U.S. patent application number 11/669647 was filed with the patent office on 2008-07-31 for laser net shape manufacturing and repair using a medial axis toolpath deposition method.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Magdi Naim Azer, Prashant Madhukar Kulkarni, Huan Qi, Prabhjot Singh.
Application Number | 20080182017 11/669647 |
Document ID | / |
Family ID | 39668303 |
Filed Date | 2008-07-31 |
United States Patent
Application |
20080182017 |
Kind Code |
A1 |
Singh; Prabhjot ; et
al. |
July 31, 2008 |
LASER NET SHAPE MANUFACTURING AND REPAIR USING A MEDIAL AXIS
TOOLPATH DEPOSITION METHOD
Abstract
A method is disclosed for laser net shape manufacturing a part
or repairing an area of a part comprising providing a CAD model of
the part to be manufactured or repaired, digitally slicing the CAD
model into a plurality of deposition layers, determining a medial
axis for each of the deposition layers of the part or repair area;
and depositing a bead of a material in a pattern centered upon the
medial axis of each of the deposition layers so as to build-up the
material by each of the deposition layer until the part is
manufactured or repaired.
Inventors: |
Singh; Prabhjot;
(Guilderland, NY) ; Qi; Huan; (Niskayuna, NY)
; Azer; Magdi Naim; (Niskayuna, NY) ; Kulkarni;
Prashant Madhukar; (Guilderland, NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY;GLOBAL RESEARCH
PATENT DOCKET RM. BLDG. K1-4A59
NISKAYUNA
NY
12309
US
|
Assignee: |
GENERAL ELECTRIC COMPANY
Schenectady
NY
|
Family ID: |
39668303 |
Appl. No.: |
11/669647 |
Filed: |
January 31, 2007 |
Current U.S.
Class: |
427/142 |
Current CPC
Class: |
B22F 5/04 20130101; F01D
5/34 20130101; F05D 2230/80 20130101; B23P 6/007 20130101; B23P
15/02 20130101; B22F 10/20 20210101; F04D 29/023 20130101; F05D
2230/31 20130101; B23K 35/0238 20130101; B23K 35/0244 20130101;
C23C 26/02 20130101; F01D 5/005 20130101; Y02P 10/25 20151101; B23K
26/34 20130101; F04D 29/324 20130101 |
Class at
Publication: |
427/142 |
International
Class: |
C23C 26/00 20060101
C23C026/00 |
Claims
1. A method for laser net shape manufacturing a part, comprising:
providing a CAD model of the part; digitally slicing the CAD model
into a plurality of deposition layers; determining a medial axis
for each of the deposition layers; and depositing a bead of a
material in a pattern centered upon the medial axis of each of the
deposition layers so as to build-up the material by each of the
deposition layer until the part is manufactured.
2. The method of claim 1, wherein the pattern comprises a zigzag
pattern or a zig pattern about the medial axis.
3. The method of claim 1, wherein the pattern comprises a single
bead centered upon the medial axis.
4. The method of claim 1, wherein the bead of material is a mixture
of different materials.
5. The method of claim 1, wherein the bead of material is formed of
a single material.
6. The method of claim 1, wherein the bead of material is formed of
different materials.
7. The method of claim 1, wherein a first layer of the deposited
material is deposited upon a substrate.
8. The method of claim 1, wherein the deposited material is
selected from a group comprising metals, metal alloys, ceramics,
and plastics.
9. The part formed by the method of claim 1.
10. The method of claim 1, wherein the part is a BLISK, turbine
blade, compressor blade or turbine component.
11. A method for laser net shape repairing a part comprising:
providing a part with an area to be repaired; shaping the area by
removing the area to be repaired to provide a smooth and continuous
surface; comparing the removed area to be repaired to a baseline
shape to determine a material deposition area; digitally slicing
the determined material deposition area into a plurality of
deposition layers; determining a medial axis for each of the
deposition layers; and depositing a material in a pattern centered
upon the medial axis of each of the deposition layers so as to
build-up the material by each of the deposition layers until the
part is repaired.
12. The method of claim 11, wherein the pattern comprises a zigzag
pattern about the medial axis.
13. The method of claim 11, wherein the pattern comprises a zig
pattern about the medial axis.
14. The method of claim 11, wherein the pattern comprises a single
bead centered upon the medial axis.
