GB2135607A - Precision bevel grinder - Google Patents

Abstract

A precision bevel grinder has a grinding tool mount 11 provided on a base 10 of the grinder at an angle of inclination to the base. The grinding tool mount 11 supports a face grinding tool 23 by means of hydrostatic bearings. The base 10 is provided with an X-axis feed saddle 14 and a Y-axis feed table 17, which are slidably supported by hydrostatic bearings and are driven by respective hydrostatic pressure feed screws 16 and (in Figure 6) 18. As a consequence, the precision bevel grinder is capable of preventing the occurrence of grinding error as would otherwise be caused by vibrations and the like, and of carrying out grinding with high precision. <IMAGE>

Description

SPECIFICATION Precision bevel grinder This invention relates to a bevel grinder suitable for grinding the beveled surface of a hard, brittle material and, more particularly, to a precision bevel grinder suitable for grinding the beveled surface contiguous with the track surface of a magnetic head for a magnetic disk unit.

Figure 1 of the drawing shows the configuration of a ferrite core 1 of a magnetic head. In the fabrication of a magnetic head employing such a ferrite-core there is required a level of grinding precision and quality considerably higher than in general applications.

This can be attributed to the following factors: 1) The surface to be machined consists of a hard, brittle material like ferrite.

2) The track width H determined by the two beveled surfaces 3 contiguous with the track surface 2 is very small and must be machined with high dimensional precision and within very strict tolerance limits.

3) The finished surface must be of high quality.

This type oF machining work is generally carried out by grinding or lapping. Because of the brittle nature of the material that has to be machined, however, even the slightest vibration of the grinder etc. used for the operation is liable to cause chipping of the machined surface and it is thus necessary to assure high precision rotation of the grinding tool and to feed the work with utmost smoothness free from the influence of any irregularities on the guide surfaces.

The general characteristics of face grinding are as follows: 1) The driving power applied to rotate the grinding tool acts in the radial direction of the tool spindle and therefore has little effect in the axial direction. Therefore, since little vibration is generated at the grinding surface of the grinding tool, it is possible to obtain a finished surface of high quality.

2) The number of passes of the grinding tool over a given unit area of the surface being ground is small so that a high-quality finished surface can be obtained simply by reducing the rate of feed of the workpiece.

For a bevel grinding operation there has been conventionally used a grinder comprising a horizontally or vertically oriented grinding tool and a control system capable of precisely controlling the vertical position of the grinding tool (grinding wheel) relative to the workpiece and of precisely feeding an X-Y feed mechanism loaded with the workpiece along the X and Y axes and the grinding operation has been conducted with the workpiece held at a prescribed angle relative to the grinding surface of the grinding tool by a workpiece clamping device mounted on the X-Y feed mechanism.

In the ordinary grinder, as guiding of the X-Y feed mechanism is conducted by contact of rollers or sliding surfaces with guide surfaces, it is impossible to consistently realize precision positioning because of vibration occurring between the guide surfaces and the rollers or because of the sticking and slipping of the sliding surfaces and changes in the state of lubrication.

Moreover, since the ball screw ordinarily used for precision positioning has poor damping characteristics and is also susceptible to vibration at the region of contact between the screw and the roller, it has been difficult to obtain a finished surface of high accuracy with such an arrangement. Further, since the bearings for supporting the spindle of the grinding tool have been of the sliding or roller type and have, therefore, been liable to minute vibrations as mentioned in the foregoing, there has been a tendency for chipping to occur easily in the case of grinding a brittle workpiece.

Also, in a grinder with longitudinal and cross feeding, it is necessary not only to control the feed of the table for supporting the workpiece along the X and Y axes but also to control the feed of the grinding tool in the vertical direction.

Generally speaking, it is difficult to consistently realize precision feeding in the vertical direction because the feed mechanism is subject to the influence of the weight of the grinding tool, its shaft and other parts of the feed system as well as to any play existing among the members of this system.

Figure 2 of the drawings shows the relationship between the conventional feed mechanism and the workpiece being fed thereby.

