Polishing apparatus

Abstract: A polishing apparatus comprising a tool (12) for polishing a surface of a workpiece (15), a table for supporting a workpiece (15), a piezoelectric element (20) for minutely driving the table (13), a load detec­ tor (19) for detecting the load applied from the tool (12) to the workpiece (15) supported on the table (13), and a load controller (21) for controlling the drive means in accordance with the load detected by the detec­ tor. The piezoelectric element (20) moves the table (13) minutely in the same direction as, or the direction opposite to, the direction in which said tool (12) applies a load to the workpiece (15).

Description: The present invention relates to a polishing (grinding) apparatus for polishing (grinding) workpieces by means of a polishing (grinding) tool comprising the features of the preamble of claims 1 and 7, respectively. A variety of components having spherical surfaces and complex curved surfaces are used in various industrial fields. Some of them, such as optical lenses and X-ray reflectors, have high-precision curved mirror surfaces. One method of forming such mirror surfaces is the high-precision polishing method, in which a soft polishing tool made of plastic or rubber is used to polish workpieces with high precision. The polishing tool can have either a concave or a convex surface. A workpiece is placed in contact with the polishing surface of the polishing tool, and is polished thereby. Recently, an automatic high-precision polishing apparatus has been developed. This apparatus comprises an NC controller, a tool for polishing a workpiece, an electric motor for driving the tool under the control of the NC controller, and a mechanism for supporting the tool and applying a load from the work point of the tool to the surface of the workpiece, under the control of the NC controller. The NC controller controls the motor in accordance with coordinates data representing the positions which the tool must take with respect to the workpiece, thereby moving the tool to a desired position. Another polishing and grinding apparatus comrising the features of the preamble of claims 1 and 7 is known from EP-A1-0 034 659. In this apparatus the tool applies a constant load to the surface of a workpiece, whereby the constant load is achieved by fine movement of the tool spindle. The fine movement is effected by means of pressure control in a hydraulic cylinder in response to signals generated by wire strain gauges. In order to polish the workpiece uniformly over its entire surface, it is necessary for the tool to apply a constant load from its work point to the surface of the workpiece, at all times during the polishing. The tool, however, cannot be moved so minutely as to move its work point along the peaks and depressions formed in the surface of the workpiece, which have heights and depths in the order of nanometers, and inevitably fails to apply the same load to every part of the workpiece surface. The parts of the workpiece are polished with different loads, and come to have different surface roughnesses. The object of the present invention is to provide a polishing (grinding) apparatus which can apply the same load to every part of the surface a workpiece even if the surface of the workpiece is complicated curved, and which can therefor polish (grind) the workpiece with high precision. According to the invention, there is provided a polishing apparatus which comprises the features of claim 1. Preferred embodiments of the invention are described in the subclaims. There is also provided a grinding apparatus comprising the features of claim 7. The detector detects the load being applied from the tool to the workpiece and generates a signal representing this load, which is supplied to the controller. The controller controls the element in accordance with the load represented by the signal, and the element moves the tool in the same direction as, or the direction opposite to, the direction in which the tool applies the load to the workpiece, the load applied to the workpiece changes to a prescribed value. In other words, the heights of the peaks formed on, and the depths of the depressions formed in, the surface of the workpiece are detected in terms of changes in the load detected by the detector, and the table is moved in accordance with these changes. Hence, the tool applies the same load to every part of the surface of the workpiece, polishing the workpiece with high precision. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a plan view illustrating a polishing apparatus according to a first embodiment of the present invention; Fig. 2 is a diagram showing, in detail, the table incorporated in the apparatus illustrated in Fig. 1; Figs. 3 and 4 show the waveforms of various signals used in the apparatus, explaining the operation of the apparatus; and Fig. 5 is a front view showing a grinding apparatus, which is a second embodiment of the invention. An embodiment of the present invention, which is a polishing apparatus, will now be described with reference to the accompanying drawings. As is shown in Fig. 1, the polishing apparatus comprises a polishing mechanism 1, a data buffer 2, and a personal computer 3. The mechanism 1 is designed to polish workpieces and is connected to the data buffer 2. The data buffer 2 is connected to the personal computer 3. The computer 3 has a memory storing numerical data for controlling the polishing mechanism, and can convert the numerical data to coordinates data. The data buffer 