15. The method of claim 11, wherein the deposited material is
selected from a group comprising metals, metal alloys, ceramics,
and plastics.
16. The method of claim 11, wherein the deposited material
comprises more than one material.
17. The method of claim 11, wherein the part is a BLISK, turbine
blade, compressor blade or turbine component.
18. The method of claim 11, wherein a first layer of the deposited
material is deposited upon a BLISK compressor disk.
19. A BLISK, turbine blade, compressor blade or turbine component
fabricated by the method of claim 1.
20. A BLISK, turbine blade, compressor blade or turbine component
repaired by the method of claim 11.
Description
FIELD OF THE INVENTION
[0001] This invention relates to laser cladding method for
fabricating or repairing a part, such as a BLISK, compressor blade
or compressor component.
BACKGROUND OF THE INVENTION
[0002] An aircraft gas turbine engine or jet engine draws in and
compresses air with an axial flow compressor, mixes the compressed
air with fuel, burns the mixture and expels the combustion gases
through an axial flow turbine that powers the compressor. The
compressor includes a disk with blades projecting from its
periphery. The disk turns rapidly on a shaft, and the curved blades
draw in and compress air.
[0003] In current manufacturing practice, the compressor is made by
forging the compressor disk as a single piece with slots at the
periphery. The compressor blades are individually forged or cast to
shape with a root section termed a dovetail that fits into slots
formed in the disk. Assembly is completed by sliding the dovetail
sections of the blades into the slots in the disk. If a blade does
not fit properly, fails, or is damaged during service, it may be
readily replaced by reversing the assembly procedure to remove the
blade, and providing a new blade.
[0004] Blades may also be formed integrally with the disk, in a
combination termed a BLISK. BLISKS may also be referred to as
integrally bladed rotors. The BLISK approach to manufacturing
offers the potential for increased performance through reduced
weight. Such an article can be forged or cast as a large disk with
an excess of metal at the periphery. The blades are then machined
from the excess metal, integrally attached to the disk. The final
product is expensive to produce, as it requires extensive
high-precision machining operations. An error in machining even one
of the blades may result in rejection and scrapping of the entire
BLISK or it may require an expensive and time consuming repair to
render the part useable.
[0005] In the past, a common repair method has included the
mechanical removal of the damaged material and essentially leaving
the BLISK as-is. While this repair method is suitable for minor
airfoil edge damage, it is not acceptable for more significantly
damaged areas due to imbalance problems and potential mechanical
property degradation.
[0006] Replacement or repair of a damaged blade portion of the
BLISK presents a difficult problem with this cast and machine
approach. If all or a portion of a blade breaks off from impact of
a foreign body during operation, for example, the BLISK becomes
unbalanced. Damaged BLISKS are often repaired by welding excess
metal into the damaged area and machining the metal to form the
appropriate shape, or by cutting out the damaged area and replacing
the cut out material by diffusion bonding a new piece of material
into the damaged area. However, such an approach is both expensive
and results in reduced performance.
[0007] A different approach to manufacture and repair BLISKS has
been disclosed in U.S. Pat. No. 5,038,014, incorporated herein by
reference. This approach utilizes a laser cladding or welding
technique that melts powders in a feed and deposits the molten
material onto a surface. Melted powder layers are built upon one
another to form new parts and to repair damaged parts.
[0008] Past laser cladding techniques have resulted in
imperfections and inclusions in the formed or repaired part
resulting from incomplete fusion of the melted layers to the
underlying substrate or previously welded material. These
imperfections and inclusions are often associated with complex
geometry of the formed or repaired part. Therefore, a need exists
to provide a layered fabrication technique that solves the problems
associated with the past manufacture and repair techniques.
[0009] Laser Net Shape Manufacturing (LNSM) provides an economical
and highly flexible method to fabricate and restore BLISKs, turbine
blades and turbine components. The LNSM technique is based on laser
cladding, wherein a laser is used to create a 3D geometry by
precisely cladding thin layers of metal powder on a base
material.
