Press work and die punch assembly
Use of mechanical and hydraulic presses for forging and extrusion has been mentioned earlier. Knuckle type mechanical presses are used widely for sheet metal work. These presses are usually of vertical configuration. These presses are provided with a heavy flywheel driven by an electric motor. A ram moves up and down the guide ways provided in the frame of the press, when the ram is connected to the flywheel through a connecting rod and a crank mechanism. The clutch for transferring the motion from the flywheel to the ram is operated by a foot operated treadle. The arrangement is somewhat similar to the mechanism of a reciprocating engine. Such presses are very useful for providing short powerful strokes.
These presses are available in two configurations:
(i) Open frame type, and
(ii) Closed frame type.
Open frame type presses are less robust as compared to closed frame type, but provide greater access for loading material as they are open in front as well as sides. Due to their appearance, they are also referred to as C-frame or gap presses as well. Closed frame type presses are used for heavier work.
The capacity of the press is indicated by the force (or tonnage), the press is capable of exerting.
A set of dies is the required tooling for working with the presses. A die set consists essentially of three parts:
(i) a punch (male tool),
(ii) a die (a female tool) and
(iii) stripper plate. The punch is fixed or
bolted to the ram and the die is fixed on the machine bed in such a manner that the two are in perfect alignment. When the punch alongwith the ram of the press moves downwards, the punch passes centrally through the die.
A die and punch assembly for making holes in metal-sheets is shown in Fig.
When the punch descends, it shears the metal-sheet. The hole punched through has the same profile as the punch. If the remaining portion of the sheet metal is the useful part, the punched out portion is thrown away as scrap. In this case, the operation is called “punching”. However, if the punch out portion is the useful part, the operation is termed “blanking” and the punched out piece is referred to as blank. The size of blank is determined by the size of hole in the die.
|Standard die set with a punch and die mounted in place|
The function of the stripper plate is to keep the sheet held down during the subsequent upward movement of the punch; otherwise, the sheet may get entangled with the punch during the upward movement of the ram and the punch.
For efficient operation and clean cut surfaces, some clearance is provided between the punch and the die. It is a function of thickness of sheet under shear and is 3–5% of thickness. Actually, after the bottom surface of the punch comes into contact with the sheet, it travels or penetrates through the sheet upto about 40% of the sheet thickness inducing higher and higher compressive stress in the sheet metal. Ultimately, the resultant shear stress at the perimeter of the blank exceeds the maximum shear strength of the material and the blanks gets sheared off through the remaining 60% of the sheet thickness. The depth of penetration-zone and shear zone are demarcated and easily seen, if the periphery of the blank is examined visually.
The shear force Vs thickness graph is typical and is shown in Fig.
The area under this curve (shown shaded) gives the energy required for the shearing operation. The die and punch are made of high quality, fine grained alloy steel. They are then heat treated to develop high hardness, wear resistance and impact-resistance.
Sometimes, when no press capable of exerting full shear force is available, the bottom surface of the punch is given a taper. This is known as “shear”. Provision of shear reduces the maximum force required as the entire periphery of the punch will not bear on the sheet metal simultaneously.
OTHER OPERATIONS PERFORMED WITH PRESSES
Apart from punching and blanking, several other useful operations are performed with the help of mechanical presses:
Some of these are listed below:
(ii) Deep drawing,
(iii) Coining, and
These operations are described briefly.
Bending means deforming a flat sheet along a straight line to form the required angle. Various sections like angles, channels etc., are formed by bending, which may then be used for fabrication of steel structures.
Three common methods of bending are illustrated in Fig.
|Types of bending dies|
The operation of bending is done with the help of a V-shaped punch, a die and press specially designed for such work. The stroke of such presses can be controlled at operator’s will and such presses are called press brakes.
In V-bending, a V-shaped punch forces the metal sheet or a flat strip into a wedge-shaped die.
The bend angle will depend upon the distance to which the punch depresses. Bends of 90° or obtuse as well at acute angle, may be produced.
Wiper bending is used only for 90° bends. Here the sheet is held firmly down on the die, while the extended portion of sheet is bent by the punch.
