Drawing Operation of Metals | Processes | Industries | Metallurgy

‘Drawing is a process of changing a flat, precut metal blank into a hollow vessel without successive wrinkling, thinning, or fracturing. This process is capable of producing cylindrical or box shaped surfaces with straight or tapered sides or a combination of straight, tapered or curved sides. It can undertake jobs of nearly any size.

In drawing operation metal flows from one place to other to give the required shape. When a metal blank is drawn into a die a change in its shape is brought about by forcing the metal to flow on a plane parallel to the die face, with the result that its thickness and surface area remain about the same as the blank.

The changes which will take place while drawing circular blanks are given below:

(i) Little or no change in the bottom area occurs because no cold work is done in this area.

(ii) All radial boundaries of the units of area remain radial in the bottom area. The units it the top flange area remain radial until they move over the die radius, then they become parallel and assume dimensions equal to their dimensions at the point where they move over the die radius.

(iii) There is a slight decrease in surface area and decrease of thickness in the units involving maximum flow. The increase in thickness is limited to the space between the punch and die.

(iv) Metal movement at all places is uniform.

On the above assumption, i.e. the thickness of drawn part and that of the blank remain same, the diameter of the blank could be calculated by the following formula, i.e.

If radius at the bottom of the shell is also taken into consideration, then:

where r = corner radius,

(r + 0.4t) = mean power radius,

d = outside dia. of shell,

h = Shell height (inside)

Further in a drawing operation, the portion below the punch is subjected to tensile stress and remaining portion is subjected to compressive stress. In order that drawing is proper and the metal does not tear away during drawing operation, these two forces should be equal, i.e. (D-d) t .f_c = d.tf_t.

where f_c = compressive strength of material of sheet

f_t = tensile strength of material of sheet.

As metal undergoes more and more working, value of f_c goes on increasing due to work hardening and ultimately it reaches such a high value that further drawing is impossible without annealing the piece.

Force Required for Drawing:

Theoretically the force is required to pull the bottom portion,

But actually in order to overcome the friction between metal and die and also to overcome other resistance force against pressure pad, a constant c is introduced to take into account all the above factors.

For mild steel, value of c = 0.3.

Effect of Punch Radius and Die Radius on Force and Thickness of Drawn Material:

These effects could be studied by plotting the curves between maximum punch load and the diameter of work for various combinations of die radii and punch radii.

From these curves it is observed that as regards the maximum punch load, the change of die radius is more predominant and change of punch radius does not produce much change.

Therefore, by giving judicial radius on the die, maximum drawing force can be reduced. In actual practice, the generous die radius is desirable to reduce the drawing load while a generous punch radius is desirable in order to maintain the strength of the shell wall, particularly at the corners.

Effect of Punch Radius Vs. Change of Thickness:

The profile of the punch, i.e. punch radius greatly affects the change in thickness of the drawn sheet. In the case of sharp punch radius (small radius), the thinning effect of the blank is more at the corner whereas larger punch radius produces a more general thinning effect over the base or bottom of the sheet. Therefore, optimum radius of punch should be chosen so that neither whole base is thin nor the corners of the shell are weak.

This effect will be very obvious by Fig. 29.19 in which curves have been drawn for various punch radii, showing the thinning effect at various places.

1 /8″ (3 mm) punch radius. Maximum thinning effect takes place at bottom corners. In case of base, thinning is very slight. In this case, therefore, the cup will fail from corners.

1″ (25 mm) punch radius. In this case thinning is gradual and uniform but most of bottom surface is very thin.

1/2″ (12 mm) punch radius. This seems to be optimum radius, as the thinning effect at corners and base is almost constant.

Generally in order to provide the uniform thickness to the wall of the shell, an operation known as ironing is carried out after drawing operation. It may be either done separately or at the time of drawing operation itself. In this operation, the extra metal comes of the top which is afterwards trimmed off.

Classification of Drawing Dies:

a. Single-Action Dies:

The simplest type of drawing die is one in which the pre-cut blank is placed in a recess on top of the die and the punch descends, pushing the cup through the die. As the punch ascends, the cup is stripped from the punch by the counter bore in the bottom of the die. The top edge of the shell expands slightly to make this possible. The punch has an air vent to eliminate suction which would hold the cup on the punch and damage the cup when it is stripped from the punch.

As successful drawing without wrinkles is very much dependent upon the control of blank-holder operation, the method by which the blank is held in position is very important. Generally spring loaded pressure pad is used for this purpose.

It may also be possible to use die in inverted position, i.e. die is fixed in the ram and the punch and pads are fixed in the die shoes so that sheet could be easily held.

b. Double-Action Dies:

In dies designed for use in double- action press, the blank-holder is fastened to the outer ram which descends first and grips the blank; then the punch, which is fastened to the inner ram descends, forming the part. These dies may be a push-through type, or the parts may be ejected from the die with a knockout attached to the die cushion or by means of a delayed action kick.

