The following points highlight the cold working processes which are generally performed on metals. The cold working processes are: 1. Shearing Operations 2. Drawing 3. Squeezing Operations 4. Bending Operations.
1. Shearing Operations:
i. Blanking:
This is the operation carried out on presses and consists of cutting the outside contour of a stamping. Production of sheet-metal blanks of flat shapes requires a single-action press equipped with tools comprising a punch, a corresponding die, stripper to keep the sheet from following the punch on its upstroke and means for aligning the sheet or strip of material and for spacing successive cuts.
ADVERTISEMENTS:
The operation of cutting inside the contours, i.e. holes and slots is called piercing. All these operations will be dealt in detail under the chapter of presses. However pressed metal parts, or stampings are recommended for mass production. Stampings combine the virtues of lightness, a high degree of uniformity, and surfaces well adopted to receive protective or decorative finishes.
ii. Punching:
This is also a press operation and consists of cutting holes of various shapes in sheets etc. The operation is generally continuous and at a time two or more holes may be punched.
iii. Perforating:
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It is also similar to punching but is done with the help of multiple punches.
iv. Trimming:
It consists of removal or cutting away of excess material left around the parting lines in the previous operations. Trimming is similar to blanking and is done in special type of trimming dies.
v. Notching:
ADVERTISEMENTS:
This is the operation of removing small amount of metal from the edges of work-pieces by the notching action usually obtained with the help of nibbler.
vi. Slitting:
It consists of making incomplete cuts in sheets of metal.
vii. Lancing:
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It is a special form of piercing operation in which the entire contour is not cut, the blanked material remains attached with the sheet. It is achieved by bending down one side of partially punched hole.
viii. Broaching:
This process is carried out by a tool having a series of stepped cutting edges just like a saw. This consists of consecutive shearing of a hole or contour and is used in slow- acting presses for accurate sizing of holes or contours, such as gear teeth and keyways. Generally it is considered as a machining operation.
ix. Burnishing:
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This is a smoothening or polishing operation accomplished by compression or friction or both. This process is sometimes also called shaving when the cut blanks are polished and sized by forcing them through an opening having sides tapered slightly inward.
x. Dinking:
This process consists of cutting or piercing any shape by pressing a sharp, thin steel edge of appropriate shape through sheet material lying on a flat hard wood platen. Generally rubber, leather, fibre, felt etc. are blanked by the process of dinking.
Bar cropping. For several applications, it is often required to prepare the billets from bar.
The various methods for this are:
i. sawing,
ii. abrasive cut-off,
iii. parting-off in leather,
iv. cropping, etc.
First three processes involve a material loss and are time consuming. These problems are overcome by cropping operation.
Fig. 6.6 shows the open blade cropping method. In this process the billet ends get distorted which is not desirable in several applications. To obtain ends with improved sureness, it is necessary to obtain blade speed as high as 10 m/sec to reduce distortion of bar.
It is also desirable to support the off-cut, or else, it should have high inertia to prevent it from bending during shearing. In one cropping machine, to obtain square edges, a high axial thrust (greater than the yield stress of the bar material) is applied before and during cropping.
Fine blanking process is a press-working technique. It utilises special press and precision tools and dies to produce parts in final assembly shape. Parts produced have clearly sheared surfaces over the entire material thickness. In conventional methods, one-third thickness is sheared and balance is fractured which has to be shaved, milled etc. The dimensional accuracy obtained is also high.
For fine blanking operation, a precise die with low punch and die clearance (1/2% of material thickness) is required. The press used is triple-action to clamp the material during the shearing operation.
The three actions in the press provide the shearing pressure, the V-ring pressure, and the counter pressure, all of which are held constant throughout the stroke to ensure good-quality parts. Shearing speed varies between 4 and 15 mm/sec.
The sequence of operations during one cycle of the fine blanking press is (Refer Fig. 6.7).
(a) Material fed into die.
(b) Up stroking ram lifts the lower die set, raising the material to the die face.
(c) As the tool closes, the V-ring (stringer) is embedded in the material, and clamped between the V-ring plate and the die plate outside the shear periphery. The counterpunch, which is under pressure, clamps, material against the blanking-punch face inside the shear periphery.
