METHOD FOR GRINDING A NON-CIRCULAR WORKPIECE

Abstract: A grinding method for grinding non-circular workpiece such as a cam. The grinding wheel is moved according to the profile data defining profile generating movement of grinding wheel and cut-in feed data defining cut-in feed movement of the grinding wheel. In a part of each revolution of the workpiece, where the acceleration of the profile generating movement become zero or a negative value, the profile generating movement according to the profile data and the cut-in feed movement according to the cut-in feed data are executed simultaneously. Then, in the remainder of each revolution, the profile generating movement is executed without cut-in feed. The above-mentioned process is repeated alterna tely until the total amount of cut-in feed reaches a predetermined value.

Description: Field of the Invention The present invention relates to a method for grinding a non-circular workpiece such as a cam shaft with a numerically controlled grinding machine. 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 of 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, in-feed and retracting feed of the grinding wheel. Workpieces are ground by the profile generating movement according to the profile data and the cut-in feed according to the machining cycle data. In the grinding machine of the aforementioned type, the time relation between the cut-in feed and the profile generation movement of the grinding wheel is very important for a desired grinding accuracy and grinding speed. In a prior grinding method, rotation of the main spindle is stopped during in-feed. Namely, the profile generating movement and the in-feed of the grinding wheel are executed alternately. For example, the profile generating movement is stopped at the original angle position where the grinding point of the workpiece moves to the centre of the base portion of the cam. Then, the grinding wheel is fed to the workpiece by a predetermined in-feed amount. After completing in-feed, the profile generating movement is resumed. The above-mentioned process is successively repeated. In such grinding method, there are problems which occur regarding grinding speed and grinding accuracy. Namely, since the main spindle is stopped for in-feed during each rotation of the main spindle, the grinding speed is decreased. Furthermore, since the amount of deflection of the workpiece changes slightly between the process of in-feed and the process of profile generating movement due to stopping of the main spindle, a step is produced at the center of the base portion of the cam. Accordingly, in the prior grinding method, it is necessary to execute spark-out grinding for a long period of time so as to eliminate the step of the base portion. In another prior method, the profile generating movement and the cut-in feed are executed simultaneously (FR-A-2 475 757). Namely, the in-feed occurs continuously while the profile generation movement is executed. In such method, the grinding point of the workpiece is moved along a path of spiral shape. Accordingly, a large amount of the non-ground portion, the thickness of which is decreased in proportion to increase of rotational angle, remains when the cut-in feed is completed. Thus, it is necessary to execute spark-out grinding while the main spindle rotates more than one revolution after finishing in-feed. Since a long period of spark-out grinding is needed in above-mentioned grinding methods, it takes a long time to grind the workpiece precisely. In the case of grinding a plurality of cam shafts which have a plurality of cams, this becomes a serious problem. SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to provide an improved grinding method for grinding a non-circular workpiece capable of increasing grinding accuracy and grinding speed. Briefly, according to the present invention, there is provided a grinding method for grinding a non-circular shaped workpiece such as a cam. The grinding wheel is moved according to the profile data calculated from the final shape of the workpiece and defining a profile generating movement of the grinding wheel, and the cut-in feed data defining a in-feed movement of the grinding wheel. In a part of each revolution of the workpiece, the profile generating movement according to the profile data and cut-in feed are executed simultaneously. Then, in the remainder of each revolution of the workpiece, the profile generation movement is executed without the in-feed. The above-mentioned process is repeated alternately until the total amount of the in-feed reaches a predetermined value. In this grinding method, there is no possibility of producing a step on the surface of the workpiece because the main spindle is not stopped during grinding operation. Furthermore, it is possible to reduce the non-ground portion due to simultaneous execution of the profile generating movement and the in-feed. Accordingly, spark-out grinding can be reduced so as to shorten the machining time required. According to another aspect of the invention, the cut-in feed of the grinding wheel is executed within a part of each rotation of the workpiece where the acceleration of the profile generating movement of the grinding wheel becomes zero or a negative value. By executing in-feed within such angular position, grinding accuracy is improved. 