WO1999047304A1

WO1999047304A1 - Large batch magnetic float polishing equipment

Info

Publication number: WO1999047304A1
Application number: PCT/US1999/005669
Authority: WO
Inventors: Ranga Komanduri; Noritsugo Umehara; Ming Jiang; Peijun Cao
Original assignee: Board Of Regents For Oklahoma State University
Priority date: 1998-03-17
Filing date: 1999-03-15
Publication date: 1999-09-23
Also published as: AU3090699A

Abstract

A dual load system (18) to achieve desired controlled polishing forces in the finishing of large batches of ceramic balls (54). The first, and primary, load system (18) is a mechanical dead weight system, and the other is a magnetic field system. The former applies the required load per ball (54) (e.g. 1N/ball) while the latter acts as a spring load to even out the polishing forces due to variations in the size of the balls (54). In principle, the apparatus can be used either as a straight polishing apparatus or as a magnetic float polishing apparatus.

Description

LARGE BATCH MAGNETIC FLOAT POLISHING EQUIPMENT

BACKGROUND OF THE INVENTION

1. Field of the Invention:

This invention relates generally to equipment used to conduct magnetic float polishing and, more specifically, to the design, construction and use of equipment for finishing large batches of ceramic balls.

2. Background:

Magnetic float polishing (MFP), sometimes termed magnetic fluid grinding, is a "gentle" polishing technique based on the magneto-hydrodynamic behavior of a magnetic fluid that can float non-magnetic abrasive grains suspended in it. The magnetic fluid is generally a colloidal dispersion of extremely fine (100 to 150 Λ) subdomain ferro-magnetic particles, usually magnetite (Fe₃O₄), in water or hydrocarbon based carrier fluids such as kerosene. The ferrofluids are made stable against particle agglomeration by the addition of surfactants. When a magnetic fluid is placed in a magnetic field gradient, it is attracted towards the side having a higher magnetic field intensity. If a non-magnetic substance (e.g., abrasive grains in this case) is mixed in the magnetic fluid, it is discharged towards the side having a lower magnetic field intensity. When the field gradient is set in the gravitational direction, the non-magnetic material is made to float on the fluid surface by the action of a magnetic buoyancy levitational force. The MFP process is considered highly effective for finish polishing because the levitational force is applied to the abrasive grains in a controlled manner. The forces applied by the abrasives to a workpiece set in the fluid are extremely small (about 1 N or less).

The development of high-performance ceramics (or advanced ceramics) is stimulating major advances in a large spectrum of industries including machine tools, electronics, manufacturing engineering, and chemical and metallurgical processing. Alumina (Al₂O₃), zirconia (Zr0₂), silicon carbide (SiC) and silicon nitride (Si₃N₄) are the most important advanced ceramic materials among high-performance ceramics with Si₃N₄ being the most promising material in this category for advanced structural bearing applications because of its high toughness among ceramics. Ceramic bearings offer significant improvements in performance and durability for a wide variety of applications ranging from inertial guidance systems to precision gimbals to turbine engine exhaust nozzle actuators and submarine pumps. Hybrid bearings with silicon nitride balls have made 100,000 rpm a possibility for high speed machine tool spindles. This is principally due to higher rigidity and greater precision associated with these bearings.

Until the advent of magnetic float polishing, ceramic balls were finished using low polishing speeds (a few hundred rpm) and diamond abrasive as a polishing medium. It takes a considerable time (some 12-15 weeks) to finish a batch of ceramic balls in this fashion, and the use of diamond abrasives at high loads often results in deep pits, scratches and microcracks on the ceramic ball surface. Magnetic float polishing allows for higher removal rates and shorter polishing cycles by using high polishing speeds with very low level controlled forces and abrasives not much harder than the workpiece.

Conventional apparatus for conducting magnetic float polishing have a chamber for holding ceramic balls in a magnetic fluid suspension, magnets beneath the chamber, and a rotatable drive shaft for contacting and turning against the balls. In some systems a nonmagnetic float, such as an acrylic float, is positioned in the suspension within the chamber. The ceramic balls are placed above the top surface of the float. The drive shaft is connected to the spindle of a milling machine, and the shaft is mated with the chamber to establish contact with the balls. When the spindle is rotated the balls are polished by the abrasive grains under the action of the magnetic buoyancy levitational force.

