GB2190862A

GB2190862A - Machining of workpieces made of brittle materials

Info

Publication number: GB2190862A
Application number: GB08711757A
Authority: GB
Inventors: Hubert Hinzen
Original assignee: GMN Georg Mueller Nuernberg AG
Current assignee: GMN Georg Mueller Nuernberg AG
Priority date: 1986-05-20
Filing date: 1987-05-19
Publication date: 1987-12-02
Also published as: GB8711757D0; FR2598955A1; KR870010922A; DE3616943C2; IT1207300B; DE3616943A1; JPS6322257A; IT8720490A0

Abstract

A process and apparatus for machining the plane-side-transverse grinding of a workpiece surface using a grinding wheel (1) rotating about a first axis at a first rotational velocity wherein the workpiece (3) or the grinding wheel is rotated about a second axis parallel to the first axis but displaced from it at a second angular velocity equal to the first angular velocity. This results in the relative translational velocity between the grinding wheel surface and the workpiece surface being of constant magnitude at all points on the workpiece surface and so the ideal grinding speed may be maintained over the entire workpiece surface. <IMAGE>

Description

SPECIFICATION Process and apparatus for the machining of workpieces made of brittle materials The invention relates to the use of the plane-side-transverse surface grinding process, especiallyforthe machining of disc-shaped workpieces made of very hard, brittle, crystalline materials with a Vickers hardness exceeding 7000 N/mm2. Of particular importance here is the machining of substrate materials for electronic components, such as, for example, silicon or gallium arsenide.

Of the grinding processes used predominantly at the present time to reduce the thickness of such workpieces, a basic distinction can be made between peripheral grinding and transverse grinding. As regards the materials relevant here, the transverse grinding process has proved particularly successful because of the planar surfaces required.

Plane-side-transverse surface grinding as described in Patent Specifications DE 3,339,942 Cl and DE 3,302,881 C2 in turn has a special position underthis heading.

In contrast to the grinding processes otherwise conventional for these materials, plane-side-transverse surface grinding only comprises two movement components, namely the rotation of the grinding wheel and thefeed movement directed perpendicularlyto the working surface. There is no need forthe advancing movement which is to be considered as a predominant process parameter in the grinding processes otherwise used for the machining of materials of this type.

The work cycle in plane-side-transverse surface grinding involves the plunging ofthe end face ofthe rotating grinding wheel into the workpiece surface, as a result of which this process is also known as "plunge-cut grinding ". The size of the grinding wheel is preferably calculated so that the entire surface to be ground on a workpiece is covered.

A characteristic of the known plunge-cut grinding is that the grinding wheel rotating at a certain angular speed gives rise to different machining speeds on the surface to be machined, because of the different distances from the axis of rotation. The attempt to keep the grinding wheel as small as possible in diameterfor reasons of both cost and space results in machining speeds which differ by a multiple. As a result of this nonuniformity caused by kinematics, the requirementforan ideal cutting speed over the entire surface to be ground, particularly with a diameter which increases, cannot be satisfied as effectively as possible.

According to the invention, this disadvantage is eliminated because the same machining speed is ensured at each point on the surface to be ground as a result of a suitable superpositioning ofthe rotation of the grinding wheel with a further rotation about an axis parallel to the axis of the grinding wheel either of the workpiece or of the grinding wheel.

Thus, according to the invention there is provided a process for machining a surface of a workpieces wherein a grinding wheel is rotated about a first axis, a surface of said grinding wheel which is substantially perpendicularto said first axis contacts and moves across said workpiece surface,said grinding wheel is moved toward said workpiece surface in a direction substantially parallel to said first axis, and either said workpiece or said grinding wheel is rotated about a second axis which is substantially parallel to and displaced from said first axis so that the magnitude of the relative translational velocity of said grinding wheel surface to said workpieces surface is subtantially constant at all points on said workpiece surface.

According to a preferred feature of the invention said first axis intersects said workpiece surface.

