Rod, wire and tube drawing

Rod, wire and tube drawing

1. Rod, Wire and Tube Drawing
Prepared by
Prof. NamanM. Dave
Assistant Professor,
Mechanical Engg. Dept.
GandhinagarInstitute of Technology
Prof. Naman M. Dave

2. Introduction
•Drawing is an operation in which the cross- sectional area of a bar
or tube is reduced or changed in shape by pulling it through a
converging die (
dies are usually tapered) .
•The drawing process is somewhat similar to extrusion, except that
in drawing, the bar is under tension, whereas in extrusion it is
under compression.
Prof. Naman M. Dave

3. •Although drawing applies tensile stress, compression also plays a
significant role since metal is squeezed as it passes through die
opening.
•Rod and wire drawing are generally finishing process, and the
product is either used as produced or is further processed into
other shapes, such as by bending or machining.
•Rods are used for various applications, such as small pistons,
structural members, shafts, spindles, and as raw material for
making fasteners such as bolt and screws.
•Wire and wire product have a wide range of applications, such as
electrical wiring, electrical equipment's, cables, springs, fencing,
welding electrodes and shopping carts.
•Wire diameter may be as small as 0.025 mm
Introduction
Prof. Naman M. Dave

4. Rod products
Prof. Naman M. Dave

5. Wire products
Prof. Naman M. Dave

6. •The concept of drawing involves pulling wire, rod, or bar though a
die, or converging channel to decrease cross-sectional area in
increase length.
•In the majority of the cases the cross section is circular, although
none-circular cross sections may be drawn/ or created by
drawing.
•In a comparison to rolling, drawing offers
1.Much better dimensional control.
2.Lower capital equipment cost.
3.And extension to small cross sections.
•In comparison to extrusion, drawing offers
1.Continuous processing.
2.Lower equipment cost.
3.And extension to small cross sections.
Drawing
Prof. Naman M. Dave

7. Drawing
•Large quantities of wires, rods, tubes and other sections are
produced by drawing process which is basically a cold working
process (Drawing is usually performed at room temperature, thus
classified a cold working process, but it may be performed at
elevated temperatures for large wires to reduce forces). In this
process the material is pulled through a die in order to reduce it to
the desired shape and size.
•In a typical wire drawing operation, once the wire is gripped and
pulled to pass through the opening of the die, its diameter
reduced to the desired one.
Prof. Naman M. Dave

8. Cold drawing properties
Improved Size and Section
•Tighter size & section tolerances
•Dimensional consistency within each bar
•Dimensional consistency from bar to bar
Improved Surface Finish / Reduces surface machining
Improved Straightness
Improved Cost Effectiveness / Production of Precision Shapes to
Precision Tolerances
Increased Mechanical Properties / Can reduce the need for
hardening.
•Yield strength
•Tensile strength
•
Hardness
Prof. Naman M. Dave

9. Wire Drawing vs. Bar Drawing
•Difference between bar drawing and wire drawing is
stock size

Bar drawing -Large diameter bar and rod stock (bar
drawing usually involves stock that is too large in
cross section to be coiled. Round bar stock may be 1
to 10 cm in diameter or even larger)
Wire drawing -Small diameter stock -wire sizes
down to 0.025 mm (0.001 in.) are possible
•Although the mechanics are the same, the methods,
equipment.
Prof. Naman M. Dave

10. 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
Prof. Naman M. Dave

11. Why not simply stretch wire, rod or bar?
•It can be argued, at least in principle, that some of the
objectivesof drawing could be achieved by simply stretching
the wire with a pulling force.
•The cross section could be reduced and elongation
accomplished, but dies would not be needed and the friction
and metal flow issues presented by die could be a voided.
•The principle problem with just stretching the wire with a
pulling force is the necking phenomena. Basically, after a
certain amount of uniform stretching, all further elongation
will be concentrated at a single location ( a neck), which will
rapidly thin and break.
•This occurs because the decrease in cross-sectional area
eventually weakens wire more than any strengthening that
occurs by work hardening.
How dose drawing work
Prof. Naman M. Dave

12. Simple explanation of drawing process.
•In the drawing process, a pulling force and a pressure force from
the die, combine to cause the wire to extend and reduce in
cross-sectional area, while passing through the die as shown in
the figure :
How dose drawing work
Prof. Naman M. Dave

