Altan T. Metal Forming Handbook

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In horizontal operating machines, the blanks drop under the force of

gravity into the indexed dial feed plate, where they are fed towards the

coining station. To carry out the coining operation, the indexed plate

stops. After coining, the coin is ejected from the minting collar by the

ejector, which is connected to the press slide, and discharged. The short

stroke of the link drive system permits the ejector movement to be

derived directly from the slide. The ejector travel is reduced again in

proportion to the slide stroke by means of a system of levers. Conse-

quently, temporal offset between the two movements, as a result of tol-

erances and clearances in the drive elements, is eliminated. The system

is not subject to wear, as neither cams nor rollers are used.

The elastic deflection of the press frame which takes place during

coining is measured by a quartz crystal transducer and fed via a charge

531

Presses used for solid forming

Fig. 6.8.22 Schematic of the rotary feed system

automatic blank

discharge

feed channel

hopper

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

amplifier with peak value memory to the programmable logic con-

troller for evaluation. The pulses are counted after the coining force

exceeds a certain minimum value.This ensures that only coining

processes which have really been carried out can be picked up and

counted. At the same time, the coining force is displayed and moni-

tored relative to a preset limiting value. In case of a force value that is

above or below the prescribed tolerance limits, the coining press is

automatically switched off.

In the case of vertically operating presses, the blanks are fed via a chan-

nel into a filling pipe. From here, they are pushed into the indexed

plate by means of a cam-controlled feeding device(Fig. 6.8.24). The dri-

ve is equipped with a backlash-free mechanical indexing gear with

extremely high indexing accuracy and optimum acceleration and

deceleration characteristics. The indexing gear is enclosed and runs in

an oil bath.

To protect the indexing drive against overloads which are not

process-related, all coining presses are equipped with a synchronous

532

Solid forming (Forging)

Fig. 6.8.23 Automatic circular blank control and discharge device

clamping lever stop cylinder

sensor

off-size

blanks

correct

blanks

lift cylinder

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

overload clutch. This overload clutch is attached without backlash to

the output drive shaft of the indexing drive. The connection between

the drive shaft and indexed plate is formed by means of several spring-

loaded rollers located in appropriate recesses. In case of an overload, the

rollers disengage and the part of the clutch which is attached to the dri-

ve shaft can still rotate in ball bearings with respect to the blocked

indexed plate. It is only possible to re-engage the clutch in a position

ensuring synchronous movement with the press.

The crank angle of approx. 120°, required for the indexed plate to

switch around, is achieved in coining presses with a normal knuckle-

joint drive system (cf. Fig. 3.2.5)by means of a sufficiently dimensioned

idle stroke, i. e. when there is no connection between the slide die and

slide. In presses with modified knuckle-joint drive (cf. Fig. 3.2.6),the

533

Presses used for solid forming

Fig. 6.8.24 Feed dial plate with overload safeguard

filling station

control

station

coining

station

indexing gear

feed pocket

dial feed plate

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

standstill of the slide is reached at the bottom or rear dead center. The

idle phase continues over one third of the total cycle time (Fig. 6.8.3).

During this comparatively long standstill phase, the produced coin can

be reliably discharged and simultaneously the next coin blank moved

precisely into the coining position. In addition, a firm connection is

provided between the slide and die. The benefit of this firm connection

is that it prevents the impact of the slide on the back of the punch

(hammer effect) and resulting unwanted vibrations.

Coining dies

To enhance the service life of coining dies, the coining punches are

hard chromium plated. The achievable service life ranges between

approx. 400,000 to 800,000 coining strokes. This figure increases three

or four-fold when using titanium nitride-coated punches (Fig.6.8.25).

