FIELD OF INVENTION
The present invention relates to an alternate method for broaching firtree groove
profiles on rotor discs. More particularly, the invention relates to a profiled milling
operation followed by a finish broaching on the discs of a gas turbine rotor.
BACKGROUND OF THE INVENTION
Gas turbine rotor consists of several discs. Firtree grooves are made on this disc by
broaching process. Broaching is a process of machining by a tool called Broach. The
material is removed progressively by each line of tooth formed on the broach face. The
tooth is formed as per the profile so that profiled form can be achieved on the job after
machining. Because of various reasons, there is a chance of breaking or damage of
broach during machining operation. In case of any failure of the broach while making fir
tree grooves on the discs of turbine, the only solution is to change the broach which is
a highly expensive tool. But in case of non-availability of broach, work stops and
ultimately there is a failure to meet the delivery schedule.
Hence there was a need to find an alternate method for broaching a firtree groove on
the discs of a large turbine.

OBJECTS OF THE INVENTION
Therefore it is an object of the invention to propose an alternative method for
broaching a fir tree groove profiles in large size gas turbine rotor discs which can
produce profiled groove on disc as per requirement with zero defect.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.l - shows special profile end mill cutter
Fig.2 - shows profile milling cutter in the fir tree groove
Fig.3 - shows the test coupon
Fig.4 - shows the portion of groove X form where the material has been removed with
specially designed end mill cutter.
Fig.5 - shows the grooves on turbine disc machined with the combination of milling and
broaching operation.
Fig.6- shows damaged broach tool with broken teeth

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
A complete set of Broach tools Is required to finish a firtree profiled groove X, as shown
in Fig.4 and 5 on the disc (C) of turbine rotor. The set consists of a semi finish and
finish broach. If one broach breaks or gets damaged, the whole operation is stopped
because broaching is a sequential cutting process where all the cutting tools are fitted
in a fixed sequence. In the absence of a semi-finish broach, the next finish broach can
not be used. These broach tools are imported items and time cycle for their
procurement is quite long.
Hence, at the time of forming the required groove when the broach gets damaged and
in absence of a replaceable broach, a new method is invented to finish the unfinished
groove. As the broken broach tool as shown in Fig.6 having for example x number of
teeth broken or damaged, a milling operation as an alternate method on CNC milling
machine with programmable indexable table and with profiled milling tool is done to
machine the material left out in the groove due to x number of damaged teeth of the
broach tool.
For this milling operation a profile milling tool (A) is manufactured to suit the profile of
the broken broach. A hardened stainless steel (HSS) end mill cutter of dia (D) 10 mm is
selected to be used for milling the groove. The end mill is then ground to manufacture

the profile on it that matches exactly to the profile of broken broach (B). The profile of
the new tool (A) is checked in shadow graph and other quality checking methods to
ensure that the profile is exactly the same as required with no deviation.
The new tool (A) is not tired directly on the turbine disc (C). To ensure required
accuracy with zero defect, the operation with the new tool (A) is carried out on a test
coupon (T). The test coupon (T) is first broached to a pre-calculated depth on a
broaching machine to establish a condition of the groove almost similar when x nos. of
teeth of the broach was damaged. The test coupon is then mounted on a CNC milling
machine with programmable indexing table for milling of groove with the new tool to
remove material equivalent to the damaged broach. After milling operation the test
coupon (T) is again mounted on the broaching machine to check the correctness of the
milled profile. The test coupon (T) is then finish broached and checked on shadow
graph to ensure the correctness of the milled profile with the new tool (A) and amount
of allowance left along the groove profile for finish broaching.
Now being confirmed with the accuracy of profile obtained on test coupon (T), the disc
(C) is mounted on indexing table of a milling machine. Setting of the disc (C) and
centering of groove is done accurately. A groove is then milled on the turbine disc (C)
and after checking the positional accuracy of the milled groove, remaining grooves are
machined successfully using the modified milling cutter (A) maintaining the required
allowance for finish broaching. An allowance of 0.2-0.25 mm is kept along the groove

profile after milling the groove with new tool (A). The finish broach is able to take cut
not more than 0.3 mm material inside the groove.
The damaged broach (B) is again used with its remaining teeth on the milled groove.
The uneven material left in the groove after milling is successfully removed with the
remaining teeth of broken broach tool and the groove is ready for finish broaching. First
a single groove is finished in the disc (C) by Finish broaching and inspected thoroughly
for its geometric perfectness or misalignment of angular position and its depth.
Complete disc is thereafter successfully finish broached and checked for its correctness.
The significant part of this combined milling and broaching operation is to overcome the
situation of lost reference. When the disc (C) is unloaded from broaching machine
without complete broaching, its reference is lost and it becomes very difficult to achieve
the same setting again. It is also difficult to achieve the same angular position of the
groove on the indexing table of the machine. As the allowance in the groove is very
small i.e. 0.2-0.25 mm, any difference in angular position might have spoiled the groove
during broaching. But the process adopted resulted in making the grooves with zero
defect.
This new idea of combined milling and broaching operation for manufacturing gas
turbine discs (C) in case of broach failure has been already implemented and such
turbine discs have been supplied to customer.

WE CLAIM
1. An alternate method for broaching fir tree groove profiled (X) in a large size gas
turbine rotor disc comprising:
forming a profile by grinding on an HSS end mill cutter (A) to match with the
profile of broken broach (B);
checking the profile formed on the tool (A);
operating the tool (A) on a test coupon (T) to check the accuracy of the
operation;
milling the grooves (X) on a test coupon (T) to remove material equivalent to the
damaged broach (B);
finish broaching the left out material on the grooves;
checking the grooves (X) on the said test coupon to check the correctness of the
milling and broaching operation;
mounting and setting the disc (C) on the machine;
milling the grooves (X) with new profiled tool leaving an allowance to finish
broach;
broaching the grooves (X) by broken/damaged broach (B) to remove uneven
material left in the groove after milling;
finish broaching the grooves (X);
inspecting the grooves (X) to check the accuracy of profile and depth;

characterised in that the combined method of milling and broaching produces the
grooves (X) with zero defect without allowing the reference to loose or any
difference in angular position to exist.


2. An alternate method for broaching fir tree groove profiles (X) as claimed in claim
1, wherein the diameter (D) of end mill cutter (A) is 10 mm.

An alternate method for broaching fir tree groove profiles (X) in a large size gas turbine
rotor discs (C) discloses a combined milling and broaching operation by introducing an
HSS end mill cutter (A) formed by grinding to match the form of the broken or
damaged broach (8). Before doing the actual operation, a trial of the cutter is made on
a test coupon (T). After producing an accurate test coupon (T) with the required form,
the discs (C) are milled with formed end mill (A) and broached with the damaged
broach (B) to remove uneven material in the groove. An allowance of 0.2-0.25 mm
material is left in the groove for finish broaching. Finally after finish broaching, the
grooves on the discs are checked for its accuracy.