ASM Metals HandBook Vol. 14 - Forming and Forging

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Fig. 4 Double-frame power hammer used for open-die forging

A typical open-die forging hammer is operated by steam or compressed air--usually at pressures of 690 to 825 kPa (100 to

120 psi) for steam and 620 to 690 kPa (90 to 100 psi) for air. These pressures are similar to those used for power-drop

hammers.

There are two basic differences between power-drop hammers used for closed-die forging and those used for open-die

forging. First, a modern power-drop hammer has blow-energy control to assist the operator in setting the intensity of each

blow. In hammers for closed-die forging, the hammer stroke is limited by the upper die surface contacting the surface of

the lower die face. In open-die forging, the upper and lower dies do not make contact; stroke-position control is provided

through control of the air or steam valve that actuates the hammer piston.

The second difference between closed- and open-die forging hammers is that the anvil of an open-die hammer is separate

and independent of the hammer frame that contains the striking ram and the top die. Separation of the anvil from the

frame allows the anvil to give way under a heavy blow or series of blows, without disturbing the frame. The anvil may

rest on oak timbers, which absorb the hammering shock.

High-Energy-Rate Forging (HERF) Machines

High-energy-rate forging machines are essentially high-speed hammers. They can be grouped into three basic designs:

ram and inner frame, two-ram, and controlled energy flow. Each differs from the others in engineering and operating

features, but all are essentially very-high-velocity single-blow hammers that require less moving weight than conventional

hammers to achieve the same impact energy per blow. All of the designs employ counterblow principles to minimize

foundation requirements and energy losses, and they all use inert high-pressure gas controlled by a quick-release

mechanism for rapid acceleration of the ram. In none of the designs is the machine frame required to resist the forging

forces.

Ram and inner frame machines are produced in several sizes, ranging in capacity from 17 to 745 kJ (12,500 to

550,000 ft · lb) of impact energy. The machine illustrated in Fig. 5(a) has a frame consisting of two units: an inner, or

working, frame connected to a firing chamber and an outer, or guiding, frame within which the inner frame is free to

move vertically. As the trigger-gas seal is opened, high-pressure gas from the firing chamber acts on the top face of the

piston and forces the ram and upper die downward. Reaction to the downward acceleration of the ram raises the inner

frame and lower die.

Fig. 5 The three basic machine concepts of high-energy-rate forging. (a) Ram and inn

er frame machine. (b)

Two-ram machine. (c) Controlled-energy-

flow machine. Triggering and expansion of the gas in the firing

chamber cause the upper and lower rams to move toward each other at high speed.

An outer frame provides

guiding surfaces for the rams.

The machine is made ready for the next blow by means of hydraulic jacks that elevate the ram until the trigger-gas seal

between the upper surface of the firing chamber and the ram piston is reestablished. Venting of the seal gas, as well as gas

pressure on the lower lip of the piston, then holds the ram in the elevated position.

Two-ram machines are available in several sizes; the largest has a rating of 407 kJ (300,000 ft · lb) of impact energy.

In a two-ram machine (Fig. 5b), the counterblow is achieved by means of an upper ram and a lower ram. An outer frame

(not shown in Fig. 5) provides vertical guidance for the two rams. Vertical movement of the trigger permits high-pressure

gas to enter the lower chamber and the space beneath the drive piston. This forces and drive piston, rod, lower ram, and

lower die upward. The reaction to this force drives the floating piston, cylinder, upper ram, and upper die downward. The

rods provide relative guidance between the moving upper and lower assemblies.

After the blow, hydraulic fluid enters the cylinder, returning the upper and lower rams to their starting positions. The gas

is recompressed by the floating pistons, and the gas seals at the lower edges of the drive pistons are reestablished. When

the trigger is closed, the hydraulic pressure is released, the high-pressure gas in the lower chamber expands through the

drive-piston ports and forces the floating pistons up, and the machine is ready for the next blow.

Controlled energy flow forging machines have been made in two sizes, with ratings of 99 and 542 kJ (73,000 and

400,000 ft · lb) of maximum impact energy. These machines (Fig. 5c) are counterblow machines from the standpoint of

having separately adjustable gas cylinders and separate rams for the upper and lower dies; however, self-reacting

principles are not employed. The lower ram has a hydraulically actuated vertical-adjustment cylinder so that different

stroke lengths may be preset.

