FIELD OF THE INVENTION
The present invention relates to the field of steam turbine blades/aero engine
blade machining technology. The utility model establishes a process and a
setting pad fixture to clamp steam turbine blade for performing root and tennon
milling operations on CNC machines. The setting pad fixture and the method
thereof can eliminate fabrication methods to manufacture blade tennon and
displacement caused by blade gravity and machining forces, can guarantee rigid
clamping during root and tennon milling and therefore greatly prolonging
service life of the blade.
BACKGROUND OF THE INVENTION
A steam turbine is a device that extracts thermal energy from pressurized steam
and uses it to do mechanical work on a rotating output shaft.
Running through the center of the steam turbine is a sturdy axle called the
rotor, which is what transfers power from the turbine to an electricity generator
called turbo generator. The blades are the most important part of a turbine.
Their design is crucial in capturing as much energy from the steam as possible
and converting it into rotational energy by spinning the rotor round. All turbines
have a set of rotating blades attached to the rotor and spin it around as steam
hits them. The blades and the rotor are completely enclosed in a very sturdy,
alloy steel casing.

Each set of blades is called a stage and works by either impulse or reaction, and
a typical turbine can have a mixture of impulse and reaction stages, all mounted
on the same rotor axle and all turning the generator at the same time. Often the
impulse stages come first and extract energy from the steam when it's at high
pressure; the reaction stages come later and remove extra energy from the
steam when it's expanded to a bigger volume and lower pressure using longer,
bigger blades. In practice, there are usually a number of stages of blades, each
one helping to extract energy from the steam before it's exhausted.
Presently many of known blade sets operating at different power stations have
completed more than their service life of 25 years, and are on the verge of
obligation. With Renovation and Modernization (R&M) of older in-service
thermal sets, large populations of whom have completed more than 25 years or
are on verge of end of their service life, being considered a more cost effective
way of augmenting power generation capacity than installing new, their
technology dates back to early 1950’s thereby providing admirable scope of
improving the efficiency and increasing the output by R&M of these sets.
But the core technical issue remains in developing different technologies, as
required, to perform successful R&M of these sets.
Typically, an advance technology based steam turbine blade consists of three
parts viz. a shroud 5' forming the top part of the blade B', the airfoil or profile
part 1’at the center of the blade B" so that the steam can smoothly glide over
the airfoil part 1’ and cause the turbine to rotate, a root 5"at the base of the
blade B" to fix the blade with turbine rotor and to restrict the blade movement
when subjected to large centrifugal forces during turbine functioning. (Refer fig.
1).

On CNC machines such as Horizontal Milling Center (HMC), manufacturing of
steam turbine blades having root, profile (airfoil) and shroud is done through
various blade clamping fixtures and pads available as per the design of root and
shroud part of the blade.
However, old technology blades do not have an integral shroud. Instead, these
blades used tennon and mortise form of blade B in which, either, a tennon 3 is
provided over the tip 2 of the blade B through fabrication instead of shroud and
a mortise ring 4 is circumscribed over plurality of individual tennon 3 to form
complete blading stage, or, the ring 4 is fitted over the blade B and the tennon
is then riveted with multiple blows or welded. (Refer figure 2)
Referring to US 20140041223 Al, tennon can be installed at the ends of turbine
blades by applying a solid state bonding procedure such as a friction welding
technique to join the tennon to the tip of the turbine blades. However,
Engineering Failure Analysis, Volume 14, Issue 8, December 2007, Pages
1476-1487, investigates about failure analysis of steam turbine and concludes
that the results of the investigations showed that improper welding of the
shroud/tennon to the blade was one of the principal causes of blade fracture.
Metallographic studies of the cracked materials in the shroud and the tip zone of
the blades, including the tennon highlights about how much the overheating
produced during the welding process contributed to the failure.
This establishes the requirement of a new process for root and tennon milling of
steam turbine blades that can replace fabrication methods to manufacture
blade tennon by advance machining methods and a fixture to rigidly clamp raw
blade blank to completely restrict displacement caused by blade gravity and
machining forces during machining and have provisions to eliminate multiple

