The content of this article contains information regarding forging presses and their use.
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A forging press functions by deploying a vertical ram that exerts gradual and controlled pressure on a die that holds a workpiece. This process resembles drop forging but utilizes a constant pressure instead of repeated strikes. The steady motion provided by the ram allows deeper penetration into the workpiece, leading to consistent plastic deformation across the material.
There are two primary types of forging press dies: open and closed. In open die forging, the die partially encloses the workpiece, allowing some movement, while in closed die forging—also known as impression die forging—the die completely encloses the workpiece.
Forging presses apply pressure using either hydraulic or mechanical forces. Mechanical forging presses employ a flywheel to store energy that is transferred through a crank system to move the ram. These presses can exert pressures reaching up to 12,000 tons.
On the other hand, hydraulic forging presses create force through high-pressure fluid. Capable of delivering up to 75,000 tons of force, these presses are ideal for more challenging and demanding tasks.
The variety of forging presses is defined by their size and the amount of force they deliver to achieve the plastic deformation necessary in the workpiece.
The core aspect of forging operations is the capability to generate and concentrate substantial force on a workpiece. High-performance forging presses deliver significant power to deform and plasticize metals to achieve precise tolerances and shapes, utilizing either open or closed dies. Numerous methods exist to produce the required force, and these methods are distinguished by their mechanisms.
Forging presses can also be categorized based on their frame design, which can be either straight-sided or C-frame. Straight-sided presses feature two parallel sides, while C-frame presses have one side open.
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 generate force using hydraulic pressure, based on Pascal's Law. By applying a small force to a fluid, a larger amount of fluid is displaced to create the significant force needed to move the ram and shape the workpiece. These presses typically operate more slowly compared to other types and have extended contact times with the workpiece.
Hydraulic forging commonly uses open dies. The process is particularly suitable for isothermal forging due to its slow compression rate. Hydraulic forging presses can handle up to 50,000 tons, with dies measuring up to 12 feet by 32 feet.
Hydraulic forging dies endure significant wear because of the extended contact time, which reduces their lifespan. The duration of contact varies depending on the extent of deformation required.
Mechanical forging presses operate using a motor and a controller equipped with a clutch and crankshaft, which ensures a consistent stroke length for the ram. The ram reaches its highest speed in the middle of the stroke, achieving maximum force at the stroke's bottom. Automatic knockout or liftout pins are used to eject the forged item from the die.
The mechanical forging process exerts significant stress on the dies while applying minimal impact load. To prevent damage and breakage, harder dies are employed. The costs associated with tooling and die fabrication for mechanical forging presses are considerable. Additionally, the time required to change dies is long and labor-intensive.
Advancements in technology have enhanced the production rates of mechanical forging presses, with many now capable of achieving up to 70 strokes per minute, thus reducing labor costs.
Similar to hydraulic presses, screw presses operate at a slower pace. A motor drives a screw that applies a continuous pressure by pushing the ram down onto the workpiece with a long stroke. Screw presses are capable of generating forces up to 31,000 tons.
A servo forging press utilizes a servo motor to drive an eccentric gear, which then produces slider movement. The motor's driving force is converted into linear motion through a system of screws, cranks, and elbow rods, which control the slider's movement. The stroke, speed, and pressure of the slider are regulated by sophisticated electronic controls. Servo forging presses are equipped with a main drive, an actuator, and an 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 forging presses offer precise control over the slider's speed and are adept at producing complex parts. These presses are designed to be energy-efficient, environmentally friendly, versatile, and equipped with advanced intelligence for manufacturing.
A pneumatic press generates force through compressed air or gas, which is directed into a cylinder attached to the ram. As the cylinder fills with air or gas, the pressure causes the ram to move downward. Once the force is no longer needed, the air or gas is expelled through an escape valve, causing the ram to release.
Upset forging, also known as heading, involves a horizontal forging machine called an upsetter. This machine utilizes a ram that moves horizontally to apply pressure against the workpiece. The workpiece is secured between two die halves, and pressure is applied along its axis by a heading tool that deforms the end of the workpiece through metal displacement. The process uses two gripper and cavity dies: one stationary and the other attached to the moving die slide, along with a puncher mounted on the header slide.
