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produced. The information given in the part drawing includes: part

identification, part numbering, sheet metal thickness, sheet metal qual-

ity, tolerances of the finished part etc. (cf. Fig.4.7.17).

To avoid the production of physical models (master patterns), the

CAD data should describe the geometry of the part completely by means

of line, surface or volume models. As a general rule, high quality surface

data with a completely filleted and closed surface geometry must be

made available to all the participants in a project as early as possible.

Process plan and draw development

The process plan, which means the operational sequence to be followed

in the production of the sheet metal component, is developed from the

data record of the finished part (cf. Fig.4.1.1). Already at this point in

time, various boundary conditions must be taken into account: the

sheet metal material, the press to be used, transfer of the parts into the

press, the transportation of scrap materials, the undercuts as well as the

sliding pin installations and their adjustment.

The draw development, i.e. the computer aided design and layout of

the blank holder area of the part in the first forming stage – if need be

also the second stage –, requires a process planner with considerable

experience (Fig.4.1.8). In order to recognize and avoid problems in

areas which are difficult to draw, it is necessary to manufacture a phys-

ical analysis model of the draw development. With this model, the

forming conditions of the drawn part can be reviewed and final modi-

fications introduced, which are eventually incorporated into the data

record (Fig.4.1.9).

This process is being replaced to some extent by intelligent simulation

methods, through which the potential defects of the formed component

can be predicted and analysed interactively on the computer display.

Die design

After release of the process plan and draw development and the press,

the design of the die can be started. As a rule, at this stage, the standards

and manufacturing specifications required by the client must be consid-

ered. Thus, it is possible to obtain a unified die design and to consider

the particular requests of the customer related to warehousing of stan-

dard, replacement and wear parts. Many dies need to be designed so that

they can be installed in different types of presses. Dies are frequently

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

installed both in a production press as well as in two different separate

back-up presses. In this context, the layout of the die clamping ele-

ments, pressure pins and scrap disposal channels on different presses

must be taken into account. Furthermore, it must be noted that draw-

ing dies working in a single-action press may be installed in a double-

action press (cf. Sect. 3.1.3 and Fig. 4.1.16).

In the design and sizing of the die, it is particularly important to con-

sider the freedom of movement of the gripper rail and the crossbar

transfer elements (cf.Sect.4.1.6). These describe the relative move-

ments between the components of the press transfer system and the die

components during a complete press working stroke. The lifting move-

ment of the press slide, the opening and closing movements of the grip-

per rails and the lengthwise movement of the whole transfer are all

superimposed. The dies are designed so that collisions are avoided and

a minimum clearance of about 20 mm is set between all the moving

parts.

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Sheet metal forming and blanking

Fig. 4.1.8 CAD data record for a draw development

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

Guides of the upper and lower dies must carry the shearing forces

and be sized according to the calculated and anticipated forces and

functions. Large scale gibs show nearly no lateral displacement and

a consistent, unchanging pressure pattern of the operational die

(cf. Sect.4.1.5). Furthermore, they reduce the tilting of the die and the

press slides (Fig.4.1.10). Thus, a consistent overall high quality of

pressed parts is achieved.

In order to avoid errors and potential problem areas later in produc-

tion, the so-called FMEA (Failure Mode and Effects Analysis) is used

with proven success during the design stage with the close collabora-

tion of all those involved with the project. The design of the pressed

parts and their arrangement and positioning in the dies have a great

influence on production safety. In sheet metal forming well-designed

press components will lead to simple, production-friendly and low cost

tools. Difficult to form press parts, on the other hand, involve expen-

sive, complicated and high cost dies, which are less easily integrated

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Principles of die manufacture

Fig. 4.1.9 Physical analysis model of a draw development

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

into the production process. Tilting of the parts in the dies can com-

pensate the negative aspects of an unfavorable part geometry. For

example, for flanging and blanking a sheet metal part can be so rotated

that the work direction corresponds to the direction of the flanging

operation e.g. the movement of the blanking punch (Fig.4.1.11).

However, it is often then necessary to increase the number of dies,

which results in production cost increases.

To meet deadlines, the forming dies must be designed first so that the

casting patterns and the finished castings are available for use as soon

as possible. After machining and try-out, dies can produce stampings

that are laser trimmed and used in prototype pre-production assemblies

(Fig. 4.1.7).

The use of CAD systems for die design has become increasingly com-

mon (Fig. 4.1.12)even though conventional drawing board design may

often be more cost effective. The reasons for this are that the drawing

system is not yet perfected, data banks are incomplete and workstation

screens only allow a perspective view in the ratio of 1:5 up to 1 : 10.

