Altan T. Metal Forming Handbook

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duced. In addition, a spotting fixture for the slide and blank holder can

be applied to the contours of the die with a precisely controlled speed.

The die mounting area, the stroke and the force on all try-out presses

are so designed that the press can accomodate the total range of large

size tools. Force and speed can be selected in the ratio of at least 1 : 10,

so that the dies can be tried out by starting with a very low ram speed.

As a rule, both the punch and the die must be reworked at the same

time during die try-out.

In order to facilitate the rework and save time, hydraulic try-out

presses can be equipped with a pivoting slide plate (Fig. 4.1.21). At first,

the moving bolster carrying the lower die moves out of the press. When

pivoted, the top die is rotated through 180°and lowered and centered

onto a movable pallet truck. After this, the slide, the clamping plate and

top die are lowered to a pre-defined height and locked into the roll-over

device. The quick-action clamps release the slide plate from the slide,

and the slide moves to the top dead center position. After the slide plate

and top die have been turned over, the slide places the slide plate with

the top die in the centering of the pallet truck and moves again to the

151

Principles of die manufacture

Fig. 4.1.20 Design of a hydraulic try-out press

hydraulic

drawcushion

deepdrawing

adjustment

stepless

slidelocking

blankholdercylinder

slidecylinder

markingstop

slidecushionwithadapter

positionsensorfor

thedrawcushion

spottingcylinderslide

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

upper stop position. After being taken out of the back of the press on

the pallet truck, the top die is accessible from all sides. This allows con-

venient and time-saving reworking to be carried out. After reverse rota-

tion, the die does not need to be repositioned as it is already positioned

by the clamping plate on the pallet truck.

The electrical system of a try-out press includes a sensor system for

measuring the setting parameters for each die. Sensors for measuring

various process parameters and a data acquisition system allow to per-

form statistical evaluation. The optimal setting parameters are saved

and made available as input into the control system of the production

press. In order to achieve production-like conditions in hydraulic try-

out presses, any elastic deformations or off-center movements of the

slide, which are detected at the die and the production line must be

considered by the control system.

During run-in, the dies need to be frequently moved in and out of

the press. A moving bolster allows this work to be carried out faster and

more safely (cf. Fig.3.4.3).In a try-out center for example, several try-

152

Sheet metal forming and blanking

Fig. 4.1.21

Rotating the slide of a hydraulic

try-out press

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

out presses may be linked together with T-tracks (Fig.4.1.22and cf.

Fig. 3.4.4). This arrangement allows comprehensive testing of the pro-

gressive forming operations so that die sets are more quickly and effi-

ciently run-in.

Try-out presses and above all try-out centers are also suitable for pro-

ducing short run parts or carrying on production on a smaller scale in

the event of the breakdown of a large production press. Depending on

the task they have to perform, such universal presses may be used, for

example, for production as well as for try-out purposes.

Parallel lift mechanisms

For any repairs and reworking operations, the top die has to be lifted off

the bottom die. For ergonomic (overhead working) and scheduling rea-

sons (working at the same time on the top and bottom dies) both dies

are laid down separately. Dies which cannot be handled manually can

be moved with the help of a hydraulic parallel lift mechanism. This

apparatus ensures quick clamping for repairs and maintenance of dies.

153

Principles of die manufacture

Fig. 4.1.22 Try-out center: try-out presses linked by T-tracks

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

The mechanism, seen in Fig. 4.1.23, lifts the top die parallel to the low-

er die, and after a 180°turn places it next to the lower die. The mecha-

nism ensures that both parts of the die are lifted safely and with con-

sistent accuracy without any tilting.

4.1.6Transfer simulators

Transfer simulators are used for the adjustment and pre-setting of part-

dependent tooling of the gripper rails and the vacuum suction crossbars

of large-panel transfer presses (cf. Fig. 4.4.29 and 4.4.34). The lower die

and the part-dependent tooling are installed in the simulator; prior to

production these will be coupled with the permanently installed trans-

fer components of the press (Fig. 4.1.24).

The movements of the transfer, part and die must be synchronized in

relation to one another so that there can be no overlapping at critical

points and consequently no collision between the transfer, the part

itself and the die. Thus, data on gripper and suction cup positions,

transport paths, transport height, part positioning and flow directions

is stored in a programme. This programme is transferred onto the pro-

duction line. The possibility of collisions of part and die, which was

already verified as part of the CAD design process, is checked out in the

simulator. The primary task for the simulator is, however, the matching

of the part-dependent fixturing to the sheet metal part. A test is per-

154

Sheet metal forming and blanking

Fig. 4.1.23

Parallel lift

mechanism

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

formed to ascertain whether the part is positioned precisely in its uni-

versal station and in the die. In addition, the energy supply and control

system of the transfer rails are checked for correct functioning. Modern

systems can also simulate the changing of part-specific templates used

in the universal station.

155

Principles of die manufacture

Fig. 4.1.24 Transfer simulator for a large-panel transfer press with crossbar transfer system

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

4Sheet metal forming and blanking

4.2Deep drawing and stretch drawing

4.2.1Forming process

In deep drawing operations forming is carried out with tools (rigid and

flexible), active media (e.g. sand, steel pellets, liquid) or active energy

(i.e. a magnetic field). The following section covers possible applica-

tions for stretch and deep drawing processes with rigid tools as a first

and subsequent operation (cf.Fig.2.1.10to 2.1.12). Sections 4.2.4 and

4.2.5 will cover deep drawing with active media (cf. Fig.2.1.13).