2 temporarily stores the coordinates data output by the personal computer 3. The polishing mechanism 1 comprises a movable stage 10, a bearing 11, a polishing tool 12, a movable table 13, a holder 14, and a pipe 16. The tool 12 is supported by the bearing 11 and connected to an electric motor (not shown) located above the movable stage 10. The table 13 is attached to the top of the stage 10. The holder 14 is fixed to the table 13, for holding a workpiece 15. The pipe 16 extends downward and slantwise to the holder 14, for supplying abrasive to the workpiece 15 held by the holder 14. The movable stage 10 can move in a horizontal plane, in the X-axis direction and the Y-axis direction, as it is driven by an electric motor (not shown) in accordance with the coordinate data stored in the data buffer 2. The polishing tool 12 is what is generally known as "polisher," made of soft material such as pitch, plastics, or rubber. The tool 12 can move up and down together with the bearing 11, and can also rotate in the direction of the arrow shown in Fig. 1. As Fig. 2 shows, the table 13 comprises two parallel plates 13a made of, for example, stainless steel and located one above the other, and two side plates 13b, each connecting the ends of the plates 13a. The plates 13a and 13b form a trapezoidal frame. The first side plates 13b is fastened to the stage 10. The table 13 further comprises a load-magnifying plate 13c which is made of the same material as the plates 13a, is located between the plates 13a, and is fastened at one end to the first side plate. Each plate 13a has two grooves 13d cut in both surfaces of the same portion, so that this portion of the plate 13a functions as a spring. Due to the spring portions the plates 13a, the table 13 can move minutely up and down, or in the directions the tool 12 is moved. When the table 13 minutely moves up or down, the holder 14, which is fixed to the table 13, also moves minutely up or down. As is shown in Fig. 1, a ball 17 is interposed between the upper plate 13a and the load-magnifying plate 13c, and a projection 18 protrudes downwards from the lower surface of the plate 13c. The ball 17 point-contacts the load-magnifying plate 13c and transmits the movement of the upper plate 13a to the plate 13c. The projection 18 has a rectangular cross section. The polishing mechanism 1 further comprises a load cell 19 and a piezoelectric ceramic member 20. As is shown in Fig. 1, the load cell 19 and the member 20 are connected, at one end, to each other and located in the gap between the lower plate 13a and the load-magnifying plate 13c. The other end of the load cell 19 is fastened to the second side plate 13b, and the other end of the piezoelectric ceramic member 20 is connected to one side of the projection 18 in order to move the load-magnifying plate 13c minutely. Hence, a load applied from the tool 12 to the workpiece 15 held by the holder 14 is transmitted to the load cell 19 via the holder 14, the upper plate 13a, the ball 17, the load-magnifying plate 13c, the projection 18, and the piezoelectric ceramic member 20. The pipe 16 is used to supply abrasive onto the surface of the workpiece 15. The abrasive is, for example, oil or aqueous solution containing particles of diamond, silicon carbide, cerium oxide (CeO₂). As is shown in Fig. 1, the polishing apparatus further comprises a polishing-load controller 21 which is designed to control the piezoelectric ceramic member 20 in accordance with the polishing load detected by the load cell 19. This circuit comprises a comparator circuit 22, a DC power supply 23, a proportional-plus-integral circuit 24, and a drive circuit 25. The power supply 23 applies a reference voltage V₂ which corresponds to a desired polishing load to be applied to the workpiece 15. The comparator circuit 22 compares the voltage V₁ output by the load cell 19 with a reference voltage V₂ applied from a DC power supply 23, generating a difference signal representing the difference between the voltages V₁ and V₂. The proportional-plus-integral circuit 24 performs proportional-plus-integral operation on the difference signals generated by the comparator circuit 22, and generates a signal representing the results of this operation. The drive circuit 25 converts the output signal of the circuit 24 to a drive voltage V₃, which is applied to the piezoelectric ceramic member 20. It will now be explained how the polishing apparatus operates. First, the tool 12 is positioned relative to the workpiece 15 held by the holder 14. Then, the personal computer 3 converts the numerical data required for polishing the workpiece 15, into the coordinates data required for driving the polishing mechanism 1. The coordinate data is stored into the data buffer 2. Thereafter, when an operator supplies a drive command to the polishing mechanism 1, the coordinates data is supplied to the mechanism 1 from the data buffer 2. The tool 12 is rotated and lowered until it contacts the workpiece 15. The stage 10 is moved in the X-axis direction and the Y-axis direction in accordance with the coordinate data. In the meantime, the abrasive is applied through the pipe 16 to the workpiece 15. Thus, the rotating tool 12 polishes the workpiece 15. The load the tool 12 applies to the workpiece 15 is hence applied to the load cell 19 through the holder 14, the upper plate 13a, and the load-magnifying plate 13c, the piezoelectric ceramic member 20. The load cell 19 generates a voltage V₁ which changes with the load applied from the tool 12 to the