Common forming processes

Blanking and piercing. As sheet is usually delivered in large coils, the first operation is to cut the blanks that will be fed into the presses; subsequently there may be further blanking to trim off excess material and pierce holes. The basic cutting process is shown in Figure I.1. When examined in detail, it is seen that blanking is a complicated process of plastic shearing and fracture and that the material at the edge is likely to become hardened locally. These effects may cause difficulty in subsequent operations and information on tooling design to reduce problems can be found in the appropriate texts.

Figure I.1 Magnified section of blanking a sheet showing plastic deformation and cracking.

Bending. The simplest forming process is making a straight line bend as shown in Figure I.2. Plastic deformation occurs only in the bend region and the material away from the bend is not deformed. If the material lacks ductility, cracking may appear on

Process Within Blanking

the outside bend surface, but the greatest difficulty is usually to obtain an accurate and repeatable bend angle. Elastic springback is appreciable.

Various ways of bending along a straight line are shown in Figure I.3. In folding (a), the part is held stationary on the left-hand side and the edge is gripped between movable tools that rotate. In press-brake forming (b), a punch moves down and forces the sheet into a vee-die. Bends can be formed continuously in long strip by roll forming (c). In roll forming machines, there are a number of sets of rolls that incrementally bend the sheet, and wide panels such as roofing sheet or complicated channel sections can be made in this process. A technique for bending at the edge of a stamped part is flanging or wiping as shown in Figure I.3(d). The part is clamped on the left-hand side and the flanging tool moves downwards to form the bend. Similar tooling is used is successive processes to bend the sheet back on itself to form a hem.

Punch

Punch

Clamp

| Flanging tool

Rolls

Clamp

| Flanging tool

Sheet

Figure I.3 (a) Bending a sheet in a folding machine. (b) Press brake bending in a vee-die. (c) Section of a set of rolls in a roll former. (d) Wiping down a flange.

If the bend is not along a straight line, or the sheet is not flat, plastic deformation occurs not only at the bend, but also in the adjoining sheet. Figure I.4 gives examples. In shrink flanging (a), the edge is shortened and the flange may buckle. In stretch flanging (b), the length of the edge must increase and splitting could be a problem. If the part is curved near the flange or if both the flange and the part are curved, as in Figure 1.4(c), the flange may be either stretched or compressed and some geometric analysis is needed to determine this. All these flanges are usually formed with the kind of tooling shown in Figure 1.3(d).

Flange Buckling Dimensions

Figure I.4 (a) A shrink flange showing possible buckling. (b) A stretch flange with edge cracking. (c) Flanging a curved sheet.

Section bending. In Figure I.5, a more complicated shape is bent. At the left-hand end of the part, the flange of the channel is stretched and may split, and the height of the leg, h, will decrease. When the flange is on the inside, as on the right, wrinkling is possible and the flange height will increase.

Stretching. The simplest stretching process is shown in Figure I.6. As the punch is pushed into the sheet, tensile forces are generated at the centre. These are the forces that cause the deformation and the contact stress between the punch and the sheet is very much lower than the yield stress of the sheet.

The tensile forces are resisted by the material at the edge of the sheet and compressive hoop stresses will develop in this region. As there will be a tendency for the outer region to buckle, it will be held by a blank-holder as shown in Figure I.6(b). The features mentioned are common in many sheet processes, namely that forming is not caused by the direct

Sheet

Blank holder i Punch

Figure I.6 (a) Stretching a dome in a sheet. (b) A domed punch and die set for stretching a sheet.

contact stresses, but by forces transmitted through the sheet and there will be a balance between tensile forces over the punch and compressive forces in the outer flange material.

Hole extrusion. If a hole smaller than the punch diameter is first pierced in the sheet, the punch can be pushed through the sheet to raise a lip as in the hole extrusion in Figure I.7. It will be appreciated that the edge of the hole will be stretched and splitting will limit the height of the extrusion.

Figure I.7 Extrusion of a punched hole using tooling similar to Figure 1.6(b).

Stamping or draw die forming. The part shown in Figure 1.8(a) is formed by stretching over a punch of more complicated shape in a draw die. This consists of a punch, and draw ring and blank-holder assembly, or binder. The principle is similar to punch stretching described above, but the outer edge or flange is allowed to draw inwards under restraint to supply material for the part shape. This process is widely used to form auto-body panels and a variety of appliance parts. Much of the outer flange is trimmed off after forming so that it is not a highly efficient process, but with well-designed tooling, vast quantities of parts can be made quickly and with good dimensional control. Die design requires the combination of skill and extensive computer-aided engineering systems, but for the purpose of conceptual design and problem solving, the complicated deformation system can be broken down into basic elements that are readily analysed. In this book, the analysis of these macroscopic elements is studied and explained, so that the reader can understand those factors that govern the overall process.

