U.S. patent application number 09/891442 was filed with the patent office on 2003-02-06 for shielded spin polishing.
Invention is credited to Shaw, James Stephen.
Application Number | 20030027495 09/891442 |
Document ID | / |
Family ID | 25398195 |
Filed Date | 2003-02-06 |
United States Patent
Application |
20030027495 |
Kind Code |
A1 |
Shaw, James Stephen |
February 6, 2003 |
SHIELDED SPIN POLISHING
Abstract
A shield is placed next to a workpiece to form a passage
therebetween. The workpiece and shield are rotated, and a stream of
abrasive shot is blasted toward the spinning workpiece and shield
to periodically enter the passage to selectively abrade the
workpiece.
Inventors: |
Shaw, James Stephen;
(Hampton Falls, NH) |
Correspondence
Address: |
FRANCIS L. CONTE, ESQ.
6 PURITAN AVENUE
SWAMPSCOTT
MA
01907
US
|
Family ID: |
25398195 |
Appl. No.: |
09/891442 |
Filed: |
June 25, 2001 |
Current U.S.
Class: |
451/29 |
Current CPC
Class: |
B24C 1/04 20130101; B24C
1/083 20130101; B24C 3/22 20130101 |
Class at
Publication: |
451/29 |
International
Class: |
B24B 001/00 |
Claims
Accordingly, what is desired to be secured by Letters Patent of the
United States is the invention as defined and differentiated in the
following claims in which I claim:
1. A method of polishing a workpiece comprising: placing a shield
next to said workpiece to form a passage therebetween; spinning
said workpiece and shield; and blasting a stream of abrasive shot
toward said spinning workpiece and shield to periodically enter
said passage to abrade said workpiece.
2. A method according to claim 1 further comprising: spinning said
workpiece around a spinning axis extending therethrough; placing
said shield radially outwardly from said spinning axis to form
forward and aft windows at opposite ends of said passage; and
blasting said shot stream alternately through said forward and aft
windows to abrade said workpiece from opposite ends during spinning
thereof.
3. A method according to claim 2 further comprising: placing a pair
of said shields on opposite sides of said workpiece to form a pair
of said passages with corresponding forward and aft windows; and
blasting said shot stream alternately through said forward and aft
windows to abrade said opposite sides of said workpiece during
spinning thereof.
4. A method according to claim 3 further comprising spacing said
shields from said workpiece to shield said workpiece opposite sides
from incidence angles of said shot stream greater than about
45.degree..
5. A method according to claim 3 wherein said opposite sides of
said workpiece include machining ridges extending substantially
perpendicular to said spinning axis, and further comprising aiming
said shot stream obliquely to said ridges.
6. A method according to claim 5 wherein said shot stream is aimed
at about 45.degree. toward said ridges.
7. A method according to claim 5 further comprising blasting a pair
of said shot streams toward said rotating workpiece and shields at
different orientation angles around said spinning axis to
alternately enter said passages during spinning of said
workpiece.
8. A method according to claim 7 wherein said workpiece comprises
an airfoil having camber, and said pair of shot streams are aimed
at different orientations to abrade said ridges from opposite edges
of said workpiece in shadow regions caused by said camber.
9. A method according to claim 5 wherein said shot is pliant.
10. A method according to claim 9 wherein said shot comprises
cellular polyurethane sponge with imbedded abrasive particles.
11. An apparatus for polishing a workpiece comprising: a shield
mounted to a turntable for supporting said workpiece next to said
shield to form a passage therebetween; means for spinning said
turntable with said workpiece and shield thereon; and means for
blasting a stream of abrasive shot toward said spinning workpiece
and shield to periodically enter said passage to abrade said
workpiece.
12. An apparatus according to claim 11 wherein: said turntable
includes a fixture to mount said workpiece at a spinning axis
extending through said workpiece and turntable; said shield is
positioned radially outwardly from said spinning axis to form
forward and aft windows at opposite ends of said passage; and said
blasting means are effective for blasting said shot stream
alternately through said forward and aft windows to abrade said
workpiece from opposite sides during spinning thereof.
13. An apparatus according to claim 12 further comprising: a pair
of said shields mounted to said turntable to form a pair of said
passages on opposite sides of said workpiece with corresponding
forward and aft windows; and said blasting means are configured to
blast said shot stream alternately through said forward and aft
windows to abrade said opposite sides of said workpiece during
spinning thereof.
