WO1995020465A1

WO1995020465A1 - Surface grinding

Info

Publication number: WO1995020465A1
Application number: PCT/EP1995/000234
Authority: WO
Inventors: Thomas H. C. Childs; Hean Joo Yoon
Original assignee: University of Leeds
Current assignee: University of Leeds
Priority date: 1994-01-27
Filing date: 1995-01-23
Publication date: 1995-08-03
Anticipated expiration: 1996-07-27
Also published as: GB9401593D0

Abstract

A magnetic fluid grinding cell is disclosed. The apparatus comprises a cell or container (1) containing a magnetic fluid (2) in which is suspended a suitable abrasive grit. A float (3) is positioned beneath drive shaft (4), both being made of a non-ferromagnetic material. An array of magnets (5) is arranged below cell (1), their poles alternately directed towards the base of the cell, as shown. Balls (6) to be finished are held in the cell whilst shaft (4) is rotated. A load (W) is applied downwards through the shaft and is resisted by magnetic forces acting upwardly on the float. The balls contact the drive shaft, the walls of the cell (1) and the float (3), the abrasive grit in fluid (2) helping to wear away the surfaces of the balls. The rotational speed of the shaft (4), Φs, and that of the float 3, Φf, are adjusted to cause skidding between the balls (6) and the grinding surfaces.

Description

SURFACE GRINDING

Field of the Invention

The present invention relates to a method and apparatus for use in grinding the surface of an object to a desired shape, in particular although not exclusively for use in the magnetic fluid grinding of ceramic balls.

Background to the Invention

It is often desired to grind the surface of an object into a desired shape or to achieve a desired level of surface smoothness. In particular, metal and ceramic balls such as are used, for instance, in bearings need to be ground to achieve not only surface smoothness but also the desired precision in shape (ie, to ensure they are as close as possible to the completely spherical) .

Hard, stiff, balls have a wide range of uses, but two major engineering ones are as components of rolling bearings and for accurate positioning and probing in metrology. A ball's ability to carry load and to roll at the same time with a low friction resistance, low wear, low distortion and without breaking is the reason for its use in bearings (ball bearings) . Its spherical symmetry, so that the space that it occupies depends only on the position of its centre and not on its orientation, is the reason for its use in metrology, for example as the anvil of a position sensor.

The combination of hardness and stiffness, combined with cheapness, needed for many applications makes hardened steel the most common ball material for engineering use, although some lightly loaded ball bearings are made from plastics, and other metals such as bronzes also find special applications. However, ceramics can have higher stiffness and hardness than steels. They can also have a lower density and thermal expansion coefficient. These properties enable ceramic rolling bearings to operate with lower distortions than metal ones, particularly at high operating speeds. Thus, despite their greater cost, ceramics are finding specialist uses as precision rolling bearings, for example in grinding and other machine tools. Their use has been suggested at extremely high speeds for gas turbine bearings. Ceramics are also more inert than metals in chemically corrosive environments and thus find use as bearings in chemical process industries and in other corrosive applications, for example in salt water. It is their chemical inertness and low thermal expansion coefficient, together with their wear resistance, which favours their use as metrology components.

A range of oxide, carbide and nitride ceramics, or mixtures of these, are used depending on the relative importance of stiffness, hardness, toughness, density, thermal expansion coefficient, thermal conductivity or chemical inertness required for a given use. Typical examples of such ceramics are silicon nitride, silicon carbide, silicon-aluminiu -oxy- nitrides, zirconia and alumina in various forms (including synthetic sapphire and ruby) .

For its use in both precision bearing and metrology a ball must be spherical within fine limits. A typical specification would be that the difference between a largest and a smallest measured diameter should be less than from 1 to 0.1 μm, depending on the grade of the ball. At this scale of accuracy it is necessary to polish a surface in order that its position be closely enough defined. There are other reasons for polishing rolling bearings. Rough surfaces touch at their high spots and at those high spots very large stresses are developed. Particularly with ceramics these concentrated contact stresses can lead to accelerated surface failure. By smoothing a surface by polishing, these concentrations may be reduced to acceptable limits: for ceramic balls, roughness values (Ra) of 0.05 down to 0.01 μm are typically desired.

Polishing is not the only finishing operation required of ceramics balls. Prior to polishing, a substantially thicker layer than 0.1 μm must be removed. Ceramic balls are typically manufactured by the pressing of powder into an approximately spherical shape, followed by solidification by heating (sintering) . During sintering shrinkage can occur so that the sintered balls may not be round. Chemical reactions may also occur with the surrounding atmosphere so that the outer layer of the ball may be out ₍ of specification with respect to its composition and/or microstructure.

As a result, the surface finishing of a ceramic ball is a three stage process. In a first stage, known as equalising, the differences in shape caused by shrinkage of the as- sintered balls are removed. In a second stage, known as rough lapping, any imperfect surface layer is removed: this can involve diameter reductions of from 300 to 1000 μm. The last stage, known as fine lapping, can typically involve a further diameter reduction of 1 to 5 μm.

The commercial manner of finishing ceramic balls, for all three stages described above, is by lapping the balls between grinding surfaces, typically in the form of rotating metal discs. A spiral groove or a series of circular grooves of different radii are cut in the face of one of the discs. The balls to be finished are placed in the grooves. The face of the other disc is pressed on to the balls, and rotation of one disc relative to the other causes the balls to roll round the grooves. An abrasive paste, known as a lapping paste, commonly containing diamond grit, is smeared on the groove surfaces, or loose abrasive is allowed into the groove region. Because the normal to the surface of the groove where it is contacted by a ball is not perpendicular to the axis of the ball's rotation, a small amount of spin is inherent in the ball's motion relative to the groove. This spin results in a relative displacement between ball and groove within the contact area between the two and results in material removal by abrasive action with the paste or loose abrasive.

