Altan T. Metal Forming Handbook

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tation. As this would render the lubricant unusable, bactericides are

generally added to the lubricants by most suppliers. The lubricant

quantities are set using simple manual control devices. Automatic

devices for controlling the lubricant quantity are available. Where

lubricants contain graphite, an agitating device in the reservoir is essen-

tial in order to prevent solid graphite particles from settling down in

the reservoir.

Spray systems with compressed air as carrier

Mist coolant application systems provide an extremely fine surface film

of coolant and lubricant for optimum cooling and lubrication. Coolants

and lubricants are fed using a joint pipe. Water is used for cooling, while

depending on the process involved, different lubricating agents can be

used. Graphite-free emulsions containing water-mixable oils are gener-

ally used for simple forming operations. Where more complex forming

processes have to be performed (such as constant velocity joint compo-

nents used in vehicle drive systems), emulsions made of oil and graphite

are used. These emulsions comprise a 1 : 1.3 to 1 : 4 mixture with water.

The water and lubricant quantities are controlled alternatively by elec-

tro-valves. Compressed air is used as a carrier medium for the lubricant.

Compressed air and water or lubricant are mixed to create a mist in a

mixing valve and fed to the die through a joint pipe. The electro-valves

are controlled individually for each station.

Cooling systems for forging

During automatic forging, high-pressure spray systems (80–120 bar) are

used for cooling purposes. There is generally no need for additional

lubrication. In order to achieve the necessary degree of heat dissipation,

considerable quantities of water are required (between 40 and 60 m

/h).

The cooling water is cleaned, recooled and then re-used. The scale col-

lected in the water must be removed using screens and filters.

Billet heating

Billets are heated by induction. For the temperature range of 760 to

800 °C, inductor outputs of approx. 0.24 kW are required per kg of steel.

Depending on the billet diameter, the frequency range lies between 2

and 4 kHz. For forging temperatures from 950 to 1,100 °C, around 0.40

to 0.45 kW/kg of steel are required at frequencies of 1 to 2 kHz. In gen-

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Presses used for solid forming

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

eral, aluminium and aluminium alloys are also heated by induction to

temperatures between 420 and 480 °C, depending on the alloy. The

required inductor output is approx. 0.35 to 0.4 kW/kg with a frequency

of 0.5 to 1 kHz.

Thyristor-controlled systems are used as converters. Considerable

quantities of cooling water are required to cool the inductors. Recool-

ing is performed in the plant’s own recooling systems or in separate

installations.

Billet preparation

Warm forming (680 to 800 °C) of shaft-shaped parts made of steel fre-

quently calls for coating (pre-graphitizing) of blanks. This measure

serves to increase die service life and is particularly beneficial for first

pressing operations (generally forward extrusion or reducing operations).

The graphite can either be tumbled on prior to heating, or sprayed on

during the heating process. This entails inductive pre-heating of the

blanks (approx. 180 °C), followed by spray application of the graphite-

water mixture. The water evaporates over a relatively short travel, during

transport to the main induction heater. Thus, the billets are heated with

a dried-on coating of graphite. Although a proportion of the applied

graphite burns, a relatively high percentage (approx. 60 %) still permits

lubrication and, together with a (very thin) scale layer, creates the

desired lubricating and separating effect in the die.

6.8.6 Sizing and coining presses

Sizing of forged and sintered parts

The dimensions, tolerances and surface quality of forged or hot formed

parts frequently fail to meet the standards expected from finished prod-

ucts. Consequently, often costly finish processing operations are neces-

sary. By introducing a sizing operation, substantially narrower toler-

ances can be achieved and subsequent machining processes can be

reduced.

Many forged parts are finish machined on machining centers. These

call for precise clamping and contact surfaces in order to assure a repro-

ducible and well defined workpiece clamping. It is frequently not pos-

sible to achieve, in forged parts, a surface quality that is required for

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Solid forming (Forging)

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

adequate clamping. In this case, cold sizing of the appropriate surfaces is

an essential pre-requisite for successful further processing (Fig.6.8.13).

