Numerically controlled machine tool

Abstract: A numerically controlled machine tool for machining a non-circular workpiece according to profile data. The numerical controller stores phase compensation values and position compensation data files in connection with the types of cam and the various rotational speeds of the main spindle. Each position compensation data file consists of a series of position compensation values for a successive rotational positions of the main spindle. The numerical controller searches a phase compensation value and a position compensation data file according to the type of cam to be ground and the rotational speed of the main spindle during the grinding operation. Thereafter, the numerical controller offsets the read-out position of the profile data by the amount corresponding to the searched phase compensation value, and compensates each profile data with position compensation data stored in the searched file so as to eliminate a machining error which changes depending upon the rotational speed of the main spindle.

Description: BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a numerically controlled grinding machine capable of machining a non-circular workpiece such as a cam shaft. Description of the Prior Art: Generally, in a numerically controlled grinding machine for grinding a non-circular workpiece, the feed of the grinding wheel perpendicular to a spindle axis is controlled according to profile data and machining cycle data. The profile data comprises the amount of movement of the grinding wheel per unit angle rotation of the spindle which defines the profile generating movement of the grinding wheel along the finished shape of the workpiece. On the other hand, the machining cycle data comprises a numerical control program to control a machining cycle which includes rapid feed, cut-in feed and retracting feed of the grinding wheel. In the numerically controlled grinding machine tool of the aforementioned type, the ability of the main spindle and the grinding wheel feed axis to accurately follow command data are needed so as to grind the workpiece precisely. In order to improve machining accuracy, a method has been proposed wherein a machine tool is operated according to ideal profile data which is calculated from the ideal final shape of a workpiece. During operation, the positional changes of the tool and the main spindle are measured in order to obtain actual movement data. After that, position compensation data is calculated by comparing the ideal profile data and the measured movement data, and the ideal profile data is compensated for with the position compensation data in order to obtain execution profile data, which is used in the actual machining operation. In such method, however, the position compensation data is set at a constant value regardless of the change in the rotational speed of the main spindle. Accordingly, machining errors arise, even if the compensated execution data is used for the actual machining operation. Namely, when a workpiece is machined without the above-mentioned compensation, a machining error which changes depending upon the rotational speed of the main spindle arises as shown in Figure 1. Accordingly, the machining error cannot be eliminated with the constant position compensation data in the case that the rotational speed of the main spindle changes during machining operation. From post-published document EP 0 265 607 A1 which falls within the terms of Article 54(3) EPC, a numerically controlled machine tool for machining a non-circular workpiece according to profile data is known. The numerical controller operates according to ideal profile data and measures the actual position change of a tool slide corresponding to the angle position change of a main spindle. Then, the numerical controller calculates the position error and phase error depending on the actual position change and ideal profile data, and produces execution profile data which are compensated for these errors. From document GB 2,124,113 a numerically controlled grinding machine is known, comprising a main spindle and a tool slide, having a tool mounted thereto, numerically controlled for grinding a non-circular workpiece according to profile data which define a profile generating movement of the tool along a final shape of said workpiece; a first motor for rotating said main spindle; a second motor for moving said tool slide; pulse generator means for detecting at least the rotational speed of said second motor and for outputting a feedback signal; profile data memory means for storing said profile data; control means for reading out said profile data from said memory means and for outputting corresponding command signals, said command signals being output in accordance with a commanded rotational speed of said main spindle; a first drive unit for driving said first motor; and a second drive unit for driving said second motor in response to said command signals and said feedback signal. Normally, the position compensation data are constant. That is, a change in the rotational speed of the spindle during machining operation is not taken into account. Therefore, machining errors cannot be prevented even if compensated data are used for the machining operation. SUMMARY OF THE INVENTION Therefore, it is an object of the invention to provide an improved numerically controlled grinding machine capable of increasing machining accuracy. According to the invention, there is provided a