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Drop Forging Design

There are different types of forging processes, today our discussing topic is drop forging design. Before drop forging production, the engineering design team will first finalise the geometry, dimension, tolerance and material requirement (normally used material is drop forged steel) for the desired component. This is based on future application of the products, including the intended performance of the part. The typical output is a machine drawing of the final required part; which include post drop forging processings such as machining, tolerance and surface finish requirements. The drop forging drawing of the considered component is shown in Fig.1.1.  And a 3D model displaying different views is shown in Fig.1.2. 

The first step for a production engineer is to convert the machine drawing into drop forging part drawing. Understanding of the function of actual part in service should be considered as a prerequisite to efficiently deal with this conversion phase. This understanding also helps to improve the decision making ability for the subsequent design stages. The selected component is a part of the universal joint used in propeller shaft for heavy commercial vehicle application. The component is known as “Flanged Yoke” in automotive terminology. It is mostly subjected to torsional loadings; as the whole propeller shaft is designed to efficiently transmit the engine torque to the rear axle. 

Fig. 1.1: Machine drawing of the component considered in the project- Flange Yoke

Fig.1.2: 3D Model of the final machined component; a) Isometric view;b) Cut sectional view showing the web thickness; c) View showing the rear details 

A.Die Design Stage: 

There are no fixed rules to design the dies for drop forging. The design method used mainly deponds on the geometry and processing of drop forging parts. But, There are a serious of recommended guidelines and principles for design, which can be adopted based on a particular situation. They are mostly empirical,and are developed from years of practical experience. 
With the recent developments in virtual process simulation using finite element method; these adopted guidelines can be further refined to suit the exact scenario. In case of closed die drop forging,drop forging design follows the following steps:

B. Determination of parting line and axis of products for drop forging manufacturing: 

This step is vital as it will affect both component quality and the method of die design. 
As per the stated guidelines, the selection of parting line should be as following: 
a) Avoid deep impressions in the die to improve die life. 
b) Minimum die side thrust to avoid die shift during the drop forging cycle. 
c) Try to place largest periphery around the parting line; so that it is easier to force metal laterally to spread into the cavity. Putting the largest flat surface on the parting line is the other variation of this criterion. 
d) Achieve desired grain orientation for the part to be drop forged. 
Reviewing the application of the component shows that, the component is continuously subjected to torsional fatigue cycle. The torque is transmitted to the body, as a coupling force acting through the holes present in the yoke. The net effect can be summed up as constant bending forces acting on the sections of the yoke. In order to improve the mechanical properties of the component, it is desirable that the grain flow orientation is perpendicular to the loading direction. This will improve the fatigue resistance of the component. Thus,reviewing the actual application of the component, one could select the parting line. 

C. Designing the drop forging part 
The first step here is to initiate necessary design changes in the drop forging drawing design so that the component can be formed with the decided parting line.Consequently some of the geometry needs to be simplified or modified. In case of the considered component,the holes in the yoke lie in the perpendicular axis of forming; so the yoke is to be formed as solid in forging and the hole will have to be generated afterwards by machining. 
Secondly,adding up the required allowances to the drop forging part design. The considered tolerances are: 
i. Machining allowances 
ii. Straightness or uneven allowances 
iii. Contraction allowances 
The standard charts and formulae is referred to finalise the allowances to the drop forging part design. 
The images of closed die drop forging part model is shown in Fig.1.3.

Fig. 1.3: 3D Model of the generated drop forged component; a) Isometric view; b) View showing the rear details; c) Cut sectional view showing the web thickness

D. Incorporating the draft angle in drop forging design 

Draft means the taper generated in the internal and external sides of a drop forging to promote easy removal of components from the die cavity. In the actual practice, the draft is generally modelled in the drop forging design and the replica is generated in the die by machining.The selection of draft angle should be optimal,as excess draft angle increases allowances in component; thus increasing the final weight of component. 
The draft angle is included in the vertical walls with respect to the axis of forming. Normally draft provided on the inner surface is larger than the outer surface drafts,as upon cooling the drop forging shrinks both radially and longitudinally. 
Draft angle value also is greatly dependent on the type of equipment used and complexity of drop forging.The draft angle value is decided as 3° for external and 5° for internal surface based on recommendation table,considering mechanical crank press and steel as drop forging material. 

Click on our drop forging design considerations page to learn more about this in detail.

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