Abstract:
An internal grinding apparatus having a grinding wheel attached to a spindle rotor radially and axially supported by electromagnetic bearings. Currents applied to the electromagnetic bearings are continuously controlled by a controlling system including a central processing unit during grinding work in order to adjust the longitudinal axis of the spindle rotor. The longitudinal axis of the spindle rotor is inclined and/or shifted at predetermined locations of the grinding wheel in a bore of a workpiece, thereby eliminating tapering of the long bore, and improving the cylindricality and straightness in grinding work.

Description:
BACKGROUND OF THE INVENTION 
     1. (Fields of the invention) 
     The present invention pertains to an internal grinding apparatus for grinding the surface of a long bore in a workpiece, and it particularly relates to an internal grinding apparatus which provides high cylindricality and straightness. 
     2. (Description of the prior arts) 
     Conventionally, when a long bore a of a workpiece 1 such as a plunger valve, is ground as shown in FIG. 5 and FIG. 6, the workpiece is rotated on the work head of the grinding machine, and a grinding wheel 4 provided at one end of a spindle rotor 2 through a grinding shaft (quill) 3 is also rotated at high speed. The rotating grinding wheel 4 is shifted (traversed) along the axis of rotation of the grinding wheel to thereby grind the surface of the long bore a. 
     However, since the quill bents during grinding work by such conventional internal grinding machine, the diameter of the long bore a of the workpiece 1 reduces as it reaches the outer end of the long bore as shown in FIG. 6, in other words, the long bore has a taper at the outer end thereof. As a result, the cylindricality and straightness are lowered. 
     In order to solve this problem, the applicant previously proposed a method in his own Japanese patent application No. 61-9769 which corresponds to the U.S. patent application Ser. No. 005,095 in which grinding work is carried out with the quill inclined at a predetermined angle after a point at which the quill starts bending. 
     However, in order to incline the quill, the mechanical clamps fastening the wheel spindle stock to the work table have to be loosened for adjusting the wheel spindle stock at a required angle using the eccentric cam, and thereafter the wheel spindle stock is fastened with the mechanical cramps again Because of this feature, this method has drawbacks: for example, it is not possible to continuously adjust the tilt angle; the mechanism is complicated; and it is difficult to make fine adjustment of the tilt angle due to backlash. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide high cylindricality and straightness in grinding work by an internal grinding machine It is another object of the present invention to provide an internal grinding machine in which a spindle rotor is supported by magnetic bearings, a grinding wheel attached at one end of the spindle rotor, and the axis of rotation of the grinding wheel is inclined by controlling currents which excite the magnetic bearings at predetermined longitudinal positions along the surface of a long bore drilled in a workpiece without interrupting grinding operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing an embodiment of an internal grinding apparatus according to the present invention; 
     FIG. 2 is a sectional schematic view of a spindle head of an embodiment of a grinding apparatus according to the present invention; 
     FIG. 3 is a block diagram showing an electrical structure of the control system in an embodiment of a grinding apparatus according to the invention; 
     FIG. 4 is a flow chart showing the operation of a grinding apparatus according to the present invention; and 
     FIG. 5 and FIG. 6 are figures presented for explaining the work of grinding a long bore in a workpiece by an internal grinding apparatus of the prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a perspective view of an internal grinder according to the present invention. The numeral 1 is a workpiece, the numeral 2 is a spindle rotor and the numeral 3 is a quill provided at one end of the spindle rotor 2, and a grinding wheel 4 is provided at the tip of the quill 3. 
     The workpiece 1 is held at a chuck 7 which is rotationally driven by a motor 6 fixed on a work head 5. The work head 5 is mounted on a work table 8. The work table 8 can be moved in the direction perpendicular to the axis of rotation of the workpiece 1. 
     The spindle rotor 2 is supported by magnetic bearings as shown in FIG. 2. 
     The spindle rotor 2 has a disk 9 fixed around the central portion thereof. The disk 9 is held in place by thrust magnetic bearings 11 which are provided on the spindle head 10. Axial direction force acting on the spindle rotor 2 is checked by the thrust magnetic bearings 11. The spindle rotor 2 is also supported by radial magnetic bearings 12 and 13 at both ends thereof. 
     These thrust and radial magnetic bearing mechanisms used for the present invention are well known in the art. The spindle rotor 2 is magnetically supported and floated between a plurality of pairs of electromagnets 12a, 12b, 13a and 13b. In this case, a gap of around 0.5 mm is maintained between the electromagnet and the periphery of the spindle rotor 2. 
