. The spindle headstock includes a motor driven spindle unit connected to an upright by a support. The shock-absorbing device comprises a yieldable structure connected on one side to the spindle unit and on the other side to the support, and is adapted to yield upon an accidental collision of the spindle headstock against a workpiece. A collision sensor or is associated with the yieldable structure for detecting the yielding thereof resulting from an accidental collision of the spindle headstock with a workpiece, and for transmitting in response thereto a signal to a machine tool control unit.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to metal cutting machine tools and, more particularly, a device for protecting the spindle headstock of a machine tool in the case of an accidental collision with a workpiece. 
     As known, in recent years manufacturers in the field of machine tools have directed their efforts to improve, in addition to the quality of machining, also the efficiency of machine tools by gradually and continuously reducing machine time and machine setup time. 
     At present, machine time has become reduced thanks to the improvements in the quality of the cutting edges of tools and, in particular, to their wear resistance, and also has become reduced because the cutting speed has increased. 
     Machine tool manufacturers are now endeavored to reduce, in the work cycle, the machine tool setup time which relates to retooling operations, workpiece loading and unloading operations, workpiece dimensional check operations and, not in the least, positioning operations of the spindle headstock in required various machining configurations. 
     It can be clearly understood that a reduction of the machine tool setup time is especially important in the case of a 24-hour work cycle, wherein a few minutes lost in the machine tool setup operation during the production of a workpiece, once they are multiplied by the daily production, can result in the loss of several hours. 
     Since each hour lost in the machine tool setup operation has a negative effect on the returns the invested capital must yield, the importance of reducing the machine tool setup time to a minimum is clearly understood. 
     For this reason, the current tendency of machine tool manufacturers is to use movable spindle headstocks having very high feed speeds. In particular, the time required for positioning the spindle headstock has been greatly reduced by increasing the feed speed of the spindle headstock to a top limit of 80 m/min by using improved drives. 
     On the one hand, the increase in the feed speed of the spindle headstock permits a great amount of machine tool setup time to be saved, but on the other hand it exposes the spindle headstock to the risk of colliding with the workpiece as a result of a wrong control, an excessive delay of the response to a control or an inaccurate positioning. 
     It must be taken in account that because of the inertia and speed involved, in the event of a collision, forces of great intensity would be developed and the resulting decrease of kinetic energy would be converted into internal energy which would be absorbed and dissipated in the workpiece and the spindle headstock, thus causing great damage to both and especially to the latter. 
     Therefore, any damage to the spindle headstock resulting from a collision with the workpiece must be absolutely avoided because the spindle headstock is easily damaged and accurately designed, and thus an expensive part of the machine tool. 
     SUMMARY OF THE INVENTION 
     The present invention is intended to reduce to a minimum the damage to a spindle headstock caused by an accidental collision with a workpiece during the positioning in an operative work configuration by providing a shock-absorbing device for the spindle headstock. 
     More particularly, the shock-absorbing device is intended to be mounted on the spindle headstock of a machine tool. The spindle headstock includes a motor driven spindle unit connected to an upright by a support. The spindle headstock comprises a yieldable structure connected on one side to the spindle unit and on the other side to the support, and is adapted to yield upon an accidental collision of the spindle headstock with a workpiece. A collision sensor is associated with the yieldable structure for detecting the yielding thereof resulting from an accidental collision of the spindle headstock with the workpiece, and for transmitting in response thereto a signal to a machine tool control unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will be clearly understood from the detailed description given for explanatory purposes only and without limitations to its extent with reference to the accompanying drawings, wherein: 
     FIG. 1 is a side elevational and partial sectional view of a first preferred embodiment of the spindle headstock of the invention, and 
     FIG. 2 is a side elevational and partial sectional view of a second preferred embodiment of the spindle headstock of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As can be seen from FIGS. 1 and 2 of the drawings, according to both embodiments of the invention, the spindle headstock, generally designated by  10 , comprises an electric motor driven spindle unit  11  and a support  12  for firmly supporting the spindle unit  11  and securing it to an upright (not shown) of a machine tool. 
     The support  12  is provided with a substantially cylindrical shaped housing  13  adapted to receive the spindle unit  11 . 
     According to the invention, for connecting the spindle unit  11  to the support  12  a special shock-absorbing device is provided which is capable of permitting the spindle unit  11  to retract inside its housing  13  in the case of a collision against a workpiece, and at the same time to provide an input signal to be transmitted to a machine tool control unit informing that a collision has occurred so that the machine tool can be stopped by decreasing the rotational speed of the spindle. The shock-absorbing device is arranged between the spindle unit  11  and the support  12  and can have different embodiments. 
     In a first preferred embodiment of the invention, shown in FIG. 1 of the accompanying drawings, the shock-absorbing device is designated by  14  and is represented in a longitudinal section. The shock-absorbing device  14  is formed of two coaxial and spaced apart tubular elements  15  and  16 . The element  15  is secured to the spindle unit  11 , and the element  16  is secured to the support  12 . 
     The two tubular elements  15  and  16  are connected to each other at their ends by two connecting annuli  17  and  18 , respectively, so as to form a closed hollow space  31  therebetween. 
     The shock-absorbing device  14  is configured so as to be capable of absorbing the energy involved in an accidental collision between the spindle unit  11  and a workpiece, by deformation or by breakage if during the collision the workpiece is subjected to a load greater than normal. 
