FIELD OF THE INVENTION
The present invention relates to a process of modification of lead screw for work
piece adaptor of CNC milling machine to replace
hexagonal head with a replaceable head.
BACKGROUND OF THE INVENTION
Bar type steam turbine blades are manufactured in 5-axis machines from metal bar stocks that is blanksofdifferent rectangular cross-sections& lengths. In single spindle CNC,5-axis milling machines, workpiece holding is realized by holding the blank horizontally in machine’s “A”- axis which can rotate continuously. One end of the blank is clamped by two numbers of rectangular jaws in workpiece adaptor in head stock of the machine. The other end is held by a centre pin in the tailstock (ref. Fig. 2) as well as jaws in some machines.
Rectangular blanks according to size of blade are loaded in the machine with the help of workpiece adaptors as mentioned above.
The locking and unlocking of clamping jaws are actuated by lead screws by rotating in clockwise direction for clamping and vice versa (ref fig. 4).
The lead screw can rotate axially and is locked for any linear motion by a groove mechanism in workpiece adaptor (ref fig. 1).

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Two numbers of clamping jaws are mounted on this lead screw from both sides. Lead screw has threads of opposite hands in its two halves. Therefore lead screw is rotated clockwise or counter clockwise to move the jaws towards each other or vice versa (ref fig. 1). Workpiece adaptors with self-locking jaws are used in CNC milling machines in new blade shop.Thelocking and unlocking of clamping jaws are actuated by lead screws.
For manufacturing of bar type turbine blades, alloy steel blanks of different cross sections and required length are clamped in the head stock by tightening the workpiece adaptor jaws (ref fig. 3 and fig 4, ). These adaptor jaws are in turn clamped by head stock of machine. The blank is clamped from tail stock side and milling operations are carried out to manufacture blades (ref fig. 2, & fig.3, ).
During prolonged usage of the lead screws, the hexagonal bolt heads at both the ends become worn out or more specifically the hexagonal shape becomes circular which makes the tightening or loosening operation by spanner impossible (ref fig.4, page 4). The tightening operation is done by rotating the lead screw by a spanner/ ratchet. As the thread directions are opposite to each other in the 2 halves, rotation causes the mounted jaws to come closer to each other or vice versa simultaneously.
Due to this, it is evident that there remains no other option, but to change the lead screw from the adaptor. The process has two distinct disadvantages:

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1. Although other segments/parts remain usable (such as threads, central slot etc.) The whole lead screw has to be replaced. This is definitely not cost effective.
2. Entire assembly of workpiece adaptor has to be dismantled which consumes much time.
OBJECT OF THE INVENTION
It is therefore an object of the invention is to propose a process of modification of lead screw for work piece adaptor of a Computer Numerical Control (CNC) milling machine to replace hexagonal head with a replaceable head.
SUMMARY OF THE INVENTION
According to the invention, the modification of the lead screw (ref fig. 5, page 5) in this context has been done in the following manner:
- a square slot of 10 mm x 10 mm with depth 10 mm has been machined in both sides of the lead screw (ref fig. 6, page 6; fig 9, page 9; fig. 11, page 11);
- a threaded hole is made on the stud as shown(ref fig. 6, page 6; fig 9, page 9; fig. 11, page 11);
- two numbers of replaceable hexagonal heads with extended blocks of square cross section to fit slots as machined on the lead screw with a counter bore hole for clearance of the bolt (ref fig. 7, page 7; fig 8, page 8; fig 10, page 10; fig 12, page 12).

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- the replaceable heads are thus fitted in the end slot of the modified lead crews and fastened by bolts (ref fig. 8, page 8; fig 12, page 12; fig 13, page 13).
- the square slot ensures maximum torque bearing capacity and low possibility of wear and tear.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE PRESENT INVENTION
1. Twisted profile steam turbine blades are manufactured from alloy steel rectangular bars in CNC5-axis milling machines in one setup. The rectangular bars are called blanks. These are prepared by cutting rectangular bars of 4 to 5 metres of different cross sections and material grades in band saw machines according to required length (blade length plus technological / holding allowances along length for holding purpose in machine).
2. The blanks are loaded in aforesaid CNC5-axis milling machines along a continuously rotating ‘A’ axis of machine (ref fig. 2, page 2; fig 3, page 3)with the help of a clamping chuck/ having two jaws at the head stock side of the machine and with the help of a centre pin at tail stock.Theclamping force applied is approximately 45 bar along ‘A’ axis of the machine.
3. Rectangular blanks according to blade requirement are loaded in the machine with the help of workpiece adaptors as mentioned above.
The locking and unlocking of clamping jaws are actuated by lead screws.

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The mentioned lead screw can rotate axially and is locked for any linear motion by a groove mechanism in workpiece adaptor (ref fig. 1, page 1; fig 4, page 4).
Two numbers of clamping jaws are mounted on this lead screw from both sides. Lead screw has threads of opposite hands in its two halves. Therefore lead screw is rotated clockwise or counter clockwise to move the jaws towards each other or vice versa(ref fig. 1, page 1; fig 4, page 4).
4. During prolonged usage of the lead screws, the hexagonal bolt heads at both the ends become worn out or more specifically the hexagonal shape becomes circular which makes the tightening or loosening operation by spanner impossible (ref fig. 4, page 4). The tightening operation is done by rotating the lead screw by a spanner/ ratchet. As the thread directions are opposite to each other in the 2 halves, rotation causes the mounted jaws to come closer to each other or vice versa simultaneously.
5. Due to this, the whole lead screw has to be replaced when the hexagonal heads wear out.Thisis definitely not cost effective. Also entire assembly of workpiece adaptor has to be dismantled which consumes much time.
6. To overcome this problem, a hexagonal replaceable type head has been designed.
The modification process of the lead screw in this context has been done in the following manner:
- a square slot of 10 mm x 10 mm with depth 10 mm has been machined in both sides of the lead screw (ref fig. 6, page 6; fig 9, page 9; fig. 11, page 11)
- a threaded hole is made on the stud as shown(ref fig. 6, page 6; fig 9, page 9; fig. 11, page 11)

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- two numbers of replaceable hexagonal heads with extended blocks of square cross section to fit slots as machined on the lead screw with a counter bore hole for clearance of bolt(ref fig. 7, page 7; fig 8, page 8; fig 10, page 10; fig 12, page 12).
- the replaceable heads are thus fitted in the end slot of the modified lead crews and fastened by bolts (ref fig. 8, page 8; fig 12, page 12; fig 13, page 13).
- the square slot ensures maximum torque bearing capacity and low possibility of wear and tear.

WE CLAIM :

1. A process for modification of lead screw for work-piece adaptor to replace a
fixed hexagonal head with a replaceable head, the process comprising the
steps of :-
- machining a square slot on both sides of the lead screw;
- configurating a through threaded hole on the stud portion of the screw;
- machining two numbers of replaceable hexagonal heads corresponding to size of said square slots, the heads having extended blocks of square cross section with a counter bore hole; and
- releasably joining the replaceable head on the end slot of the lead screw by inserting the bolts through said counter bore hole.
2. The process as claimed in claim 1, wherein the modified lead screw exhibits
enhanced torque bearing capacity and lower wear and tear.