METHOD AND APPARATUS FOR USE IN GRINDING

Abstract: Apparatus for use in controlling a grinder comprising a motor (10); a grinding shaft (12) which is driven by the motor (10) and which is adapted to support a grinding wheel (11) at one end thereof; detector means (16-19) for detecting a factor which varies under machining conditions; and control means (32-38), responsive to output signals from the detector means (16-19), for adjusting the machining conditions, characterised in that the detector means (16-19) are arranged to detect the deflection of a portion (12 a ) of the grinding shaft (12), or of means (15) secured thereto, under the machining conditions, the control means (32-38) being arranged to effect a feed adjustment in accordance with the said output signals.

Description: The present invention relates to a method of grinding a workpiece. Such grinding has involved the use of one prior grinding apparatus which is merely provided with a set of fixed points for adjusting the cutting conditions because all the working conditions, such as rough grinding, fine grinding, finishing grinding, etc., are determined in accordance with the measurements of the workpiece. However, in this case, it is not possible to grind a workpiece under optimum grinding conditions by taking account of the variation in the cutting efficiency of the grinder. Because of this, it is not possible to improve the grinding accuracy beyond a certain level in terms of circularity, cylindricality, surface roughness, etc., and it is not possible to improve the grinding work efficiency. If another prior grinder is employed, the extent of cutting is controlled in response to the electric power required by the grinding shaft during grinding. In this case, a component of the grinding resistance acting in the tangential direction relative to the grinding shaft (i.e. in a direction perpendicular to the cutting direction) is indirectly detected, and thereby the cutting speed is controlled and the cutting conditions are changed if necessary. However, in this case, a tangential force component of the grinding resistance is detected, notwithstanding the fact that detection of a normal force component of the grinding resistance is said to be the best way to determine the cutting efficiency of a grinder, and therefore it is not possible to detect changes of the cutting efficiency of the grinder adequately and this it is difficult to effect the optimum control of the grinding conditions. US-A-3 344 560 describes a grinder comprising a motor, a grinding shaft which is driven by the motor and which is adapted to support a grinding wheel at one end thereof, detector means for detecting a factor which varies under machining conditions, and control means, responsive to output signals from the detector means, for adjusting the machining conditions, whereby the detector means are arranged to detect the deflection of a portion of the grinding shaft under the machining conditions, the control means being arranged to effect a feed adjustment in accordance with the said output signals. US-A-2 930 167 describes a method for controlling a grinder having a spindle with deflection sensor means comprising the steps of (a) rough grinding a workpiece by feeding a grinding wheel in such a condition that a deflection at an end portion of a grinding shaft caused by a grinding force is at a first predetermined constant value, until the workpiece has a first predetermined size, (b) rough grinding spark-out the workpiece without feeding the grinding wheel, until said deflection is reduced to a second predetermined value. According, therefore, to the present invention, there is provided a method of grinding a workpiece in which use is made of a grinder comprising a motor; a grinding shaft which is driven by the motor and which is adapted to support a grinding wheel at one end thereof; detector means for detecting a factor which varies under machining conditions; and control means, responsive to output signals from the detector means, for adjusting the machining conditions, the detector means being arranged to detect the deflection of a portion of the grinding shaft, or of means secured thereto, under the machining conditions, the control means being arranged to effect a feed adjustment in accordance with the said output signals, the said method comprising (a) rough grinding a workpiece by feeding a grinding wheel on the grinding shaft towards the workpiece until the workpiece has a first predetermined size, the rough grinding during step (a) being carried out so that the said deflection is maintained at a first predetermined value; and (b) effecting rough grinding spark-out of the workpiece without feeding the grinding wheel until the said deflection is reduced to a second predetermined value; characterised by (c) fine grinding the workpiece by feeding the grinding wheel towards the workpiece until the workpiece has a second predetermined size, the fine grinding during step (c) being carried out so that the said deflection is maintained at a third predetermined value which is less than the first predetermined value; (d) effecting fine grinding spark-out of the workpiece without feeding the grinding wheel; (e) measuring the elapsed time taken for the transition of the said deflection from the third predetermined value to a fourth predetermined value; and (f) calculating the time required for the fine