To facilitate the drawing operation, it is usual practice to apply some lubricant which must not have corrosive effect on the metal. The most commonly used lubricants are mineral oils, soap solution, lard-oil, mineral and fatty oil emulsions, wax, etc.

Deep Drawing:

For deep drawing, the sheet is drawn radially over the surface of the die by a punch. The sheet is supported over die surface by a pressure ring which bears on the upper surface of the sheet thus preventing wrinkling of the metal while it is being drawn.

A pressure pad from bottom exerts pressure on the base of the drawn component. It may be appreciated that the type of deformation undergone by various portions of the blank in deep drawing operation varies considerably. For instance, zone a-b in Fig. 29.21 is stretched over the base of the punch but the rest of the blank is drawn radially inwards across the top of the die.

As the blank is pulled over the die radius, it is bent at plane e and unbent at plane d and is in tension and then drawn vertically downwards under tension to form the wall of cylindrical cup (zoned c). As the diameter of blank is continuously shrinking, its thickness increases towards the outer edge (zone e f).

In the plane e of bending, the upper layer is subjected to tension and bottom layer to compressive stress; the magnitude of tensile stress being more than compressive stress and hence neutral axis is shifted down from the middle position.

In the unbending planed, the layers nearer to die are in tension and layers nearer to punch are in compression; the tensile forces being higher than compressive and neutral axis being shifted from middle position towards punch. Thus as a result of bending and unbending, the strains, and hence work hardening will be maximum at outer fibers.

At the start of drawing when the blank undergoes plastic bending or unbending under tension, the instantaneous thinning of the metal is experienced. The bottom vertical portion is thus thinned about thickness of blank as further drawing increases. Near the base of the cup wall, thinning caused by bending and unbending under tension produces two necks (zone bc towards outer layer) which are the weak spots.

The drawing ratios depends on the ductility of blank material, its thickness, magnitude of punch and die radius. It can be increased by lubrication of die radius and providing differential annealing to soften the other edge of the blank etc.

Sheet metal forming is a complex job and is dependent on several factors.

The amount of deformation which sheet metal will undergo before fracture is dependent on

(i) Drawing quality of material; (material with required draw-ability should be used, drawn to fullest extent avoiding necking and fracture and keeping in mind that cost of more drawable metal is high),

(ii) Design of the tooling which is dependent on the shape of the component; sometimes redrawing which inter-stage bright annealing may have to be used, however to reduce cost maximum deformation must be achieved in each stage, thus reducing number of redraws),

(iii) Frictional conditions between the tool and work piece.

Application of lubricant in case of large radii (like hemispherical punch) is advantageous, because the material thus slips easily over punch surface. However, with small radii, the strain is not distributed evenly but is highly localised and thus fracture is likely to occur if the metal is allowed to slip away from highly strained areas due to good lubrication. Surface roughness of sheet also affects the formability.

While smooth surface which acts as lubricant is desirable, but too smooth a surface can cause seizure of the tool and work lubrication is also affected by parameters like forming speed, pressure, material properties and die design. The choice of a particular lubricant for each application needs to be made carefully. Lot of research work is being done to establish forming limits of sheet metal.

Forming limit diagrams are developed which help in determining deformation of metal without causing (i) necking or (ii) failure, Principal strains which occur at the onset of necking and at fracture are determined and plotted on a forming limit diagram. In this way, limit on onset of necking can be established.

Successive Drawing (Redrawing):

For producing a shell which is long in proportion to its diameter, it is necessary to sub-divide the process into a series of draws; starting with a shallow cup of larger diameter and progressively drawing to a longer and small shell until the desired dimensions are reached.

The amount of reduction that may be effected at each operation depends upon various factors like quality of material, die edge radius, bottom radius, material, thickness, thickening allowed etc. This property is defined by the term drawing ratio which is [(D – d)] x 100%.

As a general rule the ratio Did for the first draw varies from 45-60%; 21-25% and 10-15% in subsequent operations because the wall and bottom thickness do not remain uniform and work hardening is induced every time.

Generally when several such operations are necessary, intermediate annealing treatments are necessary to relieve the hardness induced due to several cold working operations. Sometimes ironing operation is also carried out to give uniform thickness to metal.

The general rule for number of reductions for mild steel is:

Ironing:

In deep drawing operation, the thickness of drawn cup varies along its wall. In order to reduce the wall thickness to a constant value, the drawn cap is pushed through a die (ironing). It is possible to combine drawing and ironing dies in series so that by a single punch operation, final drawn cup of uniform thickness is obtained in a single operation.

