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Metal DrawingMetal drawing is a manufacturing process that forms work by reducing its crosssection. This is accomplished by forcing the work through a mold, (die), of smaller
cross sectional area than the work. This process is very similar to extrusion, the
difference being in the application of force. In extrusion the work is pushed through
the die opening, where in drawing it is pulled through. The basic concept of metal
drawing is illustrated in the following figure.
Figure:236
Many of the same manufacturing factors of extrusion are also present in drawing.
Similar to extrusion, the die angle, amount of area reduction, and geometry of cross
sections are all essential considerations. Friction and its effects on metal flow shouldbe controlled. There is a fundamental difference between extrusion and drawing
practice, based on the fundamental difference between the two processes. Metal
extrusion can provide tremendous reductions in cross sectional area by pushing the
material through the mold. In metal drawing the amount of cross sectional reduction is
much more limited by the fact that the metal is pulled through. As in extrusion, the
greater the reduction in cross sectional area the greater the force required to form the
work. When the force needed to pull a work piece through a mold exceeds the yield
strength of the work, it will begin to yield. Yielding of the work in this manner is not
desirable in drawing manufacture.
In theory the highest possible amount of area reduction, based on preventing yielding
of the work is usually about 63%. In industrial practice area reductions generally
range from 15% to 45%. In order to obtain greater reductions in cross sectional area,
the work may be drawn through two or more drawing die in series. Metal drawing
often involves round profiles. The term draft is used to denote the reduction in
diameter of drawn round cross sections. In addition to the specific cross sectional
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reduction, the work material, and the speed at which the product is drawn are also
critical operational factors when manufacturing by metal drawing.
Metal Drawing Process
The metal drawing process in manufacturing industry is usually performed cold. Cold
working will impart the drawn product with accurate tolerances, favorable grain
structure, improved material properties, and good surface finish. Preparation of the
work prior to drawing is an important part of the operation. The work is sometimesannealed first, to recover the material from existing stresses. Next the work surfaces
are cleaned. Common industrial practice for cleaning stock includes shot blasting or
submersion in some, (typically acidic), solution. The work is then washed to remove
any solution, it may also be dried at a low temperature. After the cleaning phase the
stock may be conditioned, this can involve the application of a variety of different
chemical solutions to the surface of the work. Specific chemicals used depend on the
manufacturing situation and the work material. The main reason for these
conditioning agents is to help the work surface hold the lubrication necessary for the
process.
Once prepared the work piece is pointed at one end, which enables that end to be
inserted through the die. This end is then mechanically gripped so that the rest of the
work can be pulled through. At certain points in the process the drawn product may
require straightening. Straightening rolls can be employed as part of the
manufacturing process. Metal drawing can be either a discrete or continuous
operation, and can be very economically efficient for certain applications. In
commercial industry this process provides stock material for machining operations
and for the manufacture of such items as fences, coat hangers, nails, screws and bolts.
Metal wire drawing plays a huge roll in the manufacturing industry in the production
of cable and electrical wire.
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Drawing Dies
Metal drawing dies in manufacturing industry are usually made of cemented carbides
or tool steels. Mandrels for tube drawing are often made of similar materials as the
die. Occasionally diamond die are employed to form extremely thin wire. As the worktransverses the mold it passes through different sections. The die's first section is a
bell curved opening. This area does not contact the work, but helps filter lubricant into
the mold and allows for adequate entry of the work into the mold without damage
from die edges. Next, the forming of the work occurs in the approach section. The
approach angles down the cross sectional area, connecting with the next section, the
bearing surface. Bearing surface, also known as land, holds the precise geometric
cross section for a length of the draw. This acts as a sizing operation, ensuring tight
tolerances. The last section is the exit zone, this is a steeply angled section similar to
the entry zone. Exit zones are used to protect drawn work from the edges of the die.
Figure:237
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Defects In Metal Drawing
Defects that occur in metal drawing manufacture are similar to those that occur while
manufacturing by extrusion. Controlling metal flow is essential in preventing defects.
Mold characteristics and friction play a critical roll in the process.
Internal Cracking: Internal breakage may occur in drawn products, particularly
along the centerline. This is due to improper material flow creating high internal
stresses. Causes may be high die angles or low friction.
Surface Defect: A wide variety of surface defects can be observed in metal drawing
manufacture. Seams, scratches, and cracks are all possible defects on the surface of
drawn product. Excessive force on the surface of the work during the drawing
operation, (such as from friction), can be the cause of breakage. Also, many metal
drawing operations form at very high speeds, sufficiently designed entry and exit
zones need to be provided in order to avoid damage to the work material from the die.
