PRESS WORKING OPERATIONS(SHEET METAL)

THEORY OF CUTTING When cutting sheet metal in a die the forces applied to the sheet by the punch and die are basically shear forces, that is, equal and opposite forces spaced at a small distance apart on the metal and produce the cutting. The cutting or separation of the sheet metal is effected through the following stages of shearing Roll over Penetration Fracture From both punch side and Die side simultaneously The spacing or the small distance of the two shearing planes is called clearance.It will be generally about 8 - 10% of metal thickness for MS Sheets.

SHEET METAL CUTTING TERMINOLOGY

VARIOUS CUTTING OPERATIONS

THEORY OF BENDING AND SPRING BACK Bending is the process of folding a sheet about a straight line axis which lies in the neutral plane. Bends are made in sheet metal to gain rigidity, to produce a part of desired shape & perform a particular function etc. The cross section of the bend inward from neutral axis is in compression, outward from neutral axis is in tension as shown in the fig. SPRING BACK During bending the metal nearer to the neutral axis is stressed to the values below the elastic limit.This phenomenon creates a narrow elastic band on both sides of the neutral axis.The metal further away from the neutral axis is stressed beyond the yield strength and is plastically deformed and permanently set. When the bending force on the metal is released the elastic band tries to return to the original flat condition but cannot return fully due to the restrictions of the plastically deformed zones . Some slight return does occur as the elastic and plastic zones come to an equilibrium and this return is known as spring back. The amount of spring back mainly depends on the ratio of bending radius to stock thickness. Spring back can be reduced by over bending, Bottoming or stretch bending.

VARIOUS BENDING OPERATIONS

DESIGN CONSIDERATIONS FOR BENDING

DESIGN CONSIDERATIONS FOR HEMMING

THEORY OF FORMING Forming is the process in which the shape of the punch and die is directly reproduced in the metal with little or no metal flow. Forming, Bending or drawing actions may be combined in a die and is classified as Form die or Draw die depending on the dominant action of process The decision to use a form die instead of a Draw die will depend largely on the complexity of the shape & geometrical criteria. The use of draw die may be indicated, if the form die would cause the metal to tear because of excessive tensile strain(stretch) or form objectionable wrinkles because of excess crowding of metal etc defects.

VARIOUS TYPES OF FORMING OPERATIONS

VARIOUS TYPES OF BEADS

DESIGN CONSIDERATIONS FOR BEADS

THEORY OF DRAWING Drawing is a process in which the punch causes a flat , precut metal blank in to the die cavity to assume the shape of seamless hollow vessel without excessive wrinkling, thinning, or fracturing. METAL FLOW IN DRAWING PROCESS when the punch of draw die forces a portion of metal blank through the bore of draw ring, different forces such as radial tension, circumfrential compression and bending & frictional forces come in to action as shown in the fig.These forces cause a complicated plastic flow of the material in the blank. The volume and the thickness of the component remain constant and the final shape of the cup will be similar to contour of the punch. The progressive stages of cupping are schematically shown in fig. After a small stroke of the punch, cupping stage A, the metal elements 2, 3, 4, & 5 of the blank move radially toward the center of the blank. The flow of these elements go on till the final stage C of the cup is reached. Thus by the end of the draw of the cup area 1 is unchanged in shape and size in the bottom of the cup. The areas 2,3 & 4 change from the shape of angular segments to longer parallel-sided shapes and area 5 also changed in it’s size and shape. Due to this metal flow phenomena in Drawing operation, Blank holding pressure is to be applied on the blank in such a way that the metal flow will be a controlled one so as to avoid wrinkling, tearing, thinning etc.

DESIGN CONSIDERATIONS FOR CYLINDRICAL CUP DRAWING

DESIGN CONSIDERATIONS FOR RECTANGULAR SHELL DRAWING

DESIGN CONSIDERATIONS FOR IRREGULAR SHAPE DRAWING