(d) While the V-ring pressure and the counter pressure are held constant, the punch continues up stroking, cleanly shearing the part into the die and the inner-form slug into the punch. At the top dead centre position, all pressures cease.
(e) Ram retracts and die opens.
(f) Almost immediately after the tool opens, the V- ring pressure is reapplied. This strips the punch from the skeleton material and pushes the inner-form slugs up out of the punch. The material feed begins.
(g) Counter pressure is reposed, ejecting the part from the die.
(h) The part and slugs are removed from the die area either by an air blast or by a removal arm.
(i) Cycle is recommenced.
A fine blanked part is usually produced with a compound die so as to complete the operation in a single stroke.
An average surface finish for fine blanked part is 0.5 pm. The perpendicular of sheared edges is within 40—50 minutes.
Manufacturing to Net Shape:
The importance of manufacturing a final part in as few separate process steps as possible is increasing to conserve time and materials. Net shape manufacturing refers to a one-process operation to final precision dimension and geometry, one such example being fine blanking operation. It utilises high precision tools to clean—shear workpieces with only one press stroke.
In fine blanking a V-shape ring is pressed against the stock to lock it tightly against the die, and to force the work metal to flow toward the punch, so that the part can leave the strip without fracture or die break. CNC fine blanking processes encompass a greater variety of part configurations to include fine blanked components with bends, coining’s, semi pierced projections, and other forming operations. Both compound and progressive dies are used to produce complex shapes requiring close tolerance, dimensional integrity, and smoothly sheared faces.
2. Drawing:
Cold drawing of steel is a process of finishing hot rolled steel bars of various cross-sections by forcing them under tension through hardened dies. Within certain limits, bars of practically and cross-section and composition may be cold- drawn with varying degree of success.
The process of cold drawing involves preparation of the product by pickling, rinsing, liming and pointing. After drawing, it involves processes like straightening, shearing, sorting, inspecting, annealing and testing.
i. Blank Drawing:
This is the process of cold forming in which the metal is made to flow plastically by applying tensile stresses to the metal. In this process the blank of calculated diameter is placed on a die and held on it by a blank holder and bottom is pressed into the die by a punch and the walls are pulled in. It is used for making cup- shaped parts from the sheet-blanks.
ii. Bulging:
This process is employed for producing bulged parts, i.e. parts having larger diameter at bottom and smaller diameter at top. For this, first the part is drawn to intermediate size and then worked on with the bulging dies which require special type of fluid or may be made of rubber bulging dies.
iii. Tube Drawing:
Cold-drawing operation of tubing is employed for tubes of small diameter and where dimensional accuracies and smooth surface finish are required. First the tube is hot-rolled which is treated by pickling and washing to remove all scale and then covered with a suitable lubricant.
A cold drawing die is used as shown in Fig. 6.8 and stationary mandrel is held in the centre of the die. One end of tube is first reduced in diameter and allowed to enter the die and then gripped by tongs. The tongs are then pulled by some chain mechanism.
With the pulling of tongs, the tube is drawn through the die and proper dimensions obtained. By cold drawing generally maximum reduction in one pass cannot be more than 40% as material is heavily stressed. Hence further reductions are possible only after carrying out annealing operations.
iv. Forces in Deep Drawing:
In deep drawing, various types of forces act simultaneously. The annular portion of sheet between blank holder and the die experiences pure radial drawing, and the portions of sheet around the corners of punch and die are subjected to bending operation.
Portion of sheet between punch and die walls undergoes a longitudinal drawing. Though sheet thickening and thinning takes place at several places, but for analysis same is ignored.
Let sheet thickness be t and its radius R. Clearance between die and punch = Rd – Rp ⋍ t. Punch and die radii are rp and rd. Clearance between punch and blank holder is C.
It is also like a drawing or stretching operation and does not require much pressure like drawing and coining. It consists of producing projected or raised designs in relief on a surface of sheet. It is done with the help of two mating dies.
The sheet is first blanked and then little more force is applied by the punch which forces the metal against a mating die conforming to the same configuration as the punch. It this way very little metal is squeezed in the operation and the words are printed on the sheet in projected form.
The process consists of gripping a sheet of material, or a rolled or extruded section, at each end in suitable jaws and stretching it over a die made to the required contour until complete forming has been achieved by extending and thinning the metal over a profile. It is important the sufficient clamping force at the edge of the blank is applied to prevent increased flow of the metal.
