The content of this article contains information regarding forging presses and their use.
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A forging press uses a vertical ram to apply gradual, controlled pressure to a die holding a workpiece. It is a similar process to drop forging, but it uses slow pressure instead of a series of blows. The slow movement of the ram penetrates deeper into the workpiece so that the workpiece undergoes uniform plastic deformation.
Press forging dies can be open or closed. In open die forging, the die does not completely enclose the work-piece. In the closed die method, also known as impression die forging, the die completely surrounds the workpiece.
A forging press applies force using hydraulic or mechanical force. Mechanical force is produced by a flywheel that stores energy, which is used to move a ram attached to a crank mechanism. It can apply 12,000 tons of pressure.
A hydraulic forging press, unlike the mechanical flywheel type, creates force through the use of a high pressure fluid. Large hydraulic presses can create 75,000 tons of force.
The types of forging presses vary according to their size and the amount of force they use for plastic deformation of the workpiece.
The basic component of forging operations is the ability to produce a great deal of force and focus it on a workpiece. High performance forging presses supply a huge amount of power that deforms and plasticizes metals to reach specific tolerances and shapes using an open or closed die. There are innumerable methods available that can produce the necessary force and are differentiated by the mechanism they use.
The types of forging presses can be further divided by their frame design, which can be straight sided or C frame. Straight side presses have two sides, while C frames have one open side.
The most basic type of forging press is a mechanical press that has a ram that moves vertically to apply pressure and squeeze a workpiece into the desired shape. This type of forging press is different from the old hammer-and anvil-method of ancient times that deformed materials using a series of blows. Other types of forging presses include hydraulic, screw, and upsetters, each of which produce the same shapes and can forge alloys with moderate ductility that would shatter under the impact of a hammer.
Hydraulic presses create their force through hydraulic pressure from a fluid using Pascal‘s Law. A small amount of force is applied to the fluid, which moves a larger amount of fluid to create the force that moves the ram to shape the workpiece. They operate slower than other forging presses and have longer contact with the workpiece.
Open dies are the normal type used with hydraulic die forging. The hydraulic forging process is ideal for isothermal forging because of its very slow squeezing speed. Hydraulic forging presses are rated at 50,000 tons with dies that are 12 feet by 32 feet.
Dies for hydraulic forging presses take a great deal of abuse due to the increased contact time, which lowers the die’s useful life. Contact times vary depending on the amount of required deformation.
Mechanical forging presses are driven by a motor and controller that has a clutch and crankshaft capable of applying a constant stroke length to the ram. The speed of the ram is greatest at the center of the stroke, with maximum force achieved at the very bottom of the stroke. Knockout or liftout pins automatically eject the forging from the die.
In the mechanical forging process, a great deal of stress is placed on the dies with little impact load. Harder dies are used to prevent breakage and damage. The tooling and fabricating of dies for mechanical forging presses is a major expense. Added to the cost of dies is the long time it takes to change dies, which is a slow and tedious process.
With technological innovations and improvements, the rate of production for mechanical forging presses has gradually increased, with many presses being able to deliver 70 strokes per minute at lower labor costs.
A screw press, like a hydraulic press, works slowly. A motor turns a screw that pushes the ram down onto the workpiece with a constant pressure with a long stroke. Screw presses can produce up to 31,000 tons of force.
A servo forging press is driven by a servo motor that drives an eccentric gear to create slider movement. The driving force generated by the motor is changed into linear motion by screws, cranking rods, and elbow rods to drive the slider. The stroke, speed, and pressure of the slider is controlled by complex electrification. Servo motor forging presses have a main drive, actuator, and auxiliary mechanism.
The transmission mechanism of a servo motor forging press transfers energy from the servo motor to the actuator that drives the slider to do the reciprocating motion and complete the forging process. Servo motor forging presses have limited torque and are only suitable for low tonnage forging.
Servo motor fogging presses have precision control over the speed of the slider and are capable of manufacturing difficult-to-form parts. They are energy-saving, environmentally friendly, multipurpose, and highly intelligent manufacturing machines.
The force of a pneumatic press comes from compressed air or a gas, which is forced into a cylinder connected to the ram. When the cylinder is filled, the pressure from it forces the downward movement of the ram. The ram releases when the air or gas is released through an escape valve.
