Here is a list of sheet metal forming processes:- 1. Cutting Off, Blanking, Punching and Piercing Process 2. Bending 3. Deep Drawing.
1. Cutting Off, Blanking, Punching and Piercing Processes:
For almost all sheet metal components, blanking/punching or cutting off is the very first process.
Cutting off:
It refers to the process of shearing off across the entire width of sheet. Normally no scrap is produced in cutting off an individual piece. However in some cases scrap may be generated at the beginning or at the end of the strip (Fig. 11.1).
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Blanking and Punching:
Both the names refer to the same process, i.e. shearing a contoured piece out of a bigger sheet area. However, if the desired component is the sheared contoured piece, often called blank, the process is called blanking. On the other hand if the remaining sheet in which hole has been punched is the desired component the process is called punching.
But there are many cases in which both are the desired components. In blanking/punching operation shown in Fig. 11.2 it is clear that the edge of sheared component is not smooth. The following regions are observed on the blank-edge as well as on hole-edge.
(i) A small roll over region also called edge draw-in.
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(ii) One or two sheared portions (depending on clearance) which look bright and burnished.
(iii) A fractured region which is rough, slanting and uneven.
(iv) Burr.
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The relative sizes of these regions vary with the clearance between punch and die. The clearance is defined as the perpendicular distance between the cross sectional profiles of punch and die. The quality of edge depends on the clearance, sheet thickness, hardness of sheet and the speed at which the process is carried out.
For obtaining an edge which is straight and normal to plane of sheet, a very small clearance is recommended, i.e. ≈ 1 per cent of sheet thickness. Also higher blanking speed gives a better edge quality. As the clearance increases the size of fractured region, inclination of edge, roll over and burr increase.
These regions are illustrated in Fig. 11.2. The roll over region is bigger with softer materials than with hard materials. The dimension of the blank is the dimension at the burnished portion and this is same as die dimension. Similarly the punched hole is also not uniform. The minimum dimension or diameter in case of circular hole, is equal to the dimension or diameter of punch.
Depending upon the desired edge quality and the material of sheet, the edge clearance varies from less than 1% to 12% of sheet thickness and in some cases even more. For unimportant components like washers it may be as much as 15% of sheet thickness. Table 1.1 gives the recommended values of clearance for a few materials.
For general purpose components without the requirement of tolerances, such as washers, the die-punch clearance may be as much as 10%-15% of sheet thickness. For precision components such as motor stampings, the clearance may be 5% to 7% of sheet thickness. For high precision components the clearance may be 1.5% to 3% of sheet thickness.
The components having the requirement of straight edges perpendicular to plane of the sheet and high dimensional accuracy the clearance may vary from 0.5-1.5%. For soft materials the die punch clearance is kept lower than that for harder materials in order to obtain similar edge quality. The components in which the edges are used as bearing surfaces such as watch gears and levers, are generally made by fine blanking technique which is described below-
Flanging:
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In this process holes are enlarged by a conical indenter or punch with the result a flange is formed around the circumference of hole. The maximum possible height of flange is limited by the appearance of crack at its outer edge.
Figure 11.3 shows the process. Piercing or flanging a hole without an initial punched hole may tear the sheet. The flange so formed will have cracks and may not be strong enough for the desired application.
For obtaining a smooth edge of the flange, a hole should be punched before flanging it. The maximum enlargement without tearing is limited by the onset of necking at the flange edge. Flanged holes are needed for many applications such as threaded holes, holes with bearing surfaces etc.
Compound and Progressive Dies:
For multiple operations compound and progressive dies are often used. In compound dies the operations are carried out at the same site. For example blanking and punching may be combined as illustrated in Fig. 11.4(a) to produce washers or similar item. As illustrated in the figure the operations take place consequently (one after another) in the same stroke length. Similarly blanking and cup drawing may be combined.
In progressive dies illustrated in Fig. 11.4(b) the different operations are carried out at different sites in the same die set. After each stroke of press the strip is advanced by specified distance which is controlled by locating pins provided on the die set which position the strip for next operation. Some punched holes in the previous operation fit the locating pins and thus provide specified location. The holes punched for location may be part of component or may be provided only for the purpose of location.
Fine blanking technique is used for punching or blanking high precision components such as gears and levers that are used in watches and instruments. The edge quality in blanking can be improved by introducing compressive stress in the shearing zone of the sheet.
This can be achieved by (i) gripping the sheet between the die and a Vee shaped projection provided on the blank holding plate. Sheet may also be pressed between two opposing punches. The punch-die clearance is kept small, i.e. of the order of 1% or less. The sheared region on the edge can be increased by increasing (i) the force on the Vee and (ii) by increasing pressure between the punches if two punches are used.
