DRAWING OF RODS WIRE AND TUBES

Drawing is a metalworking process which uses tensile forces to stretch metal. Drawing is a cold working process in which the work piece (rod, wire or tube) is pulled through a tapered hole in a die so as to reduce its diameter. The process imparts accurate dimensions, specified cross-section, a clean and excellent quality of surface to the work. It is broken up into two types: sheet metal drawing and wire, bar, and tube drawing. The specific definition for sheet metal drawing is that it involves plastic deformation over a curved axis. For wire, bar, and tube drawing the starting stock is drawn through a die to reduce its diameter and increase its length. Drawing is usually done at room temperature, thus classified a cold working process, however it may be performed at elevated temperatures to hot work large wires, rods or hollow sections in order to reduce forces

The wire drawing process is quite simple in concept. The wire is prepared by shrinking the beginning of it, by hammering, filing, rolling or swaging, so that it will fit through the die; the wire is then pulled through the die. As the wire is pulled through the die, its volume remains the same, so as the diameter decreases, the length increases. Usually the wire will require more than one draw, through successively smaller dies, to reach the desired size. The American wire gauge scale is based on this. This can be done on a small scale with a draw plate, or on a large commercial scale using automated machinery. The process of wire drawing improves material properties due to cold working.

The areal reduction of small wires are 15–25% and larger wires are 20–45%.Very fine wires are usually drawn in bundles. In a bundle, the wires are separated by a metal with similar properties, but with lower chemical resistance so that it can be removed after drawing. If the reduction in diameter is greater than 50%, the process may require annealing between the process of drawing the wire through the dies. Commercial wire drawing usually starts with a coil of hot rolled 9 mm (0.35 in) diameter wire. The surface is first treated to remove scales. It is then fed into either a single block or continuous wire drawing machine.

Single block wire drawing machines include means for holding the dies accurately in position and for drawing the wire steadily through the holes. The usual design consists of a cast-iron bench or table having a bracket standing up to hold the die, and a vertical drum which rotates and by coiling the wire around its surface pulls it through the die, the coil of wire being stored upon another drum or "swift" which lies behind the die and reels off the wire as fast as required. The wire drum or "block" is provided with means for rapidly coupling or uncoupling it to its vertical shaft, so that the motion of the wire may be stopped or started instantly. The block is also tapered, so that the coil of wire may be easily slipped off upwards when finished. Before the wire can be attached to the block, a sufficient length of it must be pulled through the die; this is effected by a pair of gripping pincers on the end of a chain which is wound around a revolving drum, so drawing the wire until enough can be coiled two or three times on the block, where the end is secured by a small screw clamp or vice. When the wire is on the block, it is set in motion and the wire is drawn steadily through the die; it is very important that the block rotates evenly and that it runs true and pulls the wire at a constant velocity, otherwise "snatching" occurs which will weaken or even break the wire. The speeds at which wire is drawn vary greatly, according to the material and the amount of reduction.

Continuous wire drawing machines differ from the single block machines in having a series of dies through which the wire passes in a continuous manner. The difficulty of feeding between each die is solved by introducing a block between each die. The speeds of the blocks are increased successively, so that the elongation is taken up and any slip compensated for. One of these machines may contain 3 to 12 dies. The operation of threading the wire through all the dies and around the blocks is termed "stringing-up". The arrangements for lubrication include a pump which floods the dies, and in many cases also the bottom portions of the blocks run in lubricant.

Often intermediate anneals are required to counter the effects of cold working, and to allow more further drawing. A final anneal may also be used on the finished product to maximize ductility and electrical conductivity.

An example of product produced in a continuous wire drawing machine is telephone wire. It is drawn 20 to 30 times from hot rolled rod stock.

While round cross-sections dominate most drawing processes, non-circular cross-sections are drawn. They are usually drawn when the cross-section is small and quantities are too low to justify rolling. In these processes, a block or Turk's-head machine are used.

