FORM 2
THE PATENT ACT  1970
(39 OF 1970)
AND
THE PATENTS RULES  2003

COMPLETE SPECIFICATION
(See Section 10; rule 13)

A FIXTURE TO FACILITATE IN PERFORMING
PITCH MILLING ON THE ROOT OF THE BLADES

TRIVENI TURBINE LTD
an indian company 
of 12A  Peenya Industrial Area 
Banglore-560058.

The following specification particularly describes the present invention and the manner in which it is to be performed.

FIELD OF THE INVENTION:
The present invention relates to the field of fixture for holding blades during machining operation. Particularly  the present invention relates to a fixture that can hold blades during pitch milling operation and thereby helps in achieving a uniform pitch milled surface on the root of the blades.

BACKGROUND OF THE INVENTION:
Figure 1 illustrates a pictorial representation of fir-tree straddle root blade.

Figure 2 illustrates a side view of fir-tree straddle root blade.

Figure 3 illustrates an assembled side view of fir-tree straddle root blades.

It is evident from figure 2 that one side of the root of fir-tree straddle blade has flat face F and other side of the root of fir-tree straddle blade has tapered face T. The tapered face T of one fir-tree straddle root blade is adapted to mate perfectly with flat face F of other fir- tree straddle root blade as shown in figure 3 by means of small tilt provided to the other fir-tree straddle root blade during assembling of fir-tree straddle root blades on to a disc of a rotor.

During machining of the fir tree straddle root blades  an excess material of +0.05mm than the desired thickness is kept on every root of the fir-tree straddle blade purposefully such that the disc of the rotor would not be left with a space that is more than the thickness of the closing blade leaving a gap therein.

Figure 4 illustrates an isometric view of blades assembled on to a disc of a rotor without closing blade.

Due to provision of excess material on the root of the fir-tree straddle blades  the space left out for the closing blade in the disc of the rotor after assembling of all the blades may not be sufficient to accommodate the closing blade. In this context  the closing blade can be accommodated in the space left out in the disc of rotor by means of two methods:
1) By removing the material on the root of the closing blade and bringing the thickness of the closing blade to accommodate in the space left out in the disc of the rotor.
2) The extra space in the disc of the rotor that is required to accommodate the closing blade is provided by removing a material of uniform thickness on the blades.
The uniform thickness of the material that is to be removed on the blades is calculated by the following formula:
Uniform thickness of material that is to be removed on the blades= Extra space in the disc of the rotor that is required to accommodate the closing blade/Number of the blades on which the excess material is to be removed.

The method of removing the material only on the closing blade results in the concentration of stress on the closing blade and probability of failure of the closing blade during the rotation of the rotor is very high.

Therefore  traditionally  turbine OEM’s follow the method of removing uniform thickness of material on multiple blades. To remove uniform thickness of material on the blades  an operation called pitch milling operation is performed.

Figure 5 illustrates a pictorial representation of fir-tree straddle root blade fitted on conventional fixture.

Figure 5(a) illustrates a pictorial representation of locking fir-tree straddle root blade to conventional fixture at one position.

Figure 5(b) illustrates a pictorial representation of locking fir-tree straddle root blade to conventional fixture at other position.

The conventional fixture C is locked to rotary table R of CNC machine. A fir-tree straddle root blade is placed on the conventional fixture C with tapered face T adjoining the conventional fixture C. The fir-tree straddle root blade is fitted on to the conventional fixture C by means of allen screw 2  washer 4 and clamp 6 as shown in figure 5. The pitch milling operation is done in two passes by locking the fir-tree straddle root blade to conventional fixture C in two positions as shown in figure 5(a) and 5(b).

To remove uniform thickness of material from the root of fir-tree straddle blade  the root of the blade should have plane surface. Due to the tapered face T of the root of fir-tree straddle blade adjoining the conventional fixture C  the root of fir-tree straddle blade will not be having a plane surface. The plane surface on the root of fir-tree straddle blade is achieved by rotating the rotary table R and checking the flatness on the root by means of dial gauge. The dial gauge is made to touch at different points on the root of fir-tree straddle blade  if dial stands at zero position without any deflection then the surface of the root of fir-tree straddle blade is flat  if dial deflects from zero position then the surface of the root of fir-tree straddle blade is not flat.