In order to machine a track surface of the required configurational precision the workpiece is brought into the proper positional relationship to the grinding tool by moving the feed table along the Y axis and vertically feeding the grinding tool. With this arrangement, however, no matter how accurately the positioning is made along the Y axis, a positioning error will thereafter be introduced by the vertical feeding of the grinding tool and this error will be reflected as an error in machining precision. Moreover, after the surface 7a of the workpiece 7 has been machined with the feed table 5 in the position as shown in Figure 2, in order to machine the next surface 7b, it is necessary to bring the grinding tool 8 to the proper position by moving it laterally by the distance Y1 and vertically to the height h2.At this time, even if the feed table 5 is fed by the distance Y1 as measured on the linear scale 6, any error introduced by irregularities in the sliding surfaces or by play in the system will be magnified by the amount of the difference in height (h2-h1), making it impossible to carry out high precision machining. From this it will be understood that any attempt to increase the number of workpieces held at an angle in this manner will only serve to increase the scatter in machining precision among the workpieces machined Naturally, therefore, there is a limit to the number of workpieces that can be handled at one time so that the operational efficiency is low.

An object of the present invention is to provide a precision bevel grinder in which the aforesaid disadvantages of conventional grinders are reduced or overcome and, more particularly, to provide such a grinder which is capable of grinding the beveled surface of a hard, brittle material at high precision without causing chipping thereof. Another object of the invention is to provide a precision bevel grinder for grinding the beveled surface of a magnetic head.

In accordance with the present invention there is provided a precision bevel grinder comprising a base, a grinding tool mount provided on said base at an angle of inclination thereto, a face grinding tool fitted on a grinding tool spindle, said spindle being supported on said grinding tool mount by hydrostatic bearings, an X-axis feed saddle driven by a hydrostatic pressure feed screw and guided by X-axis hydrostatic pressure guide surfaces formed in said base, a Y-axis feed table driven by a hydrostatic pressure feed screw and engaged with Y-axis hydrostatic pressure guide surfaces formed in said X-axis feed saddle, and a workpiece clamping device fixed on the upper surface of said Y-axis feed table.

Thus, in addition to taking advantage of the general characteristics of the conventional face grinder, the precision bevel grinder according to the present invention makes it possible to provide the required high quality and configurational precision of the ground surface by supporting the spindle of a face grinder tool by means of hydrostatic bearings so as to permit highprecision rotation of the grinding tool, and employing hydrostatic pressure guide means and hydrostatic pressure feed screws in the X-axis and Y-axis feed mechanisms so as to permit highprecision positional control.

An embodiment of the invention will now be described by way of example with reference to Figures 3 to 10 of the drawings, wherein: Figure 3 is perspective view showing the overall construction of the precision bevel grinder embodying the present invention; Figure 4 is a left side view of the grinder shown in Figure 3; Figure 5 is a cross-sectional view showing a portion of the spindle of a grinding tool of the grinder; Figure 6 is a perspective view of a portion of an X-Y feed mechanism of the grinder; Figure 7(A) is a cross-sectional view showing a portion of a hydrostatic pressure feed screw of the grinder; Figure 7(B) is a cross-sectional view showing a portion of a hydrostatic pressure nut of the grinder; Figure 7(C) is a cross-sectional view showing a portion of a hydrostatic pressure table of the grinder;; Figure 8 is a perspective view of a rack for mounting workpieces on the grinder; Figure 9 is a perspective view of a ferrite core blank; and Figures 10(A) and (B) are explanatory diagrams showing grinding being conducted on the grinder of Figure 3.

An embodiment of the precision bevel grinder according to this invention is illustrated in Figures 3 and 4. This embodiment has a base 10 on an upper flat surface 1 2 of which there is fixed a grinding tool mount 11. The base 10 is further provided with X-axis guide sections 13a, 13b having engaged therein an X-axis feed saddle 14 which is driven along the X axis by an X-axis hydrostatic pressure feed screw 16 which rotates upon the receipt of command signals from a controller 1 5. The upper surface of the X-axis feed saddle 14 is provided with Y-axis guide sections which will be described later. Engaged with these Y-axis guide sections is a Y-axis feed table 17 which is driven to slide along the Y axis by the rotation of a hydrostatic pressure feed screw to be described later.On one side face of the Y-axis feed table 1 7 is provided a linear scale 19 adapted to send one pulse to the controller 15 each time the Y-axis feed table moves by a distance of 0.1 micron. The controller 15 thus receives a number of pulses proportional to the distance traveled by the Y-axis feed table.

On the top surface of the Y-axis feed table 1 7 there is fixed a set jig 21 for holding a plural number of workpieces 20 (Figure 4).