Unlike in deep drawing, in stretch formingthe sheet metal cannot

flow because it is gripped by clamps or held in a blank holder. The form-

ing process takes place with a reduction in the thickness of the sheet

metal. Pure stretch drawing is a forming process conducted under ten-

sile stresses (cf. Fig.2.1.18). Mostly large-scale work pieces such as, for

example, body panel components used in aircraft or automotive man-

ufacturing, are produced on special production lines. This is certainly

true in producing low volume or large size parts, as, for example, with

the wing elements of an aircraft. The advantage of this process is that

the tooling and machining costs are relatively modest in proportion to

the size of the part being produced.

Deep drawing is a compression-tension forming process according to

DIN 8582. The procedure with the greatest range of applications is deep

drawing with rigid tooling,involving draw punches, a blank holder and a

female die (cf. Fig.2.1.10). The blank is generally pulled over the draw

punch into the die, the blank holder prevents any wrinkling taking

place in the flange. In pure deep drawing, the thickness of the sheet

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

metal remains unchanged since an increase in the surface area does not

occur, as it occurs in a stretch drawing process.

The material flow in the drawing process can be controlled, as

required, by adjusting the pressure of the blank holder in between pure

stretch drawing, in which no sheet metal can flow out of the flange area

into the draw die, and deep drawing, in which there is a wrinkle-free and

unrestricted material flow (cf. Fig. 2.1.32).

In deep drawing, as a first step, a cup is produced from the flat blank.

This cup can then be further processed, for example by an additional

drawing, reverse drawing, ironing or extrusion process. In all deep draw-

ing processes, the pressing force is applied over the draw punch onto the

bottom surface of the drawn part. It is further transferred from there to

the perimeter in the deformation zone, between the die and the blank

holder.

The workpiece is subjected to radial tension forces F

and tangential

compression forces F

(Fig.4.2.1). The material is compressed in the tan-

gential direction and stretched in the radial direction. As the draw

157

Deep drawing and stretch drawing

Fig. 4.2.1 Pressing forces in deep drawing of a round cup with a blank holder

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

depth is increased, the amount of deformation and the deformation

resistance are also increased. The sheet metal is most severely stretched

in the corner of the draw punch. Failure normally occurs at this point

(base fracturing).

When drawing with a blank holder, the press must also transfer the

blankholding force onto the die. This is achieved either from above via

a blank holder slide or from underneath via a draw cushion. There are

basically three possible ways of applying blank holder force.

158

Sheet metal forming and blanking

Fig. 4.2.2 A double-action top down drawing die

draw

cushion

blank

press

bed

ejector

die

blank holder

slide

stroke

mounting area,

slide

blank

holder

stroke

mounting area

die

part

height

ejector

stroke

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

In double-action drawing operationsthe press has two slides acting from

above: the drawing slide with the draw punch and the blank holder slide

with the blank holder (Fig. 4.2.2and cf. Fig. 3.1.8). The blankholding slide

transfers the blankholding force via the blank holder onto the blank and

the draw die. The die and the ejector are located in the lower die on the

press bed. During forming, the blank holder brings the sheet metal into

contact against the die, the punch descends from above into the die and

shapes the part while the sheet metal can flow without any wrinkling out

of the blankholding area. In this case, the drawing process is carried out

with a fixed blank holder and moving punch. In double-action drawing

operations, the drawing slide can only apply a pressing force.

A disadvantage in double-action drawing is the fact that the parts for

further processing in successive forming and blanking dies need to

be rotated by 180°in an expensive and bulky rotating mechanism

(cf. Sect. 3.1.3). With outer body parts the danger exists, furthermore,

that the surface of the part may be damaged in the rotating operation.

Single-action drawing with a draw cushion works the other way round:

the forming force is exerted by the slide above through the die and

the blank holder onto the draw cushion in the press bed (Fig. 4.2.3and

cf. Fig 3.1.9). The draw punch and the blank holder of the drawing tool

are both located in a base plate on the press bed. Pressure pins, which

come up through the press bed and the base plate transfer the blank

holder force from the draw cushion onto the blank holder. The female

die and the ejector are mounted on the press slide. At the start of the

forming process the blank is held under pressure between the draw die

and the blank holder. The slide of the press pushes the blank holder

downwards over the draw die – against the upward-acting force of the

draw cushion. The part is formed via the downward movement of the

die over the stationary draw punch. The press slide must apply both the

pressing and the blank holder forces.

Thus, using a single-action tool, the part does not have to be rotated

after the drawing process. Furthermore, today the use of hydraulically

controlled draw cushions, even with deep drawing processes up to

depths of 250mm, produces a workpiece quality comparable to that of

double-action presses (cf. Sect. 3.1.4).

An energy-saving and cost-effective alternative in stamping is counter

drawing or reverse drawing(Fig.4.2.4and cf. Fig. 3.1.10). In this opera-

tion, once again, a single-action press with a draw cushion is used – nor-

159

Deep drawing and stretch drawing

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

mally a hydraulic one. The top die is attached to the slide. The lower die

is mounted on the press bed with the blank holder. The punch is locat-

ed in an opening in the center of the press bed on the draw cushion.

During deformation, the blankholding force is transferred via the

slide from above and the draw punch force acts from below through the

160

Sheet metal forming and blanking

Fig. 4.2.3 Single-action die with draw cushion

mounting area,

die

slide

die

blank holder

pressure

pins

slide

ejector

press

bed

draw

cushion

slide stroke

draw punch

part height

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