This preferred feature enables the workpiece to extend beyond the axis of rotation of the grinding wheel, since the kinematic conditions do not change as a result. It thereby becomes possible to use relatively small grinding wheels. At appropriately high rotational speeds, whilst the ideal cutting speed is maintained, this means that the grinding wheel does not have to be much largerthantheworkpiece itself, and consequently the skewing ofthe grinding wheel caused by the grinding pressures is also substantially reduced.

According to another aspect of the invention there is provided an apparatus for machining a surface of a workpiece comprising a grinding wheel rotatable about a first axis, a surface of said grinding wheel which is substantially perpendicularto said first axis being able to contact and move across said workpiece surface, said grinding wheel being movable toward said workpiece surface in a direction substantially parallel to said first axis, and either said workpiece or said grinding wheel being rotatable about a second axis which is substantially parallel to and displaced from said first axis so that the magnitude of the relative translational velocity of said grinding wheel surface to said workpiece surface produced is substantially constant at all points on said workpiece surface.

In a preferred arrangement said workpiece is rotatable about said second axis at an angular velocity equal to and in the same sense as that ofthe grinding wheel.

The following drawings illustrate firstly the state of the art, and secondly embodiments ofthe present invention,which are given merely byway of example: Figure 1 shows the grinding conditions according to the state of the art, Figure2 shows an embodiment ofthe grinding cycle according to the present invention, Figure 3 shows a further embodiment of the grinding cycle according to the present invention, Figure Ishows an apparatus for carrying outthe grinding process according to the present invention.

According to Figurei,avirtuallyclosedrotating plunge-cut grinding wheel 1 covers the disc-shaped workpieces 2. The speed distribution on the workpiece 2 shown forms as a result of the rotation of the grinding wheel. The length of the arrows representing diagrammatically the magnitude of the relative velocity vectors.

Appropriately, the angularspeed ofthe plunge-cut grinding wheel is selected so that the best possible cuttirg speed is obtained approximately in the centre ofthe workpiece. However, the speeds consequently deviate from this ideal value towards the upper and the lower edge of the workpiece.

According to Figure 2, the workpiece 3 is rotated about its own axis at the same angular velocity and in the same direction of rotation as that of the grinding wheel 1. As hitherto, the centre of rotation oftheworkpiece 3 again experiences no change in the machining speed as a result. However, according to the invention, as result ofthe rotation of the workpiece 3 taking place in the same direction of rotation, the regions located further inwards move at an additional relative speed which is added to the speed of the grinding wheel. Forthe regions located further outwards, the workpiece speed is subtracted from the grinding-wheel speed, so that when the angular velocity of the grinding wheel and workpiece are the same the best possible cutting speed is obtained at each on the workpiece 3.This consideration applies not only to the points along the centre line in Figure 2, butto any point on the workpiece.

The arrows represented in Figure 2 illustrate the uniformity of the mangitude ofthe relative velocity vectors resulting from this. This superpositioning of the speeds described above, can be brought about by means of the rotation of the grinding wheel together with rotation of the workpiece. Alternatively according to the invention, the rotating grinding-wheel axis can also be rotated simultaneously about a further axis parallel to this, so that, once again, the superpositioning of these two movements leads to this same goal. Such an superpositioning rotation of a grinding wheel is already known for other purposes.

The ideal cutting speed which can thus be achieved at all points on the workpiece surface allows efficient feeding and consequently a very high cutting capacity. At the same time, however, the roughness of the ground surface is improved substantially in comparison with conventional plunge-cut grinding, especially when grinding wheels ofveryfine grain are used.

According to Figure 3, in accordance with the invention the workpiece can extend beyond the axis of rotation of the grinding wheel 1, since the kinematic conditions do not change as a result. It thereby becomes possible to use relatively small grinding wheels. At appropriatley high rotational speeds the ideal cutting speed can be maintained whilst the grinding wheel does not have to be much largerthan theworkpiece itself, and consequently the skewing ofthe grinding wheel caused by the grinding pressures is also substantially reduced.

