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Title:
A METHOD OF MANUFACTURING NEAR-NET SHAPE CROWN WHEEL
Document Type and Number:
WIPO Patent Application WO/2018/055484
Kind Code:
A1
Abstract:
Invention relates to a method of producing a crown wheel with near-net shaped as-forged teeth having continuous grain flow lines and having minimal machining allowance. The method of invention comprises the steps of making ring using hammer forging and ring rolling technique followed by blocker and finisher forging step for manufacturing of gear teeth on the ring and after which a cold coining operation is performed on the finished forging to achieve near net shape of gear teeth. The cold-coining operation allows a reduced machining allowance of up to 0.3mm but with minimal dimensional variation of teeth. These near net shaped teeth's are further finished to final shape by grinding operation. The forging of near net shape gear teeth profile is carried out using any type of press capable of applying required force. Representative Figure: Figure 3

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Inventors:
KALYANI BABASAHEB NEELKANTH (IN)
KALYANI BASAVRAJ PRABHAKAR (IN)
TAKALE MADAN UMAKANT (IN)
Application Number:
PCT/IB2017/055544
Publication Date:
March 29, 2018
Filing Date:
September 14, 2017
Export Citation:
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Assignee:
BHARAT FORGE LTD (IN)
KALYANI BABASAHEB NEELKANTH (IN)
KALYANI BASAVRAJ PRABHAKAR (IN)
TAKALE MADAN UMAKANT (IN)
International Classes:
B23F15/00; B21D22/00; B21D53/28; B21H5/00; B21H5/02; B21J1/06; B21J5/02; B23P15/14; C21D9/32
Foreign References:
US20010001165A12001-05-17
US4798077A1989-01-17
US2894519A1959-07-14
CN102011852A2011-04-13
Attorney, Agent or Firm:
TASE, Vijay (IN)
Download PDF:
Claims:
Claims:

1. A method of manufacturing a crown wheel (1), characterized in that the said method comprises the steps of:

a. heating a billet from which said crown wheel is to be forged;

b. upsetting said heated billet to produce an upset preform (3):

c. piercing and punching the upset preform (3) leading to pierced and punched disc termed as a pierced preform (4);

d. ring-rolling said pierced preform (4) leading to a ring roiled disc termed as ring-rolled preform (5);

e. blocker forging the said ring-rolled preform (5) to obtain a blocker preform (6);

f. descaling said blocker preform (6) by subjecting it to a high pressure scale removal operation to obtain a descaled blocker preform (6A);

g. finish forging said descaled preform (6A) to obtain a near net shaped finish forged part (7) having near net shaped as forged teeth (7 A),

h. heat treating (iso-annealing) said near net shaped finish forged part (7)in inert atmosphere to obtain a heat-treated finish forged part (8);

h. subjecting said heat-treated finish forged part (8) to post forging operations to obtain a treated finish forged part (9);

i. cold coining said treated finish forged part (9) to obtain a cold coined part (10) having a deformation in the range of 0.2 to 0.6 mm in the teeth profile, and a machining allowance of 0.3mm; j . removing the flash formed during the blocker forging and finish forging operations using a machining operation to obtain a flashless part (11); k. subjecting said flashless part (11) to a crown wheel machining operation wherein all the surfaces of the crown wheel, except the teeth, are machined to obtain machined part (12);

1. subjecting said machined part (12) to a gear teeth grinding operation wherein the surfaces of the teeth of said machined part (12) are subjected to grinding process to obtain a near-final crown wheel (13);

m. subjecting said near-final crown wheel (13) to a post grinding heat treatment wherein said part (13) is heat treated using a press quenching method to obtain a final crown wheel (14).

2. A method as claimed in claim 1, characterized in that, in said step of billet heating, the heating of said billet is carried out in a furnace at a temperature range of 1200-1280 °C for a soaking time sufficiently long to achieve a uniform temperature in the heated billet.

3. A method as claimed in claims 1 or 2, characterized in that in said step of upsetting, said heated billet is placed on a bottom upsetting die and the upsetting operation is carried out using a forging equipment whereby said upset preform (3), which is a flat disc, of dimensions required for said piercing and punching operation, is produced.

