Tsch?tsch H., Koth A. Metal Forming Practise: Processes - Machines

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27.3 Automatic punching presses 341

As well as the geometric accuracy of the bear-

ing and guide elements, the arrangement of the

ram guide system is a decisive factor in the

accuracy of the punched parts and the tool

lives. In this press, the ideal solution was

found: the four-column guide system. The belt

feed system consists of a combined gripper and

roller feed í a Bruderer system í which can

achieve feed accuracies of 0.01 to 0.02 mm.

Figure 27.19

Operation of an automatic punching press with mass

counterbalancing í Bruderer system í with four-

column guide (Illustration from Bruderer works,

Frasnacht, Switzerland)

Figure 27.20 is an automatic double-sided shearing / forming press from the HQR series, with

a short belt conveyor and a numerically-controlled straightening and feeding device. This press

was designed for press forces of 2000 í6300 kN. The frame is a welded steel construction.

Figure 27.20 HQR-type automatic shearing and forming press with belt conveyor and numerically-

controlled straightening and feeding device (Photograph: Müller Weingarten AG , Weingar-

ten, Germany)

342 27 Special-purpose presses

It is manufactured to order and so can be adapted to suit the vertical capacity of the customer’s

tooling The frame construction has been optimised using the Finite-Element Method. The

individual elements of the press and all the know-how were integrated using a building-block

method.

The press is powered by a cross-shaft drive (Figure 27.21) with two connecting rods. The drive

moment is transmitted from the driving motor, which has a variable stroke rate, by means of

the flywheel and the pneumatic clutch/brake combination, to the pinion shaft. From here, half

the power is carried to the left eccentric shaft and half to the right one by an intermediate gear.

In this drive principle, the two eccentric shafts turn in opposite directions, each thus compen-

sating for the other’s rotational forces without any extra mass. This means that the braking

process, when set off by a safety function, for example, has the shortest possible brake angle.

Figure 27.21

Cross-shaft drive

1 driving motor,

2 flat belt,

3 eccentric bush,

4 eccentric shaft,

5 flywheel,

6 clutch/brake combination,

7 driver shaft,

8 intermediate gear,

9 herringbone gear,

10 ram,

11 pneumatic cushion,

12 ram counterweight,

13 ram mounting,

14 slideway,

15 connecting rod

(Illustration: Müller-Weingarten AG

works, Weingarten, Germany)

The ram stroke (Figure 27.22) can be automatically adjusted in steps. The required ram stroke

is set by twisting the two eccentric bushes in relation to the eccentric shafts. To do this, the

eccentric bushes are held in the locking device using synchronising discs and the eccentric

shaft is twisted by moving the sliding sleeve, by means of the main drive. The sliding sleeve

has inner dog teeth which form a friction lock between the eccentric bush and the eccentric

shaft. The ram is adjusted (ram position) by turning the ball spindle (Figure 27.23) using worm

gears. This changes the depth by which the ball spindle enters the connecting rod and thus the

distance between the ram and the table. To stop the press from being overloaded, the ball spin-

dle presses against a piston onto a cavity containing oil which is pressurised according to the

nominal press force. When there is an overload the oil below the piston suddenly escapes,

creating a space for overload relief.

The press is CNC controlled. An industrial computer controls all the functions of the press and

furthermore all peripheral devices such as the sheet conveyor, the roller feed, the workpiece

transporter and the device for ejecting finished parts. An extensive package of various software

modules allows the computer system to be adapted to a task as needed. All necessary informa-

27.3 Automatic punching presses 343

tion is visualised on the screen which at the same time acts as a programming interface for the

stored-programme control system.

