TECHNICAL FIELD This invention relates to a method and apparatus for controlling the force applied to at least one work piece by a tool element. The system has particular application to lapping machines and is used to accurately control the magnitude and rate of change of lapping force.

BACKGROUND ART

Lapping machines are well known devices employed to produce one or more flat surfaces on work pieces. To produce the desired results, lapping machines conventionally incorporate various systems for controlling lapping pressure.

Essentially, there are three lapping pressure control systems in current wide-spread usage. The first such system is the so called "dead weight" system wherein the weight of the top lap plate is employed to produce lapping pressure. The gravity induced force applied to the work piece is varied by either adding weight to the top lap plate to increase pressure or removing weight therefrom to reduce the downwardly directed force applied to the work piece.

Generally speaking, the arrangement just described is limited to smaller machine sizes where the top plate weight is relatively easily manipulated and can be lifted manually. While mechanisms have been developed to aid in raising dead weight top lap plates, thereby permitting the use of greater weight and reducing worker fatigue, the dead weight system has additional drawbacks.

Specifically, the dead weight system is relatively inconvenient and does not allow the

application of weight less than the weight of the top lap plate itself. Perhaps even more importantly, this system does not allow for convenient pressure adjustment, does not provide for gradual change of load, and does not compensate for reduction in top plate weight caused by plate wear.

Existing hydraulic and pneumatic systems, the other two principal prior art approaches for controlling lapping pressure, have some advantages over the dead weight system outlined above but still have inherent limitations which restrict their use, especially when lapping thin and fragile work pieces.

Hydraulic and pneumatic systems operate by maintaining a substantially constant pressure on a piston in either a hydraulic or pneumatic cylinder. The pressure on the piston produces a force which, in effect, counters top lap plate weight. By varying the pressure, of course, the effective weight of the top lap plate, that is, the downwardly directed pressure or force exerted thereby against the work piece, can be varied. However, control systems for accomplishing this are inherently inaccurate, due primarily to seal friction. This friction increases as pneumatic or hydraulic pressures are increased. Thus, inaccuracies are highest at the worst possible time in the lapping operation, i.e. when highly controlled lighter lapping loads are required. It is not unusual to find errors or inaccuracies in the order of about 30 to 50 pounds (plus or minus) in the highest pneumatic or hydraulic pressure ranges.

Hydraulic and pneumatic systems are further characterized by their complexity, often requiring complicated plumbing. Also, hydraulic systems can contaminate work pieces.

Although not widely utilized, various other types of systems are known in the prior art for controlling force exerted by top lap plates or other tool elements against a work piece. Such systems are, generally speaking, relatively complicated and expensive.

And, when such systems are employed in lapping-machines, they do not permit production of very thin and fragile lapped products.

The following patents illustrate representative prior art approaches for controlling application of a tool element, such as a top lap plate, to a work piece: U.S. 4,811,522, U.S. 4,272,924, U.S. 4,676,030, U.S. 4,209,949, U.S. 4,637,169, U.S. 3,455,067, U.S. 4,441,103, European Patent Application Publication 246,448, and Japanese Patent Publication 63-114869.

DISCLOSURE OF INVENTION

The apparatus and method of the present invention are for the purpose of accurately and finely controlling a force applied to at least one work piece by a tool element. The system has particular application to lapping machines, both double-sided and single-sided lapping machines.

The invention disclosed herein accurately controls and maintains the magnitude and rate of application of downward force directed to a work piece. With specific reference to the lapping process, the downward force and its corollary, lapping pressure, is a highly important variable in the lapping process, affecting both material removal rate and the flatness and parallelism of the processed sides of a work piece.

Control of the magnitude of force is important in order to produce statistically repeatable results. The ability to gently or slowly gradually change lapping pressure while the lapping machine is running is particularly crucial when lapping thin and fragile work pieces.

In a lapping machine incorporating the teachings of the present invention, downward force exerted by the top lap plate can be minimized during lapping motion starts and increased slowly and gradually to the desired lapping pressure after lapping motion has been achieved.

In planetary type lapping machines, the

magnitude of the edge forces imposed on the work pieces by the work piece carrier is directly related to the magnitude of the force imposed by the top plate on the work pieces. Edge forces are particularly acute during start-up when the static friction opposing motion of the work pieces must be overcome.

Apparatus constructed in accordance with the teachings of the present invention has a very low internal operating friction which allows very accurate load setting as compared to traditional hydraulic or pneumatic systems. Furthermore, the system has the capability of automatically compensating for lap plate wear.