4. A method as claimed in any of claims 1 to 3, characterized in that in said piercing and punching step, a hole (2) is made in said upset preform (3) by punching on said upset preform (3), followed by carrying out a piercing operation on the punched upset preform (3 A), thereby forming said pierced preform (4).

5. A method as claimed in any of claims 1 to 4, characterized in that in said step of ring-rolling, said pierced preform (4) is ring rolled in a ring rolling press to obtain said ring-rolled preform (5) of a disc shape.

6. A method as claimed in any of claims 1 to 5, characterized in that in said step of blocker forging, said ring-rolled preform (5) is forged in a forging equipment to obtain said blocker preform (6).

7. A method as claimed in any of claims 1 to 6, characterized in that in said step of descaling, the operating pressure is in the range of 300 to 400 bar.

8. A method as claimed in any of claims 1 to 7, characterized in that said finish forging operation is carried out using a forging equipment capable of producing the energy and handling force produced during the operation to produce said finish forged part (7) having a near-net shaped gear tooth profile (7A).

9. A method as claimed in any of claims 1 to 8, characterized in that in said step of providing post-forging treatment, the post-forging treatment comprises the steps of shot blasting and crack detection, leading to said treated finish forged part (9).

10. A method as claimed in any of claims 1 to 9, characterized in that said step of cold-coining, the cold coining operation is performed on said treated finish forged part (9) in a forging equipment, preferably a hydraulic press, to produce said cold-coined part (10) that produces a deformation in the range of 0,2 mm to 0.6 mm in said near-net shaped gear tooth profile (7 A).

1 1 . A method as claimed in any of claims 1 to 10, characterized in that said cold- coined part (10) is further subjected to a flash removal operation by machining to obtain a flashless part (11).

12. A method as claimed in any of claims 1 to 11, characterized in that said flashless part (11), except its teeth, is further subjected to machining to obtained a machined part (12).

13. A method as claimed in any of claims 1 to 12, characterized in that teeth of said machined part (12) are subjected to a grinding process to obtain a near- final crown wheel (13).

14. A method as claimed in any of ciaims 1 to 13 characterized in that said near- final crown wheel (13) is further subjected to heat treatment using a press- quenching method to obtain a final crown wheel (14).

15. A cold-coined crown wheel obtained using steps a to j of the process as claimed in claim 1, whereby said cold-coined part (10) has a deformation in the range of 0,2 mm to 0,6 mm in said near-net shaped gear tooth profile (7 A), and a machining allowance of 0.3mm,

Description:
A Method Of Manufacturing Near-Net Shape Crown Wheel

Field Of Invention

This invention relates to a method of producing a crown wheel. In particular, the present invention relates to production of a crown wheel with forged teeth having continuous grain flow lines and having minimal machining allowance.

Background Of The Invention

A crown wheel is commonly used in power transmission applications. In a motor vehicle, drive axles typically employ gear set comprising a drive bevel pinion and a crown wheel. These gears are of hypoid or spiral type. Manufacturing with high degree of accuracy and finishing is one of the major requirements for smooth operation of this assembly.

Conventionally, crown wheels, in particular for vehicle drive axle, are manufactured by starting with a forged blank without teeth. The teeth are then cut in this blank using special gear cutting machines. The crown wheel is then hardened and finished by lapping with the mating pinion. Lapping is a super- finishing operation which gives better surface finish to gear teeth and is helpful for better mating with drive pinion. Disadvantages of this process are excessive machining time and requirement of expensive gear cutting machine. Also, cutting tools have shorter life and are expensive to replace. Moreover, trimming of forged blank and cutting of teeth wastes large amount of the material. The conventional method of teeth cutting using machining is the most commonly used method of producing crown wheel even though it has many disadvantages and limitations as described above.

