Altan T. Metal Forming Handbook

Подождите немного. Документ загружается.

The Hydro-Mec equipment consists mainly of a pressure intensi-

fier (transmission ratio emulsion: oil = 2 : 1; max. emulsion pressure

600 bar, max. volumetric flow 20 l/s), and an emulsion tank. A 200 kW

motor and a pump in the main press drive system are used to drive the

pressure intensifier.

191

Deep drawing and stretch drawing

female die

blank

blank holder

punch

pressure medium

Fig. 4.2.12

Die layout and production

process for active hydro-

mechanical drawing

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

The quick die change system consists of quick-action clamping ele-

ments and a die ejection system, and permits to achieve minimum set-

up times. The blank holder base plate, female die and punch are

clamped using fast hydraulic clamping elements. The ejection system

transports the base plate from the machine together with the die. The

die – comprising the blank holder, female die and punch – is lifted from

the base plate and exchanged. The Hydro-Mec terminals are located in

the base plate. These are automatically coupled by means of a docking

system during inward travel into the press.

Advantages of the Active Hydro-Mec process

The Active Hydro-Mec process is characterized by a number of special

features which distinguish it from conventional production. Quality

benefits include the improved component stability and buckling resis-

tance made possible by the pre-stretching process described above.

Similar to conventional hydro-mechanical drawing processes, the sheet

metal does not rub against the punch or female die but is pressed

192

Sheet metal forming and blanking

Fig. 4.2.13 An Active Hydro-Mec press

blankholder

cylinder

stroke

limiter

active

Hydro-Mec

die

slide

slidecylinders

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

against the punch by the emulsion. At the same time the process

ensures optimum surface quality of the outer skin. Due to the lower

sheet metal thicknesses used and the elimination of reinforcements,

the overall assembly weight can be reduced. Another benefit in process

engineering terms is the ability to process different materials (steel, alu-

minium, high-strength steels) and sheet thicknesses using a single set

of dies.

In addition, the Active Hydro-Mec process represents a cost saving

over conventional production methods. Tooling costs, in particular,

can be reduced by up to 50%, as only one half of the die needs to be

machined to have the part geometry. Furthermore, dies achieve a far

longer service life due to the lower die wear involved in the hydro-

mechanical drawing method. Depending on the part contour and the

design of the die, it is often possible to save a complete processing step.

193

Deep drawing and stretch drawing

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

4Sheet metal forming and blanking

4.3Coil lines

The method of sheet metal feed plays an important role when consid-

ering the overall cost effectiveness of forming and blanking lines.

When producing small sheet metal parts, the sheet metal coil is fed

directly by a coil feed line into the press.

The function of the coil line is to unwind coils of sheet material, pass

it through the straightener and to feed it by means of a roll feed pre-

cisely to the defined position of the processing line (cf. Fig. 4.6.4).

The main components of a conventional coil line are the decoiler,

straightening machine, coil loop and roll feed (Fig. 4.3.1). The straight-

ening machine draws the coil stock from the decoiler and flattens

the material (cf.Sect.4.8.3). Using sufficiently large dimensioned roller

assemblies – in order to avoid the material from becoming bent again –

the material is transported from the straightening machine continu-

ously to the coil loop. The electronically controlled roll feed draws the

stock intermittently from the loop and feeds it, correctly positioned,

into the press. The maximum feed output of a high-performance coil

line for small and medium-sized parts (cf. Fig. 4.4.4)and a blanking

press (cf. Sects. 4.4.5, 4.4.7, 4.4.8) is indicated in Fig. 4.3.2.The stroking

rate of the press is specified depending on the feed length and the crank

angle which is available for the feed depending on the press and the

tool in use.

Compact design coil feeding lines do not require the pit for the coil

loop (Fig.4.3.3). They consist of a decoiler and a feed/straightening

machine. The decoiler unwinds the stock downwards, opposite the

direction of processing, so forming a horizontally aligned coil loop. The

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

feed/straightening machine draws the stock on an intermittent basis off

this loop, straightens it and feeds it, correctly positioned, into the press.

Whether the conventional coil feed line or the compact design is

used depends on the material to be processed. Coil stock which is thin

or has a sensitive surface can be very successfully processed using con-

ventional methods. However, in cases where generally thick coil stock

is being fed to the press, and if the material is suitable for interrupted

straightening, the compact design is generally preferred.

When working with progressive dies and compound dies, the feed/

straightening machine can be released briefly during immersion of the

pilot pins for feed correction (cf. Fig. 4.7.25). This applies equally to roll

feed. For roll feed, the release period is around 0.03s and for the

feed/straightening machine around 0.05s.