As shown in Figure 4, the grinding tool mount 11 is provided at a prescribed angle a with respect to the top surface of the workpieces 20 (this angle being matched to the angle of the beveled surface of a ferrite core in this embodiment) and supports by means of hydrostatic bearings a grinding tool spindle 22 to which there is attached a grinding tool 23. Near the grinding tool 23 is provided a microscope unit 24 for observing the ground surface. By reference numeral 25 (Figure 3) is denoted a dresser for dressing the grinding tool 23 while by numeral 26 is denoted a drive motor for rotating the grinding tool spindle 22. Although not shown, there is further provided a hydraulic unit for providing the hydrostatic pressure feed screws with hydraulic pressure.

Figure 5 shows the structure of the bearings of the grinding tool spindle 22. The tool spindle 22 has a pulley 27 at one end and a shaft 28 for the attachment of the grinding tool 23 at the other and is supported on the inclined grinding tool mount 11 by a pair of hydrostatic bearings constituted of a radial bearing 31 and a thrust bearing 30. The pulley 27 and the drive motor 26 are linked by a belt 29. In the illustrated embodiment the thrust bearing 30 is constituted of a hydrostatic pressure multipad so that thermal dislocation in the thrust direction can be minimized in spite of the fact that the thrust bearing 30 is provided as far toward the grinding tool side as possible. By employing hydrostatic bearings for the radial bearing 31 and the thrust bearing 30 in this way it is possible to realize a vibration-free grinding tool spindle having high rotational precision.

The radial bearing 31 is provided with a pair of hydrostatic pressure pockets 31 a, 31 b about its inner periphery, whereas the thrust bearing 30 is provided with a pair of static pressure pockets 30a, 30b located so as to sandwich a flange 22a of the grinding tool spindle 22. The aforesaid hydrostatic pressure pockets 31 a, 31 b and 30a, 30b are connected to a hydraulic unit (not shown) by hydraulic pressure lines 32a, 32b.

Although not shown in the drawings, the mechanism used for feeding the grinding tool in the Z direction in this embodiment is based on the so-called screw-feed system wherein sliding motion is conferred on the grinding tool 23 by the rotation of a lead screw linked to an operating knob. This type of feed mechanism is used for Zaxis feeding since in the precision bevel grinding according to this invention the Z-axis feed mechanism need only be operated for the initial setting and is thereafter left in what amounts to a fixed condition. The invention is, however, not limited to such an arrangement and there may be used instead a hydrostatic feed screw mechanism like those used for the X-axis and Y-axis feed mechanisms. In this case, both the precision and the range of application of the grinder will be enhanced.

Figure 6 is a perspective view of the essential part of the X-Y feed mechanism according to the present embodiment. The base 10 is provided with the X-axis guide surfaces 13a which engage with sliding surfaces 141 of the X-axis feed saddle 14, and the X-axis guide surfaces 13b which engage with sliding surfaces 143 provided one on either side of a projecting member 142 of the X-axis feed saddle 14. The projecting member 142 is integrally fixed to the X-axis feed saddle 14 and has formed at the center portion thereof a hydrostatic pressure nut 147 for receiving the hydrostatic pressure feed screw 16.The hydrostatic pressure feed screw 16, which is threadedly engaged with the hydrostatic pressure nut 147, rotates in response to command signals received from the controller 15 thus causing the X-axis feed saddle to travel along the X axis as guided by guide surfaces 13a, 13b.

The X-axis feed saddle 14 has formed therein in the direction perpendicular to its direction of feed a pair of guide sections 144 for the Y-axis feed table 17. It also has a pair of support members 1 45, 1 46, provided one on either side, for supporting a hydrostatic pressure feed screw 18 for precision feeding of the Y-axis feed table 17 along the Y axis.

Feed of the X-axis feed saddle 14 constructed as described in the foregoing can be controlled with outstandingly high stability in a vibrationfree manner by the guide surfaces 13a, 13b and the hydrostatic pressure feed screw 16 even at feed speeds in the vicinity of 20 mm/min at which practical problems arise in conventional feed mechanisms because of minute vibrations.

On the other hand, the Y-axis feed table 17 is, as mentioned above, mounted on the X-axis saddle 14 so as to engage with the guide section 144 provided therein. The Y-axis feed table 1 7 consists of sliding members 171 for engagement with the guide sections 144, a hydrostatic pressure nut 172 for threadedly engaging with the hydrostatic pressure feed screw 18, and a flat portion for mounting the set jig 21.

Also, as mentioned earlier, one side face of the Y-axis feed table 17 is provided with the linear scale 19 adapted to issue one pulse each time the table is fed by 0.1 micron.

