FIELD OF INVENTION
The present invention relates to a method of machining a large size aero foil cross sectioned twisted impeller blade on a CNC machine employing special cutting techniques. More particularly the invention relates to a developed method to machine a large size twisted impeller blade with varied aero foil contours on both concave and convex sides on a 3-axis CNC machine using special cutting techniques.
BACKGROUND OF INVENTION
An impeller blade of a Hydro pumps is made up of alloy steel with dimensions in the range of: length: 4.5 meters, width: 1.2 to 1.5 meters, thickness: 60 mm and weight: 2000 Kgs as shown in figure 1 and 2. It is difficult to machine such type of large blades, as there is no straight area available on the blade surface to hold it firmly on the CNC machine bed on both concave and convex sides. Improper mounting induces cutting vibrations, which is not advisable to the machine as well as the blade. A special blade holding fixtures have been designed to hold the blade on both sides firmly and reduce vibration during machining operation. Conventional milling techniques like climb up and climb down milling results in cutter collision with blade surface for these types of twisted blades. In order to address this issue, special cutting techniques have been developed. Special cutters have been used for maximum material removal in a single stroke/pass. This invention is giving a way to design and develop a fixture and establish a method to

machine a large impeller blade with the help of a 3 axis CNC Milling machine using special cutting techniques.
PRIOR ART AND ITS DISADVANTAGES:
The patent search has been carried out on this subject, but the complete information related to this invention is not directly available. The following patents (JP63241118A,US8506256B1, CN105904169A, CN106624907A, CN203604257U, CN102107295B,CN103317171B, CN102950316A etc…) related to the present invention were checked and found discrete information either related to milling methods of large impellers or blades of turbines. But there is no mention of special fixtures, cutting techniques for such a large twisted blades. Conventional fixtures cannot hold such a large blade as there is no straight portion available on the blade surface to hold. They cannot control the vibrations induced during cutting / machining process.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a method of machining a large size aero foil cross sectioned twisted impeller blade on a CNC machine employing special cutting techniques which is capable of machining varied contour profiles on both convex and concave side of the blade with reduced vibration using two special fixtures.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Fig. 1 - Shows the concave side of the impeller blade of hydro pump.
Fig. 2 - Shows the convex side of the impeller blade bottom.
Fig. 3 - Shows the special fixture to hold the concave side of the blade.
Fig. 4 - Shows the fixture to hold convex side of the blade.
Fig. 5 - Shows machining of blade concave side using a straight milling cutter (right
hand side) according to the invention.
Fig. 6 - Shows machining of blade concave side using a straight milling cutter (left
hand side) according to the invention.
Fig. 7 - Shows machining of blade concave side using a round milling cutter
according to the invention.
Fig. 8 - Shows machining of blade convex side using a straight milling cutter
according to the invention.

Fig. 9 - Shows machining of blade convex side using a straight milling cutter (right
hand side) according to the invention.
Fig. 10 - Shows machining of blade convex side using a round milling cutter according to the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
The impeller blade is very important part in the Hydro pump. It is large in dimension as shown in Fig. 1, made up of high strength stainless steel, with varied contour profiles on both concave (C1) and convex (C2) side. Initially a casting has been made with extra stock of 50 mm on all dimensions. This casting has to be machined to get the desired blade profile within the acceptable tolerance on a 3 axes CNC milling machine. The surface of the blade (B) is completely curved on both concave (C1) and convex (C2) sides. There is no straight portion available on the surface of the blade. The blade (B) is also prone to induce cutting vibrations due to its geometry. In order to address this issue, two separate fixtures (F1, F2) are designed and developed to hold the blade firmly on both sides of the blade as shown in fig 3 and 4.
The fixtures F1 and F2 consists of four (1, 1a) mild steel blocks placed horizontally. Each piece is welded (W) to another mild steel blocks (2, 2a) placed perpendicular to the horizontal mild steel block (1, 1a).