It may be possible to stretch metal over any compound contour, but formed sections or extrusions can be formed only over contours in a single plane or a very slight two-plane curvature. Two types of hydraulic stretch presses are in common use. In the first type, the sheet is gripped in stationary jaws and subsequently stretched by moving the form block vertically, with a platen actuated by hydraulic pistons, against the sheet material until the forming operation has been completed.
In the second type which is more versatile, and has more advantages, a fixed- form block platen and movable jaws attached to two hydraulic tension cylinders are employed. The cylinders are mounted on hydraulically actuated arms rotatable horizontally in a ninety degree arc simultaneously independently as may be required when forming parts of non-symmetrical configurations.
In this press the sheet is pre-stretched free of the form block upto the yield point of the material., then wrapped under tension around the form block and given a final stretch to set the material to the die contour. The second type of machine has the advantage that the jaws can be easily and rapidly positioned in close proximity to the form-block edges to eliminate excessive material waste.
Further the ability to move the arms independently allows closer control of the amount of forming being done. The stretching of the blank takes place along the tangent to the die surface, and thus friction forces developed between the material and die surface are reduced.
In this process the spring back is completely eliminated because the forming is done by introducing uniform tensile stresses in fine increments of sufficient magnitude to exceed the elastic limits of the metal, thus causing the material to take a permanent set. The process is applicable to a wider range of materials, because the ductility is the least important factor in stretch forming.
The desirable qualities in the metal for maximum stretch ability are toughness, fine grain structure and a large spread between the tensile yield and ultimate strength. It has been found that an annealed material which has high strain hardening rate is best suited to stretch forming.
Maximum cross-sectional area of stretch forming is governed by the pulling capacity of the machine; and limits to the dimensions of extrusions of formed sections beside their cross- sectional area are governed by the capacity of the jaws and maximum distance between jaws.
vii. Drop Hammer Forming:
In this process, sheet metal parts are formed by blow of a hummer with the aid of a mating punch and die conforming to be desired shape. In this process, the metal is not restrained but is allowed to flow in the direction of least resistance.
Contrary to the draw forming operation, in this case the metal is not thinned at any place rather it is compressed, resulting in the increase in thickness. Spring back of the material poses little problem. It is therefore; best to form a part in stages and not in a single blow. Either rope-type drop hammer or pneumatic type drop hammer may be employed
Aluminium alloys, stainless-steel alloys are quite successfully formed in the drop hammer. In general it is an economical way to form parts designed with compound contours. The die is made of zinc-alloy which has good impact resistance as well as excellent casting characteristics.
The punch is made of antimonial lead containing 6-8% antimony which has the advantage of shaping with ease. In case of severe forming on heavy sheets it may be necessary to use cast-iron dies with zinc alloy or cast-iron punches.
viii. Cold-Roll Forming:
It is the process of continuously shaping the ductile sheet or strip metal by means of a series of progressive forming rolls without the application of heat. This process is adopted for mass production and the cross-section or profile throughout the part length is uniform and the dimensional tolerances are held closely.
The action of rolls against the metal tends to improve the material finish besides adding strength. Section may be pierced, stencilled, or embossed during the roll- forming operation. Any ductile material can be formed from thickness ranging as low as 0.1 mm and up to 20 mm. The height and width of section are limited only by the capacity of the machine to be used and the length is limited only by the length of the coiled stock.
Use of flying cut-off units in conjunction with the roll-forming machine makes it practical to cut sections to part lengths as short as 30 cm. Each type of material to be formed will directly affect the number of passes required to form it and the type and design of tooling necessary to correct for the varying degrees of spring back.
It is feasible to roll-form a ductile material to a zero inside radius or a near sharp outside radius by forming the sharp corner into the strip before bending the matching side walls. The outside radius can then be coined almost square.
A few examples of the shapes which can be produced by cold-roll forming operation are shown in Fig. 6.16.
Cold-roll forming machines must have shafts large enough to carry the load through rolls, to form a given amount of metal to the desired shape. Many sizes of machines, having various shaft diameters and horizontal distances between shaft centres, are required to accommodate the wide variety of work encountered.
