© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Manufacturing Processes for Engineering Materials (5 th Edition in SI Units) Chapter 7: Sheet Metal Forming Processes Shearing-Blanking Bending Spinning Deep Drawing

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Sheet-Metal Characteristics Forming of sheet metals is carried out by tensile forces in the plane of the sheet. Influencing parameters include: 1.Elongation 2.Yield-point elongation 3.Anisotropy 4.Grain size 5.Residual stresses 6.Springback 7.Wrinkling

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Elongation Tension undergoes uniform elongation till UTS and begins to neck. True-stress–true-strain curve represented by Necking takes place at an angle to the direction of tension. strain at which necking begins

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Elongation Necking is localized depends on the strain-rate sensitivity, m, of the material. Parameters that affect the sheet-metal forming process: 1. Yield-point elongation Yield-point elongation involving upper and lower yield points

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Elongation 2. Anisotropy Acquired during the thermo-mechanical processing history of the sheet. 3. Grain size Important for mechanical properties and surface appearance for materials. 4. Residual stresses Due to non-uniform deformation that sheet undergoes during forming.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Elongation 5. Springback Subjected to relatively small strains during forming. 6. Wrinkling Circumferential compressive stresses that develop in the flange. 7. Coated sheet Zinc is used as a coating on sheet steel to protect sheet from corrosion.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Shearing Cutting sheet metal, plates, bars, and tubing into pieces using punch and die subjecting to shear stress. Variables are punch force, speed, edge condition of the sheet, materials, corner radii, punch-die clearance and lubrication.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Shearing As clearance increases, edges rougher and deformation zone larger. For tight clearance, die wears quickly. Deburring process

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Shearing operations 2. Fine blanking Can produce very smooth and square edges.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Shearing Punch force The punch force is product of shear strength of sheet metal and cross-sectional area being sheared. Maximum punch force is Due to plastic deformation, friction, and cracks, the punch-force vs. stroke curves can take on various shapes. UTS = ultimate tensile strength t = thickness L = total length of the sheared edge

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Estimate the force required in punching a 25-mm diameter hole through a 1.8-mm-thick 5052-O aluminium sheet at room temperature. Solution UTS for this alloy is 190 MPa. Example 7.1 Calculation of maximum punch force

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Shearing operations Common shearing operations are: 1. Die cutting Parts produced have various uses: a)Perforating b)Parting c)Notching d)Slitting e)Lancing

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Shearing operations 3. Slitting. Carried out with a pair of circular blades. 2 types of slitting equipment: 1.driven type, the blades are powered 2.pull-through type, the strip is pulled through idling blades.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Bending of Sheet and Plate Used to form parts and impart stiffness. Bend allowance is the length of neutral axis in bend area and used to find blank length for a bent part. Bend allowance is given as a = bend angle R = bend radius k = constant t = sheet thickness

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Minimum bend radius Strains at outer and inner fibers are equal in magnitude. Due to shifting of neutral axis towards the inner surface, the length of bend is in inner region. Minimum bend radii for various materials have been determined experimentally.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Minimum bend radius Studies shows that there is a relationship between the minimum R/t ratio and a mechanical property of the material. Factors affecting bendability Bendability increased by increasing its tensile reduction of area. As length increases, minimum bend radius increases.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Springback In bending, recovery is called springback.-elastic zone Relationship for pure bending is From this relationship, K s is defined as Approximate formula to estimate springback is (Handbook)

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials A 20-gage ( cm) steel sheet is bent to a radius of 1.27 cm. Assuming that its yield stress is MPa, calculate (a) the radius of the part after it is bent, and (b) the required bend angle to achieve a 1.57 rad bend after springback has occurred. Solution a.The appropriate formula is and Hence, b. Required bend angle is Example 7.2 Estimating springback

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Springback Negative springback Bend angle becomes larger after the bend is completed and load removed. Compensation for springback 1.Overbending 2.Coining 3.Stretch bending 4.Carried out at elevated temperatures

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Forces Bending force is a function of the material’s strength, length and thickness of the part. General expression for the maximum bending force is Common bending operations 1.Press-brake forming 2.Other bending operations 3.Beading

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Tube bending Tubes can be plugged with various flexible internal mandrels.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Miscellaneous Forming Processes Stretch forming Sheet metal is clamped and stretched over a die or form block. Make aircraft-wing skin panels, automobile door panels, and window frames. Cannot produce parts with sharp contours or re-entrant corners. Used for low-volume production, it is versatile and economical.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials A 38.1-cm-long sheet with a cross sectional area of cm 2 is stretched with a force, F, until α = 0.35 rad. The material has a true-stress–true strain curve σ = 100,000 e 0.3. (a) Find the total work done, ignoring end effects and bending. (b) What is α max before necking begins? Solution a)The true strain is Work done per unit volume is Since volume of workpiece is The work done is Example 7.4 Work done in stretch forming

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Solution b)The necking limit for uniaxial tension is From similar triangles, we obtain For necking to begin, Example 7.4 Work done in stretch forming

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Miscellaneous Forming Processes Bulging Process involves placing a tubular part in a split female die and expanding it with a rubber or polyurethane plug Formability enhanced by compressive stresses longitudinal to the parts.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Spinning Conventional spinning A circular sheet metal is held against a rotating mandrel where the tool deforms and shapes it over the mandrel. Tooling costs are low and economical for relatively small production runs only. Shear spinning Axisymmetric conical is generated where the diameter of the part remains constant.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Spinning Shear spinning 2 rollers are preferred to balance the radial forces and maintain dimensional accuracy. The thickness of the spun part is For ideal case in shear spinning of a cone, True strain is Max spinning reduction in thickness is F t = tangential force u = specific energy of deformation