The basic structure of the hydrostatic pressure feed mechanism used in the Y-axis feed mechanism is shown in an explanatory view in Figure 7. As will be noted from this figure, the hydrostatic pressure feed mechanism includes the hydrostatic pressure nut 1 72 which is fixed to the Y-axis feed table 1 7. This hydrostatic pressure nut 1 72 consists of a pair of female screws 33, 34 fixed adjacent to and along the hydrostatic pressure feed screw 1 8 fixed on the X-axis feed saddle 14 and hydrostatic pressure pockets 331, 341 which open onto opposite screw faces of the hydrostatic pressure feed screw 18. The hydrostatic pressure nut 172 is integrally fixed to the Y-axis feed table 17 and is driven by the rotation of the hydrostatic pressure feed screw 18.The hydrostatic pressure pockets 331,341 are supplied with hydraulic pressure by the hydraulic pressure unit (not shown) through hydraulic pressure lines 332, 342. Reference numerals 38a and 38b denote hydrostatic pressure pockets which together constitute a hydrostatic pressure thrust bearing for stabilizing the operation of the hydrostatic pressure feed screw 18.

Furthermore, the sliding member 171 of the Yaxis feed table 17 is mounted on the X-axis saddle 14 having a first trapezoidal groove in such a manner as to be guided by the inclined lateral surfaces 1 44a, 1 44b of the first trapezoidal groove. That is to say, the Y-axis feed table is guided by four surfaces of the X-axis saddle 14. At each engagement region between the X-axis saddle 14 and the Y-axis feed table 1 7 is provided one or more rows of hydrostatic pressure pockets 1 7a, 1 7b, 1 7c and 17d. A fluid delivered under pressure from an external hydrostatic fluid pressure source (not shown) in the X-axis saddle 14, a pantograph fluid pressure line adapted to expand and contact with the travel of the sliding member 17 and a fluid pressure line 17e in the table to be supplied to the hydrostatic bearing, whereby force is generated at each of the surfaces 1 44a, 1 44b, 1 44c and 144d.

As the above explanation with respect to the hydrostatic pressure feed mechanism for the Y axis feed table applies as well to that for the X axis feed saddle, an explanation of this latter mechanism will not, in the interest of brevity, be given here.

The hydrostatic pressure feed screws 16 and 18 are driven via associated transmission mechanisms by drive motors 36 and 37, respectively, and both screws are supported with high precision in both the radial and thrust directions by hydrostatic bearings.

A perspective view of the essential portions of the set jig 21 fixed on the top of the Y-axis feed table 17 for clamping the workpieces is shown in Figure 8. In the illustrated embodiment, a plurality of ferrite core blanks 35 of the type illustrated in Figure 9 are fixed in alignment on a base plate 211 which is fixed relative to reference surfaces 212 of the set jig 21 by vacuum chucks (not shown) so as to hold the beveled surfaces 3 to be machined in a fixed positional relationship with respect to the grinding tool 23.

It should be understood that prior to being subject to bevel grinding by the grinder according to this invention the blanks 35 have been processed to provide each with a plurality of grooves 351 and have been finished to make them uniform as regards their bottom surfaces and other external dimensions and features. In the illustrated embodiment, six magnetic heads are made from each blank.

The operation of the precision bevel grinder constructed as described above in accordance with this invention will now be explained with reference to a concrete example.

First, a number of blanks 35 which have been controlled for dimensional accuracy in the preceding fabrication steps are aligned on the set jig 21. For reasons of the machining method used (to be described later), in this embodiment the plurality of blanks 35 are fixed on the base plate 211 in advance by means of wax or the like and - the base plate 211 is then fixed in abutment with the reference surfaces 212 provided on the set jig 21 by vacuum chucks.

Figure 10(a) is an explanatory diagram showing the positional relationship between the blank 35 on the set jig 21 and the grinding tool 23 (a grinding wheel). The grinding tool 23 is moved along the Z axis to bring it to the desired position and the amount of grinding is then set by moving the Y-axis feed table to set the blank 35 at the desired position. After the amount of grinding has been set in this way, the grinding is conducted according to the following procedure.

In the initial setting of the amount of grinding, the grinding tool 23 and the blank 35 are first set in position for test grinding by movement along the Z axis and Y axis, respectively. Test grinding is then carried out and the amount of this test grinding is measured by microscopic observation using the microcopic unit 24 or by the linear scale 19, whereafter the amount of compensation required is determined from the measured value.

(Y, in Figure 10(A) indicates the amount of feed along the Y axis in test grinding). The amount of compensatory feed (denoted by Y2) is then set and the X-axis feed saddle 14 is fed along the X axis (i.e. perpendicularly to the plane of Figure 10(A)) to carry out grinding of a plurality of the blanks 35. Then by further feeding of the Y-axis feed table 17 by distances Y3, Y4, the beveled surfaces on one side are ground in succession.

