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Module 3_Theory of bending operation in sheet metal
- 1. Module No. 3 Theoryof Bending Lokpriya Gaikwad Mechanical - 2023 - 24 - PTD Dr. Lokpriya Gaikwad Assistant Professor Dept. of Mechanical Engineering , SIES Graduate School of Technology 1
- 2. DEFINITION • Bending refersto the operation of deforming a flat sheet around a straight axis where the neutral plane lies. • The disposition of the stresses in a bent specimen is shown. • Here, due to the applied forces, the top layers are in tension and bottom layers are in compression. Lokpriya Gaikwad 2 tension and bottom layers are in compression.
- 3. DEFINITION • Sheet bendingis defined as the straining of the metal around a straight axis. • During bending operation, the metal on the inner side of the neutral plane is compressed, and the metal on the outer side of the neutral plane is stretched. • Bending causes no change in the thickness of the sheet metal. Lokpriya Gaikwad 3 (a) Bending of sheet metal (b) both compression and tensile elongation of the metal occur in bending
- 4. TYPES OF SHEETMETAL BENDING • V-bending - performed with a V-shaped die • Edge bending - performed with a wiping die Lokpriya Gaikwad 4
- 5. TYPES OF SHEETMETAL BENDING V Bending • In V-bending, the sheet metal is bent between a V- shaped punch and die set up. • The included angles range from very obtuse to very acute values. Lokpriya Gaikwad 5 acute values. • For low production • Performed on a press brake • V-dies are simple and inexpensive
- 6. TYPES OF SHEETMETAL BENDING Edge Bending • In edge bending, cantilever loading of the sheet is seen. • A pressure pad is used to apply a force to hold the sheet against the die, while the punch forces the sheet to yield and bend over the edge of the die • For high production Lokpriya Gaikwad 6 • For high production • Pressure pad required • Dies are more complicated and costly
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- 9. BEND RADIUS • Itis the radius of curvature on the inside or concave surface of the bend. • To prevent the cracking of the material on the outer tensile surface, the bend radius cannot be made smaller than a certain value. Lokpriya Gaikwad 9 value. • Very ductile material can have zero bend radius. • i.e. they can be folded upon themselves. • To prevent damage to the punch and die, bent radius should not be less than 0.8 mm • Soft material can be bent 180˚ with a radius = or less than the stock thickness.
- 10. BEND RADIUS • Whenbending to an angle of 90, the minimum bend radius • = 2 to 5 t for C-steel = 1 t for SS Lokpriya Gaikwad 10 • = 1 t for SS • = 2 to 3.5 t for Titanium alloy • = 0.3 to 0.5 t for brass • = 0.35 t for AL
- 11. DEVELOPED LENGTH (BENDALLOWANCE) • A component is usually blanked (or sheared) before bending. • Consequently it is necessary to calculate the length before bending or the developed length of the workpiece before the blanking tool can be designed. • During bending the metal layers adjacent to the inner surface are compressed whereas the metal layers near the external surface are Lokpriya Gaikwad 11 compressed whereas the metal layers near the external surface are stretched. • The plane (layer) which is neither compressed nor stretched is called the neutral plane. • The neutral plane lies along the middle of the sheet thickness for radii exceeding twice the thickness.
- 12. DEVELOPED LENGTH (BENDALLOWANCE) • For radii less than twice the sheet thickness the neutral plane lies approximately at one-third thickness from the inner surface of the bend. • This shifting of the neutral plane should be taken into account while calculating the developed length. • The developed length should be calculated along the neutral plane. Lokpriya Gaikwad 12 plane. • This can be summarized in the following formula A = Angle of bend in degrees T = Sheet thickness R = Bend inside radius L = Development length
- 13. BEND ALLOWANCE (DEVELOPEDLENGTH) • The Bend Allowance is the arc length of the bend as measured along the neutral axis of the material. • The Bend Allowance is defined as the material that have to be added to the initial length of flat sheet in order to arrive at the length of formed part. Lokpriya Gaikwad 13 order to arrive at the length of formed part. • BA is added to the length of the straight legs of the part to get blank length. • As it was mentioned before the length of the neutral axis doesn’t change after the bending.
