Altan T. Metal Forming Handbook

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not been soft annealed: The microstructure comprises ferrite and perlite.

The hard cementite plates have to be broken through when the blanking

punch penetrates the material. The result is tearing of the cut surface.

The illustration at the top left of Fig.4.7.10shows the C45 in a soft

annealed state, spheroidized: The microstructure comprises a ferrite

matrix with spheroidal cementite embedded in it. Here, the spheroidal

cementite grains are not divided during the blanking process, but

pressed into the soft ferrite matrix: The blanking process takes place

without tear formation.

With an increasing carbon and alloy content, the tensile strength of

the material in the non-annealed and soft-annealed state (Fig.4.7.11)

increases. C45 which is not soft-annealed has a hot forming microstruc-

ture with a strength of around 700 N/mm

. Where C45 has an opti-

mum soft annealing microstructure, i.e. extra soft spheroidized, its ten-

sile strength lies at around 480 N/mm

, while the corresponding value

for the standard spheroidized quality more commonly used for fine

blanking is 540 N/mm

341

Fine blanking

Fig. 4.7.11 Dependency of tensile strength upon the proportion of carbon/alloys in

unannealed aand annealedb steels

tensile strength R

[%]

C content

350

400

450

500

550

600

650

700

750

850

0.2 0.4 0.6 0.8

1 1.4

1.2

16Cr3

16Ni14

16MnCr5

25Cr

Mo4

34Cr4

normalized

steel

steel with

90-100%

spheroidal

cementite

[ ]

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

Determining the degree of difficulty

The slide force F

[N] during fine blanking is

with the blanking force F

[N]:

To calculate the blanking force, the sheet metal thickness s [mm], the

tensile strength of the material R

[N/mm

] and the length of the cut

contour l

[mm] is required (cf. Sect. 4.5). For circular hole punching,

for example the following results:

Depending on the prevailing conditions, factor f

[–] can fluctuate

between 0.6 and 1.2. In order to ensure a sufficient blanking force, in

practice 0.9 is taken for f

. This takes into consideration the influences of

blanking edge properties (blunting and surface roughness of the blank-

ing elements, sheet metal thickness tolerance and alteration of the blank-

ing clearance as a result of abrasive wear).

The counterforce F

[N] is calculated from the surface area A

[mm

]

under pressure by the ejector and the counterpressure q

[N/mm

The value q

lies between 20 N/mm

for thin parts with a small surface

area and 70 N/mm

for larger parts. The counterforce F

must be select-

ed in such a way that the required cut surface quality and optimum

evenness of the part are achieved. As the counterforce must be over-

come directly by the blanking force, an excessively high counterforce

exercises the same effect as if the sheet were of a higher strength level

or thickness. In this way, the high counterforce also influences the ser-

vice life of the die. The punch stress and the force exerted by the punch

both increase. Depending on the part geometry, the counterforce

amounts to between 10 and 25% of the blanking force.

With the help of this information and the surface area A

[mm

] of

the hole punch with the diameter d [mm], which is:

342

Sheet metal forming and blanking

FFFN

St S G

[]

FlsRfN

SS m

=⋅⋅ ⋅

[]

ldmm

=⋅

[]

FAqN

GGG

=⋅

[]

⋅

[]

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

it is also possible to determine the mean pressure p

[N/mm

] ap-

plied on the punch:

or when punching a hole, whereby the counterforce is generally tak-

en as 10% of the punch force:

The mean pressure p

must not exceed the 0.2% compression limit R

p0.2

[N/mm

] of the perforating punch (p

p0.2

). Accordingly, on the basis

of the previous equation for the maximum ratio of sheet metal thickness

s to perforating punch diameter d, the following equation results:

If we assume that the perforating punch is made of high-speed steel

S6-5-2 with R

p0.2

= 3,000 N/mm

and HRC 63-64 and that the tensile

strength of the fine blanking material is 500 N/mm

, the following s/d

ratio results when blanking with counterpressure:

and when blanking without counterpressure:

This maximum s/d ratio is generally assumed to be 1 in normal shear-

ing practice.

Part configuration: flat and formed

The geometric shape of a part, the thickness of the sheet metal and the

characteristics of the material determine the production possibilities

available by fine blanking. In order to ascertain whether a part can be

manufactured using fine blanking, its degree of difficulty is determined:

S1 (easy), S2 (medium), and S3 (difficult). Here, the various formed ele-

ments such as slot widths, section widths, hole diameters, tooth forms,

corner angles and radius must be evaluated with the aid of Fig. 4.7.12.