14. An apparatus according to claim 13 further comprising spacing
said shields from said workpiece to shield said workpiece opposite
sides from incidence angles of said shot stream greater than about
45.degree..
15. An apparatus according to claim 14 wherein said shot is
pliant.
16. An apparatus according to claim 15 wherein said shot comprises
cellular polyurethane sponge with imbedded abrasive particles.
17. A method of polishing an airfoil to remove machining ridges
from opposite sides thereof comprising: placing a pair of shields
on opposite sides of said airfoil to form corresponding passages
therebetween; spinning said airfoil and shields; and blasting a
stream of abrasive pliant shot toward said spinning airfoil and
shields to periodically enter said passages to impinge said ridges
for abrasion thereof.
18. A method according to claim 17 further comprising: spinning
said airfoil around a spinning axis extending therethrough; placing
said shields radially outwardly from said spinning axis to form
forward and aft windows at opposite ends of said passages; and
blasting said shot stream alternately through said forward and aft
windows to abrade said ridges from opposite ends of said airfoil
during spinning thereof.
19. A method according to claim 18 further comprising: spacing said
shields from said airfoil to shield said airfoil opposite sides
from incidence angles of said shot stream greater than about
45.degree.; and aiming said shot stream obliquely to said
ridges.
20. A method according to claim 19 wherein said machining ridges
extend substantially perpendicular to said spinning axis, and
further comprising aiming said shot stream at about 45.degree.
toward said ridges.
21. A method according to claim 20 wherein said airfoil has camber,
and further comprising blasting a pair of said shot streams toward
said rotating airfoil and shields at different orientation angles
around said spinning axis to alternately enter said passages during
spinning of said airfoil to abrade said ridges from opposite edges
of said airfoil in shadow regions caused by said camber.
22. A method according to claim 21 wherein said shot comprises
cellular polyurethane sponge with imbedded abrasive particles.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to machining metal,
and, more specifically, to polishing thereof.
[0002] A gas turbine engine includes various compressor and turbine
stator vanes and rotor blades. The vanes and blades have airfoil
profiles specifically configured for use in compressing air or
expanding hot combustion gases.
[0003] Accordingly, the airfoil profiles have three dimensional
contours which vary from root to tip between leading and trailing
edges over the opposite pressure and suction sides thereof. The
airfoil has camber or arcuate curvature between its leading and
trailing edges, with the pressure side typically being generally
concave and the suction side typically being generally convex.
[0004] The airfoils may be made by various manufacturing processes.
For example, some compressor airfoils are machined to size from
metal blanks or forgings. Computer numerically controlled machines
are commercially available for precisely controlling the position
of a ball endmill tool over the complex three dimensional contour
of the airfoil. Machining typically occurs by tracing the tool
around the pressure and suction sides of the airfoil at a specific
radial or span location, and then indexing the tool radially along
the span for machining each radial section in turn from root to
tip.
[0005] Since the ball endmill tool forms a small groove as it
removes material around the circumference of the airfoil, adjoining
grooves along the span of the airfoil have sharp ridges extending
therebetween. After the machining process, these ridges must be
removed to obtain an aerodynamically smooth surface finish without
irregularity.
[0006] The ridges are typically removed by additional hand and
machine polishing processes which increase the cost of manufacture
thereof. Manual hand polishing uses grinders and buffers for
removing a majority of the ridges. However, manual polishing is
insufficient for meeting the required smooth finish requirements
for the airfoil.
[0007] Mechanical polishing follows the hand polishing process in
which a small batch of hand polished airfoils are tumbled in an
abrasive bed in a vibratory finishing machine for achieving the
desired smooth surface finish.
[0008] However, since a rotor blade airfoil typically includes an
enlarged root end with an integral platform, the platform acts as
an anchor in the polishing bed which results in directional
polishing along the span or longitudinal axis of the airfoil.
[0009] These multiple steps in polishing the machined airfoils
increase the time of manufacture and corresponding cost thereof for
achieving the desired smooth airfoil surface devoid of ridges and
grooves.
[0010] Accordingly, it is desired to provide a new process for
polishing workpieces, such as machined airfoils, with improved
effectiveness and decreased cost.
