A criterion for critical tool wear in blanking

A criterion for critical tool wear in blanking

Citation for published version (APA):

Veenstra, P. C., & Ramaekers, J. A. H. (1978). A criterion for critical tool wear in blanking. (TH Eindhoven. Afd. Werktuigbouwkunde, Laboratorium voor mechanische technologie en werkplaatstechniek : WT rapporten; Vol. WT0424). Technische Hogeschool Eindhoven.

Document status and date: Published: 01/01/1978 Document Version:

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WT rapport nr. 0424

A Criterion for Critical Tool \/ear in Blanking.

P.C. Veenstra, Eindhoven University of Technol,ogy/NL (l); J.A.H. Ramaekers, Ei,ndhoven University of Technology/tiL.

Summary: In earl ier work the statistical behaviour of blanking tools was studied in order to define the average life time of the tools. The amount of burr in the product was chosen as the criterion for the end of tool 1 ife.

In the present work the very initial phase of the process is analysed. On basis of the hypothesis that catastrophical wear develops as soon as the shape of the punch matches the bending of the blank, a criterion for critical wear is derived dependent on tool geometry and material IS properties of the blank.

Prof.Dr. P.C. Veenstra University of Technology P.O. Box 513 EINDHOVEN Netherlands

-r

I

.

m

-z

0

:r:

0 ~

m

. i Gl' - I ...:J

_OJ!

;y: _{I i} "-- '

e

_a

...

_...

--

- I I ;~ fT1 -:::;t

_rn

?':

1. I NTRODUCT I ON

In earlier work [lJ one of the present authors studied the statistical behaviour of blanking tools in order to define the average life time of the tools. By then the amount of burr was chosen as a criterion for the end of tool life.

However, to approach closer to the ultimate goal of objective production control it would be desirable to have a criterion for tool life in terms of wear of the tool related to material's properties.

Now it is observed in the blanking process that there exists a phase of rapid initial wear. After this the wear develops

steadily, however at a much lower rate. This is considered to be the phase of "normal" wear. Towards the end of tool life a third phase occurs, when the wear rate again increases rapidly.

The three phases mentioned are closely correlated with the amount of burr in the product.

To find a criterion for the onset of the third phase of catastro-phical wear and hence for the end of tool life it is accepted as a hypothesis that at this moment the worn shape of the tool matches the bending of the blank during the initial stage of the process as shown in fig. 1.

2

b

PUNCH

FLANK \lEAR BLANK

2. ANALYSIS

Fig. 1. The hypothesis for the onset of critical flank wear bs '

The worn shape of the punch matches the bending of the blank.

The blanking process is always accompanied by bending of the blank. In the very initial stage, when the punch just hits the blank the bending is due to more or less uniform load. This means that immediately after the impact of the punch in the central region the contact between punch and blank is lost and finally the load is concentrated on a narrow band at the

per i phery.

In practice this band can readily be recognized by its shining appearance, indicating that the material is plastically deformed. Assuming that the pressure band fully develops to its final

width b at the moment of maximum load F_{max , because of}

equilibrium it must hold that

0/2

2 'IT J a r d r = F (1)

0/2-b n max

where an is the normal~ressure.on the pressure band and

o

represents the diameter of the punch. In practice commonly the relationship

F _max = L _• h

0 (2)

is used to calculate the maximum force in blanking, where ho is the blank thickness, L represents the length of cut and ~sh the average shear stress. The latter is often relate~ to the tensile strength of the material by applying

3

(3)

where f his the sheari n9 factor, an experimenta 1 constant

relevant for a particular blanking operation. Its value can be taken from handbooks.

However, Ramaekers [2J showed that for a material characterized by the ~xtended relation of Nadai

I

_{- (- - n}

a = C € + € )

. 0

(4)

where

e

is the initiaf deformation of the material as received, the shegr stress in the shearing zone is uniformly distributed and has the value

T =

~

{.

JJ

In ho +

~o)n

(5)

rz

13

i

In

h

where h -h=s is the path ~f the punch. Introduging the reduced quantities

r=

F L • h C 0 and

*

s h -h 0 5 =j1=--';-o 0

(6)

(7)

4'1

it is readily found to hold

Flt =_1_ (l-slt )

(~flln

-L+

e

\n

13

lil

l";slt

oj

By differentation with respect to slt it follows

~ax ~ ~

{1m}"

exp

['0

~

-

nJ which value is achieved for

s,.