numerically controlled grinding machine, comprising a main spindle and a tool slide, having a tool mounted thereto, numerically controlled for grinding a non-circular workpiece according to profile data which define a profile generating movement of the tool along a final shape of said workpiece; a first motor for rotating said main spindle; a second motor for moving said tool slide; pulse generator means for detecting at least the rotational speed of said second motor and for outputting a feedback signal; profile data memory means for storing said profile data; control means for reading out said profile data from said memory means and for outputting corresponding command signals, said command signals being output in accordance with a commanded rotational speed of said main spindle; a first drive unit for driving said first motor; and a second drive unit for driving said second motor in response to said command signals and said feedback signal, characterized in that said numerically controlled grinding machine is further provided with compensation memory means for storing plural phase compensation values corresponding to a respective rotational speed of said main spindle, and plural position compensation data corresponding to said respective rotational speed of said main spindle, wherein each said position compensation data is composed of a series of position compensation values for successive rotational positions of said main spindle; and said control means further comprises means for searching said compensation memory means for the phase compensation value and the position compensation data that corresponds to said rotational speed of said main spindle during machining operation, and for compensating said profile data with said searched phase compensation value and position compensation data. BRIEF DESCRIPTION OF THE DRAWINGS Various other objects, features and attendant advantages of the present invention will readily be appreciated as the same becomes better understood by reference to the following detailed description of a preferred embodiment when considered in connection with the accompanying drawings, in which: FIGURE 1 is a chart which illustrates the measured error; FIGURE 2 is a schematic view of a numerically controlled grinding machine according to the present invention; FIGURE 3 is a block diagram showing the structure of the numerical controller shown in Figure 2; FIGURE 4 shows a compensation value table and compensation data files formed in the compensation value memory area shown in Figure 3; FIGURE 5 shows X axis compensation values stored in compensation data files shown in Figure 4; FIGURE 6 shows a compensation value table stored in the compensation value memory area shown in Figure 4; FIGURE 7 is a flow chart explaining the operation of the main CPU shown in Figure 3 for inputting compensation data; FIGURES 8(a) and 8(b) are flow charts explaining the general operation of the main CPU shown in Figure 3 for the machining process. FIGURES 9(a) and 9(b) are flow charts for illustrating the details of a step shown in Figure 8(a); FIGURE 10 shows a numerical control program for machining a workpiece; and FIGURE 11 is an explanatory chart which explains the method of obtaining compensation values. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings and particularly to Figure 2 thereof, a numerically controlled grinding machine is shown comprising a bed 10, on which a table 11 is slidably guided along a Z axis which is parallel to a main spindle 13. A workhead 12 is mounted on the left-hand end of the table 11. The workhead 12 has rotatably carried therein the main spindle 13, which is connected to a servomotor 14 so as to be rotated thereby. A tail stock 15 is also mounted on the right-hand end of the table 11. A workpiece W having a cam C is carried between the center 17 of the main spindle 13 and the center 16 of the tail stock 15. The left end of the workpiece is engaged with a positioning pin 18, which is fixed on the main spindle 13, so as to synchronize the phase of the workpiece W with the phase of the main spindle 13. A tool slide 20 is slidably guided on a rear portion of the bed for movement toward and away from the workpiece W along an X axis. A grinding wheel 22, rotatably supported as a tool on the tool slide 20, is rotated by a motor 21. The tool slide 20 is connected to a servomotor 23 through a feed screw (not shown) so ancing and retraction movement of the tool slide 20 is effected by the servomotor 23. As indicated in Figure 13, drive units 40, 41 are circuits to drive servomotor 23, 14, respectively, in response to command pulses, which are generated by the numerical controller 30. Pulse generators 50, 52 and tachogenerators 51, 53 are connected to the servomotor 14, 23, respectively, and output signals from the pulse generators 50, 52 and the tachogenerators 51, 53 are fed back to the drive units 40, 41 in order to control the servomotor 14, 23 with the velocity feed back signal and the position feed back signal. The numerical controller 30 controls the rotation of the servomotor 23, 14 numerically so as to grind the workpiece W. A tape reader 42 for inputting ideal profile data and machining cycle data, a keyboard 43 for inputting control data, a CRT display device 44 for displaying various information and a control panel 45 for inputting an operator command are connected to the numerical controller 30. The numerical controller 30 comprises a main central processing unit (hereinafter referred