Abstract:
A working unit intended to be mounted on a movable machine operated tool support includes a casing which is attached to the tool support and which supports a pneumatic turbine motor with a rotor a shank end mill type machining tool with a cutter portion extending out of the casing and a shank portion rotatively journalled in the casing. The machining tool shank portion is rigidly integrated with a turbine wheel to form the turbine rotor. A flow control valve is arranged to deliver pressure air to the turbine motor. The turbine rotor is provided with a generator device for delivering a speed responsive output voltage to a control unit which is arranged to make the flow control valve adjust the power supply to the rotation motox so as to maintain the rotation speed at a desired predetermined level.

Description:
This application is a U.S. National Phase Application under 35 USC. 371 of International Application PCT/SE01/02837 filed Dec. 20, 2001. 

   FIELD OF THE INVENTION 
   The invention relates to a working unit intended to be mounted on a movable machine operated tool carrier and comprises a casing attached to the tool carrier, a rotation motor supported in the casing, and a machining tool rotatively journalled relative to the casing and driven by the motor. 
   BACKGROUND OF THE INVENTION 
   Previously known working units of the type having a motor and a machining tool carrying output spindle suffer from a rather slow working action. This is due both to the fact that the working unit has a relatively large mass and to the fact that the speed of the motor is relatively low. By nature, a heavy working unit is difficult to move at high velocity either in a plain reciprocating movement or in a more or less complicated working pattern, which means that the working process time will be undesirably long. Also, a relatively slow motor limits the maximum feed speed of the working tool relative to the work piece. 
   A heavy output spindle unit including a tool chuck also means a large rotating mass, which tends to create vibration problems at high speed rotation. It is difficult to avoid resonance frequencies in such a known device. 
   One reason why previously known movable working units has a relatively large mass is a rather complex design of the motor and output spindle arrangement. 
   Another reason why previous working units has a large mass is the use of electric motors with all their heavy iron cores and copper windings. Electric motors also have a limited rotation speed which limits the rotation speed of the working tool, and, thereby, the possible feed speed of the working tool. 
   SUMMARY OF THE INVENTION 
   The main object of the invention is to solve the above problems by accomplishing an improved working unit for mounting on a movable machine operated tool carrier which due to a simple design, a low weight and a high machining tool rotation speed provides a fast working action and, thereby, shortened working process times. 
   Further objects and advantages of the invention will appear from the following specification and claims. 
   A preferred embodiment of the invention is below described in detail with reference to the accompanying drawing figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a longitudinal section through a working unit according to the invention and illustrates schematically a speed control system connected to the working unit. 
       FIG. 2  shows another longitudinal section through the working unit in FIG.  1 . 
     The working unit illustrated in the drawings comprises a casing  10  with a rear mounting neck  11  for attaching the working unit to a movable machine operated tool carrier (not shown). The casing  10  is divided into a central section  12  and a front section  13 . The front section  13  is clamped to the central section  12  by a nut  14 . The rear mounting neck  11  is secured to the central section  12  by screws  15 . 
   

