Drawing Manufacturing Process

1. Drawing Process

2. Drawing – It is a process where a
cross-section of solid rod, wire, or
tubing is reduced or changed in shape
by pulling it through a die.

3. Drawing
 Definition
◦ Cross section of a round rod / wire is
reduced by pulling it through a die.
 Variables:
◦ Die Angle, Extrusion Ratio R (A0 / Af) ,
Friction between die and workpiece,
drawing speed.
 “There is an optimum angle at which the
drawing force is minimum” for a given
diameter reduction and friction parameter.

4. Area Reduction in Drawing
Change in size of work is usually given by
area reduction:
where r = area reduction in drawing; Ao =
original area of work; and Af = final work
o
f
o
A
A
A
r



5. Drawing
 Estimation of Drawing Force required:
F = Yavg Af ln (A0/Af)
 Yavg = average true stress of material in
the die gap.
 Assumptions: no friction.

6. Drawing
 Work has to be done to overcome friction.
 Force increases with increasing friction.
 Cannot increase force too much, or
material will reach yield stress.
 Maximum reduction in cross-sectional area
per pass = 63%. In general, the reduction in
area is between - 50%.
 To produce a desired size or shape, multiple
draws may be required through a series of
progressively smaller dies. Intermediate
annealing may also be required to restore
ductility and enable further deformation.

7. Drawing Die Design
 Die angles range from 6 to 15 degrees.
 Two angles are typically present in a die:
◦ Entering angle
◦ Approach angle
 Bearing Surface (land): sets final
diameter.
 Back relief angle

8. Figure Schematic of a multistation synchronized wire-drawing machine. To prevent accumulation or
breakage, it is necessary to ensure that the same volume of material passes through each station in a
given time. The loops around the sheaves between the stations use wire tensions and feedback
electronics to provide the necessary speed control.
Figure Cross section through
a typical carbide wire-
drawing die showing the
characteristic regions of the
contour.

9. Figure Schematic of a multistation synchronized wire-drawing machine. To prevent accumulation or
breakage, it is necessary to ensure that the same volume of material passes through each station in a
given time. The loops around the sheaves between the stations use wire tensions and feedback
electronics to provide the necessary speed control.
Figure 16-39 Cross section
through a typical carbide
wire-drawing die showing the
characteristic regions of the
contour.

10. Wire, Rod, and Tube Drawing
 Reduce the cross section of a material
by pulling it through a die
 Similar to extrusion, but the force is
tensile
Figure Schematic drawing of the rod-or bar-
drawing process.
Figure Cold-drawing smaller tubing from
larger tubing. The die sets the outer
dimension while the stationary mandrel sizes
the inner diameter.

11. Wire and Bar Drawing
Cross-section of a bar, rod, or wire is reduced by
pulling it through a die opening
 Similar to extrusion except work is pulled
through die in drawing (it is pushed through in
extrusion)
 Although drawing applies tensile stress,
compression also plays a significant role since
metal is squeezed as it passes through die
opening

12. Figure - Drawing of bar, rod, or wire

13. Wire Drawing vs. Bar Drawing
 Difference between bar drawing and wire
drawing is stock size
◦ Bar drawing - large diameter bar and rod stock
◦ Wire drawing - small diameter stock - wire
sizes down to 0.03 mm (0.001 in.) are possible
 Although the mechanics are the same, the
methods, equipment, and even
terminology are different

14. Drawing Practice and Products
 Drawing practice:
◦ Usually performed as cold working
◦ Most frequently used for round cross-sections
 Products:
◦ Wire: electrical wire; wire stock for fences, coat hangers,
and shopping carts
◦ Rod stock for nails, screws, rivets, and springs
◦ Bar stock: metal bars for machining, forging, and other
processes

15. Bar Drawing
 Accomplished as a single-draft
operation - the stock is pulled through
one die opening
 Beginning stock has large diameter and
is a straight cylinder
 This necessitates a batch type operation

16. Wire Drawing
 Continuous drawing machines consisting of
multiple draw dies (typically 4 to 12) separated
by accumulating drums
◦ Each drum (capstan) provides proper force to draw wire
stock through upstream die
◦ Each die provides a small reduction, so desired total
reduction is achieved by the series
◦ Annealing sometimes required between dies

17. Figure - Continuous drawing of wire

18. Features of a Draw Die
 Entry region - funnels lubricant into the die to
prevent scoring of work and die
 Approach - cone-shaped region where drawing
occurs
 Bearing surface - determines final stock size
 Back relief - exit zone - provided with a back relief
angle (half-angle) of about 30
 Die materials: tool steels or cemented carbides

19. Figure - Draw die for drawing of round rod or
wire

20. Tube Drawing Operations
Fig : Examples of tube-drawing operations, with and without internal mandrel.
Note that a variety of diameters and wall thickness can be produced from the
same initial tube stock (which had been made by other processes).

21. Defects in Drawing
 Center cracking.
 Seams (folds in the material)
 Residual stresses in cold-drawn
products.
 If % reduction is small:
◦ (Compressive at surface / Tensile at Center)
 If % reduction is larger, opposite
occurs:
◦ (not desirable- can cause stress corrosion
cracking.)

22. Tube and Wire Drawing
 Tube sinking does
not use a mandrel
◦ Internal diameter
precision is sacrificed
for cost and a floating
plug is used
Figure 16-37 (Above) Tube drawing with
a floating plug.
Figure 16-38 Schematic of wire drawing with a
rotating draw block. The rotating motor on the
draw block provides a continuous pull on the
incoming wire.

23. Cold Forming and Impact
Extrusion
 Slugs of material are
squeezed into or
extruded from shaped
die cavities to produce
finished parts of
precise shape and size
Figure Cold-forming sequence involving cutoff,
squaring, two extrusions, an upset, and a trimming
operation. Also shown are the finished part and the
trimmed scrap. (Courtesy of National Machinery
Co., Tiffin, OH.)

24. Cold Forming and Impact
Extrusion
 Slugs of material are
squeezed into or
extruded from shaped
die cavities to produce
finished parts of
precise shape and size
Figure Cold-forming sequence involving cutoff,
squaring, two extrusions, an upset, and a trimming
operation. Also shown are the finished part and the
trimmed scrap. (Courtesy of National Machinery
Co., Tiffin, OH.)

25. Impact Extrusion
 A metal slug is
positioned in a die
cavity where it is
struck by a single
blow
 Metal may flow
forward, backward
or some combination
 The punch controls
the inside shape
while the die controls
the exterior shape
Figure 16-43 Backward and forward extrusion
with open and closed dies.

26. Cold Extrusion
Figure 16-45
(Right) Steps in
the forming of a
bolt by cold
extrusion, cold
heading, and
thread rolling.
(Courtesy of
National
Machinery Co.
Tiffin, OH.)
Figure 16-44
(a) Reverse
(b) forward
(c) combined
forms of cold
extrusion.
(Courtesy the
Aluminum
Association,
Arlington, VA.)

27. Summary
 There are a variety of bulk deformation
processes
 The main processes are rolling, forging,
extrusion, and drawing
 Each has limits and advantages as to its
capabilities
 The correct process depends on the
desired shape, surface finish,
quantity, etc.