FIELD OF INVENTION
The present invention relates to of a fixture for machining field lead core bar of large size turbo generator of a thermal power plant along with the method for setting the job on the said fixture for machining.
BACKGROUND & PRIOR ART
In turbogenerator (TG), the transmission of current from high speed rotating part called rotor (having a field winding on it) is done through two half of cylindrical bars of copper known as field lead core bar (or simply core bar). The semi-cylindrical two halves of the core bar are inserted in the bore (hollow part at the center) of the rotor. From the rotor winding, connection to the core bar is made through two current carrying bolts made of copper. The exact marking, drilling, threading and matching of the holes in the core bar for connection of current carrying bolts and connection of conducting pin in the core bar inside the rotor/exciter shaft is of prime importance because of a slight mismatch at high speed of 3000 to 3600 rpm may damage either exciter rotor or TG rotor and create out of synchronous situation and heavy damage to the total machine. A cylindrical core bar, comprising of two semi-cylindrical identical halves (because of two magnetic poles in a turbogenerator) with a separator in between,

undergoes various machining operations before being used in turbo-generator assembly. In a large size generator the practice is to join two halves of core bar by dummy/technological rings during the time of machining and the joint core bar is inserted in the TG rotor and the current carrying (CC) bolts are initially transferred on the core bar from the TG rotor. The core bar is brought out of the TG rotor and CC bolt hole is done exact as per design through boring and reaming and threads are made so that the bolts can be locked. Next the core bar is taken to a lathe machine for cutting off the technological rings.
Afterwards the job is taken for drilling of 2 M60 bolts for which 4 shoes are loaded on the machine along with the angle plate set on the job and at least 6 persons have to be engaged including the crane till the clamping is done while drilling, tapping & slot milling operation is undertaken.
Again the dismantling requires same man/machine hours and other resources before taking the job to planer machine for angular milling of corners. The next operation is assigned to marking, drilling, tapping of M20 holes and taking back to planer machine for angular machining.
The lead core bar is a very critical component in a turbo generator as all the excitation from the exciter of a generator is transferred to the field lead core bar and then to the current carrying bolt through conducting pins. The holes of these

conductor pins are arranged in a square array. There can be a transfer of load on these pins due to damage/bend in lead core bar.
Since the CC bolt performs essential jobs and inserted in the M60 threaded holes and holds the lead core bar with respect to the rotor, if there is a damage/bend in the core bar a problem would be there in locking the core bar. Also the whole of the field lead core is insulated and if there is a mismatch in CC bolt location on the bar a rework is necessary. In fact since the core bar is a critical component in order to facilitate low losses and high conductivity, copper is used to manufacture a core bar and it is soft and malleable which can easily be damaged/bent due to multiple handling leading to rejection of the job. Therefore, the constraints of the prior art can be enumerated as under. . Four machines are involved with high operating cost & cycle time
• Crane, operators get engaged for long time
• Safety & operational problem due to handling job in a vertical condition . Engagement of fitters twice for de-burring jobs

• Too much transportation of the core bar results in long waiting cycles and engagement of resources
• Frequent rework due to multi handling of soft material

OBJECTS OF THE INVENTION
The object of the invention is to develop a fixture and a method for machining of
field lead core bar for and up to 800 MW Turbo generator saving the total cycle
time in machining operation during its manufacturing process.
Further object of the invention is to bring four work centres of the whole operation
on a single machine to avoid extra amount of man/machine hours engaged in
preparation, repeated set up for job installation and transportation.
Again the object of the invention is to provide better quality of job eliminating
rework due to least transportation & handling of the job.
Also the object of the invention is to provide safe procedure leading to maximum
safety of the operator.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1 Assembly of complete fixture
Fig.2 Clamping plate
Fig.3 L-Block for assembly of 2 alignment bolts of M20
Fig.4 Core bar clamps for tight gripping of core bar on fixture with large
engagement area Fig.5 Base plate on which all the fixture items are fixed

Fig.6 Segment provided for angular milling setting
Fig.7 Fixed segment for supporting adjustable segment
Fig.8 Adjustable segment for different rating & size core bars
Fig.9 Core bar
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
The invented fixture under description comprises of the following:-Item: 1 Base plate on which all the fixture components are fitted along with
plurality of holes.
Item: 2 Plurality of M36 threaded hole
Item: 3 Plurality of 38 dia hole
Item:-4 M16 threaded hole for lifting of core bar clamps(5)
Item:-5 Core bar clamps for tight gripping of core bar on fixture with large
engagement area Item :-6 17 Dia holes on fixed segment
Item:- 7 Fixed segment for supporting adjustable segments
Item :-8 Plurality of 17 dia hole for angle plate
Item :-9 L-Block for for assembly of 2 alignment bolts of M20
Item :-10 Plurality of main block welded on base plate of the fixture