For more detailed information on internal breakage and surface defects seeextrusion
defects.
Defects In Metal Drawing
Lubrication is an important factor when manufacturing by metal drawing, its
application can help control the forces and metal flow. Lubrication will also extend
the life of the mold, reduce temperature, and improve surface finish. Different soaps
and oils may be used as lubricants. With difficult to draw metals, polymers or soft
materials may also be used as lubricants. There are two basic methods of applying
lubrication often employed in metal drawing manufacture.
Wet Drawing In wet drawing the dies and the work are completely submersed in
lubrication. Lubricant in this case is typically some kind of oil containing chemical
additives.
Dry Drawing Dry drawing applies lubrication to the material by use of a stuffing box.
The stuffing box is located in front of the mold and contains lubricant. In this case, it
may be some kind of soap. Work passes through the box and picks up lubrication
before entering the mold.
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Rod And Bar Drawing
Rod or bar drawing is a term used to denote one of two categories of metal drawing.
Rod or bar drawing refers to the drawing of work of larger cross sections, while wire
drawing refers to the forming of work of a relatively smaller profile. Due to the size of
the work, rod and bar drawing involves much more finite lengths of material than wire
drawing. This type of process is carried out as a discrete manufacturing operation.
Rod or bar drawing is usually performed on a draw bench. A draw bench consists of along table, a die stand containing the mold, and a carraige used to grip and draw the
work. The die stand may contain two or more molds, multiple dies allow more than
one part to be draw with each operation. Draw benches vary in size, and can be up to
100 feet in length. Force used to draw the metal is exerted through hydraulic or
mechanical means. Pulling force as high as 150 tons has been used in industrial
production.
Figure:238
Figure:239
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Production Of Hollow Tubes And
Drawn Shapes
The majority of drawing operations produce round or square shapes, however
different cross sections such as U-sections and other simple shapes are also
manufactured. Hollow profiles, particularly hollow round tubes of different lengths,
diameters, and wall thicknesses are common in drawing production. Many tubes and
special profiles are of larger geometry, and are drawn as a discrete manufacturing
operation. Production of drawn shapes and hollow tubes is usually performed on a
drawn bench and would be classified in the rod and bar category of operations. The
specifics of the metal deformation is important when drawing different cross sections.
Sometimes a series of operations may be needed to form a particular profile.
Often times drawing is used to finish tubes and profiles already manufactured by other
methods such as extrusion, or rotary tube piercing. When forming a tube a mandrel
may or may not be used. A tube may be formed without a mandrel if its internal
dimensions are not critical. It is often required that hollow tubes hold certain
tolerances on internal diameter and wall thickness. For that reason mandrels are often
employed. Fixed mandrels are anchored on one side, floating mandrels are not
anchored and are designed to fit in place. Floating mandrels may allow for the
production of longer lengths of tube.
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Figure:240
Figure:241
Figure:242
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Figure:243
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Wire Drawing
Wire drawing is the second major category of metal drawing operations. While rod
and bar drawing refer to the drawing of larger cross sections, wire drawing refers to
the drawing of relatively smaller cross sections. The enormous amount of electricalwire and cable produced by this method makes wire drawing a major modern
industrial process. Some wire must be manufactured to tremendously small cross
sectional areas, such as those used in electromagnets. Wire may be drawn to diameters
as low as .0001 inch. Diamond die inserts are often used in the production of
extremely fine wire.
Material work stock in wire drawing will usually undergo several reductions in
diameter, since the mechanics of the process limit the amount of reduction in a single
draw. This is accomplished by drawing the work through several die in series, each
producing an incremental reduction in the works diameter. Between dies the wire
stock is wrapped several times around a motor driven rotating drum called a capstan,
before proceeding to the next die in series. Annealing of the material may be
performed between groups of operations. The capstans provide the force for the
manufacturing process. As the diameter is reduced, the speed of the wire is increased.
Velocity of wire leaving the last mold in a series can be significantly higher than the
velocity of the work entering the first mold. Typically drawing speeds may be 20-100
feet per minute, but in some cases wire may be drawn at 10,000 feet per minute.
Pieces of stock can be end welded together as they are fed into the system of capstans
and die so that the process will be completely continuous. Industrial wire drawing
operations can manufacture miles of wire at a time.
Figure:244
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Lecture 3
SWAGING
In this process, the diameter of a rod or a tube is reduced by forcing it into a confining die. A set of
reciprocation dies provides radial blows to cause the metal to flow inward and acquire the form of the die cavity. Thedie movements may be of in and out type or rotary. The latter type is obtained with the help of a set of rollers in acage, in a similar action as in a roller bearing. The workpiece is held stationary and the dies rotate, the dies strike the
workpiece at a rate as high as 10 - 20 strokes per second.