Generally a solution of 1 part soluble oil with 5 parts of water is used as lubricant for cold-roll forming operation.
ix. Roll Forming:
The roll forming operation is carried out at a series of roll stations and a flat sheet or strip is transformed into desired section progressively in stages. The metal gauge or section thickness remains constant and only bending operation takes place in each stage.
In conjunction with roll forming operations, other machines like welding, coiling attachment etc. can be added to increase versatility and obtain products in desired form.
For better forming characteristics and maximum roll life, the ratio between the radii of bends and the gauge of material should be as large as possible (3 to 5).
It is usual to restrict the work of bending to 20-25 degrees per roll pass. Complete 90° bend may thus require 3 or 4 stages of operation. If possible, examining the shape, some steps can be combined so that the number of operations or roll stations is a bare minimum.
Section Forming or Contour Forming:
Wide variety of machines is used to obtain desired forming action for the forming of sections.
These machines are:
(i) Draw Bending Machine:
It is used for bending hollow section. The machine has a rotating die to which the piece to be bent is clamped and is drawn against a roller. A liquid or hydraulic mandrel is used to eliminate the flattening effect of tubular sections. Bending limits are governed by the ‘crinkle point’ of metals.
(ii) Roll Machine:
It comprises of a pyramid of rolls, one of which is adjustable with respect to the other. A section to be bent is passed through the rolls to produce the desired curve or radius on the piece.
(iii) Ram Machine:
A ram bender consists of two supports for the section to be bent and a central die operated by a hydraulic cylinder for applying the load. It is used for bends on extremely heavy sections like railroad rails, reinforcing bars, etc.
(iv) Compression Machines:
The compression process utilises a stationary die onto which the part to be formed is held while a rotating arm wipes the piece into the die contour. These are used where bends are severe and heavy sections have to be controlled.
(v) Stretch Machines:
In these machines the work is held in special gripping jaws and is first stretched to its elastic limit and then with constant tension is laid onto the die form.
(vi) Compression-Stretch Machines:
It is used where the elongation characteristics of a specified material are insufficient to withstand the severity of stretch forming. This process maintains the uniformity and obviates excessive spring back.
Design Considerations in Section Forming Operations:
a. Spring back:
To account for spring back in material, radii are reduced from 3% for soft A1 and soft Cu to 12% for aluminium alloys.
b. Minimum Bend Radius (R):
It is taken as
c. Offsets:
For sections which required sharp or deep offsets, radii of rolls must be generous to allow for easy traverse of reverse bends.
d. Material:
In general high ductility metal is referred for maximum economy. For semi-ductile materials, compression or compression-stretch methods are used to squeeze the material into new shape. For long shallow bends in soft materials, stretch forming is used to produce the accurate and constant results. For low-alloy carbon steel, aluminium and stainless steel, compression method is used for higher output.
ix. Press Brake Forming:
It is used for production of machine parts which are extremely long in relation to their cross-section. It is ideally suited for long bending or forming operation. It offers a highly versatile, economical means for producing parts from sheet and plate which necessitate a long, narrow forming bend. It can also be used for a wide variety of blanking, punching, notching, coping, lock seaming, beading, wiring and similar operations.
Press brakes range in bed width upto 120 cm. Press sizes range from 25 tons for light bending to 2000 tons for heavy work. Bed lengths correspondingly run from 2 m to 9 m. For length more than 9 m presses in tandem are used. Open construction is used at either end of the press brake.
Ram stroke is usually small (not more than 15 cm).
Fig. 6.17 shows a group of press brake dies sets which illustrates the multitude of forms which can be produced.
Multiple operations also can be used on one machine to achieve rapid production. Owing to the simplicity and low cost of the tooling on press brakes, trimming, punching, forming and similar operations are regularly performed in this manner in the production of cabinets, doors tops, frames and panels on refrigerators and similar equipment.
Though press brakes and dies for performing ordinary operations are extremely flexible and provide great freedom to designer, following limitations must be considered: Design should be fairly symmetrical under the centre line of the ram; material should be in sheet or plate form (upto 25 mm thick), stroke available on standard press brakes should give adequate clearance for easy and rapid handling.
High-tensile steel plate, even in the thinner gauges must be formed over die openings ten to twelve times gauge to prevent fracture. Tolerances on press brake operation must be considerably wider than those normally specified in machining, stamping or roll forming.