The conventional magnetic float polishing apparatus was originally developed for handling small batches of balls (- 15 balls of 0.5 inch diameter). However, to address the need for higher production, a radically different approach is needed. It is accordingly an object of the present invention to provide a device for finishing larger batches of ceramic balls, such as batches of 100 or more silicon nitride balls of an 0.5 inch diameter.

It is a further object of the invention that the apparatus be capable of conducting magnetic float polishing of ceramic balls under conditions similar to those of small batch equipment and that the large batch apparatus yields comparable results. SUMMARY OF THE INVENTION

These and other objects are achieved in polishing equipment that utilizes a dual load system to achieve desired polishing forces for large batches. The first, and primary, load system is a mechanical dead weight system, while the magnetic field system serves as a secondary system. The former applies the required load per ball (e.g. lN all) while the latter acts as a spring load to even out the polishing forces. In principle, the apparatus can be used either as a straight polishing apparatus or as a magnetic float polishing apparatus. As a straight polishing apparatus the mechanical load system alone is used, in which case the polishing chamber does not contain magnetic fluid but merely a polishing medium, such as water and an abrasive. But the preferred method of use is as a magnetic float polishing apparatus utilizing the dual load systems. This large batch equipment actually yields better results in terms of sphericity because of a more uniform distribution of load amongst larger numbers of balls.

The inventive apparatus has four primary component parts -- a carrier table, a polishing chamber, a rotatable drive shaft, and the mechanical load system. The polishing chamber is supported on the top surface of the carrier table, which is slidably mounted for vertical movement upon a number of carrier table support shafts. The polishing chamber has a ball track for receiving and holding a large number of workpieces, such as ceramic balls, and for holding a quantity of polishing fluid in contact with the workpieces. The drive shaft is adapted for coupling to the spindle of a milling machine has a ball contact surface for engaging the workpieces in the ball track when the polishing chamber and drive shaft are axially aligned and brought together. The mechanical load system is connected to the carrier table so as to influence its vertical movement, whereby under a controlled load the polishing chamber is moved axially (or linearly) toward the driveshaft to achieve the desired polishing force upon the workpieces.

In accordance with one aspect of the invention, the mechanical load system is a dead weight load system comprising a pair of pulley support posts mounted on opposite sides of the carrier table. A cable, such as steel wire, is fixed to each side of the carrier table and is run over the support posts and connected to a dead weight pan. The amount of weight on the dead weight pan is controlled in order to achieve the desired polishing force. The most preferred embodiment of the invention combines the large batch polishing apparatus with magnetic float polishing techniques. In this embodiment, the polishing chamber functions as a float chamber for holding a quantity of magnetic polishing fluid in contact with the workpieces. The float chamber further has a magnetic field generating means beneath the ball track so that, when a magnetic field is applied to the magnetic fluid suspension, an upward magnetic buoyancy levitational force is produced. In this embodiment, the mechanical load system provides the primary polishing force upon the workpieces while the magnetic buoyancy levitational force produced by the magnetic field system evens out uneven load forces created during polishing due to differences in the diameters of the workpieces. In one particularly preferred aspect of the invention, the magnetic field generating means comprises a bank of permanent magnetics arranged in segments to form a substantially circular ball track floor, the magnets being oriented in an alternate pole manner.

In a further aspect of the invention, the float chamber is modified to have multiple ball tracks and the drive shaft is likewise altered to have a number of ball contacting annular projections for mating with each ball track, thus increasing the capacity of the apparatus.

Results obtained with the inventive apparatus are very favorable and compare well with the results obtained with the conventional smaller apparatus. The preferred embodiment of the invention can finish approximately 100 balls per batch, and this capacity can be further increased utilizing the multiple ball tracks.

A better understanding of the present invention, its several aspects, and its objects and advantages will become apparent to those skilled in the art from the following detailed description, taken in conjunction with the attached drawings, wherein there is shown and described the preferred embodiment of the invention, simply by way of illustration of the best mode contemplated for carrying out the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a preferred embodiment of the inventive large batch magnetic float polishing apparatus.

FIG. 2 is a cross section taken along line 2-2 of FIG. 1. FIG. 3 is a cross section taken along line 3-3 of FIG. 1.

FIG. 4 is a partial cross sectional view of an alternate drive shaft and float chamber arrangement for further increasing polishing capacity.