[0010] LSNM has been used in the fabrication of new parts and the
repair of damaged parts. A Computer Aided Design (CAD) model of a
part to be fabricated is uniformly sliced along the desired
direction of material buildup. Powder is applied and fused along a
toolpath to create a bead of the deposited material. Deposited
material beads are formed adjacent to and overlapping one another
to form a layer of the deposited material, then, a plurality of
layers are built upon one another until the part is fabricated or
repaired. Alternatively, a layer of material can be formed
utilizing a single bead of material, then, a plurality of layers
are built upon one another until the part is fabricated or
repaired.
[0011] Various toolpaths have been used in depositing beads of the
material, depending on whether the material is forming an internal
area or a surface area of the part. However, prior LSNM methods
have resulted in inclusions or fusion imperfections in newly
fabricated or repaired part, requiring that the part either be
scrapped or further processed to repair the imperfections. In
addition, past laser deposition methods for repair have not focused
on producing accurate shapes and geometries.
[0012] Therefore, a need exists to develop a LNSM method that
reduces fusion imperfections to allow a part, such as a turbine
blade, bladed disk, BLISK or compressor airfoil to be manufactured
or repaired.
SUMMARY OF THE INVENTION
[0013] In accordance with a first embodiment of the invention, a
method for laser net shape manufacturing a part is disclosed, which
comprises providing a CAD model of the part, digitally slicing the
CAD model into a plurality of deposition layers, determining a
spine (hereinafter referred to as the medial axis) for each of the
deposition layers, and depositing a bead of a material in a pattern
centered upon the medial axis of each of the deposition layers so
as to build-up the material in layers until the part is
manufactured.
[0014] In accordance with a second embodiment of the invention, a
method for laser net shape repair of a part is disclosed comprising
the steps of providing a part with a damaged area; shaping the
damaged area to provide a substrate deposition surface that may be
smooth and continuous; comparing the damaged part to a non-damaged
part to determine a material deposition area; digitally slicing the
determined material deposition area into deposition layers;
determining a medial axis for each layer; and depositing material
in a pattern centered upon the medial axis of each layer so as to
build-up the material layer by layer until the part is
repaired.
[0015] In accordance with a third embodiment of the invention, a
method for laser net shape repairing a part is disclosed comprising
providing a part with an area to be repaired, shaping the area by
removing the area to be repaired to provide a smooth and continuous
surface, comparing the removed area to be repaired to a baseline
shape to determine a material deposition area, digitally slicing
the determined material deposition area into a plurality of
deposition layers, determining a medial axis for each of the
deposition layers, and depositing a material in a pattern centered
upon the medial axis of each of the deposition layers so as to
build-up the material by each of the deposition layers until the
part is repaired.
[0016] The disclosed invention presents many advantages over the
prior art method of layered deposition. By depositing a bead width
along the medial axis tool path and its offsets, the number of lack
of fusion imperfections in the solid deposit can be reduced or
eliminated.
[0017] Other features and advantages of the present invention will
be apparent from the following more detailed description of the
preferred embodiment, taken in conjunction with the accompanying
drawings which illustrate, by way of example, the principles of the
invention. The scope of the invention is not, however, limited to
this preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is an illustration of a general LNSM deposition
system.
[0019] FIG. 2 is an illustration of a BLISK.
[0020] FIG. 3 is a block diagram of an embodiment of the medial
axis toolpath method.
[0021] FIG. 4 is a sectional view of an exemplary BLISK blade
showing exemplary fabrication slices.
[0022] FIG. 5 is a cross-section layer showing an exemplary medial
axis toolpath within a slice.
[0023] FIG. 6 is an illustration of an exemplary zigzag pattern
within a fabrication slice.
[0024] FIG. 7 is an illustration of an exemplary zig pattern within
a fabrication slice.
[0025] FIG. 8 is an illustration of an exemplary damaged BLISK
blade.
DETAILED DESCRIPTION OF THE INVENTION
[0026] In Laser Net Shape Manufacturing (LNSM), the dimensions and
overall geometry of the repair are rendered in accordance with a
computer-aided design (CAD) description. The geometry of the repair
must be described mathematically. Modeling of the shapes is
achieved through the use of a CAD system, and from these
representations, tool paths are generated to drive the LNSM
process.
[0027] A CAD model of a part to be fabricated or repaired is
generated by numerically characterizing the shape of the article
such as a blade from drawings or a part prepared by more
conventional methods such as machining. Once the shape of the part
is numerically characterized, the computer generates a series of
uniform slices along the desired direction of material buildup, and
the computer determines a medial axis or spine (hereinafter
referred to as a medial axis) for each slice. Based on a
predetermined width of a bead of deposited material, it may be
necessary to apply more than one pass of material in a slice to
form a desired part width. In that case, offset toolpaths are
determined based on the bead with, to complete the slice.
