FIELD OF INVENTION
The present invention relates to an improved process on CNC lathe with newly
developed tools to reduce the process time for rough and finish machining of balancing
weight grooves in shaft of steam turbine.
BACKGROUND OF THE INVENTION
Rotor shafts of Steam Turbine have Balancing Weight Grooves which are used
for fixing of balancing weights inside these grooves for balancing of bladed rotors. Machining
of Balancing Weight grooves is a very critical operation for proper fixing of balancing weights
inside it.
After assembly of blades on the rotor, it is tested for Balancing in Over Speed
Balancing Tunnel to check for weight shifting during rotation on high RPMs of 3000 to 4000
RPM.
If the weight shift during rotation is not as per the standard, Balancing Weights
are fitted in the Balancing weight grooves at different radial locations.
In rotor shaft, no. of of these grooves varies from 4 to 6 depending upon the
design & rating of Steam Turbine. As these grooves are critical for the entire bladed rotor
balancing many in process checks are incorporated to ensure their accuracy.
Rough grooving operation for these types of grooves takes substantial time
which needs to be tapped for its reduction in order to result in total cycle time reduction.

Rough grooving operation is done using a straight grooving tool of width around
12mm. So, tool geometry and its grade plays major role in deciding cutting parameters for
machining which finally attributes to machining time and quality of surface finish.
Straight rough grooving is done leaving 0.5mm allowance on the finish face.
After straight rough Grooving, tapered profile area of groves is machined. These grooves are
finish machined with different special turning tools with high accuracy and surface finish.
Earlier indigenously developed brazed tools were used for rough and finish
machining of these types of grooves. As these grooves are on faces of rotor, tools are to be
compatible with diameter at which grooves exist. Also since these tools were brazed after
two or three times of regrinding the cutting edges of these tools are broken rendering the
whole body of the tools useless.
Cutting parameters, used for machining, were very low to suit the cutting
geometry of the brazed tools.
In the earlier process the cassette was needed to be changed frequently for
machining the left and right hand side of Balancing Weight Groove. Errors may creep in due
to this frequent change of cassette and tool setting.
Tool change time was higher due to frequent change of tool for regrinding and
their resetting and consumption of tools was also high due to frequent breakage.
In view of above, it used to take very high machining time in carrying out the
rough grooving operation.
Time taken in machining of such grooves was high in previous method leading
to less productivity.

Tool changing was required frequently which led to time delays.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose an improved process on
CNC lathe with newly developed tools to reduce the process time for rough and finish
machining of Balancing weight grooves in shaft of steam turbine which is capable of
reducing the cycle time drastically.
Another object of the invention is to propose an improved process on CNC
lathe with newly developed tools to reduce the process time for rough and finish machining
of Balancing weight grooves in shaft of steam turbine which is able to reduce tool
consumption and requirement of re-setting of tools.
A further object of the invention is to propose an improved process on CNC
lathe with newly developed tools to reduce the process time for rough and finish machining
of Balancing weight grooves in shaft of steam turbine which can produce grooves of high
accuracy and surface finish.
A still further object of the invention is to propose an improved process on CNC
lathe with newly developed tools to reduce the process time for rough and finish machining
of Balancing weight grooves in shaft of steam turbine which is capable of making a
substantial cost saving.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 - Shows the design of Balancing Weight Groove and Location of the Finished Grooves
in the Rotor
Fig. 2 - Shows the material to be removed in the balancing weight groove
Fig. 2a - Shows material removed in the First operation by straight grooving tool in the prior
art
Fig. 2b - Shows material removed in the second operation by tapered grooving tool in the
prior art