workpiece 15 as is shown in Fig. 3. The comparator circuit 22 compares the voltage V₁ with the reference voltage V₂, and generates a signal showing the difference between these voltages, i.e., V₁ - V₂. The difference signal is input to the proportional-plus-integral circuit 24. The circuit 24 processes the difference signal into a voltage signal which cancels out the difference V₁ - V₂. This voltage signal is supplied to the drive circuit 25. The circuit 25 converts the voltage signal to a drive voltage V₃. The drive voltage V₃ is applied to the piezoelectric ceramic member 20. As a result, the piezoelectric ceramic member 20 contracts in its lengthwise direction, in accordance with the drive voltage V₃. The difference V₁ - V₂ increases as the load applied to the workpiece 15 increases, as is illustrated in Fig. 3. Therefore, the drive voltage V₃ output by the drive circuit 25 increases, and the piezoelectric ceramic member 20 further contracts in its lengthwise direction. Then, the load-magnifying plate 13c is bent in the direction of the arrow shown in Fig. 1, whereby the ball 17 moves downward, and so does the upper plate 13a of the table 13. As a result, the load applied to the workpiece 15 from the tool 12 decreases to the desired value. When the tool 12 moves in contact with a stepped portion, if any, of the workpiece 15, the signal output from the load cell 19 and that of the signal input to the piezoelectric ceramic member 20 changes as is illustrated in Fig. 4. In other words, the load the tool 12 applies to the workpiece 15 changes as the tool 12 moves in contact with the stepped portion, the load cell 19 responds to the change in the polishing load, and a signal representing this change is supplied to the ceramic member 20 through the comparator circuit 22, the proportional-plus-integral circuit 24, and the drive circuit 25. As a result of this, the polishing load applied to the workpiece 15 from the tool 12 is automatically changed to the desired value. The table 13 thereby moves up and down, moving the tool 12 such that the work point thereof minutely moves along the complex curved surface of the workpiece 15. The tool 12, thus moved minutely, polishes the workpiece 15 with high precision. As has been described, in the first embodiment of the invention, the piezoelectric ceramic member 20 is driven in accordance with the difference between the desired polishing load and the polishing load being applied from the tool 12 to the workpiece 15, thereby minutely moving the table 13 in the direction identical or opposite to the direction in which the tool 12 applies the load to the workpiece 15. Hence, the tool 12 applies the desired polishing load to the workpiece 15. In other words, since the table 13 moves up and down, thus moving the work point of the tool 12 along the peaks and depressions, if any, formed in the surface of the workpiece 15, whereby the tool 12 polishes the workpiece 15 with high precision. The changes in the load applied from the tool 12 to the workpiece 15, even if very small, can be detected with high accuracy since the polishing load is applied from the workpiece 15 directly to the table 13, then to the ceramic member 20, and further to the load cell 19. The signal output by the load cell 19 and representing the polishing load is supplied, as a control signal, to the piezoelectric ceramic member 20 through the polishing-load controller 21, whereby the tool 12 applies the desired polishing load to every part of the surface of the workpiece, polishing the workpiece with high precision in the order of nanometers. Fig. 5 illustrates a grinding apparatus, which is a second embodiment of the invention. In this figure, the same reference numerals are used to designate the same components as those shown in Fig. 1. As may be understood from Fig. 5, the grinding apparatus is identical to the apparatus shown in Fig. 1, except for the following points. As is shown in Fig. 5, a bearing 33 is coupled to an electric motor (not shown) located above a workpiece 32. A cup-shaped grinding tool 34 is attached to the bearing 33. A grinding stone 35 is fastened to the tool 34. In operation, the grinding tool 34 applies a grinding load to the workpiece 32. In accordance with the grading load, a piezoelectric ceramic member 20 expands or contracts, thereby minutely moving a table 13 up or down, that is, in the direction opposite or identical to the direction in which the tool 34 is applying the grinding load to the workpiece 32. As a result of this, the load applied from the tool 34 to the workpiece 32 is changed to a predetermined, desired value. The present invention is not limited to the embodiments described above. Changes and modifications may, therefore, be made without departing from the scope of the appended claims. For instance, the load cell 19 can be replaced by a strain gauge. As has been described, the polishing apparatus according to the invention has a polishing tool, a table for holding a workpiece, a element for moving the table minutely, substantially in parallel to the direction identical or opposite to the direction in which the tool applies a load to a workpiece held by the table, and a detector for detecting the polishing load applied from the tool to the workpiece. The element is controlled in real time, in accordance with the load detected by the detector, thereby moving the table minutely such that the work point of the tool moves along the curved surface of the workpiece. As a result, the workpiece is polished with high precision.