to a "main CPU") 31, a read only memory (ROM) 33, in which control program is stored, a random access memory (RAM) 32 and an interface 34 as shown in Figure 3. In the RAM 32, there is an NC data area 321 for storing numerical control programs, an ideal profile data area 322 for storing ideal profile data calculated from the ideal final shape of the workpiece, a compensation value memory area for storing phase compensation values and position compensation values of tool feed axis (X axis). The RAM 32 also has a feed mode setting area 324, a workpiece mode setting area 325, a spark-out mode setting area 326, and a phase error compensation mode setting area 327. The compensation value memory area is composed of a compensation value table 3271 and compensation data file area 3272 as shown in Figure 4. The compensation value table 3271 stores phase compensation values and the file numbers of X axis position compensation data files in connection with various rotation speeds of the main spindle 13 and ideal profile data numbers as shown in Figure 6. Each X axis compensation data file stores a series of X axis (position) compensation values in connection with the rotational positions of the main spindle 13 as shown in Figure 5. The numerical controller 30 also comprises a drive CPU 36, a RAM 35 and a pulse distribution circuit 37 so as to distribute command pulses to drive units 40, 41. The RAM 35 stores positioning data sent from main CPU 31. The drive CPU 36 executes calculations for slow up, slow down and interpolation depend on the positioning data sent from the main CPU 31 via the RAM 35, and outputs the amount of movement data and velocity data at a predetermined interval. The pulse distribution circuit 37 distributes feed command pulses to drive units 40, 41 according to the amount of movement data and velocity data. The process for storing the compensation data will be described below. The compensation data can be obtained by a method described below. At first, a workpiece is machined according to the ideal profile data, and the cam profile of the machined workpiece is measured to obtain measured profile data. Then, the compensation data is calculated depending upon the difference between the ideal profile data and measured profile data. Namely, the phase compensation value Δϑ is calculated depending upon the angle difference between the angle position ϑ I of C axis, whereat the X axis position of the ideal profile data reaches the maximum value, and the angle position ϑ M of C axis, whereat the X axis position of the measured profile data reaches the maximum value, as shown in Figure 11. Position compensation values ΔX(I) in the X axis (hereafter referred to "X axis compensation values") are calculated depending on the difference in X axis between the ideal profile data and shifted measured profile data, which is obtained by shifting the measured profile data by the phase compensation value Δϑ. The compensation data can be also obtained by a method wherein the machine is operated according to the ideal profile data, and the positional changes of the main spindle 13 and the tool slide 20 are measured during the operation in order to obtain measured profile data. Then, the compensation data is calculated based on the ideal profile data and the measured profile data. The compensation data is obtained for various rational speeds of the main spindle. Then, the compensation data is recorded on an NC tape in connection with the profile data number of the cam and the rotational speed of the main spindle. When the switch 451 on the control panel 45 is pushed by an operator after the above described preparation, the main CPU 31 executes the compensation data read-in program shown in Figure 7. At first, a profile data number is input via the tape reader 42 at step 400 and is stored in the compensation value table 3271 at step 402. At step 404, a variable n designating the column number of the compensation value table 3271 is initially set at 1. Then, a rotational speed Rn of the main spindle 14 and a phase compensation value Qn are input via the tape reader 42 at steps 406 and 408. After that, a series of X axis compensation values ΔX (I) and a compensation data file number Pn, which designates a data file where the X axis compensation values are to be stored, are input at step 410. At step 412, the rotational speed Rn, the phase compensation value Qn and the compensation data file number Pn are stored in the column n of the compensation value table 3271, and the series of X axis compensation values ΔX (I) are stored in the compensation data file Pn. Thereafter, the process of the main CPU 31 moves to step 414 for ascertaining whether or not all compensation data for a certain profile is stored. If it is not completed, the process moves to step 416 in order to add 1 to the variable n, and then moves back to step 406 so as to input compensation data which will be stored in the next column and the next data file. By repeating the above mentioned process, compensation data for various rotational speeds (10 rpm, 20 rpm, 30 rpm) are stored in the compensation value table 3271 and compensation data files in connection with the profile data number and the rotational speed of the main spindle. Thereafter, when the switch 452 on the control panel 45 is pushed, an NC program for a machining cycle stored in the RAM 32 is executed. The NC program is shown in Figure 10. NC data in the NC program is decoded by the main CPU 31 according to steps