U.S. patent application number 10/609895 was filed with the patent office on 2004-07-22 for critical orifice gap setting for ecm grooving of flat plates.
Invention is credited to Cochran, Dustin Alan.
Application Number | 20040140226 10/609895 |
Document ID | / |
Family ID | 32718229 |
Filed Date | 2004-07-22 |
United States Patent
Application |
20040140226 |
Kind Code |
A1 |
Cochran, Dustin Alan |
July 22, 2004 |
Critical orifice gap setting for ECM grooving of flat plates
Abstract
An apparatus and method for electrochemically etching grooves in
a working surface. A frame holds a working surface about an axis
and facing a movable electrode movable along the axis. The
electrode is axially movable and has a surface carrying a groove
pattern to fix on the working surface. A source of electrolyte is
pumped at a fixed static pressure rate between the surface of the
movable electrode and the working surface. A support fixture is
provided for supporting the electrode for movement toward and away
from the working surface with minimal frictional restriction. A
force biases the electrode surface toward the working surface so
that a gap through which the electrolyte flows between the surface
of the movable electrode and the working surface is determined
primarily by the static flow rate of the electrolyte and the force
bias of the electrode toward the working surface.
Inventors: |
Cochran, Dustin Alan;
(Watsonville, CA) |
Correspondence
Address: |
James A Sheridan
Moser Patterson & Sheridan LLP
595 Shrewsbury Avenue Suite 100
Shrewsbury
NJ
07702
US
|
Family ID: |
32718229 |
Appl. No.: |
10/609895 |
Filed: |
June 30, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60441685 |
Jan 21, 2003 |
|
|
|
Current U.S.
Class: |
205/652 |
Current CPC
Class: |
B23H 7/32 20130101; B23H
9/00 20130101; B23H 3/00 20130101; B23H 2200/10 20130101 |
Class at
Publication: |
205/652 |
International
Class: |
B23H 005/00 |
Claims
What is claimed is:
1. An apparatus for electrochemically etching grooves in a working
surface, the apparatus comprising: a frame for holding the working
surface about an axis and facing a movable electrode movable along
the axis, the electrode being axially movable and having a surface
carrying a groove pattern to fix on the working surface; a source
of electrolyte to be pumped at a fixed static pressure rate between
the surface of the movable electrode and the working surface; and a
support fixture for supporting the electrode for movement toward
and away from the working surface with minimal frictional
restriction, and a force biasing the electrode surface toward the
working surface so that a gap through which the electrolyte flows
between the surface of the movable electrode and the working
surface is determined primarily by the static flow rate of the
electrolyte and the force bias of the electrode toward the working
surface.
2. The apparatus of claim 1, wherein the support fixture comprises
a hydrostatic bearing cartridge assembly.
3. The apparatus of claim 1, wherein the bias of the electrode
surface toward the working surface is established by pressure
against a distal end of the electrode.
4. The apparatus of claim 3, wherein the pressure is caused by a
substantially frictionless air cylinder.
5. The apparatus of claim 1, further comprising a source of
electric potential to be applied between the electrode and the
working surface.
6. The apparatus of claim 5, wherein the electric potential creates
a fixed current across the gap so that a rate at which an ECM
process is carried out is determined primarily by the gap.
7. The apparatus of claim 1, wherein the working surface is a
surface of a counter plate.
8. The apparatus of claim 1, wherein the working surface is a
surface of a conical element.
9. The apparatus of claim 1, wherein the electrode comprises a
plenum through which electrolyte flows.
10. A method of electrochemically etching grooves in a working
surface, the method comprising: holding, via a frame, the working
surface about an axis and facing a movable electrode movable along
the axis, the electrode being axially movable and having a surface
carrying a groove pattern to fix on the working surface; pumping
electrolyte at a fixed static pressure rate between the surface of
the movable electrode and the working surface; and supporting, via
a support fixture, the electrode for movement toward and away from
the working surface with minimal frictional restriction, and using
a biasing force to bias the electrode surface toward the working
surface so that a gap through which the electrolyte flows between
the surface of the movable electrode and the working surface is
determined primarily by the static flow rate of the electrolyte and
the force bias of the electrode toward the working surface.
11. The method of claim 11, wherein the support fixture comprises a
hydrostatic bearing cartridge assembly.
12. The method of claim 11, wherein the bias of the electrode
surface toward the working surface is established by pressure
against a distal end of the electrode.