Screwdriver blades and soldering iron tips are typical examples of swaged products.Fig 3.1shows these andother products made by swaging.
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Fig 3.1 Typical parts made by swaging.
In tube swaging, the tube thickness and / or internal dia of tube can be controlled with the use of internalmandrels. For small diameter tubing, a thin rod can be used as a mandrel; even internally shaped tubes can be
swaged by using shaped mandrels.Fig 3.2shows the process.
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Fig 3.2 (a) Swaging of tubes without a mandrel. Wall thickness is more in the die gap.
(b) Swaging with a mandrel. The final wall thickness of the tube depends on themandrel diameter.
(c) Examples of cross-sections of tubes produced by swaging on shaped mandrels.
The process is quite versatile. The maximum diameter of work piece that can be swaged is limited to about 150mm; work pieces as small as 0.5 mm diameter have been swaged. The production rate can be as high as 30 parts
per minute depending upon the complexity of the part shape and the part handling means adopted.
The parts produced by swaging have tolerance in the range 0.05 mm to 0.5 mm and improved mechanicalproperties. Use of lubricants helps in obtaining better work surface finish and longer die l ife. Materials, such astungsten and molybdenum are generally swaged at elevated temperatures as they have low ductility at room
temperature. Hot swaging is also used to form long or steep tapers, and for large reductions.
Swaging is a noisy operation. The level of noise can be, however, reduced by proper mounting of the machineor by the use of enclosure.
WIRE DRAWING
Wire drawing is primarily the same as bar drawing except that it involves smaller diameter material that canbe coiled. It is generally performed as a continuous operation on draw bench like the one shown inFig 3.3
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Fig 3.3 Wire drawing on a continuous draw block. The rotating draw block provides a continuous pull on the incomingwire.
Large coil of hot rolled material of nearly 10 mm diameter is taken and subjected to preparation treatmentbefore the actual drawing process. The preparation treatment for steel wire consists of :
Cleaning. This may be done by acid pickling, rinsing, and drying. Or, it may be done by mechanical flexing.
Neutralization. Any remaining acid on the raw material is neutralized by immersing it in a lime bath. Thecorrosion protected material is also given a thin layer of lubricant.
To begin the drawing process, one end of coil is reduced in cross section upto some length and fed through thedrawing die, and gripped. A wire drawing die is generally made of tungsten carbide and has the configuration shown
inFig 3.4for drawing very fine wire, diamond die is preferred.
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Fig 3.4 Cross section through a typical carbide wire drawing die.
Small diameter wire is generally drawn on tandom machines which consists of a series of dies, each held in a watercooled die block. Each die reduces the cross section by a small amount so as to avoid excessive strain in the wire.
Intermediate annealing of material between different states of wire may also be done, if required.
Wire drawing terms :
Where Do , Df , Lo and Lfare the original and final diameter and length. Ao and Afare original and final cross sectionalarea.
For a single cold drawing pass, the percent area reduction that can be done depends upon many factors. Theseinclude the type of material, its size, initial metallurgical condition, the final size and mechanical properties desired,
die design and lubrication efficiency. The percent of area reduction per pass can range from near zero to 50%.
Die pull
The force required to pull the stock through the die (under frictionless conditions) can be computed as follows.
Where F = die pull, i.e. the force required to pull the stock through the die
Yavg = average true stress of the material in the die gap
Ao , Af= original and final areas of cross section of material.
Alternatively, the following expression can be used
F = c t (Ao Af)
where c is a constant whose value is in the range 1.5 to 3.0 depending upon the area reduction, (lower value for
higher % reduction), and t is tensile strength of material before drawing.
The pull force determines the machine capacity needed.
TUBE DRAWING
The diameter and wall thickness of tubes that have been produced by extrusion or other processes can bereduced by tube drawing process. The process of tube drawing (Fig 3.5) is similar to wire or rod drawing except that it
usually requires a mandrel of the requisite diameter to form the internal hole.
Tubes as large as 0.3 m in diameter can be drawn.
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Fig 3.5
Drawing Equipment
Drawing equipment can be of several designs. These designs can be classified into two basic types; Drawbench, and Bull block. A draw bench (Fig 3.5) 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. The drawing speed attainable on a draw bench ranges from 5 m/min to 50 m/min.
Draw benches are available having capacities to provide pull force of upto 1 MN.
Bull block or rotating drum (Fig 3.3) is used for drawing rods or wires of very long length.
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