WO2016027206A1 - A forging die assembly with split cassettes and a method to make it - Google Patents

Abstract

The present invention is a forging die assembly comprising at least two die inserts and at least a lower cassette and an upper cassette, which are placed between a top die holder and a bottom die holder, such that the lower or upper cassette or both are of the split type and the split cassette is held together with a set of tie rods. The invention thus proposes a method of making a forging die assembly using split cassettes. The assembly made using the invention is capable of withstanding the alternate stresses developed during a closed die forging process. The invention also provides a method for reusing cassettes damaged during closed die forging.

Description

A Forging Die Assembly With Split Cassettes And A Method to Make It

Field of Invention:

The present invention relates to closed die forging. In particular it relates to the cassettes used for quick loading and unloading of dies with in-base ejection system in automated press forging.

Background of Invention:

Closed die forging requires use of cassettes during forging operation. Parts produced with forging are usually formed to final shape in multi-stage operations. Depending upon the geometry of part, preforming and final forging dies are designed and produced. The forging dies are placed into the die insert envelopes which are called 'cassette' - together, the cassettes and dies are called a die assembly. The die assembly (both upper and lower) are fixed to the die holders. The upper die holder is fixed to the ram of the forging press and lower die holder is fixed to the anvil bolster of the press.

Forging operations require high level of precision if the manufactured part is to be of acceptable quality. Forging operations also involve frequent changes of dies and resetting of die assembly, which results into several problems related to alignment. It is also known that the loads and stresses that the forged parts and indeed the die assembly are subjected to variable in nature and it introduce fatigue in the die assembly. It has been noticed that due to these issues and also due to the nature of load and stress that cassettes undergo, they tend to develop cracks at critical sections.

The conventional solid cassettes have shown a tendency of developing cracks at critical sections, in particular the center lines due to high alternate loads during blocker and finisher operations in multistage press forging. This tends to adversely affect the quality of the forged components as well as the cassette life.

It is possible to repair the cracked cassettes. However, conventional repairing of cracked cassette through welding is time consuming and costly. Moreover, the cassettes repaired using conventional methods such as welding, by their nature tend to crack repeatedly.

There is therefore a need to provide cassettes that would withstand the alternate loadings and stresses generated by them, and which have enhanced life than the conventional cassettes. There is also a need to provide a method to repair cracked cassettes which may then be reused thus reducing wastage.

Objects of Invention:

Accordingly an object of the present invention is to provide a die assembly with pre-stressed cassettes which is capable of withstanding the alternate stresses developed during a closed die forging process. Another object of the present invention is to provide a method for reusing cassettes damaged during closed die forging.

Yet another object of the present invention is to provide a method of manufacturing cassettes for closed die forging.

Brief Description of Figures:

Figure 1 shows die assembly having existing solid cassette with top and bottom die holders

Figure 2 shows existing solid cassette with crack prone regions

Figure 3 shows split cassette part of blocker side

Figure 4 shows split cassette part of finisher side

Figure 5 shows Tie rods with washer, square and hexagonal nut

Figure 6 shows Split cassettes after assembly

Figure 7 shows die assembly having split cassette with top and bottom die holder

Summary of Invention:

The present invention is a forging die assembly comprising at least two die inserts and at least a lower cassette and an upper cassette, which are placed between a top die holder and a bottom die holder, such that the lower or upper cassette or both are of the split type and the split cassette is held together with a set of tie rods. The invention also proposes a method of making a forging die assembly using split cassettes. Detailed Description Of Invention:

Figure 1 shows an existing forging die assembly with die holders. It shows top and bottom die assembly surrounded by top and bottom die holders. Different types of dies such as blocker and finisher are attached to cassettes by using clamping bolts. A die assembly has top and bottom cassettes along with the attached top and bottom dies, respectively. The top and bottom cassettes are attached to respective die holders (as shown in fig. l .) by hydraulic clamping mechanism. During set up activity the current die assembly can be simply de- clamped and unloaded and back up die assembly can be loaded and clamped.

In conventional die assembly, cassettes are manufactured from prescribed cassette material. A die assembly incorporating a split cassette is proposed. The key aspect of the invention is to identify the critical stress areas of a cassette and providing a purpose made split along the potential crack line. The multiple cassette members are then joined together using pre-loaded tie rods such that compressive load paths are developed along the cassette members to hold them together.