Another method of producing the crown wheel is to produce as-forged teeth during the forging operation itself. These as-forged teeth are given a large machining allowance (1 to 3 mm) which is subsequently removed using multiple machining operations. The normal process of machining for such crown wheels (with 1-3 mm machining allowance) consists of rough machining - finish machining - grinding of the as forged teeth.

Whenever, it has been tried to reduce the machining allowances on the as-forged and heat treated crown wheel, following technical difficulties have been faced by a forge master:

1. Scale formation - Hot forging process normally used for the production of crown wheel is carried out at temperatures higher than 1000 °C for steel parts. At this temperature heavy oxidation of the metal surface takes place which leads to heavy scaling of the part. After cooling of the hot forged parts, this scales/oxidized metal come loose from the part surface and leave scale pits. Heat treatment is also normally carried out at high temperatures (>800 °C). Hence, similar scaling/surface oxidation phenomenon is observed when the crown wheel is heat treated. Hence, enough machining allowance has to be given to the part for removal of these scale pits. 2. Fill up of the Finisher cavity - It is very difficult to ensure the complete fill up of the finisher die cavity in which the final shape of the as-forged teeth are formed. Higher machining allowances allow salvaging of light under fills during the forging operation.

3. Accuracy of Die cavity - Lower machining allowances also lead to reduced tolerances on the machined cavity in the finisher dies. The normal machines and methods used for die machining cannot maintain these tolerances and hence, hinders the process of reduction in machining allowances in the forged part.

Reducing the machining allowances have been an aim of the manufacturers as this will reduce the machining time as well as operations (elimination of rough machining and finish machining of teeth) and hence will lead to reduction in cycle time as well as the cost of the crown wheel manufacturing.

The present invention discloses a method of manufacturing of a crown wheel, and in particular a method of making of near net shape as-forged spiral or helical gear teeth using a forging and cold coining process which significantly reduces the machining allowances on the teeth thus, eliminating several machining process steps.

Summary of Invention

In accordance with the present invention, the drawback of prior art are overcome by the method of manufacturing of crown wheel with near net shape as-forged teeth. The near net shape of gear tooth is achieved by performing cold coining operation after finish forging operation. The cold coining operation reduces the machining allowance up to 0.3mm with minimal dimensional variation of teeth. This invented method allows considerable material and energy saving as compared with the prior art method. Also, strength of the produced crown wheel is higher due to continuous grain flow in comparison with conventional methods.

The present invention consists of a method of manufacturing crown wheel comprising the steps of making ring using hammer forging and ring rolling technique followed by blocker and finisher forging step for manufacturing of gear teeth on the ring and after that cold coining operation is performed on the finished forging to achieve near net shape of gear teeth. These near net shaped teeth's are further finished to final shape by grinding operation. Preferably the ring is manufactured using hammer forging and ring rolling operations, which gives better yield and better strength with continuous grain flow lines across the contours. The forging of near net shape gear teeth profile is carried out using any type of press capable of applying required force.

Objects Of Invention

Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows:

An object of the present invention is to provide a method for forging of crown wheel. It is another object of the present invention to produce as forged spiral or helical gear teeth profile on forged ring.

It is yet another object of the present invention to provide better mechanical properties and grain flow in teeth profile.

It is another object of present invention is to produce near net shape gear teeth profile by using cold coining operation with minimal dimensional variation

It is yet another object of the invention to reduce the machining (gear teeth cutting) time and cost required during the manufacturing of the crown wheel.

It is yet another object of the present invention to reduce the machining allowance on the as-forged teeth.

It is yet another object of the present invention to eliminate the rough and finish machining operation for the as-forged gear teeth.

Brief Description Of Figures

Figure 1 shows first method of conventional process sequence of manufacturing of crown wheel.

Figure 2 shows second method of conventional process sequence of manufacturing of crown wheel.

Figure 3 shows the invented process sequence of crown wheel manufacturing Figure 4(A) shows the top view and Figure 4(B) shows the 3D view of the crown wheel (final product).

Figure 5(A) shows the top view and Figure 5(B) shows the 3 D view of the upset billet.

Figure 6 shows the 3D view of the pierced and punched disc.