BRIEF SUMMARY OF THE INVENTION
[0011] A shield is placed next to a workpiece to form a passage
therebetween. The workpiece and shield are rotated, and a stream of
abrasive shot is blasted toward the spinning workpiece and shield
to periodically enter the passage to selectively abrade the
workpiece.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The invention, in accordance with preferred and exemplary
embodiments, together with further objects and advantages thereof,
is more particularly described in the following detailed
description taken in conjunction with the accompanying drawings in
which:
[0013] FIG. 1 is a schematic representation of an apparatus and
method for spin polishing a shielded workpiece in accordance with
an exemplary embodiment of the present invention.
[0014] FIG. 2 is a top view of the shielded workpiece illustrated
in FIG. 1 which is rotated about a central spinning axis thereof
for abrasive polishing the opposite sides thereof.
[0015] FIG. 3 is a schematic elevational view of the workpiece and
polishing apparatus illustrated in FIG. 1, with a flowchart
representation of an exemplary method of polishing.
DETAILED DESCRIPTION OF THE INVENTION
[0016] Illustrated schematically in FIG. 1 is an apparatus 10 for
polishing a workpiece 12 in accordance with an exemplary embodiment
of the present invention. The workpiece may have any suitable form,
such as the airfoil portion of a gas turbine engine compressor
rotor blade having a mounting dovetail at one end thereof.
[0017] The airfoil 12 has opposite pressure and suction sides 14,16
which extend radially in span from a root 18 at an inner flowpath
platform to a radially outer tip 20. The pressure and suction side
surfaces extend axially between opposite leading and trailing edges
22,24 which are relative to the direction of airflow when used in a
gas turbine engine compressor.
[0018] The airfoil typically has camber which is arcuate curvature
between the leading and trailing edges which may vary from root to
tip as the airfoil varies in twist angle according to the specific
configuration thereof. The pressure side 14 is typically concave
axially between the leading and trailing edges, with the suction
side 16 typically being convex.
[0019] As indicated above, the airfoil is typically manufactured
using a multi-axis numerically controlled machining tool having a
ball endmill tool (not shown) which is traced along both sides of
the airfoil for achieving the desired aerodynamic profile thereof.
The tool is indexed from root to tip and forms a series of
longitudinally or radially adjoining grooves having sharp ridges
26, shown magnified in part in FIG. 1 for clarity of presentation.
The ridges typically extend axially between the leading and
trailing edges and are spaced apart along the span of the airfoil
corresponding with the stacking axis thereof.
[0020] The apparatus 10 illustrated in FIG. 1 is specifically
configured for practicing a new method of abrading the ridges 26
for polishing the airfoil to an aerodynamically smooth and shiny
surface finish without irregularities therein which would
compromise aerodynamic performance.
[0021] More specifically, an individual airfoil 12 is mounted in a
suitable fixture 28 atop a rotary platter or turntable 30. The
airfoil is preferably mounted with its stacking axis coincident
with a spinning axis 32 which extends through the center of the
turntable. In this way, the airfoil is rigidly clamped to the
turntable for rotation or spinning about its center spinning
axis.
[0022] The turntable is operatively joined to a suitable motor 34
effective for rotating the turntable at any desired rotational
speed.
[0023] A pair of side shields 36 are fixedly mounted to the
turntable on opposite sides of the airfoil for rotation therewith.
Each shield is placed next to the corresponding side of the airfoil
to form a passage or channel 38 therebetween.
[0024] Means including a discharge nozzle 40 are provided for
blasting a stream of abrasive shot 42 toward the airfoil for
abrasive polishing thereof. During the polishing process, the motor
34 is operated to rotate the turntable 30 and in turn rotate and
spin together the airfoil 12 with its protective side shields 36.
As the airfoil and shields spin, the shot stream is aimed or
directed at the workpiece and periodically enters the side passages
38 for abrading selective portions of the opposite sides spinning
within the impact site of the shot stream.
[0025] One of more of the discharge nozzles 40 may be used for
blasting the shot against the workpiece, with the shields
permitting only shallow impingement of the shot against the
opposite airfoil sides as opposed to generally perpendicular
impingement. Since the shot is abrasive, abrasion and polishing of
the airfoil may be controlled by controlling the impingement or
incident angle of the shot against the airfoil surfaces.