During upset forging, the movable die slides toward the stationary die to grip and secure the workpiece. The punch or ram then moves forward, forcing the workpiece into the die cavities. Once the punch or ram retracts, the movable die opens to release the forged part. This cycle may be repeated multiple times until the desired shape is achieved.
Preheating the workpiece before upset forging is essential as it helps maintain the metal’s integrity and enhances its grain flow properties. This results in metal parts with improved tensile strength and durability due to the uninterrupted grain flow.
Upset forging is often used as an intermediate step in a multi-stage forging process. It is commonly employed to manufacture components such as bolts, screws, nuts, rivets, and flanged shafts.
The forging press process is generally faster and more cost-effective compared to other methods. It results in a grain flow that enhances the strength of the final product. The diagram below illustrates the differences in grain flow among various methods: casting, machining, and forging. Casting typically exhibits no distinct grain flow, machining maintains a straight grain flow, while forging produces a grain flow that conforms to the shape of the piece.
The texture of the forged piece is continuous, which contributes to enhanced strength in the final product.
Throughout the forging process, the grain structure of the material is compressed, which reduces stress on corners and fillets, thereby increasing the overall strength of the piece.
Forging helps minimize metallurgical defects like porosity and alloy segregation, leading to less time required for machining the finished piece and better results during heat treatment.
Since forging eliminates voids and porosity, the pieces can be machined post-forging without compromising dimensional accuracy or quality. Tolerances are typically within 0.01 to 0.02 inches (0.25 to 0.5 mm).
Forging offers cost savings through efficient raw material usage, decreased machining time, and the ability to reclaim die material.
The longevity of a die depends on factors such as the materials being processed, the strength of the material, the precision of the required tolerances, and the complexity of the design.
Forging presses come in a broad range of tonnages, from a few hundred to several thousand tons, and can achieve working speeds of up to 40 or 50 parts per minute. The process typically completes parts in a single squeeze, although complex and intricate designs may slow down production. Forging presses are suitable for mass-producing a variety of components, including nuts, bolts, rivets, screws, brake levers, bearing races, valves, and many other parts.
In press forging, dies typically feature minimal draft, enabling the production of intricate and detailed shapes with high dimensional accuracy. Forging techniques can achieve deep protrusions, extending up to six times the material's thickness. By design, draft angles can be minimized or entirely removed to enhance precision.
Various ferrous metals, such as stainless steel, can be used in forging processes. Additionally, non-ferrous metals are particularly well-suited for press forging due to their desirable properties in shaping.
In press forging, the speed, stroke length, and pressure of the die are automatically regulated to ensure precision and operational efficiency.
The press forging process offers similar capabilities as other manufacturing techniques and can utilize CNC programming for design input, including automated blank feeding and removal of forged components.
Plastic deformation penetrates deeply into the workpiece, ensuring uniform deformation throughout the metal.
Like all manufacturing processes, safety is a critical consideration. A benefit of press forging is that it generally does not require specialized training for operators, apart from ensuring safety protocols are followed.
Forged components exhibit enhanced toughness and strength due to their continuous grain structure. Press forging also improves the flexibility of the final products, making them more ductile. Additionally, forged parts are anisotropic, meaning their properties vary along different axes due to the grain alignment.
Each forged component maintains consistent structural characteristics from the initial to the final part. The controlled and monitored production process ensures uniform composition and structure, minimizing variations in machinability and eliminating transfer distortion.
Press forging can be applied to a wide variety of metals, although some are more suitable for the process than others. The metals commonly used include carbon steel, stainless steel, tool steel, aluminum, titanium, brass, and copper. High-temperature alloys containing cobalt, nickel, and molybdenum are also amenable to press forging. The selection of metal depends on the specific requirements of the final product, considering factors such as strength, durability, and weight.
Bar stock is selected based on its grain structure, mechanical properties, shape, dimensions, surface quality, and suitability for mass production.