The trigger, although pneumatically, operated, is a massive mechanical latch that returns and holds the rams through

mechanical support of the upper ram and hydraulic connection with the lower ram. With this arrangement, simultaneous

release of the two rams is ensured.

Applicability. High-energy-rate forging machines are basically limited to fully symmetrical or concentric forgings such

as wheels and gears or coining applications in which little metal movement but high die forces are required. Information

on the HERF process, as well as examples of parts forged using high-energy-rate forging, are available in the article

"High-Energy-Rate Forging" in this Volume.

Hammers and Presses for Forging

Revised by Taylan Altan, The Ohio State University

Mechanical Presses

All mechanical presses employ flywheel energy, which is transferred to the workpiece by a network of gears, cracks,

eccentrics, or levers. Driven by an electric motor and controlled by means of an air clutch, mechanical presses have a full-

eccentric type of drive shaft that imparts a constant-length stroke to a vertically operating ram (Fig. 6). Various

mechanisms are used to translate the rotary motion of the eccentric shaft into linear motion to move the ram (see the

section "Drive Mechanisms" in this article). The ram carries the top, or moving, die, while the bottom, or stationary, die is

clamped to the die seat of the main frame. The ram stroke is shorter than that of a forging hammer or a hydraulic press.

Ram speed is greatest at the center of the stroke, but force is greatest at the bottom of the stroke. The capacities of these

forging presses are rated on the maximum force they can apply and range from about 2.7 to 142 MN (300 to 16,000 tonf).

Fig. 6 Principal components of a mechanical forging press

Mechanical forging presses have principal components that are similar to those of eccentric-shaft, straight-side, single-

action presses used for forming sheet metal (see the article "Presses and Auxiliary Equipment for Forming of Sheet

Metal" in this Volume). In detail, however, mechanical forging presses are considerably different from mechanical

presses that are used for forming sheet. The principal differences are:

• Forging presses, particularly their side frames, are built stronger than presses for forming sheet metal.

• Forging presses deliver their maximum force within 3.2mm (

in.) of the end of the stroke, because

maximum pressures is required to form the flash

• The slide velocity in a forging press is faster than in a sheet metal deep-

drawing press, because in

forging it is desirable to strike the metal and

retrieve the ram quickly to minimize the time the dies are in

contact with the hot metal

Unlike the blow of a forging hammer, a press blow is more of a squeeze than an impact and is delivered by uniform

stroke length. The character of the blow in a forging press resembles that of an upsetting machine, thus combining some

features of hammers and upsetters. Mechanical forging presses use drive mechanisms similar to those of upsetters,

although an upsetter is generally a horizontal machine.

Advantages and Limitations

Compared to hammer forging, mechanical press forging results in accurate close-tolerance parts. Mechanical presses

permit automatic feed and transfer mechanisms to feed, pick up, and move the part from one die to the next, and they

have higher production rates than forging hammers (stroke rates vary from 30 to 100 strokes per minute). Because the

dies used with mechanical presses are subject to squeezing forces instead of impact forces, harder die materials can be

used in order to extend die life. Dies can also be less massive in mechanical press forging.

One limitation of mechanical presses is their high initial cost--approximately three times as much as forging hammers that

can do the same amount of work. Because the force of the stroke cannot be varied, mechanical presses are also not

capable of performing as many preliminary operations as hammers. Generally, mechanical presses forge the preform and

final shape in one, two, or three blows; hammers are capable of delivering up to ten or more blows at varying intensities.

Drive Mechanisms

In most mechanical presses, the rotary motion of the eccentric shaft is translated into linear motion in one of three ways:

through a pitman arm, through a pitman arm and wedge, or through a Scotch-yoke mechanism.

In a pitman arm press drive (Fig. 7), the torque derived from the rotating flywheel is transmitted from the eccentric

shaft to the ram through a pitman arm (connecting rod). Presses using single- or twin-pitman design are available. Twin-

pitman design limits the tilting or eccentric action resulting from off-center loading on wide presses. The shut height of

the press can be adjusted mechanically or hydraulically through wedges. Mechanical presses with this type of drive are

capable of forging parts that are located in an off-center position.

