Cold and Hot Forging: Fundamentals and Applications / Edited by Taylan Altan, Gracious Ngaile, Gangshu Shen

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CHAPTER 12

Special Machines for Forging

Pinak Barve

Fig. 12.2 Schematic of the reducer rolling operationFig. 12.1 Schematic of forging rolls for reducer rolling

12.1 Introduction

Prior to forging in an impression die, billet

stock must be often preformed to achieve ade-

quate material distribution, especially in hot

forging with ﬂash. Several special machines are

used for the purpose of preforming the incoming

stock. Some of these machines may also be used

for ﬁnish forging.

The principle of forging rolls, or reducer rolls,

is illustrated schematically in Fig. 12.1. This ma-

chine is generally used for volume distribution

in long and thin parts, prior to closed-die forg-

ing. A typical operation on reducer rolls, such

as those of Fig. 12.2, is as follows.

The operator or a robot places the heated

stock on a table in the front of the machine,

grasps the stock with the tongs, and starts the

machine with a foot pedal or automatic signal.

During the portion of the roll rotation, when the

rolls are in open position, the stock is placed

between the rolls against a stock gage and in line

with the ﬁrst roll groove. As the rolls rotate,

forging begins and the deformed stock is forced

toward the front of the machine [Altan et al.,

1973].

These sequences are repeated for the next

grooves. Thus, the shape of the die segments on

the forging rolls determines the rolled conﬁgu-

ration. An example illustrating the application

of preforming by roll forging, or reducer rolling,

is shown in Fig. 12.3. Another example, illus-

trating preforming for a truck axle forging, is

seen in Fig. 12.4.

In roll forging, the contact time between the

workpiece and the roll segments is extremely

short due to the high speed of the rolls. There-

fore, even after the ﬁnal rolling operation, the

workpiece is still hot and can be ﬁnish forged

under a hammer or press without reheating.

For high-volume production, the reducer roll-

ing operation is automated. The stock is gripped,

fed into the rolls, transferred from one die seg-

ment to the other, and released on a conveyor

using a dedicated robot. In producing long parts,

Cold and Hot Forging Fundamentals and Applications

Taylan Altan, Gracious Ngaile, Gangshu Shen, editors, p141-150

DOI:10.1361/chff2005p141

www.asminternational.org

142 / Cold and Hot Forging: Fundamentals and Applications

Fig. 12.3

Example of preforming by reducer rolling in forging of connecting rods. (a) Preforms prepared in reducer rolls. (b) Finish

forging before and after trimming. [Altan et al., 1973]

such as crankshafts and front axles, it is possible

to use vertical reducer rollers, as seen in Fig.

12.5.

The design of the roll segments needs consid-

erable experience. Recently, three-dimensional

FEM codes have been used to (a) simulate the

reducer rolling operation and (b) design and op-

timize the conﬁguration of the die segments.

Figure 12.6 shows, as an example, two roll

passes, simulated by the commercial software

DEFORM 3-D [SFTC, 2003]. The desired shape

is produced by rolling the heated billet between

two rotating dies having appropriately proﬁled

grooves. After the ﬁrst pass, the billet is fed into

the roll segments of the second pass after 90⬚

rotation.

12.2 Transverse or

Cross-Rolling Machines

Transverse rolling is used for producing pre-

forms or ﬁnish forgings from round billets. As

seen in Fig 12.7, a round billet is inserted trans-

versely between two or three rolls, which rotate

in the same direction and drive the billet [Altan

et al., 1973]. The rolls, which hold replaceable

die segments with appropriate impressions,

make one revolution while the workpiece rotates

several times in the opposite direction. Thus, the

transverse rolling method can form axially sym-

metrical shafts with complex geometry in one

operation. As an example, a forging produced

by this method is shown in Fig. 12.8. The trans-

verse rolling machines are suitable for automatic

production, using bar stock automatically fed to

the rolls through an induction heating unit.

