metal drawing

7/29/2019 Metal Drawing

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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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