Processes

Sheet metal

o Deep drawing

Bar, tube & wire

o Bar drawing

o Tube drawing

o Wire drawing

Plastic drawing

Sheet metal

The success of forming is in relation to two things, the flow and stretch of material. As a die forms a shape from a flat sheet of metal, there is a need for the material to move into the shape of the die. The flow of material is controlled through pressure applied to the blank and lubrication applied to the die or the blank. If the form moves too easily, wrinkles will occur in the part. To correct this, more pressure or less lubrication is applied to the blank to limit the flow of material and cause the material to stretch or thin. If too much pressure is applied, the part will become too thin and break. Drawing metal is the science of finding the correct balance between wrinkles and breaking to achieve a successful part.

Deep drawing

Sheet metal drawing becomes deep drawing when the workpiece is drawing longer than its diameter. It is common that the workpiece is also processed using other forming processes, such as piercing, ironing, necking, rolling, and beading.and so on......

Bar, tube & wire

Bar, tube, and wire drawing all work upon the same principle: the starting stock drawn through a die to reduce the diameter and increase the length. Usually the die is mounted on a draw bench. The end of the workpiece is reduced or pointed to get the end through the die. The end is then placed in grips and the rest of the workpiece is pulled through the die.^[1] Steels, copper alloys, and aluminium alloys are common materials that are drawn.

Drawing can also be used to produce a cold formed shaped cross-section. Cold drawn cross-sections are more precise and have a better surface finish than hot extruded parts. Inexpensive materials can be used instead of expensive alloys for strength requirements, due to work hardening

Bar drawing

Bars or rods that are drawn cannot be coiled therefore straight-pull draw benches are used. Chain drives are used to draw workpieces up to 30 m (98 ft). Hydraulic cylinders are used for shorter length workpieces

The reduction in area is usually restricted to 20 to 50%, because greater reductions would exceed the tensile strength of the material, depending on its ductility. To achieve a certain size or shape multiple passes through progressively smaller dies or intermediate anneals may be required.

Tube drawing

Tube drawing is very similar to bar drawing, except the beginning stock is a tube. It is used to decrease the diameter, improve surface finish and improve dimensional accuracy. A mandrel may or may not be used depending on the specific process used.

Wire drawing

This technique has long been used to produce flexible metal wire by drawing the material through a series of dies of decreasing size. These dies are manufactured from a number of materials, the most common being tungsten carbide and diamond.

Plastic drawing

Plastic drawing, sometimes referred to as cold drawing, is the same process as used on metal bars, but applied to plastics.

Cold drawing is primarily used in manufacturing plastic fibers. The process was discovered by Julian Hill in 1930 while trying to make fibers from an early polyester. It is performed after the material has been "spun" into filaments; by extruding the polymer melt through pores of aspinneret. During this process, the individual polymer chains tend to somewhat align because of viscous flow. These filaments still have anamorphous structure, so they are drawn to align the fibers further, thus increasing crystallinity, tensile strength and stiffness. This is done on a draw twister machine

For nylon, the fiber is stretched four times its spun length. The crystals formed during drawing are held together by hydrogen bonds between the amide hydrogens of one chain and the carbonyl oxygens of another chain

Drawing dies

Drawing dies are typically made of tool steel, tungsten carbide, or diamond, with tungsten carbide and manufactured diamond being the most common. For drawing very fine wire a single crystal diamond die is used. For hot drawing, cast-steel dies are used. For steel wire drawing, a tungsten carbide die is used. The dies are placed in a steel casing, which backs the die and allow for easy die changes. Die angles usually range from 6–15°, and each die has at least 2 different angles: the entering angle and approach angle. Wire dies usually are used with power as to pull the wire through them. There are coils of wire on either end of the die which pull and roll up the wire with a reduced diameter

Wire drawing is primarily the same as bar drawing except that it involves smaller – diameter material that can be coiled. It is generally performed as a continuous operation on draw bench like the one shown in Fig 3.3

Fig 3.3 Wire drawing on a continuous draw block. The rotating draw block provides a continuous pull on the incoming wire.

Large coil of hot rolled material of nearly 10 mm diameter is taken and subjected to preparation treatment before 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. The corrosion 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 the drawing die, and gripped. A wire drawing die is generally made of tungsten carbide and has the configuration shown in Fig 3.4 for drawing very fine wire, diamond die is preferred.