Sintered parts are also frequently subjected to a subsequent compacting

or sizing process in order to compensate for warping caused by heat

treatment (Fig.6.8.14).All these operations require a large forming

force, coupled with minimal machine and die deflection. Due to its

favorable force distribution, the knuckle-joint bottom drive press has

been shown to offer the most suitable forming system for coining appli-

cations. The basic principle of this drive type is described in Sect. 3.2.2

and 6.8.2 (cf.Fig.3.2.5).

Compared to presses for solid forming, the slide stroke and work

length can be substantially shorter. Sizing is generally performed in a

single station, so allowing the die mounting area or the press bed to be

smaller. Ejector systems in the slide and press bed are only necessary for

coining work.

Cutlery embossing presses

In addition to possible applications already outlined here, this type of

sizing press is also used with a good degree of success for the forming

and embossing of cutlery. This operation allows even the finest chis-

elling effects to be precisely embossed.

523

Presses used for solid forming

Fig. 6.8.13

Coined forgings

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

524

Solid forming (Forging)

Fig. 6.8.14 Coined sintered parts

Fig. 6.8.15 Operating sequence for the manufacture of open wrenches

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

Embossing of hand tools

Another field of application for calibrating presses is the embossing of

hand tools. Figure 6.8.15illustrates the sequence of operations involved

in the manufacture of an open wrench. Using slugs blanked from plate,

the parts are manufactured in eight operations on a knuckle-joint press

with bottom drive, nominal press force 16,000 kN (cf.Fig.3.2.5).The

production stroking rate is 35 parts per minute. The plant operates on a

fully automatic basis. The slugs are stored, and a two-axis transfer is

used as a feed system (Fig. 6.8.11). In order to assure fast die change, die

change sets are used. These can be moved in and out, controlled by

pushbutton, by a die change cart (Fig. 6.8.16).The die sets are fastened

by means of hydraulic clamps (cf. Fig. 6.7.1).

525

Presses used for solid forming

Fig. 6.8.16 Line for the manufacture of open wrenches

(nominal press force 16,000 kN, transfer feed and die change cart)

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

6.8.7Minting and coin blanking lines

Rimming and edge lettering machines

Coin blanks are manufactured on high-speed blanking presses from an

indexed sheet metal strip (cf. Fig.4.6.6and 4.6.8).Depending on the

material characteristics and the condition of the die, a rough cut surface

and/or blanking burr are formed at the outside surface of the coin blank

(cf.Fig. 4.7.8).To achieve a high level of coin quality, the outside edge

of the coin blank must be smoothed. This is achieved by a radial upset-

ting process in rimming and edge lettering machines.

When smoothing the edge, the coin blank is sized within extremely

narrow tolerance limits (+/– 0.02 mm). Reducing the diameter by 0.2 to

0.5mm causes material to accumulate at the periphery of the coin

blank. Depending on the intended purpose, different upsetting shapes

can be selected (Fig. 6.8.17).The rimming or upsetting process helps to

accumulate the material necessary for forming the rim area of a coin.

Thus, this material does not have to be forced outwards from the center

of the coin blank. As a result, the coining process, conducted after upset-

ting, requires less force. Reduction of the coining force leads in turn to

an increase in the service life of the coining dies.

Rimming machines are used for a diameter range from 14 to 50 mm

both with vertically and horizontally arranged upsetting shafts. In rim-

ming machines with a vertical upsetting shaft, the blanks are fed

towards the upset die, which comprises the upset ring and upset seg-

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Solid forming (Forging)

Fig. 6.8.17 Upsetting shapes on coin blanks

t=upsetthickness

unprocessedblank shape R :highlyrounded

shape

P :slightlyrounded

shape

M :flatknurling shape S :flatknurling

s=blankedthickness

D>D

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

ment, by means of two diametrically arranged mechanical hoppers and

channels(Fig.6.8.18).During the rolling process between the upset ring

and upset segment, diameter reduction takes place. Rimming machines

with horizontally arranged upsetting shaft can be used both for rim-

ming or upsetting, and also for embossing deep-set edge lettering and

decoration of coin blanks. Edge lettering is carried out around the

periphery of the coin blank in a separate work cycle in which the previ-

ously upset blank is rolled and deep-set between the upset ring and the

lettering jaws.