The difference between the equalising, rough lapping and fine lapping stages lies in the coarseness of the abrasive used. A problem with the process as just described is that steady rotation of the balls in the grooves would not generate a spherical ball. It would abrade tracks on the balls, rather than remove material uniformly from all over their surfaces. Thus, in the lapping process, the steady motion is disturbed in a number of ways. With spiral grooved laps, balls are fed from a reservoir into the centre of the lap (rotating disc) through a central hole in the plane-faced lap. Rotation moves the balls to the outer edge of the lap, via the spiral groove, from where they fall back into the reservoir. On their next excursion into the lap they are likely to have a different orientation from their previous one. With circular grooved laps, a radial segment of the lap is removed to allow entry and exit of the balls from the lap once per revolution.

The complete lapping system therefore consists not only of a pair of lapping discs but also of a reservoir to help randomise the material removal process. Typically a production batch may consist of several thousand balls. To avoid damage to the balls by shock loading caused by vibrations, lap rotation speed is kept fairly low: perimeter speeds are typically less than 50 m/min. The abrasive process itself is also rather slow. For all these reasons the processing time for a batch may be several weeks, even though the active processing time for any one ball may be as low as 10 hours to 1 day. The lapping process is not suited to the rapid finishing (measured in hours) of small batches (from 10 to 100 balls) .

An alternative process to the above, which is suited to the rapid finishing of small batches, has recently been proposed (Kato et al) . It is known as magnetic fluid grinding. In comparison to the batch sizes and finishing times mentioned above, it is suitable for the finishing, for example, of 10 to 100 balls in several hours. A typical magnetic fluid grinding arrangement for the finishing of ceramic balls is shown in Figure 1 (see later) . A ring of balls is placed in a container and is driven round by a conical ended drive shaft which rotates at high speeds which can be of the order of 10,000 RPM (giving surface speeds of 500 m/min) . The container additionally contains a ferromagnetic fluid and is placed above a bed of permanent magnets which are oriented alternately with their North and South poles facing the base of the container. The magnetic forces in the fluid act on the balls to press them on to the drive shaft. A circular non¬ magnetic disc beneath the balls, known as a float, is also forced upwards and serves to intensify the force pushing the balls on to the shaft. Grinding or polishing abrasive grits are also levitated in the fluid by the magnetic forces. Thus the shaft drives the balls through the fluid which is maintained as a good abrasive slurry.

Data published by Kato et al demonstrate that magnetic fluid grinding is very effective at removing material from ceramic balls. The rate of material removal is influenced by the shaft rotation speed, by the load pressing the balls on to the shaft, by the size and concentration of the grit in the fluid and by other factors: Kato particularly claims that lining the container wall with a rubber sleeve is advantageous to the process. In other published work, the chemical qualities of water as opposed to hydrocarbon-based ferromagnetic fluids are said to make water-based fluids more effective for finishing silicon nitride ceramics.

Balls do not enter or leave the container during the magnetic fluid grinding process; a reservoir for randomising motion is thus not necessary, and processing times can consequently be reduced.

There is no doubt that the published data show that magnetic fluid grinding can enable very rapid processing of ceramic balls. However, the material published by Kato does not give information that allows optimum operating conditions to be selected or surface removal rates to be predicted. Experiments by the present inventor have found different dependencies of removal rate on shaft speed, load, grit sizes and concentrations and container lining material than those observed by Kato. Indeed distinctly different removal rates have been observed between repeated experiments in the same conditions as defined by Kato's work. It is also not clear from Kato's work why the randomising action of a reservoir is not needed; nor more broadly is it clear why the process is more rapid than the traditional lapping process.

It is therefore an aim of the present invention to provide a method and apparatus for use in the grinding of the surface of an object to a desired shape (which includes to a desired surface smoothness) , which can yield improved surface removal rates and which allow greater certainty, in the selection of operating conditions to maximise removal rates, than do known methods and apparatus.

Statements of the Invention

According to a first aspect of the present invention there is provided a method for grinding the surface of an object to a desired shape, the method comprising contacting the object surface with a grinding surface; causing relative movement between the object and grinding surfaces so as to effect removal of material from the object surface; and adjusting the operating conditions during the relative movement so as to cause skidding to occur between the object and grinding surfaces.

The invention is based on the unexpected discovery that in a typical surface grinding process (here, magnetic fluid grinding of solid spheres) it is possible for skidding to occur between the spheres and the grinding surface. This type of motion has not previously been recognized. The magnitude of the velocity v' of this skidding motion affects the rate of removal of the surface of the object being ground. Thus, the invention provides, through its recognition of the occurrence of, and the significance of, v' , a way in which surface removal rates can be controlled and optimised by the monitoring and control of v' or of operating conditions which influence v' . This clearly represents an advantage over previously known surface grinding techniques, in which optimisation of surface removal rates seemed to be largely a matter of trial and error based on an inadequate appreciation of the kinetics of the grinding process.

