IMPROVED GRINDING MACHINE

Abstract: A grinding machine, in particular a face grinder for grinding the faces of silicon wafers, has a grinding wheel spindle (20) and a work spindle (22) which are mounted so as to be pivotal about axes (A and B) respectively, each axis being perpendicular to the rotational axis of its respective spindle and being orthogonal to the rotation axis of the other spindle. Three sensors or probes (1 to 3) are provided to measure the position of one spindle relative to the other and to generate signals for controlling servo motors (26, 28) for adjusting the angular orientation of each spindle about its pivotal axis.

Description: Title : Improved grinding machine Field of invention This invention concerns grinding machines and in particular a mechanism for controlling wheel infeed in dependence on angular orientation between work spindle and wheel spindle axes . Background to the invention The angular relationship between cwo such spindles can be affected by forces produced by grinding, particularly during face grinding, and this can introduce inaccuracies in the grinding process . Obj ect of the invention It is an obj ect of the present invention to provide a means to control precisely the angular orientation between two such spindles , so as to set and maintain the desired angular relationship between the two spindles and thereby increase the accuracy of the subsequent grinding process . Summary of the invention According to one aspect of the present invention there is provided a grinding machine comprising a workpiece spindle , a grinding wheel spindle , first mounting means for mounting one of the spindles for pivotal movement about a first axis which is perpendicular to the rotational axis of said one spindle , a f irst servo motor operable to position said one spindle about the first ax s , and sensors which enable the position of said one spindle to be accurately determined relative to the other spindle by generating control signals for controlling the first servo motor . Advantageously the machine may further comprise second mounting means for mounting either one of the spindles for pivotal movement about a second axis which is orthogonal to the first axis, and a second servo motor operable to position said either one of the spindles about the second axis, said sensors likewise controlling the second servo motor. The second mounting means and the second servo motor may then be provided on the other spindle. Preferably displacement sensors are employed. Preferably the information obtained from the sensors is such as to allow the angle between the two spindles to be accurately determined. The sensors may be positioned at three discrete points with respect to the spindles. The measurements at the three discrete points are preferably made continuously; alternatively they may be made on a repetitive basis. Measurements may be made at more than three positions. It is possible to use relative changes in the measurements to calculate a change in angle between the two spindles and thereby allow the control signals to be computed for adjusting one or both of the servo motors to compensate for the movement which has been detected and reposition the spindle axis so as to maintain the desired relationship between the two spindle axes . Angular movement of either spindle about its axis can be used to derive the change in linear distance between the two spindles resulting from the angular movement, and wheel feed control signals can be adjusted or corrected to compensate for any noted linear movement between the two spindles so as thereby to control precisely the infeed position between wheel spindle and work spindle. In one arrangement each spindle assembly is supported in trunnion bearings mounted within a support so as to be pivotable about one of the said two orthogonal axes . In an alternative arrangement each spindle assembly is supported by flexures which respectively define one of the two orthogonal axes about which the spindles pivot . One of the orthogonal pivoting axes may be vertical and the other horizontal. The invention is of particular application to a face grinder in which the wheel spindle and workpiece spindle are parallel and three probes are mounted so as to determine the distance between two housings the one containing the grinding spindle and the other containing the work spindle, each probe having associated therewith a sensor for generating a signal indicative of the distance between the two housings at the positions of the probes. Distance proportional signals therefore are digitised and the resulting digital signals are supplied to digital computing means programmed to compute from the digital signals the relative angular movement if any, between the two spindles as indicated by the signals from the probes, and to generate control signals for adjusting the servo motor associated with one or both of the two spindles to correct for any angular displacement detected, and further programmed to compute from the digital information from the sensors and/or from the control signals derived there from for controlling the servo motor or motors, correcting signals for adjustment of the wheelfeed control signals for adjusting the infeed position between wheel spindle and work spindle to compensate for any shift in the relative position of the two spindles . The signals from the sensors need not necessarily be digital, and could instead be analogue signals, with appropriate gain modifiers . The invention also lies in a grinding machine when fitted with pivoting spindles, servo motors and sensors as aforesaid in combination with signal processing and/or computing means for adjusting the angular positions of the spindles and if necessary the feed control signals, to improve the grinding accuracy of the machine. Brief description of the drawings An embodiment of the invention is illustrated, by