At the end of the bending operation, after the punch exerting the bending force is retrieved, due to elasticity, there is a tendency for the bend angle to open out. This is called “spring back”. The effect of spring back may be offset by slight overbending in the first place. Other methods to prevent spring back are bottoming and ironing. For low carbon steels spring back is 1– 2°, while for medium carbon steel it is 3–4°.
In deep drawing process, we start with a flat metal plate or sheet and convert it into cupshape by pressing the sheet in the centre with a circular punch fitting into a cup shaped die. In household kitchen, we use many vessels like deep saucepans (or BHAGONA), which are made by deep drawing process.
If the depth of cup is more than half its diameter, the process is termed as deep drawing and with a lesser depth to diameter ratio, it is called shallow drawing. Parts of various geometries and shape are made by drawing process. The deep drawing process is illustrated in Fig.
|Deep drawing operation|
During the drawing process, the sheet metal part is subjected to a complicated pattern of stress.
The portion of the blank between the die wall and punch surface is subjected to pure tension, whereas the portion lower down near the bottom is subject both to tension and bending. The portion of metal blank, which forms the flange at the top of the cup is subjected to circumferential compressive stress and buckling and becomes thicker as a result thereof. The flange has therefore to be held down by a pressure pad, otherwise, its surface will become buckled and uneven like an orange peel.
Deep drawing is a difficult operation and the material used should be specially malleable and ductile, otherwise it will crack under the induced stresses. The wall thickness of a deep drawn component does not remain uniform. The vertical walls become thinner due to tensile stresses. But the thinnest portion is around the bottom corner of the cup all around. This thinning of sheet at these locations is called “necking”.
After deep drawing, the component may be subjected to certain finishing operations like “irowing”, the object of which is to obtain more uniform wall thickness.
COINING AND EMBOSSING
Both coining and embossing operations are done ‘cold’ and mechanical presses with punch and die are used for these operations. In embossing, impressions are made on sheet metal in such a manner that the thickness of the sheet remains uniform all over even after embossing has been done. It means that if one side of the sheet is raised to form a design, there is a corresponding depression on the other side of the sheet. Basically it is a pressing operation where not much force is needed. The sheet is spread on the bottom die and the stroke of the punch is so adjusted that, when it moves down to its lowest position, it leaves a uniform clearance between the impressions carved in the punch and the die which is equal to the thickness of the sheet being embossed. The design is transferred on to the sheet by bending the sheet up or down without altering its thickness any where. Many decoration pieces with religious motifs are made in this way.
In coining process, a blank of metal which is softened by annealing process is placed between two dies containing an impression. The blank is restricted on its circumference in such a manner, that upon the two dies closing upon the blank, the material cannot flow laterally i.e., sideways. The material is only free to flow upwards (as a result of which it fills up the depressions in the upper die) and downwards (when it fills up depressions in the bottom die). The result of the coining operation is that the design engraved on the top and bottom dies gets imprinted on the corresponding faces of the blank in relief (i.e., raised material) without the size of the blank-circumference changing. Coins used as money in daily usage are manufactured in this manner. Here forces required are much higher, enough to cause plastic-flow of material. The embossing and coining processes are illustrated in Fig.
|Coining and embossing operations|
Readers may have noticed, that for all press work, the raw material is in the form of sheets or plates. Commercially, sheets and plates are available in sizes 2500 × 1000 mm or 2500 × 1250 mm. They have to be cut in smaller rectangular or square pieces, as per sizes required before other operations like, bending, punching etc. are performed. For cutting sheets into smaller pieces with straight cuts, guillotine shears, (which are also mechanical presses) are used.
Guillotine shears are provided with two straight blades of adequate length made of die steel. The blades are hardened and finished by grinding to give smooth and sharp edges. One blade is fixed to the ram (which is much longer in case of guillotine shear), while the other one is fixed to the edge of machine bed in the manner shown in Fig.
The sheet is placed on machine bed with one end projecting. It is held down by clamp. When the ram moves down, the blades shear the sheet along the blade length. Steel plates up to 10 mm thick can be sheared in this way on 250 tonne presses. No sheet-metal shop is complete without a guillotine shear.