There are two main types of transverse rolling

machines:

●

The two- or-three-roll machine, (Fig. 12.7)

●

A transverse rolling machine that uses two

straight wedge-shaped tools (Fig. 12.9)

12.3 Electric Upsetters

Electric upsetters are used mostly in preform

preparation for gathering a large amount of ma-

terial at one end of a round bar. The principle of

operation is illustrated in Fig. 12.10. A bar of

circular cross section (d) is gripped between the

tools (b) of the electrode (c) and is pushed by

the hydraulically or pneumatically operated up-

setting head against the anvil plate (f) on which

the other electrode (e) is secured. On switching

on the current, the rod section contained be-

tween the electrodes heats rapidly and the for-

mation of the head begins. The cold bar is con-

Special Machines for Forging / 143

tinuously fed between the gripping electrodes

(b), thus the metal accumulates continuously in

the head. The anvil electrode is gradually re-

tracted to give enough space for the formation

of the head. As soon as sufﬁcient quantity of

metal is gathered, the machine switches off and

the product can be removed by its cold end. Nor-

mally, the head is formed to ﬁnal shape in a me-

chanical or screw-type press in the same heat.

Thus, the process is suitable for manufacturing

components such as automotive exhaust valves,

or steam turbine blades [Altan et al., 1973].

The ﬂat anvil electrode can be replaced by a

water-cooled copper mold into which material is

gathered and formed to close shape and dimen-

sions. Material can be gathered at any point on

the length of the bar by placing a sheath around

one end. The only limitation on size is the avail-

ability of electric current. Commercially avail-

able equipment is capable of upsetting 0.5 to 5

in. (12.5 to 125 mm) diameter bars. As time for

upsetting a head of average size is 2 to 5 min,

several units are required for achieving high vol-

ume production.

12.4 Ring-Rolling Mills

The principle of operation of a horizontal

ring-rolling mill is illustrated in Fig. 12.11. The

vertical mills operate essentially in the same

way. The doughnut-shaped blank is placed over

a mandrel with a diameter smaller than the in-

side diameter of the blank. The mandrel, in mov-

ing laterally toward the main roll, applies pres-

sure on the blank. The main roll, which is driven,

rotates the blank and the mandrel as the cross

section of the blank is reduced. The axial rolls

provide support to the deforming ring and con-

trol its width and its squareness. As seen in Fig.

12.12, by modifying the conﬁgurations of the

mandrel and the main roll, it is possible to roll

rings with internal and external proﬁles [Beseler,

1969].

For components required in large quantities,

such as bearing races, completely automated

ring-rolling installations are available. Such an

installation may consist of a billet shear, a heat-

ing furnace, a forging press, and a ring-rolling

mill. The principles of an automatic horizontal

ring-rolling mill for manufacturing bearing races

Fig. 12.5

Schematic of a vertical reducer roller. 1, roll stand;

2, holder on manipulator; 3, slide; 4, rolls; 5, part

transfer conveyor. [Haller, 1982]

Fig. 12.4

Deformation stages in reducer rolling of a forging

to produce a truck axle. (a) Starting billet. (b)–(e)

Several reducer roll passes. (1)–(3) Locations where more mate-

rial needed in the ﬁnal forging. [Haller, 1982]

144 / Cold and Hot Forging: Fundamentals and Applications

Fig. 12.8

Forging produced in a transverse rolling machine.

[Neuberger et al., 1968]

are illustrated in Fig. 12.13. Four mandrels are

mounted in a rotating table, and the main roll,

eccentrically located within the table, is driven

independently by a variable-speed drive. The

blank placed over the mandrel, in position 1, is

rolled out into a ring as the clearance between

mandrel and main roll decreases. After having

passed the rolling zone, the hinged table seg-

ment is lifted by a cam operation, and the ﬁn-

ished ring is discharged from the machine.

12.5 Horizontal Forging

Machines or Upsetters

The horizontal forging machines are essen-

tially horizontal mechanical presses. As seen in

Fig. 12.14, these machines employ two gripper

dies, one stationary and the other movable. The

dies are closed side-to-side by a toggle mecha-

Fig. 12.7

Principle of operation of transverse rolling ma-

chines. [Neuberger et al., 1968]

Fig. 12.6

Computer simulation of reducer rolling operation

using DEFORM—3D (only two passes are shown).

(a) First pass. (b) Second pass after rotating 90⬚. (c) At the end of

the second pass. [SFTC, 2003]

Special Machines for Forging / 145

Fig. 12.10

Principle of operation of the electric upsetter.

See text for details. [Altan et al., 1973]

nism operated by a cam or eccentric located on

the eccentric shaft. Several matching die inserts

are placed in the gripper dies, while the slide

carrying the punches is moved by an eccentric-

pitman mechanism. The operational sequence of

a horizontal forging machine is illustrated in Fig.