343

Fine blanking













sRf

⋅⋅ ⋅













≤

⋅⋅

[]

–

≤

⋅⋅

3000

4450009

3000

1980

150

≤

⋅⋅

3000

450009

3000

1800

167

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

The highest single degree of difficulty determines the overall difficulty

level of the part. Under the limiting line S3, fine blanking does not offer

the necessary process reliability using classical tooling technology.

344

Sheet metal forming and blanking

Fig. 4.7.12 Dependency of the degree of difficulty of a fine blanking part upon the thickness/

geometrical shape: hole diameter/section width (A); tooth module (B);

slot/section (C); radii (D)

S1 degree of difficulty 1

min.

= 0,6 s

min.

= 0,6 s

S2 degree of difficulty 2

S3 degree of difficulty 3

0,5

2 3 4 5 6 7 8 9 10 11 12 13

14 15

0.1

0.2

0.3

0.4

0.5

1.0

1.5

2.0

3.0

4.0

6.0

8.0

10.0

tooth module m

S1 degree of difficulty 1

min.

= 0,6 s

min.

= 0,6 s

max.

= approx. 15 a

degree of difficulty

0,6

2 3 4 5 6 7 8 9 10 11 12 13

14 15

R = 0.6 R

r = 0.6 R

r=R

123456789

0.1

0.5

1.0

1.5

2.0

2.5

3.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

m =

z: no. of teeth

hole diameter d, section width a [mm]

sheet metal thickness s [mm]sheet metal thickness s [mm]

slot a, section b (mm)

radius r

[mm]

120° corner angle a

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

Example:

We wish to produce an index cam with a thickness of 4mm in C15 (spher-

oidized) with a tensile strength of 420 N/mm

(Fig. 4.7.13). The following form-

ing elements are included and assigned to their respective degrees of difficulty

in accordance withFig. 4.7.12:

–hole diameter d (mm): 4.1 S1

–section width b (mm): 3.5 S3

–module m: 2.25 S2

–radius R

(mm) with

an angle of 80°: 0.75 S1/S2

The greatest degree of difficulty is presented by the section width (S3). This sets

the total degree of difficulty of the part at S3, which means that the part can be

produced.

By constructing compound progressivediesand transfer dies with part trans-

fer,it is possible to combine forming processes such as deep drawing,

countersinking, semi-piercing, offsetting, bending and embossing on

workpieces with fine blanking(Fig. 4.7.14 and 4.7.15,cf. Sect. 4.1.1).

345

Fine blanking

Fig. 4.7.13Indexing cam: fine blanked part with differing degrees of difficulty

radius R

section b

module m

hole diameter d

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

Properties of the cut surface

The cut surface of fine blanked parts can be blanked smooth over the

entire workpiece thickness (100% of s). However, at times tearing and

fracture may occur. While tearing depends mainly on the microstruc-

ture of the material (Fig. 4.7.10),fracture behavior is influenced by the

magnitude of the blanking clearance (Fig. 4.7.16).

For configuration of the dies and on-line in-process quality control,

the cut surfaces of a part must be described and defined in accordance

with the functional requirements.

346

Sheet metal forming and blanking

Fig. 4.7.14 Examples of different forming techniques which can be combined with

fine blanking

bevelledcut

offsetting

bending

embossing

holewithtapered

countersink

one-sided

shearing

flanging

cylindrical

countersink

holewithcountersink

oneitherside

holewith

roundededges

semi-piercing

weldedbuckle

cupdrawing

coining

Fig. 4.7.15

Fine blanked part featuring the

forming processes semi-piercing,

offsetting, cup drawing, coining,

bending, taper sinking and flanging

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

In the part drawing, the designer is required to define the necessary

smooth cut ratios, surface roughness characteristics, admissible tearing,

die-roll heights and widths and also blanking burr formation. The spec-

ifications are made either on the basis of the cut surface standard

according to VDI 3345

or as per VDI 2906 page 5: whereby E represents the admissible

degree of tearing according to VDI 3345 in accordance with no. 1, 2, 3

or 4. In order to avoid the need for repeated complex specifications, the

blanked surfaces are marked with simple symbols , starting

with the last letter of the alphabet (z). The length of the cut surface is

indicated by a dotted line (Fig. 4.7.17).

