FIELD OF INVENTION
The present invention relates to the development of a new process of machining
steam turbine blades. Moreover the invention relates to a process for machining
aerofoil of steam turbine blades by using circular inserts in place of two
rhomboidal inserts to eliminate the twisting in the profile of 3DS & F type blade
due to excessive load generated by the rhomboidal inserts during machining the
aerofoil of blade in order to increase productivity and to reduce manufacturing
cost.
BACKGROUND OF THE INVENTION
The efficiency and reliability of turbine is dependent upon the design and
manufacturing accuracy of blades and its aerofoil. Turbine blades play an
important role in deciding the thermal efficiency of the steam turbines. In a
modern machine shop having CNC facilities, the blanks are cut to the required
sizes based on the blade length and cross-section and loaded on Work piece
station of multiple axis CNC machining centers
Complete machining of required blades is carried out step by step as per CNC
program. In the first step, roughing operation of Hub and Hub slot, Shroud and
aerofoil is carried out and after completion of roughing operation, finishing
operation is carried out on the semi-finished blades. The machine automatically
selects the required tool/ cutter from tool magazine for different operations and
measure the tool length and diameter in laser Unit inside the machine. Important
dimensions of blades are automatically checked by Renishaw probe during
roughing operation and automatic correction is carried out to maintain the
required dimensions. After completion of machining, the finished blade
automatically comes out of the machining area while the next new blank comes
for machining,
The process of prior art suffers with some disadvantages such as twisting of
profile of blades due to excessive load coming on profile during profile roughing
operation by dia 32 mm cutter having two edges rhomboidal inserts.
The previous dia 32 mm cutter consisting of two edges rhomboidal insert were
exerting too much load on the aerofoil of the blades as well as on the spindle of
the machines resulting in damage of spindle of various 5-axis machines.
Therefore it was necessary to reduce the
OBJECTS OF THE INVENTION
1. It is therefore an object of the invention to propose a process of machining
steam turbine blades which can eliminate the twisting of aerofoil on some stages
of blades.
2. Another object of the invention is to propose a process of machining steam
turbine blades which can reduce the excessive load coming on spindle of
machines.
3. A still another object of the invention is to propose a process of machining
steam turbine blades which enable to increase productivity and to reduce cycle
time.
SUMMARY OF THE INVENTION
The invention relates to the rough cutting of free cutting hub and aerofoil cutting
in order to avoid excessive load on aerofoil blade as well as to reduce the
excessive load on the spindle of machine. The operation uses circular cutter
insert having 4 cutting edges spaced apart by an angle of 90° in place of
rhomboidal inserts having diagonally opposite 2 cutting edges. Free cutting hub
is done towards hub side in the first step and then the tool is taken back.
Tool is again taken to the shroud side for material cutting for roughing air foil
operation. The difference is when the cutter reaches from shroud side to hub
side, no material will be available for cutting to the proximate end of hub which
was already clear by cutter in free hub cutting operation.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1- Process flow diagram of aerofoil machining.
Fig. 2- Rhomboidal inserts having 2 cutting edges per insert in a set of 3 inserts.
Fig. 3- Circular inserts having 4 cutting edges per insert in a set of 3 inserts in
the new invention.
Fig. 4- Comparison of cutting edges between the inserts in the prior art and new
invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The original technology for manufacturing advanced type turbine blades was
provided by the supplier of the CNC machining centre. The dia 32 mm cutter
consisting of two edges rhomboidal inserts (C of Fig.-l) are also the part of that
technology given by machine supplier.
Previously we had few design of blades but now more variety of new design
blades of various sizes have been introduced. Some blades cross-sections which
are very thin and very difficult to machine with the existing process of machining
blades provided by the machine supplier.
After experiencing problems of blade aerofoil twisting and spindle damaging on
various 5-axis machines, it was analyzed and found that the dia 32 mm cutter
consisting of two edges rhomboidal inserts is exerting excessive load on blade
aerofoil and also on the spindle of machine. It was analysed and concluded that
damaging of spindles of some of the machines which occurred earlier are only
due to the excessive load of dia 32 mm cutter which consisted of two edged
rhomboidal inserts.
However the first operation of Rhomboid roughing of rectangular bar of bladeare
done by the circular inserts with Cutter dia 50 mm & it has been observed that
the spindle load coming on the machine was very less. From here an idea was
clicked for aerofoil roughing of the blades by cutter diameter 32 mm with circular
inserts (D of Fig -lc). Circular inserts of cutter dia 32 mm was selected for
aerofoil rough machining of blades on 5-axis machine & accordingly changing the
program for machining the aerofoil of blades.
During the machining aerofoil of blades it was observed that load on spindle
appearing in machine screen was very less &. finally the blade was for sent
3DCMM checking.
Report of 3DCMM was positive, twisting of blades was not present & finally the
implementation of new technology to the entire CNC 5-axis machine took place.
Life of circular inserts is approximately double to that of 2 edged rhomboidal
inserts. In circular insert 4 cutting edges are available for machining the blades
but in Rhomboidal inserts only 2 cutting edges are available. Finally 4 times life
of inserts increases by the use of circular inserts instead of use of rhomboidal
inserts. Inserts indexing time is also saved.
COMPARISON OF LIFE BETWEEN RHOMBOIDAL INSERT AND
CIRCULAR INSERT
Rhomboidal inserts:
Life of insert = 3 blades /cutting edge (where available cutting edge = 2)
= 3 x 2- 6 blades / set
Total blade machine by one set (consisting of 3 inserts)
= 6 blades / set
Circular inserts:
Life of inserts = 6 blades/ cutting edge (where available cutting edge = 4)
= 6 x 4 = 24 blades / set
Therefore total blade machine by one set (consisting of 3 inserts) = 24
blades/set.
OPERATION SUMMERY DETAIL BETWEEN THE PREVIOUS AND
PRESENT INVENTION
1. COMPONENT NAME - LG 2R_56 K
MACHINE -CNC 5 AXIS MACHINNING CENTRE (LIECHTI)
Operation List with 17 mm 2 edge insert

2. COMPONENT NAME: LG2R_56K
MACHINE: CNC 5- AXIS MACHINING CENTRE (LIECHTI)
Operation list with the present invention with 12mm circular insert having 4
cutting edge

WE CLAIM:
1. A process for machining Aerofoil of 3 DS and F- Type steam turbine blades on
CNC machines using circular inserts comprising
- Preparing the blanks to the required size in suitable cutting machine as per
blade length and cross section.
- Loading the blanks on work piece station of 5- Axis CNC machine centre.
- Feeding the cutter having circular insert from hub side of fillets to undertake
material cutting of free cutting hub in circular motion.
- Lifting the cutter back in this position after executing the free cutting hub
operation.
- Positioning the cutter in the shroud side of the fillet and
- increasing the depth of cut for executing metal cutting operation in circular
motion.
- proceeding the movement of cutter from shroud side of fillet to hub side while
undergoing material cutting known as Roughing air foil characterized in that
while the cutter with circular insert moves from shroud side to hub side material
cutting, it does not cut any material at the hub portion since the material already
removed by cutter in free hub cutting operation thus minimizing the thrust load
on the machine spindle while eliminating the twisting of aerofoil on some stages
of blades.
2. The process as claimed in claim 1, where during changing the cutting edge,
the insert screw is undamped with the help of key and indexing the insert with
fresh edge.
3. The process as claimed in claim 1 where the insert is made of carbide.
4. The process as claimed in claim 1 where the insert is circular having 4 cutting
edges, each cutting edges at 90° interval.