The shape of material can be transformed by forging with the aid of the following operations:
1. Drawing Down: It is also known as Drawing Out. This operation is performed to increase the length of the workpiece in forging by decreasing the cross-sectional area.
This process is performed by hammering the hot workpiece lengthwise to reduce cross-section.
2. Up Setting: This is the reverse of Drawing Down operation. In this operation, the cross-section of workpiece is increased at the expense of length.
This process is performed by hammering one end of hot workpiece while other end is supported against the anvil.
3. Bending: Bending is done by holding the workpiece between two fixtures and desired bend can be given by striking the workpiece with the help of hammer. This operation can also be carried out on the anvil break.
4. Punching and Drafting: Punching operation is performed by a tool called punch, for producing holes in the workpiece, when it is in the hot state; and drafting is an operation carried on by a special tool known as draft to enlarge the hole.
Estimation procedure varies from shop to shop and person to person but for a general procedure, following factors may be considered:
For estimation of net weight of the forged component, following procedure is adopted:
a. Break up the job drawing into suitable geometrical section, whose volumes can easily be calculated by using mensuration.
b. Next, find the value of each section, neglecting rounded corners and taking suitable assumptions.
c. Now, find total volume of material required by subtracting volume of the hollow spaces.
d. Lastly, calculate the weight of the component by multiplying the total volume with its density.
Certain amount of material is lost during different forging operations. The exact estimation of losses is very difficult, but by practical experience, the losses can be calculated during forging as accurate as possible. Various losses in forging are:
(i) Tong Loss: While performing forging operations, some length of stock is required for holding the job in tong. This length is an extra length, which is removed after completion of the job. For estimation purposes, the weight of the extra length is also considered and is known as Tong loss. 2 to 3 cm of the stock length.
(ii) Scale Loss: The outer surface of the hot metal is generally oxidized, and when hammering is done oxidized film is broken and falls down in the form of scale. It reduces the dimensions of the job, and therefore, this loss must be considered for estimation purposes. Generally, it is taken as 6% of the net weight.
(iii) Flash Loss: It is the surplus metal, which comes out between the two meeting surfaces of the dies. For getting finished product, this surplus metal is required to be trimmed off.
This loss may be calculated by assuming it to be 20mm wide and 3mm thick all around the periphery of the dies.
Thus, volume of flash loss = periphery x 20x 3 cu mm nearly.
(iv) Shear Loss: The required sizes of workpiece for forging operations are obtained from long bars by sawing or shearing. In sawing operation, some material is always lost. If last piece of bar is not to be required length, it is rejected. This loss of material is taken as 5% of the net weight.
(v) Sprue Loss: The portion of metal between the length held in the tong and the material in the die is called sprue. This is also a metal loss and can be taken as 7% of the weight.
Thus we can see that nearly 15-20% of the net weight of metal is lost during forging. Therefore, in estimation their consideration is very essential and total weight will be net weight of job plus sum of the weight of different losses occurred during forging. Thus this gives the amount of weight of material required for forging.
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