FIG. 5 is a partial top view of the float chamber showing a preferred magnet orientation.

FIG. 6 is a schematic showing the preferred polar orientation of magnets.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Before explaining the present invention in detail, it is important to understand that the invention is not limited in its application to the details of the construction illustrated and the steps described herein. The invention is capable of other embodiments and of being practiced or carried out in a variety of ways. It is to be understood that the phraseology and terminology employed herein is for the purpose of description and not of limitation.

Referring initially to FIGS. 1-3, there is shown a preferred embodiment of the inventive apparatus 10. The apparatus 10 has four primary components— a carrier table 12, a float chamber 14, a drive shaft 16, and a mechanical load system 18.

The carrier table 12 is slidably mounted for vertical movement upon four carrier table support shafts 20. The support shafts 20 are mounted on a support base 22 with a lower support shaft mount plate 24 and bolts 26. An angled upper support shaft mount plate 28 receives a linear bearing 30 for coaxial placement over the support shaft 20. The linear bearing 30 and upper support shaft mount plate 28 are secured to the top surface of the carrier table 12, such as by bolts 32.

The float chamber 14, also referred to herein as a polishing chamber, is supported upon the carrier table 12 by three float chamber supports 34. A number of float chamber alignment clamps 36 are used to fix the float chamber 14 in place once it is properly aligned (i.e., the axis of the float chamber 14 coincides with the axis of the drive shaft 16). A bore gauge is used to ensure proper concentricity of the chamber 14.

In its preferred embodiment, the float chamber 14 consists of a lower steel yolk 38 and an upper guide ring 42 secured together by bolts 40 and a raised, centered disc 46. The guide ring 42 has an upright, angular wall portion 44 that, along with the edge of centered disc 46, form the walls of a ball track 48. The floor of the ball track 48 comprises a bank of permanent magnets 50 arranged in segments to form a substantially circular ball track floor. The magnets are oriented in an alternate pole manner as illustrated in FIGS. 5 and 6.

Above the ball track floor is a circular non-magnetic float 52 . In operation, the ball track 48 contains a plurality of workpieces 54 and a quantity of magnetic polishing fluid. The magnetic field created by the magnets 50 acts on the magnetic polishing fluid to produce an upward magnetic buoyancy levitational force. The float serves to evenly distribute this magnetic buoyancy levitational force to the workpieces 54. The magnetic polishing fluid contains non-magnetic abrasive grains which are forced upward under the magnetic buoyancy levitational force into polishing contact with the workpieces 54.

The drive shaft 16 has a drive shaft head 56 and a shank 58. The drive shaft head 56 has an outer annular wall 60 with a beveled edge 62. In operation, the shank 58 is connected to the spindle of a milling machine, such as a Bridgeport Interact 412 Numerical Control Milling Machine. When the float chamber 14 and drive shaft 16 are axially aligned and brought together, the ball contacting surface 62 of the drive shaft head 56 engages the workpieces 54 in the ball track 48. The rotation of the drive shaft 16 facilitates the finishing of the workpieces 54. In the most preferred embodiment, the float chamber 14 is approximately 12 inches in diameter. As the float chamber 14 is much larger in the inventive apparatus than in smaller conventional apparatus, the spindle can be run at a much lower speed, such as 400 RPM instead of the 2,000 RPM used in connection with smaller apparatus.

The mechanical load system 18 is operatively connected to the carrier table 12 so as to influence its vertical (axial) movement along the carrier table support shafts 20. With the load system 18, a calculated load is used to control the axial movement of the carrier table 12 toward the drive shaft 16 to achieve the desired polishing force upon the workpieces 54. Since the drive shaft 16 is stationary, the vertical movement of the carrier table 12, and thus the float chamber 16, provides the primary polishing force upon the workpieces 54. Heretofore in smaller apparatus, the polishing forces were supplied solely by the abrasive grains acting under the magnetic buoyancy levitational force generated by a bank of permanent magnets. However, when larger batches are desired, permanent magnets are not sufficient to supply the polishing force. A magnetic buoyancy levitational force is useful, however, even when the primary polishing force is provided by the load system 18. The magnetic buoyancy force evens out uneven load forces created during polishing due to differences in the diameters of the as- received workpieces 54. Thus, in the most preferred embodiment of the invention, the load system 18 is combined with magnetic float polishing techniques to achieve the desired results.