Upset forging or heading uses an upsetter that is a horizontal forging machine using a ram that moves horizontally against the workpiece. The workpiece is held in place between die halves while pressure is applied in the direction of its axis by a heading tool that spreads, or upsets, the end of the workpiece by metal displacement. The process requires two gripper and cavity dies, with one being stationary while the other is fastened to the moving die slide, and a puncher that is attached to the header slide.
During the upset forging process, the movable die slides to the stationary die to grip and hold the workpiece. The punch or ram moves forward and forces the workpiece into the die cavities. When the ram or punch retracts, the movable die is in an open position to release the forging. The process may be repeated multiple times before it is completed.
The workpiece for upset forging is preheated, which helps the metal retain its integrity and optimizes its grain flow properties. The unbroken grain flow produces metal parts with superior tensile strength and durability.
Upset forging is referred to as free forming since the workpiece is reduced in height and form. It is often used as an intermediate process as part of a multiple step forging process. Upset forging is commonly used to produce bolts, screws, nuts, rivets, and flanged shafts.
The process of a forging press is quicker and less expensive than other methods. It produces a grain flow that makes the final piece stronger. The diagram below compares the grain flow for casting, machining, and forging. In casting, there is no grain flow, while the grain flow with machining remains straight. With forging, the grain flow follows the contour of the piece.
The texture of the piece is continuous, creating improved strength for the final piece.
During the forging process, the grain structure of the piece becomes compressed, which creates reduced stress on the corners and fillets increasing the strength of the piece.
Forging reduces metallurgical defects such as porosity and alloy segregation, which reduces the time for machining the completed piece and a positive response to heat treatment.
With the absence of voids and porosity, pieces can be machined after forging without a loss of dimensional accuracy or quality. Tolerances are within 0.01 to 0.02 inches (0.25 to 0.5 mm).
The saving factors for forging include raw material usage, reduced machining time, and reclamation of die material.
The number of cycles for a die varies depending on the types of materials being shaped, the strength of the material, the need for close tolerances, sharp corners, and intricate designs.
Forging presses have a wide range of tonnage from several hundred to several thousand with working strokes per minute as high as 40 or 50 parts per minute. Parts are completed in a single squeeze, which can be slowed by complex and intricate designs. The forging press process can be used for mass production of nuts, bolts, rivets, screws, brake levers, bearing races, valves, and many other parts.
Dies in press forging have less draft, which makes them able to produce complex and complicated shapes with excellent dimensional accuracy. Forging can create deep protrusions, up to six times the thickness of the material. Draft angles are made less or eliminated by designing.
Some ferrous metals can be forged including stainless steel. Non-ferrous metals are highly suited for press forging.
The speed, travel distance, and pressure of a press forging die are automatically controlled for accuracy and efficiency.
The forging press process has the same options as any other manufacturing method and can use CNC programming to enter designs, which can include blank feeding and forged piece removal.
Plastic deformation goes deep into the workpiece, creating a uniform deformation throughout the metal.
As with any form of manufacturing and production, safety is an initial concern. The positive side of press forging is that it does not require any form of special training for the operator except for concerns for safety.
Due to the uninterrupted grain structure of forged parts, they are tougher, stronger, and defect-free. Press forging improves the elasticity of completed pieces making them more ductile. Parts produced from forging are anisotropic and, due to the grain structure, have different properties in traverse and axial directions.
Each forged part has exactly the same structure from the first to the last. Parts are made by a controlled and monitored production process, which creates uniformity of composition and structure as well as minimal variations in machinability and the elimination of transfer distortion.
Most metals can be press forged though some metals are more adaptable to the process. The range of metals includes carbon steel, stainless steel, tool steel, aluminum, titanium, brass, and copper. High temperature metals containing cobalt, nickel, and molybdenum can also be press forged. Since every metal has its individual strength, endurance, and weight, the type of metal chosen for a process depends on the needs of the person who will use the completed piece.
Bar medium is used for its grain structure, mechanical properties, shape, dimensions, quality of its surface, and the ability to be mass produced.
Steel needs to be heated to 2200° F (1200° C) to be able to press forged. The heating process makes steel more ductile and malleable for shaping under pressure. A billet of steel can be permanently formed without cracking because of its plasticity.