[0026] More specifically, and as shown in more detail in FIG. 2,
each of the two shields 36 is placed radially outwardly from the
spinning axis 32 to form forward and aft apertures or windows 44,46
at opposite ends of the respective passages 38. The nozzle 40 is
held substantially stationary in space relative to the rotating
turntable 30 so that as the turntable spins the opposite windows
44,46 each face the discharge end of the nozzle once per revolution
for permitting corresponding periodic abrasion of the airfoil.
[0027] In this way, the shot stream is blasted periodically through
the forward windows 44 permitting access to the opposite sides of
the airfoil at the leading edge, and then through the aft windows
46 permitting access of the shot stream to the opposite sides of
the airfoil at the trailing edge.
[0028] By placing the pair of shields 36 on respective opposite
sides of the airfoil, a pair of the access passages 38 with
corresponding forward and aft windows 44,46 are created.
Accordingly, as the workpiece spins during the polishing process,
the shot stream is blasted alternately through the forward and aft
windows to abrade and polish both sides of the airfoil from
opposite ends thereof during each revolution. In each revolution,
therefore, the airfoil 12 may be polished along both sides thereof
as the shot stream flows past the spinning leading and trailing
edges 22,24.
[0029] Since the airfoil 12 is a slender or thin member having a
chord length many times greater than the maximum thickness thereof,
the two side shields 36 may be used to advantage to both
preferentially protect the opposite sides of the airfoil while
permitting selective polishing thereof at shallow impingement
angles controlled by the opposite forward and aft access windows
44,46.
[0030] In an alternate embodiment, either side of the airfoil may
be protected from polishing by a suitable cover thereover such as
mounting one of the side shields directly against the airfoil side,
with polishing performed on only one side thereof using a
corresponding side shield space therefrom. One side polishing in
this manner may be desirable for other workpiece configurations,
whereas the airfoil configuration is preferably polished along both
sides using the pair of side shields for this purpose.
[0031] Although various forms of abrasive shot may be used, in the
preferred embodiment illustrated in the figures the shot 42 is
resilient or pliant for controlled selective abrasion. As shown in
FIG. 3, the shot is preferably a cellular polyurethane sponge with
imbedded abrasive particles 42b. This form of pliant shot has
substantial advantages in selectively abrading the intended ridges
from the airfoil surfaces while protecting from excessive abrasion
the resulting smooth surfaces formed during the polishing
process.
[0032] More specifically, the present invention is one of a series
of inventions specifically configured for using the abrasive
characteristics of the pliant shot. In U.S. patent application Ser.
No. 09/358643 a process for Sustained Surface Scrubbing is
disclosed in which pliant shot is blasted over a workpiece surface
for selectively abrading protrusions thereon with little, if any,
abrasion of the adjoining flat surfaces.
[0033] In that process, shallow incidence angles of impingement
including 30.degree. and 45.degree. and up to about 60.degree. to
the flat workpiece surface results in scrubbing of the pliant shot
along the flat surface with little significant abrasion thereof,
but with substantial abrasion of protrusions such as burrs or sharp
edges extending normally to the flat surface. Impingement angles
greater than about 60.degree., including perpendicular impingement,
are undesirable for their excessive abrasion effect as well as the
possibility of imbedding abrasive particles into the metallic
surface being treated.
[0034] As illustrated in FIG. 3, the preferred form of the pliant
shot 42 is a light-weight, resilient material such as a sponge,
rubber, felt, plastic, foam, or other resilient material. The shot
may have open or closed cells. The shot preferably includes the
abrasive particles 42b imbedded therein, although in alternative
embodiments abrasive may be omitted. Suitable abrasives include
particles of various minerals, metal oxides, plastics, and black
walnut shell, for example.
[0035] One type of suitable pliant shot is commercially available
from Sponge-Jet Inc. of Eliot, Me. under the tradename of Sponge
Media. This sponge media includes a polyurethane open-cell carrier
in which is impregnated different types of abrasive material for
different abrasive performance. And, one form of the sponge media
is without abrasive.
[0036] Equipment for discharging the pliant shot is also
commercially available from Sponge Jet Inc., but is modified and
operated differently for the purposes of the present invention. In
conventional practice, the sponge media is blasted generally
perpendicularly against a surface of a workpiece for removing
coatings thereof while profiling the underlying surface for
improving adherence to replacement coatings.