Steel must be heated to approximately 2200°F (1200°C) to be suitable for press forging. This heating process enhances the steel's ductility and malleability, allowing it to be shaped effectively under pressure. The increased plasticity of the steel ensures that a billet can be formed permanently without the risk of cracking.
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 boasts superior weight-to-strength and strength-to-density ratios, along with exceptional corrosion resistance. To enhance its inherent toughness and strength, titanium is heat-treated before being subjected to press forging.
Stainless steel, known for its corrosion resistance and remarkable strength, is versatile for forging into various shapes. Among its many grades, 304(L) and 316(L) are particularly used in press forging. Due to its strength, stainless steel demands higher pressures during forging and is processed at temperatures ranging from 1706°F to 2300°F (930°C to 1260°C).
Once brass is trimmed to the desired lengths, it is heated to approximately 1500°F (815°C) and then forged using either a closed or open die. Brass can be molded into various shapes and sizes, ranging from just a few ounces to several tons. Forged brass offers enhanced strength and durability.
Before forging, copper bars are heated to prepare them for shaping. Once heated, the bars are molded into their final form. Forged copper is known for its outstanding electrical and thermal conductivity. Copper forgings are categorized into high conductivity types and non-electrical grades, which are used for engineering purposes.
Magnesium is known for its low density while offering strength and stiffness that surpasses both steel and aluminum. However, it is more expensive and challenging to forge. The magnesium alloys most suitable for forging include AZ31B, AZ61A, AZ80A, ZK60A, M1A, and HM21A. Pure magnesium poses a risk of ignition, which is why it is typically alloyed with other metals to mitigate this issue.
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 a significant consideration in press forging. The machinery used for this process is large and must be robust to generate the necessary force. The tools and dies used must be custom-made from specific metals.
Press forging is not suitable for producing highly intricate parts and designs. While it can handle components with complex external shapes, it struggles with parts requiring internal cavities and detailed features.
Only parts that can be shaped by pressing two dies together can be manufactured. Delicate features, overhangs, and special additions are not feasible with this method.
Forging press dies are costly and challenging to produce, especially for complex parts. They are crafted from a specific steel type, which must be heat-treated, roughly machined, and finely finished.
Pressing a part in a forging press requires immense pressure, necessitating large and costly equipment.
If a metal needs to be heated before press forging, additional finishing steps are required after the process.
Press forging is limited to producing parts of specific sizes, which excludes large-scale designs.
The types of metals suitable for press forging are restricted. Cast iron, chromium, and tungsten cannot be forged using this method due to their brittleness.
Despite eliminating shrinkage and porosity, press forging can still result in defects such as laps, piping, and die failures in the final product.
Forging presses cannot achieve millimeter-level tolerances.
Metals that require heating before pressing can develop residual stress if not cooled properly after the process.
Scale pits occur when the surface to be forged is inadequately cleaned, particularly in open environment forgings.
Flakes are internal cracks that appear during the cooling phase after heating and pressing, weakening the final product.
Press forging applies pressure gradually, with the die remaining in contact with the workpiece for an extended period, which slows down production.
Forging presses play a crucial role in the manufacturing processes across various industries, including automotive, aerospace, agricultural machinery, oilfield components, tools and hardware, and military ordnance.
In automobiles, press forged components are used at locations subjected to shock and stress. A single car or truck may contain more than 200 press forged parts. Some examples are illustrated in the diagram below.
Both ferrous and nonferrous forgings are utilized in helicopters, piston-engine planes, commercial airliners, and supersonic military jets. Aircraft are engineered with press forged components and may include over 400 distinct forged parts. Examples are shown in the diagram below.
Forged components used in farm tractors and earthmovers include engine and transmission parts, gears, sprockets, levers, shafts, spindles, ball joints, wheel hubs, rollers, yokes, axle beams, bearing holders, and links.
There are more than five hundred forged components in tanks.
Forging press components are crucial in the construction of oil platforms because of their durability, absence of porosity, and capacity to endure high-pressure environments.
The Occupational Safety and Health Administration (OSHA) has established guidelines and standards that manufacturers must adhere to for the safe operation of forging presses.
OSHA's regulations for operating forging presses are detailed in standard 29 CFR Part 1910.
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