SYSTEM FOR CONTROLLING THE TAPPING OPERATION

Abstract: The screw tapping operation is controlled by turning the tap with a digital spindle motor (1) in synchronism with the drive tap in the axial direction by a servo motor (2). A speed instruction is supplied from a position controller (4) to a digital control circuit (5) which drives a spindle motor, the speed instruction being based on a distribution pulse from an interpolation circuit (3a). The two motors are turned forward and reverse repeatedly to reciprocate the tap according to a fixed cycle, or a switch (7) supplies the distribution pulse and the feedback signal intermittently to perform the tapping.

Description: The present invention relates according to the precharacterising part of claim 1 to a machine tool using a digital spindle motor for its principal-axis motor and, more particularly, to a tapping control system in which rotation of a tapping tool (tap) by means of a digital spindle motor is synchronized with axial movement of the tap by means of a servomotor, thereby ensuring ideal tapping work. In tapping work, a tap must be advanced over a predetermined distance while it makes one revolution. Thus, the rotation and axial advance of the tap must be synchronized completely. In association with this, high-accuracy screw or gear cutting work is conventionally performed in a manner such that rotation of a tool and its feed relative to a workpiece are synchronized. For example, a principal-axis motor for tool rotation is rotated at a predetermined command speed, and rotation of a tool-feed motor is controlled in accordance with a pulse train, generated from a pulse coder attached to the tool-rotation motor, so that the two motors are rotated in synchronism with each other. In synchronous control of this type, speed commands are given to the individual motors; that is, a speed control loop is used as a control loop. The righting moment (correction ability) of the speed control loop against variations of speed is small, and the rotational speed of the tool-rotation motor is liable to change as the load varies. As the rotational speed of the tool-rotation motor varies in this manner, the synchronism of the two motors lacks accuracy, so that the working accuracy is lowered. When tapping a deep hole by means of a small-diameter tap, the tap may sometimes be broken. In connection with the synchronous control of the aforementioned type, moreover, a multifunction machine tool is conventionally used for various types of machining. Various tools, such as a tapping tool, cutter, etc., can be replaceably mounted on a tool holder of the machine tool. However, this machine tool has problems of working accuracy for the above reasons. In a system disclosed in Japanese Patent Application No. 60-155319 by Brother Kogyo K.K., a feed driver is used to move a workpiece relative to a tool, and rotation of a rotation driver, which is used to relatively rotate the workpiece and the tool in accordance with the amount of feed by the feed driver, is controlled. This system, however, is intended for screw working only. Although it can therefore perform high-accuracy screw working, the disclosed system, which uses a servomotor incapable of high-speed rotation, cannot effect any other type of machining even if adapted for the attachment of various tools. In particular, the system cannot actually perform deep cutting in which the tool is subjected to a substantial load. EP-A-0 025 652 by the present applicant discloses a control system for synchronizing machine tool motors for tapping operations, having the features of the preamble of accompanying claim 1. In this system, an arithmetic circuit determines the feed speed based on the tap rotational speed and commanded pitch. US-A-3 878 742 discloses a numerically controlled lathe capable of performing various machining operations including drilling of a screw thread. According to the present invention, there is provided a tapping control system for a machine capable of carrying out machining work on a workpiece including tapping a screw with a tap, and having a spindle motor for spindle rotation controlled by a rotation control circuit, and a feed motor for tool feed controlled by a feed control circuit, the system comprising:- command means for supplying speed commands to the rotation control circuit and feed control circuit; and a detector responsive to rotation of the spindle motor for outputting a feedback signal indicative of a rotational speed of the spindle motor; wherein said spindle motor and said feed motor are driven in synchronism with each other; characterised in that:- said spindle motor is a digital spindle motor, said rotation control circuit is a digital control circuit, and said detector is a pulse coder; said feed motor is a servomotor, and said feed control circuit is a servo-circuit; and in that said command means comprise:- an interpolating circuit for distributing pulses individually to said digital control circuit and the servo-circuit to effect linear interpolation in accordance with a pitch value of tapping work to be carried out; position control means for delivering a speed command to said digital control circuit in accordance with the distributed pulses from the interpolating circuit and the feedback signal outputted from the pulse coder to effect feedback control of the rotational position of said spindle motor; and reversing control means which control said interpolating circuit, said position control means and servo circuit in a tapping mode such that said spindle motor and said servomotor are each forwardly rotated to carry out a predetermined depth of tapping work on a workpiece, and then reversely rotated to retract the tap through a distance less than said predetermined depth, said forward and reverse rotations being repeated alternately until completion of tapping work on said workpiece. An embodiment of the present invention may provide a tapping control system