Figure 7 shows the 3D view of the ring rolled disc

Figure 8(A) shows the 3D view and Figure 8(B) shows the section view of the blocker preform shape.

Figure 9(A) shows the top view and Figure 9(B) shows the 3D view of the finish forged part.

LIST OF PARTS

1. Crown wheel

2. Central hole

3. Upset preform

3 A. Punched upset preform

4. Pierced preform

5. Ring-rolled preform

6. Blocker preform

6A. Descaled blocker preform

7. A finish forged part

7A. As-forged teeth

8. Heat Treated finish forged part 9. Treated finish forged part

10. Cold-coined part

11. Flashless part

12. Machined part

13. Near-final crown wheel

14. Final crown wheel

Detailed Description Of Invention

The present invention is related to the method of manufacturing a crown wheel used in automobile and industrial applications.

The key inventive feature of the present invention are the continuous grain flow and the consequent high strength achieved due to near net shaped as-forged gear teeth achieved during manufacturing of the crown wheel. Further the process design developed in this invention allows for minimum machining allowance (in the range of 0.2 - 0.5 mm) which eliminates the rough and finish machining on the crown wheel teeth.

According to the invented process, the manufacturing process starts with a forged billet as raw material. This billet is converted into the final shape of the crown wheel by taking it through various intermediate processes.

As shown in Figure 4, a crown wheel (1) has ring type structure with a big hole in the centre (2). When the crown wheel (1) is made with the conventional forging process of upset, and blocker and finisher steps, lot of material at the centre of the part has to be discarded during the piercing operation. Moreover, large amount of material flow caused during the blocker and finisher operation, which tries to push material from the centre towards the periphery, causes large wear and tear of the dies and tooling's in which the forging is being done.

To overcome these problems, this invention discloses the use of a ring rolling operation as one of the steps in the forging process flow. The output of the upset operation, an upset preform (3), is shown in Figure 5. After the upset operation, piercing of the upset preform (3) is done in order to remove some material in the centre. This pierced preform (4, see Figure 6) is then put in the ring rolling machine for next process which produces a rolled ring or ring-rolled preform (5). The ring rolling machine basically consists of three rollers of which two rollers maintain the height of the ring while the third roller increases the diameter of the part. The ring-rolled preform (5, see Figure 7) is then provided as input to the blocker operation.

The use of ring rolling gives following benefit:

1. This significantly reduces the loss of material which was happening in the conventional method. This reduces the input material requirement.

2. This also gives better material distribution which leads to better material flow in blocker and finisher. This further reduces the input material requirement. 3. The better material distribution and flow, as explained above, also reduces the wear and tear of the blocker and finisher dies leading to increased die life.

The ring rolling operation is followed by a blocker operation to produce a blocker preform (6, see Figure 8) which is followed by the finisher operation to produce a finish forged part (7, see Figure 9). The ring rolled preform (5) has some inherent defects on its surface called wrinkles. If the ring rolled preform (5) is directly given as input to the finisher, these wrinkles lead to formation of cracks and cold shuts in the finished parts. To avoid this problem, as an aspect of the invention, a blocker operation has been introduced before the conventional finisher operation. The purpose of the blocker operation is to iron out the wrinkles thus avoiding any cracks or coldshuts in the final part. The advantages of introduction of blocker are as follows:

1. The blocker operation improves the material flow during the forging process and hence reduces the wear and tear of the finisher die. This helps in getting a consistent teeth profile in the final part.

2. The blocker operation irons out all the surface defects formed during the ring rolling process like wrinkles and thus completely avoids formation of the cracks and coldshuts (MPI indications) in finished part.

It should be noted that rough teeth shape is not formed in the blocker dies as any small mismatch in the blocker and finisher impression will lead to formation of multiple coldshuts and cracks on the teeth surface. Based on the finisher die cavity shape, the blocker die cavity shape has to be designed. The shape of the blocker die cavity is designed to give required material distribution and shape to the ring rolled preform.

