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are changed in solid forming (cf. Fig.6.7.1). For dies weighing under 10

to 15kg, this is generally performed manually. For heavier dies, when

changing individual die blocks or for hot forming, die changing arms

with a hoisting power of 50 to 800kg are used (Fig.3.4.5). The up and

down movement is powered by electric motor in mechanical presses,

while hydraulic presses are fitted with hydraulic lifting cylinders. The

swivel or traversing movement into the die space can be executed man-

ually, or controlled by pushbuttons to the preselected die position.

Alternatively, fully automatic systems are also available.

3.4.2Die clamping devices

The clamping of dies onto the bed and slide clamping plates must be per-

formed as quickly and reliably as possible. Clamping devices are either

electrically or hydraulically powered, in some cases also in combination

with manual actuation by means of spring power, threaded spindles, tog-

gle lever, wedge action or self-locking mechanisms. Whichever means is

used, safety is always a prior concern (Fig.3.4.6). Different clamping

systems are often used for the upper and lower die, since the lower dies

Changing dies

Fig. 3.4.5 Die changing arm for solid forming

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

are pushed over the press bed, while the slide is lowered vertically on

the upper dies. The task of classifying the different clamping systems is

excessively complex, as the selection depends heavily on the required

clamping force, the type of die and press involved and the degree of

press automation.

Small dies are generally held by push-in clamps or clamping rails.

The rails are fixed on the clamping plate, while push-in clamps are

inserted into the T-slots on the clamping plate. The advantage of work-

ing with this type of small, lightweight fixture is that it restricts the

usable table surface only minimally and permits easy handling. If the

clamping force of the push-in clamps is not sufficient when connected

to the hydraulic system of the press at 250bar, it can be increased to

50kN by raising the line pressure to 400bar.

T-slot clamps are frequently used to secure medium-sized dies

(Fig. 3.4.7). A pressure of 250 bar is applied to the clamping cylinders by

the press hydraulic system. As a result, a clamping force of up to 200kN

can be generated. The clamps can be released by means of spring pow-

er after a drop in the oil pressure. The safety of the fixture is guaranteed

by positive locking action in the T-slot and monitored by pressure sen-

sors which bring the press to an immediate standstill in case of a drop

in pressure. For large dies with differing widths, it is necessary to use

Fundamentals of press design

Fig. 3.4.6 Different die clamp designs

clamped released

swivel-pullclamp

hydraulic

push-in-clamp

hydraulically

electro-

mechanically

electro-

mechanically

rotary-pullclamp

slide

clamping

plate

upperdie

T-slot

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

clamps that can be adjusted automatically to the dimensions of the die

used (Fig.3.4.8). These adjustable fixtures can be combined with all

types of clamps – hydraulic, electric and hydromechanical. If there are

no adjusting devices available, the clamps are mounted outside the

clamping plates, and the dies must be adjusted to the clamp spacing by

using adapter plates.

Changing dies

Fig. 3.4.7 T-slot clamp for upper and lower tool

mechanical release

slide

top die

T-slot bar

hydraulic clamping

T-slot bar

mechanical release

hydraulic clamping

bolster

plate

bottom die

Fig. 3.4.8 Automatic die clamping device with adjusting facility:

electromechanical clamp (left), hydraulic clamp (right)

adjustmentspindle

upperdie

slide

clampingplate

largestdie

smallestdie

T-slot

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

3 Fundamentals of press design

3.5 Press control systems

3.5.1 Functions of the control system

The functions performed by press control systems include the reading

of signals and data transmitted from the control elements and sensors

located on the machine and their conversion into control commands

to the machine’s actuators. The generation of control commands

depends on the selected operating mode such as “set-up”, “automatic

die change” and “automatic continuous operation”.

Functions to be monitored or initiated include, for example, safety

devices, clutch/brake, main drive system, lubrication, draw cushions,

die clamps, moving bolsters, part transport and automatic die change.

In addition, the control system executes machine functions and fault

diagnostics.

3.5.2 Electrical components of presses

The electrical components of presses and their peripherals are broken

down into three main areas:

– operating and visualization system,

– electrical control (located centrally in the switch cabinet or decen-

trally distributed in the unit),

– sensors and actuators in the press.

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

3.5.3Operating and visualization system

Process visualization is an instrument which makes the production

sequences of machines and plants ergonomically accessible. It supports

the operation, observation and monitoring of technical plant and

equipment. The control units are positioned in various locations

around the machine and are basically broken down according to three

categories:

–free-standing operator consoles,

–operator stations permanently integrated in the machine (Fig. 3.5.1)

and

–movable pendant operator stations.

Press control systems

Fig. 3.5.1

Operator station

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

Control functions can be subdivided into central and local systems:

central control functions refer to higher-level press functions, while

local control functions are used for manual operation of individual

functions directly at the press.

Control units are comprised of input and display units for operation

and visualization and can range from simple keys with pilot lamps

through to industrial PCs with high-resolution color monitors. The

main function of the visual display system is to support the press oper-

ator on start-up and repeat start-up of the press, for example following

a die change or machine fault, and to permit the input of various oper-

ating parameters (Fig.3.5.2). In addition, the visual display system can

be used to store die and machine data and for logging functions. Infor-

mation system functions which are integrated into the visual display

system offer the user support both for maintenance work and trou-

bleshooting. There is an increasing tendency for this type of system to

make use of multi-media technology (cf. Sect.4.6.7).

