 |
Description  |
|
|
BACKGROUND OF THE INVENTION
This invention relates to the manufacturing of abrading tools, and
particularly to forming the abrading surfaces of lapping plates used in
high precision lapping of magnetic transducing heads.
Magnetic transducing heads, used to store and retrieve data on rotatable
magnetic recording discs, call for fine manufacturing tolerances, often
measured in microinches (millionths of an inch). Thin film heads typically
are formed by applying layers of an electrically conductive material and a
magnetic flux conducting core or pole-piece material along one side of a
comparatively large body or slider. In use, a finely machined planar
bottom surface of the slider is spaced vertically apart from a horizontal
magnetic recording surface of the rotating magnetic disc, supported by a
thin film of air. To form the transducer bottom surface, high precision
abrading equipment is used, including a rotating lapping plate having a
horizontal lapping surface in which abrasive particles such as diamond
fragments are embedded. An abrasive slurry, for example a water soluble
glycol base containing diamond fragments or other abrasive particles, is
applied to the lapping surface as the lapping plate is rotated relative to
the slider or sliders maintained against the lapping surface. The diamond
fragments can be from one to two hundred and fifty microinches in
diameter. An example of such lapping plate, along with a carrier arm for
maintaining a slider bar or other workpiece against the lapping plate, is
disclosed in U.S. Pat. No. 4,536,992 (Hennenfent et al).
Common practice is to periodically refurbish the plates with a lapping
grit, to produce a surface texture suitable for the embedding and
retention of the appropriate size of diamond grit being used with the
lapping process. A problem with this is that the surface is susceptible to
a rapid change in smoothness as it is used to lap a workpiece, principally
due to fragments removed from the workpiece during lapping. The change in
smoothness affects the hydrodynamic bearing film provided by the liquid
component of the abrasive slurry, creating a "hydroplaning" effect which
raises the workpiece from the lapping surface, to diminish the abrasive
action of the particles and substantially increase abrading time.
The general idea of interrupting the lapping surface, for example by
forming grooves in a lapping plate, is known in the art. For example, U.S.
Pat. No. 3,921,342 (Day) shows a lapping plate 12 in which a plurality of
troughs are formed in the lapping surface. A filler of material can be
placed in the troughs, so that unspent abrasive liquid is maintained
adjacent the working surface of the lapping plate, while spent abrasive
fluid is centrifugally removed beyond the lap plate periphery. In U.S.
Pat. No. 4,037,367 (Kruse), grooves are formed between working surface
areas in which an abrasive such as diamond particles are embedded in a
metallic coat. The grooves sweep beneath the workpiece to remove abrasive
particles as the abrasive disc rotates. Kruse teaches the depth of the
groove should be at least twice the nominal diameter of the particles, and
the groove width should be at least ten times the nominal diameter. U.S.
Pat. No. 3,683,562 (Day) also discloses a grooved lapping plate.
A problem with grooved plates, however, is due to excessive width and depth
of grooves. Abrasive particles entering excessively deep grooves are in
effect lost, as they become too far removed from the workpiece surface to
provide any further abrasive action. This removal of the grit may be
caused by steep, nearly vertical side walls of the grooves, as well as the
groove depth. Further, the wide grooves provide a surface discontinuity
too severe for small workpieces. Forming such grooves is costly and time
consuming. Even if the grooves can be sized properly, substantial segments
of the lapping surface remain ungrooved, or alternatively a prohibitively
large number of grooves are required. Surface uniformity--on the
microscopic scale suitable for lapping small workpieces--could be achieved
only with extreme care. Refurbishment of such a lapping surface would
require renewal of the grooves as well, further adding to the expense.
Therefore it is an object of the present invention to provide a lapping
tool having a selected texture for discontinuity over its lapping surface,
and on a microscopic scale appropriate for lapping small workpieces.
Another object is to provide a textured lapping surface which is
substantially uniform.
Another object is to provide a process for forming, in a lapping tool, a
substantially uniform textured lapping surface, while avoiding the expense
of cutting grooves in the lapping plate.
Yet another object of the invention is to provide a lapping tool having a
uniformly textured lapping surface amenable to repeated refurbishment by
conventional processes.