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 water – cooled 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 D_o , D_f , L_o and L_f are the original and final diameter and length. A_o and A_f are original and final cross sectional area.

For a single cold – drawing pass, the percent area reduction that can be done depends upon many factors. These include 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

Y_avg = average true stress of the material in the die gap

A_o , A_f = original and final areas of cross section of material.

Alternatively, the following expression can be used

F = c s_t (A_o - A_f )

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 s_t is tensile strength of material before drawing.

The pull force determines the machine capacity needed.

The diameter and wall thickness of tubes that have been produced by extrusion or other processes can be reduced 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.

Fig 3.5

Drawing Equipment OR DRAW BENCHES

Drawing equipment can be of several designs. These designs can be classified into two basic types; Draw bench, 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.

Fig 3.5

Bull block or rotating drum (Fig 3.3) is used for drawing rods or wires of very long length.

Fig 3.3 Wire drawing on a continuous draw block. The rotating draw block provides a continuous pull on the incoming wire.

Rod or bar drawing:

Rod drawing is similar to wire drawing except for the fact that dies are bigger because of the rod size being larger than the wire. In rod drawing or bare drawing, the product must remain straight.

Bars or rods that are drawn cannot be coiled therefore straight-pull draw benches are used. Chain drives are used to draw work pieces up to 30 m (98 ft). Hydraulic cylinders are used for shorter length work pieces

The reduction in area is usually restricted to 20 to 50%, because greater reductions would exceed the tensile strength of the material, depending on its ductility. To achieve a certain size or shape multiple passes through progressively smaller dies or intermediate anneals may be required

Process of rod drawing

Machine capacity

1. 1 MN draw bench

2. 30m run out

3. 150-1500 mm/sec draw speed.

Application of wire drawing

In addition to direct applications such as electrical wiring, wire is the starung material for many products such as wire frame structures(shopping carts, coat hangers etc.), nails, screw bolts, rivets and wire fencing wire mesh, wire ropes, electrods, chainpin, umbrella ribs, springs, hand toools and needles etc.

Applications of tubes drawing

1. Boliers and heat exchangers: for carrying water

2. For pies lines in petro chemical and fertilizer complexes

3. Automobile industry: for rear axle of heavy commercial vehicales, steering columns of cars and main frames of motor cycles and scooters etc.

4. Fine caplillary tubes, and clas tubes for special applications in instrumentation, elctronics, aircraft, medical and nuclear inductries etc.

Main variables of drawing process:

1. Strength of work material

2. Temperature

3. Speed of drawing

4. Co-efficient of friction

5. Die-angle

6. Reducation of area

Degree of drawing

The degree of drawing is measured in terms of reducation of area(RA) which is defined as the ration of the difference in cross sectional area before and after drawing to the original area, expresses in percent.

RA = (A_o – A_f) / A_o*100

= (D_o² – D_f² ) / D_o² *100

Where A₀ = original cross sectional area of the wire or rod before operation of drawing

A_f = final cross sectional area of the wire or rod after operation of drawing

For fine wires, this value is 15 to 25%

For corase wire, theis value is 30 to 35 %

For rods, this value can be upto 40%

Lubrication

Proper lubrication is essential in drawing, in order to improve die life, reduce drawing forces and temperature, and improve surface finish.

The following are different methods of lubrication:

§ Wet drawing: the dies and wire or rod are completely immersed in lubricant

§ Dry drawing: the wire or rod passes through a container of lubricant which coats the surface of the wire or rod

§ Metal coating: the wire or rod is coated with a soft metal which acts as a solid lubricant

§ Ultrasonic vibration: the dies and mandrels are vibrated, which helps to reduce forces and allow larger reductions per pass

Various lubricants, such as oil, are employed. Another lubrication method is to immerse the wire in a copper (II) sulfate solution, such that a film of copper is deposited which forms a kind of lubricant. In some classes of wire the copper is left after the final drawing to serve as a preventive of rust or to allow easy soldering.