Altan T. Metal Forming Handbook

subsequently quenched and tempered. Gear teeth and various chain

wheels can also be highly economically fine blanked.

4.7.3Fine blanking tools

Tool types

Internally and externally contoured parts are produced in a single

stroke of the slide using complete blanking dies.The blanking burr is

located on the same side for both the inside and outside contour. Parts

manufactured in this way exhibit a high degree of flatness. The dimen-

sional tolerances of the parts depend largely on the manufacturing

quality of the die. Tolerances in the coil feed do not influence the

dimensional tolerances of the blank.

Due to the complex part geometries involved, in the case of progres-

sive blankingdies, blanking of part contours takes place in a number of

stages (cf. Sect. 4.1.1). For internal transfer from one station to the next

within the die, the part remains in the strip or scrap web. These are

referred to as pre-piercing dies whereby the pre-piercing process can

take place in one or more stages and is performed prior to blanking of

the outside contour. Here, the differences in feed exert an influence on

351

Fine blanking

Fig. 4.7.20 Fine blanked parts used in the automobile industry

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

the position of the inside contours relative to the outside contour. The

blanking burr on the inside contours is located on the opposite side to

that of the outside contour. The compound progressive die also permits

forming operations such as bending, offsetting, drawing, etc. in the con-

secutive stations (Fig4.7.21and 4.7.22).Compared to complete blank-

ing, parts produced using this method exhibit generally larger dimen-

sioned tolerances and lower flatness.

In the case of transfer dies, the part is not held in the scrap web. The

blanks are transported from one workpiece station to the next by

means of a gripper rail, which is frequently coupled to the press (cf.

Fig. 4.4.24or as illustrated in 4.7.22,by means of a transverse feeder).

Die systems

In fine blanking technology, the dies belonging to the various die types

are subdivided into “moving punch” and “fixed punch” systems.

The system making use of a moving punch is used mainly for com-

plete blanking dies in the production of small to medium-sized parts

with few inner forms (Fig. 4.7.23).The blanking plate 2 is mounted in the

upper die-set 16 on the die block. For the purpose of positioning, it is

positioned and fastened and additionally supported by the base plate 14,

piercing punch retaining plate 10, back-up plate 8 and holding ring 9.

352

Sheet metal forming and blanking

Fig. 4.7.21

Two-station compound

progressive fine blanking die:

station:

semi-piercing from

below and hole punching,

station:

embossing the cylin-

drical countersinks at the inside

hole, blanking the slots and outside

contour.

Tooling system ”fixed punch”, four-

column steel frame with ball gibs,

active elements are constructed

and coated in segments

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

The ejector 3 is guided in the blanking plate. The ejector in turn guides

the inner form punch 5. Via pressure pins 7 and pressure pad 11, coun-

terpressure, i.e. ejection force, is applied to the ejector 3.

In lower die-set 17, the guide plate or vee-ring plate 4 is mounted

directly on the frame, positioned and fastened. The moving punch 1,

which is connected to the press via the punch base 12, runs in guide

plate 4. The inner form slugs are ejected by the press hydraulic system

via pressure pins 7 of ejector bridge 13, and the inner form ejector pin

6 from punch 1. The upper die-set 16 and lower die-set 17 are precisely

353

Fine blanking

Fig. 4.7.22 Two-station compound progressive fine blanking die with part transport by means

of transverse shear pusher:

station:

processing sequence in vertical direction – cup drawing by blanking

and drawing

station:

processing sequence in vertical direction – setting cup radii, piercing

inner form, trimming outside toothing. Active elements are structured in ring for-

mation and coated

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

positioned by means of the press frame's column gib 18. The blanking

and guide plate are fixed using the latch bolt 15.

Thefixed punch systemis suitable for all die types – in particular for the

manufacture of thick, large parts (Fig. 4.7.24).The blanking punch 1 is

positioned on a hardened pressure plate 9 on the upper die-set 16, and

is permanently positioned and fastened to the upper die-set 16. The pre-

cisely fitted guide plate 15 and the vee-ring plate 8, which bear the latch

bolts 14, guarantee the fitting precision of the blanking punch 1 relative

to the blanking plate 2 by means of the column gib unit 18 of the frame.

The inner form slugs are stripped from the blanking punch 1 by the

inner form ejectors 6 and 7. Pressure is applied on the ejectors 6 and 7

and the vee-ring plate 8 by the vee-ring piston of the press via pressure

pins 13. Pressure pins 13 transmit the force of the vee-ring during the

blanking process and the stripping force during the stripping cycle.