13. The method of claim 12, wherein the pressure is caused by a
substantially frictionless air cylinder.
14. The method of claim 10, further comprising a source of electric
potential to be applied between the electrode and the working
surface.
15. The method of claim 14, wherein the electric potential creates
a fixed current across the gap so that a rate at which an ECM
process is carried out is determined primarily by the gap.
16. The method of claim 10, wherein the working surface is a
surface of a counter plate.
17. The method of claim 10, wherein the working surface is a
surface of a conical element.
18. The method of claim 10, wherein the electrolyte flows through a
plenum in the electrode.
19. An apparatus for electrochemically etching grooves in a working
surface, the apparatus comprising: means for holding the working
surface about an axis and facing a movable electrode movable along
the axis, the electrode being axially movable and having a surface
carrying a groove pattern to fix on the working surface; means for
pumping electrolyte at a fixed static pressure rate between the
surface of the movable electrode and the working surface; means for
supporting the electrode for movement toward and away from the
working surface with minimal frictional restriction; and means for
biasing the electrode surface toward the working surface so that a
gap through which the electrolyte flows between the surface of the
movable electrode and the working surface is determined primarily
by the static flow rate of the electrolyte and a force bias of the
electrode toward the working surface.
20. The apparatus of claim 19, wherein the means for biasing the
electrode surface toward the working surface is adapted to apply
pressure against a distal end of the electrode.
21. The apparatus of claim 20, wherein the means for biasing the
electrode surface toward the working surface comprises a
substantially frictionless air cylinder.
22. The apparatus of claim 21, wherein the air cylinder is located
distally of the electrode.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Application No. 60/441,685, filed Jan. 21, 2003 by Cochran
(entitled "Critical Orifice Gap Setting for ECM Grooving of Flat
Plates"), which is herein incorporated by reference.
FIELD OF THE INVENTION
[0002] This invention pertains generally to the field of fluid
dynamic bearings, and more particularly to etching grooves in a
counter plate used in a disc drive.
BACKGROUND OF THE INVENTION
[0003] Disc drives, including magnetic disc drives, optical disc
drives and magneto-optical disc drives, are widely used for storing
information. A typical disc drive has one or more discs or platters
that are affixed to a spindle and rotated at high speed past a
read/write head suspended above the discs on an actuator arm. The
spindle is turned by a spindle drive motor. The motor generally
includes a shaft having a thrust plate on one end, and a rotating
hub having a sleeve and a recess into which the shaft with the
thrust plate is inserted. Magnets on the hub interact with a stator
to cause rotation of the hub relative to the shaft.
[0004] In the past, conventional spindle motors frequently used
conventional ball bearings between the hub and the shaft and the
thrust plate. However, over the years the demand for increased
storage capacity and smaller disc drives has led to the read/write
head being placed increasingly close to the disc. Currently,
read/write heads are often suspended no more than a few millionths
of an inch above the disc. This proximity requires that the disc
rotate substantially in a single plane. To provide a stable
rotating system and avoid non-repeatable run-out, the latest
generation of disc drives utilize a spindle motor having fluid
dynamic bearings on the shaft and the thrustplate to support a hub
and the disc for rotation.
[0005] In a fluid dynamic bearing, a lubricating fluid such as gas
or a liquid or air provides a bearing surface between a fixed
member and a rotating member of the disc drive. Dynamic
pressure-generating grooves formed on a surface of the fixed member
or the rotating member generate a localized area of high pressure
or a dynamic cushion that enables the spindle to rotate with a high
degree of accuracy. Typical lubricants include oil and
ferromagnetic fluids. Fluid dynamic bearings spread the bearing
interface over a large continuous surface area in comparison with a
ball bearing assembly, which comprises a series of point
interfaces. This is desirable because the increased bearing surface
reduces wobble or run-out between the rotating and fixed members.
Further, improved shock resistance and ruggedness is achieved with
a fluid dynamic bearing. Also, the use of fluid in the interface
area imparts damping effects to the bearing that helps to reduce
non-repeat runout. However, to be effective, the
pressure-generating grooves must be very accurately defined, both
as to shape and depth, on a high-speed basis.