The illustrated load system is a simple dead weight system wherein a pulley support post 64 is mounted on either side of the carrier table 12. The pulley support post 64 has an upright 66 and a cross arm 68. A pair of pulleys 70 are affixed at the top of each cross arm 68. A cable 72, such as steel wire, is fixed at one end to the carrier table 12 and is run above and over the pulleys 70 to a dead weight pan 74. A pan hook 76 connects the dead weight pan to the cable 72.

The amount of dead weight necessary to accomplish polishing with the present apparatus depends upon the number of workpieces 54 and can be easily calculated. The normal load per ball is approximately 1 N.

FIG. 4 illustrates an alternate embodiment of the invention wherein the float chamber 14 is subdivided by annular dividers 78 to form additional ball tracks 48. The annular dividers 78 may be glued or otherwise fixed to the floor 50 of the float chamber 14. A number of acrylic floats 52 of diameters complimentary to the ball tracks 48 are placed in the ball tracks 48 above the floor 50. A rubber ring 80 reduces wear on the guide ring 42 and the ball contacting surfaces of the annular dividers 78. In connection with this embodiment, the drive shaft head 56 is modified to have several intermediate annular projections 82 to compliment the outer annular wall 60 and to provide a number of beveled end, ball contacting surfaces 62. Examples

The preferred embodiment of the present invention is best illustrated through the following examples conducted on uniaxially pressed Si₃N₄ balls (CERBEC NBD-200 from Norton Advanced Ceramics) in the as-received condition having a nominal diameter of 13.4 mm. These balls also contained nearly a 200 μm thick x 5 mm wide band of material around the periphery at the parting plane resulting from the hot pressing process. The balls were to be finished to a final size of 12.7 mm (0.5 inch), a sphericity of <0J5 μm, and best finish

7 achievable (a few nanometers Ra). The large difference in the diameter between the as- received condition to the final size required is made necessary in order to remove all the reaction material that is formed on and near the surface during the hot pressing process.

Table I and Table II give the nominal chemical composition and the mechanical properties, respectively of the Si₃N₄ (NBD 200) balls, and Table III gives various properties of the abrasives used. Table IV lists the test conditions used for the different stages of polishing.

Table I - Chemical Composition of NBD-200 Silicon Nitride

Al C Ca Fe Mg O Si₃N₄

< 0.5

. < 0.88 ^0.04 <.0.17 0.6-1.0 23-33 97.1-94.1

Table II - Mechanical and Thermal Properties of Si₃N₄ Workplace

PROPERTY VALUE

Flexural Strength, MPa 800

Weibull Modulus 9.7

Tensile Strength, MPa 400

Compressive Strength, GPa 3.0

Hertz Compressive Strength, GPa 28

Hardness, Hv (10kg), GPa 16.6

Fracture Toughness, K_lc, MNm^"3'² 4.1

Density, g/cm² 3.16

Elastic Modulus, GPa 320

Poisson's Ratio 0.26

Thermal Expansion Coefficient at 20-1000°C,/°C 2.9x10-⁶

Thermal Conductivity at 100°C, W/m-K 29

Thermal Conductivity at 500°C, W/m-K 21.3

Thermal Conductivity at 1000°C, W/m-K 15.5 Table III - Properties of Various Abrasives

Abrasive Density Knoop Hardness Elastic modulus Melting point, g/cm² kg/mm² GPa °C

B₄C 2.52 2800 450 2450

SiC 3.2 2500 420 2400

CeO₂ 7.13 625

165 2500

Table IV - MFP conditions for finishing big batch balls

Uniaxially Hot Pressed Si₃N₄ balls (CERBEC)

Workmaterial Diameter: 13.4 mm (0.5 inch) Initial Sphericity: 200 μm

B₄C, #400

Abrasive SiC #1200 B_rC #1500 Ce0₂ (5 μm)

Water base (W-40)

Magnetic Fluid Saturation Magnetization at 25 °C: 400 Gauss

Viscosity at 27 °C: 25Cp

Abrasive vol% 5%

Polishing Load 1 N/ball

Polishing Speed 400 rpm (12 inch diameter driving shaft)