SUMMARY OF THE INVENTION
To achieve these and other objects, there is provided an abrading tool
comprising a lapping body having a substantially horizontal lapping
surface and a plurality of first abrasive particles fixed to the lapping
surface. The lapping surface is adapted for surface engagement with a
workpiece and further for supporting an abrasive slurry, with the lapping
body being movable horizontally with respect to the workpiece for lapping
a surface of the workpiece through the abrasive action of the first
abrasive particles and of a plurality of second abrasive particles
suspended in the abrasive slurry. A plurality of generally spherical
depressions are formed in the lapping surface, spaced apart from one
another, generally uniformly distributed over the lapping surface, and
combining to comprise from twenty-five percent to sixty-five percent of
the surface area of the lapping surface. The depressions have diameters in
the range of from two to twenty thousandths of an inch, with a depth of
each depression being less than one-fourth of its diameter.
Preferably, the depressions comprise from forty to fifty percent of the
lapping surface area, with the depressions having diameters ranging from
three to six thousandths of an inch and depths of less than one-sixth the
diameter. As one example, the depressions or cavities can have a diameter
of about five thousandths of an inch, and a depth of seven hundred
microinches, and together cover approximately forty-five percent of the
lapping surface. So arranged, the depressions interrupt the planarity of
the lapping surface to reduce the hydrodynamic film from the abrasive
slurry, permitting the workpiece to interact more intimately with the
lapping plate. This substantially reduces the above-mentioned
hydroplaning, a particular advantage when curved surfaces are to be formed
in the sliders, as described in the aforementioned U.S. Pat. No.
4,536,992, for more uniform curvature. The cavities provide volumes for
removal of particulate contaminants from the workpiece being lapped, and
thus reduce scratching of the workpieces. At the same time, it is believed
that the spherical shape of the depressions, combined with the high
diameter to depth ratio, causes a turbulence in the flow of slurry within
the depressions, especially near their peripheries. The result is a more
effective use of the abrasive particles suspended in the abrasive slurry,
increasing the lapping rate, particularly as compared to the expected rate
for a similar surface area provided with steep-walled grooves.
Another aspect of the present invention is a process for manufacturing an
abrading tool having a desired surface texture for a lapping surface of
the tool. The process comprises the steps of:
(a) machining a lapping surface of an abrading tool to a desired planarity;
(b) forming in the lapping surface a plurality of generally spherical
depressions, spaced apart from one another, generally uniformly
distributed over the lapping surface, and together comprising from
twenty-five percent to sixty-five percent of the surface area of the
lapping surface, with the 25 remainder of the lapping surface comprising a
substantially planar surface portion; and
(c) fixing a plurality of abrasive particles to the planar surface portion.
Preferably, the depressions are formed by propelling a plurality of
substantially spherical members against the lapping surface, with the
spherical members constructed of a material such as glass, harder than the
material forming the lapping surface. Glass beads with a nominal diameter
of about ten one-thousandths of an inch have been used to form depressions
of a diameter of approximately five thousandths and a seven hundred
microinch depth. A further refinement in the process involves machining
the lapping surface for planarity after forming the depressions with the
glass beads and prior to fixing the abrasive particles. This restores the
desired planarity of the plateau portion of the lapping surface,
principally by removing any extrusion ridges at the boundaries of the
depressions.
To distribute the cavities with the desired uniformity, the glass beads
preferably are propelled serially against the lapping surface of the
lapping disc while the disc is rotating, and with the nozzle propelling
the glass beads traversing an arcuate path in a plane which also contains
the lapping plate rotational axis. Cavity density can be controlled by
regulating the application time or the amount of glass beads supplied to
the nozzle.