in which rotation and axial advance of a tap are synchronized completely, and the tap is subjected to less resistance such that even a relatively thin tap is not broken during tapping work. An embodiment of the invention may also provide a tapping control system in a multifunction machine tool which is adapted to be replaceably mounted with various tools, whereby both deep cutting, such as milling, and high-accuracy tapping can be performed. As described above, the digital spindle motor for rotating the tap and the servomotor for axially moving the tap are driven individually in response to the distributed pulses linearly interpolated by the interpolating circuit. The two motors are driven synchronously so that the rotational position and the axial position of the tap correspond to each other. After a predetermined amount of tapping work is finished, moreover, the tap is retracted through the distance less than the predetermined amount. The tapping work is performed on the basis of such an alternating motion. Accordingly, chips are eliminated, so that the tap is subjected to less resistance. Thus, the tapping work can be effected with ease, and even a relatively thin tap need not be broken by the tapping work. In a tapping mode, the digital spindle motor, used as the principal-axis motor for tool rotation, and the motor for tool feed, are driven in response to the distributed pulses from the interpolating circuit for linear interpolation in accordance with a pitch command value read from a working program. Therefore, the two motors can be rotated in synchronism with each other. Further, the digital spindle motor is digitally controlled so that it can satisfactorily follow up the command. Accordingly, high-accuracy rigid tapping can be accomplished without using a floating holder as the tool holder. In a deep-cut mode, on the other hand, a position control loop for the spindle motor is released, and the spindle motor is enabled to rotate with large torque and at high speed. Thus, deep cutting can be securely performed resisting the load. Reference is made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a control block diagram of a machine tool to which is applied a tapping control system according to an embodiment of the present invention; Fig. 2 is a diagram showing the follow-up performance of a conventional spindle motor; Fig. 3 is a diagram showing the follow-up performance of a digital spindle motor; and Fig. 4 is a diagram showing a fixed cycle used in the embodiment of the invention. Fig. 1 shows a multifunction machine tool to which is applied a tapping control system according to an embodiment of the present invention. This machine tool, which operates in various working modes including tapping and deep-cut modes, comprises an AC digital spindle motor 1, for use as a principal-axis motor for tool rotation, and an AC servomotor 2 for tool feed. A tap 21 mounted on a tool holder 20 (Fig. 4) is rotated relative to a workpiece (not shown) by means of the digital spindle motor 1, and is driven in the working direction, with respect to the workpiece, by means of the feed motor 2. The spindle motor 1 is enabled to rotate at high speed so as to resist a substantial load at the time of deep cutting, e.g., milling. The tool holder 20 is replaceably fitted with a plurality of tools, such as the tap 21 and a milling cutter (not shown), by means of an automatic tool replacer. The holder 20 can also hold a tap 21 for rigid tapping in a nontelescopic manner. Numeral 3 denotes a numerical control unit including an interpolating circuit 3a, and symbol pC1 designates a pulse coder which produces a feedback pulse Fs with every rotation of the spindle motor 1 through a predetermined angle. Numeral 4 denotes position control means which delivers, as a speed command value, the difference between a distributed pulse Ps from the interpolating circuit 3a and the feedback pulse Fs from the pulse coder PC1. The means 4 includes an error register (not shown) and a D/A converter (not shown). Numerals 7 and 8 denote switching means which are shifted in response to a switching command CS from the numerical control unit 3. These means 7 and 8 are adapted to be shifted to a feedback control position, as shown in Fig. 1, when the spindle motor 1 is subjected to feedback control at the time of tapping work. More specifically, the switching means 7 applies the output of the position control means 4 to a digital control circuit 5, while the switching means 8 applies the feedback pulse Fs to the position control means 4. When the feedback control is not expected, on the other hand, the means 7 and 8 are shifted to an open control position. In other words, the switching means 7 is shifted to apply a speed command SV for the spindle motor 1, delivered from the numerical control unit 3, to the digital control circuit 5, while the switching means 8 is shifted to prevent the feedback pulse Fs from being applied to the position control means 4. The digital control circuit 5 includes a speed control section 5a, current control section 5b, and a PWM (pulse width modulation) control section 5c. The speed control section 5a delivers a torque command in response to the difference between the speed command from the position control means 4 or the numerical control unit 3 and a present speed based on the feedback pulse Fs from the pulse coder PC1. On receiving this torque command, the current control section 5b delivers a command current to each phase of the spindle motor 1 in accordance with a present driving current from current detectors 9 and 10. Thus, the PWM control section 5c modulates the pulse width of the command current output for each phase. Thereupon, the digital control circuit 5 delivers the modulation output thus obtained to a power circuit 6, thereby driving the same. In this manner, the spindle motor 1 is driven at a command speed. The digital control circuit 5 is of a conventional type (e.g., Japanese