Note: Descriptions are shown in the official language in which they were submitted.
CA 02366325 2001-12-27
TITLE: METHOD OF FORMING TURBINE BLADE ROOT
FIELD OF THE INVENTION
The present invention relates to a process for making
turbine blades for use in power generation, aerospace,
the pumping industries and the like. In particular it
relates to a process for forming the root of the turbine
blade.
BACKGROUND OF THE INVENTION
The turbine blade, for example, in the jet engine, has a
very simple purpose, to re-direct and compress air. The
turbine blade is attached to a ring. along with a varying
number of identical turbine blades. When the turbine
blades are all installed to the ring, they form a perfect
circle. Several stages of rings are mounted, one behind
the other, each with fewer and fewer blades. As the rings
are rotated, each blade moves air using the principal of
an airfoil and/or bucket. As the air is passed on to the
next stage, the area, or volume decreases, thereby
compressing the air into a smaller and smaller area. Once
the air finally reaches the required volume, it is mixed
with~fuel and ignited causing a massive controlled
explosion, which is then directed in some cases into w
another ring of turbine blades.
The turbine blade has a blade or foil and a root. The
root allows the. blade to be affixed to the rotating disk
or ring. The root is a critical feature, as the root must
be a near perfect fit in order to prevent separation from
the disk.
The current methods of manufacturing a root are generally
broken down into three categories: grinding, broaching or
machining with a form tool. Using one of these methods a
root can be produced relatively quickly at an acceptable
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price. The turbine blade and root are usually made from
stainless steel ranging in many different grades. Blades,
while less common can also be made .from super alloys and
ceramics with high nickel content. It is for this reason
that machining.or removing material to form the root is
extremely difficult.
When forming a root by grinding, a blank is mounted into
a CNC (computer numerical controlled) grinder. The root
shape is ground in with a grinding wheel having the
opposing shape of the root to be formed.
The broaching process is also quite a simple concept.
Again, a blank is mounted into a broaching machine, where
a long flat cutting tool moves perpendicular to the part.
On the flat bar, cutting teeth remove the material, as
the bar moves from top to bottom.
Finally, machining with a form tool, also simple to
explain is one of the most common practices. A form tool,
described as it sounds, is a cutting tool, which has the
desired shape of the root ground in. This cutting tool is
used in various styles of machining centers, and
basically is spun at the required RPM (revolutions per
minute) and moved perpendicular into the blank to machine
in the root. ' . . ' . .. .,. ._ .
Manufacturing the root with a broach or a form tool w
raises two immediate problems. First, in both instances,
if the broach or the form tool breaks in the middle of a
production run, it is nearly impossible to reproduce or
repair the broaching tool or form tool to the identical
shape. This will cause mis-match in a production run.
Second, if a form tool or a broach breaks, and deliveries
are critical, in almost every circumstance unless a spare
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tool is on premises, a new tool can take a long period of
time to acquire.
When supplying blades to the service industry, many times
with only a used broken part as the pattern to
manufacture new pieces, reversing engineering a component
is many times the only practical method. The root will be
mounted into a ring with has lost a varying amount of
material, and the new blade has to fit onto that ring.
The practice is common to supply the service industry
with a range of sample parts to fit onto the ring, in
order to achieve the best fit. If one were to produce
these samples with a broaching tool or a form tool, the
cost to supply a great number of samples would be high.
This is due in part from the fact that a set of form
tools and or broaching tools would require to be ground
for each sample.
The methods of machining a root with either broaching,
grinding or use of a form tool, all are susceptible to
wear. These three processes DO NOT have any means of
compensating for tool wear. Subsequently, the
manufacturer must rely on frequently changing the tool or
redressing the grinding wheel in order to keep the
process stable. _
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a
process for forming the root of a turbine blade that
overcomes the problems associated with tool breaks in the
middle of a production run.
It is a further object of the present .invention to
provide a process for forming the root of a turbine blade
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that can provide a range of sample parts to fit onto the
ring quickly and cost effectively.
It is a further object of the present invention to
provide a process for forming the root of a turbine blade
that eliminates problems associated with tool wear.
Accordingly the present invention provides a process for
forming the root of a turbine blade using the EDM method.
EDM machining of the root is done in a manner that any
re-cast layer left behind is less than 1 micron. After
EDM machining the turbine blade is subjected to MPI to
check for depth or presence of surface cracks. A liquid
tracer coating is then applied to the root, which allows
the ability of the next process to be verified as
complete. The root is then subjected to glass heeding to
remove any re-cast and insure the surface finish prior to
shot peening is consistent and contains no scratch marks
or machine marks. The root is then subjected to shot
peening to impart to the surface small indentation or
dimples and produce a compressed surface which resists
further surface cracks.