grinding spark-out of step (d) from the said elapsed time, the third predetermined value and the fourth predetermined value. The step (e) may further comprise the steps of:- (e1) comparing the elapsed time with a predetermined time value; and (e2) dressing the guiding wheel after fine grinding spark-out of step (d) whenever the elapsed time is greater than the predetermined time value. Preferably, the means secured to the portion of the grinding shaft comprises a cylindrical member which is mounted on said portion and which is spaced by a gap from the detector means. The control means may comprise sizing means for measuring the size of a workpiece being machined; and a controller which is arranged to receive signals both from the sizing means and from the detector means, the controller being arranged to produce a feed signal in response to the signals from the sizing means and the detector means. The said controller may be a main controller whose feed signal is outputted to a feed controller, the feed controller being arranged to output a driving signal to a servo motor which is adapted to effect feeding of the grinding wheel. The grinder may have temperature-compensation means for ensuring that the feed adjustment effected by the control means is substantially unaffected by temperature. The temperature-compensating means preferably comprises temperature sensing means disposed adjacent to the detector means; means connected to the temperature sensing means for generating a temperature-compensation signal, the signal adjusting means being arranged to produce a feed signal which is substantially unaffected by the temperature adjacent to the detector means. The use of the method of the present invention enables the accuracy and efficiency of a grinding operation to be improved by controlling the grinding work conditions based on the amount of deflection of the grinding shaft occurring in a direction normal to the grinding shaft. The detector means may be arranged to detect a deflection of the grinding shaft caused by a component of the grinding load acting against a workpiece in the normal direction (in the cutting direction) relative to the grinder. Because of this feature, the cutting efficiency of the grinder may be precisely determined, grinding work may be carried out under optimum grinding conditions taking account of any variation in the cutting efficiency, and the grinding accuracy and efficiency may be improved. Moreover, if the output signal from the detector means is temperature corrected as indicated above, an initial machining condition can be properly adjusted. The invention is illustrated, merely by way of example, in the accompanying drawings, in which:- Figure 1 is a diagram illustrating the amount of feed of a grinder and the amount of deflection of its grinding shaft in relation to the machining time in an embodiment of the present invention; Figure 2 is a control flow chart of the said embodiment of the present invention; Figure 3 is a block diagram of a control apparatus used in the said embodiment of the present invention; Figure 4 is a block diagram of a detecting and temperature-compensating circuit for deflection sensors of the said embodiment of the present invention; Figures 5 and 6 are diagrams showing the relationship between the output voltage from the said sensors and temperature; Figure 7 is a sectional view of a grinder used in the said embodiment of the present invention; and Figure 8 is a sectional view taken on the line II-II shown in Figure 7. Figures 7 and 8 show an embodiment of a spindle apparatus 5 for use in controlling a grinder employed in the method according to the present invention. As shown in Figure 7, a high-frequency motor 10 is mounted adjacent the central portion of the length of the spindle apparatus 5. The high-frequency motor 10 has a rotor 8 having a through-hole 8 a provided at the radial centre thereof. A grinding shaft or spindle 12 is provided with a grinding wheel 11 at an end portion thereof for grinding a workpiece W shown on the left hand side of Figure 7. The grinding shaft 12 is firmly fixed within the through-hole 8 a of the rotor 8, and they rotate together. The high-frequency motor 10 is provided within a casing 14 forming the outer surface of the spindle apparatus 5. Opposite ends of the grinding shaft 12 are supported at respective ends of the casing 14 through bearings 13 a and 13 b . A sensor holder 21 is provided in the casing 14 on the side of the workpiece W, and sensors 16-19 (deflection sensor means) are carried by the sensor holder 21 radially of the grinding shaft 12 as shown in Figure 8. Each of the sensors 16-19 detects a gap between itself and the outer peripheral surface of a cylindrical target 15 which is fixed around the outer peripheral surface of a portion 12 a of the grinding shaft 12. Each of the sensors 16-19 thus detects the deflection at the portion 12 a of the grinding shaft 12 which occurs during machining. This deflection is detected in the radial direction, particularly the normal direction (the cutting direction of the grinding wheel 11, i.e. the direction X as shown in Figure 8), and in the tangential direction of the grinding shaft 12. A shielding member 20 formed with a heat insulating material such as Bakelite (Registered Trade Mark) etc. is provided on and closer to the workpiece side of the sensor holder 21. The shielding member 20 