Theory of bending

TOOL DESIGN

UNIT 4

THEORY OF BENDING

BY

PURNESH ALONI

ROLL NO. 11

MTECH(PRODUCTION ENGG)DEPARTMENT OF

MECHANICAL ENGINEERING

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CONTENTS

• Introduction• Types of bending• Spring back in bending• Compensation for spring back• Variations in bending operations• Design principles

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INTRODUCTION

• BENDING :- Bending is defined as the straining of metal around a straight axis. The metal on the inside of the neutral axis is compressed. The metal on the outside of the neutral axis is stretched.

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V-bending :

• In V-bending the sheet metal blank is bent between a V-shaped punch and die

• The figure below shows a front view and isometric view of a V-bending setup with the arrows indicating the direction of the applied force

TYPES OF BENDING

Figure courtesy of Engineering Research Center for Net Shape Manufacturing

Edge or wipe bending:• Edge or wipe bending involves cantilever loading of the

material• A pressure pad is used to apply a Force to hold the blank

against the die, while the punch forces the workpiece to yield and bend over the edge of the die

TYPES OF BENDING

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Spring back in bending

• Spring back is defined as the increase in included angle of the bent part relative to the included angle of the forming tool after the tool is removed.

• When the bending stress is removed at the end of the deformation process, elastic energy remains in the bent part causing it to partially recover to its original shape. In bending, this elastic recovery is called springback.

• It increases with decreasing the modulus of elasticity, E, and increasing the yield strength, Y, of a material.

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Spring back in bending

αi: bend angle before springback αf: bend angle after springback Ri: bend radius before springback Rf: bend radius after springback

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Spring back in bending

In order to estimate springback, the following formulacan be used:

Manufacturing processes by S. Kalpakjian and S. Schmid

where:Ri, Rf: initial and final bend radii respectively

Y: Yield strengthE: Young’s modulust: Sheet thickness

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Compensation for Springback

Overbending

• When overbending is used in V-bending (for example), the punch angle and radius are fabricated slightly smaller than the specified angle and raduis of the final part. This way the material can “springback” to the desired value.

Bottoming (coining)

• Bottoming involves squeezing the part at the end of the stroke, thus plastically deforming it in the bend region.

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Variation in Bending Operations

• Flanging is a bending operation in which the edge of a sheet metal is bent at a 90° angle to form a rim or flange. It is often used to strengthen or stiffen sheet metal. The flange can be straight, or it can involve stretching or shrinking as shown in the figure below:

(a)Straight flanging(b)Stretch flanging(c)Shrink flanging

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Variation in Flanging

In stretch flanging the curvature of the bending line is concave and the metal is circumferentially stretched, i.e., A > B. The flange undergoes thinning in stretch flanging.

In shrink flanging the curvature of the bending line is convex and the material is circumferentially compressed, i.e., A < B. The material undergoes thickening in shrink flanging.

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Variations in Bending Operation

•Hemming involves bending the edge of the sheet over onto itself in more than one bending step. This process is used to eliminate sharp edges, increase stiffness, and improve appearance, such as the edges in car doors.•Seaming is a bending operation in which two sheet metal edges are joined together.•Curling (or beading) forms the edges of the part into a roll. Curling is also used for safety, strength, and aesthetics.

(a)Hemming(b)Seaming(c)Curling

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Design Principles

BEND RADIUS: It is defined as the radius of curvature on the inside or concave surface of bendWhen bending to an angle of 900 ,the minimum bend radius = 2 to 5t for C-steel = 1t for stainless steel = 2 to 3.5t for Ti alloys = 0.3 to 0.5 t for brass = 0.35t for Al

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Design Principles

BENDING ALLOWANCE: To calculate the blank length for bending ,the length of material in the curved section or bend area has to be calculated. This length in the bend area which will be more than corresponding length before bending is call Bending Allowance. B.A,mm B.A = Bending allowance along neutral axis, mm θ = Bend angle in degree r = inside radius of bend, mm k = Distance of neutral axis from the inside surface of bend.

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Design Principles

STOCK MATERIAL Where r<2t Where r>2t1.MILD STEEL (i)Edge Bending (ii) V-bending, U-bending

0.2t 0.33t

0.33t 0.5t

2.SOFT COPPER, SOFT BRASS,SOFT TO HALF HARD ALUMINIUM

0.33t 0.5t

3. HALF HARD COPPER ,HALF HARD BRASS, HALF HARD ALUMINIUM

0.4t 0.5t

4.HARD COPPER, HARD BRASS , HARD STEEL

0.5t 0.5t

Values of k:-

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Design Principles

WIDTH OF DIE OPENING :- The width of die opening controls spanking area, the amount of press exertion and the length off effective press stroke.

w= R1+R2+c – V-bending

w=R1+R2+c+t -Edge bending

R1 - Punch edge radius

R2 - Die edge radius

c - Clearance=t

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Design Principles

BENDING FORCE : Bending force depends upon the thickness of the stock, the length of the bend , the width of die operation and the type of bend. F, N L=Total length of bend = Ultimate tensile strength of material ,MPA w= Width of die opening, mm t=m Thickness of blank, mm K= Die-opening factor

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Design Principles

VALUES OF DIE-OPENING FACTOR• For V-bending K=1.20 for w=16t =1.33 for w=8t• For U-bending K=0.67• For Edge bending K=0.33

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