3. Squeezing Operations:
i. Cold Rolling:
This process is principally adopted on wrought iron products in order to improve surface finish, surface properties and to impart better tolerances; or in other words; it is a finishing operation for wrought products such as rods, sheets and strip. Cold rolling is done below the recrystallization temperature of the metal.
For making sheets, hot-rolled metal is used, which prior to cold rolling is properly cleaned by immersing in an acid solution, washed in water and then dried. This clean steel is passed through a number of rollers thereby producing a slight reduction in area in each pass, until the required thickness is obtained.
ii. Sizing:
This operation is used for surfacing or flattening flats and bevels on drop forgings. Malleable castings are sized by heavy presses.
iii. Swaging or Cold Forging:
This operation consists of applying compressive or impact forces on the metal below the recrystallization temperature. It causes the metal to flow in the predetermined shape according to the design of the dies. Rotary swaging and cold heading are the two important processes of swaging.
(a) Rotary Swaging:
It is a process of reducing the diameters of rods, tubes and bars by rotating the dies which open and close rapidly on the work. This causes the rod or tube to be tapered at one end or reduced in size by the combination of pressure and impact.
Swaging operation is severe and it requires annealing if much reduction in area is to be carried out. This swaging operation is performed in a swaging machine which consists of a hollow spindle carrying die section as shown in Fig. 6.22.
(b) Cold Heading:
It is another form of swaging operation and is used for the manufacture of bolts, rivets, hexagonal socket screws, screws and similar heated items. The importance of cold heading operation lies principally in the saving of material; very little or no scrap being produced. A header being a high speed automatic production machine, it results in low labour cost with proper production runs.
Because of the method of flowing the material, if often results in better structure of the metal and hence stronger parts. The machine produces parts to close tolerances and fairly smooth finishes. Cold heading can be done on a wide variety of materials (capable of being cold worked without hardening too rapidly e.g. steel having 0.45% carbon or less and alloys which work readily).
A typical example for manufacturing the bolt from cold headed operation is described below. A ductile material in the form of rod is fed on the machine, where it is cut in standard length, held in a pair of jaws, and subjected to two or three blows to form the head roughly.
It is then repositioned in another die for final shaping and sizing and then thread rolling is done to cut threads on the shank. The manufacture of the bolt is done on automatic machines whose output ranges from 50 to 150 pieces per minute.
iv. Orbital Cold Forging:
In this process, extremely high degree of deformation is attained with little force. Finished workpiece is obtained with high accuracy in a single forging operation. It uses both fixed and movable die. The upper die makes contact with rolling motion over the total workpiece surface.
The lower portion acts as fixed die and contains the workpiece. An orbiting upper die, held at a slight angle (1—2°) imparts desired shape by displacing metal by constant pressure as the workpiece in lower die is raised.
Upper die can be made to orbit in a number of patterns (planetary, orbital, spiral, straight line, etc.) Orbital motion enables concentration of forming force in a small area of upper die. This area is constantly shifted in the selected pattern across the entire part surface. The lower die presses the blank against the orbiting tipper die. The workpiece is forged between upper and lower dies. Fig. 6.23 shows the sequences.
v. Coining:
Coins, metals and other similar parts are produced by this process. It requires the use of die and punch which are of robust structure as high pressure is required during the operation. The dies used are such that they confine the metal and restrict its flow in the lateral direction. Since high pressure is required in the operation, its use is limited to fairly soft alloys.
vi. Stamping:
It is an operation of printing letters, numbers and other figures on metal sheets using stamping or roller dies.
vii. Hobbing:
It is a process of producing cavities into softer metals by forcing a hardened steel form or hob. This operation is performed on heavy presses of capacities ranging from 250 to 8000 tonnes. During the operation the flow of metal in the blank is restrained from lateral movement by the heavy retainer ring placed around it. A typical set up of hobbing operation is shown in Fig. 6.24.
The advantages of this process are as follows:
(1) A number of identical cavities can be produced economically.
(2) No machining is required as the surfaces of the cavity are highly polished finished.