13. •Because of this combined effect , the pulling force or drawing
force is less than the force that would cause the wire to stretch,
or neck and break downstream from the die.
•On the other hand, if a reduction too large in cross- sectional
area is attempted at the die, the drawing force may break the
wire.
•In commercial practice, engineering pulling loads are rarely
above 60% of the as- drawn strength, and the area reduction in a
single drawing pass is rarely above 30% or 35%, and is often
much lower.
•
A particular common reduction in none ferrous drawing is the
American Wire Gage (AWG) number, or about 20.7%. So many
drawing passes are needed to achieve large overall reduction
How dose drawing work
Prof. Naman M. Dave

14. Wire Drawing
•Wire drawing involves stock that can be easily coiled and
subjected to sequential or tandem drawing operations
with as many as a dozen or more draws occurring with a
given drawing machine.
•Each drawing operation or “pass” will involve delivery of
the wire to the die from a coil on capstan OR drum,
passage through the die, and take-up on a capstan that
pulls the wire through the die
•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 of dies.
Prof. Naman M. Dave

15. •Fine wire drawing typically refers to round
wire with a diameter of less than 0.1 mm.
•Ultra fine wire drawing typically refers to
round wire as fine as 0.0025 mm in diameter.
Wire Drawing
Prof. Naman M. Dave

16. •Annealing before each drawing operation permits large area
reduction (Annealing sometimes required between dies to
relieve work hardening). Because of the strain hardening,
intermediate annealing between passes may be necessary in
cold drawing in order to maintain sufficient ductility to the
material and avoid failure.
•Tungsten Carbide dies are used to for drawing hard wires, and
diamond dies is the choice for fine wires.
Wire Drawing
Prof. Naman M. Dave

17. Bar Drawing
•Bar drawing usually involves stock that is too large in cross
section, and hence must be drawn straight.
•Round bar stock may be 1 to 10 cm in diameter and even larger.
•Accomplished as a single-draft operation -the stock is pulled
through one die opening
•Beginning stock has large diameter and is a straight cylinder
•Hydraulically
operated draw
bench for
drawing metal
bars
Prof. Naman M. Dave

18. •The reduction in area is usually
restricted to 20 to 50%, because
greater reductions would exceed
the tensile strength of the
material, depending on
itsductility
. To achieve a certain
size or shape multiple passes through progressively smaller dies or intermediate
annealsmay be
required
Bar Drawing
Prof. Naman M. Dave

19. Tubes produced by extrusion or other process (such as shape rolling)
Also tubes can be reduced in thickness or diameter by tube drawing.
The shape of tubes can be changed by using dies and mandrels with
various profiles
Tub Drawing
Prof. Naman M. Dave

20. •Tube drawing is also similar to wire drawing,
except that a mandrel of appropriate diameter is
required to form the internal hole.
•Here three arrangements are shown in figure (a)
with a floating plug and (b) fixe plug drawing(c)
with a moving mandrel
Tube drawing
Prof. Naman M. Dave

21. Fixed plug drawing
•This is the oldest tube drawing method.
•Fixed plug drawing, also known as stationary mandrel
drawing, uses a mandrel at the end of the die to shape the
ID of the tube.
•This process is slow and the area reductions are limited
(lengths of tubes are limited), but it gives the best inner
surface finish of any of the processes.
Prof. Naman M. Dave

22. Floating plug drawing
Floating plug drawing, also known as floating mandrel drawing, uses
a mandrel that is not anchored whatsoever to shape the ID of the
tube.
The mandrel is held in by the friction forces between the mandrel
and the tube.
The greatest advantage of this that it can be used on extremely long
lengths, sometimes up to 1,000 feet (300 m).
The disadvantage is it requires a precise design otherwise it will give
inadequate results. This process is often used for oil -tubing
Prof. Naman M. Dave

23. Floating plug drawing
Prof. Naman M. Dave

24. •Is the process that draws the tube with a mandrel inside the tube;
the mandrel is drawn with the tube (moving mandrel). Tubes as large
as (30 m)in diameter can be drawn.
•
The advantage to this process is that the mandrel defines the ID and
the surface finish.
•The disadvantages are that lengths are limited by the length of the
mandrel, usually no more than 100 feet (30 m), and that a second
operation is required to remove the mandrel, called reeling. This
type of process is usually used on heavy walled or small ID tubes.
•Common applications include super-high pressure tubing and
hydraulic tubing
•This process is also use for precision manufacturing of trombone
hand slides
Moving mandrel drawing
Prof. Naman M. Dave

25. Moving mandrel drawing
•Also known as semi-floating mandrel drawing, is a mix
between floating plug drawing and fixed plug drawing. The
mandrel is allowed to float at the end of tube, but it still
anchored.
•This process gives similar results to the floating plug process,
except that it is designed for multiple diameter of straight
tubes. It gives a better inner surface finish than rod drawing.
Prof. Naman M. Dave

