In the first four articles in this series, the operation and use of four types of forging equipment – hammers, mechanical presses, hydraulic presses and screw presses – were reviewed. In this fifth and final article, a general comparison of these four types of forging equipment is made. Each type of equipment can perform well or poorly depending on conditions and circumstances. It is hoped that the comparisons made in this article will help readers select the right type of equipment for the specific job.

Figure 1. A powered forging hammer


Figure 2. This set of bomb lugs is a typical part produced on a forging hammer in a platter.

Overview of Forging Equipment

Hammers – Forging hammers use energy from a moving ram and die to deform a hot workpiece. The typical gravity hammer is similar to the physics of a hammer driving a nail into wood. The die movement is controlled by energy. Once the energy in the dropping die is consumed, the die stops. Hammer dies normally have multiple cavities. In each die cavity, multiple hits are common for production forging operation.

Figure 1 is a schematic of a hammer with its various components labeled. A typical hammer is a relatively simple piece of equipment. To forge a component, the ram is raised and then dropped. This dropping action can be augmented with air or steam power. In spite of its simple design, a forging hammer must be a very rugged machine in order to withstand the large impacting blows during operation.

Figure 2 shows a typical part made on a forging hammer. Note that multiple parts are made on the single platter. There is also an extensive amount of flash to ensure that proper die fill is achieved during the forging process.

Figure 3. Schematic of a mechanical press

Mechanical Presses – Mechanical presses convert the rotational energy of a flywheel into the linear motion of a ram. The top die is mounted to the moving ram. A clutch connects the flywheel to the eccentric (or crank) shaft. For a mechanical press, the total stroke length is fixed. The load capability and ram speed are dependent on the position of the ram. At bottom dead center the speed of the die goes to zero as the motion goes from the downward deformation to upward return.

Figure 4. Typical parts forged on a mechanical press

Figure 3 shows a mechanical forging press with its major components labeled. In contrast with a hammer, a mechanical press is more complex. The various mechanical devices on the press are used to convert flywheel energy into useful deformation work and to control its operation.

Figure 4 shows parts forged on a mechanical press. Note the similarity to components produced on a hammer. Dies for a mechanical press can have deeper cavities, allowing deep extrusion-type forgings to be produced.

Figure 5. A hydraulic press with the major components labeled

Hydraulic Presses – Hydraulic presses use fluid to apply pressure to a hydraulic cylinder, resulting in ram motion. The speed and load can be accurately controlled with the proper system. Hydraulic presses generally operate at relatively low strain rates. One important aspect of hydraulic presses is that they are force limited, with a power and speed limit based on the hydraulic system employed.

Figure 6. Typical forged parts on a hydraulic press

Figure 5 is an illustration of a hydraulic press. Although the press itself is not very complicated, the necessary hydraulic and control systems add to the complexity. Figure 6 shows some typical components produced on a hydraulic forging press. Because of the large tonnage available on some hydraulic presses, the forged components can at times be very large and very long. The ability to produce long extrusions or isothermal forgings is unique to this type of press.

Figure 7. Schematic of a forging screw press

Screw Presses – A screw press converts the rotational energy from a flywheel into the linear motion of a ram. A clutch usually connects the flywheel to a screw mechanism, which drives the screw toward the workpiece. Like a hammer, the die movement is controlled by energy. Once the energy is dissipated, the ram stops. A screw press normally operates with only a single hit per part.

Figure 7 shows a screw press with its various components. Like a mechanical press, the screw press converts the flywheel energy into useful deformation work. The conversion of the rotational energy into the necessary linear motion requires some added complexity to the drive train. Figure 8 shows typical parts forged on a screw press. Screw presses are able to produce thin forgings with a fair amount of detail.

Figure 8. Typical parts forged on a screw press

Thermal Considerations

Table 1 lists the typical forgings (hot, warm or cold) that are produced on the four types of equipment. All four machines produce hot forgings. Hammers are rarely used for warm forgings and never for cold forgings. The strength required in cold-forging tooling compromises toughness and ductility, which is required for hammer forging dies. Screw presses are also not used for cold forgings. Hydraulic presses are used for hot and warm forging and cold coining or extrusion. Mechanical presses are used through the full range of forging temperatures.