prevents grinding fluid from splashing from the workpiece side onto the sensors 16-19 and onto the sensor holder 21 so that the sensor function is protected therefrom. Each of the sensors 16-19 is comprised of a copper wire wound around an iron core, and detects a change of the inductance caused by a change of the gap between the tip of the iron core and the outer peripheral surface of the target 15, and thereby detects a deflection (displacement) at the portion 12 a of the grinding shaft 12 in its radial direction. In the spindle apparatus 5 as described hereabove, the grinding shaft 12 is rotated at high speed by the high-frequency motor 10, and the grinding wheel 11 grinds the internal surface of the workpiece. At the same time, the sensors 16-19 detect deflection at the portion 12 a of the grinding shaft 12 which is caused by the grinding force. Based on the detected amount of deflection of the spindle 12, the cutting speed and the amount of cutting are controlled, and an optimum moment for the correction of the tool (dressing) is determined, whereby grinding under optimum conditions is effected. The control is effected by the control apparatus shown in the block diagram of Figure 3, the said control apparatus being responsive to output signals from the sensors 16-19. A deflection at the portion 12 a of the grinding shaft 12 is detected by the detecting sensors 16-19. Signals detected by the sensors 16-19 and by a sizing device 32 for measuring the size of the workpiece W are inputted via a sizing amplifier 33 into a main controller 34 which is also arranged to receive signals from the sensors 16-19 and to produce a feed signal in response thereto. The feed signal from the main controller 34 is outputted to an X-axis feed controller 36, the feed controller 36 being arranged to output a driving signal to a servo driver 37 and thence to a servo motor 35 which effects feeding of the grinding wheel 11, by way of a rotary encoder 38. Thus the servo motor 35 is controlled by a control block comprising the main controller 34, the X-axis feed controller 36, the servo-driver 37 and the rotary encoder 38, whereby the feed of the grinder is controlled (by means not shown). A grinding process using the apparatus of Figures 7 and 8 is described with reference to Figure 1 and Figure 2. The grinding shaft 12 is rotated by the high-frequency motor 10 together with the grinding wheel 11. The grinding wheel 11 is fed (by means not shown) from a position at which it is not in contact with the workpiece W, at a gap feed speed for rough grinding of V GR . When the grinding wheel 11 touches the workpiece W,and is fed further, the portion 12 a of the grinding shaft 12 starts deflecting. A deflection of the grinding shaft 12, i.e. a deflection δ at the portion 12 a of the grinding shaft 12 in the normal direction (cutting direction) relative to the grinding shaft 12, is detected by the sensors 16 and 18. A detection signal corresponding to the deflection amount δ is transmitted to the main controller 34 through a grinding shaft deflection detection amplifier 39. When the deflection amount δ becomes δ GR , the control block comprising the main controller 34, the x-axis feed controller 36, the servo driver 37 and the rotary encoder 38 controls and changes the gap feed speed for rough grinding to an initial rough grinding feed speed V RI . A rough grinding feed speed V R , after the feed speed is changed to the initial rough grinding feed speed V RI , is controlled so that the deflection at the portion 12 a of the grinding shaft 12 in the normal direction thereof becomes constant at δ GR . Therefore the rough grinding feed speed V R is not constant. When the workpiece W is ground to a predetermined size, the sizing device 32, which detects changes of the size of the workpiece W during grinding, outputs a first sizing signal to the main controller 34 through the sizing amplifier 33, whereby the rough grinding is completed and a rough grinding spark-out is started. The time for the rough grinding spark-out T R continues until the deflection δ at the portion 12 a of the grinding shaft 12 in the normal direction thereof becomes δ RSP , when the grinding wheel 11 is temporarily retracted and is separated from the workpiece. Then the grinding wheel 11 is fed at the gap feed speed for fine grinding V GF . When the grinding wheel 11 touches the workpiece W again, and the grinding wheel 11 is further fed, the portion 12 a of the grinding shaft 12 starts deflecting. The deflection δ in the normal direction is detected by the sensors 16 and 18, and the detected signal is inputted into the main controller 34. The gap feed speed for fine grinding V GF is changed to an initial feed speed for fine grinding V FI in response to a detected signal outputted when the deflection amount δ becomes δ F . After the feed speed is changed to the initial feed speed for fine grinding V FI , the feed speed for fine grinding V F is controlled so that the deflection δ at the portion 12 a of the grinding shaft 12 in the normal direction thereof is constant at δ F . Therefore the feed speed for fine grinding V F is not constant. When the workpiece W is ground to a second predetermined size, the sizing device 32 outputs a second sizing signal to the main controller 34 through the sizing amplifier 33, whereby the feed for fine grinding is stopped, and a spark-out of fine grinding is started. The