   DETAILED DESCRIPTION 
   In the casing  20  there is provided a rotation motor in the form of a pressure air operated turbine with a rotor  19 , and a machining or milling tool  20  in the form of an end cutter. At its forward end, the milling tool  20  is formed with a cutter portion  21  extending out in front of the casing  10 , and a shank  22  extending into the casing  10 . A turbine wheel  24  carrying a circumferential row of drive blades  23  is rigidly secured to the inner end of the shank  22  which means that milling tool  20  and the turbine wheel  24  together form the turbine rotor  19  and the shank  22  forms the rotation axle of the rotor  19 . The turbine wheel  24  is secured to the milling tool shank  22  by a press and/or fit. 
   By forming the motor rotor as a separate turbine wheel  24  rigidly mounted on the tool shank  22  as described above a standard type shank end mill can be used as machining tool  20 . This means that a working unit according to the invention is simple and inexpensive to manufacture. 
   The turbine rotor  19 , i.e. the tool shank  22 , is radially supported relative to the casing  10  by two hydro-dynamic bearings  25 , 26 . These bearings  25 , 26  comprise two bushings  27 , 28  which are rigidly secured in a support sleeve  29  which in turn is rigidly secured in the front section  13  of the casing  10 . As illustrated in  FIG. 2 , both bushings  27 , 28  are supplied with a liquid via two radial passages  31 , 32 , an axially extending passage  33  and a supply opening  34  in the casing  10 . Preferably, the liquid being used as bearing medium is the liquid used for lubricating and cooling the cutter end portion  21  of the milling tool  20  during operation. 
   As illustrated in  FIG. 1 , the bushings  27 , 28  are provided with outer circumferential grooves  35 , 36  which via small diameter radial openings communicate fluid from the radial passages  31 , 32  to the inner circumference of the bushings  27 , 28 . Between the two bushings  27 , 28 , the support sleeve  29  is provided with a radial outlet  38  for draining fluid into an outlet passage  39 . See FIG.  2 . 
   The turbine motor comprises a nozzle sleeve  40  which is located in the central casing section  12  and defines an air communication chamber  41 . The nozzle sleeve  40  is axially locked in one direction by a couple of axial extensions  42 , 43  on the mounting neck  11  extending into the casing section  12  and in the opposite direction by a shoulder  44  on the nozzle sleeve  40  in co-operation with a shoulder  45  in the casing  10 . The fluid communication chamber  41  is supplied with pressure air from a pressure air source  46  via a conduit  37  and an inlet opening  47  and a flow control valve  48 . See FIG.  1 . The nozzle sleeve  40  is formed with radial air feed openings  50  and a number of nozzles  51  for directing a flow of pressure air onto the drive blades  23  of the turbine wheel  24 . 
   At its rear end, the turbine rotor  19  is provided with a permanent magnet  52  which together with a stationary winding coil  53  rigidly secured in the nozzle sleeve  40  forms a tacho-generator. This generator delivers a speed responsive output voltage and forms a part of a speed governing device which also includes the air inlet flow control valve  48  and a control unit  55 . The control unit  55  is connected to the winding coil  53  by means of a cable  60 . The routing of the cable  60  inside the casing  10  is schematically illustrated only. 
   The flow control valve  48  is activated by a reversible electric servo motor  54  which is connected to the control unit  55 . At a desired predetermined turbine speed, the generator delivers a voltage of a certain magnitude. Depending on the actual voltage magnitude of the generator being above or below this certain voltage magnitude, the control unit  55  will deliver either a positive or a negative voltage to the servo motor  54 , thereby making the latter activate the flow control valve  48  in an opening or a closing direction, respectively. 
   Moreover, the turbine rotor  19  is axially supported by two aerodynamic bearings  56 ,  57  located on opposite sides of the turbine wheel  24 . Pressure air is supplied to the forward bearing  56  via a passage  58  communicating with the air communication chamber  41  and is drained from the bearing via a radial opening  61 . The passage  58  communicates with an annular groove  59  at the rear end surface of the support sleeve  29  to supply pressure air and create an air layer between the turbine wheel  24  and the sleeve  29 . 
   The rear axial bearing  57  is formed between the turbine wheel  24  and the winding coil  53 , and pressure air is supplied to this bearing  57  from the air communication chamber  41  via a flow restricting opening  62  in a transverse wall  63  of the nozzle sleeve  40 . 
   In operation, it is assumed that the working unit is attached to a movable machine operated tool carrier of any suitable type, and that pressure air is supplied from the pressure air source  46  via the flow control valve  48 , the conduit  37 , the inlet opening  47  and the air communication chamber  41  to start the turbine. Before the turbine wheel  24  has reached the desired predetermined rotation speed, the flow control valve  48  is maintained wide open to deliver a powerful pressure air flow to the working unit. The supplied air passes the air communication chamber  41  and reaches the turbine nozzles  51  via the radial openings  50  to activate the turbine wheel  24 . Downstream the turbine wheel drive blades  23 , the air flow enters the outlet passage  39  in the front section  13  of the casing  10  and is exhausted from the working unit. 
   At the same time, pressure air enters the passage  58  and reaches the annular groove  59  of the forward bearing  56  to form an aerodynamic axial support in one direction for the turbine rotor  19 . At the rear end of the rotor  19 , pressure air reaches the rear end surface of the turbine wheel  24  via the opening  62  in the wall  63 , thereby forming an aerodynamic support in the opposite direction for the rotor  19 . 
   In parallel with pressure air being supplied for starting up the turbine rotor  19 , lubricating and cooling liquid is supplied to the milling tool cutter end  21  in a common non-illustrated way. Lubricating and cooling liquid is also supplied to the radial bearings  25 , 26  via passages  33 , 31 , 32  and openings  35 , 36  where the liquid creates a hydrodynamic radial support for the milling tool shank  22 , i.e. the turbine rotor  19 . The liquid is drained successively to the outlet passage  39  via the radial opening  38 . 
   Now, the rotor  19  is properly supported in the radial as well as in the axial directions and the supplied pressure air makes the turbine rotate at an increasing speed. As long as the speed has, not reached the desired level, the output voltage from the generator winding coil  53  is below the desired certain magnitude, and the flow control valve  48  is kept open to a relatively large extent to make the turbine wheel  24  accelerate. However, as the desired speed level is reached and exceeded to some extent the generator voltage becomes too high, and the control unit  55  will start to deliver a negative output voltage to the flow valve servo-motor  54  such that the latter will start moving the flow control valve  48  towards a smaller opening area to reduce the pressure air flow to the turbine nozzles  51  and, thereby, reducing the turbine speed. Should the turbine speed decrease too much under the desired predetermined level, the control unit  55  would again change the output voltage to be positive and make the servomotor  54  rotate in the opposite direction, thereby shifting the flow control valve  48  to a larger opening area to increase the turbine speed. When the turbine wheel  24  is running at the desired predetermined rotation speed, the control unit  55  will not deliver any voltage at all. 
   By the working unit according to the invention there is obtained a structurally simple and light device which is capable of operating at a very high speed thanks to a rotor spindle which is not only small and light but which is fully integrated with the turbine motor to an integrated unit. The device according to the invention is also capable of high speed rotation and long service life because of the frictionless bearings supporting the rotor both in the radial direction and the axial direction. By forming the turbine motor as a separate turbine wheel rigidly mounted directly on the machining tool shank portion by a simple press fit or a shrinkage fit a standard type end mill may be used without any special adaptation for this particular purpose.