Patent Application No. 60-36051). For comparison, Figs. 2 and 3 show motor follow-up performances for the cases in which the spindle motor is driven by means of the digital control circuit 5 and by the conventional system, individually. According to the prior art system, as shown in Fig. 2, the spindle motor follows up the speed command with a considerable delay. On the Other hand, the spindle motor controlled by the digital control circuit 5 can accurately follow up the speed command without any substantial delay. Meanwhile, a servo-circuit 12, like the prior art one, detects the difference between a distributed pulse Pz from the interpolating circuit 3a and a feedback pulse Fz from the pulse coder PC2, by means of an error register in its position control section 12a. The output of this error register is converted into an analog signal by means of a D/A converter of the control section 12a, and is delivered as a speed command. The difference between this speed command and an actual speed from an F/V converter 12c is amplified by means of a speed control section 12b, and is delivered as a torque command. In a current control section 12d, a driving current command for each phase is outputted in accordance with this torque command and an actual driving current value from current detectors 14 and 15 which are used to detect the driving current for the servomotor 2. On receiving this driving current command, a PWM control section 12e modulates the pulse width of the command. This modulation output is used to drive a power circuit 13, thereby driving the AC servomotor 2. Numerals 11 and 16 denote three-phase power sources, individually. The tapping operation will now be described. As shown in Fig. 4, tapping work is performed in a fixed cycle such that the tap 21 is first fed quickly to a working start point R, and the spindle motor 1 for tool rotation and the servomotor 2 for tool feed are then rotated forwardly, thereby effecting tapping for a predetermined distance d1 in a tapping direction (hereinafter referred to as Z-axis direction). Subsequently, the spindle motor 1 and the servomotor 2 are rotated reversely, so that the tap 21, while being rotated, is retreated or moved reversely to the work advancing direction, by a predetermined distance d2 (d1 > d2) shorter than the depth of finished work. Then, the spindle motor 1 and the servomotor 2 are rotated forwardly, thereby effecting tapping again for the predetermined depth d1 (moved distance in the Z-axis direction is d1 + d2), in the same manner as aforesaid. Thereafter, the tap 21 is retreated again over the distance d2. When the depth of the tapping work, accompanied with such reciprocation, reaches a predetermined depth as instructed, the motors 1 and 2 are rotated reversely, so that the tool 4 is disengaged entirely from a tapping hole. Then, the tool is fed quickly to a position at a predetermined height, where it stands ready for the next working process. The tapping work is performed on the basis of the aforementioned fixed cycle. Referring now to Fig. 1, the tapping operation will be described. First, if a tapping command is read from a working program by the numerical control unit 3, a tapping tool is mounted on the tool holder by means of the automatic tool exchanger (not shown), and the switching command CS is delivered from the numerical control unit 3. In response to the command CS, the switching means 7 and 8 are shifted to the feedback control position, and the speed commands Vs and Vz for the spindle motor 1 and the servomotor, which correspond to a pitch value designated by the working program, are applied to the interpolating circuit 3a. Thereupon, the distributed pulses Ps and Pz, interpolated linearly, are delivered from the interpolating circuit 3a to the position control means 4 and the position control section 12a of the servo-circuit 12, respectively. More specifically, if the pitch or the distance between each two adjacent threads of the screw to be worked is P, and if the number of threads to be formed on the workpiece is i, the aforementioned pulse distribution by the interpolating circuit 3a is performed in a manner such that the servomotor 2 advances the tap 21 axially by a distance iP while the spindle motor 1 causes the tap 21 to make one revolution. Subsequently, the position control means 4 delivers, as the speed command, the difference between the distributed pulse Ps and the feedback pulse Fs from the pulse coder PC1, to the digital circuit 5. Thereupon, the digital circuit 5 executes speed control such that the spindle motor 1 is rotated forwardly at the command speed by means of the power circuit 6. Meanwhile, the servo-circuit 12 also executes the aforesaid position control and speed control in accordance with the distributed pulse Pz and the feedback pulse Fz from the pulse coder PC2, so that the servomotor 2 is rotated forwardly by means of the power circuit 13. As a result, the spindle motor 1 and the servomotor 2 are rotated forwardly in synchronism with each other and at a required rotational speed ratio. Thus, in working the screw with i threads, for example, the tap 21 is axially driven by the distance iP for each revolution thereof, thereby effecting the tapping work with the pitch provided by the working program. When the moved distance of the tap 21 (or amount of pulse distribution to the servomotor 2) attains d1, the speed commands Vs and Vz for the spindle motor 1 and the servomotor 2 are delivered in the opposite (or negative) direction. Thereupon, the interpolating circuit 3a delivers the distributed pulses Ps and Pz in the opposite direction through the same linear interpolation as aforesaid, thereby reversely rotating the spindle motor 1 and the servomotor 2 in synchronism with each other. This reverse operation is continued until the moved distance of the tap 21 attains d2. When the retreated distance of the tap 21 attains d2, thereafter, the tap 21 is rotated forwardly