The edge obtained by this process does not require additional finishing operation. More over the material on the edge suffers severe shear deformation and gets work hardened. In sheet metal gears and levers in which edge surface is also a wearing surface, the work hardening is helpful in increasing the life of the components. Another method of increasing the sheared area on the edge uses two punches and two dies and carries out partial blanking on either side.
Generally the skeleton left after blanking the sheet, is treated as scrap. It is, therefore, very important that the skeleton area or waste should be minimized and maximum possible sheet area be utilized. This will lower the cost of blanks. Nesting is the arrangement of components on the sheet surface so that maximum of sheet area is utilized.
Figure 11.7 shows some examples of nesting. In Fig. 11.7(a) the inclined edges of the component have been matched to reduce the consumption of sheet. The minimum spacing between adjacent blanks with sheet thickness t should be 1.5t or more. Figures 11.7(b) and (c) show the nesting using geometrical features of components.
Figures (11.8 A, B & C) show the blanking of circular blanks. In the three figures the width of strip has been adjusted for blanking (i) a single row, (ii) two rows and (iii) three rows of blanks. Let D be the diameter of blank, ‘a’ the edge margin and ‘b’ the margin between two adjacent blanks.
The minimum values of a and b are 1.5 times the sheet thickness. For thinner stocks the values of a and b may be increased to 2-2.5 times the sheet thickness.
Utilization of sheet for the three cases is analyzed below:
(i) Efficiency for Blanking Single Row of Blanks (Fig. 11.8 (A)):
(ii) Efficiency for Blanking Double Row of Blanks (Fig. 11.8(B)):
The lines joining the centers of adjacent circles shown shaded in Fig. 11.8(B), make an equilateral triangle having side equal to D + b, therefore, its height is equal to (D + b). cos 30°.
(iii) Efficiency for Blanking 3 or More Rows (Fig. 11.8(C)):
Let there be ‘n’ rows of circles. There would be (n – 1) equilateral triangles as discussed above.
Width of strip (wn) = D + 2a + (n – 1) (D + b). cos (30°)
Length of strip used for n circles (ln) = D + b
Efficiency of nesting ƞ is given as follows:
80 mm diameter blanks are to be punched out of 1.2 mm thick strips. If the margin between blank and strip-edge is 2.5 mm and that between two blanks is 2 mm determine the % utilization of strip. The strip width can be adjusted to accommodate (i) single row of blanks, (ii) double row of blanks, and (iii) 3 rows of blanks. Also find the efficiency if the margins are limited to 1.5 times the sheet thickness.
Let l be the length of cut and t the sheet thickness. The maximum force (Fs) required during blanking, is given by-
where Ks is the shearing resistance of sheet. The experimental work of several investigators have shown that value of Ks depends on the following factors-
(i) Strengths of material.
(ii) The clearance between die and punch.
(iii) Condition of tool (sharp or worn).
(iv) Thickness of sheet.
(v) Size of blank.
(vi) Curvature of contour.
The shearing resistance observed in blanking/punching is 20% to 30% higher than the value of shear yield strength of material observed in tensile testing. The punch force is also increased due to frictional stress between the edge of sheet and die.
Thus the value of Ks is generally taken equal to ultimate tensile strength of material. For proper operation of blanking die and press, die designer should ensure that centre of forces due blanking of different holes should coincide with the central line of press ram.
Shear Angle on Dies and Punches:
The face of punch or die may be machined and ground at an angle other than 90 with respect to the axis of hole. The purpose is to reduce the load on the press, besides it also reduces blanking noise and shock to the press. Figure 11.9 shows examples of shear. If the shear is provided on die, it should be symmetrical with respect to the central plane so that the sheet may be placed horizontally on it.
Blanks of 50 mm diameter are to be punched out of 2 mm thick steel sheet. The edge (radial) clearance between punch and die is 5% of sheet thickness. Determine the dimensions of punch and die and the punch force if cutting resistance of the material is equal to 300 N/mm2.
2. Bending:
Bending is a common process in sheet metal working. Large scale bending operations are done on brake presses with the help of Vee shaped tools.
In bending operation the surface layers of the sheet in the bend area suffer maximum stresses. The outer layers of the bend region suffer tensile stress and inner layers suffer compressive stress. The surface layers are also the first to reach the plastic state. As the bending proceeds the layers below the surface successively reach the plastic state.