This process is extensively used in plastic and die casting industries.
viii. Crimping:
This process is used for producing flutes or corrugations. It is very often used for joining metal pieces together as for stove pin joints.
ix. Thread Rolling:
This is the method of producing external threads by-rolling the part to be threaded between hard serrated plates. The threads are formed by squeezing action and no machining is required. The dies consist of a pair of plane threading rolls. On their faces are made long, straight projecting flutes having the profile and pitch of the desired thread and inclined to the line of travel by an angle equal to the helix angle of the screw.
The screw blank on which threads are to be produced is held between the dies and is of diameter equal to pitch circle diameter. The dies are set, so that the distance between the roots of the die teeth equals the outside diameter of the finished screw.
The rolling of screw blank between the dies results in the displacement of metal from the region below the pitch circle to form the peaks of the threads beyond the pitch circle and thus threads are formed on screw blank. In other words, the dies in penetrating the surface of the blank, displace material to form the roots of the thread and force the displaced material radially outward to form the crests of the thread.
The process of thread rolling increases the tensile strength of the threads. The fibres of the material are not severed as in cutting but are reformed in continuous unbroken lines following the contours of the threads, as in good forging.
Rolled threads, therefore, resist stripping, because shear failures must take place across rather than along the direction of the grain. Rolling is the fastest method of producing screw threads.
The fatigue resistance of threads produced by cold rolling is increased considerably because of the following reasons:
(i) Rolling between smooth dies leaves the thread with smooth burnished roots and flanks, free from tears, chatter, cutter marks that would serve as focal points of stress concentration and, therefore, starting points for fatigue failures.
(ii) Rolling leaves the surface layers of the thread, particularly in the rods, stresses in compression.
In this process no chips are produced and, therefore, there is saving in the material. By cold rolling it is possible to produce a wide variety of threads and non-threading operations on many different materials. This process has been conceded to be the fastest method of producing screw threads.
Threads by cold rolling process are produced either on thread- rolling machines or automatic screw machines. Thread rolling machines use flat and cylindrical dies, and the automatic screw machines use cylindrical thread rolls. In most instances, the entire length of thread is formed by the in feed method without endwise feeding of the blanks or dies. Through feeding of the blank is used on cylindrical die machines for continuous threading of long bars and short headless parts.
Since the rolling does not remove or compress material, the diameter of the blank should not be more than that required, otherwise the dies will be over-loaded. In rolling the threads, the dies penetrate the surface of the blank to form the roots to the threads and in so doing force the displaced material radially outward to from the crests and the major diameter of the thread.
In the threads which have a balanced thread form—where addendum and reddendum are equal, diameter of blank may be taken as the pitch diameter of the finished thread; in any case it should be less than the maximum pitch diameter. Blank should be as round and straight as possible. Their ends should be bevelled to prevent excessive chipping of the threads on the dies or rolls.
Roll ability of materials i.e. their behaviour during the thread-rolling process depends on the properties of materials such as chemical composition, hardness, strength, ductility and toughness.
It may also be judged from the considerations like:
(a) resistance of the material to plastic deformation,
(b) behaviour of the material during displacement and
(c) degree of smoothness of the material rolled.
The materials which can be easily thread rolled are plain carbon steels, structural alloy steels, stainless steels, non-leaded brasses, copper etc.
The accuracy of cold rolled threads is dependent on the tolerances of the blank diameter, the thread form of the dies, and the set up and rigidity of the equipment used. The surface finish of threads is dependent on the finish of the surface of dies.
x. Knurling:
It is a cold working operation in which surfaces are made rough by passing the surface between sharp serrations on hard steel rollers. Knurling is obtained by displacement of the materials on the surface of the work blank when the knurl is pressed against a rotating blank. This operation is carried out on lathes or screw machines.