     Therefore, the axis of the spindle rotor 2 can be inclined and/or shifted by controlling currents to excite the electromagnets 12a, 12b, 13a and 13b of the radial magnetic bearings 12 and 13. For example, when an excitation current applied to the electromagnets 12a and 13b is made larger than that applied to the electromagnets 12a and 13a, the spindle rotor 2 is held in a position with its axis shifted toward the electromagnets 12b and 13b by an amount corresponding to the difference in excitation current between these pairs; and when an excitation current applied to the electromagnet 12b is made larger than that applied to 12a, and at the same time an excitation current applied to the electromagnet 13b is made smaller than that applied to the electromagnet 13a, the axis of the spindle rotor 2 can be inclined relative to the central axis of the magnetic bearings. 
     By the tilt of the spindle rotor 2, the grinding wheel 4 can be inclined in the direction T at a tilt angle 8. 
     A motor stator 50 effects to rotate the spindle rotor 2 when a current flows through a coil 14. Therefore the spindle rotor 2 functions as a rotor of a motor and the motor stator 50 as a stator of a motor. 
     Position sensors 15 and 16 are provided at the right and left end portions of the spindle rotor 2. They are respectively composed of a pair of sensors 15a and 15b and a pair of sensors 16a and 16b. The position sensors detect radial positions of the spindle rotor 2 at the two supporting points, i.e., at the points between the electromagnets 12a and 12b, and between the electromagnets 13a and 13b. A position sensor 17 composed of a pair of sensors 17a and 17b detects the position of the disk 9. 
     The spindle 10 is fixed to a work table 19 through a spindle stock 18. The work table 19 is mounted on a guide member 21 fixed on a bed 20 in such a manner that it can be moved along the axis of the spindle rotor 2. 
     The work table 19 is moved by a thread screw 23 which is rotated by a servo motor 22, and the shift amount is detected by an encoder 24 directly connected to the servo motor 22. 
     FIG. 3 is a block diagram showing an electrical composition of an embodiment according to the present invention. The embodiment has a CPU 27 which carries out a required computation using a ROM 25 storing a system program and a RAM 25 storing a set of working memory. The CPU 27 totally controls all of the system of the equipment. 
     The servo driver 29 receives a control signal which is outputted from the CPU 27 and D/A converted by a D/A converter 28, and thereby rotationally drives the servo motor 22. The revolution speed of the servo motor 22 is detected by an encoder 24, and the detected signal is processed by a pulse shaping and direction determination circuit 30 and then sent through a position counter 31 to the CPU 27. 
     The CPU 27 provides an output signal through a D/A converter 32 to a magnetic bearing controller 33, and controls currents to excite electromagnets 12a 12b, 13a  and 13b of the magnetic bearings 12 and 13. The radial position of the spindle rotor, which is controlled by the applied currents for excitation, is detected by the position sensors 15 and 16, and the detected signal is sent through a position detection circuit 34 in the magnetic bearing controller 33 and an A/D converter 35 to the CPU 27. 
     The controlling operation of the CPU is hereunder described with reference to the flow chart in FIG. 4. 
     Once grinding work is started with both the workpiece 1 and the spindle rotor 2 being rotated in predetermined directions (STEP 100), reciprocal grinding, or reciprocation, is started in which the grinding wheel 4 is alternately advanced and retrieved in and along the long bore a of the workpiece 1. The reciprocation is carried out by controlling the servo motor 22 with the encoder 24 detecting the position of the work table 19 (STEP 102). 
     The RAM 26 stores data of positions of the grinding wheel 4 in the long bore a, and data of tilt angles θ of the grinding wheel 4. Based on a shift amount of the work table 19, a tilt angle θ of the grinding wheel is computed using the data stored in the RAM 26, and thereby the current to excite the magnetic bearings 15 and 16 is controlled to incline the grinding wheel at the computed tilt angle θ (STEP 104, 106). 
     The surface of the long bore a is ground to a predetermined dimension with the tilt angle of the grinding wheel being adjusted (STEP 108, 110). 
     In the above embodiment, the shift amount of the table 19 is detected by the encoder 24. However, table shift detection equipment composed of a proximity switch device can also be used instead. The proximity switch device is comprised of contactless switches which do not require mechanical contacts for switching operation. 
     Further, although in the above embodiment, control of the tilt angle of the grinding wheel is continuously carried out in response to the shift of the work table 19, the tilt angle θ of the grinding wheel can also be adjusted at intervals of predetermined shifts of the work table. However it should be noted that by continuously adjusting the tilt angle of the grinding wheel in the manner described here with reference to the embodiment, a long bore can be ground with high cylindricality and straightness. 
     It is further understood by those skilled in the art that the foregoing description is a preferred embodiment of the disclosed apparatus and that various changes and modifications may be made in the invention without departing from the spirit and scope thereof.