Chapter 16 Lecture 2 Sheet Metal Forming

Figure 16.14a: Major Strain and Minor Strain During stretching in sheet metal, Volume constant –  l +  w +  t = 0 Major strain always larger than minor strain –Major strain always larger than 0 –Minor strain can be either positive and negative or zero Plane strain –Minor strain is 0 –  l +  w +  t = 0, thus  l +  t = 0

Figure 16.14b: Forming-limit Diagrams (FLD) Engineering strains Although the major strain is always positive (stretching), the minor strain may be either positive or negative or zero

Safe Zone and Failure Zone for Al Although the major strain is always positive (stretching), the minor strain may be either positive or negative or zero Safe zone for aluminum Failure zone for Al

Safe Zone and Failure Zone Although the major strain is always positive (stretching), the minor strain may be either positive or negative or zero Failure zone for high strength steel Safe zone for high strength steel

Safe Zone and Failure Zone Although the major strain is always positive (stretching), the minor strain may be either positive or negative or zero Safe zone for low carbon steel Failure zone for low carbon steel

Figure 16.14b: Forming-limit Diagrams (FLD) ? ?

Figur 16.31: Steps in Manufacturing an Aluminum Can

Methods for Reducing the Diameter of Drawn Cups Conventional redrawing

Methods for Reducing the Diameter of Drawn Cups Reverse redrawing

Process: Die reduces workpiece thickness around punch Application: Tubular products, cans Key Process Variables: Friction Thickness reduction Punch velocity Tooling geometry Process sequence Punch Workpiece Die Ironing

Deep Drawing Figure (a) Schematic illustration of the deep-drawing process on a circular sheet-metal blank. The stripper ring facilitates the removal of the formed cup from the punch. (b) Process variables in deep drawing. Except for the punch force, F, all the parameters indicated in the figure are independent variables.

Video clip for Drawing process

Deep Drawing Process factors influencing the force in Deep Drawing –Blank diameter –Clearance –Material –Thickness –Blank holder force –Blank holder friction –Lubrication

Deep Drawing Operation

Drawbeads Figure (a) Schematic illustration of a draw bead. (b) Metal flow during the drawing of a box- shaped part, while using beads to control the movement of the material. (c) Deformation of circular grids in the flange in deep drawing.

Deep Drawability Limiting Drawing Ratio (LDR) –LDR = D 0 /D p D 0 : Maximum Blank diameter D p : Punch Diameter Unfortunately, Very difficult to get this ratio (time consuming) Fortunately, closely related to Normal Anisotropy

Normal Anisotropy Figure Strains on a tensile-test specimen removed from a piece of sheet metal. These strains are used in determining the normal and planar anisotropy of the sheet metal. Normal anisotropy: R =  w /  t –Remember:  l +  w +  t = 0 –Simple tension, R =1.0 Determines thinning behavior of sheet metals during stretching; important in deep- drawing operations Tensile tests determine normal anisotropy

r 0 o r 45 o r 90 o Rolling Direction Normal anisotropy (r) Average Normal Anisotropy R ave = (R 0 + 2R 45 +R 90 )/4 Average Normal Anisotropy

Average Normal Anisotropy Vs Limiting Drawing Ratio Figure The relationship between average normal anisotropy and the limiting drawing ratio for various sheet metals Limiting Drawing Ratio (LDR) –LDR = D 0 /D p D 0 : Maximum Blank diameter D p : Punch Diameter

Typical Range of Average Normal Anisotropy, R, for Various Sheet Metals

r 0 o r 45 o r 90 o Rolling Direction Normal anisotropy (r) Planar Anisotropy R ave = (R 0 -2R 45 +R 90 )/2 Planar Anisotropy (Earing Tendency)

Figure 16.45: Earing

Rolling Direction      R<0  R>0 RR RR R R h h Earing Effect of Planar Anisotropy on Earing R =  w /  t

Bending Figure Bending terminology. Note that the bend radius is measured to the inner surface of the bent part.

R/T Ratio versus % Area Reduction Equation 16.5*, minimum bend radius, “R”, –R = T [50/r a – 1] –The r a is tensile reduction area Figure Relationship between R/T ratio and tensile reduction of area for sheet metals. Note that sheet metal with a 50% tensile reduction of area can be bent over itself, in a process like the folding of a piece of paper, without cracking

Springback Figure Springback in bending. The part tends to recover elastically after ending, and its bend radius becomes larger. Under certain conditions, it is possible for the final bend angle to be smaller than the original angle (negative springback).

Reducing or eliminating Springback