Figure 4 explains in detail an advantageous embodiment of a machineworking in accordance with the present invention. The machine illustrated consists of an essentially cuboid machine bed 12 which, on the upper horizontal surface, carries an intermittently rotatable or horizontally displaceable table 13which can be clamped during the grinding process.

The workpieces 3 are each arranged on a rotatable unit4,these preferably being screwed on to a direct drive 5. Projecting beyond the table 13 is a stand 14, on which are arranged a drive motor 6 and a grinding spindle 7 carrying the grinding wheel 1 with the working surface 9 facing the workpieces 3. The stand 14 is fastened to the bed 12 by means of exact rigid guide elements 10 so as to be movable vertically up and down. The vertical up-and-down movement of the stand 14can beinduced,forexample,byan electrically driven threaded spindle 11. The spindle 11, by means of the stand 14, feeds the grinding wheel 1 downwards to the desired dimension ofthe workpiece 3 and, after sparking out, lifts it off again in rapid motion.During this return movement, the ready-machined workpiece 3 is removed from the region of the grinding wheel 1 as a result of the intermittent rotation or displacement of the table 13, and at the same time a newworkpiece 3to be machined is brought underneath the grinding wheel 1.

This simultaneous loading and unloading takes place in coordination with the return movement of the stand 14, so that there can be no contact between the newworkpiece 3 and the grinding wheel 1 until the transport movement is completed and the table 13 clamped.

According to the invention, it is possible to machine workpieces consisting of hard, brittle/friable, metallic and non-metallic materials, such as, for example, ceramic, quartz, silicon, germanium, sapphire, spinel, gallium-gadolinium-garnet (GGG), A(lll) - B- (V) compounds and super-hard sintered materials, especially silicon nitride, boron carbide, corundum and tungsten carbide.

Claims

1. Aprocessformachiningasurfaceofa workpiecewherein a grinding wheel is rotated about a first axis, a surface of said grinding wheel which is substantially perpendicularto said first axis contacts and moves across said workpiece surface, said grinding wheel is moved toward said workpiece surface in a direction substantially parallel to said first axis, and either said workpiece or said grinding wheel is rotated about a second axis which is substantially parallel to and displaced from said first axis so that the magnitude of the relative translational velocity of said grinding wheel surface to said workpieces surface is substantially constant at all points on said workpiece surface.

2. A process as claimed in claim 1 wherein said first axis intersects said workpiece surface.

3. A process as claimed in claim 1 or 2 wherein said grinding wheel surface completely covers said workpiece surface.

4. A process as claimed in any preceding claim wherein said workpiece is a disc-shaped workpiece.

5. A process as claimed in any preceding claim wherein said workpiece is of very hard, brittle/friable, crystalline materials with a Vickers hardness of more than 7000 N/mm2.

6. Anapparatusformachining a surface of workpiece comprising a grinding wheel rotatable about a first axis, a surface of said grinding wheel which is substantially perpendicularto said first axis being able to contact and move across said workpiece surface, said grinding wheel being movabletowardsaidworkpiecesurfaceina direction substantially parallel to said first axis, and either said workpiece or said grinding wheel being rotatable a bout a second axis which is substantially parallel to and displaced from said first axis so that the magnitude of the relative translational velocity of said grinding wheel surface to said workpiece surface produced is substantially constant at all points on said workpiece surface.

7. An apparatus as claimed in claim 6 wherein said first axis intersects said workpiece surface.

8. An apparatusasclaimed in6or7whereinsaid grinding wheel surface may completelycoversaid workpiece surface.

9. An apparatus as claimed in any of claims6to 8 wherein said workpiece is a disc-shaped workpiece.

10. An apparatus as claimed in any of claims 6to 9 wherein said workpiece is of very hard, brittle/friable, crystalline materials with a Vickers hardness of morethan 7000 N/mm2.

11. An apparatus as claimed in any of claims 6to 10 wherein said workpiece is rotatable about said second axis at an angularvelocity equal to and in the samesenseasthatofthegrindingwheel.

12. A process substantially as hereinbefore described with reference to Figure 2 or 3 ofthe accompanying drawings.

13. An apparatus substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.