[0006] Accordingly, there is a need for an apparatus and method for
forming grooves in a work piece made of a hard metal to manufacture
fluid dynamic bearings suitable for use in a disc drive. It is
desirable that the apparatus and method allow the grooves to be
formed quickly and cheaply. It is also desirable that the apparatus
and method not require expensive equipment or the use of a
metal-removing tool that must be frequently replaced. It is further
desirable that the apparatus and method not use an etch-resistant
material during manufacture that could contaminate the work piece
leading to the failure of the bearing and destruction of the disc
drive.
[0007] As the result of the above problems, electrochemical
machining (ECM) of grooves in a fluid dynamic bearing has been
developed. A broad description of ECM is as follows. ECM is a
process of removing material metal without the use of mechanical or
thermal energy. Basically, electrical energy is combined with a
chemical to form a reaction of reverse electroplating. To carry out
the method, direct current is passed between the work piece which
serves as an anode and the electrode, which typically carries the
pattern to be formed and serves as the cathode, the current being
passed through a conductive electrolyte which is between the two
surfaces. At the anode surface, electrons are removed by current
flow, and the metallic bonds of the molecular structure at the
surface are broken. These atoms go into solution with the
electrolyte as metal ions and form metallic hydroxides. These
metallic hydroxide (MOH) molecules are carried away to be filtered
out. However, this process raises the need to accurately and
simultaneously place grooves on a surface across a gap which must
be very accurately defined, as the setting of the gap will
determine the rate and volume at which the metal ions are carried
away. Even in simple structures, this problem can be difficult to
solve. When the structure is the interior surface of a conical
bearing, the setting of the gap width can be extremely difficult.
Manufacturability issues associated with conical parts often make
it difficult to control the diameter of the cones. Therefore, it is
very difficult to make a tool with fixed electrodes that will
guarantee a continued consistent work piece to electrode gap. As
noted above, the distance is paramount to the accuracy of grooved
depth.
[0008] In known designs, the gap is varied to yield a predetermined
mass flow, and the position of the electrode relative to the work
piece is adjusted mechanically to establish the gap. This takes up
to thirty seconds in time, which translates directly into
manufacturing costs.
[0009] The present invention provides a solution to these and other
problems, and offers other advantages over the prior art.
SUMMARY OF THE INVENTION
[0010] The present invention relates to a critical orifice gap
setting for ECM grooving of flat plates.
[0011] In one embodiment according to the present invention, an
apparatus and method are provided for electrochemically etching
grooves in a working surface. A frame holds a working surface about
an axis and facing a movable electrode movable along the axis. The
electrode is axially movable and has a surface carrying a groove
pattern to fix on the working surface. A source of electrolyte is
pumped at a fixed static pressure rate between the surface of the
movable electrode and the working surface. A support fixture is
provided for supporting the electrode for movement toward and away
from the working surface with minimal frictional restriction. A
force biases the electrode surface toward the working surface so
that a gap through which the electrolyte flows between the surface
of the movable electrode and the working surface is determined
primarily by the static flow rate of the electrolyte and the force
bias of the electrode toward the working surface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] So that the manner in which the above recited embodiments of
the invention are attained and can be understood in detail, a more
particular description of the invention, briefly summarized above,
may be had by reference to the embodiments thereof which are
illustrated in the appended drawings. It is to be noted, however,
that the appended drawings illustrate only typical embodiments of
this invention and are therefore not to be considered limiting of
its scope, for the invention may admit to other equally effective
embodiments.
[0013] FIG. 1 is an exploded perspective view of the basic elements
of a disc drive in which a motor incorporating a counter plate
formed by embodiments according to the present invention is
especially useful;
[0014] FIG. 2 is a sectional side view of a motor incorporating a
counter plate formed by embodiments according to the present
invention;
[0015] FIG. 3 is a cross-sectional side view of a system used to
etch grooves in a counter plate;
[0016] FIG. 4 is a perspective view with a partial carve-out of a
hydrostatic bearing cartridge assembly according to an embodiment
of the present invention; and
[0017] FIG. 5 is a bottom view of an exemplary counter plate having
grooves etched therein by an embodiment of an apparatus and method
according to the present invention.