According to another aspect of this invention, the finisher dies are designed in such a way that, this will produce required teeth profile in the as-forged condition (see Figure 9). For this to happen, the impression in top die is designed in such a way that it has cavities shaped similar to the teeth profile of the crown wheel. The design of the teeth profile in the finisher dies is a very critical process. The shape of the cavities has to be designed in such a way that it has compensation for the forging temperature, shrinkage, scale loss, non-uniform temperature distribution, coining allowance, machining allowance, warpage allowance etc.

An iterative simulation approach for evaluation of numerous manufacturing concepts was used to optimize the part geometry of near net shape, forging die design and manufacturing process sequence using virtual manufacturing techniques. Forging part geometry i.e. near net shape and process was optimized using 3D metal flow simulation. Based on simulation results, an optimal die design and process sequence was developed for manufacturing crown wheel.

The formation of the teeth profile in finisher operation gives following benefits:

1. This significantly reduces the machining to be done for teeth formation.

This reduces the machining time as well as input material requirement. 2. As forged teeth profile ensures that the grain flow lines remain continuous along the contour of gear teeth profile and are not broken in the machining operation. This significantly enhances the strength of the teeth which leads to better performance of the crown wheel with higher service life.

One important aspect of this invention is to provide minimum machining allowances in the finish forged crown wheel teeth. Conventionally a machining allowance of 1 to 3 mm is kept in the finish forged crown wheel's teeth. This high amount of machining allowance is required to compensate for the scale pit and oxidation which forms during the hot forging operation. Further if the machining allowance is reduced, the fill up of the finisher die cavity becomes utmost important. Even a very slight underfill on as-forged teeth will lead to rejection of the part due to insufficient machining allowance. Generally, in the finisher operation light underfill happens due to the entrapment of coolant/lubricant or gasses in between the material being forged and the die cavity surface. Moreover the machining method used for the manufacturing of the finisher dies is not accurate enough to achieve the level of accuracy required to reduce the machining allowances in the crown wheel teeth. To overcome these technical difficulties following steps were added in the invented process:

1. Machining of the Finisher dies: In order to maintain the accuracy of the teeth profiles in the finisher operation, high grade machining equipment has to be used. The capability of this machine has to be as follows: Spindle accuracy should be <5μπι; the machine should be a direct drive machine and not belt or gear driven. During the machining of the finisher dies the process parameters should be as follows: Spindle speed should be between 50,000 to 1, 00,000 RPM; feed rate should be 5 to 8 m/min.

Provision for vent holes in the finisher die cavity - As the machining allowances on the as-forged teeth in the present invention is very less (-0.3 mm), the fill up of the finisher die cavity is of utmost importance. Even a very slight underfill on as-forged teeth will lead to rejection of the part. Generally in the finisher operation light underfill happens due to the entrapment of coolant/lubricant or gasses in between the material being forged and the die cavity surface. This entrapment is avoided in the finisher die using an innovative concept of providing a vent hole of 0.3- 1.5mm from the deep cavity surface (as-forged teeth cavity in this case) to the bottom surface of the finisher dies. This vent hole lets the entrapped gas/lubricant/coolant to escape thus allowing complete fill up of the die cavity.

Scale removal prior to finisher forging - Conventional method of hot forging uses only one scale removing operation and that is done after the billet heating operation in the furnace. Even after this scale removal, more scale formation takes place during the blocker and finisher operation which later leads to scale pit in finish forged crown wheel. To overcome this, a special high pressure jet scale remover is used just prior to the finish forging operation. This removes the scale present on the surface of the blocker preform. A special scale removal machine having a operating pressure between 350 to 400 bar is used for this operation.

The finisher operation is followed by flash trimming operation which is followed by post forging operations which also includes the heat treatment process (iso- annealing). The trimming operation is done at high temperature which leads to distortion in the finish forged part. This distortion later leads to problems in machining. This is overcome in the conventional method by keeping larger machining allowances (1 to 3 mm) on the crown wheel teeth. According to another aspect of the present invention, the hot trimming operation is not done after the finisher forging operation. The excess flash produced in finisher operation is removed through flash machining after finisher operation or the cold coining operation.