26. •Annealing–to increase ductility of stock
•Annealing:This is a thermal treatment generally
used to soften the material being drawn, to
modify the microstructure, the mechanical
properties and the machining characteristics of
the steel and/or to remove internal stresses in the
product. Depending on the desired characteristics
of the finished product, annealing may be used
before, during (between passes) or after the cold
drawing operation, depending on material
requirements.
Preparation of the Work for
Wire or Bar Drawing
Prof. Naman M. Dave

27. •Cleaning-To prevent damage to work surface and draw die
(Abrasive scale (iron oxide) on the surface of the hot rolled
rough stock is removed) it is done by pickling process.
•Pointing–to reduce diameter of starting end to allow
insertion through draw die (Several inches of the lead ends of the bar or coil are reduced in size by swaging ,so that it
can pass freely through the drawing die. Note: This is done
because the die opening is always smaller than the original
bar or coil section size) and also to prevent die from wearing.
•Coating: The surface of the bar or coil is coated with a
drawing lubricant to aid cold drawing.
Preparation of the Work for
Wire or Bar Drawing
• Raw Stock: Hot rolled steel bar or rod coils are used as raw material.
Because the hot rolled products are produced at elevated temperatures
(1700 –2200 i.e. hot rolling), they generally have a rough and scaled surface
and may also exhibit variations in section and size.
Prof. Naman M. Dave

28. Lubrication
•Proper lubrication is important in drawing operations.
•The basic methods of lubrication used in wire drawing are:
Dry drawing : in dry drawing, the surface of the wire is
coated with various lubricants, depending on the strength
and frictional characteristics of the material. The rod to be
drawn is first surface treated by pickling, which removes the
surface scale that could lead to surface defects and
considerably reduce die life (because of it’s abrasiveness).
The bar then goes through a box (stuffing box) filled with
soap powder to coat it
•Picklingis ametalsurface treatment used to remove impurities,
contaminants,rustorscalefromferrousmetals,copper,
andaluminumalloys.A solution called pickle liquor, which
containsstrong acids, is used to remove the surface impurities. It
is commonly used to descale or cleansteelin
varioussteelmakingprocesses
Prof. Naman M. Dave

29. Wet drawing : in wet drawing, the dies and rod are
completely immersed in a lubricant. Typical lubricants
include oils and emulsion (containing fatty or chlorinated
additives) and various chemical compounds.
Metal coating : for high -strength materials, such as steels,
stainless steels, and high- temperature alloys, the surface of
the rod may be coated with a softer metal or with conversion coating. Conversion coatings may consists of sulfate oxalate coatings on the rod, which typically are then coated with soap, as a lubricant.
•Copper or tin can be chemically deposited as a thin layer on surface of the metal, whereby it acts as a solid lubricant
•Polymers may also be used as a solid lubricants, such as in
drawing titanium
Lubrication
Prof. Naman M. Dave

30. 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 (land) -determines final stock size. The purpose of the
land is to size, that is, to set the final diameter of the product
•Back relief -exit zone –if the exit of die bearing has a sharp edge, this
can shave the wire. This will produce metal particles, dust or fines,
which will
Pollute drawing lubricant
Block the entrance of the next die
Prof. Naman M. Dave

31. •Back relief
•The friction against sharp edge also imparts
vibration to the wire, which can be transmitted
through the machine. The negative results of this
are:
1.Abnormal wear of the die
2.Poor wire quality
3.Numerous wire breaks
Features of a Draw Die
•To avoid this sharp edge, it is necessary to produce A well
smooth transition zone between the bearing and the exit zone
Prof. Naman M. Dave

32. Die materials
•Die materials for drawing are generally alloy tool steels,
carbides, or diamond.
•For drawing fine wires the die may be diamond, either a single
crystal or a polycrystalline diamond.
•Carbide and diamond dies are made as inserts or nibs, which are then supported in a steel casting as shown:
Prof. Naman M. Dave

33. •A typical wear pattern
on a drawing die is
shown below.
•The die wear is highest
at the entry. Although
the pressure is highest
in this region and may
be partially reasonable
for wear of die.
Die Wear
Prof. Naman M. Dave

34. Other factors that are involved in wearing of die
include:
1.Variations in the diameter of the entering wire.
2.Vibration, which subjects the die-entry contact
zone to fluctuating stresses
3.The presence of abrasive scale on the surface
of entering wire.
Die Wear
Prof. Naman M. Dave