Thus formed, the spherical cavities have a uniformity and density of
distribution over the lapping surface superior to that of prior art
grooves. As a consequence, lapping plates textured in accordance with the
present invention provide more consistent lapping action throughout their
useful lives, and when used to lap relatively large workpieces, have been
found to last nearly ten times as long as a comparable lapping plate with
a flat lapping surface. At the same time, given the small nominal pit
depth, such a disc may be refurbished repeatedly by simply abrading the
lapping surface to remove all pits, then retexturizing. Better
co-planarity is achieved when lapping composite heads. Pole tip recession,
a problem described in copending U.S. patent application Ser. No. 123,967
(Holmstrand), filed concurrently herewith, has been reduced from an
average of 2.4 microinches to 1.5 microinches in accordance with the
apparatus described in that application, and has been further reduced to
1.1 microinches when utilizing a lapping surface textured in accordance
with the present invention.
IN THE DRAWINGS
For a better appreciation of the above and other features and advantages,
reference is made to the following detailed description and drawings, in
which:
FIG. 1 is a perspective view of a lapping plate constructed in accordance
with the present invention;
FIG. 2 is an enlarged side sectional elevation of a lapping plate of the
prior art;
FIG. 3 is an enlarged side sectional elevation of a portion of the lapping
plate in FIG. 1;
FIG. 4 is a top plan view of a portion of the lapping plate of FIG. 1;
FIG. 5 is a top plan view similar to that of FIG. 4 showing a portion of an
alternative embodiment lapping plate;
FIG. 6 is a schematic view illustrating apparatus employed in forming a
lapping surface of the lapping plate of FIG. 1;
FIG. 7 illustrates a spherical glass bead used in forming the lapping
surface; and
FIGS. 8-11 illustrate a portion of the lapping plate of FIG. 1 in various
stages of formation of the lapping surface.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning now to the drawings, there is shown in FIG. 1 a lapping plate 16
rotatable about a vertical axis and having a substantially flat and
horizontal lapping surface 18. A workpiece carrier arm 20 supports a
workpiece 22 against the lapping surface, so that the bottom surface of
the workpiece is abraded as lapping plate 16 is rotated. A container 24
supplies an abrasive slurry 26 to the lapping surface. Particles (e.g.
diamond fragments one to ten microinches in diameter preferred, but
possibly up to two hundred fifty microinches) in the slurry contribute to
the abrading action. Abrasive slurry 26 is carried to the workpiece by the
rotating lapping plate. For a further explanation of the abrading system
employing lapping plates such as plate 16, reference is made to the
aforementioned U.S. Pat. No. 4,536,992.
Usually there are two sources of abrasion: abrasive grit suspended in the
abrasive slurry, and further abrasive particles embedded in the lapping
surface. These latter particles are shown at 28, embedded in a
substantially flat lapping surface 30 of a prior art lapping plate 32
(FIG. 2). Lapping plate 32 experiences rapid degradation during abrading,
due to build-up of material removed from the workpiece and the
hydroplaning effect of the abrasive slurry in elevating the workpiece.
An enlarged portion of lapping plate 16 is shown in FIG. 3. To reduce
hydroplaning and extend the useful life of the lapping plate 16, a
plurality of cavities or depressions 34 are formed in the lapping plate
along lapping surface 18. Cavities 34 effectively divide lapping surface
18 into two separate regions, including a cavitated region and a
substantially planar plateau 36. Abrasive particles such as diamond
fragments 28 are embedded in plateau 36. Cavities 34 preferably have a
diameter (taken along the plane of plateau 36) of about five thousandths
of an inch. However, cavity diameters can range from about two to twenty
thousandths of an inch, and it is not critical that the cavity diameters
be uniform. The maximum cavity depth should be less than one-fourth of its
diameter, and preferably is less than one-sixth of a diameter. For
example, given a typical cavity diameter of five one-thousandths of an
inch, the typical depth is approximately seven hundred microinches.
As is best seen in FIG. 4, depressions 34 are generally spherical and cover
a substantial area of lapping surface 18. The portion of the lapping
surface shown in FIG. 4 is small (approximately 0.1 inches square) but
representative of the lapping surface, as cavity distribution is
preferably uniform over the entire lapping surface. Cavities 34 are for
the most part spaced apart from one another, although occasionally a pair
of cavities may be formed adjacent one another. Cavities 34 together cover
approximately forty to fifty percent of lapping surface 18, with the
remainder of the lapping surface consisting of plateau 36. The cavities
thus represent a substantial portion of the lapping surface not embedded
with abrasive particles 28. However, this has not been found to
substantially reduce abrading efficiency. It is believed that cavities 34
create turbulence in the abrasive slurry, particularly near the periphery
of each cavity, which increases the abrading action of particles suspended
in the slurry and counters the effect of reducing the surface area of
plateau 36.
