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Lecture 10

Lecture 10 Sheet-metal Forming Processes.pdf

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Mechanical Engineering
Alokesh Pramanik

Lecture 10: Sheet-metal Forming Processes Thursday, 31 October 2013 7:58 AM • Bulk Deformation(mostlyhot): Rolling, forging, extrusion, drawing (wire, bar, tube), other squeezing processes • Sheet-Metal Forming (cold): Bending, shearing, Drawing (sheet, plate), stretching • Rolling: Slab or plate squeezed between rolls • Extrusion: Work squeezed through die opening • Forging: Work squeezed and shaped between dies • Wire, bar and tube drawing: Work pulled through die opening • Introduction • Classification of Major Cold-Working Processes • Sheet-metalforming processes:Bending, shearing, drawing (including stretching) • DVD on Sheet-metalshearing and bending • Overview/summaryof manufacturing processes Cold Working • • Recrystallizationtemperature varies and depends on material chemical composition,material melting temperatureand prior %CW • No heating • Good surface finish • Good dimensional control • Net or near net shape • Work (strain) hardening • Directional properties • Reproducibility • Little contamination • Higher forces, powerful equipment • Pre-cleaning required • Small deformation,low ductility • Intermediateannealing • Work hardening, residual stresses • Directional properties • Cold working Processes: •Squeezing •Rolling •Swaging •Cold forging •Swaging •Cold forging •Extrusion •Riveting •Coining •Hubbing •Thread rolling •Bar and tube drawing •Wire drawing Sheet-metalforming • Bending, shearing (cutting), drawing (sheet, plate), stretching • Operations on one sheet (0.4-6mm)or plate stock • Commonmaterial is low C steel (0.06-0.15%) • Usually cold work, some operations warm, rarely hot • Performedon various presses, except roll bending, stretching and roll forming • Significant commercialimportance Bending • Plastic deformation(straining) of metals around a linear axis with little or no change in surface area • Multiple bends can be made simultaneously,but to be classified as true bending each axis must be linear and independent of the other • If multiple bends are made with a single die, the process is called forming • When the axes of deformationare not linear or are not independent, the process is known as drawing • Deforming material around a straight axis • Little change to thickness or surface area • Permanentdeformation • Bending methods:V-bending, edge bending • Related operations:Roll bending, roll forming • V-Bending: for low production, performed on a press brake, V-dies simple and inexpensive, from acute to obtuse angles, air-bend and bottoming die • Edge bending: Also call wipe bending, for high production, cantilever loading, pressure pad required, dies morecomplicated and costly • Both compressionand tensile deformationoccur, neutral axis displaced, small thinning in centre, stretching of bend • • Bending force: wher F = bending force, TS = Tensile strength of sheet metal, w = part width in direction of bend axis, t = stock thickness, D = die dimension (eg. Width of V- channel, K = 1.33 for V-bending and K=0.33 for edge bending • Bend allowance:If the bend radius is small relative to stock thickness, the material tends to stretch during bending; it is important to estimate the amount of stretch so the final part length can meet specifications, where if R<2t, K=0.33 if R>=2t, K = 0.50 • Springback: is the increase in the included angle (decrease in the bend angle) of a bent part relative to the included angle of a forming tool after the tool is removed • Reason for springback: when bending pressure is removed,elastic energy that remains in the bent part is recovered and this causes the part to partially revert toward its original shape Roll bending Roll bending • Continuous form of 3-point bending where metal sheets, plates, pipes, extrusions, etc. are formed into curved sections using forming rolls • Roll Forming • Progressivebending of metal strip through a series of forming rolls at high speed (<80m/min) • All be bends are parallel to each other uniform in cross-section • Variety of different shapes possible, eg. Gutters, downspouts, car beams and bumpers • Shearing • Mechanical cutting of materials without the formation of chips or use of burning or metals • When two cutting blades are straight the process is called shearing • When blades are curved, processes have special names such as blanking, piercing, nothing and trimming • In terms of tool design and material behaviour, all are shearing type operations Clearance in shearing: • Clearance, c, in a shearing operation is the distance between the punch cutti
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