35. Equipment
•Drawing equipment can be of several designs. These designs can be
classified into two basic types; Draw bench, and Bull block:
1.
A draw bench : A draw bench uses a single die and the pulling
force is supplied by a chain drive or by hydraulic means. Draw bench is used for single length drawing of rod or tube with diameter greater than
20mm (may be 1 to 10 cm in diameter and even larger). Length can be as
much as 30 m. similar to a long horizontal tensile testing machine but
with hydraulic or chain-drive mechanism, is used for single draws of
straight rods with large cross sections and for tubes with length up to 30 m.
1.
Bull block : smaller cross sections are usually drawn by a bull block,
which is basically a rotating drum around which wire is wrapped. The tension in the setup provides the force required to draw the wire
Prof. Naman M. Dave

36. •A draw bench uses a single die and the pulling force is supplied by
a chain drive or by hydraulic means. Draw bench is used for single
length drawing of rod or tube with diameter greater than 20mm.
Length can be as much as 30 m
•Drawing equipment
can be of several
designs. These designs
can be classified into
two basic types
1.Draw benchfor rod
and tube
2.and Bull block for wire
Equipment
Prof. Naman M. Dave

37. Draw bench
Prof. Naman M. Dave

38. Bull block
Prof. Naman M. Dave

39. Drawing Defects
Defects in drawing are similar to those observed in
extrusion especially center cracking.
•The factors influencing center cracking are:
1.The tendency for cracking increase with increasing die
angle.
2.Decreasing reduction per pass.
3.Friction
4.The presence of inclusions in the material.
A type of surface defect in drawing is the formation of seams
. These are longitudinal scratches or folds in the
material which can open up during subsequent forming operation, such as by upsetting, heading, thread rolling, or by bending of the wire or rod.
Prof. Naman M. Dave

40. •Because of inhomogeneous
deformation that the material
undergoes, a cold drawn rod, wire,
or tube usually contains residual
stresses.
•Typically a wide range of residual
stresses can be present within the
rod in three principles directions :
1.Transverse direction
2.Longitudinal direction.
3.
Radial direction.
Residual stress in drawing operation
Drawing Defects
Prof. Naman M. Dave

41. For very light reductions, the surface residual stresses are
compressive. Note that light reductions are equivalent to
shot peening or surface rolling which induce compressive
residual stresses on surface, thus improving fatigue life.
•Residual stress can be significant in stress application
causing cracking or in warping of the component when a
layer is subsequently removed, as by machining or
grinding
Residual stress in drawing operation
Prof. Naman M. Dave

42. shot peening
•Shot peeningis a cold working process used to produce
acompressiveresidual stresslayer and modify mechanical
properties ofmetals. It entails impacting a surface with
shot (round metallic, glass, or ceramic particles) with force
sufficient to createplastic deformation
Prof. Naman M. Dave

43. Mechanics of rod and wire drawing
The major variables in drawing process are:
1.The reduction in cross-sectional area
2.Die angle
3.Friction also plays a major role
Prof. Naman M. Dave

44. The drawing stress, σ
d, for the simplest case of ideal deformation- no
strain hardening ( that is no friction or redundant work ) is obtained by :
For the material that exhibits strain hardening with the true- stress- strain
behavior, Yis replaced byan average flow stress Ȳ:
Where Yis the yield stress
of the material
The quantity, Ȳ, is obtained from the expression:
Where Ȳis the average flow
stress
Idea drawing force-with no friction
Prof. Naman M. Dave

45. Idea drawing force-with no friction
•The ideal drawing force ( that is no friction or redundant work ) is
obtained by :
•Where iisthe average flow stress and can be obtained by :
A
0: is the initial cross sectional area.
A
f:is the final cross sectional area.
K is the stress (strength) coefficient, n is the strain hardening (work
hardening) exponent
Prof. Naman M. Dave

46. Friction at the die-workpieceinterface increases the drawing
force because work has to be supplied externally to overcome
friction.
Idea drawing force-with friction
The drawing stress with friction is obtained by following
expression: .
Prof. Naman M. Dave

47. Noting that the compressive stresses in the two directions are
equal to p, the die pressure along the die contact length can
be obtained:
Die pressure
σ−=
fYp
Y
f
Y
f:is the flow stress , σ :is the tensile stress in the
deformation zone.
Prof. Naman M. Dave

48. Example
•A round rod of annealed 302 stainless steel is
being drawn from a diameter of 10 mm to 8 mm
at s speed of 0.5m/s. assume that frictional and
redundant work together constitute 40% of the
ideal work of the deformation.
Calculate the power required for this operation?
Calculate the die pressure at the exit of the die?
Prof. Naman M. Dave

49. Comparison of bulk deformation
processes
Prof. Naman M. Dave

50. Comparison of bulk deformation
processes
Prof. Naman M. Dave

51. Comparison of bulk deformation
processes
Prof. Naman M. Dave