The theory behind the invention is as follows. It is known (Archard's wear law) that when two surfaces are pressed together by a load W, with abrasive grits trapped between them, and one of the surfaces is made to slide past the other with a sliding speed v, if the hardness H of the sliding surface is less than that of the abrasive, then the volume V abraded from the sliding surface increases linearly with time t according to the equation:

V = KWv t/H (1)

where K is a coefficient that depends on the shape of the abrasive and can therefore vary with time if the abrasive shape changes due, for example, to wear. K typically has a value of between 0.1 and 0.01 if the abrasive is fixed to one of the surfaces, as occurs when material is removed from a surface by a grinding wheel or abrasive paper. K has a lower value, say 0.01 to 0.001, if abrasive is free to move between both sliding surfaces, as it would be if it were loose. For a given material being finished, one should therefore strive to control the product KWv.

The general belief regarding the state of motion of balls driven round a container by a shaft, as in the magnetic fluid grinding process, is that the balls roll round the container and that wear is due to a small amount of slip associated with the spinning motion that is inherent to rolling. The unexpected discovery behind the present invention is that the balls do not always roll. Sometimes there is skidding between the balls and the drive shaft. The value of the skidding velocity, V, varies with the operating conditions. It can be calculated that in the absence of skidding between the balls and the shaft (v' = 0) , there is a relation between the angular velocity Ω₅ of the shaft, Ω_b of the balls rotating around the container (as distinct from ω_b of the balls rotating round their own axes) and Ω_f of the float, which relationship depends on the geometry of the container (see Figure 2) :

Ω_b/Ω, = A Ω_f/Ω, + B (2)

where A = cos(9 / (1 + sin( + cosfl) and B = (R./R₀) / (1 + sinø + cosø) .

Further, if a skid velocity V occurs at the shaft, then:

Ω_b/Ω_s = A Ω_f/Ω_s + B - (C/Ro) (V/Ω (3)

where C = 1 / (1 + sinι9 + cost?) .

Figure 3 shows a particular example of these equations in graphical form, for the case θ = 30°, R_Q = 15.4 mm and for a ball of radius 6.35 mm that gives R, = 12.22 mm. The Figure shows the variation of Ω_b/Ω_s with Ω_f/Ω_s for a range of values of v'/Ω_s. The values of v'/Ω_s quoted are for v' in m/s and Ω, in rev. /min. Superimposed on this figure, as solid dots, are experimental data obtained with a container and balls of sizes close to those indicated for the example. The experimental data demonstrate that under certain operating conditions the container was operating at much lower values of v'/Ω_s than under others. In particular, conditions giving low values of Ω_b/Ω_s gave rise to relatively high values of v'/Ω_s and hence (see the explanation below) will turn out to be operating conditions to be aimed at if efficiency is to be improved.

The present invention is based around the principle that if v' can be optimised, then so too can the overall surface removal rate V/t (see equation (1) above with v' a particular instance of the sliding speed v) . Thus, the monitoring and control of the skid velocity v' can be used at least partially to control the surface grinding process. In particular, the aim should be to operate under conditions where at least some skidding occurs between the surface of the object being polished and the grinding surface (ie, v' > 0) .

Various measurements may be taken that can lead to the calculation of the ratios Ω_b/Ω_s and Ω_f/Ω_s and hence to the monitoring of the skid velocity v' . Further, a practicable and easier measurement is that of Ω_b/Ω_salone. The data of Figure 3 demonstrate that Ω_f/Ω_s is often close to zero, particularly for Ω_b/Ω_s < 0.25 in the example shown. This unexpected observation enables a one-to-one relation to be deduced between v'/Ω,. and Ω_b/Ω,, as shown by the solid line of Figure 4 for the same numerical example as for Figure 3. Alternatively, as illustrated by the dashed lines in Figure 4, one-to-one relations that may be used as control curves exist between v'/Ω_s and Ω_b/Ω_s for any particular value of Ω_f/Ω_s.

The more or less linear relationship between Ω_b/Ω_s and v'/Ω.,, as shown in Figure 4, is unexpected and leads to a further advantageous embodiment of the present invention, in which values of Ω_b/Ω_s may be measured and used to monitor and control v' so as, in turn, to optimise surface removal rates.

The magnitude of Ω_b/Ω_s depends on many variables. Principle ones, but not necessarily the only ones, are the shaft rotation speed Ω_s; the contact load W between the ball and the shaft; the effective viscosity of the magnetic fluid containing the abrasives (ie, the combined effects of the fluid viscosity and the concentration and size of the grits in the fluid) ; and the material of the container wall and of the abrasive grits. Two further factors might be (i) (diameter of container - diameter of float) /radius of balls, which conrrols the clearance between the container and the float relative to the size of the balls; and (ii) grooving of the container wall, the float and the shaft due to their wear by the balls, which grooving changes the contact geometry between the balls and corresponding parts of the container. Control of any of these variables, in order to control Ω_b/Ω_s or to control v' directly, is a critical part of the method of the invention. Such operational factors as are listed above may be monitored, for the purpose of control, either by monitoring Ω_b/Ω_s or by monitoring directly their effect on surface removal rate, V/t.

This invention thus represents the application of new understanding of the surface grinding process that was not previously available. Clear evidence of this comes from published work by Kato (for example Wear, 1990, Vol 136, pp.

117-134) in which he discusses the important properties of the ferromagnetic fluid in a magnetic fluid grinding process. No mention is made of viscosity. Instead water based fluids are said to be best for the finishing of silicon nitrides because of the well-known chemical reactions between water and silicon nitride, leading to its oxidation. The present work has shown that in fact oil-based fluids are as good as water based ones in terms of removal per unit amount of sliding. It has also shown (through an understanding of the importance of skidding) the importance of viscosity in the magnetic fluid.