Item :-11 Clamping Plate (Fig.2)
Item :-12 Block for angular milling (Fig.6)
Item-1 Base of the fixture holds and supports all other items (Fig.5) having
plurality of M16 threaded holes required for its threaded connection with angle
plate(9) through 17 dia hole(8) (Fig.3).
Plurality of main block (10) is welded on the base of the fixture (1) at a distance
so that they can carry the load of the core bar without causing buckling/bending in
the core bar(Fig. 7).
L-Block (9) is fixed to assemble 2 alignment bolts of M20 with the base (1) of the
fixture using threaded joints of M16 (Fig.5).
The semi circular curve of the fixed segment (7) and core bar clamp (5) matches
with the field lead core bar while fixed segment (7) for supporting adjustable
segments is assembled with the main block (10) through 17 dia hole (6)(Fig. 8).
The corresponding M16 threaded hole is made on main block (10) (Fig.7)
Core bar clamps (5) for tight gripping of core bar on fixture with large engagement
area having 38 dia hole (3) is assembled with the main block (10) through
threaded joint between holes of M36 (2) and holes of 38 dia hole (3) (Fig.7). M16
threaded hole (4) is used for lifting of core bar clamp (5) (Fig.4).

After assembly of the fixture as above the machining job of the field lead core bar
is undertaken only on one single machine and set up with the fixture described as
under.
Step 1: As earlier 20 dia hole for current carrying bolt is transferred from rotor to
the core bar.
Step 2: Putting the field lead core bar in the multiple fixed segment (7) such that
the curve of the fixed segment matches with that of core bar.
Step 3: Putting the core bar clamp (5) over the core bar in such a way that the
curve of the clamp matches with that of the field core bar.
Step 4: The core bar clamp (5) is assembled with main block (10) through
threaded joint between holes of M36 (2) and 38 dia hole (3). Core bar clamp (5)
grips the core bar tightly on fixture with large engagement area.
Step 5: The alignment of the core bar CC bolt hole (as done in step 1) with
respect to machine spindle is done using L- Block (9). This is done by using a
clamping plate (11) which is assembled on the technological hole made on the
core bar.
Step 6: 2 nos of M60 threaded holes are made to assemble current carrying bolts
along with drilling & tapping of 2 nos. of lifting bolt holes on the side of the core
bar.

Step 7: Core bar clamp (5) is released and the core bar is rotated by 180 degree
along the longitudinal axis so that the step-6 can be repeated for second CC bolt
(Fig.10).
Step 8: The technological rings that hold the two halves of the core bar are cut off
.
Step 9: Since the core bar is in two separate halves, core bar upper and lower
half are removed and placed for angular milling(12) on fixed segment (7) one after
another.
Step 10: Angular milling of core bar corners in both the halves along with the
separator is undertaken.
The above method is found effective using the fixture on a single machine and
setting which leads to least cycle time and use of heavy capacity crane, elimination
of rework and mismatch of holes without any damage to the soft material during
transportation.

WE CLAIM
1. A fixture for machining the field lead core of a turbo-generator up to 800 MW capacity comprising a base plate, core clamps, fixed and supporting segments, angle plates, L-Block, plurality of welded blocks, clamping plates, block for angular milling, alignment & fixing bolts characterized by minimum cycle time and transportation of the job with maximum accuracy & safety.
2. The fixture as claimed in claim 1, wherein, a base (1) having threaded holes (8) for connection of angle plates (9), holding & supporting the items comprising the fixture.
3. The fixture as claimed in previous claims, wherein, plurality of main blocks (10) welded on the base of the fixture (1) at a distance capable of carrying load without causing buckling/bending in the core bar.
4. The fixture as claimed in previous claims, wherein, L-Block (9) for assembly of alignment bolts with the base of the fixture using threaded joints.

5. The fixture as claimed in previous claims wherein, semicircular curve of the fixed segment (7) matching with the field core bar for supporting adjustable segments.
6. The fixture as claimed in previous claims wherein, core bar clamp(5) matching with the field lead core for tightly gripping of core bar fixture with large engagement area
7. (7) A method of optimization of multi activity machining & handling jobs for a lead core bar of a turbo-generator up to 800 MW capacity through development of a novel fixture comprising of the following steps.
. Transfer of the hole diameter for current carrying bolt from rotor to core bar
• Putting the field lead core bar in the multiple fixed segments (7) of a fixture (1) in matching with the curve of the core bar.
• Putting the core clamp (5) over the core bar to match the two curves of clamp & core.
. Fastening the Core bar clamp (5) with the main block (10) through threaded joints between hole & bolt.

• Aligning of the current carrying bolt hole with respect to the machine spindle using L-Block (9) and a clamping plate (11)
• Making two threaded holes 180 degree apart by rotating the core bar along the longitudinal axis for assembly of current carrying bolts.
• Cutting the technological rings which holds the two halves of the core bar
• Angular milling of core bar corners in both halves