Figure 27.22

Stroke setting (stroke length)

1 herringbone gear,

2 eccentric shaft,

5 synchronising disc,

6 connecting rod,

7 eccentric bush,

8 ring gear,

9 sliding sleeve,

10 ring gear,

13 hydraulic cylinder,

14 hydraulic piston

(Illustration: Müller-Weingarten AG

works, Weingarten, Germany)

Figure 27.23 Elements for adjusting the ram (ram position)

1 connecting rod, 2 ball spindle, 3 ball flange, 4 worm gear, 5 worm shaft, 6 roller chain, 7

electric motor, 8 sliding block, 9 piston, 10 toothed belt, 11 angle encoder, 12 oil cavity (Il-

lustration from Müller Weingarten AG works, Weingarten, Germany)

344 27 Special-purpose presses

Table 27.3 Technical specifications for HQR automatic punching presses

Nominal press force kN

2000 – 6300

Stroke rate per min.

80 – 150

Ram stroke mm

200 – 250

Bolster plate mm

Length

1900 – 2800

Depth

1100 – 1400

27.4 Exercise on Chapter 27

1. What are the distinctive features of a transfer drawing press and what workpieces is it

used for?

2. How do transfer presses for bulk forming differ and what are they used for?

3. What number of strokes per minute do automatic punching presses have?

4. Why do automatic punching presses require mass counterbalancing?

5. How is the stroke adjusted on an eccentric press?

6. Why are hydraulic extrusion presses used for long extruded workpieces?

7. What is the advantage of transfer presses?

8. What different types of transfer press construction are there?

9. Which workpieces are manufactured only on transfer presses?

10. Which elements of a transfer press transport the workpieces from die set to die set?

11. What is meant by a flexible manufacturing system?

28 Workpiece and stock feed systems

All presses which work automatically in continuous mode need automatic workpiece feed

devices.

Some feed devices are powered indirectly from the press.

Others have their own drive. All of them, however, are controlled from the press.

Feed devices can be sorted according to their application into:

28.1 Feed devices for piercing or blanking operations

In automatic piercing and blanking, the operation is carried out from a metal strip. There are

two strip feed systems: the roll feed and the gripper feed system.

28.1.1 Roll feed system

This consists of two pairs of rollers which

move the stock by friction (Figure 28.1). The

movement is derived from the eccentric shaft

as a link drive. At the start of the feed process,

the pusher rod is pulled upwards. This turns

the lever l

anticlockwise. This direction o

rotation activates the freewheeling clutch c

(roller clutch). This then drives the bottom roll

clockwise by means of the gears g

and g

This moves the strip stock from left to right.

The link rod r transfers the infeed movement

to the outfeed side. In another construction

(Figure 28.2) the roller clutch is driven di-

rectly by the pusher rod, the lower part o

which is designed as a toothed rod.

Figure 28.1 (above right) Plan of the roller

feed system, e eccentric shaft, a crank arm, p

pusher rod, g gears, c roller clutches, t top roll, b

bottom roll, s spreader ring brake, l lever, r link

rod

Figure 28.2 (below) Roller feed system with

bottom roll drive by means of toothed rod

346 28 Workpiece and stock feed systems

28.1.2 Gripper feed system

This is a feed system where the strip stock

eing moved is clamped between two pairs o

gripper jaws. Here, the movement itself, as

shown in Figure 28.3, can again be derived

from a link drive, but there are also hydraulic

and pneumatic drives. In Figure 28.3 the

usher rod swings round a crank lever which

carries out the feed movement. The link rod

transfers the gripper infeed movement to the

clamping jaws on the outfeed side. The outer

transport grippers are closed pneumatically.

When the sheet is clamped tight, the feed

movement takes place, controlled synchro-

nously with the ram movement of the press.

Both inner clamping rods are opened during

the feed phase. They are for holding the sheet

tight during the working phase (blanking

operation).

Figure 28.3 The principle of a gripper feed,

a) clamping jaws open, b) shut

28.2 Transport devices in deep drawing transfer presses

In deep drawing transfer presses the work-

pieces are transported from stage to stage with

gripper rail systems. These gripper devices

consist of two rails which open and close

laterally, moving in the direction of feed by

one feed step. Gripper elements which are

adapted to the workpiece shape at each stage

are mounted on the gripper rails (Figure 28.4).