In none of his writings on the finishing of balls does Kato discuss, or even refer to, the existence of skidding. It seems that he may have believed that any slip in ball finishing was due to the inherent spin associated with rolling in a geometry such as that of the grinding cell (already mentioned above) . In this sense, the present invention represents a new and unexpected deviation from known techniques.

In the method of the invention, the object whose surface is to be ground may be a three dimensional object of any suitable shape (eg, a sphere, a cylinder..), although the invention is of particular use where the object is, or is intended to be after grinding, a sphere. The object may be made from any material; it may, for instance, be a metal (preferably non- ferromagnetic) , a plastics or a ceramic ball. More preferably, however, the object is made from a ceramic material such as an oxide, carbide or nitride ceramic, since the method of the invention is particularly suitable for use in grinding such materials.

The grinding surface preferably comprises the surface of grinding means such as a rotating shaft which, according to the method, is caused to rotate relative to the object surface whilst the two surfaces are in contact. The grinding surface should be of a material which is harder than that of the object surface. It may be a uniformly harder surface, or in the form of hard abrasive particles embedded in a softer surface, for example austenitic stainless steel, aluminium, nickel or copper alloys, etc.

The relative movement between the object and the grinding surfaces may for instance be caused by rotating one or both of the surfaces relative to the other. Where grinding means is caused to rotate, the object itself is also preferably caused to move around a container in which the object and grinding means are enclosed, as in known magnetic fluid grinding processes.

Preferably, the object and grinding surfaces are immersed in a fluid containing particles of an abrasive material such as diamond or other hard grit, whilst the surfaces are moving relative to one another. In this case, the grinding surface is preferably of a material sufficiently soft as to allow particles of the abrasive material to become embedded in the grinding surface during use. During relative movement of the two surfaces, a load is preferably applied to either the grinding surface or the object in such a way as to urge the surfaces into contact with one another. For instance, where the grinding means comprises a rotating shaft, the load may be applied along the axis of the shaft, in a direction urging the shaft into contact with the object surface.

The method of the invention may be used to grind a surface to any desired level of smoothness; it may thus be used in an "equalising", "rough lapping", "fine lapping" or "polishing" stage of production of an object. The operating conditions, such as for instance the applied load, the hardness of the grinding surface, the amount and type of abrasive material (if any) used, the length of time for which the two surfaces are in contact and the speed of their relative movement, should be varied according to the degree of smoothness desired to be achieved.

The method of the invention is particularly suitable for use as part of a magnetic fluid grinding process, in which case the object and grinding surfaces are immersed in a ferromagnetic fluid (preferably containing an abrasive grit) during their contact with one another, the method including the step of exerting magnetic forces on the fluid so as to urge the two surfaces into contact with one another, by means of appropriately placed magnets. As in knpwn magnetic fluid grinding techniques, a magnetic float may also be used to help urge the two surfaces into contact with one another. In the case where the method of the invention is to be used as part of such a magnetic fluid grinding process, the object whose surface is to be ground, the grinding surface and any other grinding means and any float used in the method must be made of non-ferromagnetic materials. Other aspects of such a process may be conventional, the operating conditions being varied so as to cause skidding between the object and grinding surfaces in accordance with the invention.

The method of the invention preferably comprises the step of adjusting the operating conditions so as to maximise as far as possible the value of the velocity v' of the skidding which is caused to occur between the object surface and the grinding surface.

The operating conditions which might be controlled so as to achieve this aim include (by way of a non-exhaustive list) the type and speed of the relative movement between the two surfaces (for instance, their speeds of rotation where a rotating drive shaft is used as a grinding means) ; the viscosity of any fluid surrounding the surfaces during their contact with one another; the nature and size of abrasive particles which may be included in that fluid or otherwise present between the two surfaces during their contact; the load which is applied to force the surfaces into contact with one another; and the materials from which the grinding surface, any container in which the object and grinding surface are contained and any magnetic float are made.

The geometry of apparatus used in the method, for instance, clearances between various relatively moving components, may also be adjusted in a way that affects the value of V .

By way of example, if the method is to be used as part of a magnetic fluid grinding process for grinding ceramic balls, it has been found preferable to work under at least the following operating conditions:

Viscosity of magnetic fluid: > 0.01 to 0.02 Pa.s.

Drive shaft rotation speed: > approx 2000 rev/min (the surface speed of the shaft where it contacts the object surface should be greater than about 2.5 m/s) .

Load : typically between 0.2 and 5 N per ball. Higher values may be possible, but optimum values are thought to be around IN.

Drive shaft material austenitic stainless steel,

Float material : a metal or plastics material,

Float clearance : it can be beneficial that the difference in diameter between the inner surface of the fluid container and the float is almost equal to but not greater than the radius of the ceramic balls. Container wall material : aluminium alloy.

Where the method of the invention is used to grind the surface of a spherical object by means of a magnetic fluid grinding process, the method preferably additionally comprises the steps of monitoring values for Ω_b/Ω_s and Ω_f/Ω_s (where Ω_b is the angular velocity of the object rotating around a rotating shaft carrying the grinding surface, Ω_s the angular velocity of the shaft and Ω_f the angular velocity of a magnetic float used to urge the object and grinding surfaces together) ; preferably calculating therefrom values for v' ; and utilising the measured and/or calculated values to adjust (and preferably to maximise) the value of v' .

Where values for v' are calculated, these can be used to calculate the length of time for which grinding must be carried out to achieve a desired volume of surface removal. This allows effective control of the grinding process without the need periodically to cease grinding and remove and check the object being ground.