again, and is moved by (d1 + d2) in the Z-axis direction. Then, the reverse direction is started in the same manner as aforesaid. Thereafter, the processes in the same fixed cycle are executed, and the workpiece is worked to the designated fixed depth position (Z-axis position). Thus, the spindle motor 1, used to rotate the tap 21, and the servomotor 2 are driven individually in accordance with the pulses distributed as a result of the linear interpolation in the interpolating circuit 3a. Accordingly, the spindle motor 1 and the servomotor 2 are synchronized, and the respective rotational positions of the motors 1 and 2, that is, the rotational position and the axial position of the tap 21, correspond entirely to each other. Even though the spindle motor 1 and the servomotor 2 are rotated in either direction, therefore, the aforesaid predetermined positional relationship can be maintained continually, thereby permitting accurate tapping work. Moreover, the tap 21 advances in the working direction for tapping while rotating forwardly, and then retreats by the fixed distance while rotating reversely. Thus, chips are eliminated, so that the tap 21 is subjected to less resistance, and can less frequently be broken. If a deep cutting command, e.g., a milling command, is read from the working program, on the other hand, a milling tool is mounted on the tool holder 20. Thereafter, the switching command CS is delivered from the numerical control unit 3, whereupon the switching means 7 and 8 are shifted to the open control position. Thus, the feedback pulse Fs from the pulse coder PC1 is stopped from being applied to the position control circuit 4, while the speed command Sv from the numerical control unit 3 is applied to the digital control circuit 5 through the switching means 7. Consequently, the spindle motor 1 starts to rotate at a speed corresponding to the speed command Sv, that is, at high speed, thus effecting deep cutting, such as milling.

Claims: A tapping control system for a machine capable of carrying out machining work on a workpiece including tapping a screw with a tap, and having a spindle motor (1) for spindle rotation controlled by a rotation control circuit (5), and a feed motor (2) for tool feed controlled by a feed control circuit (12), the system comprising:- command means (3, 3a; 4, 12a) for supplying speed commands to the rotation control circuit (5) and feed control circuit (12); and a detector (PC1) responsive to rotation of the spindle motor (1) for outputting a feedback signal (Fs) indicative of a rotational speed of the spindle motor; wherein said spindle motor (1) and said feed motor (2) are driven in synchronism with each other; characterised in that:- said spindle motor (1) is a digital spindle motor, said rotation control circuit (5) is a digital control circuit, and said detector (PC1) is a pulse coder; said feed motor (2) is a servomotor, and said feed control circuit (12) is a servo-circuit; and in that said command means (3, 3a, 4) comprise:- an interpolating circuit (3a) for distributing pulses individually to said digital control circuit (5) and the servo-circuit (12) to effect linear interpolation in accordance with a pitch value of tapping work to be carried out; position control means (4) for delivering a speed command to said digital control circuit (5) in accordance with the distributed pulses from the interpolating circuit (3a) and the feedback signal (Fs) outputted from the pulse coder (PC1) to effect feedback control of the rotational position of said spindle motor (1); and reversing control means in the form of a numerical control unit (3) which control said interpolating circuit (3a), said position control means (4) and servo circuit (12) in a tapping mode such that said spindle motor (1) and said servomotor (2) are each forwardly rotated to carry out a predetermined depth of tapping work on a workpiece, and then reversely rotated to retract the tap through a distance less than said predetermined depth, said forward and reverse rotations being repeated alternately until completion of tapping work on said workpiece. A tapping control system according to claim 1, wherein said spindle motor (1) and said servomotor (2) are driven with a predetermined rotational speed ratio corresponding to said pitch value. A tapping control system according to claim 1 or 2, further comprising switching means (7, 8) for switching the system between a feedback control mode and an open control mode, said numerical control unit (3) being operable to switch said switching means to the feedback control mode for performing tapping work, and to the open control mode for performing deep cutting work. A tapping control system according to claim 3, wherein said numerical control unit (3) is operable to rotate said spindle motor (1) at a high speed for deep cutting when the system is switched to the open control mode. A tapping control system according to any preceding claim, further comprising a tool holder (20) for nontelescopically mounting a tool such as a tap (21) or a deep cutting tool. A tapping control system according to claim 3, arranged such that when said switching means (7, 8) is in the switching position corresponding to said open control mode, said distributed pulses and said feedback signal (Fs) are prevented from being applied to said position control means (4). A tapping control system according to any preceding claim, wherein said digital control circuit (5) includes a speed control section (5a), which receives a speed command from either said position control means (4) or directly from said numerical control unit (3) depending on the selected mode, a current control section (5b) coupled to said speed control section (5a), and a pulse width modulation control section (5c) for delivering a torque command to the spindle motor (1).

Inventors: IWAGAYA, TAKASHI
Applicants: FANUC LTD.
Application year: 1987
Main classification: B23G1
IPC codes: G05B19, B23G1