Test Time 60 min step

As in most finishing operations, there are three stages involved in magnetic float polishing, namely, 1) roughing to remove as much material as possible without imparting serious damage to the surface, 2) an intermediate stage of semifinishing where size and sphericity have to be carefully monitored, and 3) final finishing where all three, namely, size, sphericity, and finish have to closely controlled to meet the final requirements. The preferred methodology for finishing ceramic balls utilized in connection with the present invention is as follows: A coarser, harder abrasive, such as B₄C (500 grit) and/or SiC (400 grit) (i.e. compared to the Si₃N₄ work material), or combination of abrasives is used during initial stages of polishing to reach the desired diameter at high removal rates and at the same time improve the sphericity for proper ball motion without seriously damaging the surface and subsurface due to brittle fracture. After reaching a diameter closer to the desired diameter, an intermediate (semifinishing) stage is utilized as a transition between the roughing and finishing stages, as the material removal rate is of prime concern in the first stage and removal rate and sphericity in the intermediate stage. Harder abrasives with a finer grit size are used for this intermediate stage, namely, SiC (1000 grit) and SiC (1200-1500 grit). During this stage, the removal rates are lower and the sphericity much better than roughing, as the emphasis during this stage is the improvement of sphericity with reasonable removal rates. In a final intermediate stage (prior to chemo-mechanical polishing), fine SiC abrasive (8000 grit) is used to approach the required diameter and sphericity and remove almost all the deep valleys from the surface. This is followed by final polishing using a softer, chemo-mechanical abrasive, such as CeO₂ or ZrO₂ to produce balls having a desired diameter and sphericity and a final surface finish which is extremely smooth and almost damage-free.

The MFP apparatus utilized in the example was that illustrated in FIGS. 1-3. The drive shaft 16 was driven by the spindle of a Bridgeport CNC machine tool. The magnetic field was measured by a Gauss/Tesla meter. The pH value of the polishing environment was measured by a pH/Temperature meter. The polishing load was set up by the dead weight load system 18. To calculate material removal rates, the weight reduction in the balls was measured by measuring the weight before and after polishing at every test stage using a precision balance. The surface finish of the polished balls was analyzed using a Form Talysurf 120 L, ZYGO laser interference microscope, and an ABT 32 scanning electron microscope (SEM). The finished balls were characterized for roundness using a TalyRond 250 (cut-off:

50 upr, Filter: 2CR), and for surface features using a scanning election microscope (SEM), a Form TalySurf 120 L (cut-off: 0.25 mm and 0.8 mm, evaluation length: 4-6 consecutive cut-off, Filter: ISO 2CR), and an AFM. Although the latter three instruments measure or illustrate slightly different surface features, they are basically complimentary in nature. Their combined use provides confidence in the data obtained. In these examples, three randomly selected balls from each batch were traced 3 times using the TalyRond and Form Talysurf to

10 provide the roundness and surface roughness, respectively. This means that altogether 9 traces were made per ball. The TalyRond trace measures the maximum departure from a true circle of assumed magnitude and as such it denoted roundness. The sphericity of each ball, according to the Antifriction Bearing Manufacturers Association, is defined as the maximum value of the roundness measured on three orthogonal planes of the ball. Similarly, the surface finish of each ball is taken as the maximum value of three traces along three orthogonal planes of the ball.

Results obtained with the inventive apparatus were very favorable and comparable with the results obtainable with the conventional smaller apparatus. Table V gives the diameter, sphericity and material removal rates (MRR) obtained at progressive stages of polishing. A material removal rate of 1 μm min per ball was achieved in the initial stages of polishing and a final sphericity of 0J5 μm was obtained upon finishing to their final condition.

Table V - Polishing procedure and results

Abrasive Time Min Diameter (mm) Sphericity (μm) MRR before after before after (μm/min)

B4C #400 12 x 60 13.1 12.732 200 0.6 0.8-1.0

SiC #1200 2 x 60 12.732 12.706 0.6 0.3 0.2

B4C #1500 1 x 60 12.708 12.702 03 0.2 0.1

CeO2 2 x 90 12.702 12.700 0.2 0.15 0.01

It has thus been demonstrated that the above described inventive apparatus is capable of finishing large batches of ceramic balls to a quality comparable to that achieved by conventional smaller apparatus. And, in fact, the large batch equipment with more balls provides a more uniform load per ball and, consequently, better sphericity. This broadens the commercial viability of this new technology, namely, magnetic float polishing, for the finishing process, starting from the as-received balls, as the large batch apparatus promotes favorable efficiencies in time and labor in the manufacturing process.