blade settings and rework during machining on a CNC machine. Also, tennon
blade have T-slot in root which are different from present generation bladesthat
also have to be taken care whilst establishing the technology for proper
clamping and machining tennon blade (Refer figure 3).
OBJECTS OF THE INVENTION
An object of the invention is to propose a process for root and tennon milling of
steam turbine blades on HMC machines, in which machine root and tennon part
of the steam turbine blade through milling operation is done in single setting on
CNC machine with more accuracy to maintain the performance of the steam
turbine itself.
Another object of the invention is to propose a process for root and tennon
milling of steam turbine blades on HMC machines, which saves time and reduce
operator fatigue by completely eliminating the requirement of setting-up steam
turbine blades with multiple settings on conventional (non-CNC) machines.
Yet another object of the invention is to propose a process for root and tennon
milling of steam turbine blades on HMC machines is to reduce the overall cycle
time in root &tennon milling operation performed on a steam turbine blade.
A further object of establishment of process for root and tennon milling of steam
turbine blades on HMC machines is to set forth an overall process improvement
for minimizing general and process related errors and therefore reduce
reworking.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
These together with other objects and advantages which will become
subsequently apparent reside in the details of construction and operation as
more fully hereinafter described and claimed, reference being had to the
accompanying drawings forming a part hereof, wherein like numerals refer to
like parts throughout, and in which :
Figure 1 shows schematic diagram of an advance class steam turbine blade B'
with integrated shroud 2' root 5' and airfoil 1'.
Figure 2 tennon and mortise form of blade B in which a tennon 3 is fabricated or
riveted over the tip 2 of the blade B instead of shroud 2'and a mortise ring 4 is
circumscribed over plurality of individual tennon 3 to form complete blading
stage.
Figure 3 shows a prior art blade with T-root and integrated tennon through
machining.
Figure 4 shows Blade root thrust pad: on which blade internal profile can rest &
also mount in fixture from tenon end.

Figure 5 shows Blade root Top Clamping Pad to clamp the external profile of
blade from tennon end.
shows Internal profile clamping pad on which blade internal profile can
Figure 6
restand also mount in fixture from tennon end.
Figure 7 shows External profile clamping pad on which blade external profile
can rest.
DETAILED DESCRIPTION OF THE INVENTION
Referring to figure 4 in detail, Blade root thrust pad 8holds the blade root 5so that
blade Bcan be clamped in the fixture properly. LMW blades have T-root having
rhomboid angle of 90° which are different from the advance class blade's root
having rhomboid angle 25°-33°, so there is rhomboid angle difference between a
LMW and an advance class blade. In blades where* rhomboid angle is 90° the
face is rectangular and not rhomboid, therefore, the design the pad needs to be
in such a manner that the blade can be clamped on blade root thrust pad 8firmly
and no angle variation can take place during machining operation.
While deciding the height of blade root thrust pad 8,the blade airfoil 1dimensions
are also considered so that blade can rest on the blade root thrust pad 8with
same center axis at both ends root andtennon.

The rhomboid angle difference also affects the design of external profile
clamping pad 11and internal profile clamping pad 10because the machine
Fixture base pads are designed for 29° rhomboid angle blades. So this factor is
taken care of by determining co-ordinates of internal profile clamping pad by
rotating the blade airfoil coordinates with 29° while designing the external profile
clamping pad 11andinternal profile clamping pad 10.
The main purpose of this Blade root top clamping Pad 9is to hold the blade root
5 from upper side i.e., right below the airfoil part so that blade Bcan clamp in
fixture properly.
Similar to root thrust pads 8being designed for 90° rectangular rhomboid angle,
root top clamping pads 9are also designed for this rhomboid angle. So we have
taken care this factor while designing these pads in such manner so that blade
Bcan be clamped on root top clamping pads 9firmly and no angle variation can
takes place while operation running.
The main purpose of this Internal profile clamping pad 10to hold the internal
profile of LMW blade Bso that blade can clamp in fixture properly. As LMW blades
do not have rhomboid shape towards shroud side as in advance class blades,
there lies a challenge to clamp the blade B from its airfoil 1internal profile with
complete stability. For this we have taken out the co-ordinates of internal profile
of blade & generate the NC program as per requirement to manufacture the
Internal profile clamping pad 10 as per internal profile of blade on CNC machine
such as Vertical Milling Center with self-generated program. Then the slope of
airfoil was extended so that blade can clamp firmly in machine fixture.

In this Internal profile clamping pad 10, the possibility to extend the pad length
as per requirement with elliptical shape bolt clamping in machine fixture is given.
We can adjust the pad 10 length as per requirement with different positions of
bolts in fixture. The rectangular rhomboid (90°) angle factor was also considered
while designing pad 10by rotating the coordinates of internal profile with 29°
because machine fixture base pads are designed for 29° rhomboid angle.
The main purpose of External profile clamping pad 11is to hold the external
profile of LMW blade with variable blade length, especially, small length blade. In
small length blades, top clamping pad design becomes totally changed. As blade
length does not reach the top clamping position in fixture so we have to design
the pad in such manner that external profile clamping pad 11can clamp the blade
from inside the machine fixture.
As LMW blades do not have rhomboid shape towards shroud side as in advance
class blades, there lies a challenge to clamp the blade Bfrom its airfoil 1external
profile with complete stability. For this we have taken out the co-ordinates of
external profile of blade & generate the NC program as per requirement to
manufacture the external profile clamping pad 11as per external profile of blade
on CNC machine such as Vertical Milling Center with self-generated program.
Then the slope of airfoil was extended so that blade can clamp firmly in machine
fixture. In this pad 11,we have given the possibility to clamp the small length
blades also as per requirement with increase the pad length so that we can clamp
the various lengths blade in same settings. The rectangular rhomboid (90°) angle
factor was also considered while designing pad 11by rotating the co-ordinates of
internal profile with 29° because machine fixture base pads are designed for 29°
rhomboid angle.