Fig. 2c - Shows material removed in the third operation by straight grooving tool according
to prior art.
Fig. 3, 3b and 3c - Brazed tools used according to prior art.
Fig. 4 - Shows the both types of tool together (LHS and RHS) for machining according to the
invention.
Fig. 5 - Shows both types of tapered grooving tool together according to the invention.
Fig. 6 - Shows both types of tapered grooving tool together according to the invention.
Fig. 7 - Shows the developed and improved straight grooving tool, according to the
invention.
Fig. 8 and 9 - Shows the developed tool for cutting the taper material of the Balancing
Weight Groove according to the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
The object of the invention being reduction of process time on CNC lathe for
rough and finish machining of Balancing Weight grooves in shafts of Steam Turbine, new
CNC parametric program, newly designed tool and improved process for the said machining
are incorporated.
Earlier some brazed tool was used to rough plunging of Balancing Weight
grooves. Three number of cuts were involved in rough machining of Balancing Weight
groove. First cut in middle of groove left equal allowance on both sides. Some finish cut was
taken with 0.5mm allowance left on Right hand and Left hand side of neck width. This
process takes much time for roughing of Groove as the cutting parameter are very less such
as cutting speed = 40-60 m/min and feed = 0.08 to 0.1 mm/rev.
A new parametric CNC program is developed for optimum numbers of Plunging
cuts per groove depending upon its size for rough and finish machining of Balancing Weight
groove in shafts of a turbine. At the same time new grooving tools are developed with
improved cutting geometry and grade for highly productive machining suiting the developed

tool. The cutting geometry and grade of the new grooving tool enabled to incorporate very
high cutting parameters such as cutting speed 75-90mm/min and feed of 0.15 to 0.22
mm/revolution during the rough machining of Balancing Weight groove.
This application reduced cycle time drastically and tool consumption and
requirement of resetting of tools reduced substantially. The new developed tool is clamped in
both downward and upward direction to impart rigidity in the tool. An arrangement of
coolant is made to improve surface finish and life of the tool.
The following are the benefits accrued after changing the process of machining
and the use of an improved tool.
1) Productivity: Total machining time of rotor shafts on CNC Lathe reduced substantially.
2) Financial Benefit: As cycle time is reduced critical CNC machine availability is increased
to take up more nos. Of rotors.
3) Power Saving: Overall Productivity & Utilization of machine is increased hence power
consumption was reduced.
4) Quality: With use of improved process and tools, quality of machining is also
improved.
5) Health and working condition: As clamping and unclamping was reduced so machine
operators were less fatigue.
6) Amount of material removed
Estimated area of unit section of Bal. Wt. Groove(Width = 35.6 Depth= 23) =
1100.628 mm2
Average locating dia of Bal. Wrt. Groove = 1545.6mm

Perimeter with reference to above dia = 4853.184 mm
Volume of material removed = 5390.084 cm3
Density considered = 7.9 gm/cm3
Total material removed in a single balancing weight groove = 5390*7.9 = 42.58 kg
Total material/Shaft removed = 42.58*6 = 255.48 kg
7) Financial Saving: Time saved in machining of 1 Balancing Weight Groove = 1.2 hr
Time saved in machining of Balancing Weight Groove=(6*1.2) = 7.2 hrs/shaft
Machining Cost = Rs. 6500 per hour.
Total saving = 6500X7.2 = Rs. 46800.
Average No. Of shaft machined per year = 40
Total saving per year = 48,00 X 40 = Rs. 19,20,000
= 19.20 lacs.
Advantage of using the new process and improved tool from safety and working
condition point of view.
8) Elimination of frequent Cassette Change
This process eliminates the frequent requirement of changing the cassette to machine
the left and right balancing weight grooves. This in turn reduces the machining cost.
Also this eliminates errors and rectification which may creep in due to frequent
cassette change and tool setting.
9) Tool Safety
As tool having new geometry with insert was used which was much convenient and
safer. Previously, grinding of tool was required as it get blunt frequently but new
method is easier and safer as only changing of insert is required. Also for different
sides of groove no new attachment is required as the same tool is able to machine on
both the sides.