12.15 for the upsetting process: (a) the hot end

of the bar is placed into the stationary gripper

die against a stop, (b) the moving gripper die

closes and the stop retracts, (c) the heading tool

begins to deform the bar, and (d) completes the

upsetting at the end of its stroke.

The upsetters are used for upset forging,

piercing, and reducing of bars and tubes. In au-

tomatic operation, the parts are transported by

ﬁnger-type cam-operated devices or walking-

beam-type transfer devices from one die cavity

to the next. Most horizontal forging machines

are designed such that the gripper dies are ori-

ented vertically, i.e., during closing action, the

movable gripping die moves horizontally

[Lange, 1958].

12.6 Rotary or

Orbital Forging Machines

The principles of rotary forging machines are

illustrated in Fig. 12.16 for a simple upsetting

operation.

Instead of the direct pressing action between

two ﬂat platens, the workpiece is subjected to a

combined rolling and pressing action between a

ﬂat bottom platen and a swiveling upper die with

a conical working face. The cone axis is inclined

so that the narrow sector in contact with the

workpiece is parallel to the lower platen. As the

cone rotates about the cone apex, the contact

zone also rotates. At the same time, the platens

are pressed toward each other so that the work-

piece is progressively compressed by the rolling

action. Press loading is appreciably less than that

of conventional upsetting because of relatively

small area of instantaneous contact. The appli-

cation of a rotary forging machine to a closed-

die forging operation is illustrated in Fig. 12.17.

12.7 Radial Forging Machines

In many applications, it is necessary to forge,

as ﬁnal product or as a preforming step, solid

Fig. 12.9

Principles of and tooling for transverse rolling ma-

chine with straight dies. (a) Operation. (b) Assem-

bly of simple die. [Altan et al., 1973]

146 / Cold and Hot Forging: Fundamentals and Applications

Fig. 12.12 Horizontal ring-rolling mill for producing rings with internal and external proﬁles. [Beseler, 1969]

Fig. 12.11 Operational principles of a horizontal ring-rolling mill. [Beseler, 1969]

shafts with varying or constant diameter along

the length and tubes with internal or external

varying diameter. All these parts require sym-

metrical reduction of cross sections. For some

of these applications, open-die forging presses

cannot be used economically because they are

limited in number of strokes per minute and in

the speed of feed and manipulation of the stock

during forging. In addition, large reductions be-

tween two ﬂat dies may cause excessive chilling

at the billet corners and cracking at the center of

the billet. Although these latter problems may

be reduced by using V-dies, each V-die set can

be used only for a certain dimension range and

frequent die changes during forging increase the

production costs. Figure 12.18 indicates that the

best solution for high-production symmetrical

reduction of cross sections is a forging system

that squeezes the material from all sizes simul-

taneously. There are several machine types de-

signed speciﬁcally for this purpose of radial or

draw-forging of axisymmetric parts [Haller,

1971].

Automated and computer-numerical-con-

trolled (CNC) radial forging machines can be

used for hot or cold forging, with two, three, or

four dies to produce solid or hollow, round,

square, rectangular, or proﬁled sections. Some

of the principal applications are production of

stepped shafts or tubes, sizing of solid bars such

Special Machines for Forging / 147

Fig. 12.14

Schematic of a horizontal forging machine. 1,

stationary gripping die; 2, movable gripping die;

3 and 4, end-die cavities; 5, eccentric shaft; 6, slide carrying the

punches; 7, upsetting and piercing punch. [Lange, 1958]

Fig. 12.13

Principle of semiautomatic ring-rolling machine

for manufacturing of bearing races. [Beseler,

1969]

as pilger mandrels, and sizing of the bores of

tubes to exact round or proﬁled shapes. The tol-

erances in hot forged tubes are about Ⳳ0.004 in.

(Ⳳ0.1 mm) on the inside diameter (ID). In cold

forging the outside diameter (OD) and ID tol-

erances are about Ⳳ0.004 (Ⳳ0.1 mm) and

Ⳳ0.001 in. (0.025 mm), respectively [Walter,

1965].

There are two types of radial precision forging

machines: vertical and horizontal. Both models

use essentially the same design principles. The

vertical models are suitable only for relatively

short components and are difﬁcult to automate.

The horizontal models are built in several vari-

ations depending on the application. The hori-

zontal machines consist of a forging box with

gear drive, one or two chuck heads to manipu-

late the workpiece, centering devices, and nec-

essary hydraulic and electronic control compo-

nents (Fig. 12.19). The core of the machine is a

robust cast steel forging box that absorbs all

Fig. 12.15 Operating sequence in upsetting on a horizontal forging machine

148 / Cold and Hot Forging: Fundamentals and Applications

Fig. 12.16

Principle of rotary or orbital forging machines.