354

Sheet metal forming and blanking

Fig. 4.7.23 Moving punch system, complete blanking die:

1 blanking punch; 2 blanking plate; 3 ejector; 4 guide/vee-ring plate;

5 inner form punch; 6 inner form ejector pin; 7 pressure pins; 8 back-up plate;

9 holding ring; 10 piercing punch retaining plate; 11 pressure pad;

12 punch base; 13 ejector bridge; 14 base plate; 15 latch bolt;

16 upper frame; 17 lower frame; 18 gib unit

2

7

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

Blanking plate 2 is located, positioned and fastened on the lower die-

set 17. In special cases, for example where a split blanking plate is used or

extremely thick materials are being processed, the blanking plate 2

accommodates a shrink ring 19. The ejector 3, which is guided in the

blanking plate 2, is also responsible for guiding the inner form punches

4, and transmits the counterpressure and ejection force from the coun-

terpressure piston of the press via the pressure pins 13. The inner form

punches 4 are mounted on the piercing punch retaining plate 10 and

supported by the back-up plate 12.

The underlying principle of the fixed-punch die system is also used

for progressive blanking dies, compound progressive dies and transfer

dies (Fig.4.7.25).In the case of progressive and compound dies, an addi-

tional coil guidance system 1, an initial blanking stop 3, as well as pilot

355

Fine blanking

Fig. 4.7.24 Fixed punch system, complete blanking die:

1 blanking punch; 2 blanking plate; 3 ejector; 4 inner form punch;

5 piercing punch; 6 inner form ejector; 7 ejector pins; 8 vee-ring plate;

9 pressure plate; 10 piercing punch retaining plate; 11 intermediate plate;

12 back-up plate; 13 pressure pins; 14 latch bolt; 15 guide plate; 16 upper frame;

17 lower frame; 18 gib unit; 19 shrink ring

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

pins 7/8 and positive press-off pins 4 are required. The coil guide not

only ensures coil guidance but also stripping of the scrap web from the

pilot pins 7/8. The initial blanking stop 3 ensures precise initial blanking

of the sheet metal at the start of a coil or strip. The pilot pins 7/8 are

responsible for precisely positioning the sheet metal in each die station,

i.e. ensuring that the prescribed feed step is adhered to precisely. The

task of the positive press-off pins 4 is to compensate for the counter-

pressure so that it does not have to be absorbed fully by the individual

piercing punches 6 during initial blanking in the first die station.

Die design

The factors which determine the design layout of a fine blanking die are

the geometrical shape and size of the part and the type and thickness of

the sheet metal being processed. These parameters determine both the

type of die used and also the die system in general.

Further definition of the type of die design must be carried out

according to the following conditions, which are basically the same as

those also valid in standard blanking. In order to achieve optimum

blanking conditions, the line of application of the slide force must

coincide as far as possible with that of the blanking or forming force (cf.

Sect. 4.5). The more precisely this condition is complied with, the bet-

ter the blanking result and the lower the degree of die wear.

The part drawing must contain all the dimension and tolerance spec-

ifications, and the cut surface properties of the part must be indicated

(Fig. 4.7.17). Depending on the type of die used, the strip layout must

indicate the position of the part in the sheet metal or the processing

sequence (Fig. 4.7.2and cf. Sect. 4.1.1). The most important aspect of

the strip layout on the sheet metal is always the achievement of opti-

mum material utilization (cf. Fig. 4.5.2to 4.5.7). The strip layout is also

used to define the strip width and feed step.

Calculation of press forces

The following forces are exerted during fine blanking: the blanking

force, vee-ring force, counterforce, stripping force and ejection force

(Fig.4.7.3).The first three forces determine the configuration of the

press, adding up to create the total machine force:

356

Sheet metal forming and blanking

FFFFN

Ges S R G

=++

[]

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

The blanking force F

[N] and the counterforce F

[N] were already

calculated when establishing the degree of difficulty. The vee-ring force

[N] depends on the tensile strength of the material R

[N/mm

the length l

[mm] and the height h

resp.H

[mm] of the vee-ring

(Fig. 4.7.7) as well as an offset factor f

[–]:

For f

, experiments have produced the value 4. This value only applies

in cases where the geometrical conditions of the vee-ring described in

sect. 4.7.1, are adhered to.

The stripping force F

[N] is calculated from the blanking force and

the offset factor f

[–], which lies between 0.1 and 0.2 :

‘

357

Fine blanking

Fig. 4.7.25 Compound progressive die

1 coil guidance unit; 2 coil guidance bolts; 3 initial blanking stop;

4 positive press-off pins; 5 latch bolts; 6 piercing punch;

7 feed step pilot/embossing punch; 8 feed step pilot/embossing punch

FflhRN

RRRm

=⋅⋅⋅

[]

FFfN

Ra S

=⋅

[]

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

The ejection force F

[N] is approximately the same as the stripping

force:

With a cut contour of the same length, a toothed shape requires a

greater ejector force than a round contour. Both forces are additionally

influenced by the lubrication, the surface roughness of the blanking

elements and the elastic recovery of the sheet metal.

Die lubrication

Fine blanking is impossible to perform without lubrication. The process

would lead to cold welding of the active elements to the blank and rapid

wear and blunting of the die. Accordingly, the necessary precautions must

be taken to ensure that the active elements are supplied with sufficient

lubricant at every point of the cut contour and forming areas in the die.