Deep drawing. In stamping, most of the final part is formed by stretching over the punch although some material around the sides may have been drawn inwards from the flange. As

Figure I.8 (a) Typical part formed in a stamping or draw die showing the die ring, but not the punch or blankholder. (b) Section of tooling in a draw die showing the punch and binder assembly.

there is a limit to the stretching that is possible before tearing, stamped parts are typically shallow. To form deeper parts, much more material must be drawn inwards to form the sides and such a process is termed deep drawing. Forming a simple cylindrical cup is shown in Figure I.9. To prevent the flange from buckling, a blankholder is used and the clamping force will be of the same order as the punch force. Lubrication is important as the sheet must slide between the die and the blankholder. Stretching over the punch is small and most of the deformation is in the flange; as this occurs under compressive stresses, large strains are possible and it is possible to draw a cup whose height is equal to or possibly a little larger than the cup diameter. Deeper cups can be made by redrawing as shown in Figure I.10.

Figure I.9 (a) Tooling for deep drawing a cylindrical cup. (b) Typical cup deep drawn in a single stage.

Tube forming. There are a number of processes for forming tubes such as flaring and sinking as shown in Figure I.11. Again, these operations can be broken down into a few elements, and analysed as steady-state processes.

Fluid forming. Some parts can be formed by fluid pressure rather than by rigid tools. Quite high fluid pressures are required to form sheet metal parts so that equipment can

Sheet

Sheet

Required Force For Flaring Tube
Figure I.11 (a) Expanding the end of a tube with a flaring tool. (b) Reducing the diameter of a tube by pressing it through a sinking die.

be expensive, but savings in tooling costs are possible and the technique is suitable where limited numbers of parts are required. For forming flat parts, a diaphragm is usually placed over the sheet and pressurized in a container as in Figure I.12. As the pressure to form the sheet into sharp corners can be very high, the forces needed to keep the container closed are much greater than those acting on a punch in a draw die, and special presses are required. Complicated tubular parts for plumbing fittings and bicycle frame brackets are made by a combination of fluid pressure and axial force as in Figure I.13. Tubular parts, for example frame structures for larger vehicles, are made by bending a circular tube, placing it in a closed die and forming it to a square section as illustrated in Figure I.14.

Coining and ironing. In all of the processes above, the contact stress between the sheet and the tooling is small and, as mentioned, deformation results from membrane forces in the sheet. In a few instances, through-thickness compression is the principal deformation force. Coining, Figure I.15, is a local forging operation used, for example, to produce a groove in the lid of a beverage can or to thin a small area of sheet. Ironing, Figure I.16, is a continuous process and often accompanies deep drawing. The cylindrical cup is forced through an ironing die that is slightly smaller than the punch plus metal thickness dimension. Using several dies in tandem, the wall thickness can be reduced by more than one-half in a single stroke.

Pressure container

Part

Hydroforming Tubes

Diaphragm

Figure I.12 Using fluid pressure (hydroforming) to form a shallow part.

Tube

Punch /

Tube

Punch /

Punch

Pressure

Punch

Pressure

Figure I.13 Using combined axial force and fluid pressure to form a plumbing fitting (tee joint).

Tube

Tube

Figure I.14 (a) Expanding a round tube to a square section in a high pressure hydroforming process. (b) A section of a typical hydroformed part in which a circular tube was pre-bent and then formed by fluid pressure in a die to a square section.

Coining tool \

Figure I.15 Thinning a sheet locally using a coining tool.

Summary. Only very simple examples of industrial sheet forming processes have been shown here. An industrial plant will contain many variants of these techniques and numerous presses and machines of great complexity. It would be an overwhelming task to deal with all the details of tool and process design, but fortunately these processes are all made up of relatively few elemental operations such as stretching, drawing, bending, bending

under tension and sliding over a tool surface. Each of the basic deformation processes can be analysed and described by a 'mechanics model', i.e. a model similar to the familiar ones in elastic deformation for tension in a bar, bending of a beam or torsion of a shaft; these models form the basis for mechanical design in the elastic regime. This book presents similar models for the deformation of sheet. In this way, the engineer can apply a familiar approach to problem solving in sheet metal engineering.

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