Apparatus constructed in accordance with the teachings of the present invention includes a work piece support for supporting at least one work piece. The work piece support, for example, may be a bottom lap plate. A tool element support is provided for supporting the apparatus tool element, which may be, for example, a top lap plate.

Transport means is operatively associated with the tool element support for moving the tool element support and the tool element relative to a work piece. Sensing means is provided for sensing the effective weight of the tool element as applied to the work piece.

Finally,, control means is operatively associated with the sensing means and the transport means, the control means controlling movement of the tool element support and the tool element by the transport means responsive to the effective weight of the tool element sensed by the sensing means.

Other features, advantages, and objects of the present invention will become apparent with reference to the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a side view, in somewhat schematic fashion, of a lapping machine constructed in accordance

with the teachings of the present invention and illustrating the top lap plate thereof in an elevated position;

Fig. 2 is a view similar to Fig. l but illustrating the top lap plate thereof lowered to perform the lapping function;

Fig. 3 is an enlarged side view, in partial section, illustrating selected components of the lapping machine;

Fig. 4 is an enlarged side view, in partial section, of a preselected portion of top lap plate support structure and related machine components; and

Fig. 5 is a simplified schematic diagram relating to the lapping machine control mechanism.

BEST MODE FOR CARRYING OUT THE INVENTION Referring to the drawing, a lapping machine constructed in accordance with the teachings of the present invention is generally designated by reference numeral 10. For illustration purposes, the lapping machine 10 is a double-sided planetary lapping machine including a work piece support in the form of a bottom lap plate 12 and a tool element in the form of a rotatable top lap plate 14. It will be appreciated, however, that commercial designs exist in which neither the top nor bottom plate rotate and that the teachings of the present invention have application to these latter designs as well.

It will be appreciated that, as in conventional, work pieces (not shown) usually having flat top and bottom surfaces are placed on the upper surface of bottom lap plate 12. Conventionally, the work pieces are positioned on the bottom lap plate by work holders (not shown) called carriers. The carriers, as is well known in the art, are flat circular parts of lesser thickness than the work pieces with holes into which the work pieces are placed. The carriers also rest on the bottom lap plate.

Through a suitable gear arrangement (not shown)

a desired motion is imparted to the carriers. 'The carriers interact with the edges of the work pieces carried thereby, causing same to be pushed about the bottom lap plate, motion of each work piece corresponding to the motion of the carrier hole in which it resides.

In order to produce lapping on both side ^' s of the work piece or work pieces positioned on the bottom lap plate, top lap plate 14 is brought into contact with the top surface of each work piece. Since the work pieces are thicker than the carriers, the top plate rests on the work pieces only.

A slurry of hard abrasive particles in a liquid vehicle is conventionally introduced between the lapping plates. As a result of the work piece motion and the downward top lap plate force, a fine grinding action occurs at the interface between the work pieces and top lap plate as well as the interface between the work pieces and bottom lap plate.

The structure described thus far is conventional and for that reason it has not been illustrated in detail. Suffice it to say that the magnitude of the edge forces imposed on the work pieces by the carriers is directly related to the magnitude of the force imposed by the top plate on the work pieces. Edge forces are particularly acute during start-up when the static friction opposing motion of the work pieces must be overcome.

It is therefore advantageous when lapping thin or fragile work pieces where edge forces must be minimized, to have a lapping machine in which the downward force can be minimized during lapping motion starts and increased gently to the desired lapping pressure after lapping motion has been achieved, and this is one of the objects of the present invention which is attained by the structure and method hereinafter described.

Top lap plate 14 is supported by a tool element support which includes a support member 16 extending upwardly from the top lap plate, a platform 18, and

spring means 20. The support member 16 comprises a rotatable support shaft 22 having a splined lower portion, as shown in Fig. 3.

The upper end of the support shaft 22 passes through an aperture 26 formed in platform 18. The spring means 20 is a coil compression spring positioned on the platform about shaft 22 with the upper end of the spring in engagement with a flange element 24. Flange element 24 does not engage support shaft 22 and defines an aperture through which the support shaft passes, as perhaps best seen in Fig. 4.

Disposed immediately above flange element 24 is sensing means in the form of a load cell transducer 30. A suitable transducer is load cell Model ALD-W-3 made available by A. L. Design, Inc. of Amherst, N.Y. Transducer 30 is sandwiched between an enlarged washer 32 and flange element 24. The shaft 22 passes through washer 32 and is freely rotatable relative thereto.