More preferably, the method comprises the steps of monitoring values for Ω_b/Ω_s and adjusting the operating conditions of the method so as to minimise the value of Ω_b/Ω_s and hence to maximise the value of v' . This may be achieved, for instance, by monitoring the variation of Ω_b with Ω_s, and selecting such values of Ω_b and Ω_s and such other operating conditions as to operate the method in that region of the Ω_b/Ω_s graph in which skidding can be seen to occur between the object surface and the grinding surface.

According to a second aspect of the present invention there is provided apparatus for use in grinding the surface of an object to a desired shape, the apparatus comprising a grinding surface which may be brought into contact with the surface of the object in such a way that the grinding surface and the object surface are moveable relative to one another; means for causing relative movement of the surfaces when in contact with one another; and control means for monitoring and/or controlling the operating conditions of the apparatus in use, so as to cause the relative movement of the two surfaces to give rise to skidding between the surfaces.

The grinding surface is preferably provided on a grinding means such as a rotatable shaft. The apparatus may additionally comprise an abrasive material, which may be suspended in a fluid, located between the object surface and the grinding surface whilst the surfaces are moving relative to one another. Thus, the apparatus preferably comprises a container in which the grinding surface and/or means and the object itself may be contained, preferably immersed in such a fluid containing particles of an abrasive material.

The apparatus is preferably adapted for use in a magnetic fluid grinding process; in other words, it comprises a container in which a rotating drive shaft, a magnetic float and the object to be ground may be contained, the container also containing in use a ferromagnetic fluid which preferably contains abrasive particles. Such apparatus will also comprise means by which magnetic forces may be applied to the magnetic fluid in such a way as to urge the object and the grinding surface into contact with one another during use. In such apparatus, the drive shaft surface (grinding surface) , the inner walls of the container and the float must be made of a non-ferromagnetic material. The drive shaft surface is preferably made from austenitic stainless steel, the container wall from aluminium and the float from any suitable metal or plastics material. However, these and other practical details will be chosen and varied in any particular case, with the assistance of the control means, in such a way as to cause skidding to occur and preferably so as to maximise the value of the skidding velocity v' . In such apparatus for use in a magnetic fluid grinding process, the container may comprise a liner (which must also be of an appropriate non-ferromagnetic material, for instance aluminium) . Such a liner is more easily replaced when worn away due to its contact with the object being ground during use.

The control means of the apparatus preferably comprises a computer, which may be used to monitor and/or to calculate values for operating conditions which may affect the value of v' . The apparatus may additionally comprise other apparatus and instruments which may be used to measure, monitor and control such operating conditions. For any particular operating condition to be controlled, conventional apparatus and instruments may be used. However, the control means of the apparatus preferably allows the user to control several such operating conditions together in a manner which allows him to monitor the value of v' and/or parameters which are dependent on or influence v' .

Preferably, the apparatus comprises means for monitoring (ie, for measuring and/or calculating) the values of Ω_b/Ω_s and Ω_f/Ω_s (meanings as before) so as to monitor and preferably to maximise the value of v' .

More preferably, the apparatus additionally comprises means for monitoring the value of Ω_b/Ω_s, to enable the user to adjust operating conditions so as to minimise Ω_b/Ω_s and hence to maximise v' . Such control of the operating conditions may be effected automatically by the control means of the invention. The apparatus preferably comprises measuring means for measuring the value of Ω_b at any instant during use, so as to allow calculation of values of Ω_b/Ω,. Such measuring means may comprise, for instance, a strain gauge associated with a wall of the container, which gauges the strain caused in the container wall by the object passing the gauge as it moves around the inside of the container. The container wall is preferably of reduced thickness at that point around its circumference where the strain gauge is situated. In such apparatus, the need for a replaceable liner in the container is even greater, due to the reduced thickness and hence increased vulnerability of the container wall.

Alternatively, the measuring means may comprise a source of ultrasonic or electromagnetic radiation and an appropriately positioned detector, the arrangement of source and detector being such that movement of the object within the container disturbs the passage of the radiation between the source and detector, to which disturbance the detector is sensitive.

As a further alternative, the measuring means could comprise a "cage" located within the container and moveable with the object whose surface is being ground, the speed of movement of the cage being detectable from outside the container.

According to a third aspect of the present invention there is provided apparatus for use in grinding the surface of an object to a desired shape, the apparatus comprising grinding means, having a grinding surface with which the object surface may be brought into contact; means for causing relative movement of the object surface and the grinding surface when in contact with one another; a container in which the object and grinding means are housed during use; a removable liner for the container; and liner drive means for causing movement of the liner relative to the object during use, so as continually to vary the position on the liner at which the object contacts the liner. The liner drive means preferably operates to cause the liner to execute a reciprocating motion, for instance in an up and down direction or from side to side around the inside of the container. Such apparatus is preferably adapted for use in a magnetic fluid grinding process.

Because the liner may be caused in use to move relative to the object being ground, wear on the liner caused by contact between the object surface and the liner surface may be spread out over a greater proportion of the liner surface, thus prolonging the effective life of the liner. In conventional apparatus, the liner remains fixed in position, and this results in grooves being worn at particular positions around the surface of the liner due to contact with the object being ground.

The arrangement of the grinding means and the liner in the apparatus may be such that the object whose surface is being ground is positioned between the liner surface and the grinding surface, in contact with both, during use. In such a case, the surface of the liner is preferably shaped (for instance, tapered) such that its movement relative to the object also causes movement of the object relative to the grinding surface in such a way that the object is forced to adopt a range of different positions relative to the grinding surface. In such apparatus, wear on the grinding surface is also spread out over a greater proportion of that surface.