Installation of fixture pads
We can do the fixture setting for one blade at single time so we need each type
of above mentioned pads in one no. With the help of these pads we can
proceed the setting in machine fixture.
First clamp the blade root thrust pad in machine fixture with plurality of M6
bolts then clamped the blade root locater with plurality of M6 bolts & also
clamped the blade root top clamping pad with a M12 bolt. After that clamp the
internal profile clamping pad with plurality of M6 bolts & then clamp external
profile clamping pad with a M12 bolts. All bolts are made of alloy steel material.
Now adjust the machine fixture arms as per the blade length as machine fixture
has plurality of Arm which moves on lead screw. After that put the blade in
machine fixture & hydraulically clamped the LMW blades in machine fixture.
Procedure for Root & Tennon milling Process
After completion of pads arrangement on machine fixture. Develop the CNC
program for Root & tennon milling operation on HMC machine. In this program
we put the input values from blade drawing i.e. blade height, blade radius at
root, tennon dimensions & tennon radius etc. After that run the program in
machine with full clamping of blade. Initially, finish the hub dimension in blade
then maintain the tennon dimensions & total height.
Since, the foregoing is considered as illustrative only of the principles of the
invention. Further, since numerous modifications and changes will readily occur
by those skilled in the art, it is not desired to limit the invention to the exact
construction and operation shown and described, and accordingly all suitable
modifications and equivalents may be resorted to, falling within the scope of
the invention as claimed.

WE CLAIM
1. A clamping device for holding a steam turbine blade in a blade root and
tennon manufacturing process, the device comprising :
- a blade root thrust pad 8 to hold the blade root 5 so that the blade B can
be clamped in the device;
- a blade root top clamping Pad 9 to hold the blade root 5 from upper side
such as below an airfoil part to firmly clamp the blade B in the device;
- an internal profile clamping pad 10 to hold the internal profile of the
blade B so that the blade can be clamped in the device; and
- an external profile clamping pad 11 to hold the external profile of the
blade with variable blade length, especially, small length blade.
2. A process of machining a steam turbine blade in particular the blade root
and tennon, the process comprising :-
- clamping the blade root thrust pad, internal profile clamping pad and
blade root locater in the milling machine fixture with a plurality of
bolts;
- clamping the blade root top clamping pad including external profile
clamping pad in the machine fixture with a plurality of differently sized
bolts;

- adjusting the machine fixture arms connecting acrossthe blade length,
the machine fixture arms moving over a lead screw;
- placing the blade in the machine fixture and hydraulically clamping the
blades in the machine fixture;
- developing a CNC program for Root and tennon milling operation by
feeding the input values such as blade height, blade radius including
tennon dimensions;
- operating the program in the CNC machine maintaining a rigid clamping
of the blade; and
- Initially finishing the hub dimension in the blade then continuing the
machine by maintaining the tennon dimensions including the total
height.

3. The device as claimed in claim 1, wherein the blade root thrust pad 8 is
designed considering the blade airfoil 1 dimensions and the height of the
Blade root thrust pad 8 is so selected that the blade can rest on the blade
root thrust pad 8 with same center of axis at both ends of the root and
tennon.
4. The device as claimed in claim 1, wherein the root thrust pads 8 and root
top clamping pads 9 are designed at 90o rhomboid angle.

5. The device as claimed in claim 1, wherein theinternal profile clamping pad
11 are manufactured on the CNC machine such as a Vertical Milling Center
with self-generated program based on the co-ordinates of internal and
external profile of the blade.
6. The device as claimed in claim in any of claims 3 to 5, wherein the internal
profile clamping pad 10 has provisions to extend the pad length through
elliptical shape bolt clamping.
7. The device as claimed in claim 6, wherein the internal profile clamping
pad 10 and the external profile clamping pad 11 are so designed to
maintain the rectangular rhomboid (90°) angle factor by rotating the
co-ordinates of the internal profile by 29° rhomboid angle.
8. The device as claimed in claim 1 wherein when the blade length does not
reach the top clamping position in the device, the external profile clamping
pad (11) is designed in such a manner that the pad can clamp the blade
from inside the machine fixture.