Spherical Lapping Method

The present invention relates generally to prosthetic joints and, particularly to a spherical head used in such applications. More particularly, the present invention relates to a method for producing a spherical head having a very smooth surface for prosthetic applications.

The recent advances in the use of implantable prosthetic joints to replace natural joints in humans have produced significant improvements. Many designs for such prosthetic implants, such as hip prosthesis, require a spherical head having a highly polished surface to reduce friction and increase the life of the joint.

It is known to machine parts from bar stock on a ball grinder to produce parts that are spherical, but have a relatively rough surface. The parts are then buffed on a buffer to achieve a polished surface. A problem with the buffing process, however, is that surface irregularities, such as grinding and buffing lines, remain on the finished part.

According to the present invention, a method of lapping a spherical head is provided which reduces surface irregularities on the finished part. The method comprises the steps of rotatably mounting a plurality of lapping fixtures having generally semi-spherical cavities, positioning polishing cloths or liners in the generally semi-spherical cavities, sequentially bringing the spherical head into engagement with the spinning lapping fixtures and varying the angle between the longitudinal axis of the spherical head and the rotational axes of the lapping fixtures between forward and reverse rotations of the spherical head about its longitudinal axis.

In accordance with preferred embodiments, the subject method further includes the step of applying a wetting agent and diamond suspensions varying from a relatively coarse grit to a relatively fine grit to the lapping fixtures. Also in accordance with further preferred embodiments, the subject method includes the step of rinsing the lapping compounds from the spherical head.

In the illustrated embodiments, the subject method further includes the step of positioning a generally daisy-shaped polishing cloth having adhesive backing in each of the semi-spherical cavities. The adhesive backing ensures good engagement between the mating surfaces of the polishing cloths and the respective semi-spherical cavities.

According to the present invention, each lapping fixture comprises a central core having a top end defining a generally semi-spherical cavity and an outer sleeve having an opening adapted for receiving the central core. According to still further preferred embodiments, the subject method further includes the steps of positioning the polishing liners in the respective semi-spherical cavities with the peripheral portions thereof extending over the top ends of the lapping fixtures and mounting the outer sleeves over the central cores to clamp the peripheral portions of the polishing liners in place.

In illustrated embodiments, the lapping fixture includes a plurality of modular inserts for lapping a corresponding plurality of heads having different diameters. Each modular insert has the same predetermined outside diameter to securely fit the opening in the central core of the lapping fixture, but a different diameter semi-spherical cavity.

Additional features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the inventions presently perceived.

The detailed description particularly refers to the accompanying figures in which:

FIG. 1 is a diagrammatic view of a robotic spherical lapping apparatus and a method for lapping spherical modular heads in accordance with the present invention,

FIG. 2 is a perspective view of a portion of the FIG. 1 lapping apparatus including a robot for positioning a spherical head into a lapping fixture mounted on a support spindle, the lapping fixture comprising a central core, a modular insert received in the central core, a polishing cloth with adhesive backing inserted into a generally semi-spherical cavity in the modular insert and a hollow outer sleeve mounted over the central core to clamp the polishing cloth in place,

FIG. 3 is a cross-sectional view of the lapping fixture mounted on the support spindle and a portion of the robot carrying the spherical head,

FIG. 4 is a cross-sectional view of the modular insert,

FIG. 5 is a top view of the polishing cloth prior to its installation in the modular insert,

FIG. 6 is a top view of the support spindle rotatably mounted about a rotational axis,

FIG. 7 is a perspective view of the central core/modular insert/polishing cloth assembly,

FIG. 8 shows an operator control panel of the lapping apparatus, and

FIG. 9 diagrammatically shows the rinsing/air drying station for rinsing, cooling and air drying the spherical heads.

Turning now to the drawings, FIG. 1 is a diagrammatic view of a robotic spherical lapping apparatus 10 and a method for lapping spherical heads in accordance with the present invention. A robot 12 is used for lapping a modular spherical head 14. The spherical head 14 is mounted on an arbor 16 and placed in a loading fixture 20 at a load/unload station 22 in an operator work area 24. Illustratively, the robot 12 may be of the type manufactured by Mitsubishi, Model No. RV-E2.