According to a fourth aspect of the present invention, there is provided a container liner and liner drive means in combination, as described above, suitable for use in apparatus for use in grinding the surface of an object. The liner may be shaped as described above.

The present invention will now be described by way of example only and with reference to the accompanying illustrative drawings, of which:

Figure 1 shows a longitudinal cross-section through part of typical magnetic fluid grinding apparatus; Figure 2 shows in more detail part of the cross-section shown in Figure 1, indicating the geometry of certain components of the apparatus;

Figure 3 is a graph showing the variation of Ω_b/Ω_s with Ω_f/Ω_s in a magnetic fluid grinding process; Figure 4 is a graph showing the expected variation of v'/Ω_s with Ω_b/Ω_s in a magnetic fluid grinding process;

Figure 5 is a graph showing observed variations of Ω_b with Ω, in grinding methods in accordance with the present invention; Figure 6 is a graph showing surface removal rate against WV in methods carried out in accordance with the present invention;

Figure 7 illustrates by means of cross-sections various types of apparatus in accordance with the invention;

Figure 8 shows, by means of a front view and a part longitudinal cross-section, further apparatus in accordance with the invention; and

Figures 9 and 10 are longitudinal cross-sections through parts of yet further types of apparatus in accordance with the invention.

Detailed Description

The theory behind the present invention, as described above, was supported by experimental studies on motion and finishing of balls of a ball bearing grade of silicon nitride (Vickers Hardness 14GPa) , from 12.5 to 8 mm in diameter, by grits in a magnetic fluid. The apparatus used for the studies was a magnetic fluid grinding cell such as is shown in part cross- section in Figure 1. The apparatus comprises a cell or container 1 containing a magnetic fluid 2 in which is suspended a suitable abrasive grit. A float 3 is positioned beneath drive shaft 4, both being made of a non-ferromagnetic material (eg, stainless steel for the shaft 4; a metal or plastics material for the float 3) . An array of magnets 5 is arranged below cell l, their poles alternately directed towards the base of the cell, as shown.

In use, balls such as 6, to be finished, are held in the cell whilst shaft 4 is rotated at a rotation speed Ω_s. A load W is applied downwards through the shaft and is resisted by magnetic forces acting upwardly on the float. The balls contact the drive shaft, the walls of the cell 1 and the float 3, the abrasive grit in fluid 2 helping to wear away the surfaces of the balls. In all the tests, the load W ranged from about 0.2 to 1.0N per ball and the shaft rotation speed from about 2000 to 8000 rev/min. The rotation speed of float 3, Ω_f, took whatever was its natural value, usually in the range 0 to 1000 rev/min. Cell and fluid materials were varied as indicated in Table 1, but in all tests the shaft 4 was made of an austenitic grade of stainless steel and the float 3 of an aluminium alloy. In series D and E, the difference between the cell and float diameters was varied from less than to approximately equal to the ball radius.

Table 1. Conditions of magnetic fluid grinding tests.

Series Magnetic fluid Container wall material

Carrier Viscosity at 25°C liquid Pa.s

A Water 0.01 Elastomer

B Water 0.02 Elastomer

C Hydrocarbon 0.005 Elastomer

D1,D2 Water 0.01 Aluminium alloy

E1,E2 Water 0.02 Aluminium alloy

F Hydrocarbon 0.005 Aluminium alloy

Example observations of motion measurements are presented in Figures 5a-5d. In each panel of the Figure, the hatched line marked v' = 0 is the expected variation of Ω_b with Ω_s with no skidding at the shaft contact with the ball. In every case there is a range of low Ω, for which v' = 0, but there is a critical value at which deviations from v' = 0 occur, ie, at which skidding starts.

Figure 5a shows that the onset of skidding is displaced to larger values of Ω_s by increasing W: this is a universal observation. Referring to equation (1), removal rate V/t depends on the product of v' and W; Figure 5a, with Figure 4, indicates that, at a given Ω_s, Ω_b/Ω_s increases and hence v' reduces as W increases; hence removal rate will not increase in proportion to W, but more slowly. In fact it is possible that an increase of W will lead to a reduction of removal rate, if that increase suppresses sliding sufficiently. There will be an optimum load W at each shaft speed Ω_s for maximum removal rate, in any particular experiment. Figure 5b shows that the onset of skidding with increasing Ω₅ also depends on the viscosity of the magnetic fluid in the cell. The viscosities quoted are those measured for the fluids at 25°C, with no abrasive grit suspended in them. Subsidiary tests have demonstrated that the amount of abrasive in the fluid also influences the amount of slip, perhaps through its effect on viscosity.

Figure 5c demonstrates an effect of the container wall which may be interpreted as an effect of friction at the contact point between the container and balls. Abrasive grits were observed to embed in an aluminium alloy container wall, but they do not embed in an elastomer wall. The embedded grit impedes the motion of the ball, so that more slip occurs at the (driving) shaft contact than when the wall is made from an elastomeric material.

Finally, Figure 5d shows the effect of clearance between the container and float, that only occurs for particular fluid viscosities. In the present tests, slip was difficult to induce with the fluid of viscosity 0.01 Pa.s when the diametral difference between container and float was much less than the ball radius (curve (i) in Figure 5d) , but when clearance was increased to equal the ball radius, curve (ii) was obtained. In this latter case the balls could almost roll off the edge of the float and it is speculated that the float then increased the resistance to motion of the balls. However when the fluid viscosity was increased to 0.02 Pa.s this float size effect was not needed to create slip (although a large clearance appeared to aid ball sphericity) . When fluid viscosity was reduced to 0.005 Pa.s, the float size effect was not large enough to create slip. Thus it appears that, in these tests, the viscosity range 0.01 to 0.02 Pa.s is critical for the process; viscosities larger than this are needed if skidding is to be induced to an adequate extent.