The amounts of feed along the X and Y axes are controlled electrically by output signals from the controller 15.

Once grinding of the beveled surfaces on one side has been completed, the base plate 211 having the blanks 35 fixed thereon is reset as reversed with respect to the reference surfaces 212 of the set jig 21 (Figure 10(B)) and the grinding operation described above is repeated.

As is clear from the above explanation, when the precision bevel grinder according to the present invention is used, a feeding operation along the Z axis is required only at the time of setting the amount of grinding at the initial stage, and for setting the amount of grinding thereafter it suffices simply to feed the Y-axis feed table 1 7 along the Y-axis by amounts equal to the distance between the grooves 351 of the blank 35.

Therefore, as there are fewer causes for error than in the conventional system shown in Figure 2, it is possible to carry out grinding with high precision.

Moreover, as the X-axis feed saddle and the Yaxis feed table on which the workpieces are mounted are both driven by mechanisms employing static pressure feed screws, vibration can be prevented even during feeding at speeds for fine grinding and, moreover, positioning can be controlled with high precision. Also, since hydrostatic bearings are used for the grinding tool spindle, the rotational precision of the grinding tool (grinding wheel) is improved and vibration thereof is prevented so that precision grinding of highly brittle materials can be carried out under optimum conditions. Further, since the spindle of the grinding tool is held at a prescribed angle of inclination, the surface for supporting the workpieces can be made horizontal.Because of this, and the fact that the intervals on the linear scale and between the grinding points are constant, it is possible to minimize error due to pitching and yawing even when the number of workpieces is increased. Then again, since the workpieces are supported parallel to the linear scale provided on the Y-axis feed table, the amount of feed can be read directly. As a result, it is easy to dimensionally control the amount of grinding and, moreover, since the positional relation between the linear scale and the workpieces is maintained constant, any sliding error that may arise in the Y-axis feed table will not be magnified depending on the position at which the workpiece is ground. The precision bevel grinder according to this invention thus provides highly advantageous effects from the practical standpoint.

In the embodiment described, a belt drive is used between the spindle of the grinding tool and the motor which drives it so as to make it possible to eliminate the effect of heat and vibration from the motor. It should be noted, however, that a direct connection between the motor and the spindle is also possible in cases where motor vibration and heat does not have to be taken into consideration.

Claims (7)

1. A precision bevel grinder comprising a base, a grinding tool mount provided on said base at an angle of inclination thereto, a face grinding tool fitted on a grinding tool spindle, said spindle being supported on said grinding tool mount by hydrostatic bearings, an X-axis feed saddle driven by a hydrostatic pressure feed screw and guided by X-axis hydrostatic pressure guide surfaces formed in said base, a Y-axis feed table driven by a hydrostatic pressure feed screw and engaged with Y-axis hyrostatic pressure guide surfaces formed in said X-axis feed saddle, and a set jig for fixing workpieces on the upper surface of said Yaxis feed table.

2. A precision bevel grinder according to Claim 1, wherein said hydrostatic pressure bearings for supporting said spindle on said grinding tool mount are a hydrostatic pressure radial bearing and a hydrostatic pressure thrust bearing provided in said grinding tool mount.

3. A precision bevel grinder according to Claim 2, wherein said radial bearing has a pair of hydrostatic pressure pockets which surround said spindle and said thrust bearing comprises a hydrostatic pressure multipad and is located so as to sandwich a flange on said spindle.

4. A precision bevel grinder according to any preceding Claim, wherein said grinding tool mount is provided with a hydrostatic pressure feed screw for feeding said spindle in its axial direction.

5. A precision bevel grinder according to any preceding Claim, wherein one side face of said Yaxis feed table is provided with a linear scale which issues one pulse each time said Y-axis feed table is fed by 0.1 micron.

6. A precision bevel grinder according to any preceding Claim, wherein said hydrostatic pressure feed screws for driving said X-axis feed saddle and said Y-axis feed table are engaged with hydrostatic pressure nuts fixed on said X-axis feed saddle and said Y-axis feed table, respectively, each of said hydrostatic pressure nuts having a pair of female screws, provided with respective hydrostatic pressure pockets which open onto opposite screw faces of the associated static pressure feed screw and which are supplied with hydraulic pressure.

7. A bevel grinder substantially as hereinbefore described with reference to Figures 3 to 10 of the drawings.