FIELD OF INVENTION
The present invention relates to a process of machining by a twin tool device internal
thread of a blind hole in field lead core bar adaptable to rotor shaft of generators.
BACKGROUND OF THE INVENTION
While manufacturing of rotor shaft particularly in the field lead core bar for machining
of thread in a blind hole, a particular problem was encountered for high capacity
generator like 600 MW TG rotor. The field lead core bar of the rotor is required a full
thread hole of M60 X 2 of a blind hole.

At present these holes and threading are done with a boring bar. The boring bar has a
provision for fixing and tightening the tool bit. The cutting edge of the tool bit is kept
perpendicular to the boring bar is always 8-10 mm lower than the top edge of boring
rod. The boring bar fouls with the end wall of the hole and the tool is not allowed to
access up to the end wall. In such case threading is possible only up to a point 10 mm
before the end wall of the hole.
But in case of higher capacity generator like 600 MW TG rotor, the field core bar
requires a hole to be full threaded for firmly gripping for current carrying bolt. Now
this rotor runs at a high speed of 3000 rpm in normal running and increases upto
3600 rpm in certain cases. At such a high speed insufficient griping of current carrying
bolt may not with stand the centrifugal force on the bolt and the bolt may come off
from the rotor shaft which shall be a disaster for the machine.

Moreover rotor is a very sensitive part of the generator and have high on the vibration
performance of the machine and less possibility to change any design feature without
the consent of original designer maker of the company.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a process for machining
by a twin tool device internal thread in a blind hole which eliminates the existing
problem in machining.
Another object of the present invention is to propose a process for machining by a
twin tool device internal thread in a blind hole which reduces the machining time.
A further object of the present invention is to propose a process for machining by twin
tool device internal thread in a blind hole which increases productivity.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
Fig.1a: shows a schematic diagramme of elevation view of a groove tool
mounted on a tool bar.
Fig.1b: shows a schematic diagramme of end view of a groove tool mounted on
a tool bar.
Fig.lC & lb: shows a schematic diagramme of isometric view of groove tool mounted
on a tool holding bar.
Fig.2a: shows a schematic dagramme of elevational view of a thread tool with
holding bar
Fig.2b: shows a schematic diagramme of end view of a thread tool with holding
bar.

Fig.2C & 2d: shows a schematic diagram of isometric view of the thread tool mounted
with a tool holding bar.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The twin tool device comprises one groove tool and one threading tool mounted on a
holding bar. As shown in Fig.1(a, b, c, d) - The newly developed grooving tool
mounted with a holding bar for machining a blind hole in field lead core bar comprises
a square based parallelepiped shaped holding bar (1); Each side the square base of
the bar having a length 'd'. One end the mounted bar is affixed with an inverted 'L'
shaped tool bit (2) at one face. The length of the long arm of 'L' shaped tool bit (2) is
equal to 'd' same as that of the one side on the square base of the holding bar (1).
The length of the short arm of the 'L' shaped tool bit (2) is d/2. The long arm of 'L'
shaped tool bit (2) is disposed parallel to the longitudinal axis of the holding bar (1)
and protruded a small part (3) at a distal

end of the longitudinal axis. The top face (4) of tool bit of protruded part (3) is
configured at an angle 2° aligned from horizontal line and the front plane (5) of the
protruded part is disposed at an angle 2° with the vertical line. The width and length
of the protruded part is 'd/2' and 'd' and the tool bit is affixed with the tool bar (1) by
brazing in one the faces of the tool holding bar at junction point (6).
As shown in Fig.2, the newly developed thread tool mounted with a holding bar
comprises a square base parallelepiped shaped holding bar (10). Each side of the
square base of the holding bar having a length 'd'. One end of the holding bar is
affixed with a polygonal shaped tool bit (20) having an apex (40) of a triangular part
(30) at top part with a with a square base part (20) at the bottom. The triangular part
(30) and a part of the square base (20)are protruded from the top face of the holding
bar (10). The apex (40) of the triangle (30) is parallel to the longitudinal axis of the
holding bar (10) and the protruded apex of the triangle (30) is disposed at a distal
end from the longitudinal axis of the holding bar (10).

