Grinding machine spindle flexibly attached to platform

Abrading – Abrading process – Glass or stone abrading

Reexamination Certificate

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Details

C451S006000, C451S023000, C451S056000, C451S443000

Reexamination Certificate

active

06217420

ABSTRACT:

The invention concerns grinding and polishing machines and methods of grinding and polishing discs such as wafers of silicon for use in the construction of semi-conductor devices, and discs of glass or other brittle materials on which magnetic material is to be deposited for forming magnetic memory disks for computer disk drives and the like.
BACKGROUND TO THE INVENTION
When grinding discs for either of the above purposes, it is important that the outside diameter of the disc is finished to a high level of accuracy and often to a particular cross-sectional form. In the case of memory disk a circular opening is also required again to an accurately controlled diameter and circularity. In the case of silicon wafers, registration in later manufacturing steps requires registration devices to be formed around the periphery of the disc such as the formation of flats and notches.
Conventionally edge grinding and polishing machines have incorporated linear slideways for all axes. Whether incorporating re-circulating rolling element bearings or air bearings, all such axes share a common failing when grinding and polishing brittle material namely they permit significant relative motion between grinding wheel and component. This arises from the need to provide for orthogonal movements of grinding wheels usually obtained by stacking one linear axis above another. This motion requires the use of hard wearing grinding wheels to minimise loss of form due to wear, but such wheels tend to produce poor quality surfaces with deep damage.
When grinding silicon wafers, the depth of sub-surface damage arising during grinding should be minimised since to be used in subsequent manufacturing steps, a wafer must be delivered with zero damage. When sub-surface damage has arisen, this means a post grinding step of acid etching before polishing. Both processes are expensive, and the less the damage the shorter the polishing time that is needed.
SUMMARY OF INVENTIVE ASPECTS
Machine construction
According to one aspect of the present invention, a grinding machine as aforesaid comprises a rigid platform on which a workhead spindle and a grinding wheel spindle are located, and wherein the grinding wheel spindle is mounted on a sub-assembly which is attached to the platform by means of flexures which permit limited movement of the sub-assembly in a direction generally parallel to the direction in which the wheel has to advance to achieve grinding or polishing of a workpiece mounted on the workhead spindle, the flexures generally inhibiting movement of the sub-assembly in all other directions.
Where two grinding wheels are to be mounted on the platform, preferably two sub-assemblies are provided, one for each grinding wheel, and each sub-assembly is attached to the platform by flexures as aforesaid.
Axis reversal hysteresis can cause damage during the grinding of a disk or wafer as the grinding axes are reversed after spark-out. By mounting the sub-assemblies on flexures which provide very smooth motion and only permit movement in one direction, axis reversal hysteresis is much lower than is the case where conventional linear axes are used.
According to another feature of the invention, the positions of the flexures mounting the sub-assemblies to the machine base are selected so as to reduce the effect of torsional vibration within the machine base on the sub-assemblies carried by the flexures.
Preferably flexure mounting points are preferably generally central of the machine and are preferably located on either side of and close to the machine centre line.
According to another aspect of the invention, the flexures on which the sub-assemblies are mounted, themselves may be attached to a solid mass, preferably containing polymer concrete and preferably forming part of the base of the machine. In this way any motion of the sub-assembly or carriage is highly damped.
According to a further preferred feature of the invention, hollow cavities within selected parts of the machine may be filled or at least partially filled with polymer concrete so as to raise the natural frequency of vibration of the machine parts with the objective of providing minimum mass with optimum distribution of the damping/stiffening material.
The flexures typically comprise stress relieved steel links which are directly jointed to the platform on the one hand and spindle bearing sub-assemblies on the other.
Spindle mounting
The grinding wheel and workpiece spindles preferably are mounted in air bearings.
Workhead mounting
Preferably the workhead is mounted upon a preloaded slideway driven by servo motors and fitted with a high resolution position encoder.
Where the sub-assemblies are mounted on a horizontal extension of the machine base, towards one end of the mass of the machine, rigidity of the horizontal extension may be improved by mounting a rigid bracket between the extension and an end of the remaining mass of the machine.
Where the sub-assemblies are mounted symmetrically about the centre line of the base and are spaced apart, the bracket immediately extends on the centre line of the machine between the two sub-assemblies.
So-called coulomb damping is obtained by forming the bracket from three pieces and bolting them together.
Mounting for platform
In a preferred arrangement the platform is itself mounted through vibration absorbing means to a fixed machine base.
The machine base may be a single piece fabrication, but where vibration isolation is to be optimised it may comprise a two-part structure with vibration absorbing means between the two parts.
In a two-part structure in which one part is mounted above the other by the vibration isolation (absorbing) means, the upper of the two parts may be a metal fabrication which contains or is at least in part formed from polymer concrete so as to provide a direct, highly damped link between the mountings for the cams which move the spindles and the sub-assemblies and the workhead spindle guideway means.
Preferably the effect of torsional vibrations of the machine base on the grinding wheel stability is reduced by mounting the grinding wheel carrying sub-assemblies in a central position on the machine base. At that point in the machine base there is preferably included a section of polymer concrete which provides high damping to the sub-assemblies.
Subassembly drive means
According to another aspect of the present invention, drive means whereby each sub-assembly is movable to advance or retract the grinding wheel spindle towards and away from a workhead spindle, comprises cam means.
Preferably solid cams are employed.
The cam means may operate in one sense only in which event return spring means may be provided for providing a restoring force in the opposite sense.
Alternatively the cam means may operate in both senses so that positive drive is provided to both advance and retract the sub-assembly towards and away from the workhead spindle.
In one arrangement the two-subassemblies are spring loaded apart and the grinding feed is provided by appropriate inward movement of the cam arms. This feature of the invention allows rapid interpolation around the corners of silicon wafer flats, since the maximum sub-assembly acceleration forces act outwards, towards the cams.
Conventional machines use ball screw feeds which are inherently noisy as the recirculating balls engage and disengage the ball mount. The noise appears as imperfections in the grinding process. By driving the sub-assemblies through solid cams and mounting the cams between two pairs of high precision angular contact bearings, additional stiffness is introduced into the assembly. A machine constructed thus will not introduce mechanical rumble during grinding.
By mounting the cams very close to the workhead axis bearing rails and directly linking them by polymer concrete, a very tight and highly damped stiffness loop is produced.
Where drive to the relatively movable parts of a machine is achieved through ball screws, such a drive has to be disconnected and a secondary mechanism must be engaged if for example for control is to be imp

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