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WO1995020465A1 - Meulage de surfaces - Google Patents

Meulage de surfaces Download PDF

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Publication number
WO1995020465A1
WO1995020465A1 PCT/EP1995/000234 EP9500234W WO9520465A1 WO 1995020465 A1 WO1995020465 A1 WO 1995020465A1 EP 9500234 W EP9500234 W EP 9500234W WO 9520465 A1 WO9520465 A1 WO 9520465A1
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WO
WIPO (PCT)
Prior art keywords
grinding
balls
container
liner
shaft
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/EP1995/000234
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English (en)
Inventor
Thomas H. C. Childs
Hean Joo Yoon
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
University of Leeds
Original Assignee
University of Leeds
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Filing date
Publication date
Application filed by University of Leeds filed Critical University of Leeds
Publication of WO1995020465A1 publication Critical patent/WO1995020465A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B11/00—Machines or devices designed for grinding spherical surfaces or parts of spherical surfaces on work; Accessories therefor
    • B24B11/02—Machines or devices designed for grinding spherical surfaces or parts of spherical surfaces on work; Accessories therefor for grinding balls
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B1/00—Processes of grinding or polishing; Use of auxiliary equipment in connection with such processes
    • B24B1/005—Processes of grinding or polishing; Use of auxiliary equipment in connection with such processes using a magnetic polishing agent

Definitions

  • 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.
  • 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) .
  • 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.
  • 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) .
  • Polishing is not the only finishing operation required of ceramics balls. Prior to polishing, a substantially thicker layer than 0.1 ⁇ m 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.
  • the surface finishing of a ceramic ball is a three stage process.
  • a first stage known as equalising
  • the differences in shape caused by shrinkage of the as- sintered balls are removed.
  • a second stage known as rough lapping
  • any imperfect surface layer is removed: this can involve diameter reductions of from 300 to 1000 ⁇ m.
  • the last stage known as fine lapping, can typically involve a further diameter reduction of 1 to 5 ⁇ m.
  • 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.
  • 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.
  • a production batch may consist of several thousand balls.
  • 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) .
  • FIG. 1 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.
  • the shaft drives the balls through the fluid which is maintained as a good abrasive slurry.
  • 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.
  • a method for grinding the surface of an object to a desired shape 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.
  • 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.
  • V KWv t/H (1)
  • 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 Figure shows the variation of ⁇ b / ⁇ s with ⁇ f / ⁇ s for a range of values of v'/ ⁇ s .
  • the values of v'/ ⁇ s quoted are for v' in m/s and ⁇ , 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'/ ⁇ s than under others.
  • conditions giving low values of ⁇ b / ⁇ s gave rise to relatively high values of v'/ ⁇ 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) .
  • the monitoring and control of the skid velocity v' can be used at least partially to control the surface grinding process.
  • 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) .
  • ⁇ b / ⁇ s depends on many variables. Principal ones, but not necessarily the only ones, are the shaft rotation speed ⁇ 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 ⁇ b / ⁇ 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 ⁇ b / ⁇ s or by monitoring directly their effect on surface removal rate, V/t.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • a magnetic float may also be used to help urge the two surfaces into contact with one another.
  • 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 may also be adjusted in a way that affects the value of V .
  • Viscosity of magnetic fluid > 0.01 to 0.02 Pa.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.
  • 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.
  • the method preferably additionally comprises the steps of monitoring values for ⁇ b / ⁇ s and ⁇ f / ⁇ s (where ⁇ b is the angular velocity of the object rotating around a rotating shaft carrying the grinding surface, ⁇ s the angular velocity of the shaft and ⁇ 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' .
  • v' 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.
  • the method comprises the steps of monitoring values for ⁇ b / ⁇ s and adjusting the operating conditions of the method so as to minimise the value of ⁇ b / ⁇ s and hence to maximise the value of v' .
  • This may be achieved, for instance, by monitoring the variation of ⁇ b with ⁇ s , and selecting such values of ⁇ b and ⁇ s and such other operating conditions as to operate the method in that region of the ⁇ b / ⁇ s graph in which skidding can be seen to occur between the object surface and the grinding surface.
  • 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.
  • 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.
  • 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.
  • the container may comprise a liner (which must also be of an appropriate non-ferromagnetic material, for instance aluminium) .
  • 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.
  • 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' .
  • the apparatus comprises means for monitoring (ie, for measuring and/or calculating) the values of ⁇ b / ⁇ s and ⁇ f / ⁇ s (meanings as before) so as to monitor and preferably to maximise the value of v' .
  • the apparatus additionally comprises means for monitoring the value of ⁇ b / ⁇ s , to enable the user to adjust operating conditions so as to minimise ⁇ b / ⁇ s and hence to maximise v' .
  • 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 ⁇ b at any instant during use, so as to allow calculation of values of ⁇ b / ⁇ ,.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • wear on the grinding surface is also spread out over a greater proportion of that surface.
  • 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.
  • 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 ⁇ b / ⁇ s with ⁇ f / ⁇ s in a magnetic fluid grinding process
  • Figure 4 is a graph showing the expected variation of v'/ ⁇ s with ⁇ b / ⁇ s in a magnetic fluid grinding process
  • Figure 5 is a graph showing observed variations of ⁇ b with ⁇ , 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
  • FIG. 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.
  • Figures 9 and 10 are longitudinal cross-sections through parts of yet further types of apparatus in accordance with the invention.
  • the theory behind the present invention 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.
  • balls such as 6, to be finished, are held in the cell whilst shaft 4 is rotated at a rotation speed ⁇ 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.
  • 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, ⁇ f took whatever was its natural value, usually in the range 0 to 1000 rev/min.
  • Figure 5a shows that the onset of skidding is displaced to larger values of ⁇ s by increasing W: this is a universal observation.
  • removal rate V/t depends on the product of v' and W;
  • Figure 5a indicates that, at a given ⁇ s , ⁇ b / ⁇ s increases and hence v' reduces as W increases; hence removal rate will not increase in proportion to W, but more slowly.
  • an increase of W will lead to a reduction of removal rate, if that increase suppresses sliding sufficiently.
  • Figure 5b shows that the onset of skidding with increasing ⁇ 5 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.
  • Figure 5d shows the effect of clearance between the container and float, that only occurs for particular fluid viscosities.
  • 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.
  • 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) .
  • 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 ⁇ b with ⁇ s , this region can more easily be located.
  • 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.
  • the material of the container wall is also preferably such as to allow the abrasive to embed in it.
  • apparatus which preferably comprises means for monitoring ⁇ b (angular velocity of the balls moving around the container in apparatus for carrying out magnetic fluid grinding) .
  • ⁇ 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 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;
  • 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.
  • 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.
  • 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.
  • 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 ⁇ 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • FIGS. 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.
  • centrifugal forces 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.
  • magnetic fluid grinding the magnetic fluid not only provides viscosity, it also carries the abrasive.
  • 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.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Finish Polishing, Edge Sharpening, And Grinding By Specific Grinding Devices (AREA)