The front face of the square part of the tool bit (20) remain with the same plane with
the holding bar (10). The front face (50) of the triangular part (30) is aligned at an
angle 2° with the vertical line. The apex line (40) of the triangular part (30) is
configured at an angle 3° aligned to the horizontal line and the sliding face (60) is
configured at 60° alignment with horizontal line. The square base part (20) of the tool
bit is brazed (70) with the tool holding bar (10). This tool is used machining the
thread inside a blind hole of the field core bar.
The process of machining by a twin tool device internal thread of the blind
hole are described below:-
- setting a groove tool mounted with a tool holding bar at tool post;
- machining of internal bore of the blind hole by the groove tool;
- replacing the groove tool with a threaded tool;
- machining an internal thread on said internal bore by the threading tool;
- dismantling the threading tool after completion of said thread machining
upto the end wall of internal hole;

WE Claim
1) A process of machining by a twin tool device an internal thread in a blind hole
of field core bar, the blind hole being otherwise inaccessible by the standard
tool, the field core being adaptable to a motor shaft the process comprising:
- setting a groove tool mounted with a tool holding bar at tool post;
- machining of internal bore of the blind hole by the groove tool;
- replacing the groove tool with a threaded tool;
- machining an internal thread on said internal bore by the threading tool;
- dismantling the threading tool after completion of said thread machining
upto the end wall of internal hole;
2) A twin tool device for implementing the process as claimed in claim 1, wherein
the twin tool device comprises at least one groove tool for machining the
internal blind hole, the configuration of the groove tool

corresponds to the respective configuration of the holding bar the groove tool
further comprising:-
- a square base parallelepiped bar having a length of its base is 'd'
- an inverted 'L' shaped tool bit having short arm affixed with one face of a
holding bar and the long arm is protruded on top face of the holding bar;
- the protruded part of the tool bit disposed parallel to the axis of a
longitudinal axis of the holding bar;
- the protruded part of the tool bit having a height 0.5d is disposed at a distal
end from the longitudinal axis of the holding bar.
- the top surface of tool bit is configured at an angle 2° alignment with
horizontal and the front face of tool bit is configured at an angle 2°
alignment with the vertical.

- the short arm of 'L' shape tool bit is affixed with one face at the top of the
holding device.
3) A twin tool device as claimed in claim 2 further comprising a threading tool for
machining of the internal thread of a blind hole of the field core, wherein the
threading tool further comprises: -
- a square based parallelepiped bar having a length of a base in 'd';
- a polygonal shaped tool bit having a triangular shaped upper part and a
square base bottom part;
- the triangular part and a small part of the square base of tool bit in
protruded from the top face of the holding bar and disposed at a distal end
from the longitudinal axis of the holding bar;
- the apex of the triangular part of tool bit is disposed parallel to the
longitudinal axis of the holding bar;
- the front face of the triangular part of tool bit is aligned at an angle 2° with
the vertical line and the apex line of the triangular part of

tool bit is configured at an angle 3° aligned to the horizontal line and the siding
face of the triangular part of tool bit is configured at 60° alignment with
horizontal line.
- the front face of the square base of tool bit remains in the same plane of
the holdings bar.
4) The device as claimed in claim 2 wherein the groove tool bit is affixed with one
face of the holding bar by properly brazing at the contact point.
5) The device as claimed in claim 2 wherein the thread tool bit is affixed with one
face of the holding bar by properly brazing at junction point.

A process of machining an internal thread in a blind hole of field core bar by a twin
tool device, the blind hole being otherwise inaccessible by the standard tool, the field
core being adaptable to a rotor shaft the process comprising setting a groove tool
mounted with a tool holding bar at tool post; machining of internal bore of the blind
hole by the groove tool; replacing the groove tool with a threaded tool; machining an
internal thread on said internal bore by the threading tool; dismantling the threading
tool after completion of said thread machining upto the end wall of internal hole;