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 an explanatory chart which explains the grinding method according to the present invention; FIGURE 2 is a schematic view of a numerically controlled grinding machine capable of grinding a non-circular workpiece by the grinding method according to the present invention; FIGURE 3 is a block diagram showing the structure of the numerical controller 30 shown in FIGURE 2; FIGURE 4 shows a numerical control program for machining a workpiece; FIGURES 5(a) and 5(b) are flow charts explaining the general operation of the main CPU 31 shown in FIGURE 3; FIGURE 6 is a flow chart illustrating the details of step 136 shown in FIGURE 5(a); FIGURE 7 is an explanatory chart which explains the positional change of the grinding wheel relating to the angle position change of the main spindle; and FIGURE 8 is an explanatory chart which explains the amount of change of the lift, velocity of the lift, and acceleration of the lift. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings and particularly, to FIGURE 1, thereof, a grinding method is illustrated for grinding a workpiece W, which is rotated about the main spindle axis O, with a grinding wheel G. The grinding wheel G is moved toward and away from the workpiece W along the X axis synchronously with the rotation of the main spindle which is rotated by an angle O. Accordingly, the grinding wheel G is moved relative to the direction C about the workpiece W. L indicates the initial locus of the centre of the grinding wheel G when the profile generating movement is executed. In the method according to the present invention, the profile generating movement is executed continuously, and the in-feed is added to the profile generating movement at a part of each revolution of the workpiece W without stopping the main spindle. Namely, the grinding wheel G is moved by cut-in feed from a point P1 on a initial locus L to a point P2 on an inner locus M during the rotational section 01 of the workpiece w. During such in-feed, the profile generating movement and the in-feed are executed simultaneously. Then, the grinding wheel is moved along the inner locus M by the profile generating movement without the cut-in feed. The above-mentioned in-feed is executed repeatedly as shown in FIGURE 1. Since the in-feed is executed within a narrow area, the non-ground portion of the workpiece after the above-mentioned grinding, which is proportional to the area A shown in FIGURE 1, becomes a small amount. Theoretically, such non-ground portion can be eliminated by extending the movement of the grinding wheel along the final locus N to the point E which corresponds to the end of the rotational section 01. Practically, the non-ground portion is ground by spark-out grinding after the above-mentioned grinding. Since the amount of the non-ground portion is small, the non-ground portion can be eliminated by very short spark-out grinding. It is to be particularly noted that the machining accuracy can be improved by executing the in-feed within an area such as the top portion of the cam where the acceleration of profile generating movement becomes zero or a negative value. Namely, if cut-in feed is executed when the grinding wheel is accelerated, the follow delay of the grinding wheel, which causes inaccuracy of positioning, is increased. Thus, it is necessary for accurate grinding to execute cut-in feed within a predetermined angle section where the acceleration of the profile generating movement becomes zero or a negative value. FIGURE 8 shows a lift of a finished cam, the velocity of the lift and the acceleration of the lift. The velocity of the lift is calculated by differentiating the amount of lift, and the acceleration of the lift is calculated by differentiating the velocity of the lift. The profile generation movement of the grinding wheel G is determined according to the amount of lift, and the acceleration of the profile generation movement is proportion to the acceleration of the lift. By experimentation it is ascertained that the machining accuracy is improved by executing the cut-in feed within an angle section where the acceleration of the lift becomes zero or a negative value (i.e.: the acceleration of the profile generating movement becomes zero or a negative value). A numerically controlled grinding machine capable of grinding a cam by the above-mentioned method is described hereinbelow. Referring now to FIGURE 2, 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 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 centre 17 of the main spindle 13 and the centre 16 of the tail stock 15. The left end of the workpiece W 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 11 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 that an advancing and retracting movement of the tool slide 20 is effected by the servomotor 23. 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. 