way of example only, in the accompanying drawings in which: Figure 1 is a plan view of a face grinding machine; Figure 2 is an end view of the machine shown in Figure 1; and Figure 3 is a side view ' thereof . The viewing directions are shown by the arrows II and III in Figure 1. Detailed description of embodiment Referring to Figure 1, the grinding machine comprises a base 10, a track 12 along which a grinding spindle support 14 can slide and a track 16 along which a support 6 for a workpiece spindle can slide. The grinding spindle assembly is shown at 20 and the work spindle assembly at 22, and the pivoting axis of the grinding spindle is shown at axis A and the orthogonal axis about which the work spindle can pivot is shown at B. Three probes, each incorporating a sensor, are shown in the drawings. Probes 1 and 2 can be seen in Figure 1, and probe 3 which is hidden by probe 1 in the plan view in Figure 1 can be seen in Figures 2 and 3. The three probes 1, 2 and 3 are respectively mounted at the corners of a triangular bracket 23 secured to the body of the work spindle 22, and they act against anvils (not shown) mounted on a corresponding bracket 25 secured to the body of the grinding spindle 20. However, the probes may alternatively be mounted on the bracket 25, and vice versa the anvils are mounted on the bracket 23. The probes lie outside the rotation zone of both the work wheel and the cup grinding wheel (not shown) . They may be of any type capable of measuring distance and providing an electric signal output, eg linear displacement probe, capacitive gauge, inductive gauge, air gauge, Linear Variable Differential Transformer (LVDT) , or laser interferometer. In the example shown, the probes are linear displacement probes containing glass scale encoders. A preferred proprietary probe is the Certo range type C60M made by Dr Johannes Heidenhain GmbH of D- 83301 Traunreut, Germany. The infeed axis is identified in Figure 3 at 24. A servo motor for adjusting the work spindle assembly about axis B is denoted by 26 and a servo motor for adjusting the grinding spindle assembly about axis A is shown at 28. However, any of a variety of actuators or prime movers may be utilised in place of servo motors. The control electronics is not shown in detail, nor is the control for the motors associated with the grinding and work spindles contained within the assemblies 20 and 22. Axes A and B are defined by stub-shafts extending from either side of the assemblies 20 and 22 respectively and which are carried in aligned bearings (not shown) fitted into the upper ends of support members 30 and 32 in the case of axis B, and into support numbers 34 and 36 in the case of axis A. Linear ball screws providing lever-actuation about pivots, usually stub-shafts on bearings may be used. In the example shown, axis A is vertical and axis B is horizontal . Furthermore, in the example shown, probes 1 and 3 are mounted vertically above and below the axis of the work spindle and probe 2 is mounted midway between the two probes 1 and 3 but displaced in a horizontal plane containing the axis of the work spindle . In use, the servo motors 26 and 28 move their respective assemblies about the axes A and B in order to maintain the signal from each probe at a constant value. Thus the movement about axis B is controlled by the difference in the output signals between probe 1 and probe 2, while the movement about the axis A is controlled by the difference in the output signal between probe 2 and that -of the average of probes 1 and 3. The linear infeed of the cup grinding wheel is controlled by the average of all three probes. Where the probes are not symmetrically disposed around the wheel, each probe reading has to be adjusted by a gain modifier. In the illustrated embodiment, either spindle could be the grinding spindle, the other in each case being the work spindle . In a modified arrangement (not shown) one of the spindles may be fixed, while the other spindle is supported within a double gimbal device so as to be able to be pivoted by respective servo motors about the two orthogonal axes . Furthermore, in some applications the machine may be arranged to be relatively stiff about one of the orthogonal axes, so that angular errors or inaccuracies only require to be corrected in the other axis. In such cases it may be sufficient for one of the spindles to be pivoted only about such other axis, and for the other spindle to be relatively fixed. The servo motor may be replaced, for example by on-axis servo motors, off-axis servo motors through gears, ball screws, hydrostatic screws, air cylinders, hydraulic drives, linear motors, piezo stacks, poisson-pushers or thermal displacement drives . The pivot axes may, for example be real, flexured, or generated by means of two linears and a rotary, or by tripod or hexapod legs, or any combination thereof. A particular application of the present invention is in the grinding of the faces of silicon wafers. It will be appreciated that in a conventional arrangement even a small change in angle between the spindle axes of the grinding wheel and the workpiece component results in the component face being ground incorrectly, either slightly in a conical form or conversely in a slightly "gothic arch" form, or a combination of the two forms. Since all grinding machines inherently have a compliance between the wheel and the component, the resulting change in angle between axes, if uncorrected, must inevitably produce such a form error. However, by measuring this change in angle in accordance with the invention, as above described, and using this measurement to control the correction of the angle, so the form error can at least be significantly reduced. Such reduction in error is limited only by the accuracy of measurement and by the bandwidth of the control loop, and is independent of the stiffness of the machine.