Again, the heat treatment process (i so-annealing) is carried out at high temperature and hence, scale pit issue is generated by it. In the conventional method of heat treatment where a normal furnace is used a scale pit of 0.1 to 0.6 mm is formed on the surface of crown wheel. This variable thickness of scale pit leads to unequal coining allowance for the next operation. For keeping the machining allowance as low as 0.2-0.5 mm (preferably 0.3 mm) this scale pit formation has to be avoided. According to another aspect of this invention, the heat treatment operation is carried out in a closed pit furnace having an inert atmosphere. This avoids the formation of scale pit during this process.

The teeth profile formed in the conventional hot forging process is not very precise and accurate due to the inherent nature of the forging process. It is found that the machined crown wheel teeth profile gets distorted during machining and post machining heat treatment. This minor distortion leads to inappropriate contact pattern between crown wheel and mating pinion leading to premature fatigue failure in service.

To overcome this technical difficulty of the conventional processes, in one aspect of the invention, a cold coining operation is introduced in the manufacturing process of the crown wheel. The cold coining operation has following benefits:

1. The minor cold plastic deformation done during the cold coining operation produces very precise and accurate teeth profile.

2. Due to this, during machining process uniform machining allowance is present along the complete profile of the teeth. This significantly reduces the distortion of teeth during machining.

3. The uniform machining allowance also reduces the uneven residual stress pattern generated during the machining process thus significantly reducing the distortion during post machining heat treatment process. Due to all the above steps, it becomes possible to keep a machining allowance of

0.2 to 0.5 mm (preferably 0.3 mm) on the as-forged teeth of crown wheel. Due to this, the conventional machining steps of rough teeth machining and finish teeth machining are completely eliminated. After the cold coining operation only teeth grinding has to be done to achieve the final shape, size, finishing of the crown wheel teeth. After the teeth grinding, press quenching heat treatment is done followed by hard facing and ID (internal diameter) grinding.

The crown wheel (1) showed in Figure 4 is the final finished product which goes into the assembly in the vehicle. The crown wheel shown in Figure 9 is the output of the finisher operation, or in other words it is as-forged crown wheel (7). A nominal machining allowance is left on the as-forged crown wheel. During the machining operation this allowance is removed giving it the required surface finish and geometric tolerances and the holes shown in Figure 4 are produced.

Crown wheel manufacturing process is disclosed in the following sections.

As shown in Figure 3, the invented crown wheel manufacturing process typically involves the following steps:

1. Billet Heating: A billet of the required material chemistry is taken for this process. The section and length of the billet taken for this operation is predefined based on the material requirement for the part to be produced. This billet is heated in the furnace in the temperature range of 1200-1280°C for sufficient soaking time to achieve uniform temperature in the billet. Upsetting: This operation is carried out on the heated billet of step 1 using forging equipment for e.g. in the hammer. Two flat dies of predefined dimensions are used for the upsetting operation. The heated billet is placed on the bottom die and then upsetting operation is carried out. It is the operation to reduce the height and increasing the diameter so as to form a flat disc of dimensions required for next operation. The upsetting step results into an upset preform (3) as shown in Figure 5.

Punching and piercing: In this process firstly a blind hole or simply a hole (2) is made in the upset preform (3) of step 2 by punching on the disc to produce a punched upset preform (3 A), which is further subject to a piercing operation. This step leads to a pierced and punched disc termed as pierced preform (4) as shown in Figure 6. The pierced preform (4) has a ring type shape. The outer as well as the inner diameter of the pierced preform (4) is not as per the final dimensional requirement of the crown wheel. Hence, this pierced disk has to be further processed in a ring rolling mill.