Further features of the invention will be described or
will become apparent in the course of the followingv _
detailed description. r ' - .,
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The electrical discharge machining (EDM) method,
according to the present invention, of forming the
turbine blade root is very much different from
conventional methods based on the fact that all other
methods of removing the material effectively rip, tear,
or score the material. EDM, as mentioned earlier breaks
the material down by conducting a controlled amount of
r
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voltage and current through an electrode in the form of;
in the case of wire EDM a thin wire. Rapidly occurring
electrical discharges from the electrode break down the
bonding material in the piece being machined. However the
EDM method does leave behind on the surface a very small
amount of "slag". Vvlhhile manufacture of the blade using
EDM is not uncommon, processes proposed to EDM the root
have never been commercially acceptable. EDM removal of
metal, particularly at higher removal speeds, leaves a
re-cast layer on the surface of the. piece being machined.
The re-cast layer is susceptible to cracking and any
residual re-cast left on the piece being machined can
transfer structural cracks thereby causing component
failure.
The speed of removing the material has always been the
main driver for EDM manufacturing. The EDM process, of
the present invention, to form a root of a turbine blade
accomplishes the following objectives:
The root of all the turbine blades manufactured fit
uniformly within the mating ring. As noted previously,
manufacturing the root with a broach or a form tool
raises problems, if the broach or the form tool breaks in
the middle of a production run. Using these methods it is
nearly impossible to reproduce or repair the broaching
tool or form tool to the identical shape. This will cause --
mis-match in a production run. Further, if a form tool or
a broach breaks, and deliveries are critical, in almost
every circumstance unless a spare tool is on premises, a
new tool can take a long period of time to acquire. By
using the EDM process of the present invention, tool
breaks do not create a problem. Replacement electrodes
will produce identical pieces and are readily available.
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Blades supplied to the service (repair) industry are not
generally made to a single specification. Also, when
supplying blades to the service industry, many times only
mused broken part is provided and reversing engineering
a component is the only practical method of manufacture.
The root will be mounted into a ring with has lost a
varying amount of material, and the new blade has to fit
onto that ring. The practice is to supply the service
industry with a range of sample parts to fit onto a ring,
in order to achieve the best fit. If one were to produce
these samples with a broaching tool or a form tool, the
cost would be higher to supply a large number of samples.
This is due in part from the fact that a set of form
tools and or broaching tools would require to be ground.
The EDM process of the present invention eliminates the
need for a large number of form tools or broaching tools.
The same EDM machine can be programmed to produce
different roots to accommodate ring wear.
When machining a root with broaching, grinding or use of
a form tool, the tools all are susceptible to wear. These
three processes do not have any means of compensating for
tool wear. Subsequently, the manufacturer must rely on
frequently changing the tool or redressing the grinding
wheel in order to keep the process stable. The.EDM'v
process of the present invention to form the root of the _ . ._-:._ _ . _. ._
turbine blade is not susceptible to tool wear. The wire:
being used in the process is continuously wound through
and disposed.
In known processes of making turbine blade roots after
machining, the.root is subjected to: MPI (magnetic
particle inspection) and shot peeving. The turbine blade
is subjected to MPI to check for depth or presence of .
surface cracks. By shot peeving, the material is
effectively bombarded with small media called shot. Each
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piece of shot striking the material acts as a tiny ball
peen hammer, imparting to the surface small indentation
or dimples. The effect of this process ultimately
produces a compressed surface which resists further
surface cracks.
The process of the present invention eliminates any re-
cast from the EDM process prior to MPI and shot peeving,
In the present invention, EDM machining of the root from
a stainless steel workpiece is done in a manner that any
re-cast layer left behind is less than 1 micron, After
EDM machining, the turbine blade is subjected to MPI to
check for depth or presence of surface cracks. A liquid
tracer coating is then applied to the root, which allows
the ability of the next process to be verified as
complete. The root is then subjected to glass heeding to
remove any re-cast. By glass heeding the surface prior to
shot peeving, the surface finish prior to shot peeving is
consistent and contains no scratch marks or machine
marks, which are always evident after conventional
machining. The root is the subjected to shot peeving to
impart to the surface small indentation or dimples. The
effect of this step ultimately produces a compressed
surface which resists further surface cracks.
The process of the present invention is applicable to
forming a turbine blade root in a completely vertical cut
(straight cut) or in a similar fashion using multiple
cuts at varying angles in order form radial surfaces.
Although various preferred embodiments of the present
invention have been described herein in detail, it will
be appreciated by those skilled in the art, that
variations may be made thereto without departing from the
spirit of the invention or the scope of the appended
claims.