However even in severe bends, the layers very near to neutral section of sheet may remain elastic. On unloading the elastic deformation tries to recover thus unbending the sheet by a small angle. This unbending is called spring back.
The magnitude of spring back depends on the following factors:
(i) Angle of bend.
(ii) Thickness of sheet.
(iii) Type of tooling.
(iv) Work-hardening characteristics of the material.
The effect of spring back can be minimized by the following methods:
(i) By over bending the sheet so that on unloading the sheet recovers to the desired angle of bend.
(ii) By setting (compressing) the material at the bend region so that the recovery is minimum.
The process of bending is extensively used in industry to make different products. For manufacturing welded tube the strip may be bent by rotating roller dies or by pulling through a conical die into tubular form, which is subsequently welded to produce welded tubes. In the old plants the forge welding technique is quite popular.
The strip is continuously heated in a furnace to the welding temperature, it is bent into shape and press welded by roller dies. The technique is used for tubes of diameters 13 mm to 114 mm.
The developments in oil industry created a demand for large diameter pipes for laying long distance pipe lines. Submerged arc welding (SAW) is commonly used for making pipes of diameters from 200 mm up to 2500 mm. For smaller diameter, electric resistance welding is commonly used. In many plants high frequency induction heating is also employed.
During pressure welding, ridges are formed both on inside as well outside the tube. For small diameters only outside ridge is cut off by cutting tool. On big diameter pipes both the inside and outside ridges are cleaned off while the weld is still hot. After welding and cleaning of ridges the pipes are finished by rounding them to the tolerable limits followed by other finishing operations.
3. Deep Drawing:
Deep drawing or cup drawing is the process of forming hollow shaped components from flat pieces of sheet metal or from already drawn components in which case it is also called redrawing. A large variety of components such as kitchen wares, boxes of various shapes, components of car bodies, domestic gas cylinders, etc. are made by deep drawing.
The equipment consists of a die with a central hole having a profile radius, a blank holding plate which keeps the sheet pressed flat on to the die surface and a punch with suitable diameter and corner radius, which presses the sheet into the die hole.
The dimensions of various tools are designed as per requirement of the process. The radial clearance between punch and die is equal to sheet thickness + thickening of sheet during drawing which is nearly 12 % of sheet thickness. If ironing of cup wall is desired the clearance may be decreased.
The motion of punch pushes the bottom of cup into the die. The sheet around the punch pulls the remaining sheet into the die to form the cup. The flat sheet between the die and blank holding plate, at diameters more than die hole diameter, moves towards the die center against the frictional forces between sheet and die surface and between sheet and blank holding plate. The sheet then bends over the die profile radius, slips over it against the frictional forces and then unbends into the cup wall.
Each of the above process increases tensile stress in the cup wall. The maximum tensile stress occurs at the punch profile radius where the sheet bends over it under tension. Generally the failure takes place near the punch profile radius.
The tensile stress here is the cumulative stress due to drawing of flange, bending and unbending of sheet at die profile radius, frictional effect on die profile radius and lastly due to bending on punch profile radius (Fig. 11.13). If the stress goes beyond the ultimate tensile strength of the material, the localized necking starts, which may lead to fracture.
From the above description of the process, it is clear that the sheet in the flange which moves towards center of die, suffers plastic deformation under the following stresses.
(i) A compressive circumferential stress.
(ii) Tensile radial stress.
(iii) Compressive blank-holding stress.
Figure 11.12(b) also shows the variation in thickness of drawn cup. The bottom of cup has nearly same thickness as that of blank. At the punch profile radius the sheet suffers thinning due to bending under tension. One or two thinning zones may be observed in the sheet at punch profile radius.
Cup wall thickness increases from bottom to the top. The increase in thickness at the top edge may be as much as 12% of the initial thickness. If sufficient clearance is not given in die diameter the wall of cup may be ironed out resulting in uniform thickness of cup wall.
The sheet tends to buckle when subjected to simple compressive force in the plane of the sheet. If buckling is not prevented during flange drawing it will form ‘wrinkles’ in the cup wall. The purpose of blank holding plate is to inhibit buckling and to keep the sheet flat. The pressure of blank holding plate should be just sufficient so that there is no circumferential buckling and wrinkling.
The excessive pressure will, however, increase the frictional force which will increase the stress in cup wall and may lead to early fracture at punch profile radius. During drawing, the increase in sheet thickness at the edge of blank is the maximum. Hence the contact of blank holding plate is limited to a small width of sheet along the outer rim of blank.
The blank holding force (Fb) may be calculated as follows-