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 profile data and machining cycle data, a keyboard 43 for inputting control data and a CRT display device 44 for displaying various information are connected to the numerical controller 30. The numerical controller 30 comprises a main central processing unit (hereafter referred to as "main CPU") 31, a read only memory (ROM) 33, in which the 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 a NC data area 321 for storing numerical control programs and a profile data area 322 for storing profile data calculated from the ideal final shape of the workpiece. The RAM 32 also has a feed mode setting area 323 and a workpiece mode setting area 324. The numerical controller 30 also comprises a drive CPU 36, a RAM 35 and a pulse distribution circuit 37 so as to distribute the 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 slowing up, slowing down and interpolation depending 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. Operation of the numerical controller 30 is described hereinbelow. Before grinding operation, an operator inputs an NC program shown in FIGURE 4 in the NC data area 321 of RAM 32 via keyboard 43 or tape reader 42. When a start switch on the control panel (not shown) is pushed by the operator, the NC program shown in FIGURE 4 is decoded block by block according to the steps specified in the flow chart shown in FIGURES 5(a) and 5(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 ends 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 in the main CPU 31 is moved 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 in the main CPU 31 is moved to step 106 so as to ascertain the command code in greater detail. In step 106 through step 126, mode flags in the mode setting area 323, 324 of the RAM 32 are set, depending on the command code. If code G00 is ascertained at step 106, the flag in the feed mode setting area 323 is set at step 108 so as to set a feed mode in a rapid feed mode. On the other hand, if code GO1 is ascertained at step 110, the flag in the feed mode setting area 323 is reset at step 112 so as to set a feed mode in a grinding feed mode. Similarly, if code G50 is ascertained at step 120, the flag in the workpiece mode setting area 324 is reset so as to set the workpiece mode in a regular mode. If code G51 is ascertained at step 124, the flag in the workpiece mode setting area 324 is set to set the workpiece mode in a cam mode. After the above-mentioned mode setting has occurred, the process of the main CPU 31 is moved to step 132 for certain steps depending on the NC program and mode. If code x is ascertained at step 130, the process of the main CPU 31 is moved to step 132, 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 136. If not, a regular pulse distribution, which is not synchronized with main spindle rotation, is executed at step 134. Since code G00 is programmed in block NO10 of the NC program shown in FIGURE 4, the feed mode is set in a rapid feed mode at step 108. Then, the grinding wheel G is moved to programmed absolute position X25.0 at step 134 in response to code X25.0 in block N10. By such rapid feed, the grinding point of the grinding wheel G reaches the grinding start point which is distanced 25.0 mm from the main spindle axis. Afterward, in response to code G51 in block N020, the workpiece mode is set in a cam mode, and one set of the profile data designated by profile number P2345 is selected at step 126. Then the feed mode is set in a grinding feed mode in response to the code G01 in block N030 at step 112, and the cam grinding procedure is completed at step 136 in response to cut-in feed data X22.5. Data with code F in block N030 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. Accordingly, if F0.25 and R1.5 are programmed as shown in FIGURE 4, the grinding wheel G is fed at 0.25 mm per revolution at the speed of 1.5 mm per revolution at every cut-in feed angle section (0 ∼ π/3, 2π∼7π/3 ...) as shown in FIGURE 7. The main spindle 13 rotates ten revolutions until the total amount of the cut-in feed reaches 2.5 mm. The cam grinding operation is accomplished by the process at step 136. The details of the process at step 136 are shown in FIGURE 6. The process is started at the time when the angle position of the workpiece W reaches the original angle position shown in FIGURE 1 where the top portion of the cam C is positioned against grinding wheel G. First, the pulse number corresponding to the in-feed amount per unit rotational angle (0.5°) is calculated depending upon the data with code R at step 200. Afterwards, the pulse-distribution-finished signal is inputed at step 202 so as to ascertain whether or not the previous pulse distribution is finished. If the previous pulse distribution is finished, it is ascertained whether or not the data read at the previous cycle is the last data of the profile data at step 204 so as to detect one revolution of the main spindle 13. If the previous data is not the last data of the profile data, the process of the main CPU 31 is moved to step 206, then the next profile data is read out from the profile data area 322 of RAM 32. The