Claims: A polishing apparatus comprising a tool (12) for polishing a surface of a workpiece (15); a table (13) for supporting the workpiece (15), load-detecting means (19) for detecting the load applied from said tool (12) to said workpiece (15) and generating an electrical signal representing the load; and load-controlling means (21); characterized in that said table (13) is minutely movable in the same direction as, or the direction opposite to, the direction in which said tool (12) applies a load to the workpiece (15) and comprises: a substantially trapezoidal frame comprised of upper and lower plates (13a) each having first and second ends and functioning as a spring, and two side plates (13b), one interposed between the first ends of the upper and lower plates (13a), and the other interposed between the second ends of said upper and lower plates (13a); a load-magnifying plate (13c) located below said upper plate (13a) and functioning as a spring; and a ball (17) interposed between said upper plate (13a) and said load-magnifying plate (13c) and point-contacting both said upper plate (13a) and said load-magnifying plate (13c); there is provided an electromechanical transducer means (20) connected to said table (13), for minutely moving said table (13) in accordance with an electrical signal; and said load-controlling means is controlling said electromechanical transducer (20) means in accordance with the electrical signal generated by the load-detecting means (19). The polishing apparatus according to claim 1, characterized in that said upper plate (13a), said lower plate (13a), and said load-magnifying plate (13c) have grooves, thereby functioning as springs. The polishing apparatus according to claim 1, characterized in that said electromechanical transducer means (20) is a piezoelectric element connected to said load-magnifying plate (13c) and said load-detecting means (19). The polishing apparatus according to claim 3, characterized in that said load-detecting means (19) is a load cell connected to said piezoelectric element (20). The polishing apparatus according to claim 3, characterized in that said load-detecting means (19) is a load cell. The polishing apparatus according to claim 1, characterized in that said load-controlling means (21) comprises a comparator circuit (22) for comparing a prescribed load with the load detected by said load-detecting means (19), and generating a difference signal representing a difference between the loads, a proportional-plus-integral circuit (24) for performing a proportional-plus-integral operation on the difference signal and generating an integration signal, and a drive circuit (25) for drive said electromechanical transducer means (20) in accordance with the integration signal. A grinding apparatus comprising a tool (34) for grinding a surface of a workpiece (32); a table (13) for supporting the workpiece (32); load-detecting means (19) for detecting the load applied from said tool (34) to said workpiece (35); and load-controlling means (21); characterized in that said table (13) is minutely movable in the same direction as, or the direction opposite to, the direction in which said tool (34) applies a load to the workpiece (32); and comprises: a substantially trapezoidal frame comprised of upper and lower plates (13a) each having first and second ends and functioning as a spring, and two side plates (13b), one interposed between the first ends of the upper and lower plates (13a), and the other interposed between the second ends of said upper and lower plates (13a); a load-magnifying plate (13c) located below said upper plate (13a) and functioning as a spring; and a ball (17) interposed between said upper plate (13a) and said load-magnifying plate (13c) and point-contacting both said upper plate (13a) and said load-magnifying plate (13c); there is provided an electromechanical transducer means (20) connected to said table (13), for minutely moving said table (13) in accordance with an electrical signal; and said load-controlling means (21) is controlling said electromechanical transducer means (20) in accordance with the electrical signal generated by the load-detecting means (19).

Inventors: TAKAMATSU HIROSHI C O INTELLEC, UEDA KATSUNOBU C O INTELLECTUA, TAKAMATSU, HIROSHI, UEDA, KATSUNOBU, TAKAMATSU, HIROSHI, C/O INTELLECTUAL PROPERTY, UEDA, KATSUNOBU, C/O INTELLECTUAL PROPERTY
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: B24B49
IPC codes: B24B37, B24B49, B24B13