Metal Drawing process (Bar, Wire and Tube Drawing) 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. Introduction • 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 Wire products Metal Drawing process (Bar, Wire and Tube Drawing) Metal Drawing process (Bar, Wire and Tube Drawing) Metal Drawing process (Bar, Wire and Tube Drawing) Metal Drawing process (Bar, Wire and Tube Drawing) 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. 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 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. Wire Drawing • 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 • 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. 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 Tube Drawing ❑ 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 Tube drawing • 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) fixed plug drawing(c) with a moving mandrel 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. 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 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. Preparation of the Work for Wire or Bar Drawing • 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 • Raw Stock: Hot rolled steel bar or rod coils are used as raw material. • 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. Lubrication ❑ 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 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: Die Wear • 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 ❑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. Drawing-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. 2. 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. Drawing-Equipment • Drawing equipment can be of several designs. These designs can be classified into two basic types 1. Draw bench for rod and tube 2. and Bull block for wire • 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 Draw bench Bull block Rod and wire drawing process- affecting factors ❑The major variables in drawing process are: 1. The reduction in cross-sectional area 2. Die angle 3. Friction also plays a major role 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.