The disadvantages of conventional method of pitch milling are as follows:
a) The fir-tree straddle root blade is not locked firmly to the conventional fixture. Therefore  pitch milling operation on the root of the fir-tree straddle blades cannot be performed at greater depths.
b) It is very difficult to rotate the rotary table R with accuracy.
c) It takes a lot of time to do setup.

Therefore  there is felt a need for development of a fixture to overcome the drawbacks of the prior art.

OBJECTS OF THE INVENTION:
An object of the present invention is to provide a simple fixture to facilitate in performing pitch milling on the root of the blades.

Another object of the present invention is to provide a fixture that can hold the blades firmly during machining operation.

One more object of the present invention is to provide a fixture wherein set up of the blades can be done in less time.

BRIEF DESCRIPTION OF THE DRAWINGS:
The invention will now be described with reference to the accompanying drawings in which:

Figure 1 illustrates a pictorial representation of fir-tree straddle root blade;

Figure 2 illustrates a side view of fir-tree straddle root blade;

Figure 3 illustrates an assembled side view of fir-tree straddle root blades;

Figure 4 illustrates an isometric view of blades assembled on to a disc of a rotor without closing blade;

Figure 5 illustrates a pictorial representation of fir-tree straddle root blade fitted on conventional fixture;

Figure 5(a) illustrates a pictorial representation of locking fir-tree straddle root blade to conventional fixture at one position;

Figure 5(b) illustrates a pictorial representation of locking fir-tree straddle root blade to conventional fixture at other position;

Figure 6(a)  6(b)  6(c) and 6(d) illustrates a pictorial representation of fixture in accordance with the present invention fitted with a fir-tree straddle root blade;

Figure 7 illustrates a top view of rectangular block of figure 6(a); and

Figure 8 illustrates a side view of rectangular block of figure 6(a).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:
Figure 6(a)  6(b)  6(c) and 6(d) illustrates a pictorial representation of fixture in accordance with the present invention fitted with a fir-tree straddle root blade.

Figure 7 illustrates a top view of rectangular block of figure 6(a).

Figure 8 illustrates a side view of rectangular block of figure 6(a).

In accordance with the present invention  there is provided a fixture comprising a rectangular block 12  two allen screws 14  a washer 16  plurality of grub screws 18  two pins 20 and a clamp 22.

The length  breadth and height of the rectangular block 12 is of 168mm  126mm and 90mm respectively and is generally indicated by reference numerals X1  Y1 and Z1 respectively. The rectangular block 12 includes a slot S of L-shaped cross-section provided at one edge of the block 12 as shown in figure 8. The width and depth of the slot is of 40mm and 45mm respectively and is generally indicated by reference numerals W1 and D1 respectively. Three equidistantly spaced through threaded holes H1 of 16mm diameter and separated by a distance of 32mm are provided in parallel to each other on the slot S of the rectangular block 12 at a distance of 31mm and 20mm from the horizontal edge and vertical edge of the rectangular block 12 respectively as shown in figure 7 and is generally indicated by reference numerals D2  Y2  Y3 and X2 respectively. Plurality of grub screws 18 are inserted in to equidistantly spaced through threaded holes H1 of the rectangular block 12. Three equidistantly spaced threaded holes H2 of 8mm diameter  22mm depth and separated by a distance of 30mm are provided in parallel to each other on the top surface of the rectangular block 12 at a distance of 50mm and 30mm from the horizontal edge and vertical edge of the rectangular block 12 respectively as shown in figure 7 and is generally indicated by reference numerals D3  D4  X3  Y4 and X4 respectively. Two holes H3 separated by a distance of 15mm are provided on either side  perpendicular to the extreme threaded hole H2 in form of a T-shape and is generally indicated by a reference numeral Y5. Two pins 20 are inserted in to the holes H3 to provide butting to fir-tree straddle root blade. A threaded hole H4 is provided on the side of the rectangular block 12 at a distance of 25mm from the top surface as shown in figure 8 and is generally indicated by a reference numeral Z2.

A fir-tree straddle root blade is held to butt against two pins 20 and hold firmly in position by means of allen screw 14 passing through washer 16 and threaded hole H2 of the rectangular block 12 such that the washer 16 adjoins the fir-tree straddle root blade tightly. A clamp 22 is adapted to butt against the fir-tree straddle root blade from the bottom by means of allen screw 14 passing through the clamp 22 and threaded hole H4 of the rectangular block 12.

