Mechanical Forging
Among the basic mechanical concepts press forging is a technique that exerts gradual pressure on a die which holds a workpiece. The process can be either open die or closed die forging operation. In an open die press forging process, the die is only on one side of the workpiece. While in closed die forging methods, the metal workpiece is bounded by a die, and pressure is applied on the die. Plastic deformation of the metal occurs, filling die cavities.
Closed die forging is preferred over open die forging for metals as it leads to less flash and needs less draft. Common press forging methods where a closed die is used are coining and hubbing.
Coining
The metal is placed inside a closed die and high pressure (almost 5-6 times the strength of the metal) is applied to obtain dimensional accuracy in the product. This process is used to make coins.
Hubbing
In this type of press forging, a pattern flows into the die cavities by pressing it onto the die. Silverware is made using this process.
Mechanical Forging Presses generally consist of a ram that moves vertically to apply compression on the workpiece. Mechanical forging can produce similar results as hammer forging and, in addition, can forge various alloys of moderate ductility that are not suitable for hammer forging as they tend to shatter due to fast impact of a hammer.
These presses are driven by a motor and controlled with an air clutch and have an eccentric type of crankshaft that exerts a constant length stroke to a vertically operating ram. They operate by converting rotation of the motor into linear motion of the ram. Ram speed is extremely high at the center of the stroke, but the maximum force is not attained until near the bottom of the stroke. Because of the fixed length of the stroke, care must be taken to ensure that the closure allowed is adequate in size to avoid sticking of the press at the bottom of the stroke, leaving the stroke incomplete. Such an occurrence can cause severe damage to the press. It can also cause substantial downtime to free the press by burning the dies apart.
Mechanical Presses are suitable for low profile forgings and usually incorporate knockout/liftout pins in the dies which help in automatically ejecting the forging from the die. This helps in allowing the die and thus forging, to be designed with minimum draft allowance, thus reducing weight and consequent machining. Stresses in press dies are usually high, but impact load is low so harder dies can be used without the risk of damage that is expected in a power hammer.
Mechanical Forging Presses are cost-effective on longer runs as tooling costs are usually high and the tool change and setup time are slow. However, with rapid change in technology, mechanical presses can achieve high production rates on certain part configurations as compared to hammer forging. Specific types of mechanical forging presses can deliver up to 70 strokes per minute. In general, mechanical presses involve less operator skill and provide better results than forging hammers.