Alternatively, a series of non-uniform slices along the desired
direction of the material build-up could be made, with the computer
determining a medial axis or spine for each slice.
[0028] The medial axis and its offsets can be connected leading to
two distinct cases. In the first case, the offsets may be connected
to the medial axis in such a way that the toolpath becomes a
continuous zigzag toolpath. Alternately, the offsets may be
discontinuous from the medial axis, in which case the toolpaths
consist of a series of beads which include a medial axis bead and
offsets to that bead. This is referred to as a medial discontinuous
zig toolpath. The zigzag pattern is preferred as it is continuous
and may lead to a more stable deposition. The zig pattern is most
often used in fabricating narrow layers. The deposition may include
depositing material only upon the medial axis, for example in a
narrow tip section of a part. Adjacent beads of deposited material
overlap to a determined extent to form a uniform layer. Adjacent
beads may overlap between about 10% to about 90%. Medial axis
toolpath deposition reduces the number of changes in the direction
of laser motion during material deposition, thereby stabilizing
material deposition and improving fabrication quality.
[0029] The movement of the deposition head, or equivalently, the
part, is then programmed using available numerical control computer
programs to create a pattern of instructions, to deposit material
along the determined medial axis path within a uniform or adaptive
thickness slice. Multiple offset medial slices may be deposited
within a slice as determined by the bead width of the deposited
material. The actual laser deposition parameters that are used to
deposit material will have been determined through prior
experimentation. The developed instructions maintain a desired bead
width and overlap of a deposited material in accordance with the
invention. The resulting article reproduces the shape of the
numerical characterization very accurately, to net shape or
near-net shape specifications, including complex curves of a BLISK,
turbine blade, compressor blade or turbine component.
[0030] Process parameters such as laser power and toolpath speed
are varied along the length of the toolpath depending on the
thickness of the slice cross-section. Stable build-up is achieved
using medial axis toolpaths because of the few changes in direction
of deposition within the slice cross-section and the variation of
laser process parameters along the medial axis. The bead width can
be varied by increasing the laser power and reducing the deposition
speed. Typically, laser power is increased where the layer cross
section is thicker, for example at the center of a blade, and the
laser power is reduced when approaching the thinner sections of the
layer, for example at leading and trailing edges of a blade.
[0031] For the repair of articles, including BLISKS, turbine blades
and turbine components, material is deposited to repair the damaged
area or section. If a compressor blade breaks near the midpoint, it
may be necessary to grind a smooth and continuous surface onto the
blade corresponding to the closest remaining undamaged area or
section, and then to deposit material upon the surface until the
part is repaired. If an area or section of a part is damaged, it
may be necessary to remove material to form a notch upon which
material may be deposited. The repaired blade is virtually
indistinguishable from the original fabricated blade, as it is
accomplished by the same apparatus and with the same
shape-controlling pattern.
[0032] Often, damage to a part is in the form of uneven and
irregular shaped damage. In order to prepare the part for repair,
the damaged area may be prepared by machining away material in the
area approximate to the damage in order to form a smooth and
continuous surface. Machining away the damage is preferably
conducted automatically in a multi-axis numerically controlled
milling machine that is programmed to form a predetermined smooth
surface or notch proximate to the damaged area. The repair region
is then cleaned, as needed, by aqueous cleaners and/or solvents,
and dried, followed by the computer controlled deposition of
material to form a repaired part. The repaired portion of the part
has no macroscopically detectable bond line after finishing or
discontinuity to the base portion of the part. The damaged part may
be a turbine blade, bladed disk, BLISK or compressor airfoil, but
is not limited to these parts.
[0033] In fabricating or repairing a part by the present approach,
the composition of the powder feed may be maintained constant
throughout the entire part. Alternatively, the composition of the
powder feed may be intentionally varied within any bead or as
between successive beads, to produce controllable composition
variations throughout the article. For example, in a compressor
blade, a strong, tough alloy composition may be used near the base,
and a hard, wear resistant or abrasive alloy near the tip.