However, considering the total production costs, high quality CAD

produced die designs and die casting pattern data are provided as a by-

product. Shrinkage and expansion allowances can be determined by

computer and used for NC machining of casting models, an economic

alternative to NC machining of castings. A further advantage of CAD

design, in so far as it is based on a volume model, is the possibility

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Sheet metal forming and blanking

Fig. 4.1.10 Influence of the guidance system on the reduction of slide tilting and offset

5,9

6,5

13,5

10,5

offset

[%]

columns

160mm[

columns

160mm[

4lugs

polyamid

4x10mm

columns

50mm[

without

gibs

4lugs

brass

4x10mm

reductionof

slidetilt

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

135

Principles of die manufacture

Fig. 4.1.12 CAD design of pivoting blanking tool

Fig. 4.1.11 Improvement to the positioning of the part by means of swivel movement in the

folding operation:

a simple die construction for press parts with simple geometry;

b complex high cost die construction with a sliding element for difficult press

part geometry;

c simple die construction via appropriate swivelling of the pressed part in an

improved working position

abc

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

of ongoing collision testing, which clearly contributes to the reduction

of errors. Jaw layout plans, die construction plans and single part draw-

ings can be made available by this system. The general use of CAD/CAM

systems in tool making have many inherent advantages that are linked

to the process. Therefore, in the medium term, CAD/CAM techniques

will be increasingly used to increase the total cost effectiveness of part

production.

Mechanical processing

Today’s technical possibilities in NC processing require ever more easy

to manufacture designs from the die designer. In particular the use of

five-axis milling machines allows to machine completely complex

castings with a single fixturing operation. In the past, because of

restricted conditions, extended operating and assembly activities were

carried out with fixtures and additional clearances. Today, for example,

three-dimensionally located sliding and guiding components can be

cast (Fig.4.1.17).

High speed milling (HSM) is used increasingly for machining of

sculptured surfaces in which economical cutting speeds of 2,000

m/min are attainable. Even strongly patterned surfaces for inner parts

can still be machined with medium feed rates of around 5m/min.

Cermets and special PVD-coated carbides are capable of attaining sur-

face qualities of R

= 10mm, which reduces the need for secondary man-

ual polishing to a minimum.

Assembly and try-out

More accurate machining and finishing operations together with the

above-mentioned advantages of a CAD supported die design, simplify

the assembly task considerably. The general processing of data gives

access to further theoretical and error-free blanking and contour infor-

mation without having to resort to the use of costly aids. The expert

knowledge of the qualified diemaker is, however, still necessary. In par-

ticular, in the pre-run or try-out phases when the die developer tries to

produce a good part after the installation of the dies in the press, it

becomes evident whether or not the theoretical design effort has been

correct (cf. Sect.4.1.5). Prototype tools or simulation methods provide,

of course, considerable help. There are, however, additional factors

which contribute towards the quality of the stampings obtained with

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Sheet metal forming and blanking

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

the production dies. The machining of the die surfaces and the gibs, the

milling of the free form surface using HSM operations and the try-

out on a precisely guided try-out press with adjustable hydraulic draw

cushions should all be carried out efficiently (Fig.4.1.19and 4.1.20).

Process control

In order to assure that the part dependent process plan and the

draw development are correct, prototype dies or soft tools are produced

(Fig.4.1.13). In so doing, the forming behavior of the sheet metal mate-

rial, especially when the part geometry is complex and more prone to

failure, must be investigated at the preliminary stages of the die design.

However, the practise of using soft tools dedicated exclusively for this

purpose is becoming less significant as computer-aided simulation

processes are being increasingly used for die design and development.

Prototype dies

The preparation of prototype or soft dies is, however, worthwhile if

further objectives need to be attained. At a very early point in the pro-

duction process, the sheet metal parts are made available, which can be

used, for instance, to construct vehicles for assembly and crash test pur-

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Principles of die manufacture

Fig. 4.1.13 Prototype drawing die in aluminium for the production of the roof of a station

wagon

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

poses. In certain cases, these parts may be used as pre-production series

(Fig. 4.1.7). A further consideration which justifies the production of

prototype dies is when the timing of their manufacture is close to the

schedule for the beginning of series production. Thus, information on

the formability and tolerances of the parts can be obtained, which con-

tributes to goal oriented and cost-effective die manufacture.