Non-Circular Drawing (Rectangular Shape):

The drawing of rectangular shell involves varying degrees of flow sensitivity. This operation is combination of bending of straight portion and drawing of corners. The blank for drawing rectangular shape could be considered as divided into parts due to variation in flow in different parts of the shell.

The corners comprise the drawing area, which includes all the metal in corners of the blank necessary to make a full corner on the drawn shell. The sides and ends constitute the forming area, which includes all the metal necessary to make the sides and ends to full depth. The shape of the blank is as shown in Fig. 29.22.

Relationship between length and width of blank for rectangular drawing where two or more draws are required is as given below:

Length = rk + I;

Width = rk + w

where I = length of preceding die,

w = width of preceding die

k – constant,

r = corner radius if corner radius is less than 12 mm; and beyond corner radius of 12 mm, r is taken equal to 12 mm

Amount of pressure required for rectangular drawing = pressure for bending straight portions and pressure for drawing the corners and also to overcome some friction

.^.. P = l .t .f_t c₂ + 2π rt f_t . c₁

where I = total length of straight sides of rectangular shell

t = thickness of metal,

f_t = ultimate tensile strength

r = corner radius of rectangular shell

c₁ = constant (= 0.5) for low shell height upto 50 mm

(= 2.0) for shell having depth more than 50 mm.

c₂ = constant (depends upon the amount of drawing) = 0.2 (generally)

Automation of Power Press:

Automation of power press results in increase of number of components produced every hour. Automatic feeding of one press results in same production as by 3 to 4 manually operated presses. Since the cost of feed supply system is much less than the presses eliminated, it is advisable to use automatic presses. The important aspects to be kept in mind are the minimum wastage of material, proper selection of equipment, particularly stock supply machinery and feeders, and adoption of all safety measures.

Before attempting design of automation, complete information and specification of press must be ascertained (like press speed, stroke, feed line height, direction of feed, longest and shortest feed lengths required, range of thickness and width of stock to be processed, maximum weight of coiled stock and its dimensions). The coiled stock is less expensive to buy and easy for handling. Coiled stock is available in the form of reels or cradle. Cradle is used when stock is at least 1.25 mm thick.

Reel is used for thinner sheet or whose finish is likely to be easily marred. In the case of reel, the coil is supported by its aye and weight of the coil is supported internally by an arbor, thus stock surfaces open up freely and are not defamed.

However, in this case, the coil weight should not be too heavy so as to deform its edges and surfaces. In order to avoid deflection of arbor on which reel is supported, this reel should never be overloaded. A deflection of arbor would result in streaking of strip of stock on the low side and thus leading into tracking problems. For quick and easy loading, reels with a single-pedestal supported self-centring arbors are used.

In order to control the intermittent movement of stock form roll, either brakes (spring-loaded, electric, air-operated, or water cooled type) are provided or automatically controlled variable speed and d.c. motors are used. The coil acts as a heavy flywheel and has high inertial posing problems in start and stop.

The sheet from the rolls is first fed to a straightener which removes the ‘set’ or coil curl, particularly from tight wound coil. Due to variation of curl from outer to inner diameter, straightener should function adjustably so that it works harder on inner coils to produce uniformly flat stock.

Straighteners could be pull through type (used for thin sheet), or powered type, equipped with smoothly ground, lightly spring-loaded pinch rolls, or high-quality powered type equipped rolls with a textured gripping surface.

The peeling and threading of this stock from coil into the straightener is mechanised. The stock from the straightener is fed to a feeder located close to press. Feeder grips the stock and advances the stock in short increments.

The problem of frequent start and stop of coil to feed sheet intermittently to press can be overcome by installing the coil on a payoff reel located 3 to 6 m from the press and its feeder. The reel is powered by a motor and driven slowly. When the stock is drawn from the cut, it forms a slack loop which acts as an accumulator.

The amount of feedstock accumulation can be greatly increased by suspending the slack loop in a pit. Arrangements are made to sense height of loop and change the speed of stock to keep the loop within limits.

The feeders should be designed to have enough of surplus power to take care of minor interferences’ in the dies as a result of burns on the stock, coil camber and changing tensions due to rising/falling loop of stock. Some automatic presses use solid state electronic controls instead of mechanical stops. Automation should also include automatic section of the stamped parts and automatic chopping and removal of scrap.

The proper entry of stock into the die is influenced by alignment of the coil-handling and stock-feeding equipment. Die set should be properly aligned with the press feed. When designing for automation, safety of press operator and the protection of equipment and tooling are of paramount consideration. It is essential to provide physical harriers and presence-sensing press-shut off systems for protection purpose.