time for spark-out for fine grinding T FSP is calculated by, for example, T F x δ F /(δ F - δ FSP ) , based on the time T F which is required for the transition of the deflection δ at the portion 12 a of the grinding shaft 12 in the normal direction thereof from δ F to δ FSP . When the time T FSP passes, the fine grinding step is completed, and the grinding wheel 11 is separated from the workpiece W. The control block measures the time T F which is required for the transition of the deflection δ detected by sensors 16 and 18 from δ F to δ FSP after the spark-out for fine grinding is started. The time T F is used to determine the sharpness of the grinding wheel 11. When it exceeds a predetermined value, it is determined that the sharpness of the grinder has deteriorated, and a command for the dressing is outputted. Based on the amount of deflection in the normal direction of the grinding shaft 12 and in the manner described hereabove, the feed speed and cutting amount are controlled, and the command for the correction of the tool is outputted whereby the sharpness of the grinding wheel 11 is always maintained at a satisfactory level, a workpiece can be ground under optimum conditions, and therefore the grinding accuracy and efficiency are improved. Figure 4 illustrates a signal detecting function and a signal temperature-compensation function for the sensors 16-19. In Figure 4, a sine wave oscillator 31 generates an alternating current which flows through the coils of the sensors 16-19, thereby exciting the iron cores thereof. The normal direction sensors 16 and 18 detect a change in inductance caused by a displacement (deflection) of the portion 12 a of the spindle 12 in the direction X (the normal direction or the cutting direction), and output a signal voltage to an alternating current amplifier 24. The output voltage from the sensors 16 and 18 is amplified by the alternating current amplifier 24, and full-wave rectification of the signal in the direction X is carried out by a synchronizing detector 25. A synchronizing signal generator 27, which produces a synchronizing signal corresponding to the phase of the alternating current generated by the sine wave oscillator 31, outputs a synchronizing signal to the synchronizing detector 25. Then, an output signal voltage from the synchronizing detector 25 is inputted into an adder 26. Further, in the circuit in Figure 4, a direct current power source 22 is provided in parallel with the sensors 16 and 18, and 17 and 19, and a reference resister 23 is provided in series with the direct current power source 22. The direct current power source 22 and the reference resister 23 are used for detecting pure resistance of the coils of the sensors 16-19. Both terminals of the reference resister 23 are connected to a pure resistance detector 28. The pure resistance detector 28 detects, based on a change in voltage between the two terminals of the reference resister 23, a change in pure resistance of the coils of the sensors 16-19 caused by a thermal change. Namely, since the voltage from the direct current power source 22 is constant, the pure resistance of the coils of the sensors 16, etc. increases with the rise of temperature, and the voltage between the two terminals of the reference resister 23 falls with the rise of temperature, as shown by a graph (b) in Figure 5. As a result, the pure resistance detector 28 outputs a signal voltage corresponding to the changed amount of the fall of voltage caused by the decrease of the pure resistance value. Then a signal component, outputted from the oscillator 31, in the signal voltage is cut by a low-pass filter 29, and the signal is inputted into the adders 26. The direct current power source 22, reference resister 23, pure resistance detector 28, low-pass filter 29 and adders 26 form signal error compensation means. An output voltage from the synchronizing detector 25 to be inputted into the adders 26 has a drift as shown bv the graph (a) in Figure 5 when the temperature of the sensors 16, etc. rise, notwithstanding the fact that the deflection of the spindle 12 remains the same, and consequently an output signal value has an error caused by the thermal change. Therefore a signal voltage from the low-pass filter 29 (as shown by the graph (b) in Figure 5) is added to a signal from the synchronizing detector 25 in order to compensate the error, whereby the adder 26 outputs a signal which is unaffected by temperature, as shown in Figure 6, and a constantly accurate information is provided to realize correct adjustment of the machining conditions. In the above embodiment, the direction X in Figure 5 is the cutting direction, and the normal direction sensors 16 and 18 detect a deflection amount δ for control purposes. However, it is also possible to arrange that the direction Y is placed in the cutting direction, and in this case the sensors 17 and 19 detect a deflection amount, and the control is carried out by a Y-axis motor. In the embodiment described above, the feed of the grinder is controlled, and the sharpness of the grinder is determined based on changes of the detected amount of deflection during spark-out. Moreover, an error in the signal from a deflection detector caused by a thermal change can be compensated, whereby grinding work can be carried out under optimum conditions, and the grinding accuracy and efficiency can thereby be improved.