Abstract

L'invention se rapporte à une cellule de meulage par fluide magnétique. L'appareil comprend une cellule ou cuve (1) contenant un fluide magnétique (2) dans lequel sont en suspension des particules abrasives appropriées. Un flotteur (3) est positionné sous l'arbre d'entraînement (4), les deux étant fabriqués dans un matériau non ferro-magnétique. Un groupe d'aimants (5) est placé sous la cellule (1), leurs pôles dirigés alternativement vers la base de la cellule, comme indiqué. Les billes (6) à polir sont maintenues dans la cellule alors que l'arbre (4) tourne. Une charge W est appliquée vers le bas à travers l'arbre et est maintenue par des forces magnétiques agissant vers le haut sur le flotteur. Les billes viennent en contact avec l'arbre d'entraînement, les parois de la cellule (1) et le flotteur (3), les particules abrasives dans le fluide (2) facilitant l'usure des surfaces des billes. La vitesse rotative de l'arbre (4), Φs, et celle du flotteur (3), Φf, est réglée de façon à provoquer un glissement entre les billes (6) et les surfaces de meulage.
PCT/EP1995/000234 1994-01-27 1995-01-23 Meulage de surfaces Ceased WO1995020465A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB9401593.0 1994-01-27
GB9401593A GB9401593D0 (en) 1994-01-27 1994-01-27 Surface grinding

Publications (1)