A pneumatic band conveyor 26 transports the loading fixture 20 to a pickup/dropoff station 28 near the robot 12 inside a safety enclosure 30. The robot 12 picks up the spherical head 14 from the loading fixture 20 and sequentially moves it through a series of three (3) spinning lapping fixtures 32 to produce a highly reflective finish on the spherical surface 34 (shown in FIGS. 2 and 3) of the spherical head. The spherical head 14 may be made from any suitable high strength, lightweight and rustproof material, such as cobalt chrome. Illustratively, the pneumatic band conveyor 26 may be of the type manufactured by Tolmatic, Model No. BC2 Series.

The loading fixture 20 is equipped with 2 positions: a position 21 for loading an unfinished part to be lapped in the loading fixture and a position 23 for unloading a finished part from the loading fixture when the loading fixture is at the load/unload station 22 in the operator work area 24. On the other hand, the robot 12 picks up the unfinished part from the position 21 of the loading fixture and drops off a finished part at the position 23 of the loading fixture when the loading fixture is at the pickup/dropoff station 28 near the robot. As a safety measure, the robot 12 is programmed not to start a new cycle unless an unfinished part to be lapped is in the position 21 of the loading fixture 20 and the loading fixture is at the pickup/dropoff station 28 near the robot. Also, the robot 12 will not release a finished part unless the position 23 of the loading fixture 20 is empty and the loading fixture is at the pickup/dropoff station 28 near the robot.

The lapping fixtures 32 are each provided with a generally semi-spherical cavitv 74 (shown in FIGS. 2-4) and are rotatably mounted about their respective rotational axes 38. The semi-spherical cavities 74 are lined with polishing cloths 40, which are provided with adhesive backing to ensure a good contact between the polishing cloths and the semi-spherical cavities 74. The polishing cloths 40 are sprayed with a wetting agent and lapping compounds, varying from a relatively coarse grit (e.g., 3 microns) for the first lapping fixture to a transition mixture of a relatively coarse grit (e.g., 3 microns) and a relatively fine grit (e.g., 0.25 microns) for the second lapping fixture to a relatively fine grit (e.g., 0.25 microns) for the third lapping fixture. A series of three (3) spray guns 42 are mounted on a motor driven band cylinder 44. The first gun sprays the wetting agent. The second gun sprays a relatively coarse grit lapping compound. The third gun sprays a relatively fine grit lapping compound. The band cylinder 44 is programmed to move the respective one of the spray guns 42 to an appropriate one of the lapping fixtures 32 for the application of a wetting agent and a lapping compound. For the very first time for each batch of parts, each lapping fixture 32 receives application of both the wetting agent and the lapping compound. After the very first application of both the wetting agent and the lapping compound, each lapping fixture 32 receives alternate applications of the wetting agent and the lapping compound between successive cycles. The amount of lapping compound applied in each application is controlled by the nozzle diameters of the spray guns, the application time programmed for each application and the air line pressure.

Illustratively, the motor driven band cylinder 44 may be of the type manufactured by Tolmatic, Model No. BCES 100. The wetting agent may be a water based lubricating fluid--such as, Lapmaster Kemet Type W. The lapping compounds may be diamond suspensions--such as, Metadi Supreme Polycrystalline Diamond Suspensions, 3.0 micron size for the first lapping fixture (coarse grit) and 0.25 micron size for the third lapping fixtures (fine grit). The second or transition lapping fixture receives a mixture of the coarse grit and fine grit diamond suspensions. The spray application time is about 0.1 seconds. About 8 ounces each of 3.0 micron and 0.25 micron mixtures of the diamond suspensions and wetting agent are needed for processing 400 parts.