The working cycle consists of four move-

ments:

1. close gripper rails and grasp workpieces;

2. gripper system moves one feed step to the

right, carrying the workpieces;

Figure 28.4

The operating principle of a gripper rail sys-

tem.

1 close, 2 feed movement, 3 open, 4 return

28.3 Transport devices for transfer presses for bulk forming 347

3. the gripper rails open and deposit the conveyed workpieces in the operating position on the

next processing die.

4. The open gripper rails drive back into their initial position during the working movement of

the ram.

According to the shape of the initial blanks, they are fed to the first station of the gripper rails

by stacking magazines, disc feed systems or other suitable conveyors.

28.3 Transport devices for transfer presses for bulk forming

On these machines, grippers are used to carry the workpieces from one forming stage to the

next.

The grippers (Figure 28.5) are opened and moved linearly by means of cams. The jaws are closed

by spring tension (Figure 28.6). There are some gripper jaws which pivot instead of moving line-

arly (Figure 28.7), allowing them to turn a workpiece 180°. Figure 28.8 shows a gripper system

for a transverse feed press with four working stages.

Figure 28.5 Transporting workpieces with sim-

ple spring-actuated jaws

Figure 28.6 The drive of the gripper system

to produce the linear movement.

1 gripper, 2 cam

Figure 28.7 Pivotable grippers (180°) Figure 28.8 Grippers on a four-stage trans-

verse feed press

348 28 Workpiece and stock feed systems

28.4 Feed devices to supply round blanks for backward extrusion

This kind of device usually consists of three

arts: a vibrating conveyor, a baffle (guide

rails) and a pusher. The vibrating conveyor is

for sorting the blanks. The vibrations of the

container separate the parts and they move

upwards in spiral grooves. From there they fall

through a mechanism which is controlled syn-

chronously with the ram of the press, falling

into a chute with guide rails which they slide

down due to gravity (Figure 28.9).

The pusher device then moves the blanks in

front of the die. The formed parts are knocked

out by an ejector which is part of the press.

Stud-

olts, rivets and other shaped parts are

conveyed in the same way.

Figure 28.9 Vibrating conveyor

28.5 Feed devices to convey single workpieces in steps

To convey single workpieces, rotary indexing

tables are also used. The workpieces are placed

in the front area of the table, outside the danger

zone, either manually or with a feed device.

The rotation, in steps (one move per press

stroke), is produced by a Geneva drive (Mal-

tese Cross) and transmitted from the crankshaft

y means of toothed and bevel gears. Figure

28.10 shows the drive layout of this kind o

rotary indexing table.

Figure 28.10

Drive layout of a rotary indexing table.

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28.7 Exercise on Chapter 28 349

28.6 Feed devices to supply forging presses

Today, impression-die forging in forging lines is also often carried out fully automatically.

The blanks, heated to the forging temperature, are placed in the die using industrial robots

(known as manipulators). The manipulator removes the finished or pre-formed forgings from

the die after forming and conveys them to the next machine. These industrial robots (Figure

28.11) are built in various sizes (related to the weight they transport and the distance they mo-

ve). According to requirement, they can carry out linear movements in three axes and also turn.

The gripper elements are adapted to the shape of the workpiece.

The lengthwise movements are mainly pneumatic and the turning movements produced using

electricity (an electric motor with a downstream gear drive). All the manipulators are NC con-

trolled and can be programmed.

Figure 28.11 Industrial robot with two linear and two turning movements (Illustration from Bosch

Material Handling Engineering, Stuttgart, Germany)

28.7 Exercise on Chapter 28

1. What material feeding systems do you know for piercing and blanking operations?

2. Which workpiece transport devices are used with drawing transfer presses?

3. Which workpiece transport devices are used with transfer presses for bulk forming?

4. Where are industrial robots used in metal forming?

Tsch?tsch H., Koth A. Metal Forming Practise: Processes - Machines - Tools

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