FIG. 5 illustrates an approximately 0.1 inch.times.0.1 inch portion of an
alternative lapping plate 38 in which a lapping surface 40 is formed of
approximately seventy percent plateau 42, and about thirty percent of the
area covered by cavities 44, which are about the same size as cavities 34.
Textured surfaces may be formed with cavities occupying from about
twenty-five to about sixty-five percent of the lapping surface area. Where
surface discontinuity to reduce hydroplaning is the primary concern, the
cavity density is higher, while a lower density is preferred when there is
a need to maximize the plateau area where abrasive particles 28 may be
embedded.
A salient feature of the present invention is the uniformity of the lapping
surface, even over the relatively small scale of the surface areas shown
in FIGS. 4 and 5. Prior art texturizing such as the cutting of grooves
illustrated in the aforementioned U.S. Pat. Nos. 4,037,367 (Kruse) and
3,921,342 (Day) cannot achieve this degree of uniformity, nor even
approach it without inordinately expensive and time consuming cutting or
grinding.
FIG. 6 illustrates a bead blasting apparatus 46 utilized to form cavities
34 to achieve the desired density and uniformity. In particular, a
plurality of spherical glass beads, loaded into a bead container 48, are
supplied to a guide tube or nozzle 50 mounted to pivot about a pivot axis
52 with respect to a fixed support 54. An air compressor 56 provides air
at an elevated pressure sufficient to project the glass beads rapidly and
serially through nozzle 50 and onto lapping surface 18 of lapping plate
16. The lapping plate is supported, by means not illustrated, on a
rotational axis 58 which appears as a point in the figure.
A motor 60 rotates lapping plate 16 about axis 58, and a second motor 61
reciprocates nozzle 50 over an arcuate path, the extremes of which are
indicated by the upper position of the nozzle shown in solid lines, and
the lower nozzle position indicated in broken lines. The use of separate
motors to rotate lapping plate 16 and to reciprocate nozzle 50 enables
their asynchronous operation. The rate of nozzle reciprocation, the plate
rotation rate, and their precise relationship do not appear critical.
However, synchronizing these rates should be avoided, for more random
cavity distributions, which tend to be more uniform.
As motor 60 rotates the lapping plate and reciprocates the nozzle, the
glass beads are projected onto lapping surface 18 with sufficient force to
partially penetrate it, thus forming cavities 34. Nozzle 50 is spaced
apart from lapping plate 16 a desired distance in the direction of axis
58, i.e. normal to the plane of FIG. 6. The beads are projected toward the
plate in this direction. It is preferred that the glass beads be generally
uniform in diameter, resulting in cavities or depressions of a generally
uniform diameter and depth. Controlling the cavity density is controlled
in a straightforward manner, either by controlling the number of glass
beads loaded into container 48, or in setting the operating time of
apparatus 46. Thus, cavity density can be reduced as shown in FIG. 5 as
compared to FIG. 4, or alternatively increased.
Shown in FIG. 7 is a glass bead 62 typical of the beads used to form
cavities 34. Bead 62 has a diameter A of about .01 inches, and is
propelled against the lapping surface at a speed sufficient for
penetration of the lapping plate a distance C, so that the cavity formed
will have a depth equal to C and a diameter equal to B. Typically, B is
approximately half of diameter A, or about 0.005 inches, to yield a depth
C of about seven hundred microinches and a ratio B/C of about seven. If
desired, the cavity depth and diameter can be varied, by changing the bead
size, the speed at which the beads are projected onto the lapping surface,
or both.
As noted above, beads 62 are preferably of glass and spherical in shape.
Glass beads are substantially harder than the lapping plate material (e.g.
lead), ensuring that cavities conform to the shape of the beads, and also
minimizing the possibility of cavity contamination from bead fragments
breaking off during formation. The smooth, spherical bead configuration
reduces the chances for bead fragmenting as the beads form cavities having
the desired smoothness and shape.