Claims: A method of grinding a workpiece (W) in which use is made of a grinder comprising a motor (10); a grinding shaft (12) which is driven by the motor (10) and which is adapted to support a grinding wheel (11) at one end thereof; detector means (16-19) for detecting a factor which varies under machining conditions; and control means (32-38), responsive to output signals from the detector means (16-19), for adjusting the machining conditions, the detector means (16-19) being arranged to detect the deflection of a portion (12 a ) of the grinding shaft (12), or of means (15) secured thereto, under the machining conditions, the control means (32-38) being arranged to effect a feed adjustment in accordance with the said output signals, the said method comprising (a) rough grinding a workpiece (W) by feeding a grinding wheel (11) on the grinding shaft (12) towards the workpiece (W) until the workpiece (W) has a first predetermined size, the rough grinding during step (a) being carried out so that the said deflection is maintained at a first predetermined value (δ GR ); and (b) effecting rough grinding spark-out of the workpiece (W) without feeding the grinding wheel (11) until the said deflection is reduced to a second predetermined value (δ GSP ); characterised by (c) fine grinding the workpiece (W) by feeding the grinding wheel (11) towards the workpiece (W) until the workpiece (W) has a second predetermined size, the fine grinding during step (c) being carried out so that the said deflection is maintained at a third predetermined value (δ F ) which is less than the first predetermined value (δ GR ); (d) effecting fine grinding spark-out of the workpiece (W) without feeding the grinding wheel (11); (e) measuring the elapsed time (T F ) taken for the transition of the said deflection from the third predetermined value (δ F ) to a fourth predetermined value (δ FSP ); and (f) calculating the time (T FSP ) required for the fine grinding spark-out of step (d) from the said elapsed time (T F ), the third predetermined value (δ F ) and the fourth predetermined value (δ FSP ). Method as claimed in claim 1 characterised in that step (e) further comprises the steps of:- (e1) comparing the elapsed time (T F ) with a predetermined time value; and (e2) dressing the guiding wheel (11) after fine grinding spark-out of step (d) whenever the elapsed time (T F ) is greater than the predetermined time value. Method as claimed in claim 1 or 2 characterised in that the means (15) secured to the portion (12 a ) of the grinding shaft (12) comprises a cylindrical member (15) which is mounted on said portion (12 a ) and which is spaced by a gap from the detector means (16-19). Method as claimed in any preceding claim characterised in that the control means (32-38) comprise sizing means (32) for measuring the size of a workpiece (W) being machined; and a controller (34) which is arranged to receive signals both from the sizing means (32) and from the detector means (16-19), the controller (34) being arranged to produce a feed signal in response to the signals from the sizing means (32) and the detector means (16-19). Method as claimed in claim 4 characterised in that the said controller (34) is a main controller whose feed signal is outputted to a feed controller (36), the feed controller (36) being arranged to output a driving signal to a servo motor (35) which is adapted to effect feeding of the grinding wheel (11). Method as claimed in any preceding claim characterised in that the grinder has temperature-compensating means (23,26,28,29) for ensuring that the feed adjustment effected by the control means (32-38) is substantially unaffected by temperature. Method as claimed in claim 6 characterised in that the temperature-compensating means (23,26,28,29) comprises temperature sensing means (23) disposed adjacent to the detector means (16-19); means (28,29) connected to the temperature sensing means (23) for generating a temperature-compensation signal; and signal adjusting means (26) arranged to receive both thermally influenced signals from the detector means (16-19) and the said temperature-compensation signal, the signal adjusting means (26) being arranged to produce a feed signal which is substantially unaffected by the temperature adjacent to the detector means (16-19).

Inventors: KIHARA, HIROYUKI C/O SEIKO SEIKI KABUSHIKI KAISHA, OMORI, HIDEKI C/O SEIKO SEIKI KABUSHIKI KAISHA
Applicants: SEIKO SEIKI KABUSHIKI KAISHA
Application year: 1988
Main classification: B24B49
IPC codes: B24B41, B24B5, B24B49