10) Personal Safety
Frequent handling of tool is reduced. In previous Brazed tool complete cassette and
accessories was to be taken out whereas in new method only insert has to be
changed which is more safe.
11) Process Safety
In suggested development of tools, coolant is supplied from two nozzles whereas in
previous designed tools there was no provision of coolant supply which was less
effective. Proper coolant supply improves the finish of groove and absorbs heat
developed during machining.
Fig. 7 shows the developed and improved straight grooving tool made
indigenously. Fig. 8 shows that the working length of the tool has been increased to X=
30mm from X=25mm so as to accommodate more variety of Balancing weight grooves in
higher rating/new design sets.
Also high grade insert which can cut at higher speeds and feeds has been
developed for the same.
The length of the tool is sufficient to be held by the screws of the cassette.
This also adds to the rigidity of the tool eliminating the chance of vibration and breakage. All
these dimensions are finalized after some experimental research.
Fig. 8 and 9 shows the new tools developed for cutting the taper material of
the Balancing Weight Groove. The clearance angle in the tool as shown in Fig. 8 has been
increased to d=17.5 deg from b=15 deg earlier so as to increase the clearance from the
taper of the balancing weight groove. Also the present designs of the balancing weight
grooves have 11.5 deg and 13.8 degrees which can easily be accommodated in this. As
shown in fig. 9 the length "X' is increased to 200mm from 150mm so as to accommodate
tools for machining of LHS and RHS balancing weight grooves.

Earlier different tools were used for right hand side and left hand side
balancing weight grooves.
Earlier a set of two tools were used to make Balancing Weight Groove. Tool 3.b
was used to make outer profile and 3.c was used to make inner profiles on left hand side.
Tool 3.c is then used to make outer profile and tool 3.b is used to make inner profile on the
right hand side. Tool change is done frequently. To make the left hand groove, the tools
were mounted on the left hand side of the ram and on the right hand side for the other side
groove.
Use of new tool with insert for plunging of balancing groove was new as tool of
such geometry was previously not available.
As shown in fig. 2a, in the first operation material with a straight grooving tool
is removed from the Balancing Weight Groove. The cassette has to be shifted from the right
hand side to the left hand side and also the tool has to be changed from the earlier cassette
to the new cassette. Tool setting has to be done again which increases the machining time
and also the chances of errors creeping in increases.
In the second operation, as shown in fig. 2b material with a tapered grooving
tool is removed from the Balancing Weight Groove. The cassette has to be shifted from the
right hand side to the left hand side and also the tool has to be changed from the earlier
cassette to the new cassette. Tool setting has to be done again which increases the
machining time and also the chances of errors creeping in increases.
As shown in fig. 2c, in the third operation material with a tapered grooving tool
is removed from the Balancing Weight Groove. The cassette has to be shifted from the right
hand side to the left hand side and also the tool has to be changed from the earlier cassette
to the new cassette. Tool setting has to be done again which increases the machining time

and also the chances of errors creeping in increases.
In the invented methodology the cassette length was sufficiently increased and
also the design was changed after some experimentation to accommodate tools for
machining of both types (LHS and RHS) of Balancing Weight Grooves.

WE CLAIM
1. An improved process on CNC lathe with newly developed tools to reduce the
process time for rough and finish machining of Balancing weight grooves in shaft
of steam turbine, the said process comprising;
developing new grooving tools with improved grade and cutting geometry;
selecting a high and improved cutting parameters on the CNC machine suiting the
said tool wherein a new CNC program is incorporated based on the said improved
cutting parameter to introduce optimum numbers of plunge cuts per groove
depending upon its size for rough and finish machining of Balancing weight
groove in shafts of a turbine to reduce the process time.
2. The improved process as claimed in claim 1, wherein the implemented high
cutting parameters are cutting speed of 75-90 mm/min and feed of 0.15-0.22
mm/revolution.
3. The improved process as claimed in claim 1, wherein the number of plunge cut
implemented is 3.

ABSTRACT

The invention relates to an improved process on CNC lathe with newly developed
tools to reduce the process time for rough and finish machining of Balancing weight
grooves in shaft of steam turbine. The process comprising developing new grooving tool
with improved grade and cutting geometry and then selecting a high and improved cutting
parameters on the CNC machine suiting the said tool when the tool is rigidly clamped in
both upward and downward direction wherein a new CNC program is incorporated based
on the said improved cutting parameter to introduce optimum number of plunge cuts per
groove depending upon its size for rough and finish machining of balancing weight groove
in shafts of a turbine.