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- 15. 1. Calculate thedeveloped length for the component shown in the figure. L1 = 20 – (2 + 3) = 15 mm L3 = 30 – (2 + 3) = 25 mm Lokpriya Gaikwad 15 As the inside radius of bend is less than 4 mm (Twice of t) L2 = 0.0174 x 90 (3 + 0.33 x 2) = 5.749 = 5.75 mm Total length of blank = 15 + 5.75 + 25 = 45.75 mm
- 16. SPRING BACK • Atend of the bending operation • Pressure on the metal is released • Results in Elastic Recovery by the material. • This causes a decrease in the bend angle • The phenomenon is termed as spring back. • Spring back is the ER following plastic deformation Lokpriya Gaikwad 16 • Spring back is the ER following plastic deformation during bending.
- 17. SPRING BACK • Bendangle is the exterior angle from horizontal in case of edge bending. • It is between v from outside. i.e. exterior Lokpriya Gaikwad 17
- 18. SPRING BACK • SBresults in • increase in included angle of bent part • relative to included angle of tool after tool is removed. • Reason for spring back: Lokpriya Gaikwad 18 • Reason for spring back: • When bending pressure is removed • elastic energy remains in bent part, • causing it to recover partially toward its original shape
- 19. SPRING BACK Lokpriya Gaikwad 19 Figure:SB shows decrease in bend angle and an increase in bend radius: (1) during bending, the work is forced to take the radius Rb and included angle Ab' of the bending tool. (2) after punch is removed, the work springs back to radius R and angle A'
- 20. SPRING BACK For lowcarbon steel, it can be 1° to 2° medium carbon steel, it can be 3° to 4°, phosphor bronze and spring steel = 10 to 15°. To compensate for spring back Punches and mating dies are made with included Lokpriya Gaikwad 20 Punches and mating dies are made with included angles less than required in component. Result Due to this, component will be bent to greater angle than desired, but it will SB to the desired angle.
- 21. SPRING BACK • Inductile materials, • SB is much lower than in Brittle. • SB depend on the modulus of elasticity. • Amount of SB increases with greater yield strength or with the material’s strain-hardening tendency. Lokpriya Gaikwad 21 with the material’s strain-hardening tendency. • Cold working and heat treatment both increase the amount of SB in material. • SB of low-strength steel material will be smaller than that of high-strength steel • SB of aluminum will be two or three times higher.
- 22. SPRING BACK For othertypes of bending, the part is overbent by an angle equal to spring-back angle by having the face of the punch undercut or relieved. The values of spring back given above are for 90° bends Lokpriya Gaikwad 22 The values of spring back given above are for 90° bends and are usually greater for greater angles.
- 23. SPRING BACK SB dependsupon: a)Tensile strength of the material bent b)Bend radius c)Bend angle SB is directly proportional to a,b,c • Harder metal will have more degree of SB due to higher EL Lokpriya Gaikwad 23 • Harder metal will have more degree of SB due to higher EL • Thicker metal will have less degree of SB due to more plastic deformation. • Smaller radius would reduce SB by creating larger plastic deformation and could cause tearing due to higher stresses at outer surface.
- 24. Bending Force Maximum bendingforce estimated as follows: D TSwt K F bf 2 Lokpriya Gaikwad 24 where F = bending force; TS = tensile strength of sheet metal; w = part width in direction of bend axis; and t = stock thickness. For V- bending, Kbf = 1.33; for edge bending, Kbf = 0.33
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- 26. DEFECTS IN SHEETMETAL BENDING 1. Cracking • A bad crack is called a fissure. • Cracking occurs when the inside radius is too small in relation to sheet thickness. • Cracking is influenced by the tensile strength of the Lokpriya Gaikwad 26 • Cracking is influenced by the tensile strength of the work material, the directions in which it was rolled and bent, the condition of its cut face, etc. Cracks on Al sheet bent at rt angle
- 27. DEFECTS IN SHEETMETAL BENDING Cracking Following precautions are observed to prevent cracking. 1. Bend work with the sheared side (lustrous side) facing outward. 2. Pay attention to the direction, the work material is rolled. The work is apt to crack when it is bent in parallel with the Lokpriya Gaikwad 27 direction of roll. 3. In case of aluminum, the punch tip R should be greater than the sheet thickness. 4. Material with great extensibility (high ductility) is hard to crack. 5. Use a slow bending speed.