Fig. 7 Principle of operation of a mechanical press driven by a pitman arm (connecting rod)

A wedge drive (Fig. 8) consists of a massive wedge sloped upward at an angle of 30° toward the pitman, an adjustable

pitman arm, and an eccentric driveshaft. The torque form the rotating flywheel is transmitted into horizontal motion

through the pitman arm and the wedge. As the wedge is forced between the frame and the ram, the ram is pushed

downward; this provides the force required to forge the part. The amount of wedge penetration between the ram and

frame determines the shut height of the ram. The shut height can be adjusted by rotating the eccentric bushing on the

eccentric shaft by means of a worm gear. A ratchet mechanism prevents the adjustment from changing during press

operation.

Fig. 8 Principle of operation of a wedge-driven press. See text for details of operation.

Wedge drives transmit the forging force more uniformly over the entire die surface than pitman arm drives. Wedge drives

also reduce ram tilting due to off-center loading. Increases in forging accuracies during on-center and off-center loading

conditions and the ability to adjust the shut height are the main advantages of wedge-driven mechanical presses. A

disadvantage is the relatively long contact time between the die and the forged part.

The Scotch-yoke drive (Fig. 9) contains an eccentric block that wraps around the eccentric shaft and is contained

within the ram. As the shaft rotates, the eccentric block moves in both horizontal and vertical directions, while the ram is

actuated by the eccentric block only in a vertical direction. The shut height of the ram can be adjusted mechanically or

hydropneumatically through wedges.

Fig. 9

Principle of operation of mechanical press with a Scotch yoke drive. (a) The ram is at the top of the

stroke; the Scotch yoke is centered. (b) Scotch yoke is in the extreme forward position midway through the

downward stroke. (c) At bottom dead cen

ter, the Scotch yoke is in the center of the ram. (d) Midway through

the upward stroke, the Scotch yoke is in the extreme rear position.

This press design provides more rigid guidance for the ram, which results in more accurate forgings. Forging of parts off-

center is also possible with this type of drive. Because the drive system is more compact than the pitman arm drive, the

press has a shorter overall height.

Capacity

Mechanical presses are considered stroke-restricted machines because the forging capability of the press is determined by

the length of the stroke and the available force at the various stroke positions. Because the maximum force attainable by a

mechanical press is at the bottom of the work stroke, the forging force of the press is usually determined by measuring the

force at a distance of 3.2 or 6.4 mm (

in.) before bottom dead center. Table 2 compares the capacities of

mechanical presses with those of hydraulic and screw presses. More information on determining the capacities of

mechanical presses and other types of forging equipment is available in the article "Selection of Forging Equipment" in

this Volume.

Table 2 Capacities of forging presses

Force Pressing speed Type of press

MN tonf m/s ft/s

Mechanical 2.2-142.3

250-16,000

0.06-1.5

0.2-5

Hydraulic 2.2-623 250-70,000

0.03-0.8

0.1-2.5

Hammers and Presses for Forging

Revised by Taylan Altan, The Ohio State University

Hydraulic Presses

Hydraulic presses are used for both open- and closed-die forging. The ram of a hydraulic press is driven by hydraulic

cylinders and pistons, which are part of a high-pressure hydraulic or hydropneumatic system. After a rapid approach

speed, the ram (with upper die attached) moves at a slow speed while exerting a squeezing force on the work metal.

Pressing speeds can be accurately controlled to permit control of metal-flow velocities; this is particularly advantageous

in producing close-tolerance forgings. The principal components of a hydraulic press are shown in Fig. 10.

Fig. 10 Principal components of a four-post hydraulic press for closed-die forging

Some presses are equipped with a hydraulic control circuit designed specifically for precision forging (see the article

"Precision Forging" in this Volume). With this circuit, it is possible to obtain a rapid advance stroke, followed by

preselected first and second pressing speeds. If necessary, the maximum force of the press can be used at the end of the

second pressing stroke with no limits on dwell time. The same circuit also provides for a slow pullout speed and can

actuate ejectors and strippers at selected intervals during the return stroke.