11 While the invention has been described with a certain degree of particularity, it is understood that the invention is not limited to the embodiment(s) set for herein for purposes of exemplification, but is to be limited only by the scope of the attached claim or claims, including the full range of equivalency to which each element thereof is entitled.

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Claims

WHAT IS CLAIMED IS:

1. A large batch ball polishing apparatus, comprising: a carrier table slidably mounted for vertical movement; a polishing chamber supported upon said carrier table, said polishing chamber having a ball track for receiving a plurality of workpieces; a drive shaft adapted for coupling to a spindle of a milling machine, said drive shaft having a ball-contacting surface for engaging said workpieces in said ball track when said polishing chamber and said drive shaft are axially aligned and brought together; and a load system operatively connected to said carrier table so as to influence its vertical movement, whereby under a controlled load said polishing chamber is moved axially toward said drive shaft to achieve a desired polishing force upon said workpieces.

2. The apparatus according to claim 1 , wherein said load system is a deadweight load system comprising a pair of pulley support posts mounted on opposite sides of said carrier table and a pair of cables, each fixed to a side of said carrier table and run over a support post to a deadweight pan.

3. A large batch magnetic float polishing apparatus, comprising: a carrier table slidably mounted for vertical movement; a float chamber supported upon said carrier table, said float chamber having a ball track for receiving a plurality of workpieces and for holding a quantity of a magnetic polishing fluid in contact therewith, said float chamber further having a magnetic field generating means beneath said ball track whereby when a magnetic field is applied to said magnetic fluid suspension an upward magnetic buoyancy levitational force is produced; a drive shaft adapted for coupling to a spindle of a turning machine, said drive shaft having a ball-contacting surface for engaging said

13 workpieces when said float chamber and said drive shaft are axially aligned and brought together; and a load system operatively connected to said carrier table so as to influence its vertical movement, whereby under a controlled load said float chamber is moved axially toward said drive shaft to achieve a desired polishing force upon said workpieces; said load system providing the primary polishing force upon said workpieces while said magnetic buoyancy levitational force evens out uneven load forces created during polishing due to differences in the diameters of said workpieces, thereby providing a more uniform load per ball and improved sphericity.

4. The apparatus according to claim 3, wherein said magnetic field generating means comprises a bank of permanent magnets arranged in segments to form a substantially circular ball track floor, said magnets being oriented in an alternate pole manner.

5. The apparatus according to claim 4, further comprising a non-magnetic float ring positioned above said ball track floor.

6. The apparatus according to claim 5, wherein said non-magnetic float ring is a lightweight acrylic.

7. The apparatus according to claim 3, wherein said float chamber has multiple ball tracks and said drive shaft has a ball-contacting annular projection for mating with each said ball track.

8. The apparatus according to claim 3, wherein said load system is a deadweight load system comprising a pair of pulley support posts mounted on opposite sides of said carrier table and a pair of cables, each fixed to a side of said carrier table and run over a support post to a deadweight pan.

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9. A large batch magnetic float polishing apparatus, comprising: a carrier table slidably mounted for vertical movement upon a plurality of support shafts; a float chamber supported upon said carrier table, said float chamber having a ball track for receiving a plurality of workpieces and for holding a quantity of a magnetic polishing fluid in contact therewith, said ball track having a floor formed of a substantially circular bank of permanent magnets arranged with alternating poles whereby when a magnetic field acts upon said magnetic fluid suspension an upward magnetic buoyancy levitational force is produced; a drive shaft adapted for coupling to a spindle of a turning machine, said drive shaft having a ball-contacting surface for engaging said workpieces when said float chamber and said drive shaft are axially aligned and brought together; and a load system operatively connected to said carrier table so as to influence its vertical movement, whereby under a controlled load said float chamber is moved axially toward said drive shaft to achieve a desired polishing force upon said workpieces, said load system comprising a pair of pulley support posts mounted on opposite sides of said carrier table and a pair of cables, each fixed to a side of said carrier table and run over a support post to a deadweight pan; said load system providing the primary polishing force upon said workpieces while said magnetic buoyancy levitational force evens out uneven load forces created during polishing due to differences in the diameters of said workpieces, thereby providing a more uniform load per ball and improved sphericity.

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