Dimensional and form tolerances

The achievable tolerance levels depend on the material, the workpiece

thickness and geometrical shape of the part. The blanking press, the die

and the lubricant used are also significant in determining the achiev-

able part quality. The guideline values for achievable tolerances are

indicated in Table 4.7.2.

347

Fine blanking

Fig. 4.7.16 Terms defining the fine cut surface:

min. smooth cut section in % of the sheet metal thickness s in case of fracture;

min. smooth cut section in % of the sheet metal thickness s in case of shell-

shaped fracture; b

shell-shaped fracture width (sum of all b

can be defined as

required by the user); h

die-roll height; b

die-roll width; E admissible tear in

accordance with VDI Guidline 3345/2906 part 5 relative to size no. 1, 2, 3 or 4

achievable smooth cut ratio

tearing E

fracture

h/sin%

100 100

75 50

ZDIN

SS12

,,…

()

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

Table 4.7.2: Achieveable tolerances in fine blanking

Sheet metal

thickness [mm]

0.5 to 1 6 to 7 7 ± 0.01 7 8 ± 0.01

1 to 2 7 7 ± 0.015 7 to 8 8 ± 0.015

2 to 3 7 7 ± 0.02 8 8 ± 0.02

3 to 4 7 8 ± 0.02 8 9 ± 0.03

4 to 5 7 to 8 8 ± 0.03 8 9 ± 0.03

5 to 6 8 9 ± 0.03 8 to 9 9 ± 0.03

> 6 8 to 9 9 ± 0.03 9 9 ± 0.03

Inside

contours

ISO

quality

Outside

contours

ISO

quality

Hole

distance

tolerances

[mm]

Inside

contours

ISO

quality

Outside

contours

ISO

quality

Hole

distance

tolerances

[mm]

Tensile strength over 500 N/mm

Tensile strength up to 500 N/mm

The die-rollwith its width b

and height h

depends on a variety of

blank and material-related factors (Fig.4.7.8, 4.7.16and 4.7.18).The

angle and radius of inward and outward pointing corners, the material

and the microstructure, strength and sheet metal thickness, for instance,

exert a considerable influence. Also important in determining the die-

roll amount is the edge preparation of the blanking plate and inner form

348

Sheet metal forming and blanking

Fig. 4.7.17 Example dimensions for a fine blanked part in accordance with VDI 3345

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

punches, as well as the effect of the vee-ring. The die-roll width b

depends on the die-roll height h

. In most cases, the following applies:

Burr-free fine blanking is not possible. The blanking burris located

opposite to the die-roll, and occurs as soon as the first part is blanked

due to edge preparation at the blanking plate and the blanking punch.

As a result of wear of the active elements, the height and width of the

burr increase with the increasing number of blanking operations. The

blanking burr is generally removed by belt or flat grinding.

The cut surfacesare not at absolute right angles to the plane of the

sheet metal. The outside contours of a blank on the burr side are greater

than at the die-roll side – inner contours are smaller on the burr side

than on the die-roll side. As a guideline, the difference amounts to

0.0026mm per 1mm of blank thickness, and depends on a number of

influencing variables, such as dimensioning of the blanking plate, con-

figuration of the blanking plate with or without shrink ring, prepara-

tion and coating of active elements.

Application examples

The following pictures provide examples of fine blanked parts. In

Fig. 4.7.19,an automatic car transmission is illustrated to indicate the

349

Fine blanking

bhmm

≈⋅

[]

Fig. 4.7.18 Geometry and die-roll on a fine blanked part:

R corner radius; b

die-roll width;

corner angle;

s blank thickness; h

burr height; h

die-roll height

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

variety of fine blanked parts. High demands concerning cut surface

quality, dimensional tolerances and flatness are required from the lam-

inations, drive boxes and intermediate plates. Extremely small holes in

the intermediate plate are also fine blanked, although the s/d ratio lies

at around 2.

Several fine blanked parts are also required for subassemblies used in

the brake and drive system (Fig.4.7.20),for instance brake disks with

their many holes which have a diameter smaller than half the sheet

metal thickness. These small perforations could not be executed using

standard blanking methods. In addition, a tear and fracture-free cut

surface is essential in order to avoid notching effects, as the disks are

350

Sheet metal forming and blanking

Fig. 4.7.19 Fine blanked components in the automatic transmission of a passenger car

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