DETAILED DESCRIPTION
[0018] FIG. 1 is an exploded perspective view of a magnetic disc
drive for which a spindle motor having a fluid dynamic bearing
manufactured by the method and apparatus for the present invention
is particularly useful. Referring to FIG. 1, a disc drive 100
typically includes a housing 105 having a base 110 sealed to a
cover 115 by a seal 120. The disc drive 100 has a spindle 130 to
which are attached a number of discs 135 having surfaces 140
covered with a magnetic media (not shown) for magnetically storing
information. A spindle motor (not shown in this figure) rotates the
discs 135 past read/write heads 145 that are suspended above
surfaces 140 of the discs by a suspension arm assembly 150. In
operation, the spindle motor rotates the discs 135 at high speed
past the read/write heads 145 while the suspension arm assembly 150
moves and positions the read/write heads over one of several
radially spaced tracks (not shown). This allows the read/write
heads 145 to read and write magnetically encoded information to the
magnetic media on the surfaces 140 of the discs 135 at selected
locations.
[0019] FIG. 2 is a sectional side view of a spindle motor 155 of a
type which is especially useful in disc drives 100. Typically the
spindle motor 155 includes a rotatable hub 160 having one or more
magnets 165 attached to a periphery thereof. The magnets 165
interact with a stator winding 170 attached to the base 110 to
cause the hub 160 to rotate. The hub 160 is supported on a shaft
175 having a thrustplate 180 on one end. The thrustplate 180 can be
an integral part of the shaft 175, or it can be a separate piece
which is attached to the shaft, for example, by a press fit. The
shaft 175 and the thrustplate 180 fit into a sleeve 185 and a
thrustplate cavity 190 in the hub 160. A counter plate 195 is
provided above the thrustplate 180 resting on an annular ring 205
that extends from the hub 160. An O-ring 210 seals the counter
plate 195 to the hub 160.
[0020] A fluid, such as lubricating oil or a ferromagnetic fluid,
fills interfacial regions between the shaft 175 and the sleeve 185,
and between the thrustplate 180 and the thrustplate cavity 190 and
the counter plate 195. One or more of the thrustplate 180, the
thrustplate cavity 190, the shaft 175, the sleeve 185 or the
counter plate 195 have pressure generating grooves (not shown in
this figure) formed to create fluid dynamic bearings. In one
embodiment, the grooves are formed in inner surfaces 215 of the hub
160. In another embodiment, the grooves are formed in the sleeve
185 and in the thrustplate cavity 190. The grooves in the
thrustplate cavity 190 form a fluid dynamic thrust bearing 220 by
generating a localized region of dynamic high pressure to form a
dynamic cushion that rotatably supports the hub 160 in the
direction of thrust. Grooves in the inner surface 215a of the
sleeve 185 form one or more fluid dynamic journal bearings 225
having dynamic cushions that rotatably support the hub 160 in a
radial direction.
[0021] Fluid dynamic bearings, as previously implied, are generally
formed between rotatable and non-rotatable members having
juxtaposed surfaces between which a layer or film of fluid is
induced to form a dynamic cushion as an anti-friction medium. To
form the dynamic cushion, at least one of the surfaces is provided
with grooves that induce fluid-flow in the interfacial region and
generate the localized region of dynamic high pressure referred to
previously.
[0022] As mentioned herein, it is difficult to make a device with
fixed electrodes that guarantees a continued consistent work piece
to electrode gap. The distance of the gap is paramount to the
accuracy of grooved depth.
[0023] Given the above, it is necessary to create or define a tool
used to form the grooves incorporating moving electrodes. Utilizing
moving electrodes gives rise to another problem (i.e., how to set
the gap between the electrode and the working surface on which the
grooves are to be defined). The electrode/work piece gap itself is
in many instances the "critical orifice." Critical orifice flow
measurement is utilized because the setting of the gap will
determine the rate and volume at which the metal ions are carried
away, all other parameters being unchanged, and thereby determines
the shape and depth of the grooves being formed.
[0024] In known designs, as mentioned herein, the gap is varied to
yield a predetermined mass flow and the position of the electrode
relative to the work piece is adjusted mechanically to establish
the gap. This takes up to thirty seconds in time, which translates
directly into manufacturing costs. It is desirable to be able to
set a gap quickly and accurately with a consistent gap width each
time the gap is set.
[0025] Referring to FIG. 3, one embodiment according to the present
invention provides a method and apparatus for forming the pressure
generating grooves in a working surface of the counter plate 195. A
system 310 comprises counter plate 195, electrode 312, plenum 314,
insulation 316, gap 318 (sometimes referred to as "critical orifice
gap" or "machining gap") and injection port 320.