Ring rolling: The pierced preform (4) disc is transferred to the ring rolling press. The ring rolling operation is carried out using three rolls. Two rolls maintain the height of the pierced preform (4) during the rolling operation while the third roll increases the internal diameter of the pierced preform (4). At the end of the ring rolling process, a ring with required dimensions is produced also termed as a ring-rolled preform (5) as shown in Figure 7. Blocker forging: The ring-rolled preform (5) is further forged in forging equipment (hammer, for example) in order to obtain the blocker preform (6) of the required shape as shown in Figure 6. Blocker preform descaling: The blocker preform is then descaled using a high pressure jet scale remover. An operating pressure in the range of 350 to 400 bar is used for this. The product resulting from this step is a descaled blocker preform (6A). Finish forging with as-forged teeth formation: The descaled blocker preform (6 A) is subjected to a finisher (or finish forging) operation on a forging equipment capable of producing required energy and handling force produced during the operation. A near-net shape gear teeth profile is forged by finisher forging operation. This operation produces a finish forged part also termed as a finish forged part (7) as shown in Figure 9. The finish forged part has near- net shaped as-forged teeth profile (7A) having a minimum machining allowance of 0.2 to 0.5 mm. Heat treatment - The finish forged part is next heat treated (i so-annealing) in a closed pit furnace having inert atmosphere. This process not only imparts the required microstructural and mechanical properties to the finish forged part but also avoids formation of scale pit during heat treatment. The output of this operation is termed as heat-treated finish forged part (8). Post forging treatments: After the heat treatment operation, post forging operations are conducted on the heat-treated finish forged part (8) leading to a treated finish forged part (9). These operations include shot blasting, crack detection etc.

Cold coining: The treated finish forged part (9) is subjected to cold coining operation on forging equipment, preferably a hydraulic press. In the cold coining operation a final very small deformation, in the range of 0.2 to 0.6 mm, is given to the teeth profile (denoted by 7 A in Figure 9) to correct any distortion present in them. The resultant product is a cold-coined part (10). The cold-coining operation is carried out such that the machining allowance on the cold-coined part is approximately 0.3 mm. Flash machining - The flash formed during the blocker and finisher operation is removed by machining operation. This gives a flashless part (11). Crown wheel machining - In this operation all the surfaces of the flashless part (11), except the teeth, are machined as per the machined part dimensional requirement to obtain a machined part (12).

Gear teeth grinding: The teeth surfaces of the machined part (12) are subjected to grinding process to obtain the near-final crown wheel (13) (as shown in Figure 4).

Post-grinding heat treatment: The near-final crown wheel (13) is given heat treatment through press quenching method to obtain the final crown wheel (14). This method avoids distortion of the final crown wheel (14). The hole (2) made in the punching and piercing operation evolves into a larger size through the various steps of manufacture, and the final crown wheel is as shown in Figure 4.

ADVANTAGES OF INVENTION

1. This new product will ensure the integrity of the metal flow lines and improved strength.

2. This forging product can save material, cost and resources.

3. The invented process will reduce the rejection of parts due to cold shuts, cracks etc.

4. This process also produces parts having better fatigue performance under the service condition as it has better contact pattern with the mating pinion drive.

5. This process eliminates the rough machining and finish machining operations of the as-forged gear teeth thus, reducing the cycle time and cost for machining.

It is evident that the invention comprises a number of embodiments.

1. A method of manufacturing a crown wheel (1), characterized in that the said method comprises the steps of:

a. heating a billet from which said crown wheel is to be forged;

b. upsetting said heated billet to produce an upset preform (3); c. piercing and punching the upset preform (3) leading to pierced and punched disc termed as a pierced preform (4);

d. ring-rolling said pierced preform (4) leading to a ring rolled disc termed as ring-rolled preform (5);

e. blocker forging the said ring-rolled preform (5) to obtain a blocker preform (6);

f. descaling said blocker preform (6) by subjecting it to a high pressure scale removal operation to obtain a descaled blocker preform (6 A);

g. finish forging said descaled preform (6A) to obtain a near net shaped finish forged part (7) having near net shaped as forged teeth ( A),

h. heat treating (iso-annealing) said near net shaped finish forged part (7)in inert atmosphere to obtain a heat-treated finish forged part (8);

b. subjecting said heat-treated finish forged part (8) to post forging operations to obtain a treated finish forged part (9);