Depending on the machine type and the diameter of the coin blank,

rimming machines achieve an output of 130,000 and 600,000 pcs/hour.

Minting presses

For minting applications (change coins with a diameter of between 14

and 40mm) preferably presses with knuckle-joint drive or with modi-

fied knuckle-joint drive system are used (cf.Sect.3.2) with nominal

press forces from 1,000 to 2,000 kN and stroking rates between 650 and

850/min (Fig. 6.8.19).

527

Presses used for solid forming

Fig. 6.8.18

High-performance rimming

(upsetting) machine

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

Minting presses are designed in a vertical and also horizontal version.

Horizontally working presses are used to produce round coins. This

machine design is characterized by simple operation and a low number

of coin-specific exchange parts (Fig. 6.8.20).Vertically operating presses

can be universally applied for the production of round, multiple-edged

and bimetal coins (Fig. 6.8.21);however, a greater number of coin-spe-

cific exchange parts is required here.

The monobloc press frame made of nodular cast iron offers good

damping properties (cf. Fig.3.1.1).The press is driven by means of a

controllable electric motor, flywheel, eccentric shaft and knuckle-joint.

The bearing points in the knuckle-joint (friction bearing) are config-

ured for a maximum specific compression stress of 110 N/mm

. The

slide is mounted without backlash in pre-tensioned roller gibs. The

indexed dial feed plate for infeed and outfeed transport of the coin

blanks and coins is driven by means of a synchronous timing belt from

the eccentric shaft. The coin blanks are fed by means of mechanical

hoppers and channels towards the indexed dial feed plate. The ejector

system is directly linked to the slide and safeguarded against overload-

ing. The coining dies can be exchanged (coining punch and minting

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Solid forming (Forging)

Fig. 6.8.19 Press shop of a modern mint

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

collar) within 2 to 3 min. The machine rests on vibration absorbing ele-

ments. As a result, no more than 10 % of the static load is transmitted

into the foundation as dynamic loading. This means that the presses can

be set up on upper floors. These presses are generally equipped with

sound enclosures in order to minimize noise generation.

The coin blanks are fed towards a mechanical hopper via an inclined

conveyer with storage space or other metering systems (Fig.6.8.22). The

hopper operates according to the centrifugal principle, which ensures

particularly gentle handling of the blanks. The possibility of scouring is

minimal, and noise development is also accordingly low.

The hoppers have a slight incline. The feed plate is directly driven by

an adjustable gear motor. The outfeed channel is tangentially located.

Depending on the diameter of the blanks, this arrangement permits

feed rates of up to 3,000 blanks/min.

When resetting for the production of different coin sizes, only the

blank outfeed channel and thickness need to be adjusted. No exchange

parts are required. Depending on the press design, the hopper and feed

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Presses used for solid forming

Fig. 6.8.20 Horizontal minting press

monobloc press frame

punch holder

slide

link drive system

main drive

dial feed plate

feed hopper

blank control and

discharge device

punch holder

electronic

press

control

ejector

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

channel can be swivelled out or moved completely out of the press for

die change.

In the following channel, the blanks are checked for diameter and

thickness related defects, and ejected if found to be outside the permissi-

ble tolerance (Fig.6.8.23).The standstill periods caused by off-size blanks

would seriously impair the efficiency of high-speed machines. Therefore,

coin blanks with defects are automatically segregated in a control station

while the machine is running. If a defective blank reaches the control sta-

tion, the flow of blanks is interrupted. The interruption triggers a sensor,

which disables further blank feed from the hopper by means of a stop

cylinder. The gauge jaws are opened by means of a lift cylinder and the

off-size blank is ejected into a collecting tray. The gauge jaws return to

their home position, the stop cylinder opens and blank feed continues.

The described cycle takes only fractions of a second, ensuring a continu-

ous supply of blanks and preventing production disruptions.

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Solid forming (Forging)

Fig. 6.8.21 Vertical minting press

ejector

electronic press

control system

punch holder

dial feed plate

punch holder

monobloc press frame

link drive system

blank control and

discharge device

feed hopper

slide

main drive

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998