FIGS. 8-11 illustrate the process for texturizing lapping plate surfaces in
accordance with the present invention. The first step, illustrated in FIG.
8, is to abrade the top of lapping plate 16 to form smooth, planar lapping
surface 18. This is accomplished by moving an abrading tool 64 relative to
lapping plate 16, usually rotationally, with the tool maintained against
the lapping surface.
Next, the apparatus of FIG. 6 is employed to propel glass beads 62 against
lapping surface 18, with the bead size and speed (a function of air
pressure) selected to form the cavities of the desired diameter and depth,
and with either operating time or bead supply controlled to determine the
density of the cavities. As a result, lapping surface 18 consists mainly
of cavities 34 and plateau 36. Also, however, there may be an undesirable
build-up of lapping plate material ridges near the cavity edges, due to
the impact of the beads against the lapping surface, as illustrated at 66.
Consequently, it may be desirable or necessary to use abrading tool 64
once again, to remove ridges 66 from the remainder of the lapping surface,
with the results of this further machining shown in FIG. 10.
Finally, lapping plate 16 is charged with abrasive particles. Charging is
accomplished by providing an abrasive slurry 68 over the lapping surface,
which slurry contains a grit such as diamond particles. Slurry can be,
though need not be, the same as slurry 26. Then, a pressure member 70,
constructed of a material harder than the lapping plate, is pressed
against the lapping surface to cause at least some of the abrasive
particles in slurry 68 to become embedded into the lapping surface,
particularly over plateau 36. At this point, lapping plate 16 is ready for
use in abrading workpieces, as explained in the aforementioned U.S. Pat.
No. 4,536,992.
When lapping plate 16 requires refurbishment, lapping surface 18 is
machined with abrading tool 64 and returns to the form illustrated in FIG.
8 to restore flatness, whereupon it is re-blasted, and re-charged using an
abrasive slurry and pressure member as discussed in connection with FIG.
11.
Cavities 34 substantially increase the useful life of lapping surface 18,
as they provide receptacles for workpiece fragments, loose abrasive
particles and any other matter which otherwise would accumulate between
embedded grit particles on a flat lapping surface, reducing scratching of
workpieces. The maximum cavity depth, for example seven hundred
microinches as discussed above, is substantially larger than the nominal
grit size used in abrading sliders, e.g. one--ten microinches. It has been
found that lapping plate 16, as opposed to totally planar lapping plates,
can be used to abrade more than ten times the number of workpieces before
refurbishment, and with greater lapping consistency. The cavities are
sufficiently large in diameter to reduce hydroplaning of workpieces, yet
are sufficiently small to permit use of lapping plate 16 to abrade
relatively small workpieces. The reduced hydroplaning effect is
particularly noticeable when lapping plate 16 is employed to generate
curved sliders, and results in more uniform curvature.
Finally, lapping surface 16 yields improved co-planarity of features,
particularly in connection with lapping composite materials. In the
aforementioned co-pending U.S. patent application Ser. No. 123,967, it was
noted that use of the wiping guide reduced pole tip recession from 2.4
microinches to 1.5 microinches. It has been found that use of the wiper
guide, augmented by use of a lapping plate textured in accordance with the
present invention, results in a further reduction in pole tip recession,
to 1.1 microinches.
A further advantage of forming cavities with spherical glass beads, in
combination with the shallow penetration of the lapping surface, is the
formation of cavities with smooth, gradually inclined side walls, as
opposed to the nearly vertical side walls of the prior art grooves. It is
believed that the gradually inclined peripheral cavity walls assist in
causing turbulent flow in the abrasive slurry, particularly near the
cavity boundaries, effectively increasing the plateau portion of the
lapping surface to improve abrading efficiency. This turbulent flow also
is believed to reduce hydroplaning of workpieces. As a contrast to steep
walled grooves, the shallow, spherical depressions can occupy a larger
share of the lapping surface area without unduly sacrificing abrading
efficiency, while substantially eliminating workpiece hydroplaning.
* * * * *
|
|
 |
|
 |
Description |
|