[0037] However, the pliant shot disclosed above is used for
selectively abrading the ridges 26 to polish the airfoil sides for
meeting the precise dimensional sectional profiles thereof.
[0038] The blasting apparatus illustrated in FIG. 1 is conventional
in structure and includes a hopper 48 in which the pliant shot 42
is stored. The hopper is joined in flow communication with a
respective delivery conduit or hose 50 extending to each of a pair
of the nozzles 40.
[0039] An air compressor or pump 52 is operatively joined to the
hopper and hoses by a suitable mixing device for providing air as a
carrier fluid 54 under suitable pressure for carrying and
discharging the shot in a stream through the respective nozzles 40.
The nozzles may have any suitable configuration for discharging the
shot in a suitably wide stream for decreasing overall processing
time.
[0040] As initially illustrated in FIG. 2, the two side shields 36
are positioned generally parallel to the opposite sides of the
airfoil and extend from leading to trailing edges thereof to
provide the forward and aft windows 44,46 general coplanar with the
corresponding leading and trailing edges 22,24 of the airfoil. In
this way, as the workpiece and shields rotate together on the
turntable, the side shields prevent the shot stream from impinging
the sides of the airfoil at large incidence angles approaching
perpendicular, while permitting impingement at the desired shallow
angles of incidence.
[0041] The width of the side shields illustrated in FIG. 2 along
with their spacing from the opposite sides of the airfoil control
the total angular range A of impingement or incidence angles at
which the shot impinges the sides of the airfoil from either the
leading or trailing edges thereof. The total range A of incidence
angles is a cone corresponding with the angular extent that the
forward or aft windows face the corresponding nozzle during
spinning of the airfoil through which windows the pliant shot may
impinge the airfoil surfaces. The individual shields therefore act
as shutters which rotate into position to temporarily block the
shot stream from impinging the airfoil over a majority of the
360.degree. motion of the airfoil in each revolution.
[0042] As the airfoil spins during the polishing process, the
instantaneous angle of incidence B correspondingly varies from
about 0.degree. when either the leading and trailing edges are
aligned with the shot stream to a maximum angle of incidence at the
outer extremes of the two windows just as the shields enter the
shot stream and interrupt the flow thereof to the corresponding
sides of the airfoil.
[0043] The preferred shallow angle of incidence B is illustrated in
more detail in FIG. 3 and is preferably less than or equal to about
45.degree.. Correspondingly, the two side shields 36 extend in
width and spacing from the opposite sides of the airfoil to shield
those sides from the shot stream at incidence angles greater than
about 45.degree.. In this way, the stream of shot 42 impinges the
airfoil sides at shallow angles in the carrier air 54. As the shot
impinges the protruding ridges, substantial abrasion thereof occurs
until the ridges are removed.
[0044] Without the ridges, the airfoil sides become smooth and
further impingement by the pliant shot merely scrubs along the
surface without appreciable further abrasion thereof. The abrasion
or polishing effect is thereby self-limiting and substantially ends
upon removal of the ridges notwithstanding the continuing stream of
shot. The shot is resilient and compresses upon impingement for
protecting flat surfaces yet is effectively abrasive upon impinging
protruding surfaces such as the ridges.
[0045] In the exemplary embodiment illustrated in FIG. 3 the ridges
26 extend substantially perpendicular or normal to the longitudinal
spinning axis 32. In order to maximize the abrasion thereof, the
shot stream 42 is preferably aimed obliquely to the ridges 26 in a
cross grain orientation. In FIG. 3, the grain of the ridges is
horizontal and the nozzle 40 is preferably inclined at an oblique
inclination angle C for directing the shot obliquely across the
ridge grain.
[0046] Since the airfoil 12 spins during the polishing process, the
pliant shot 42 from the corresponding nozzle 40 impinges the ridges
obliquely thereto at both leading and trailing edges of the airfoil
in each revolution.
[0047] Note the distinction between the surface incidence angle B
in the radial section illustrated in FIG. 2 and the ridge incidence
angle C in the span plane illustrated in FIG. 3. The shot reception
cone angle A in FIG. 2 is limited; with the spinning side shields
36 periodically interrupting the shot stream from engaging the
workpiece.