As shown in FIG. 3, the tilt angle .alpha. between the longitudinal axis 46 of the spherical head 14 and a line 39 perpendicular to the rotational axes 38 of the lapping fixtures 32 is varied from about 41.degree. to about 45.degree. between the forward and reverse rotations of the robot wrist joint 151 to obtain full coverage of the spherical surface 34. Thus, the spherical head angle .alpha. changes from about 41.degree. to about 45.degree. during clockwise or forward rotation of the spherical head 14 through about 270.degree.. The spherical head angle .alpha. then changes back 41.degree. during anticlockwise or reverse rotation of the spherical head 14 again through about 270.degree.. Each paired rotation cycle of forward and reverse rotations of the spherical head 14 takes about 0.05 minutes. The number of paired cycles of forward and reverse rotations of the sperical head 14 for the first lapping fixture 32 vary between about 3 sets of 19 paired rotation cycles for 36 mm diameter sperical head to about 3 sets of 16 paired rotation cycles for a 22 mm diameter sperical head. The number of paired cycles of forward and reverse rotations of the sperical head 14 for the second lapping fixture 32 is about 2 sets of 14 paired rotation cycles. The number of paired cycles of forward and reverse rotations of the sperical head 14 for the third lapping fixture 32 is about 2 sets of 13 paired rotation cycles. For the very first time for each batch of parts, each lapping fixture 32 receives application of both the wetting agent and the lapping compound. (In the particular embodiment described, each batch comprises 40 spherical heads.) After the very first application of both the wetting agent and the lapping compound, each lapping fixture 32 receives alternate applications of the wetting agent and the lapping compound between successive sets of paired rotation cycles. The lapping fixtures 32 are continuously rotating at one of the three (3) preset speeds of 500, 600 and 700 rpm, depending on the diameter of the spherical head 14. Illustratively, the rotational speeds are 500 rpm, 600 rpm and 700 rpm for spherical heads having diameters of 36 and 32 mm, 28 and 26 mm and 22 mm, respectively.

The lapping apparatus 10 further includes a rinsing/air drying station 48. The spherical head 14 is rinsed and cooled with one micron-filtered RO (Reverse Osmosis) water after each lapping cycle. This step rids the spherical head 14 of the lapping compounds to minimize cross contamination. An air jet blows off excess moisture from the spherical head 14 before the robot 12 either moves the spherical head to the next lapping fixture 32 or to the pickup/dropoff station 28 to release it in the loading fixture 20.

As diagrammatically shown in FIG. 9, the rinsing/air drying station 48 comprises a round opening 50 in the floor 52 that supports the pneumatic band conveyor 26 and the motor driven band cylinder 44. A six (6) inch pipe 54 is placed flush to the bottom edge of the Lexan floor 52, and four (4) rows of nozzles 56 are mounted along the inside of the pipe in a circular configuration between the opening 50 in the floor 52 and an effluent tank 58 that is used for collecting the discharge. The three (3) lowermost rows of nozzles 56 are connected to filtered RO water, and are used to rinse and cool the spherical heads 14. The uppermost row of nozzles 56 is connected to an air line, and is used to air dry the spherical heads 14. The rinsing and air drying procedure is repeated after polishing in each lapping fixture (i.e., 3 times per part). Illustratively, in this particular embodiment, the floor 52 is a Lexan sheet, the 6" pipe is a PVC pipe and the nozzles 56 are Loc-Line Circle Flow Nozzles.

As illustrated in FIG. 3, each lapping fixture 32 includes a central core 60 having a side wall 62 with a top end 64 and an opposite bottom end 66. The top end 64 includes a tapered mouth portion 68 that defines a cavity 70 for selectively receiving an interchangeable modular insert 72. As shown in FIG. 4, the modular insert 72 has an outer wall 76 that engages the inner surface of the side wall 62 of the central core 60 to provide a tight fit.

A plurality of modular inserts 72 having the same predetermined outside diameter, but different diameter semi-spherical cavities, may be provided for lapping a corresponding plurality of spherical heads having different diameters. The same predetermined outside diameter of the modular inserts 72 allows their secure reception in the cavity 70 in the central core 60 regardless of the diameters of the semi-spherical cavities therein. Thus, the same set-up can be used to produce different diameter spherical heads by simply changing the modular insert.

As shown in FIG. 5, the polishing cloths 40 are generally daisy-shaped and include a center portion 80 and a plurality of arms 82 radially extending from the center portion. As can be seen from FIG. 3, the polishing cloths 40 are positioned in each of the modular inserts 72 such that the radially extending arms 82 thereof drape over the tapered mouth portions 68 of the central cores 60.