profile data consists of movement data for the grinding wheel G at each rotation of 0.5 degree. Then, it is ascertained whether or not the cut-in feed within one revolution is finished or completed at step 208. In this case, this is ascertained by detecting whether or not accumulated in-feed amount per unit angle of rotation reaches 0.25 mm. If the in-feed within one revolution is not finished, the movement data is calculated by adding the in-feed amount per unit angle to profile data at step 210. Then, the in-feed amount per unit angle of rotation is accumulated at step 212. Afterwards, the positioning data is outputed to the drive CPU 36 at step 216. On the other hand, if the in-feed within one revolution is finished, the amount of movement is calculated from only the profile data at step 214 and then the positioning data is output at step 216. Afterward, the process of the main CPU 31 returns to step 202. If it is ascertained that the data previously read is the last data of the execution profile data at step 204, it is ascertained whether or not the total in-feed amount has reached a programmed total-feed amount, which is programmed at step 218. In this case,this is ascertained by detecting whether or not the grinding point of the grinding wheel G reaches the position where it is 22.5 mm away from the main spindle axis. If the total cut-in feed amount does not reach the programmed amount, the process moves to step 206 via steps 220, 222 so as to execute again the process as mentioned above. At step 220, the read-out position of the profile data is set to the start position of the profile data so as to produce a series of positioning data in the next rotation cycle. Furthermore, the accumulated in-feed amount per unit angle is cleared at step 222. Since accumulated cut-in feed is cleared, the process is moved to step 210 vie step 206, 208. Accordingly, cut-in feed of the grinding wheel G is accomplished again. By repeating the process, mentioned above, an intermittent in-feed at the top portion of the cam as shown FIGURE 7 is accomplished. On the other hand, if the total cut-in feed amount reaches the programmed amount, the process for grinding cam according to the NC program in block N030 is finished. Referring to FIGURE 7, it is understood that the in-feed is executed during an angle of 0 ∼ π/3 radian in each rotation of the main spindle 13. In these angle sections, both the profile generating movement and the in-feed are executed, and in the remaining part of the revolution, only the profile generating movement is executed. The profile generating movement and the in-feed between point q1 and q2 are executed in control cycles which pass the step 210, and the profile generating movement between point q2 and q3 are executed in control cycles which pass the step 214. Furthermore, point q3 corresponds to step 218 and point qe corresponds to step 224. As mentioned earlier, since the in-feed is executed at the top portion of cam, where the acceleration of the profile generating movement becomes a negative value, there is no possibility of reduced machining accuracy during execution of the simultaneous in-feed movement. After the above-mentioned grinding operation is completed, short spark-out grinding is executed so as to eliminate the non-ground portion of the workpiece W. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the inventions may be practiced other than as specifically described herein.

Claims: A method for grinding a cam or like non-circular workpiece by profile generating movement of a grinding wheel according to profile data calculated from the desired final shape of said workpiece, and the simultaneous radially advancing infeed movement of said grinding wheel according to infeed data, while rotating said workpiece, characterized by: executing in a predetermined part of each revolution of said workpiece, said simultaneous profile generating movement of said grinding wheel and said infeed movement of said grinding wheel, executing only in the remaining part of each revolution of said workpiece, said profile generating movement of said grinding wheel without infeed movement, and repeating said rotating of the workpiece and moving of the grinding wheel until the total amount of infeed of said grinding wheel reaches a predetermined value. A method for grinding a cam or like non-circular workpiece as set forth in claim 1, wherein said infeed movement of said grinding wheel is executed within a predetermined part of each revolution of said workpiece, in which the acceleration of said profile generating movement of said grinding wheel becomes zero or a negative value. A method for grinding a cam or like non-circular workpiece as set forth in claim 2, further comprising: advancing and retracting said grinding wheel thereby executing only said profile generating movement of said grinding wheel without infeed movement until said workpiece rotates a predetermined number of revolutions.

Inventors: MARUYAMA, TOSHIO, NAKANO, HIROSHI, SAKAKIBARA, YASUJI
Applicants: TOYODA KOKI KABUSHIKI KAISHA
Application year: 1987
Main classification: G05B19
IPC codes: B23Q15, B24B19, G05B19