Fundamentals of press design

Fig. 3.5.2 Tooling data input mask for cam values

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

3.5.4Structure of electrical control systems

Electrical control systems are broken down into four main areas:

–the conventional part,

–the programmable logic controller (PLC),

–input and output systems and

–the communication system.

The conventional part of the electrical system consists of the power

supply, drive controllers, the individual electrical and electronic con-

trol devices and the low-voltage switchgear for power control monitor-

ing and attaching the components in the control cabinets and the

machine. The safety control represents a particularly important com-

ponent of the conventional part for all functions relevant to safety such

as the emergency stop, monitoring of safety gates or safety doors and

the engagement of the clutch/brake in mechanical presses.

In addition, the electrical control system must reliably prevent the

press from starting up or running through independently, following the

occurrence of any individual fault. In case of failure of any component,

it must ensure that no further press closing movement can be initiated.

The safety function of the conventional part of a hydraulic press

monitors the valves responsible for reliable standstill of a movement

which could lead to injury. These include the valves located in the low-

er cylinder area which control die closure. Hydraulic redundancy is

achieved by means of two monitored valves. Failure of one of these

does not lead to the execution of a hazardous movement. If one valve

fails, the press’s valve monitoring circuit automatically brings the press

to a standstill.

The PLC controls and monitors all machine functions not covered by

the conventional control circuits and automatic die change, performs

fault diagnostics and provides the interface to other machine sections.

The sensors and actuators of the machine are connected to the cen-

tral PLC by means of the input/output system. There are two different

technologies being used: first the central input/output modules which

are integrated in the PLC racks or in additional units of the control cab-

inet; second, on an increasing basis, local input/output modules within

the press connected via fast communication systems to the central PLC

unit (Fig. 3.5.3).

Press control systems

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

Fundamentals of press design

Fig. 3.5.3 PLC with central and distributed input/output systems

distributedI/O

... ... ... ...

switchcabinet

PLC

machine

sensorsandactuators

distributed

I/Omodule

distributed

I/Omodule

distributed

I/Omodule

distributed

I/Omodule

serialbus

centralI/O

... ... ... ...

switchcabinet

PLC

terminalbox

machine

paral

ndi

dual

gnal

paral

lel

indi

dual

gnal

sensorsandactuators

I/O

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

The communication system connects the PLC, and provides data

exchange with the visual displays within the machine and also com-

munication with external systems.

3.5.5Functional structure of the control system

Depending on the type of machine or press, the control system is bro-

ken down according to the following functional groupings:

–feed,

–main drive system,

–safety control,

–slide functions (interlocking, overload safeguard, weight compen-

sation, die clamping system, adjustment),

–

draw cushion or blank holder functions (pressure control, adjustment),

–moving bolster functions (traversing, lifting/lowering, clamping),

–functions of the workpiece transport devices and dies,

–hydraulic, pneumatic, lubrication functions,

–special die changing functions.

Cam operated limit switches play a special role in press control systems.

These press-specific functional units generate switching signals for cer-

tain functions depending on the position of the crank angle. A differ-

ence must be drawn between electromechanical rotating cam limit

switches (Fig.3.5.4), which are used for safety-relevant functions, and

electronic cam switches that are programmable. The latter comprise a

measuring system for the crank angle of the press and an electronic

evaluating module.

In contrast to mechanical presses, valves for the downstroke and

upstroke of the slide are separate in a hydraulic press, so eliminating the

need for a cam operated safety device.

3.5.6Major electronic control components

Safety control systems used in Europe are generally still the dual-chan-

nel, self-monitoring conventional type using contactors. In contrast,

since the mid-eighties, the safety control systems used in large-panel

Press control systems

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

presses for example in the US market have been frequently designed to

use two PLC systems, each monitoring the other. American PLC manu-

facturers have offered suitable solutions for this type of application for

some time. In Germany too, press safety control solutions based on

redundant PLC systems are becoming increasingly popular.

The systems used are standard commercially available PLCs

(Figs. 3.5.5and 3.5.6). They are adapted to the functional requirements

of the press by means of intelligent modules such as rotating cam limit

switches, fast positioning modules, fast and conventional communica-

tion modules and, where required, by independently operating task-

specific control systems.

Operations involving critical real-time functions and complex data

processing tasks which are not suitable for the PLC are processed by

independent subsystems. These include the control of the draw cush-

ions and multiple-axis part transport devices or fast feed systems, acqui-

sition, processing and display of sensor outputs associated with the

drawing process as well as acquisition and evaluation of machine and

production data. Hydraulic presses, for example, are equipped with

quick-acting pressure systems to control the parallelism of the slide.

Digital rectifiers and DC or three-phase asynchronous motors are used

for main, transfer and ancillary drive systems (Fig.3.5.7). Workpiece han-

dling systems are being increasingly equipped with dynamic digital ser-

vo drive systems, in which control intelligence is often integrated into

the drive system electronics (Fig. 3.5.8).

100

Fundamentals of press design

Fig. 3.5.4 Electromechanical rotary cam switch

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