[0026] In use, an electrolyte is supplied (as described herein)
through the electrode 312 and into the plenum 314. In FIG. 3, the
plenum 314 is shown as having a smaller diameter at a proximal end
and a larger diameter at a distal end; however, this need not be
the case.
[0027] Before or after the electrolyte is supplied, the electrode
312 is moved into contact with or proximate the counter plate 195
via a constant downward force F. In one embodiment, F is due to a
constant pressure P.sub.ac applied by a (substantially)
frictionless air cylinder. In other embodiments, F is due to the
gravitational pull on a mass or the like.
[0028] Electrolyte is supplied through the electrode 312 and into
the plenum 314. It is envisioned that the electrolyte is supplied
into the plenum 314 by penetrating the electrode in one embodiment.
In another embodiment, the electrolyte is supplied into the plenum
314 without penetrating the electrode. The electrolyte is supplied
at a constant pressure P.sub.e and with a constant flow rate
Q.sub.e.
[0029] The electrolyte exits the plenum 314 via an injection port
320. The electrolyte comes into contact with the counter plate 195
and disperses in a radial fashion through the gap 318. The force of
the electrolyte displaces the electrode 312 in a distal (upward)
direction until an equilibrium is reached with the downward force F
on the electrode 312. The gap 318 then becomes a critical orifice
as the width of the gap 318 will directly affect grooves that will
be formed in the counter plate 195.
[0030] If P.sub.e, Q.sub.e and F are constant then the
cross-sectional flow area of the gap 318 will remain constant. In
this case, the electrode 312 will hover over the counter plate 195.
The gap 318 is automatically established without the need to make
an external adjustment.
[0031] The insulation 316 prevents unwanted areas of the counter
plate 195 from being scathed. The insulation 316 covers all areas
of the electrode 312 that are proximate the counter plate 195 for
which it is desired that the electrode 312 areas be made
ineffectual in forming grooves in the counter plate 195. An
electric potential is applied between the electrode 312 and the
counter plate 195. Desired grooves are thus formed in the counter
plate 195 as described herein.
[0032] FIG. 4 is a perspective view with a partial carve-out of a
hydrostatic bearing cartridge assembly 410 according to an
embodiment of the present invention. The electrode 312 is slidably
positioned within the hydrostatic bearing cartridge assembly 410
and protrudes from a proximal end thereof. The hydrostatic bearing
cartridge assembly 410 provides a (substantially) frictionless way
for the electrode 312 to slide up and down.
[0033] As mentioned herein, a (substantially) frictionless air
cylinder 412 imparts a force F to the electrode 312 in a proximal
(downward) direction. The electrode 312 is free to slide up and
down with substantially no friction due to hydrostatic bearings
414. Electrolyte is supplied into the plenum 314 via a first inlet
416. Electrolyte is supplied to the hydrostatic bearings 414 via a
second inlet 418. P.sub.ac and P.sub.e are controlled and
maintained constant via a super-precision regulator(s), which is
known to those of ordinary skill in the art.
[0034] FIG. 5 is a bottom view of an exemplary counter plate 195
having grooves etched therein by an embodiment of an apparatus and
method according to the present invention. FIG. 5 merely depicts an
exemplary embodiment of grooves 510 formed according to methods
described herein. The grooves, which are separated by ribs or
raised lands, can have a depth of from about 0.009 to 0.015 mm,
although they are not limited to this range. Generally, the grooves
are shaped and arranged to form a chevron or herringbone pattern.
That is, the grooves are made up of two straight segments that meet
at an angle to define a "V" shape. Alternatively, the grooves
define a pattern that has an arcuate or sinusoidal shape, or may be
of any other pattern; the present invention is useful to form any
desirable pattern.
[0035] Thus the present invention represents a significant
advancement in the field of fluid dynamic bearing motor design.
Wear is significantly reduced by providing an accurate and
relatively inexpensive method of forming grooves on a counter plate
195. It is contemplated that embodiments of the apparatus and
methods described herein can be used to etch grooves of varying
configurations. Moreover, it is envisioned that embodiments of the
apparatus and methods described herein can be used to etch grooves
in any suitable plate, conical element or the like.
[0036] While the foregoing is directed to embodiments of the
invention, other and further embodiments of the invention may be
devised without departing from the basic scope thereof, and the
scope thereof is determined by the claims that follow.
* * * * *