Split cassette invention is also related to novel method of repairing of cracked cassette as well as method of utilizing scrap cassette material. The key aspect of the invention is to split a cassette thereby removing the possibility of a failure along the potential crack line which would have developed in an integral cassette. The cassette surface along the split are machined and joined together by three tie rods under pre-tension. The size, material and pre- tension of tie rods are determined by finite element analysis methodology. For the purpose of the analysis, external loads at blocker and finisher are kept constant and tie rod pre-tension is optimized by cassette parts contact analysis. It is observed that tie rod pre-tension remains almost constant even after the application of external fluctuating loads at blocker and finisher dies which increases the fatigue life of split cassette.

The split cassettes used in the present invention are either new cassettes that are deliberately split along the potential crack lines or the ones already cracked. Depending on the type of cassettes that is used, the die assembly manufacturing process varies.

Starting with a new cassette, the following process is proposed:

Cutting the split cassette material along the potential crack lines or center lines,

- Machining of individual parts of split cassette (two parts) and accessories

(Tie rods, nuts, washers, dowels), - Drilling holes (total of 6 or more) in both parts of split cassette, followed by aligning both parts of the split cassette using dowels,

Joining the two parts of the split cassettes using tie rods (total of 3 or more) of suitable diameter heated to desire temperature which are inserted in the holes to join the two parts of split cassettes,

Applying required torque to the nuts of tie rods to clamp the cassettes, Inspecting the split cassettes after joining

Starting with an already cracked cassette, the following process is proposed:

Inspecting a cracked cassette, Cutting the cassette along the center line, If the cracks are not along the center line then gouging (material removal process) of cracked area,

- Welding in cut faces, If the cracks are not along the center line then

carrying out welding also in the gouged area,

- Finish machining the welded region and cutting the faces of cassettes,

- Drilling holes (Total 6 or more) in both parts of split cassette,

Aligning both parts of the split cassette using dowels,

Joining the two parts of the split cassettes using tie rods (total 3 or three) of suitable diameter heated to desire temperature which are inserted in the holes to join the two parts of split cassettes,

Applying required torque to the nuts of tie rods to clamp the cassettes, Inspecting the split cassettes after joining Starting with this split cassette prepared either way, the complete die assembly is put together as described earlier. Once the split cassettes are ready, the die assembly is put together as follows:

- Placing the dies on the respective top or bottom cassette.

Clamping the dies to respective cassettes by using clamping bolts to form top and bottom die assemblies.

Inverting and lifting the top die assembly (top cassette and top die).

- Placing the top die assembly in its inverted position onto the bottom die assembly to form the complete die assembly (only placement not joined)

- Loading and clamping the complete die assembly to the die holders

wherein the top die assembly is clamped to the top die holder and the bottom die assembly is clamped to the bottom die holder.

Non-linear finite element analysis is performed to calculate contact gap and contact pressure distribution over contact area of cassette members. Finite element models for split cassette joint analysis included five components: blocker and finisher dies, two cassette parts, three tie rods, nuts and bottom die holder. A number of load cases are analyzed by varying pre-tension in the tie rods between 0 and 1700KN. For all cases external loads applied at blocker and finisher dies are kept constant and contact gap, contact pressure distribution of cassette parts was examined and quantified.

Integrity of the tie rods is important to the integrity of the die set. The stress distribution on tie rod after applying pre-tension and then after applying external loads on dies was observed. The total stresses on tie rods varies by very little amount from pre-tensioning to external loading. It was also observed that gap between the contact surfaces at the split in the cassettes is almost zero , which ensures effective attachment between cassette parts even after the application of external loads at blocker and finisher dies.

It is observed that alternating stresses of solid cassette and that of split cassette are almost same. The split cassette of the invention is typically made from cassettes which were cracked during working. A scrapped cassette which already has central crack is cut along the center into two parts and each of the two cassette parts were repaired by welding and then machined separately. Three tie rods are machined along with their standard square and hexagonal nuts as shown in Figure 5.

Figure 4 shows finisher side split cassette part in which two dowels are shown for locating purpose. Dowels are provided to achieve perfect alignment between two cassette parts assembly as there is clearance fit between tie rod and cassette part holes.

The invention provides the following advantages over the conventional methods of cassette formation:

1. It is observed that stresses on tie rods of split cassette remains almost

constant from pre-tensioning to external loadings at blocker and finisher dies. Tie rod pre-tension relives the stress concentrations caused by making a hole inside cassette parts.

2. From finite element analysis of split cassette it is concluded that the present method can be used as novel method of repairing of cassettes which will improve repaired cassette life and make cassette repairing process economical.