195

Coil lines

Fig. 4.3.1 Coil line for direct feed of stock material into the press

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

196

Sheet metal forming and blanking

Fig. 4.3.2 Feed performance diagrams of a blanking press and of a high-performance

line for small and medium-sized parts

500

feed step [mm]

feed performance of a blanking press

stroking rate [1/min]

1000 1500 2000 2500 3000 3500 4000

100

300°

270°

240°

210°

180°

crank angle available

for transport

100

200

300

400

500

feed step [mm]

stroking rate [1/min]

40 60 80 100 120 140 160 180 200

600

700

800

900

1000

300°

270°

240°

210°

180°

crank angle available

for transport

feed performance of a high-performance line for small and medium-sized parts

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

197

Coil lines

Fig. 4.3.3 Compact design coil line

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

4Sheet metal forming and blanking

4.4Sheet metal forming lines

4.4.1Universal presses

Universal presses are characterized by their high degree of flexibility.

They are suitable both for blanking and forming operations (Fig.4.4.1).

In combination with individual, progressive blanking, compound and

transfer dies, this opens up a wide range of application possibilities for

the production of small to medium-sized parts (cf.Sect.4.1.1). To

increase the number of formed parts and so enhancethe cost-effective-

ness of the production process, universal presses are equipped with coil

lines (cf.Fig.4.3.1), scrap disposal systems and stacking units for fin-

ished products (Fig.4.4.41). Fully automatic die change plays an impor-

tant role in ensuring economical production (cf.Sect.3.4).

Comparison: mechanically and hydraulically driven universal presses

Generally speaking, universal presses are driven either mechanically or

hydraulically. In the case of mechanical universal presses, the move-

ment of the eccentric shaft is transmitted to the slide by means of the

connecting rod. Accordingly, the sinusoidal curve of the stroke is pre-

scribed by the geometry of the eccentric or crank shaft (cf.Fig.3.2.3).

For a mechanical and a hydraulic press with a forming stroke of

approx. 100 mm, the stroke versus time, the time required for one work

cycle (cycle time) and the time available for part handling (transporta-

tion time), are shown in Fig. 4.4.2.It is seen that the handling time

available is 2.4 s, the cycle time of the mechanical press is 3.6 s, so that

the corresponding stroking rate is around 17 strokes per minute.

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

In the case of hydraulic universal presses, the slide velocity is adjust-

ed to the process (Fig.4.4.3). Compared to mechanical presses, the

impact speed of the slide on the workpiece can be drastically reduced.

The desired gentle impact of the top die leads to lower tool stresses and

smoother material flow, but increases the cycle time. In hydraulic press-

es, the pressure has to be released at the bottom dead center (BDC)

without causing an impact, and the hydraulic system has to be reversed.

Therefore, the workpiece contact time increases (cf.Fig.3.3.1). As a

result, with a handling time of 2.7 s, the cycle time is 1.4 s longer for the

hydraulic press compared to its mechanical counterpart. In our exam-

ple, this corresponds to 12 strokes per minute. The difference in output

between hydraulic and mechanical presses can be smaller than this but

also greater depending on the specific production processes involved.

The energy consumption of the two press types also differs

(Fig. 4.4.2). The slide velocity of hydraulic presses is controlled in line

with the motor output. In contrast, in mechanical presses energy is

extracted from the flywheel, causing a drop in flywheel speed. If a

199

Sheet metal forming lines

Fig. 4.4.1

Hydraulic universal press

(nominal press force 5,000kN)

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

greater amount of energy is required for a work cycle, for example when

working with high drawing depths, the drop in flywheel speed must

not be more than 20% of the idle running speed. A constant supply of

energy for the drive motor is drawn from the electrical net. The energy

capacity of the drive motor is selected such that the motor can bring the

flywheel up to its idle speed after each working stroke. If the energy

requirement of both press types is compared, it becomes apparent that

hourly power requirement for mechanical presses can be some 30% low-

er than that of hydraulic presses despite their lower output. However, this

drawback can be compensated by changing the forming technology

used, for example through counter-drawing(cf. Fig. 3.1.10).

High-performance transfer presses

The high-performance transfer press is derived from a mechanical uni-

versal press with particularly high output, developed for high-precision

200

Sheet metal forming and blanking

Fig. 4.4.2 Slide displacement and power requirement versus time in mechanical and

hydraulic presses with cycle and transport times

hydr. cycle time approx. 5

transport time approx. 2,7

transport time approx. 2,4

hydraulic

press

forming area

hydraulic

press

eccentric

press

eccentric

press

TDC

BDC

mech. cycle time approx. 3,5

1000

900

800

700

600

500

400

300

200

100

300

200

100

displacement [mm]power requirement [kW]

time [s]

0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0

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