     For this purpose, the two connecting annuli  17  and  18  are connected to the tubular elements  15  and  16  by screws  19 , which have a rated breaking load. As known, the screws of this kind are adapted to break when they are subjected to a load greater than their rated breaking load. For this purpose, a V-shaped notch  19   b  is provided in the screw shank  19   a  for reducing the resistance of the screw so as to cause it to break when it is subjected to a tensile or shear stress having an intensity greater than the corresponding limit values it generally undergoes in the case of a collision between the spindle unit  11  and a workpiece. 
     In the event of such a collision, the breaking of the screws  19  permits the spindle unit  11  to retract inside the housing  13  and, therefore, to drive the tool spindle away from the workpiece. 
     In order to detect the breakage of the screws  19 , having the rated breaking load, in at least one of the screws a longitudinal bore is provided through which a breakage detector passes. Such a detector can be formed of a fluid under pressure. When breakage of the screws  19  occurs, the resulting pressure change of the fluid is sensed, and converted by a transducer into an output signal to be transmitted to a closed-loop machine tool control unit of the spindle headstock  10 . In the retraction of the spindle unit  11  inside the housing  13 , the machine tool control unit causes the rotational speed of the tool spindle to be gradually reduced until operation of the machine tool is stopped. 
     Referring now to FIG. 2 of the drawings, there is shown a second embodiment of the shock-absorbing device according to the present invention. According to this second embodiment, the shock-absorbing device, in this case designated by  20 , is formed of two coaxial and spaced-apart tubular elements  21  and  22 , wherein the element  21  is secured to it the spindle unit  11  and the element  22  is secured to the support  12 . 
     The tubular element  21  is firmly maintained in position around the spindle unit  11  by an attachment  23 . This attachment  23  is provided at one end of the tubular member  21 , integrally therewith, and is secured to the spindle unit  11  by screws  24 . 
     The tubular element  22  is secured to the support  12  by screws  25 . A sleeve  26  extends beyond the tubular member  22  and into engagement against the attachment  23  of the tubular element  21 . The sleeve  26  is coaxially inserted inside the tubular member  22  and is axially movable therein. An annulus  27  connected to the tubular element  22  by screws  28  acts as a stop for the sleeve  26 . 
     An annulus  29  is connected by screws  30  to a free end of the tubular element  21  in order to maintain the spindle unit  11  firmly in position inside the housing  13  of the support  12  during normal work operation of the machine tool. The annulus  27  secured to the tubular element  22  contacts the annulus  29 . 
     The coaxial and spaced-apart tubular elements  21  and  22  defme a substantially cylindrical closed chamber  31  which is filled with fluid under pressure. 
     The two tubular elements  15  and  16  are connected to each other at their ends by means of two connecting annuli  17  and  18 , respectively, so as to form a closed hollow space ( 31 ) therebetween. 
     During normal work operation of the machine tool, the shock-absorbing device  20  maintains the spindle unit  11  fly in position because of the mutual engagement of the tubular elements  21  and  22 . As already mentioned, the chamber  31  is closed and is filled with a fluid under pressure. 
     When the spindle  11  unit is struck suddenly and violently as a result of a collision against a workpiece, the spindle unit  11  retracts inside the housing  13  of the support  12  and at the same time urges the sleeve  26  backwards. As a result of the retraction of the spindle unit  11 , the annulus  29  disengages from the annulus  27 , thus causing the chamber  31  to open and the fluid contained under pressure therein to flow out. 
     The resulting fluid pressure change is detected by a pressure transducer and is converted into an output signal to be transmitted to a closed-loop machine tool control unit of the spindle headstock  10 . In the retraction of the spindle unit  11  inside the housing  13 , the machine tool control unit causes the rotational speed of the tool spindle to be gradually reduced until the operation of the machine tool is stopped. 
     Once the collision between the spindle unit  11  and the workpiece has occurred, the shock-absorbing device  20  can be easily and readily brought back to the initial condition by filling the chamber  31  with pressurized fluid, thereby making its servicing easy and economical. 
     Of course, other yieldable structure capable of absorbing and internally dissipating the impact energy resulting from an accidental collision of the spindle headstock against a workpiece, in order to protect the spindle headstock, can be used instead of the shock-absorbing device shown in the Figures. 
     Also the sensors used for detecting a collision between the spindle headstock and a workpiece can be different from those described above and can operate according to different principles. For instance, photoelectric sensors, position sensors, etc. can be used. 
     From the foregoing, it can be understood that the invention accomplishes the proposed object and, in particular, it provides a spindle headstock  10  having a spindle unit  11  connected to a support  12  by a shock-absorbing device  14 , 20  that is capable of imparting to the spindle unit  11  the required stability and resistance, together with positioning accuracy, during the normal work operation of the machine tool, and at the same time is capable of absorbing the impact energy involved in an accidental collision between the spindle headstock  10  and a workpiece in order to protect the integrity of the spindle unit  11 . 
     Such a shock-absorbing device, in its preferred embodiment described above, has a relatively simple configuration and its cost is extremely low compared to that of the spindle headstock as a whole. 
     Of course, to the shock-absorbing unit according to the invention various modifications and changes can be made without departing from the inventive idea; furthermore, all the accessory parts can be substituted with other parts that are technically equivalent.