Typical Operational Speeds

Although the ram (die) speed during a forging stroke is given as a velocity, the workpiece deformation rate is given as a strain rate. Strain rate is important because it influences the flow stress, flow characteristics and microstructure of the forging. The engineering strain rate is nominally the speed of the die divided by the length of deformation. Table 2 is a chart with typical strain rates associated with each of the four types of forging presses. The hydraulic press is the slowest of the four. At the other extreme is the hammer, which deforms metal at very high strain rates – typically two orders of magnitude greater than a hydraulic press.

If the metal being forged is very strain-rate sensitive, like many titanium or aluminum alloys, then forging them at a slower strain rate (hydraulic press or screw press) is preferred. This is compounded when the material exhibits strain softening at high (forging) temperatures, which can lead to flow localization. Flow localization can occur when the workpiece is rapidly deforming in a local region. The deformation heat can build up, resulting in reduced local strength. This results in concentrated deformation in this “soft” region with a very pronounced shear band. Flow localization generally results in a very undesirable microstructure and mechanical properties. As seen in Table 3, high-speed equipment is more prone to flow localization. Screw and mechanical presses typically operate at intermediate speeds.

One of the consequences of the forging speed of the equipment is heat buildup. Table 3 lists the risks of heat buildup in the dies for each type of equipment. Heat buildup varies indirectly with speed – the higher the speed, the less the risk due to the lower contact time between the hot workpiece and the (relatively) cold tooling.

Control Aspects

Table 4 provides an overview of which parameters can normally be controlled in each type of equipment. It is often desirable to control one or more of the parameters of position, load, speed and/or energy. Because of the individual designs of each type of forging equipment, an operator or a system can use only a limited number of these variables for control purposes. Position, load and speed can normally be used as control variables on a hydraulic press, making it a very versatile machine. Mechanical presses can use position and speed as control variables.

The consequences of not being able to directly control load on a mechanical press needs to be understood. If, for example, a workpiece is too large, the load may become excessive and cause the clutch to slip, the motor to stall, or breakage of tooling or press components. All of these are obviously undesirable. Because of its design and basic physics of operation, a screw press can use load or energy as a control variable. In some screw presses, position can also be used. In contrast, the simple operational mode of a forging hammer, which relies on the dropping of a large weight to deform the workpiece, typically permits the control of energy only. The height of the drop is directly related to the energy content of the blow.

In terms of automation, mechanical presses are the most popular. Their stroke cycle can easily be synchronized to robots or feeding devices. Virtually all cold forming is perfomed on automated mechanical presses, with four to six die stations. High-volume automotive production is set up on automated lines of mechanical presses.

Screw presses and hydraulic presses are most popular for aerospace and medical applications of aluminum, titanium and nickel-based superalloys. These generally offer the best process control for alloys that are very sensitive to the process conditions. Steel can be successfully forged in a wide range of process conditions. For many steel applications at low to moderate volume, the hammer is an economical solution.

Summary

This article’s objective is to compare the four types of forging presses – hammer, mechanical, hydraulic and screw. Each type has been the focus of a previous paper that gave the details of operation as well as advantages and limitations. In making the comparison in this article we have looked at the complexity of each type of equipment and the typical types of parts produced on each. The temperature of normal use, speed of operation, effects of temperature on dies and within the workpiece, and the means to control each type of equipment have been presented. The final point that should be gained from these articles is that a single type of forging equipment is not suitable for every possible job. It is incumbent on the forging engineer to understand the advantages and limitations of each type of press and choose the one most suitable for the job at hand.

Acknowledgements
The support for these papers from the Forging Industry Association, the Forging Defense Manufacturing Consortium, Scientific Forming Technologies Corporation, the Colorado School of Mines and the PRO-FAST Program is appreciated. The PRO-FAST Program is enabled by the dedicated team of professionals representing both the Department of Defense and industry. These teammates are determined to ensure that the nation’s forging industry is positioned to meet the challenges of the 21st century. Key team members include: R&D Enterprise Team (DLA J339), Logistics Research and Development Branch (DLS-DSCP) and the Forging Industry Association (FIA).

Co-author John Walters is vice president of Scientific Forming Technologies Corporation, Columbus, Ohio. He may be reached at 614-451-8330 or jwalters@deform.com. Co-author Dr. Chet Van Tyne is FIERF Professor, Department of Metallurgical Engineering, Colorado School of Mines, Golden, Colo. He may be reached at 303-273-3793 or cvantyne@mines.edu.