The daisy-shaped polishing cloths 40 vary in size to accommodate different size modular inserts 72. They are changed after processing about forty (40) spherical heads. After changing the polishing cloths 40, the operator must push a reset button to restart the count-down and to indicate that the polishing cloths have been changed in order to continue the robot operation.

As shown in FIGS. 2 and 3, a hollow outer sleeve 84 is mounted over each central core 60 to hold the respective polishing cloth 40 in place. The top end 86 of each outer sleeve 84 defines an annular ledge portion 88 forming an opening 90 providing access to the generally semi-spherical cavity 74 in the modular insert 72. In addition, the top end 86 of each outer sleeve 84 forms a splash guard 92 to prevent splashing of the lapping mixture during the lapping process and during compound application. The annular ledge portion 88 extends over the tapered mouth portion 68 of the central core 60 and the top wall of the modular insert 72. When the outer sleeve 84 is mounted over the central core/modular insert/polishing cloth assembly, a lug 94 secured to the central core 60 is received in a cam-shaped slot 96 in the outer sleeve 84. The outer sleeve 84 is rotated relative to the central core 60 to draw the two together to clamp the modular insert 72 in place and to clamp the outwardly extending arms 82 of the polishing cloth 40 between the underside of the ledge portion 88 and the topside of the central core 60.

As shown in FIG. 3, each lapping fixture 32 is rotatably and compliantly mounted to a support spindle 100 for rotation therewith by means of a set of 3 equally spaced shoulder bolts 102 and a pair of opposing springs 104 and 106 in the manner depicted. The support spindle 100 is, in turn, secured to a shaft 108 of a drive motor 110 by a pair of set screws 112. FIG. 6 is a top view of the support spindle 100 and the three shoulder bolts 102 spaced 120.degree. apart. Illustratively, the drive motor 110 may be of the type manufactured by Reliance Electric, Model No. VM-3558-2PH-1725.

As can be seen from FIG. 3, one end of each of the shoulder bolts 102 is screwed to the central core 60 and its head end is received in a slot 114 in the support spindle 100. One spring 104 is disposed between the underside of the central core 60 and the topside of the support spindle 100. The other spring 106 is disposed in the slot 114 between a wall 116 of the support spindle 100 and the head 118 of the shoulder bolt 102. The ability of the compliantly-mounted lapping fixture 32 to move side-to-side and up-and-down causes the lapping fixture to return to the same place and to remain in alignment. The compliant mounting also compensates for allowable part tolerances and produces uniform and consistent pressure between the spherical head 14 and the lapping fixture 32 during the lapping process.

As shown in FIGS. 2 and 3, the arbor 16 carrying the spherical head 14 is inserted in a slot 120 of a chuck 122 and held in place by a spring-loaded latch 124. The chuck 122 is mounted on the arm 126 of the robot 12.

FIG. 8 shows the operator control panel 130 of the lapping apparatus 10. On the right hand side of the control panel 120 are a plurality of warning lights 132-142 to warn the operator of various fault conditions--(a) spray gun shuttle 44 fault, (b) lapping compound mixer fault, (c) the door 1 in the safety enclosure 30 providing access to the lapping fixtures 32 for changing the polishing cloths 40 open, (d) the door 2 in the safety enclosure 30 providing access to the compound tanks and the pump filter open, (e) needs compound or polishing cloth change and (f) lapping fixture drive motor 110 fault. The compound mixer keeps the lapping compound mixed and prevents settling of the diamond suspensions. On the left side of the control panel 130 are a cloth counter 144 and the switches 146 and 148 for the doors 1 and 2 in the safety enclosure 30. The numeral 150 designates the power on/off button. The robot 12 checks the machine components for faults and will automatically shut down if it identifies a fault condition.