c. cold coining said treated finish forged part (9) to obtain a cold coined part (10) having a deformation in the range of 0.2 to 0.6 mm in the teeth profile, and a machining allowance of 0.3mm;

d. removing the flash formed during the blocker forging and finish forging operations using a machining operation to obtain a flashless part (11); e. subjecting said flashless part (11) to a crown wheel machining operation wherein all the surfaces of the crown wheel, except the teeth, are machined to obtain machined part (12); f. subjecting said machined part (12) to a gear teeth grinding operation wherein the surfaces of the teeth of said machined part (12) are subjected to grinding process to obtain a near-final crown wheel (13);

g. subjecting said near-final crown wheel (13) to a post grinding heat treatment wherein said part (13) is heat treated using a press quenching method to obtain a final crown wheel (14).

method as disclosed in embodiment 1 , characterized in that, in said step of billet heating, the heating of said billet is carried out in a furnace at a temperature range of 1200-1280 °C for a soaking time sufficiently long to achieve a uniform temperature in the heated billet.

A method as disclosed in embodiments 1 or 2, characterized in that in said step of upsetting, said heated billet is placed on a bottom upsetting die and the upsetting operation is carried out using a forging equipment whereby said upset preform (3), which is a flat disc, of dimensions required for said piercing and punching operation, is produced.

A method as disclosed in any of embodiments 1 to 3, characterized in that in said piercing and punching step, a hole (2) is made in said upset preform (3) by punching on said upset preform (3), followed by carrying out a piercing operation on the punched upset preform (3 A), thereby forming said pierced preform (4).

A method as disclosed in any of embodiments 1 to 4, characterized in that in said step of ring-rolling, said pierced preform (4) is ring rolled in a ring rolling press to obtain said ring-rolled preform (5) of a disc shape. A method as disclosed in any of embodiments 1 to 5, characterized in that in said step of blocker forging, said ring-rolled preform (5) is forged in a forging equipment to obtain said blocker preform (6).

A method as disclosed in any of embodiments I to 6, characterized in that in said step of descaling, the operating pressure is in the range of 300 to 400 bar. A method as disclosed in any of embodiments 1 to 7, characterized in that said finish forging operation is carried out using a forging equipment capable of producing the energy and handling force produced during the operation to produce said finish forged part (7) having a near-net shaped gear tooth profile (7A).

A method as disclosed in any of embodiments 1 to 8, characterized in that in said step of providing post-forging treatment, the post-forging treatment comprises the steps of shot blasting and crack detection, leading to said treated finish forged part (9).

A method as disclosed in any of embodiments 1 to 9, characterized in that said step of cold-coining, the cold coining operation is performed on said treated finish forged pari (9) in a forging equipment, preferably a hydraulic press, to produce said cold-coined part (10) that produces a deformation in the range of 0.2 mm to 0.6 mm in said near-net shaped gear tooth profile (7 A). A method as disclosed in any of embodiments 1 to 10, characterized in that said cold-coined part (10) is further subjected to a flash removal operation by machining to obtain a fiashJess part (11). 12. A method as disclosed in any of embodiments 1 to 1 1, characterized in that said flashless part (11), except its teeth, is further subjected to machining to obtained a machined part (12).

13. A method as disclosed in any of embodiments 1 to 12, characterized in that teeth of said machined part (12) are subjected to a grinding process to obtain a near-final crown wheel (13).

14. A method as disclosed in any of embodiments 1 to 13 characterized in that said near-final crown wheel (13) is further subjected to heat treatment using a press-quenching method to obtain a final crown wheel (14).

15. A cold-coined crown wheel obtained using steps a to j of the process as disclosed in embodiment 1, whereby said cold-coined part (10) has a deformation in the range of 0.2 mm to 0.6 mm in said near-net shaped gear tooth profile (7 A), and a machining allowance of 0.3mm.

While the above description contains much specificity, these should not be construed as limitation in the scope of the invention, but rather as an exemplification of the preferred embodiments thereof. It must be realized that modifications and variations are possible based on the disclosure given above without departing from the spirit and scope of the invention. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.