A bearing 34 is positioned on washer 32. Washer 32 defines a pocket accommodating the outer race of bearing 34. The inner race of bearing 34 is close fitted to shaft 22. The threaded upper end of shaft 22 has an enlarged fastener 36 secured thereto. The fastener 36 is positioned on the inner race of bearing 34 and thus provides vertical support for the shaft.

It will be appreciated that with the arrangement just described, coil spring 20 continuously exerts an upward bias or force against the support member 16. It will further be appreciated that force from spring 20 is accurately transmitted to load cell 30 since spring 20, flange element 24, load cell 30 and washer 32 do not touch shaft 22.

Below platform 18 the splined portion of support shaft 22 passes through a spline bearing 38 mounted in the framework of the lapping machine. Spline bearing 38 permits free up or down motion of the support shaft 22, but is operable to transmit rotational torque to the support shaft for rotating the top lap plate 14 during the lapping operation. A pulley 40 (Fig. 3) is

affixed to the spline bearing 38 and rotation of pulley 40 rotates the spline bearing 38, the shaft 22 and the top lap plate. A motor 42 rotates a pulley 44. A transmission belt 46 extends between pulleys 40, 44 whereby the support shaft 22 and top lap plate 14 are rotated at the desired speed.

Transport means is provided for moving the top lap plate into engagement with the one or more work pieces positioned on the bottom lap plate 12. In particular, means is provided to raise and lower platform 18. Platform 18 includes a ball nut 50 threadably engaging a ball screw 52. The lower end of ball screw 52 is journaled in a bearing 54 and the upper end _. thereof is journaled in a bearing 56. A pulley element 58 is affixed to the upper end of the ball screw 52.

Ball screw 52 is substantially parallel with support shaft 22. It will be appreciated that rotation of the ball screw 52 will result in either the raising or lowering of platform 18. This, in turn, results in a corresponding lowering or raising of spring 20 and all the components bearing on and depending from spring 20, including support shaft 22 and top lap plate 14. Rotatable motion is imparted to pulley element 58 and thus ball screw 52 by a toothed belt 60 extending between the pulley element 58 and a pulley 62. Pulley 62 is affixed to the rotatable output shaft of an electric stepping motor 64. Motor 64 is attached to and depends from a portion 66 of the lapping machine framework which also accommodates the upper end of ball screw 52. A suitable motor is stepping Motor Model #802D3450B02W made available by Pacific Scientific of Rockford, Illinois.

A housing 70 is affixed to platform 18 on the side of the platform opposed to the side thereof supporting the top lap plate 14 and associated structure. A linear bearing 72 is positioned in housing 70 and slidably disposed on a guide shaft 74. Guide shaft 74 is affixed at its upper end to framework portion 66 and the lower end of guide shaft 74 is secured to another portion 76 of the framework, i.e. the same framework portion

accommodating the bearing 54 and spline bearing 38.

The guide means just described stabilizes the platform 18 by countering the moment imparted to the platform by the force of spring 20. Since guide shaft 74, ball screw 52 and support shaft 22 are essentially parallel and vertical, only a vertical force and no moments are applied to the ball nut 50.

When the ball screw 52 is rotated by the stepping motor 64, platform 18 supporting spring 20 is lowered. Top lap plate 14, due to the fact that support shaft 22 is free to slide through spline bearing 38 follows in a downwardly direction until contact is made between the top lap plate and the work pieces on bottom lap plate 12. This latter position is illustrated in Fig. 2. Also with reference to such figure, it is noted that a bellows-type cover 80 or the like is disposed about support shaft 22 under platform 18 to provide safety and protection.

After contact with the work pieces, descent of the top lap plate 14 ceases. Continued rotation of the ball screw causes the platform 18 to continue descending, but, of course, the top plate can no longer follow. Relative motion between the platform 18 and the flange element 24 then occurs. This results in lengthening of the compression spring 20, reducing the upward force it imparts through the support shaft 22 to the top lap plate.

The force differential between the downwardly directed weight of the top lap plate-support shaft assembly and the upwardly directed force of the compression spring is the force imposed on the work pieces by the top lap plate. The actual force applied to the work pieces is, in essence, the effective weight of the top lap plate. It will be appreciated that the load sensed by the load cell transducer 30 is also decreased by this differential.