- 28. DEFECTS IN SHEETMETAL BENDING 2. Warping • Fig illustrates a phenomenon in which the bent work warps. • When a material is bent into another shape, its texture changes. • It becomes thinner on the outer Lokpriya Gaikwad 28 • It becomes thinner on the outer surface, so part of the material flows from the longitudinal direction to fill the thinner part. • As a result, the material contracts in the longitudinal direction.
- 29. DEFECTS IN SHEETMETAL BENDING Warping • This event occurring on the inner surface, together with the force on the outer surface, causes the bent work to warp in the longitudinal direction. • This phenomenon varies with the type, quality, sheet thickness and inside radius of the work. Lokpriya Gaikwad 29 thickness and inside radius of the work. • Warping is more severe with longer bending lengths. • If the bending length is shorter, it can be corrected more easily. • In the case of long products, such corrections are made by hand in most factories.
- 30. DEFECTS IN SHEETMETAL BENDING Marring • The work can be marred by slippage or scraping at the tooling joints during bending. • Marring is a serious problem with stainless steel, aluminum, vinyl-coated steel, and painted steel sheets Lokpriya Gaikwad 30 aluminum, vinyl-coated steel, and painted steel sheets which are easily scratched. • Easily scratched material is usually coated with vinyl before bending; however, the vinyl coating sometimes breaks, leaving scratches on the base metal.
- 31. DEFECTS IN SHEETMETAL DRAWING Lokpriya Gaikwad 31 Sheet Metal defects in drawn parts: (a) Wrinkling in the flange (b) Wrinkling in the wall (c) Tearing (d) Earing (e) Surface scratches.
- 32. DEFECTS IN SHEETMETAL DRAWING Lokpriya Gaikwad 32
- 33. DEFECTS IN SHEETMETAL DRAWING Wrinkling • It is a kind of buckling of the undrawn part of the blank. • Is is caused by the excessive compressive stresses if the slenderness ratio is higher than a certain value. Lokpriya Gaikwad 33 the slenderness ratio is higher than a certain value. • This may occur in the vertical walls. • If this defect occurs on the punch nose when drawing a domed cup, it is known as Puckering.
- 34. DEFECTS IN SHEETMETAL DRAWING Wrinkling (a) Wrinkling in the flange: It is a series of ridges that form radially in the undrawn flange of the workpart due to compressive Lokpriya Gaikwad 34 the workpart due to compressive buckling. (b) Wrinkling in the wall: If and when the wrinkled flange is drawn into the cup, these ridges appear in the vertical wall.
- 35. DEFECTS IN SHEETMETAL DRAWING Tearing: • Tearing is an open crack in the vertical wall. • It is usually near the base of the drawn cup. • The tearing defect usually occurs in the radius connecting the cup bottom and the wall. • This type of failure can also occur as the metal is pulled Lokpriya Gaikwad 35 • This type of failure can also occur as the metal is pulled over a sharp die corner. • It is due to high tensile stresses that cause thinning and failure of the metal at this location. • Also due to the obstruction of the metal flow in the flange.
- 36. DEFECTS IN SHEETMETAL DRAWING Earing: • Earing is the formation of irregularities in the upper edge of a deep drawn cup. • If the material is perfectly isotropic, earing doesn’t form. Lokpriya Gaikwad 36 earing doesn’t form. • As its name indicates, the formation of ears at the free edges of a deep-drawn cylindrical cup is known as earing defect. • This defect is caused by the anisotropy of the sheet metal.
- 37. DEFECTS IN SHEETMETAL DRAWING Surface scratches: Surface scratches can occur on the drawn part if the punch and die are not smooth or if lubrication is not sufficient enough. a) Surface Marks: These defects include draw marks, burnishing, step Lokpriya Gaikwad 37 These defects include draw marks, burnishing, step rings, etc. This defect is caused by improper punch-die clearance and poor lubrication. b) Surface Irregularities: This defect is caused by non-uniform yielding of metal due to non-uniform forces.