Generally rings with a carbide insert are used as minting collars. They

have a service life of between 70 and 100 million coining processes. The

admissible specific compressive strain for the coining punch ranges

between 1,500 and 1,800 N/mm

Universal minting press for coins and medals

In cases where both change and collectors’ coins with improved surface

quality have to be produced, a vertical universal minting press with a

nominal press force of 3,000 kN is used. The design of this press is the

same as that used for coining presses for the production of change

coins. However, this press also offers the facility to preselect several

coining blows in order to improve surface quality. Up to four coining

blows can be pre-selected at the control desk. In contrast to standard

minting presses, the dial feed plate is driven in this press type by a ser-

vo motor in conjunction with an indexing gear. The dial feed plate

comes to a standstill for execution of the coining impacts. The same

applies to the ejector. To manufacture coins with a particularly high

standard of surface quality, the press is equipped with a manual feeding

device. The press also provides a suitable facility for punch cleaning.

The machine produces within a diameter range of 14 to 50mm, and

reaches stroking rates of 400/min when a single coining blow is used,

and between 75 and 150/min in multiple blow operation.

534

Solid forming (Forging)

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

Universal press for joining, minting and punching

Based on the underlying concept of the vertical 1,500 kN minting press,

in addition to the minting of standard, multi-sided and bimetal coins,

two further application possibilities are offered:

–The press can be used to separate the rings and centers of used bimet-

al coins which are being withdrawn from circulation. Separation of

the different materials offers clear advantages for recycling.

–Designed for piercing, the press permits the manufacture of outer

rings from upset coin blanks.

Normally, rings for bimetal coinsare produced at high stroking rates in

multiple dies on a high-speed blanking press (cf. Fig. 4.6.6and 4.6.8).

Due to the minimal stability of the rings, however, subsequent upset-

ting is only possible to a limited degree.

Experience has shown that the reduction in diameter which takes

place here lies around 0.1 to a maximum of 0.2mm. Due to the low web

width, it may be necessary to upset the edge in several passes in order

to achieve the specified diameter. In practice, a cylindrical upsetting

shape with a 45°chamfer on each side was found to be successful

(Fig. 6.8.17).In the case of rings produced on universal presses in mints,

however, any optional upsetting shapes and edge decoration can be

used if the coin blanks are rimmed or edge lettering is carried out prior

to piercing.

535

Presses used for solid forming

Fig. 6.8.25

Titanium nitride-coated coining punches

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

Figure 6.8.26indicates the individual phases required for the piercing

process:

–The upset coin blank is centered in the piercing station by the feed

insert. The slide is positioned in the bottom dead center.

–The slide moves upwards, the coin blank is held between the female

piercing die and the spring-mounted stripper.

–The coin blank is inserted in the holding collar and pressed against

the fixed piercing punch.

–At the top dead center, the piercing process is completed, the scrap

web is pushed through the piercing punch into the scrap hole of the

female piercing die.

–On the return stroke of the slide, the spring-mounted stripper push-

es the pierced coin blank out of the holding collar back into the feed

insert. The pierced coin blank is discharged by the indexed dial feed

plate, while a new upset unpierced coin blank is fed for the next

cycle.

536

Solid forming (Forging)

Fig. 6.8.26 Piercing sequence

down stroke,

blank feed

contact with

ejector

contact with

piercing punch

up stroke,

blank pierced

down stroke,

discharge of

pierced blank

holding collar

spring-loaded

stripper

piercing punch

ejector

piercing die

feeder insert

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

Medal minting presses

Presses with a nominal force of 2,000, 3,600 and 10,000 kN are used to

produce medals and commemorative coins. Depending on the press

type, the maximum medal diameter is between 38 and 80mm. The

stroking rates range from 45 to 120/min. The resulting output depends

on the number of coining strokes:

Single stroke: 25 – 40 medals/min

Double stroke: 16 – 30 medals/min

Triple stroke: 12 – 24 medals/min

Complex designs and even difficult medal contours can thus be manu-

factured in a single stroke. For high-grade medals, knuckle-joint presses

are configured for multiple stroke operation, as it is only with repeated

coining that the required standard of surface quality can be achieved.