The tests detailed above show that a whole range of operating conditions may be adjusted so as to increase v' and hence surface removal rate. The essential feature of the present invention is the need to operate in that region of the graphs shown in Figure 5 where skidding occurs and v' > 0. By monitoring the variation of Ω_b with Ω_s, this region can more easily be located.

It appears that for a given ball material and abrasive grit type in a magnetic fluid grinding process, surface removal rate is completely controlled by the product of v' and W. This is demonstrated in Figure 6, in which V/t (refer to equation (1)) is plotted against Wv' , where v' is deduced from Ω_b/Ω_s, for all the measurements from series A to F of Table 1. Despite the wide range of motions illustrated in Figure 5, all data plot on to a single relationship that is represented by equation (1) , with K = 0.07 ± 0.02.

The size of K is significant. It is in the range of 0.1 to 0.01 expected when an abrasive is fixed to an abrading surface. In the present case, abrasive was observed to be embedded in the austenitic stainless steel drive shaft surface. A further preferred feature of the method of the invention is thus that the grinding surface used be sufficiently soft for abrasive to embed in it. As mentioned above, the material of the container wall is also preferably such as to allow the abrasive to embed in it.

It should be noted that the data presented so far refer to removal rates with new grits and unworn cells. Over a period of use, deterioration occurs both in the grit and in the geometry of the cell. Thus, measured values of Ω_b/Ω₅ should be interpreted with care, bearing in mind that the value of K will decrease slightly with time due to blunting of the abrasive.

Another aspect of the present invention is apparatus which preferably comprises means for monitoring Ω_b (angular velocity of the balls moving around the container in apparatus for carrying out magnetic fluid grinding) . One such means allows detection of the passage of a ball round the container wall b the strain that the passing ball causes in the wall. One way of doing this, which has been successfully used in the present work, is by attaching a strain gauge to the outer surface of the container. For this to be effective, the container wall must be thin; an example of apparatus modified in this way is shown in Figure 7. Figure 7a shows the lower part of the apparatus in longitudinal cross-section (15 is the drive shaft, 14 the balls being ground) ; Figure 7b an axial cross-section through the container 10 of the apparatus; Figure 7c a transverse cross-section through the container; and Figure 7d a transverse cross-section showing the use of an optional liner 13.

Figure 7 shows an example of a container 10, in this case 48 mm in outer diameter, made from tube with a wall thickness of 5mm, with the wall thinned to 0.5 mm in one segment 11. It is to this thinned part of the wall that a strain gauge 12 is attached.

In order to protect the strain-gauged wall from damage due to wear or otherwise, an expendable liner 13 may be sleeved inside the container, as illustrated in Figure 7d. A metal liner is preferred, but an elastomer or polymer liner is also possible. To maintain sensitivity of the strain gauge response to the passing balls 14, the liner 13 should be thin. For a metal liner a maximum thickness of 1.5 mm is recommended, but up to 3 mm is allowable with elastomer or polymer liners.

Passage of balls round a container may be detected by other means. Non-contacting means could rely on a ball's movement disturbing the passage of an ultrasonic or electromagnetic wave. A wave source and detector would then form part of a measuring unit associated with the grinding apparatus.

A third means of measuring Ω_b might involve adding a cage to the apparatus, as shown in Figure 8. Figure 8a shows a front view of part of such modified apparatus, showing the arrangement of balls 20 and cage 21. Figure 8b is part of a longitudinal cross-section through the apparatus, showing the drive shaft 22, magnetic float 23, container wall 24 and liner 25. The cage 21 is of a tubular construction.

In use, the balls 20 carry the cage 21 with them, and rotation of the cage can then be detected from outside the cell by appropriate means. A subsidiary advantage of a cage could be an additional drag on the ball motion and an enhancement of skidding between the balls and the shaft 22.

In every case the use of the measurement of Ω_b might be firstly to calculate Ω_b/Ω_s and hence to determine v' from a graph such as Figure 4; and secondly to multiply v' by the load W to determine the product v'W. For given material conditions, operating conditions of W and Ω_s, and possibly of the drive shaft cone angle θ (see Figure 2) , would be altered to obtain a maximum or other desired value of v'W. This could be either a manual calculation or obtained using a suitable automated software program as part of a control loop to change W and Ω_s so as to maximise v' and hence surface removal rate.

In use of a method or apparatus in accordance with the present invention, in order to achieve operation in the skidding region, the following operating conditions have been found to be preferred:

Ferromagnetic fluid : Any chemical composition, but a viscosity greater than 0.01 to 0.02 Pa.s is preferred. Container wall (or liner of container wall) : Any non- ferromagnet ic material, but a material in which abrasive grits can become embedded is preferred. Metal materials may be aluminium, copper or austenitic iron or their alloys. Elastomer and plastics materials are also possible.

Drive shaft Any non-ferromagnetic metal; austenitic stainless steel is an example that has been used, but aluminium, nickel or c opper a l l oy s are alternatives.