In the system just disclosed, displacement of platform 18 may be thought of as being the manipulated variable, and the force from spring 20 the controlled

variable. The spring rate, which is a design choice, defines the change in controlled variable per unit change of manipulated variable. The smaller the change in controlled variable that results from a given change in manipulated variable, the less sensitive the accuracy of the controlled variable is to the accuracy of the manipulated variable. By appropriate selection of the spring rate, any degree of force accuracy can be achieved per a given degree of displacement control.

In the disclosed embodiment, the system incorporates a microprocessor or central processing unit 84 which initiates and controls the stepping motor 64 and thus top lap plate descent. The microprocessor is preferably programmed to accomplish relatively rapid descent to a preselected location near contact with the work pieces but out of actual engagement therewith. At that point, descent of the top lap plate is terminated and a reading of the load cell is taken and stored. A start-up load "set point" is retrieved from the lapping recipe stored in the microprocessor memory.

Through a suitable control associated with the apparatus, the operator then initiates lowering of the top plate by the stepping motor at a slower rate of descent. With each step of the stepping motor 64, readings of the load cell transducer 30 are made. The differential between the current reading and the initial reading are calculated and compared by the microprocessor with the set point. When the set point reading is achieved, the stepping motor is de-energized and descent of the platform 18 is terminated.

During the lapping process, a lapping force change can be made uniformly over a specified period of time. Since each step of the stepping motor represents a fixed load change, a given load change can be converted to a fixed number of steps. The microprocessor may, for example, determine the number of 10 millisecond periods in the requested time interval and divide same into the required number of steps. Every ten milliseconds the microprocessor commands the stepping motor to take that

number of steps until the total time period is' achieved. At the end of the time period, the actual load is compared to the set point load and the stepping motor is commanded to step every 0.1 seconds until the set point is matched exactly.

Fig. 5 illustrates in diagrammatic fashion the operation of the control system utilized in the disclosed preferred embodiment of the invention.

In the disclosed embodiment, the system incorporates a microprocessor used to, among other tasks, monitor the load cell measurements and control the stepping motor 64 in response thereto and to raise and lower the top lap plate 14 in response to "UP" and "DOWN" switches (not shown) on the machine control panel. A suitable microprocessor is No. M50734SP made available by Mitsubishi Electronics, America Inc., Hackensack, N.J.

As previously indicated, when the machine is powered up, the top plate is in the raised position. The microprocessor is waiting for the operator to close a push button switch (not shown) signaling the command to lower the top plate 14.

In response to closure of the "DOWN" switch, the microprocessor sends pulses at the required time interval to the stepping motor controller. The stepping motor controller, in response to the pulses, generates the required voltage pattern between A+, -A and B+ and B- to produce motor rotation. The stepping motor controller produces, for example, one 1.8° motor step for each pulse received from the microprocessor. Also, the microprocessor puts the correct voltage level on its output connected to the direction selection input on the stepping motor controller to produce the required rotational direction to cause the top lap plate to descend. A suitable motor controller is Model 5430 from Pacific Scientific of Rockford, Illinois.

In actual practice, the top lap plate is preferably lowered swiftly to an elevation close to, but not touching the work pieces. This elevation is called the float position. From this float position elevation

downward motion is slower to minimize the shock caused by the top lap plate contacting the work pieces.

The top lap plate dwells briefly at the float position during which time the microprocessor signals the A/D converter to make a conversion. The A/D converter out puts digital signals to the microprocessor data bus which represent the magnitude of the load on the load cell 30. The microprocessor stores this data in its memory. The data represents the weight of the top plate assembly. A suitable A/D converter is Part No. AD7578 from Analog Devices, Norwood, MA.

With each step of the stepping motor, as the top plate is lowered from the float position into contact with the work pieces, the microprocessor signals the A/D converter for a data conversion, such conversion representing the magnitude of force on the load cell 30. The new data is subtracted from the initial data and the result compared to the desired load. If the calculated result is less than the desired load another step is taken. When the calculated result equals the desired load no steps are taken. At such time the start up load is being applied to the parts and the lapping cycle can be started.

The "desired load" is a lapping recipe variable stored in the microprocessor memory prior to lapping. The desired load can be programmed to change as the lapping cycle progresses.

As the lapping cycle progresses the microprocessor frequently samples load cell readings and makes the comparison of actual load to desired load at the time in the cycle. The microprocessor initiates stepping motor action as a result of the comparison to either increase, decrease or not change top plate load on the work pieces.