as specified by the flow chart shown in Figure 8(a) and 8(b). One block of an NC program is read out from the NC program area of the RAM 32 at step 100, and it is ascertained whether or not the data of the block shows the end of the program at step 102. If the data shows completion of the NC program, the execution of this program is ended. But if not, the process of main CPU 31 moves to step 104, and it is ascertained whether or not the block includes code G at step 104. If code G is ascertained at step 104, the process of main CPU 31 moves to step 106 so as to ascertain the command code in greater detail. In step 106 through step 116, mode flags in the mode setting area 324 ∼ 326 of the RAM 32 are set, depending on the detail command code. If code G01 is ascertained at step 106, the flag in the feed mode setting area 324 is set at step 108 so as to set a feed mode in a grinding feed mode. Similarly, if code G04 is ascertained at step 110, the flag in the spark out mode setting area 326 is set at step 112 so as to set the feed mode in a spark-out mode. If code G51 is ascertained at step 114, the flag in the workpiece mode setting area 325 is reset to set the workpiece mode in a cam mode at step 116. Furthermore, if code x is ascertained at step 118, the process of the main CPU 31 moves to step 120, and it is ascertained whether or not the workpiece mode is set in the cam mode and the feed mode is set in the grinding feed mode (hereafter referred to as the cam grinding mode). If the mode is the cam grinding mode, pulse distribution for grinding the cam is executed at step 128. If not, a regular pulse distribution, which is not synchronized with main spindle rotation, is executed at step 122. On the other hand, if code x is not ascertained at step 118, it is ascertained whether or not the spark out process is commanded at step 124. If the spark out process is commanded, the process of the main CPU 31 moves to step 126 in order to execute pulse distribution for spark-out grinding. The process for actual cam grinding with compensation will be explained according to the NC program shown in Figure 10. When the switch 452 on the control panel 45 is pushed, the NC program for the machining cycle shown in Figure 10 is decoded block by block according to the process shown in Figure 8(a) and Figure 8(b). First, in response to code G51 in block N010, the workpiece mode is set in a cam mode, and one set of the ideal profile data designated by profile number P1234 is selected at step 116. Then the feed mode is set in a grinding feed mode in response to the code G01 in block N020 at step 108, and the cam grinding procedure is executed at step 128 in response to feed data X-0.1. Data with code F in block N020 indicates the feed amount per revolution of the main spindle 13, and data with code R indicates the feed speed per revolution of the main spindle 13. Data with code S indicates the rotational speed of the main spindle 13. Since data with code F and data with code R are the same value in the NC program shown in Figure 10, the grinding wheel G is fed continuously at a constant speed until the total cut-in feed amount reaches a programmed total feed value with code X. The cam grinding operation with compensation is accomplished by the process at step 128. The details of the process at step 128 are shown in Figure 9(a) and 9(b). First, the pulse number corresponding to the cut-in feed amount per unit rotational angle (0.5°) is calculated depending upon the data with code R at step 200. Then, a phase compensation value Δϑ and compensation data file number P are searched from the compensation value table 3271 according to the commanded profile data number and the commanded rotational speed of the main spindle, and compensation data file designated by the compensation data file number P is searched from the data file area 3272, at step 202. The compensation data file stores a series of X axis compensation values ΔX (I) for successive rotational angle positions of the main spindle 13, wherein each rotational position is located at intervals of 0.5 degrees. The phase error can be compensated for by offsetting the read out position of the execution profile data ahead from a theoretical read out position, which corresponds to the commanded angle position of the main spindle, by an amount corresponding to the phase compensation value Δϑ. At step 202, the offset address IO corresponding to the phase compensation value Δϑ is also calculated. Then, the read out address I is initially set to the offset address IO at step 204. The read out address I is used for designating the read out positions of ideal profile data and the X axis compensation values. Afterwards, the pulse distribution finish signal is input at step 206 so as to ascertain whether or not the previous pulse distribution is finished. If the previous pulse distribution is finished, the execution profile data D(I) and X axis compensation value ΔX (I) are read out, and then execution profile data A (I) is calculated with the equation described below. A (I) = D (I) - ΔX (I) Thereafter, it is ascertained whether or not the cut-in feed within one revolution is finished or completed at step 210. This is ascertained depending on the data with code F. In this case, it is ascertained whether or not the cut-in feed is finished by ascertaining whether or not the feed amount within one revolution reaches 0.1 mm. If the cut-in feed within one revolution is not finished, the amount of movement data is