Aluminum is ideal for forging because it is lightweight, corrosion resistant, and durable. Forgings of aluminum are used in applications requiring performance and the ability to endure excessive stress. Aluminum has high thermal conductivity, design flexibility, and fracture toughness. It can be forged using open or closed dies and does not require preheating before being forged.
Titanium has excellent weight-to-strength and strength-to-density ratios and corrosion resistance. Prior to press forging, titanium is heat-treated to improve its natural toughness and strength.
As with several other metals, stainless steel is corrosion resistant, has excellent strength, and can be forged into multiple shapes. Of the many grades of stainless steel, 304(L) and 316(L) are used for press forging. Stainless steel requires greater pressure due to its strength and is forged at temperatures of 1706° F to 2300° F (930° C to 1260° C).
After being cut to lengths, brass is heated to 1500 ° F (815° C) and is forged using a closed or open die. Brass can be shaped into any type of form from a few ounces to several tons. Forged brass is stronger and more durable.
Copper bars are heated prior to the forging process. After heating, the bars are pressed into the desired shape. Forged copper has excellent electrical and thermal conductivity. Copper forgings are divided by high conductivity and non-electrical, which is an engineering grade.
Magnesium has a low density with strength and stiffness that is greater than steel or aluminum, but it costs more and is difficult to forge. The magnesium alloys that are best for forging are AZ31B, AZ61A, AZ80A, ZK60A, M1A, AND HM21A. The problem with pure magnesium is its tendency to ignite, which is why it is used in combination with other metals.
The press forging process has many qualities that have made it an excellent means for producing high volumes of parts at low cost. Regardless of its wide use, there are drawbacks, limitations, and disadvantages to the process.
Cost is the major factor regarding press forging. The equipment for the process is very large and has to be durable to be able to create the necessary force. Tools and dies for the process have to be specially made using a select number of metals.
Highly complex parts and designs cannot be produced using press forging. Though parts with complex exterior designs can be manufactured using the process, parts that have internal cavities and intricacies cannot be forged.
Only parts that can be formed by pressing two dies together can be produced. Delicate features, overhangs, or special add-ons cannot be forged.
Forging press dies are very expensive and difficult to make for complicated parts. A special type of steel is used to make the dies, which has to be heat-treated, rough machined, and have special finishing.
Tons of pressure are necessary to form a part in a forging press, which requires very large and expensive equipment.
If a metal has to be heated before being press forged, it will require after processing finishing.
Only parts of a certain size can be produced by press forging, which eliminates any large designs.
Press forging is restricted by the types of metals that can be forged. Cast iron, chromium, and tungsten cannot be press forged because they are too brittle.
Though press forging eliminates shrinkage and porousness, there are still defects that can occur in the final product, such as laps, piping, and die failure.
Tolerances in a forging press cannot be forged down to a millimeter.
Residual stress takes place in metals that have to be heated before being pressed. If the parts are not cooled properly, residual stress occurs.
Scale pit happens when the surface to be forged has not been properly cleaned and is common when forging is done in an open environment.
Flakes happen when heated, pressed, and forged metals are cooling. They are internal cracks that reduce the strength of the product.
Press forging slowly applies pressure to the workpiece. The die has to remain in contact for an extended period of time. The length of this part of the process slows down production.
Forging presses are an essential part of the manufacturing processes of several industries, which include automotive, aerospace, agricultural equipment, oilfield parts, tools and hardware, and military ordnance.
Press forged parts in automobiles are found at points of shock and stress. There can be over 200 press forged parts in a car or truck. Some of them can be seen in the diagram below.
Ferrous and nonferrous forgings are used in helicopters, piston-engine planes, commercial jets, and supersonic military aircraft. Aircraft are designed using press forged parts and can contain over 400 different forged parts. Some can be seen in the diagram below.
Engine and transmission parts, gears, sprockets, levers, shafts, spindles, ball joints, wheel hubs, rollers, yokes, axle beams, bearing holders, and links are produced using forged parts for farm tractors and earthmovers.
Tanks have over five hundred forged parts.
Forging press parts are an essential part of the construction of oil platforms due to their lack of porosity and ability to withstand high-pressure conditions.
The Occupational Safety and Health Administration (OSHA) has developed practices and standards that manufacturers must follow regarding the safe use of a forging press.
The regulations for the operation of a forging press are contained in OSHA standard 29 CFR Part 1910.
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