The method of working of the fixture in accordance with the present invention is as follows:
The fixture is placed on the bed of the CNC machine. A fir-tree straddle root blade placed on the fixture with tapered face T of the fir-tree straddle root blade mating with the top surface of the rectangular block 12 is adapted to butt against two pins 20 and locked by means of allen screw 14 and washer 16. The fir-tree straddle root blade is supported from the bottom by means of a clamp 22. As tapered face T of the fir-tree straddle root blade abuts the rectangular block 12  the flat face F of the fir-tree straddle root blade will not be having a plane surface. Plane surface on the flat face F of the fir-tree straddle root blade is required for removal of uniform thickness of material. Plane surface on the flat face F
of the fir-tree straddle root blade is achieved by adjusting the plurality of grub screws 18 to and fro and checking the flatness on the surface of the fir-tree straddle root blade by means of a dial gauge. The pitch milling operation on the root of the fir-tree straddle blades is done in two passes by locking the blades to the fixture in two positions as shown in figure 6(c) and figure 6(d) respectively.

Although the invention has been described herein above with reference to the embodiments of the invention  the invention is not limited to the embodiments described herein above. It is to be understood that modifications and variations of the embodiments can be made without departing from the spirit and scope of the invention.

We claim:
1) A fixture to facilitate in performing pitch milling on the root of the blades  said fixture comprising:
a) a rectangular block having a L-shaped slot provided at one edge of the block  said block provided with a plurality of equidistantly spaced through threaded holes on the base of said L-shaped slot  said block provided with a plurality of equidistantly spaced threaded holes on its top surface  said block provided with two holes on either side  perpendicular to said extreme threaded hole on the top surface in the form of a T-shape  said block provided with a threaded hole on its side surface at a predefined depth;
b) two pins inserted in to said two holes provided on the top surface of said block facilitates in providing butting to said blades;
c) a clamp attached to said block by means of an allen screw passing through said clamp and said hole provided on the side surface of said block facilitates in providing butting to said blades from bottom;
d) a washer inserted in to an allen screw  said allen screw in turn inserted in to said threaded hole on the top surface of said block  wherein said washer abuts and locks the root of said blades firmly; and
e) a plurality of grub screws inserted in to said threaded holes provided on the base of said L-shaped slot of said block  said grub screws provided in a position to move forward and backward in said threaded holes by means of rotation of said grub screws facilitates in achieving a plane surface on the root of said blades during said pitch milling operation.

2) A fixture to facilitate in performing pitch milling on the root of the blades as claimed in claim 1  wherein the length  breadth and height of said fixture is of 168mm  126mm and 90mm respectively.

3) A fixture to facilitate in performing pitch milling on the root of the blades as claimed in claim 1  wherein the width and depth of said L-shaped slot of said block is of 40mm and 45mm respectively.

4) A fixture to facilitate in performing pitch milling on the root of the blades as claimed in claim 1  wherein said plurality of equidistantly spaced through threaded holes on the base of said L-shaped slot are provided at a distance of 31mm and 20mm from the horizontal edge and vertical edge of said block respectively.

5) A fixture to facilitate in performing pitch milling on the root of the blades as claimed in claim 1  wherein said plurality of equidistantly spaced through threaded holes on the base of said L-shaped slot are separated by a distance of 32mm from each other.

6) A fixture to facilitate in performing pitch milling on the root of the blades as claimed in claim 1  wherein plurality of equidistantly spaced threaded holes on the top surface of said block are provided at a distance of 50mm and 30mm from the horizontal edge and vertical edge of said block respectively.

7) A fixture to facilitate in performing pitch milling on the root of the blades as claimed in claim 1  wherein plurality of equidistantly spaced threaded holes on the top surface of said block are separated by a distance of 30mm from each other.

8) A fixture to facilitate in performing pitch milling on the root of the blades as claimed in claim 1  wherein said predefined depth of said threaded hole on the side surface of said block is of 25mm.

9) A fixture to facilitate in performing pitch milling on the root of the blades as claimed in claim 1  wherein the diameter of said grub screws is of 16mm.

10) A fixture to facilitate in performing pitch milling on the root of the blades as claimed in claim 1  wherein the diameter of said allen screws is of 8mm.

Dated this 16th day of August  2012 (for Triveni Turbine Ltd)


Dr.Sunil Jajit
GM-IPR