The press frame serves as a slide which is moved by the knuckle-joint,

whereby a rigid connection exists between the press bed and the press

housing (cf. Fig. 3.2.5).This results in practically shock-free closure of

the dies.

Automatic medal minting machines are equipped with mechanical,

cam-driven ejectors. The drive cam disk sits directly on the crankshaft

and ensures synchronous operation with the press work cycle. For the

medal minting process, the ejector is controlled to wait until the num-

ber of blows, preselected in the multiple stroke device, has been com-

pleted.

For the manufacture of top quality medals, ultra-clean, dust-free oper-

ation must be guaranteed. The machines are equipped with ventilating

and filtration devices which permit only filtered air to enter the die area.

The die mounting area is also enclosed and is slightly pressurized. The

highest degree of quality is achieved in medal minting presses using

manual feed methods (Fig. 6.8.27).

In order to ensure the quality of the medals, the punches must be

cleaned at short intervals. By pressing a button, the ejector moves to

the cleaning position, the bottom die is raised around 2 mm out of the

minting collar and dirt particles or spilled cleaning materials can be

wiped away.

Medals with enhanced surface quality for collectors are often pro-

duced in high piece numbers. A cassette feed system is ideally suited for

537

Presses used for solid forming

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

this type of application (Fig.6.8.28).In view of the stringent quality

requirements, an automatic cassette feed system must ensure particular-

ly gentle transport of the medal blanks. Only one blank is removed from

the cassette magazine at a time and lowered onto the transfer level. Plas-

tic grippers lift the blank into an intermediate deposit station and trans-

port it to the minting station.

The subsequent blank remains clamped in the medal magazine until

the transport area is clear. After the minting process, the grippers lift the

minted medal from the bottom die, which is located in the ejection

position, and place it on a conveyor belt which transports it out of the

minting area. A turning station, connected to the conveyor belt, per-

mits the operator to inspect both sides of the medal during running

production.

538

Solid forming (Forging)

Fig. 6.8.27

Medal minting press with

manual feed

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

CNC-controlled hydraulic medal minting presses

These presses permit path or pressure-dependent control of each stroke

of the slide, as well as synchronous operation of the slide and ejector by

two CNC axes. In case of multiple stroke operation, the die remains

closed without opening, i. e. in contact with the medal. Only the coin-

ing stress between the upper and lower die is released. As a result, pen-

etration of abraded matter between the dies and the coined piece is pre-

vented.

The nominal press force of the machine series ranges between 4,000

and 12,000 kN. In addition to minting of medals, coins and jewellery,

the machine can also be used for the cold hobbing of tools, for example

in the manufacture of coining punches, die sinks, jewellery moulds,

dies for fasteners, dies for cutlery, etc. The die sinking velocity required

for this technique ranges between 0.01 and 4 mm/s (Fig. 6.8.29).

539

Presses used for solid forming

Fig. 6.8.28 Cassette feed for medal production

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

To comply with the very highest quality requirements and when pro-

ducing difficult and complex reliefs, it is possible to use an oscillating

technique with varying press forces (Fig. 6.8.30).

The hydraulic double-action press comprises a press frame with a

closed frame structure, an integrated drive cylinder and separate power

drive. The press slide (coining cylinder) is CNC-controlled and has a

multiple-surface design.

Based on the principle of differential pressure surfaces, optimum lev-

els of output are achieved with low energy consumption. The ejector

cylinder, also controllable by CNC, is integrated underneath in the

press frame. The cheeks of the press yoke are positioned directly against

the die system and are slightly larger than the piston rod, so guarantee-

ing optimum rigidity. The traversing motions are controlled by means

of servo control valves with incremental setpoint inputs and mechani-

cal actual value feedback.

540

Solid forming (Forging)

Fig. 6.8.29 Hydraulic medal minting press

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