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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 BLANKING
It is the operation of cutting or shearing a piece out of sheet to a predetermined contour
PIERCING
It is the operation of producing the holes of round or contoured shape in the blanks or in formed parts.
NOTCHING
In Notching operation a small piece or pieces of metal are removed from the edges/ corners of a Strip or a Blank
TRIMMING
It is the operation of cutting off the excess metal after the Drawing or Forming operation
LANCING
It is the operation of slitting on three sides and opening out by bending the lug about an axis on the uncut fourth side with in the blank.
THEORY OF BENDING AND SPRING BACK •
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BENDING 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 SIMLPE BENDING
The operation of Folding the sheet about a straight-line axis is called simple bending
FLANGING
It is similar to above in which the height of bend is shorter compared to the overall size of the part. • It strengthens the edges of sheet metal parts • It provides flanges required for assembling parts by spot welding or any other joining processes.
HEMMING
Hemming is an operation in which the edge 0 of a component gets folded by180 . • It improves the rigidity of the edge • It facilitates joining of two parts as in the case of Bonnetassy. /Doorassy.
DESIGN CONSIDERATIONS FOR BENDING Radius of bend ≥ T Minimum height of bend ≥ 3T
Relief Notches for bend
DESIGN CONSIDERATIONS FOR HEMMING Where appropriate provide an offset • To minimise area of fitting • To give additional rigidity to inner panel • To reduce potential assembly distortion
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Height of flange after 0 +/- 1.0 90 flanging = 9.0 ( except at corners and feature lines)
• Maximum permissible radius of bend 0 during 90 flanging = 0.5T 0 • Preferred radius of bend during 90 flanging = 0 (or as minimum as possible)
THEORY OF FORMING •
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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
ANGULAR BEADS
V-BEADS
BEAKS
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 • Empirical Rule to decide required no. of Draws for Cylindrical cup (Where h = inside height of shell d = Mean dia. Of shell) • Drawing Force for Cylindrical Cup (Fd)
If λ =h/d No. of Draws Up to 0.75 --------- 1 0.75 -- 1.5 --------- 2 1.5 -- 3.0 --------- 3 3.0 -- 4.5 --------- 4 Fd = Π d t (S u + Sy)/2 Kg. Where d = Punch diameter (mm.) 2 Su = Ultimate Tensile strength (Kg/mm ) 2 Sy = Yield strength (Kg/mm ) T = Thickness of sheet. (mm.)
• Blank Holder Force( Fb)
Fb = 1/3 of Drawing force(Approx.)
• Empirical Rule for Punch and Die radii.
Punch Radius = Die radius =
5t 6t
DESIGN CONSIDERATIONS FOR RECTANGULAR SHELL DRAWING • Empirical Formulae to decide If λ =h/r No. of Draws required no. of Draws for Up to 7 --------- 1 Rectangular shell 7 -- 13 --------- 2 (Where h = inside height shell 13 -- 18 --------- 3 r = Corner radius Of shell) 18 -- 24 --------- 4 • Drawing Force Rectangular Shell (Fr)
Fr = t Su (2Πr + C1+L C2 )
Kg.
Where 2 Su = Ultimate Tensile strength (Kg/mm ) T = Thickness of sheet. (mm.) r = Corner radius Of shell. (mm.) L = Total length of straight sides of rectangular shell C1 = 0.5 – 2(lower values for shallow draw Higher values for h/r > 0 C2 = 0.2 – 0.3 ( For easy to sever Draw Conditions)
DESIGN CONSIDERATIONS FOR IRREGULAR SHAPE DRAWING • Empirical Formulae to If e% < 5 , Part can be produced by Forming analyze the formability If 5 < e% < 30 , Part requires Draw of Irregular shaped If e% > 30 , Part cannot be formed shells If ∆e% > 5 Wrinkles tend to be shown
• General guide lines related to Drawing Operation
Where e% = Percentage of elongation = (L – L 0) x 100 L0 = Blank length before forming L = Blank length after forming ∆e% = Difference in percentage of elongation between neighboring sections • Draft angle on draw panel wall = 2 0 Min. • Maximum permissible material thinning = 10% • Material utilization Target = 70% and above • Drawn corners to be achieved in one Draw