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Spinning Shear spinning For an ideal case in shear spinning of a cone, u is the area under the true-stress–true-strain curve, As the thickness decreases, the max spinning reduction in thickness is

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Spinning Tube spinning Tubes or pipes reduce thickness by spinning them on a cylindrical mandrel using rollers. Ideal tangential forward force is Friction will double the actual force using the equation above.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials High-Energy-Rate Forming Explosive forming The peak pressure generated in water is given by the expression Compressibility of the energy-transmitting medium and acoustic impedance is important for peak pressure. p = peak pressure K = constant for type of explosive W = weight of the explosive R = distance of explosive from workpiece a = constant

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Calculate the peak pressure in water for kg of TNT at a standoff of m. Is this pressure sufficiently high for forming sheet metals? Solution Peak pressure is This pressure is sufficiently high to form sheet metals. Example 7.5 Peak pressure in explosive forming

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Deep Drawing Flat sheet-metal blank is pressed, using punch, into the die cavity. Bank is held in place with a blankholder under a certain force.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Deep Drawing Variables in deep drawing are: 1.Properties of the sheet metal. 2.Ratio of the blank diameter to the punch diameter. 3.Sheet thickness. 4.Clearance between the punch and the die. 5.Corner radii of the punch and die. 6.Blankholder force. 7.Speed of the punch. 8.Friction at the punch, die and workpiece interfaces.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Deep Drawing During the deep-drawing, workpiece is subjected to the states of stress. Important to know how much pure drawing and stretching is taking place. Stresses increase with an increasing D o /D p ratio and can eventually lead to failure.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Deep drawability (limiting drawing ratio) LDR is defined as the max ratio of blank diameter to punch diameter that can be drawn without failure, D o /D p. Normal anisotropy of the sheet metal is Based on volume constancy, it reduce to Average R value is where the subscripts refer to angular orientation

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Estimate the limiting drawing ratio (LDR) that you would expect from a sheet metal that, when stretched by 23% in length, decreases in thickness by 10%. Solution From volume constancy, From the information given, we obtain Hence, Thus, Example 7.7 Estimating the limiting drawing ratio

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Show that, assuming no thickness change of the sheet during drawing, the theoretical limiting drawing ratio is Solution The diametral change from a blank to a cup involves a true strain of Drawing stress is, in the limit Consequently, Example 7.8 Theoretical limiting drawing ratio

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Deep drawability (limiting drawing ratio) Earing Planar anisotropy causes ears to form in drawn cups. The controlling parameters: 1.alloying elements 2.processing temperatures 3.annealing cycles after processing 4.thickness reduction in rolling 5.cross (biaxial) rolling of plates to make sheets

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials A steel sheet has R values of 0.9, 1.3, and 1.9 for the 0 rad, rad, and 1.57 rad directions to rolling, respectively. For a round blank 100 mm in diameter, estimate the smallest cup diameter to which it can be drawn. Will ears form during this operation? Solution Substituting the given values To determine whether or not earing will occur in this operation, Ears will form in deep drawing this material. Example 7.9 Estimating cup diameter and earing

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Deep drawability (limiting drawing ratio) Maximum punch force Work consists of deformation, redundant, frictional and ironing. Approximate formula for the maximum punch force is The punch force is supported by the cup wall. Tearing occurs when force is excessive.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Deep-Drawing practice Considerations in deep drawing: 1.Clearances and radii 2.Draw beads 3.Blankholder pressure 4.Redrawing 5.Drawing without a blankholder 6.Tooling and equipment 7.Lubrication

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Formability of Sheet Metals and Modeling 3 factors that influence formability: 1.properties of sheet metal 2.friction and lubrication 3.characteristics of the equipment, tools and dies used Testing for formability: 1.Tension tests 2.Cupping tests 3.Bulge test 4.Forming-limit diagrams 5.Limiting dome-height test

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials A thin-walled spherical shell made of an aluminium alloy is being expanded by internal pressure. If the original shell diameter is 200 mm, what is the maximum diameter to which it can safely be expanded? Solution Max allowable engineering strain is about 40%, thus, Example 7.10 Estimating diameter of expansion

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Dent resistance of sheet-metal parts A dent is a small, permanent, biaxial deformation. The factors in dent resistance is yield stress, thickness, and shape of the panel. Dent resistance is expressed as S is the panel stiffness defined as Denting needs higher energy levels as compared to static conditions. Dynamic forces cause more localized dents than static forces.

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Equipment for Sheet-Metal Forming Operations consists of mechanical, hydraulic, pneumatic, or pneumatic-hydraulic presses. Press selection includes the considerations of: 1.types of forming operation, 2.size and shape of the parts 3.length of stroke of the slides 4.number of strokes per minute 5.press speed 6.shut height

© 2008 Pearson Education South Asia Pte Ltd Chapter 7: Sheet-Metal Forming Processes Manufacturing Processes for Engineering Materials Design Considerations Guidelines for design issues of sheet-metal forming operations: 1.Blank design 2.Bending 3.Stamping and progressive-die operations 4.Deep drawing