[0034] A wide variety of materials may be deposited using the
approach of the invention. For example, metals and metal alloys
including titanium and titanium alloys, nickel and nickel alloys,
cobalt and cobalt alloys, and iron and iron alloys, superalloys
including Ni-based, Co-based, and Fe based superalloys, ceramics,
cermets and plastics may be deposited. The deposited material may
be a single material or a mixture of different materials. Also, the
deposited material may be varied or changed during the deposition
such that the bead of material is formed of different materials or
more than one material. The toolpath file is generated from
commercial computer-aided manufacturing (CAM) software containing
commands that are understandable to the computer numerically
controlled (CNC) operating system. The commands are loaded into
memory and executed. Typical commands are move commands, which tell
the CNC to move to a new point at a given speed, turn on/off the
laser, designate laser power and powder flow. These commands are
all embedded directly within the part program when it is created,
and are triggered at specific points in the program. Some
parameters that control the process must be changed dynamically
during the processing of a part, including but not limited to laser
power, tool velocity, powder feed rate, and overlap ratio. Although
specific embodiments discussed below are directed to BLISKS, the
invention is equally applicable to the LNSM of other parts,
including a variety of turbine parts including but not limited to
turbine blades.
[0035] A Laser Net Shape Manufacturing (LNSM) system is illustrated
in FIG. 1. As shown in FIG. 1, a powder supply (not shown) feeds a
powder nozzle 2 for deposition upon a substrate 3. A laser 4 melts
the powder as it is fed upon the substrate surface and also melts
the substrate surface to create a melt pool 5 in the vicinity where
the laser 4 is directed on the powder and the surface of the
substrate 3. The system 1 and substrate 3 are moved relatively to
form a layer of a solidified deposited material 7 as the melt pool
5 cools.
[0036] The path the laser 4 takes along the substrate 3 is referred
to as a toolpath. The deposited material 7 is referred to as a bead
of material. The width of deposited material 7 along the toolpath
is referred to as a bead width. The formed melt pool 5 cools and
solidifies as the laser 4 moves along the substrate 3. More than
one powder feed may be used to form the deposited material 7, and
in this illustration, a second powder nozzle 8 is shown
contributing to the solidified deposited material 7. The laser 4,
by melting both the powder feed and the surface of the substrate 3,
forms a strongly bonded deposited material 7.
[0037] Upon completion of a first bead of the deposited material 7,
the nozzle 2 and laser 4 are positioned and moved relative to the
substrate 3 so that an adjacent second bead of deposited material 7
may be deposited along side of the first bead, the width of the
second bead overlapping the width of the first bead. The process is
repeated until a layer of the deposited material 7 is formed.
[0038] In accordance with a specific embodiment of the invention, a
BLISK 10 as shown in FIG. 2 was fabricated. The BLISK 10 shown in
FIG. 2 is illustrative, and the invention is not limited to a
specific BLISK design but rather finds application in the
fabrication and repair of a variety of components including
compressor blades and airfoils, turbine blades and BLISKS. The
BLISK 10 is formed of BLISK blades 20 formed upon a BLISK
compressor disk 30. The BLISK 10 was fabricated according to the
process flowchart of FIG. 3 and the detailed inventive process as
provided herein.
[0039] In accordance with the specific embodiment, a BLISK 10 was
fabricated according to the invention. As shown in FIG. 4, a CAD
model 310 was digitally sliced into individual deposition layers
320. A medial axis was determined for each layer. FIG. 5 shows an
exemplary layer 440 with a medial axis 460. Material was deposited
in a pattern centered upon the medial axis 460 of each layer in
accordance with the teaching of the invention. The first layer of
material was deposited upon the BLISK compressor platform surface
330, so as to build-up the material by each layer until the BLISK
10 was manufactured.
[0040] The material was deposited by varying the control deposition
parameters including, but not limited to laser power, laser
scanning velocity, and powder feed rate so as to maintain a
constant bead overlap of approximately 50 percent while varying the
deposited bead width in a given layer. The bead width varied in a
given layer from between about 0.035 inches and about 0.056 inches.
In this specific embodiment, the deposited material was a
nickel-based superalloy Inconel 718, also known as IN718. However,
the material may be selected from any of the known structural
materials in the field of the invention.