Fine blanking oils can be applied to the sheet material using rollers or

sprayjets. Both the top and the bottom surface of the sheet metal must

be evenly wetted by a film of lubricant. For this purpose wetting sub-

stances are added to the fine blanking oils.

Where a spray system is used, oil mist is created which must be

removed using a suitable extraction device. If high-viscosity oils are used

for thick, higher strength materials, it is also possible to apply lubricant

through the rollers.

To ensure that the fine blanking oil applied to the sheet metal surface

also reaches the friction partners during the blanking process, special

design measures are called for at the die. The guide plate, the ejector

and the inner form ejection pins must be chamfered or prepared along

the cut contours in order to create lubrication pockets (Fig.4.7.26).

Recesses are provided to expose the vee-ring and blanking plate, so

ensuring that oil-wetted sheet material reaches the active part of the die

with each feed step. In the case of closed dies, the blanking oil on the

sheet metal is pressed into the lubrication pockets and serves as a sup-

ply for lubrication of the punch and blanking plate.

No universal fine blanking oil exists which satisfies equally all

requirements. The fine blanking oil used in each case must be specially

adjusted according to the material, the thickness and strength of the

part. The viscosity of the oil must be coordinated to the specific types

of stress occurring during fine blanking.

358

Sheet metal forming and blanking

FFfN

Ga S

=⋅

[]

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

Depending on the sheet metal thickness and material strength, the oil

is subjected to different pressure and temperature levels. The oil must be

capable of withstanding these levels of stress to the extent that the lubri-

cant film remains intact during the blanking process. A low viscosity oil,

for example, is suitable for low material strengths of 400 N/mm

up to

a thickness of 3 mm, while for materials with a strength of 600 N/mm

the same oil would only be suitable up to a sheet thickness of 1mm. A

high-viscosity oil can be used for a blank thickness of 14mm and a

strength of 400 N/mm

, or for a thickness of 10 mm with a strength of

600 N/mm

In the past, organic chloride compounds were used as extreme pres-

sure (EP) additives in fine blanking oils. However, due to the toxicologi-

cal effects of the chlorine additives, chlorine-free oils are now increas-

ingly used. The present developments in this field only permit this

requirement to be fulfilled up to a sheet metal thickness of around

6mm and a strength of around 450 N/mm

without detriment to the

service life of dies. Where high temperatures and pressure levels occur,

the use of chlorine-free oils involves a significant compromise in terms

of tool life, although coating the active die elements can help to coun-

teract this effect.

4.7.4Fine blanking presses and lines

Requirements

Fine blanking is only possible through the effect of three forces: the

blanking force, the vee-ring force and the counterforce (Fig.4.7.3).

Accordingly, special triple-action presses are used for fine blanking

359

Fine blanking

Fig. 4.7.26 Formation of the lubricant film in the die

lubricant

vee-ringplateblankingpunch

ejector blankingplate

innerformejector

innerformpunch

lubricationpockets

recesses

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

operations. The machines require a controlled movement sequence

with a precise top dead center (Fig.4.7.4).The narrow blanking clear-

ance of the dies must not change even under high levels of stress. Fine

blanking presses are, therefore, required to comply with stringent preci-

sion requirements for example regarding slide gibs, high frame rigidity

and the parallelism of die clamping surfaces. Both mechanical and

hydraulic systems are used for the main slide drive.

Machine layout and drive system

In standard configuration, mechanical presses (Fig.4.7.27)are equipped

with a “combination bed” for the tool systems of moving and fixed punch

(Fig. 4.7.23and 4.7.24). Making use of the straight-side principle, the

monobloc press frame as a welded construction offers good dimensional

rigidity and freedom from vibrations (cf. Fig. 3.1.1). A clearance-free, pre-

tensioned slide gib is used (cf. Fig. 3.1.5).A central support in the upper

and lower die clamping plate ensures optimum die support and introduc-

tion of forces to the die. A controlled, infinitely variable DC motor drives

the press via the flywheel, a disk clutch and a worm gear pair on two syn-

chronously running crankshafts with different eccentricity (Fig. 4.7.27).

The crankshafts drive a double knuckle-joint system which generates the

movement sequence of the slide required for fine blanking.

This drive system is particularly suited for material thicknesses

between approx. 1 and 8 mm, and total press forces of up to 2,500 kN.

The vee-ring force and counterforce are applied by hydraulic systems.

The mechanical drive system is characterized by the following factors:

–a fixed slide movement sequence with constant stroke and precise

position of the top and bottom dead center,

–low energy consumption,

–high output depending on the size of the press with stroking rates of

up to 140/min,

–minimal setting and maintenance input.

The press body of hydraulic presses comprises a robust monobloc welded

straight-side construction with high rigidity (Fig. 4.7.28andcf. Fig. 3.1.1).

This is achieved by implementing suitable design measures such as selec-

tion of large upright cross-sections and large ribs. The slide and main

piston together form a unit which is integrated in the press bed. The

slide is guided in the press body and the lower part of the slide piston.

360

Sheet metal forming and blanking

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

Altan T. Metal Forming Handbook

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