Thumb screw and hand wheel surface are made by this process. Knurling tools are used for producing straight, diagonal, or diamond knurling having teeth of uniform pitch on cylindrical surface. A knurled tooth is V- shaped, and the depth of the tooth is less than the depth of a theoretical V-form. The tooth has a rounded root and crest. The blank diameters for knurling are approximately equal to the knurled diameter minus the depth of booth.
xi. Riveting:
It is a permanent fastening process in which the end of the metal pin is pressed over or spread out by hammering operation. Riveting can be cold or hot worked. Hot riveting is carried out for heavy duty work and cold riveting for light work, wrought iron and steel are used for hot riveting and copper, brass and aluminium are used for cold riveting.
xii. Cold Extrusion:
The principle of cold extrusion is exactly similar to that of hot extrusion. Of the various processes of extrusions, impact extrusion is essentially the cold working process.
xiii. Impact Extrusion:
The application of impact extrusion is limited to making of small work-pieces such as tooth paste, shaving cream and collapsible medicine tubes from the ductile materials. The work material is placed into the blind hole of the die and a ram punch with clearance is forced into the die, which causes the metal to flow plastically around the punch and finally in tube as shown in Fig. 6.28. Lead, aluminium, zinc and tin are some of the common materials used for impact extrusion.
The steel used for cold extrusion should have a low yield stress, a slow rate of work hardening, and a considerable extension before fracture. A change in cross-sectional area of at least 25% should be possible with punch stress of 25,000 kg/cm2. At present cold extrusion is limited to low and medium carbon steels.
Cold extrusion severely distorts the grain structure of the steel and increases its yield strength considerably. Thus plain carbon steel can be used in place of costly nickel- chromium steels. The residual stress in the component depends on the degree of deformation and the temperature attained (which may be as high as 300°C and it produces some stress relief).
The steel slugs before extrusion are annealed (softening), cleaned by pickling in dilute sulphuric acid to remove surface oxide, phosphate and lubricated (usually sodium stearate). The purpose of phosphate contain is to act as an effective barrier between tool and work to prevent metal contact.
The ends of punches which are subjected to fatigue caused by the rise and fall of direct and bending stresses during extrusion and wear are designed so that these can be easily replaced. Punches and dies are made from high speed steel.
where, sy = mean yield stress
A0 and Ae are the original and extruded cross-sectional areas.
xiv. Shot-Peening:
Short-peening has been developed recently to improve the fatigue resistance of metal by setting up compressive stresses in the surface. The process is carried out by blasting or hurling a rain of small shots pneumatically or mechanically at high velocity against the worked surface.
Small indentations are produced due to striking of shots, which causes the metal to flow plastically to a depth of few tenths of mm. This process is adopted to remove stress concentration on parts of irregular shapes or at local area.
xv. Planishing:
It uses smooth dies to flatten blanks.
4. Bending Operations:
Bending is the cold working process involving plastic deformation in which the total surface area remains constant. Bending of pipe, tube, rolled or extruded shapes involves practices and theories of the simple supported beams, the cantilever beam and many of the problems of forming and deep drawing. In bending flow of metal occurs in plastic range, and there is a permanent change in shape.
Thinning of outer wall of a bent tube or other hollow section and corresponding thickening of the inner wall of the bend are noticeable effects of plastic flow. The neutral axis also gets displaced. When bending thin walled tubes, the material has a tendency to buckle and wrinkle which can be avoided by supporting the material by a shoe which is located opposite to the direction of die pressure.
It is observed that bent material tends to recover elastically when the deforming force is removed. This spring-back can be overcome by bending the metal a few degrees more than the desired angle or by setting the punch travel so that it compresses the bent metal beyond its yield stress (this operation being known as planishing).
In sheet metal work bending is sometimes called forming also. In this case, a plastic deformation of metal takes place and the metallic sheet is stressed in both tension and compression. The bottom-most layer is under tension and topmost layer under compression, when the punch is forced over the sheet.
The metal must be bent at right angles to its fibre. The sheet is more liable to fail along the direction of fibres, i.e., along the length of impurities, then across them. Hand brake and press brake dies are most commonly used for bending operations.
In addition to the above two dies forming dies of different designs are also used for various forming operations. Forming is also similar to bending operation with the difference that a number of bends are formed about the linear axis by making the metal confine to die shape.
Operation of bending can be done on ram-type machine or rotary-type machine. The ram bending machine follows the principle of simple supported beam closely. In this machine two pressure dies are mounted in fixed positions on the frame of the machine, and are free to rotate about their own mounting pins, while the form is mounted directly to the piston rod of the hydraulic cylinder.
Fig. 6.29 shows punch of radius r bending a sheet of thickness t. This die must have radius of r + t. The portion of the die in contact with the job is provided with radius of rd. The angle between two faces of punch is α. The angle between the two bent surfaces of the job is π-2θ. The bending force F is maximum at some intermediate stage, depending on the frictional characteristics.