The following are some of the points which must be taken into account

here:

–the die material

–the complexity of the sheet metal part

–the blank material

–the design of the draw development

–the geometrical parameters such as degree of shrinkage, cambering,

prestressing

–the process-related variable such as lubrication, blank holder forces,

work-flow direction, draw beads

–the size and geometry of the blanks

–special factors and parameters which influence the results of the

stamping operation

The type and design of the prototype dies are influenced by different

factors. The choice of prototype material will be in line with the sheet

metal specification, thickness and the number of parts to be produced.

Today, plastics, aluminium alloys (Fig. 4.1.13), different qualities of cast

metals as well as rolled steel can be used as prototype or soft die

material. A summary of the choice and application criteria is given in

Table 4.1.1.

Methods of process simulation

Computer-aided simulation techniques, e.g. the analysis of the form-

ing process without any physical dies, will increasingly replace the use

of prototype and soft dies. This development is becoming more practi-

cal through rapidly growing hardware developments which allow the

processing of ever increasing amounts of computerised data in ever

shorter time spans. In addition, the computer software used for process

simulation has become more reliable and more user friendly.

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139

Principles of die manufacture

Die type Plastic Aluminium alloys Iron alloys

without additives alloyed or Zamak Cerrotru GS/GG cast iron Plate steel

fiber-reinforced

Manufacturing complete structure cast steel frame model production precise model model manufacture for punch construction of die

process in plastic, as basic structure for punch and manufacture, casting and female die (possibly from plates and jaws,

cast in layers for plastic panels female die the punch, insertion blank holder), casting of screwed and pinned,

or with back as (punch), casting of (possibly blank holder), of sheet metal thickness, models, reworking of reworking of functional

the front casting female die shape casting of models, casting of female functional surfaces, surfaces, integration

reworking of die (no reworking) integration of gib elements of gib elements

functional surfaces

Repair and machinable shape punch shape weldable, melt down welding on, welding on,

possibility for or new front machinable, inserts are inserts machinable existing die screw-mounted inserts, new jaws

modification casting seamlessly glued in, and recast reworking by means or inserts

casting of of milling

female die

Range of parts outer bodywork outer bodywork parts outer bodywork parts outer bodywork parts reinforcement and reinforcement and

parts in steel and reinforcements in and reinforcements and easily formable chassis parts with chassis parts

and aluminium steel and aluminium parts complex shapes with flat shapes

Sheet metal sheet steel up to up to 2.5 mm up to 1.5 mm up to 1.2 mm up to 3 mm up to 3 mm

thickness 2 mm, aluminium

up to 4 mm

Piece numbers with max. sheet metal for outer easily deform- easily deform- depending on stress level and sheet metal

thickness, extremely bodywork able parts able parts material used, possibly with series capability

low piece numbers, parts, up to up to 100 pieces, up to 50 pieces,

otherwise suitable 1,000 pieces complex parts complex parts

for small or pre-series up to 50 pieces up to 20 pieces

Product. period 2 to 3 weeks 4 to 6 weeks 3 to 4 weeks 2 to 3 weeks 7 to 10 weeks 5 to 7 weeks

Table 4.1.1: Allocation of prototype dies

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

For the evaluation of the geometry data, which has been established

by the processing plan and the draw development as well as the pro-

duction parameters, CAE solutions are available which are based upon

mathematical and plasto-mechanical equations or on experimentally

proven data. Analytical and numerical simulation belong to these solu-

tions. The numerical methods, and here in particular the Finite

Element Method (FEM), deliver today the most accurate results.

Stampings can be almost completely interactively analysed on a work-

station in relation to the most important failure criteria. The results are

the distribution of stress (tears) (Fig.4.1.14), the tendency towards

wrinkling (Fig.4.1.15)and the variation of the sheet metal thickness.

The necessary steps and the prerequisites for a simulation run are as fol-

lows:

–preparation of CAD data for the punch, female die and blank holder

as well as the total geometry of the draw development in the work-

ing direction (Fig. 4.1.8),

–determination and input of the blank size (cf. Sect. 4.2.1 and 4.5),

–input of the process parameters such as sheet metal material, rolling

direction, blank holder force, frictional conditions,

–definition i.e. establishment of the FE mesh (partly automatic) and

–initiation of the calculation process.

By varying the process and die parameters, the simulation can be

repeated as often as required until a failure-free drawing part is pro-

duced. The advantage of this process lies in the fact that any objective

and necessary component changes can be introduced at any time prior

to the actual manufacture of the dies. Specific measures for the control-

ling metal flow, for example through the shape of the blank or through

the use of draw beads, can be simulated. Similarly, calculations can be

made to predict and compensate for spring back (cf. Sect. 4.8.1).

Through process simulation, process planners and die designers re-

ceive reliable information at an early stage in the die development,

which really contributes to the production of fully operational and reli-

able dies.

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