The supports (6, 4, 7, 3 and 5 and 6a, 4a, 7a, 3a and 5a) of the fixtures (F1, F2) are made of 100 X 100 mm square cross section mild steel beams. This supports of the fixtures (F1, F2) are distributed evenly so that the complete blade surface area is covered and provides support to the blade and reduce vibrations. Since, there is no straight portion available on the blade surface, the supports of the fixtures are machined (8) on a CNC milling machine exactly as per the blade contour, so that the blade gets the maximum support from the fixture when it is mounted on it.
Simulation studies have been conducted on the blade profile to generate tool path without any collision using simulation software. Postprocessor suitable to the 3 axes CNC machine has been developed and CNC part programs for roughing, semi finishing and finishing operation have been developed. Conventional milling technique will result into plunging of the cutter into the blade surface. Hence, two special cutting techniques have been developed for straight cutter (9) and round cutter (10).
For Concave surface (C1) of the blade (B) using a straight cutter (9), passage / cavity (11) is created at the centre of the blade surface by removing the material. For right hand side of the blade (B) the cutter enters from the top end and exit at the centre passage as shown in Fig 5. For left hand side of the blade (B) it enters at the top side of the blade (B) and exits at the centre (12) as shown in Fig 6. This way we can avoid plunging of the cutter into the blade surface.
Using a circular type milling cutter (10) the tool enters at one side of the blade and exits at the other side as Shown in Fig 7.

Before machining, of concave side the blade (B) is placed on the support members (6, 4, 7, 3, 5) of the fixture F-1 with contour of the bottom side of the blade matching with the machine profile of said support members.
The bottom seating portions of the blade are then welded (W2) to the supporting members as an alternate to rigid clamping of the job for machining as it is not possible to clamp the job. The welding causes arresting of vibration during machining. After the machining is complete, the weldings (W2) are cut to release the job from the fixture F-1. The blade (B) is then placed over the fixture F-2 for machining the convex side (C2) of the blade (B). Similar procedure of clamping the job by welding (W2) with support members (6a, 4a, 7a, 3a, 5a) are carried out and then finally machined. The weldings (W2) of the bottom of the blade (B) with the support members of the fixture F-2 are then cut to release the blade (B) from the fixture. The blades are then finally dressed with small grinding wheels to remove the welding spot on the blade, if there is any.

WE CLAIM
1. A method of machining a large size aero foil cross sectioned twisted impeller blade on a CNC machine employing special cutting techniques, the said method comprising the steps of;
providing fixture (F1) on a CNC machine to allow the bottom of blade (B) to rest on machined faces of supporting members (6, 4, 7, 3, 5) of the said fixture;
welding the bottom of the blade (B) with the support members (6, 4, 7, 3, 5) at the place of resting to arrest vibration during machining;
creating a cavity (11) / passage at the centre of the blade surface to allow the cutter to exit after machining without fouling; wherein,
a straight milling cutter (9) enters from the top end and exits at the centre passage to machine profile of both left hand side and right hand side of the blade (B), when after profile machining a circular type milling cutter (10) enters at one side of the blade (B) and exits at the other side, wherein,
the blade (B) is released from the fixture (F1) by cutting the welding and placed on the fixture (F-2) for machining convex portion of the blade employing the said method adopted for concave machining.

2. The method as claimed in claim 1, wherein the fixture (F1, F2) are made up of 100 X
100 mm square cross section mild steel beams.
3. The method as claimed in claim 1, wherein the four horizontal base members (1, 1a)
of fixtures F1 and F2 are welded with three base members (2, 2a) perpendicular to said horizontal members when vertical supporting members (6, 4, 7, 3, 5 and 6a, 4a, 7a, 3a, 5a) are welded to the base members (2, 2a).
4. The method as claimed in claim 1, wherein the top faces of vertical supporting
members (6, 4, 7, 3, 5 and 6a, 4a, 7a, 3a, 5a) are machined to provide a profile (8,
8a) matching with the contour of the concave (C1) and convex (C2) profile of the
blade (B).