This degree of bending operation is normally specified in terms of the strain in the outer fibre. The neutral plane shifts towards the centre of curvature, usually by 5%. The smallest punch radius r for given job thickness can be found by the strain in the outer fibre which, of course has a limiting value of beyond which a fracture takes place.
Fig. 6.30 shows the distribution of bending stress in the job.
(M = bending moment, and E = coefficient of elasticity).
Thus to produce a bend of included angle a, the pinch angle should be α-2ɸ).
In the rotary machine, the form is exactly centered and keyed to a shaft with one end of the material securely clamped to the form. A pressure die retains the free end and confines movement of the material along a straight line. Under these conditions, the material is wiped into the form groove by the pressure die as the shaft rotates and is bent to the same shape as the form.
Sometimes mandrels of suitable shapes are used to support hollow sections internally preventing collapse of the wall during bending. Usually ball type mandrels are used in which the balls are pivoted so that the mandrel may be removed from the finished bend by withdrawal of the straight part of the mandrel.
The strain level in the zone on either side of neutral plane is within the elastic range and it increases from zero at neutral plane upto yield limit. The plastic deformation starts beyond the yield limit. Let the yield stress be σy; and the maximum stress at outer and inner fibres be σ0 and σi.
It may be noted that values of σ0 and σi are different due to shifting of neutral plane by around 5%.
The loading due to stress distribution as shown in Fig. 6.27 be represented by a bending moment M and force per unit width P so that:
Problem:
A 5 mm thick sheet of 20 mm width is to be bent into an angle of 90° so that two sides of angle are 50 mm. Length of die is 100 mm. Take values of fracture strain of sheet material as 0.2.
E = 200 kN/nm2,
yield tensile stress σy = 350 N/ mm2,
strain hardening rate n = 500 N/mm.2, µ = 0.1.
Determine (i) the stock length to make 90° bend, (ii) the minimum possible radius of the corner of bend (iii) the maximum bending force, (iv) the required punch angle, (v) die length if the maximum force is to be restricted to 3200 N.
Solution:
Bending with Linear Tool Motion:
Fig. 6.38 shows various examples of formation by bending with linear tool motion.
Bending with Rotary Tool Motion:
Fig. 6.39 shows the examples of formation with rotary tool motion.
Bending Operation in More than One Stage:
It is possible to produce complicated shapes by bending operation in stages (Refer Figs. 6.40 and 6.41) and continuous bending operation using a series of contoured rolls. (Refer Fig. 6.42).
Miscellaneous Rotary Bending Operations:
Tubes and other hollow sections are bent by wrapping the job around a form block using wiper roll which may be hinged at the centre of the curvature to be produced. (Refer Fig. 6.43). In order to prevent the tube from collapsing, the inside space is filled with sand. Fig. 6.44 shows the operations of stretch bending, draw bending, press bending and roll bending.
i. Roll-Forming:
Passing the strip stock between two rollers to form the desired shape is called roll forming. The process is made continuous by the use of guides between set of rollers.
Most ductile sheets or strips (0.1 to 20 mm) are used for roll forming. This process is particularly suitable for producing large quantities of long strips of desired shape, with minimum handling operations.
The machines used for roll-forming are of double housing and over-hung type. The former is used for forming heavier sections and has bearings on either ends, while the latter is used for lighter sections and has both the bearings on the side of roller.
Another type of roll forming operations is called roll- bending operation. It is employed to bend or curve sheets and plates into cylindrical shape with the help of three long and big rolls.
The coil bending machine consists of three rollers of same diameter, two of them are held in a fixed position and the third being adjustable. A metallic sheet or plate is passed through these rolls. The final shape of bending depends upon the position of adjustable roller as shown in Fig. 6.47.
ii. Seaming:
Seaming is the process of interlocking sheet metal products such as drums, cans etc. Generally two types of seam joint are used, one is called lock seam and the other is called the compound seam.
The lock seam is used for longitudinal joints where absolute tightness is not necessary. The compound seam, sometimes called as box seam is much stronger and tighter than lock seam and is suitable for holding finer materials. Both of these joints are formed and closed either by hand or power seaming process.