=

1 - exp

[;:0

~

-

n

J

with the condition

e

$

~3

o n

It is remarked that from this result and the relation 0B

~

C (:)"

:0

(e

$ n)

o the shearing factor

Flt f C ~ sh

=

O'a

(8)

(10) (11) (12) (13)

can be calculated as a function of material's constants as shown in fig. 2. .s:::. If) \4- l-0 .j.J u I\J \4-en e: I-I\J (j,) .s:::. If) 1.or---~

__

~0 0.2 0.8 0.4

0.6

0.8 1 It..) e: 1/13 .j.J 0 e: =n I\J 0.4 0 E I-0 . \4-0.2

o

o

0.2 0.4 0.6

0.8

n

Fig.

2.

The shearing factor fsh as a

function of material's properties.

(j,) "'0

-

I\J .j.J

.-

_e: 0

5

When substituting eq. 9 into eq. 1 it is clear that if the distribution of normal stress over the pressure band is known, the width of the band can be expressed in material1s properties. Next the place of action of the resultant force can be calculated and together with the clearance of the tool the bending of the blank is found according to the theory of bending of thin plates due to closely neighbouring opposite forces.

According to the hypothesis assumed and as shown in fig. 1 the critical flank wear of the punch is expressed in terms of vertical displacement of the bla~k, according to

b = IW(r=D/2) - W(r=D/2-b)I

s (14 )

For the sake of simplicity the distribution of on over b is taken to be linea r •

Next, since the maximum load occurs when the blanking process has developed up to about 30%, it .is assumed that the average

pressure in the pressure zone equals the average of the yield stress in the shearing zone at the moment of maximum force and the yield stress of the material as received.

Since, according to Ramaekers [2J it holds

Ii =

-'3

In

--L

(15)

1il

1 _-s It

it follows with eq. 10 that

e + €

=

I3n

+

e

o max 0 and thus

°

= -

21

c[(

13ri)

n + if the As eq. nAVE

definition of the 0.2% strain yield limit is applied. 1 simply reduces to

L • b •

°

=

FHA·

X

nAVE

it is found through substitution of eqs. 9 and 17 that

( 16) ( 17) ( 18) blt

=

..£..

=

~

(l3n)n

r-

-'n]

(19) ho

13

(l3ri)n +

(e

+0.002)n eXPL€o13 - n . 0

As shown in fig. 3 this reduced width of the pressure band is not very strongly dependent on material's properties. In particular,

if there is some initial deformation in the material, the reduced width becomes virtually constant.

Because of its shining appearance the width can easily be measured. The average of 28 samples of different thickness and different material proved to be blt=O.57±O.07, which is in fair agreement with eq. 19.

6

0.8

~ ~o o

•

b REDUCED WIDTH

0.7

0.4

0.3

0.2

o.

1 ( . PRESSURE ZONE 0.1 o.~

_0.3

0.4

e =0.02 o

STRAIN HARDENING EXPONENT

n

0.5

0.6

0.7

0.8

Fig.

3.

The reduced width of the pressure zone ~

as a' function of material's properties according to eq. 19.

Now, when substituting eq. 19 in Prescott's [3J formula for bending of a thin plate under the particular loading condition of blanking and applying eq. 14 it is obtained for the reduced critical flank wear of the tool

where \I Young's reduced blt = bs = t3(§n)n (1-,})

£.

exp

[~ J~

-

nJ s ho E 0

""3

It (blt+ult )

11

+ 1-v ( Olt_blt

)21

bltDlt (20) 1 +v Dlt +blt+2ult

is Poisson's ratio,

~

the ratio of specific

str~ss

and modulus, ult

=

~ the reduced clearance and

0*=

~ the

d' lameter of the punch. 0 0

The formula is visualized in fig.

4,

which again shows the relatively strong influence of initial deformation. However, since technical materials as applied in blanking practically always have a few percent of initial deformation and moreover the technically relevant value of the strain-hardening exponent ranges in 0-0.6, it may be concluded that eq. 20 provides a safe upper limit for critical tool wear.

7

It appears that the data obtained from eq. 20 agree pretty well with experience in the shop, as far as mass production of small components of mainly Ni.Cr steels is involved.

8

7 6,

REDUCED

CRITi~AL

TOOL

~EAR 5 b* =...! 5 h o 4 3 2 -'£

=0

o

-Ni-Cr STEEL

C/E

=

1500/21.10

4

v

=

0.3

PUNCH OIA

CLEARANCE

e:

=0.03

.0 £ =0.20 o 1 • 10-3

STRAiN HARDENING EXPONENT

n

0

Q.1 _0.2

_0.3

0.4

0.5

.0.6 0.7 Fig. 4. The critical reduced flank wear as

a function of material's properties.

REFERENCES

[1] A.G.M. Buiteman, F. Doorschot,

P.C.

Veenstra. On the life time of die sets.

Ann. CIRP, vol. 24/1, 1975. [2] J.A.H. Ramaekers.

Thesis, Eindhoven, University of Technology, 1970.

[3] J. Prescott.

Applied elasticity, Dover, Publ. New York.