Publication Number Publication Date
WO1995020465A1 true WO1995020465A1 (fr) 1995-08-03

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PCT/EP1995/000234 Ceased WO1995020465A1 (fr) 1994-01-27 1995-01-23 Meulage de surfaces

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GB (1) GB9401593D0 (fr)
WO (1) WO1995020465A1 (fr)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1999019115A1 (fr) * 1997-10-10 1999-04-22 Axen Niklas Procede et dispositif permettant de donner a des boules en ceramique ou autres materiaux durs une forme spherique
WO1999033610A1 (fr) * 1997-12-30 1999-07-08 Board Of Regents For Oklahoma State University Polissage par flottabilite dans un fluide magnetique d'un materiau magnetique
WO1999047304A1 (fr) * 1998-03-17 1999-09-23 Board Of Regents For Oklahoma State University Machine a flotteur magnetique de polissage d'objets en grandes series
WO1999016577A3 (fr) * 1997-09-30 2003-05-08 Univ Oklahoma State Procedes de polissage par fluide magnetique et materiaux associes
US7252576B1 (en) 2006-02-21 2007-08-07 The Board Of Regents For Oklahoma State University Method and apparatus for magnetic float polishing
EP3819400A1 (fr) * 2019-11-08 2021-05-12 The Swatch Group Research and Development Ltd Cuve de polissage avec chaussette amovible et capteurs integres

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2398573A1 (fr) * 1977-07-26 1979-02-23 Newall Eng Procede et appareil d'usinage de cames
US4216629A (en) * 1978-09-15 1980-08-12 Degaeta Albert M Method and apparatus for producing balls
JPS57205067A (en) * 1981-06-08 1982-12-16 Toyota Motor Corp Barrel for barrel-finishing equipment
DE4022009A1 (de) * 1989-07-11 1991-02-07 Nippon Sheet Glass Co Ltd Verfahren zur kontaktlosen, maschinellen bearbeitung sphaerischer flaechen
US5046287A (en) * 1991-02-20 1991-09-10 Lin Chao Tong Wet grinding type of grinder with improved material discharge mechanism

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2398573A1 (fr) * 1977-07-26 1979-02-23 Newall Eng Procede et appareil d'usinage de cames
US4216629A (en) * 1978-09-15 1980-08-12 Degaeta Albert M Method and apparatus for producing balls
JPS57205067A (en) * 1981-06-08 1982-12-16 Toyota Motor Corp Barrel for barrel-finishing equipment
DE4022009A1 (de) * 1989-07-11 1991-02-07 Nippon Sheet Glass Co Ltd Verfahren zur kontaktlosen, maschinellen bearbeitung sphaerischer flaechen
US5046287A (en) * 1991-02-20 1991-09-10 Lin Chao Tong Wet grinding type of grinder with improved material discharge mechanism

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 7, no. 59 (M - 199)<1204> 11 March 1983 (1983-03-11) *
T.H.C. CHILDS, H.J. YOON: "Magnetic Fluid Grinding Cell Design", ANNALS OF THE CIRP, vol. 41, pages 343 - 346 *

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1999016577A3 (fr) * 1997-09-30 2003-05-08 Univ Oklahoma State Procedes de polissage par fluide magnetique et materiaux associes
WO1999019115A1 (fr) * 1997-10-10 1999-04-22 Axen Niklas Procede et dispositif permettant de donner a des boules en ceramique ou autres materiaux durs une forme spherique
WO1999033610A1 (fr) * 1997-12-30 1999-07-08 Board Of Regents For Oklahoma State University Polissage par flottabilite dans un fluide magnetique d'un materiau magnetique
WO1999047304A1 (fr) * 1998-03-17 1999-09-23 Board Of Regents For Oklahoma State University Machine a flotteur magnetique de polissage d'objets en grandes series
US7252576B1 (en) 2006-02-21 2007-08-07 The Board Of Regents For Oklahoma State University Method and apparatus for magnetic float polishing
EP3819400A1 (fr) * 2019-11-08 2021-05-12 The Swatch Group Research and Development Ltd Cuve de polissage avec chaussette amovible et capteurs integres
WO2021089724A1 (fr) * 2019-11-08 2021-05-14 The Swatch Group Research And Development Ltd Cuve de polissage avec chaussette amovible et capteurs integres

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