calculated based upon the cut-in amount per unit angle and execution profile data A (I) at step 212. Then, the positioning data is output to the drive CPU 36 at step 214. On the other hand, if the cut-in feed within one revolution is finished, the moving amount is calculated from only the execution profile data A (I) at step 213. Then, it is ascertained whether or not read out address I reaches the final address I max of the profile data at step 216. If I is equal to or larger than I max the read out address I is reset to an initial value 1 at step 218 so as to return the read out address to the beginning address. If not, the read out address I is incremented by 1 at step 220. Then, it is ascertained whether or not read out address I reaches the offset address IO so as to detect one revolution of the main spindle 13. If read out address I reaches the offset address IO, it is ascertained whether or not the total cut-in feed amount reaches a programmed total feed amount, which is programmed with code X. If the total cut-in feed amount does not reach the programmed amount, the process moves to step 206 so as to continue the above-mentioned process repeatedly. If the total cut-in feed amount reaches the programmed amount, the process for grinding the cam according to the NC program in block 20 is finished. Furthermore, the process for spark-out grinding is executed in response to code G04 in block N030. This process is similar to the process shown in Figure 9(a) and 9(b). However, there is no cut-in feed in this process and spark-out grinding is finished when the number of revolutions of the main spindle 13 reaches a programmed value. Namely, a phase compensation value Δϑ and a series of X axis compensation value ΔX (I) are searched according to the profile data number and rotational speed of the main spindle during spark-out grinding. Then, ideal profile data, which are offset by the phase compensation value Δϑ from the commanded angle position of the main spindle, are read out successively, and is compensated by X axis compensation value ΔX (I). The compensated ideal data is output successively during the programmed rotation of the workpiece so that the spark-out grinding is accomplished with compensation for the errors, which changes depend upon the speed of the main spindle.

Claims: A numerically controlled grinding machine, comprising a main spindle ( 13 ) and a tool slide ( 20 ), having a tool ( G ) mounted thereto, numerically controlled for grinding a non-circular workpiece ( W ) according to profile data which define a profile generating movement of the tool ( G ) along a final shape of said workpiece ( W ); a first motor ( 14 ) for rotating said main spindle ( 13 ); a second motor ( 23 ) for moving said tool slide ( 20 ); pulse generator means ( 52 ) for detecting at least the rotational speed of said second motor ( 23 ) and for outputting a feedback signal; profile data memory means ( 32 ) for storing said profile data; control means ( 31, 36, 37 ) for reading out said profile data from said memory means ( 32 ) and for outputting corresponding command signals, said command signals being output in accordance with a commanded rotational speed of said main spindle ( 13 ); a first drive unit ( 41 ) for driving said first motor ( 14 ); and a second drive unit ( 40 ) for driving said second motor ( 23 ) in response to said command signals and said feedback signal, characterized in that said numerically controlled grinding machine is further provided with compensation memory means ( 327 ) for storing plural phase compensation values ( Θ1 - Θ6 ) corresponding to a respective rotational speed of said main spindle ( 13 ), and plural position compensation data ( P1 - P6 ) corresponding to said respective rotational speed of said main spindle ( 13 ), wherein each said position compensation data ( P1 - P6 ) is composed of a series of position compensation values for successive rotational positions of said main spindle ( 13 ); and said control means ( 31, 36, 37 ) further comprises means ( 31; 202 ) for searching said compensation memory means ( 327 ) for the phase compensation value ( Θ1 - Θ6 ) and the position compensation data ( P1 - P6 ) that corresponds to said rotational speed of said main spindle ( 13 ) during machining operation, and for compensating said profile data with said searched phase compensation value and position compensation data. A numerically controlled machine tool, according to claim 1, characterized in that said profile data memory means ( 32 ) stores a series of positional data of said tool slide ( 20 ) in a successive memory address; said position compensation data memory area ( 3272 ) stores a series of compensation values in a successive memory address; and said control means ( 31, 36, 37 ) comprises means ( 202, 204, 208 ) for offsetting the read out position of said profile data and compensation values from a theoretical memory address by an amount corresponding to said searched phase compensation value, and for reading out said positional data and position compensation values successively from an offset memory address, and compensating said read out positional data with read out position compensation values.

Inventors: ISOMURA KOUICHI, OHTA NORIO, YOSHIMURA MASATOMO, YOTSUI YOSHINOBU, ISOMURA, KOUICHI, OHTA, NORIO, YOSHIMURA, MASATOMO, YOTSUI, YOSHINOBU
Applicants: TOYODA MACHINE WORKS LTD, TOYODA KOKI KABUSHIKI KAISHA
Application year: 1988
Main classification: G05B19
IPC codes: B23Q15, G05B19, B24B19