In operation, a spherical head 14 to be lapped is mounted on the arbor 16, placed in a loading fixture 20 at the load/unload station 22 and shuttled to the pickup/dropoff station 28 near the robot 12. The robotic arm 126 is manipulated to move the spherical head 14 through the three spinning lapping fixtures 32. A coarse grit diamond suspension (e.g., 3 microns) is used in the first lapping fixture. The second lapping fixture receives a transition mixture of a relatively coarse grit (e.g., 3 microns) and a relatively fine grit (0.25 microns) diamond suspensions. The third lapping fixture receives a relatively fine grit diamond suspension (0.25 microns) to produce a fine surface finish and minimize surface irregularities. For the very first time for each batch of parts, each lapping fixture receives application of both the wetting agent and the lapping compound. After the very first application of both the wetting agent and the lapping compound, each lapping fixture receives alternate applications of the wetting agent and the lapping compound between successive sets of rotation cycles. The angle between the spherical head 14 and the lapping fixture 32 is varied between fonvard and reverse rotations of the robot wrist joint 151 to enable full coverage of the spherical surface 34. The rinsing/air drying station 48 cools the spherical head 14, rinses the diamond suspensions from the spherical head and blows off the excess moisture after polishing in each lapping fixture. The part is then either returned to the next lapping fixture 32 or released in the loading fixture 20 and transported to the load/unload station 22. The compliant support eliminates the need for exact spherical head-to-lapping fixture alignment and produces uniform and consistent pressure therebetween.

A typical cycle for lapping a spherical head is as follows: (1) check for faults, (2) pick up an unfinished spherical head, (3) move the spherical head to the first lapping fixture, (4) spray a coarse grit lapping compound and/or a wetting agent in the first lapping fixture (Note: For the very first part, both the coarse grit lapping compound and the wetting agent are sprayed. For the next 39 parts, the coparse grit lapping compound and the wetting agent are alternately sprayed.), (5) engagement of the spherical head with the first lapping fixture while it is spinning and while the angle between the longitudinal axis of the spherical head and the rotational axis of the lapping fixture is varied between the forward and reverse rotations of the spherical head, (6) lift the spherical head and spray the coarse grit lapping compound in the first lapping fixture, (7) engagement of the spherical head with the first lapping fixture while it is spinning and while the angle between the longitudinal axis of the spherical head and the rotational axis of the lapping fixture is varied between the forward and reverse rotations of the spherical head, (8) lift the spherical head and spray the wetting agent in the first lapping fixture, (9) engagement of the spherical head with the first lapping fixture while it is spinning and while the angle between the longitudinal axis of the spherical head and the rotational axis of the lapping fixture is varied between the forward and reverse rotations of the spherical head, (10) rinse, cool and air dry the spherical head, (11) move the spherical head to the second lapping fixture, (12) spray an intermediate grit lapping compound (or a mixture of coarse grit and fine grit lapping compounds) and/or the wetting agent in the second lapping fixture (Note: For the very first part, both the lapping compound and the wetting agent are sprayed. For the next 39 parts, the lapping compound and the wetting agent are alternately sprayed.), (13) engagement of the spherical head with the second lapping fixture while it is spinning and while the angle between the longitudinal axis of the spherical head and the rotational axis of the lapping fixture is varied between the forward and reverse rotations of the spherical head, (14) lift the spherical head and spray the wetting agent in the second lapping fixture, (15) engagement of the spherical head with the second lapping fixture while it is spinning and while the angle between the longitudinal axis of the spherical head and the rotational axis of the lapping fixture is varied between the forward and reverse rotations of the spherical head, (16) rinse, cool and air dry the spherical head, (17) move the spherical head to the third lapping fixture, (18) spray a fine grit lapping compound and/or the wetting agent in the third lapping fixture (Note: For the very first part, both the fine grit lapping compound and the wetting agent are sprayed. For the next 39 parts, the fine grit lapping compound and the wetting agent are alternately sprayed.), (19) engagement of the spherical head with the third lapping fixture while it is spinning and while the angle between the longitudinal axis of the spherical head and the rotational axis of the lapping fixture is varied between the forward and reverse rotations of the spherical head, (20) lift the spherical head and spray the wetting agent in the third lapping fixture, (21) engagement of the spherical head with the third lapping fixture while it is spinning and while the angle between the longitudinal axis of the spherical head and the rotational axis of the lapping fixture is varied between the forward and reverse rotations of the spherical head, (22) rinse, cool and air dry the spherical head, (23) move the spherical head to the load/unload station near the operator and unload spherical head.

Although the present invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the present invention as described and as defined in the following claims.