Claims: CLAIMS 1. A grinding machine comprising a workpiece spindle, a grinding wheel spindle, first mounting means for mounting one of the spindles for pivotal movement about a first axis which is perpendicular to the rotational axis of said one spindle, a first servo motor operable to position, said one spindle about the first axis, and sensors which enable the position of said one spindle to be accurately determined relative to the other spindle by generating control signals for controlling the first servo motor. 2. A machine as claimed in claim 1 further comprising second mounting means for mounting either one of the spindles for pivotal movement about a second axis which is orthogonal to the first axis, and a second servo motor operable to position said either one of the spindles about the second axis, said sensors likewise controlling the second servo motor. 3. A machine as claimed in claim 2 in which the second mounting means and the second servo motor are provided on the other spindle . 4. A machine as claimed in any one preceding claim in which the sensors are displacement sensors. 5. A machine as claimed in any one preceding claim in which the sensors provide information which allows the angle between the two spindles to be accurately determined. 6. A machine as claimed in any one preceding claim in which the sensors are positioned at three discrete points with respect to the spindles. 7. A machine as claimed in claim 6 in which continuous measurements are made at the three discrete points. 8. A machine as claimed in claim 6 in which that the measurements at the three discrete points are made on a repetitive basis. 9. A machine as claimed in any one of claims 1 to 5 in which measurements are made at more than three positions. 10. A machine as claimed in any one preceding claim in which relative changes in the measurements are employed to calculate a change in angle between the two spindles whereby control signals are computed for adjusting one or both of the servo motors to compensate for the movement which has been detected, and to reposition the spindle axis so as to maintain the desired relationship between the two spindle axes. 11. A machine as claimed in any one of claims 3 to 9 in which angular movement of either spindle about its axis is employed to derive the change in the near distance between the two spindles resulting from the angular movement, and in which wheel feed control signals are adjusted or corrected to compensate for any noted linear movement between the two spindles so as thereby to control precisely the infeed position between wheelfeed spindle and the work spindle. 12. A machine as claimed in any one of claims 3 to 11 in which each spindle assembly is supported in trunnion bearings mounted within a support so as to be pivotable about the respective first or second axes. 13. A machine as claimed in any one of claims 3 to 11, in which each spindle assembly is supported by flexures which respectively define one of the two orthogonal axes about which the spindles pivot . 14. A machine as claimed in any one of claims 2 to 13 in which the first axis is vertical, and the second axis is horizontal. 15. A face grinding machine in which a grinding spindle and a workpiece spindle are carried by respective housings, the workpiece and wheel spindle axes are parallel and three probes are mounted for determining the distance between the two housings, each probe having associated therewith a sensor for generating a signal indicative of the distance between the two housings at the positions of the probes. 16. A machine as claimed in claim 15 in which fields proportional to the distance are digitised and the resulting signals are supplied to digital computing means programmed to compute from the digital signals any relative angular movement between the two spindles, as indicated by the signals from probes, and to generate control signals for adjusting a servo motor associated with one or both of the housings containing the spindles to correct for any angular displacement detected, and further programmed to compute from the digital information from the sensors and/or from the control signals derived there from, for controlling the servo motor or motors, correcting signals for adjustment to the wheelfeed control signals for adjusting the infeed position between wheel spindle and workpiece spindle to compensate for any shift in the relative position of the two spindles . 17. A machine as claimed in claim 15 in which the signals from sensors are analogue signals and appropriate gain modifiers are provided. 18. A machine fitted with pivoting spindles and servo motors and sensors as claimed in any one of claims 3 to 17, in combination with signal processing and computing means for processing signals from the sensors to control the servo motors and thereby to reduce errors during grinding due to misalignment of the spindles . 19. A machine as claimed in claim 18 in which processed and computed signals adjust the feed control signals in addition to the angular positions of the spindles. 20. A grinding machine as claimed herein, constructed, arranged and adapted to operate substantially as herein described with reference to, and as illustrated in, the accompanying drawings.

Inventors: BENT RUSSELL GRAHAM, BENT, RUSSELL, GRAHAM
Applicants: BENT RUSSELL GRAHAM, UNOVA UK LTD, BENT, RUSSELL, GRAHAM, UNOVA U.K. LIMITED
Application year: 1998
Main classification: B23Q17
IPC codes: B24B7, B24B27, B24B41, B24B49