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Wire and Bar Drawing:

Drawing is an operation in which the cross section of a bar, rod, or

wire is reduced by pulling it through a die opining as in figure (40).

The general features of the process are similar to those of extrusion.

The difference is that the work is pulled through the die in drawing,

whereas it is pushed through the die in extrusion.

The basic difference between bar drawing and wire drawing is the

stock size that is processed. Bar drawing is the term used for large-

diameter bar and rod stock, while wire drawing applies to small-

diameter stock. Wire sizes down to 0.001 in (0.03 mm) are possible in

wire drawing.

Bar drawing is generally accomplished as a single-draft operation: the

stock is pulled through one die opining because the beginning stock has

a large diameter, it is in the form of a straight piece rather than coiled.

By contrast, wire is drawn from coils consisting of several hundred (or

even several thousand) feet of wire and is passed through a series of

draw dies. The number of dies varies typically between 4 and 12.

Figure (40) Drawing processes.

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In drawing operation the change in size of the work is usually given by

the area reduction, defined as follows:

AAAr f

Where r= area reduction in drawing.

A = original area of work in^2(mm^2).

fA = final area of work in^2(mm^2).

In bar drawing, rod drawing, and in drawing of large-diameter wire

for upsetting and heading operations, the term draft is used to denote

the before and after difference in size of the processed work. The draft

is simply the difference between original and final stock diameters:

fDDd

Where d= draft, in(mm).

D = original diameter, in(mm).

fD = final work diameter, in(mm).

Analysis of Drawing:

Mechanics of drawing, if no friction work occurred in drawing,

true strain could be determined as follows:

rA

A

f

1

1lnln

The stress that results from this ideal deformation is given by:

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f

ffA

AYY

ln

Where

fY = average flow stress based on the value of strain because

friction is present in drawing and work experiences in homogeneous

deformation, the actual stress is larger than provided by the above

equation.

In addition to the ratio fAA / , other variables that influence draw

stress are die angle and coefficient of friction at the work-die interface.

We present the equation suggested by Schey:

f

fdA

AY

ln)tan

1(

Where d = draw stress,lb/in^2(MPa).

= die-work coefficient of friction.

= die angle (half angle) as defined in figure 1.

= factor that accounts for in homogeneous deformation.

CL

D12.088.0

Where D = average diameter of work during drawing, in(mm).

CL = contact length of the work with the draw die, in(mm).

sin2,

2

f

C

f DDL

DDD

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The corresponding draw force is then the area of the drawn cross

section multiplied by the draw stress;

f

ffdfA

AYAAF

ln)tan

1(

Where F= draw force, lb(N).

The power required in a drawing operation is the draw force

multiplied by exit velocity of the work.

Example:wire is drawn through a draw die with entrance angle =

15

starting diameter is 0.1 in, and final diameter 0.08 in. the coefficient of

friction at the work-die interface =0.07. The metal has a strength

coefficient K=30000lb/in^2, and a strain hardening exponent n=0.2.

Determine the draw stress and draw force in this operation?

Solution:

inDD

D f

09.02

08.01.0

2

inDD

L f

C 0386.015sin2

08.0_1.0

sin2

16.10386.0

09.012.088.012.088.0

CL

D

222 00786.01.044

inDA

222 00503.008.0

44

hnDA ff

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446.000503.0

00786.0lnln

fA

A

22.0

/212752.1

)446.0(30000

1inlb

n

KY

n

f

f

fdA

AY

ln)tan

1(

2/13882)446.0)(16.1)(

15tan

07.01(21275 inlbd

lbAF fd 7000503.0*13882