Theory

The upsetting process is a metal forming operation in which the diameter of a workpiece is increased by compressing its length. It is one of the most widely used forging processes, particularly for mass production of components such as engine valves, bolts, screws, couplings, and fasteners.

Upset forging is generally performed using high-speed crank presses, although vertical crank presses and hydraulic presses are also used. These machines are commonly arranged horizontally to enable rapid transfer of workpieces between stations.

Upsetting Machines and Operation

The initial workpiece is usually a wire or rod, although bars up to 25 cm in diameter can also be processed using heavy-duty machines with capacities exceeding 1000 tons. Standard upsetting machines employ split dies containing multiple cavities. These dies open to allow the workpiece to move from one cavity to the next and then close while a heading tool or ram moves longitudinally to upset the material into the cavity.

Design Rules for Upset Forging

The following rules must be followed to avoid defects during upsetting:

• The unsupported length of metal that can be upset in one blow without buckling should not exceed three times the diameter of the bar.
• Lengths greater than three times the diameter may be upset successfully if the upset diameter does not exceed 1.5 times the original diameter.
• If the stock length exceeds three times the diameter and the cavity diameter is not more than 1.5 times the bar diameter, the unsupported length beyond the die face must not exceed the bar diameter.

Pre-formed Defects

Pre-formed defects are defects introduced during the preparation of the pre-form before final forging. These defects are often coincidental and difficult to detect.

Characteristics of Pre-formed Defects

• Pre-form defects are rarely detected during inspection.
• If a defect is found, it is difficult to determine how many defective pre-forms may have already passed inspection.
• Low-quality pre-forms frequently cause problems during subsequent forming operations.
• Most pre-form defects are not easily visible to the naked eye.
• Pre-forms found insufficient by inspectors are typically returned to the supplier.

Understanding and controlling pre-formed defects is essential to ensure sound forging quality and reduce rejection rates.