Float Any non- f erromagnet ic material, but metal and plastics materials are preferred. As far as the float is concerned there is a preferred diameter relative to the container wall or liner diameter and the ball radius. Beneficially, the difference in diameter between the inner surface of the container or liner and the float is almost equal to the ball radius although not greater.

Shaft linear or rotation speed The surface speed of the shaft where it contacts the balls should be greater than 2.5 m/s. This corresponds to a rotational speed of 2,000 rev/min in the example data of Figure 5. There is no upper speed limit.

Load per ball Typical loads per ball in this process will be in the range 0.2 to 5N, though higher values may be possible. Optimum values may be around IN. The numerical values of viscosity, shaft speed and load quoted above are all based on the finishing of balls of around 10 mm diameter. It is expected from published studies of magnetic force generation in the process (Childs and Yoon, Annals. CIRP, Vol 41/1/92, pp.343-346) that either much larger or smaller balls may also be finished using the method of the invention. For larger balls, lower viscosity fluids and higher loads might be applicable, while for smaller balls larger viscosities and lower loads might apply.

Turning now to Figures 9 and 10, there are shown part cross- sections of apparatus in accordance with the third aspect of the invention, which incorporate reciprocating container liners. Each apparatus, for use in the magnetic fluid grinding of balls such as 30, comprises a container 31, a drive shaft 32 and a magnetic float 33. Liners 34 (Figure 9) and 35 (Figure 10) are caused, by a mechanism not shown but conventional in construction, to oscillate up and down relative to the container 31, as shown by the arrows.

The liner 35 is tapered at its lower end at an angle to the vertical.

The advantage of apparatus such as that seen in Figures 9 and 10 is as follows. A shortcoming of the geometrical arrangement sketched in Figures 1 and 2 is that grooves worn in the container, shaft or float, due to contact with the balls, limit the life of these parts of the cell. If the clearance between the float and container is increased, the float oscillates in its rotation round the cell and the groove worn in its surface becomes spread out, which can thus increase its life. A similar spreading of wear may be achieved in conjunction with the cell wall liner. Figure 9 shows a liner 34 with clearance between it and the container wall, that may be oscillated up and down to spread the wear on its surface. If the face of the liner is inclined slightly to the vertical, as shown in Figure 10, oscillation of the liner will also cause the balls 30 to move up and down the face of the drive shaft 32, thus spreading the wear on this contact too.

In the processes described in the previous sections, the container was stationary and the shaft rotated. Centrifugal forces thus throw the balls onto the container or its liner. If the idea of Figure 10 is adopted, centrifugal forces will limit the slope of the liner because if the slope is too large balls will roll down it and lose contact with the drive shaft 32. In that case it might be advantageous to limit centrifugal forces by rotating both the shaft and the liner in opposite directions, so as to reduce the absolute value of

The concept of increasing surface removal rate by increasing skidding between an object and an abrasive surface, particularly by adding a viscous substance to the system, may have a wider applicability than to magnetic fluid grinding alone, for instance in traditional lapping processes. However, in magnetic fluid grinding the magnetic fluid not only provides viscosity, it also carries the abrasive. Thus if the effect of viscous drag on V were to be utilised in a traditional lapping process, a fluid with more of the properties of a plastic gel or jelly might be necessary, able to carry abrasive grits in it without the grits settling out under gravitational forces.

Claims

1. A method for grinding the surface of an object to a desired shape, the method comprising contacting the object surface with a grinding surface; causing relative movement between the object and grinding surfaces so as to effect removal of material from the object surface; and adjusting the operating conditions during the relative movement so as to cause skidding to occur between the object and grinding surfaces.

2. A method according to claim 1 in which the grinding surface comprises the surface of a rotating shaft.

3. A method according to claim 2 in which the object is caused to move around a container in which the object and shaft are enclosed.

4. A method according to any preceding claim in which the the object and grinding surfaces are immersed in a fluid containing particles of an abrasive material such as diamond or other hard grit, whilst the surfaces are moving relative to one another.

5. A method according to claim 2 for grinding the surface of a spherical object by means of a magnetic fluid grinding process, the method additionally comprising the steps of monitoring values for Ω_b/Ω_s and Ω_f/Ω, (where Ω_b is the angular velocity of the object rotating around the rotating shaft Ω the angular velocity of the shaft and Ω, the angular velocity of a magnetic float used to urge the object and grinding surfaces together) ; and utilising the measured values to adjust the value of v' , where v' is the velocity of the skidding motion.

A method according to claim 5 wherein the operating conditions of the method are adjusted so as to minimise the value of Ω,,/Ω,.

7. Apparatus for use in grinding the surface of an object to a desired shape, the apparatus comprising a grinding surface which may be brought into contact with the surface of the object in such a way that the grinding surface and the object surface are moveable relative to one another; means for causing relative movement of the surfaces when in contact with one another; and control means for monitoring and/or controlling the operating conditions of the apparatus in use, so as to cause the relative movement of the two surfaces to give rise to skidding between the surfaces.

8. Apparatus according to claim 6 including means for monitoring the value of Ω,,/Ω .

9. Apparatus for use in grinding the surface of an object to a desired shape, the apparatus comprising grinding means, having a grinding surface with which the object surface may be brought into contact; means for causing relative movement of the object surface and the grinding surface when in contact with one another; a container in which the object and grinding means are housed during use; a removable liner for the container; and liner drive means for causing movement of the liner relative to the object during use, so as continually to vary the position on the liner at which the object contacts the liner.

10. Apparatus according to .claim 9 in which the liner drive means operates' to cause the liner to execute a reciprocating motion, for instance in an up and down direction or from side to side around the inside of the container.