From the foregoing discussion, it is evident that the present invention has the following embodiments.

1. A forging die assembly with split cassettes comprising at least two die inserts and at least a lower cassette and an upper cassette, which are placed between a top die holder and a bottom die holder, characterised in that the lower or upper cassettes or both are of the split type and the spilt cassette is held together with a set of tie rods. A forging die assembly as disclosed in embodiment 1, characterised in that said split cassette is made from cassettes which were already cracked. A forging die assembly as disclosed in embodiment 1, characterised in that said split cassette is made from solid raw material.

A forging die assembly as disclosed in embodiments 1 and 3, characterised in that said split cassettes are made by identifying critical stress areas of a cassette and providing a purpose made split along the potential crack line or center line.

A forging die assembly as disclosed in embodiments 1, 3 and 4, characterised in that the cassette surfaces along the split are machined and drilled throughout for holes for tie rods, following which pre-loaded tie rods are used, such that compressive load paths are developed along the cassette members to hold them together.

A forging die assembly as disclosed in embodiments 1 and 2, characterised in that said split cassettes are made by inspecting a cracked cassette and providing a purpose made split along the center line of cracked cassette. A forging die assembly as disclosed in embodiments 1, 2 and 6, characterised in that the cassette surfaces along the split are optionally gouged and welded which is then machined and drilled throughout for holes for tie rods, following which pre-loaded tie rods are used, such that compressive load paths are developed along the cassette members to hold them together. 8. A forging die assembly as disclosed in embodiments 1 to 7, characterised in that two locator dowels are provided.

9. A method of making a forged die assembly with split cassettes, characterised in that said method comprises the step of providing split cassettes and holding the split cassette parts together with a set of tie rods.

10. A method of making a forging die assembly as disclosed in embodiment 9 characterised in that it further comprises the steps of:

placing the dies on the respective top or bottom split cassette

clamping the dies to respective split cassettes by using clamping bolts to form top and bottom die assemblies,

inverting and lifting the top die assembly (top split cassette and top die), placing the top die assembly in its inverted position onto the bottom die assembly to form the complete die assembly (only placement not joined), loading and clamping the complete die assembly to the die holders wherein the top die assembly is clamped to the top die holder and the bottom die assembly is clamped to the bottom die holder.

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.

Claims

1. A forging die assembly with split cassettes comprising at least two die inserts and at least a lower cassette and an upper cassette, which are placed between a top die holder and a bottom die holder, characterised in that the lower or upper cassettes or both are of the split type and the split cassette is held together with a set of tie rods.

A forging die assembly as claimed in claim 1, characterised in that said split cassette is made from cassettes which were already cracked.

A forging die assembly as claimed in claim 1, characterised in that said split cassette is made from solid raw material.

A forging die assembly as claimed in claims 1 and 3, characterised in that said split cassettes are made by identifying critical stress areas of a cassette and providing a purpose made split along the potential crack line or centre line.

A forging die assembly as claimed in claims 1, 3 and 4, characterised in that the cassette surfaces along the split are machined and drilled throughout for holes for tie rods, following which pre-loaded tie rods are used, such that compressive load paths are developed along the cassette members to hold them together

A forging die assembly as claimed in claims 1 and 2, characterised in that said split cassettes are made by inspecting a cracked cassette and providing a purpose made split along the center line of cracked cassette. A forging die assembly as claimed in claims 1, 2 and 6, characterised in that the cassette surfaces along the split are optionally gouged and welded which is then machined and drilled throughout for holes for tie rods, following which pre-loaded tie rods are used, such that compressive load paths are developed along the cassette members to hold them together.

A forging die assembly as claimed in claims 1 to 7, characterised in that two locator dowels are provided.

A method of making a forged die assembly with split cassettes, characterised in that said method comprises the step of providing split cassettes and holding the split cassette parts together with a set of tie rods.

A method of making a forging die assembly as claimed in claim 9, characterised in that it further comprises the steps of:

placing the dies on the respective top or bottom split cassette

clamping the dies to respective split cassettes by using clamping bolts to form top and bottom die assemblies,

inverting and lifting the top die assembly (top split cassette and top die), placing the top die assembly in its inverted position onto the bottom die assembly to form the complete die assembly (only placement not joined), loading and clamping the complete die assembly to the die holders wherein the top die assembly is clamped to the top die holder and the bottom die assembly is clamped to the bottom die holder.