[0048] The ridge incidence angle C in FIG. 3 permits cross grain
abrasion of the ridges and is held constant as the airfoil spins
relative to the nozzle. And, in each revolution of the airfoil, the
ridges are abraded in opposite cross grain directions on each side
of the airfoil as the leading edge and trailing edge of the airfoil
are sequentially rotated within the impingement zone of the
corresponding nozzle.
[0049] In the preferred embodiment illustrated in FIGS. 1 and 2,
two of the nozzles 40 are used for blasting a pair of the shot
streams 42 toward the rotating airfoil and protective shields at
different orientation angles around the spinning axis. As shown in
FIG. 2, the two nozzles 40 are oriented obliquely from each other,
but preferably not diametrically opposite to each other, so that
the common airfoil 12 may be polished by multiple shot streams in
each revolution of the airfoil.
[0050] However, only one shot stream is permitted to enter the
windows at any one time during rotation of the airfoil in this
preferred configuration to prevent collision of the two streams.
The shot streams thusly alternately enter the passages 38 during
spinning of the airfoil and increase the total rate of ridge
abrasion.
[0051] As shown in FIG. 3, the two nozzles 40 may both be oriented
with the common ridge incidence angle C for maximizing cross grain
abrasion of the ridges.
[0052] Irrespective of the circumferential location of the
individual nozzles 40 around the spinning axis 30, each nozzle
still enjoys the benefit of the limited surface incidence angle A
for effecting the selective surface scrubbing or abrasion of
protrusions, such as the ridges, without significant abrasion of
adjoining flat or smooth surfaces as they are formed.
[0053] A particular advantage of the two nozzles 40 illustrated in
FIG. 2 is the ability to improve the polishing process of an
arcuate workpiece such as the airfoil. The airfoil has camber or
curvature between its leading and trailing edges which may be
substantial in defining the concave pressure side and convex
suction side. Since the suction side is convex, the incident shot
stream illustrated in FIG. 2 may cause a shadow region on the
downstream sides of the convex surface in which little if any
abrasion of the ridges occurs.
[0054] However, by correspondingly aiming the two nozzles 40 at
different orientations for the airfoil suction side, the
corresponding shot streams may more effectively abrade the ridges
from the opposite edges of the airfoil in the respective shadow
regions due to the opposite nozzle and the camber of the airfoil.
Accordingly the leading edge portion of the airfoil and trailing
edge portion of the airfoil are best polished from the
corresponding shot streams as they enter the respective forward or
aft windows 44,46.
[0055] The individual nozzles 40 illustrated in FIG. 2 may be
suitably sized to discharge the stream of pliant shot 42 with a
sufficient width fitting within the maximum width of the individual
forward and aft windows 44,46. In this way, the ridges may be
abraded from the leading edge 22 to the midchord region in the
airfoil forward portion when the shot enters the forward windows,
and the aft portion of the airfoil is abraded from the trailing
edge to the midchord when the shot enters the aft windows.
[0056] As shown in FIG. 3, the shot is also directed preferably
downwardly at the cross grain inclination angle C with the abrasive
effect of the shot extending over a finite portion of the radial
span of the airfoil. In this way, a radial band of the airfoil may
be effectively polished for each radial position of the
corresponding nozzles 40.
[0057] As shown in FIG. 1, the nozzles 40 are preferably mounted to
suitable translation carriages 56 mounted next to the turntable 30
for automatically moving the nozzles along the span length of the
airfoil so that all radial sections of the airfoil from root to tip
may be automatically polished. In the preferred embodiment, the
airfoil spins on the turntable 30 as the two nozzles 40 slowly move
radially back and forth between the root and tip of the airfoil for
removing all of the ridges from root to tip of the airfoil and
leaving a smooth and polished surface.
[0058] In a single operation, therefore, an individual airfoil may
be polished on both sides for removing the ridges therefrom. The
polishing operation is effectively self-terminating since once the
ridges are removed, the remaining surface is smooth and subject to
little, if any more, additional abrasion from the pliant shot
impinging the smooth surface at shallow incidence angles. The
resulting airfoil profile may be precisely controlled in dimensions
for maximizing aerodynamic performance.
[0059] While there have been described herein what are considered
to be preferred and exemplary embodiments of the present invention,
other modifications of the invention shall be apparent to those
skilled in the art from the teachings herein, and it is, therefore,
desired to be secured in the appended claims all such modifications
as fall within the true spirit and scope of the invention.
* * * * *