[0041] The material deposition pattern was the continuous zigzag
pattern 500 as shown in FIG. 6. In this BLISK fabrication, a
discontinuous zig pattern 600, formed of a medial axis 610 and two
exterior offsets 620 as shown in FIG. 7, was not used, but may have
been used, especially in the narrow tip section of the BLISK.
[0042] Process parameters such as laser power and toolpath speed
were varied along the length of the medial axis depending on the
thickness of the slice cross-section. Stable build-up was achieved
using the medial axis toolpaths because of the few changes in
direction of deposition within the slice cross-section and the
variation of laser process parameters along the medial axis. Laser
power was increased toward the center of the medial axis, and
decreased as moving away to the ends of the medial axis and
approaching trailing edges.
[0043] In this manner, a BLISK blade 10 was fabricated that was
substantially free of imperfections, including gap imperfections.
The resulting article reproduced the shape of the CAD model very
accurately, to net shape or near-net shape specifications,
including complex curves of the airfoil.
[0044] In accordance with a second specific embodiment of the
invention, a BLISK was repaired in accordance with the invention. A
damaged BLISK 700, shown as a cut away BLISK blade, was provided as
shown in FIG. 8. The damaged BLISK 700 had an area to be repaired
740 and an area not needing repair 710. The area to be repaired 740
was removed to surface 750 by machining away the area to be
repaired. The machining was conducted automatically in a multi-axis
numerically controlled milling machine that was programmed to form
a predetermined notch approximate to the area to be repaired. The
notch was then cleaned, as needed, by aqueous cleaners and/or
solvents, and dried. The invention, while directed to a BLISK
repair in this embodiment, is not limited to BLISK repair, but may
be applied to repairing and fabricating components including
compressor blades.
[0045] A measurement was made of the remaining undamaged portion
710 and was compared to a CAD model containing a baseline shape of
the area of a desired BLISK shape. From the comparison, a material
deposition volume corresponding to the damaged area was determined
by the CNC, and sliced into deposition layers.
[0046] A medial axis was determined for each layer. Material was
deposited in a pattern centered upon the medial axis of each layer
in accordance with the teaching of the invention, starting with a
first layer upon the surface 750, so as to build-up the material by
each layer until the BLISK 10 was repaired.
[0047] The material was deposited by varying the control deposition
parameters including, but not limited to laser power, laser
velocity, and powder feed rate so as to maintain a constant bead
overlap of approximately 50 percent while varying the deposited
bead width in a given layer. The bead width varied in a given layer
from between about 0.035 inches and about 0.056 inches. In this
specific embodiment, the deposited material was a nickel-based
superalloy Inconel 718, also known as IN718, and the compressor
disk was formed also of IN718.
[0048] The material deposition pattern was the continuous zigzag
pattern 500 as shown in FIG. 6. In this BLISK repair, a
discontinuous zig pattern was not used, but may have been used,
especially in the narrow blade section of the BLISK.
[0049] Process parameters such as laser power and toolpath speed
were varied along the length of the medial axis depending on the
thickness of the slice cross-section. Stable build-up was achieved
using the medial axis toolpaths because of the few changes in
direction of deposition within the slice cross-section and the
variation of laser process parameters along the medial axis. Laser
power was increased in the center of the layer cross-section and
decreased upon approaching the leading and trailing edges.
[0050] In this manner, a damaged BLISK blade 700 was repaired that
was substantially free of imperfections, including gap
imperfections. The resulting repaired article reproduced the shape
of the CAD model very accurately, to net shape or near-net shape
specifications, including complex curves of the airfoil. The
performance of the BLISK was not reduced as a result of the repair
according to the invention.
[0051] The performance of the BLISK is not reduced as a result of a
repair according to the invention. This approach allows the blades
of the BLISK to be repaired multiple times, without loss of the
functionality of the BLISK due to an excessive reduction in its
dimensions in the non-repaired regions to below the minimum
specified values.
[0052] The disclosed invention presents many advantages over the
prior art method of layered deposition. By depositing a bead width
along a medial axis tool path and its offsets, the number of lack
of fusion imperfections in the solid deposit can be reduced or
effectively eliminated.
[0053] While the invention has been described with reference to a
preferred embodiment, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiment disclosed as the best mode contemplated for
carrying out this invention, but that the invention will include
all embodiments falling within the scope of the appended
claims.
* * * * *