Advantages and Limitations

The principal advantages of hydraulic presses include:

• Pressure can be changed as desired at any point in the stroke by adjusting the pressure control valve

•

Deformation rate can be controlled or varied during the stroke if required. This is especially important

when forging metals that are susceptible to rupture at high deformation rates

•

Split dies can be used to make parts with such features as offset flanges, projections, and backdraft,

which would be difficult or impossible to incorporate into hammer forgings

• When excessive heat transfer from the hot workpiece to the dies is not a pro

blem or can be eliminated,

the gentle squeezing action of a hydraulic press results in lower maintenance costs and increased die life

because of less shock as compared to other types of forging equipment

• Maximum press force can be limited to protect tooling

Some of the disadvantages of hydraulic presses are:

• The initial cost of a hydraulic press is higher than that of an equivalent mechanical press

• The action of a hydraulic press is slower than that of a mechanical press

• The slower action of a hydraul

ic press increases contact time between the dies and the workpiece. When

forging materials at high temperatures (such as nickel-

base alloys and titanium alloys), this results in

shortened die life because of heat transfer from the hot work metal to the dies

Press Drives

The operation of a hydraulic press is simple and based on the motion of a hydraulic piston guided in a cylinder. Two types

of drive systems are used on hydraulic presses: direct drive and accumulator drive. These are shown in Fig. 11.

Fig. 11 Schematic of drive systems for hydraulic presses. (a) Direct drive. (b) Accumulator drive.

See text for

details.

Direct drive presses for closed-die forging usually have hydraulic oil as the working medium. At the start of the

downstroke, the return cylinders are vented allowing the ram/slide assembly to fall by gravity. The reservoir used to fill

the cylinder as the ram is withdrawn can be pressurized to improve hydraulic flow characteristics, but this is not

mandatory. When the ram contacts the workpiece, the pilot operated check valve between the ram cylinder and the

reservoir closes, and the pump builds up pressure in the ram cylinder. Modern control systems are capable of very smooth

transitions from the advance mode to the forging mode.

In modern direct drive systems used for open die work (see Fig. 11a), a residual pressure is maintained in the return

cylinders during the downstroke by means of a pressure control valve. The ram/slide assembly is pumped down against

the return system backpressure, and dwell inherent in free fall is eliminated. When the press stroke is completed, that is,

when the upper ram reaches a predetermined position or when the pressure reaches a certain value, the oil pressure is

released and diverted to lift the ram. With this drive system, the maximum press load is available at any point during the

working stroke.

Accumulator-drive presses (Fig. 11b) usually have a water-oil emulsion as a working medium and use nitrogen or

air-loaded accumulators to keep the medium under pressure. Accumulator drives are used on presses with 25 MN (2800

tonf) capacity or greater. The sequence of operations is essentially similar to that for the direct-drive press except that the

pressure is built up by means of the pressurized water-oil emulsion in the accumulators. Consequently, the ram speed

under load is not directly dependent on pump characteristics and can vary, depending on the pressure in the accumulator,

the compressibility of the pressure medium, and the resistance of the workpiece to deformation.

Accumulator-drive presses can operate at faster speeds than direct-drive presses. The faster press speed permits rapid

working of materials, reduces the contact time between the tool and workpiece, and maximizes the amount of work

performed between reheats. Pressure build-up is related to workpiece resistance. Modern pumps can fully load in 100 ms-

-not much different than the opening time for large valves.

Capacity and Speed

Hydraulic presses are rated by the maximum amount of forging force available. Open-die presses are built with capacities

ranging from 1.8 to 125 MN (200 to 14,000 tonf), and closed-die presses range in size from 4.5 to 640 MN (500 to 72,000

tonf). Ram speeds during normal forging conditions vary from 635 to 7620 mm/min (25 to 300 in./min). Press speeds

have been slowed to a fraction of an inch per minute to forge materials that are extremely sensitive to deformation rate.

Hammers and Presses for Forging

Revised by Taylan Altan, The Ohio State University

Screw Presses

Screw presses are energy-restricted machines, and they use energy stored in a flywheel to provide the force for forging.

The rotating energy of inertia of the flywheel is converted to linear motion by a threaded screw attached to the flywheel

on one end and to the ram on the other end.

Screw presses are widely used in Europe for job-shop hardware forging, forging of brass and aluminum parts, precision

forging of turbine and compressor blades, hand tools, and gearlike parts. Recently, screw presses have also been

introduced in North America for a wide range of applications, notably, for forging steam turbine and jet engine

compressor blades and diesel engine crankshafts.

The screw press uses a friction, gear, electric, or hydraulic drive to accelerate the flywheel and the screw assembly, and it

converts the angular kinetic energy into the linear energy of the slide or ram. Figure 12 shows two basic designs of screw

presses.