[Altan et al., 1973]

Fig. 12.17

Illustration of closed-die forging with a rotary

forging machine. 1, rotating upper platen; 2,

workpiece; 3, lower die; 4, ejector

Fig. 12.18

Deformation of a round cross section in stretch

forging. P, load. (a) Between ﬂat anvils. (b) Be-

tween four curves of a radial forging machine. [Haller, 1971]

forging forces (Fig. 12.20). It is mounted with

the gear box in a support bolted to the founda-

tion frame. The forging box contains four rotat-

able adjustment housings in which the eccentric

shafts are mounted. The eccentric shafts, which

are driven by an electric motor through a gear

system, actuate the connecting rods and the forg-

ing dies at a rate of 250 to 1800 strokes/min.

The stroke position of the connecting rods (or

dies) is adjusted in pairs independently (for rec-

tangles or special shapes) or in unison (for

rounds and squares) by rotating each adjustment

housing through a link, screw, adjustment nut,

and worm gear drive powered by one or two

hydraulic motors. Each adjustment nut rests,

through a piston, on an oil cushion of a hydraulic

cylinder. During operation, the forging pressure

generates in this oil cushion a pressure propor-

tional (about 20%) to the forging pressure. The

pressure in this oil cushion is continuously

monitored, and if it exceeds a certain limit the

adjustment housings are immediately rotated to

bring the dies to open position while the move-

ment of the chuck heads is stopped simulta-

neously. This system protects the machine from

overloading [Altan et al., 1973].

Depending on the machine type, one or two

hydraulically driven chuck heads are provided

Fig. 12.19 Schematic of a GFM radial precision forging machine with two chuck heads. [Walter, 1965]

Special Machines for Forging / 149

Fig. 12.20

Forging box of a radial precision forging machine illustrating the tool function and adjustment. (a) Dies. (b) Pitman arm.

(c) Guides. (d) Eccentric shaft. (e) Adjustment housing. (f) Adjustment screw. (g) Worm gear drive. (h) Adjustment input.

(i) Adjustable cam. (k) Forging box. [Altan et al., 1973]

Fig. 12.21

Typical examples of stepped shafts produced in

precision radial forging machines. [Altan et al.,

1973]

for holding and manipulating the workpiece dur-

ing forging. Two chuck heads are used when the

workpiece must be forged over its entire length,

including the chucked ends, in one heat. During

forging, round components are rotated while

square, rectangular, and proﬁled sections are

forged at ﬁxed positions. The movement of the

chuck heads and the variation of the forging

stroke in forging stepped components can be

controlled by numerical control. For the forging

of tubular parts, the chuck heads are provided

with stationary or movable mandrels, which are

cooled internally in hot forging applications.

Central water cooling and lubrication of critical

machine components are carried out automati-

cally during the operation of the machine. The

radial precision forging machines are capable of

producing parts similar to those shown in Fig.

12.21.

REFERENCES

[Altan et al., 1973]: Altan, T., Boulger, F.W.,

Becker, J.R., Akgermon, N., Henning, H.J.,

“Forging Equipment, Materials, and Prac-

tices,” Metals and Ceramics Information Cen-

ter, Battelle Columbus Laboratories, 1973.

[Beseler, 1969]: Beseler, K.H., “Modern Ring-

Rolling Practice,” Met. Form., Vol 36, Feb

1969, p 1.

[Haller, 1971]: Haller, H.W., Handbook of Forg-

ing (in German), Hanser Verlag, 1971.

[Haller, 1982]: Haller, H.W., Practice of Im-

pression Die Forging (in German), Hanser

Verlag, 1982.

150 / Cold and Hot Forging: Fundamentals and Applications

[Lange, 1958]: Lange, K., Closed-Die Forging

of Steel (in German), Springer-Verlag, Berlin,

1958.

[Neuberger et al., 1968]: Neuberger, F., et al.,

“Transverse Rolling,” Met. Form., Vol 35, Oct

1968, p 1.

[SFTC, 2003]: Scientiﬁc Forming Technologies

Corp., DEFORM 2D and 3D Software, Co-

lumbus, OH, 2003.

[Walter, 1965]: Walter, L., “Use of Precision

Forging Machines,” Met. Treat., Aug 1965, p

296.