Sheet Metal Forming Minoan gold pendant of bees

Sheet Metal Forming Minoan gold pendant of bees encircling the Sun, showing the use of granulation, from a tomb at Mallia, 17 th century BC. In the Archaeological Museum, Iráklion, Crete.

Historical Note; Sheet metal stamping was developed as a mass production technology for the production of bicycles around the 1890’s. This technology played an important role in making the system of interchangeable parts economical (perhaps for the first time).

Steps in making Hub Steps in Sprocket making

Stress Strain diagram – materials selection

Basic Sheet Forming Processes (from http: //www. menet. umn. edu/~klamecki/Forming/mainforming. html) Shearing Drawing Bending

Shear and corner press

Brake press

Finger press

Shearing Operation Force Requirement Sheet Punch D Die T Part or slug F = 0. 7 T L (UTS) T = Sheet Thickness L = Total length Sheared UTS = Ultimate Tensile Strength of material

Yield Criteria t Y/2 s Y t max = (1/2) Y Tresca t max = (2/3)1/2 Y Mises

Schematic of a Blanked Edge

Bending Force Requirement Force Punch Workpiece T T = Sheet Thickness W = Total Width Sheared (into the page) L =Span length UTS = Ultimate Tensile Strength of material Die L Engineering Strain during Bending: e = 1/((2 R/T) + 1) R = Bend radius Minimum Bend radius: R = T ((50/r) – 1) r = tensile area reduction in percent

Stress distribution through the thickness of the part Y s Elastic s Y y. Y -Y Elastic-plastic s h -Y Fully plastic

Springback • Over-bend • Bottom • Stretch

Pure Bending tension compression Bending & Stretching

Stretch Forming Loading Wrapping * source: http: //www. cyrilbath. com/sheet_process. html Pre-stretching Release

Stretch Forming

Stretch forming

Stretch Forming Force Requirement F = (YS + UTS)/2 * A F = stretch forming force (lbs) YS = material yield strength (psi) UTS = ultimate tensile strength of the material (psi) A = Cross-sectional area of the workpiece (in 2) • Example of Force Calculation Calculate the force required to stretch form a wing span having a crosssectional area of. 50 X 120” made from 2219 aluminum alloy having a yield strength of 36, 000 psi and a UTS of 52, 000 psi: F = 88000/2 * 60 = 2, 640, 000 lbs = 1320 tons Calculate the force required to shear a 10” diameter, 1/8” thick blank from mild steel with a UTS of 45, 000 psi: F = 0. 7 (. 125)(p)(10) 45, 000 = 62 tons

Auto body panels 10 - 11 panels • 3 to 5 dies each • ~$0. 5 M each • ~$20 M investment

Tooling for Automotive Stamping

Machines

Material Selection Material selection is critical in both product and process design. Formability is the central material property. This property must be balanced with other product and process considerations such as strength, weight, cost, and corrosion resistance. Auto Body Panel Progressive stamping 1010 Steel, cold-rolled. 04” sheet, custom order Double-sided Zinc clad Cost ~ $. 35 -. 45/lb UTS ~ 300 MPa YS ~ 185 MPa Elongation ~ 42% n =. 26 vs. Aerospace Example Airplane Body Panel stretch forming 2024 Aluminum, T 3 temper. 08” sheet, oversize mechanically polished Cost ~ $4. 0/lb UTS ~ 470 MPa YS ~ 325 MPa Elongation ~ 20% n =. 16

Comparison of representative Parts: Aero and Auto

Aerospace Stretch Forming Body Panel Process Parts Received Mylar Protection Applied Clad and Prime Surfaces ‘Burr’ Edges in tension Stretch Forming ‘Burr’ Edges and Inspect Hand Trim Chemical Milling Index to Block Process Flow for Automobile Door Stamping Operation Raw material Blank material starting dimensions Drawing Pierce Restrike Flange

Design: Stretch Forming vs. Stamping Stretch Forming Advantages over Stamping n n n Tighter tolerances are possible: as tight as. 0005 inches on large aircraft parts Little problem with either wrinkling or spring back Large, gently contoured parts from thin sheets Stretch forming Disadvantages over Stamping n n n Complex or sharply cornered shapes are difficult or impossible to form Material removal – blanking, punching, or trimming – requires secondary operations Requires special preparation of the free edges prior to forming

Springback

Elastic Springback Analysis y x h L b r = 1/K M M y 1. Assume plane sections remain plane: ey = - y/r 2. Assume elastic-plastic behavior for material (1) s s. Y E ey e s= E e e e � s Y e e

3. We want to construct the following Bending Moment “M” vs. curvature “ 1/r” curve M Loading MY EI EI 1/r. Y 1/R 1 Springback is measured as Permanent set is Unloading 1/R 0 – 1/R 1 1/r (2)

4. Stress distribution through the thickness of the beam Y s Elastic s Y y. Y -Y Elastic-plastic s h -Y Fully plastic

d 5. M = A y d. A y b dy h Elastic region (3) At the onset of plastic behavior = - y/r E = - h/2 r E = -Y (4) This occurs at 1/r = 2 Y / h. E = 1/r. Y (5) Y s Substitution into eqn (3) gives us the moment at on-set of yield, MY MY = - EI/r. Y = EI 2 Y / h. E = 2 IY/h (6) After this point, the M vs 1/r curve starts to “bend over. ” Note from M=0 to M=MY the curve is linear.

Y In the elastic – plastic region s (7) Note at y. Y=h/2, you get on-set at yield, M = MY And at y. Y=0, you get fully plastic moment, M = 3/2 MY y. Y

To write this in terms of M vs 1/r rather than M vs y. Y, note that the yield curvature (1/r)Y can be written as (see eqn (1)) (8) Where e. Y is the strain at yield. Also since the strain at y. Y is -e. Y, we can write (9) Combining (8) and (9) gives (10)

Substitution into (7) gives the result we seek: (11) Eqn(11) M Loading MY EI EI 1/r. Y 1/R 1 Elastic unloading curve Unloading 1/R 0 1/r (12)

Now, eqn’s (12) and (13) intersect at 1/r = 1/R 0 Hence, Rewriting and using 1/r = 2 Y / h. E, we get (13)

New developments Tailored blanks Binder force control Segmented dies Quick exchange of dies Alternative materials; cost issues

The Shape Control Concept

Conventional Tooling Tool Pallet Parking Lot

60 Ton Matched Discrete Die Press(Robinson et al, 1987) Tool Setup Actuators Press Motion Passive Tool Programmable Tool

1. 6 1. 4 50 MAX 1. 2 RMS 40 1 30 0. 8 0. 6 20 0. 4 10 0. 2 SYST EM ERROR T HRESHOLD 0 0 P 1 P 2 P 3 PART CYCLE P 4 RMS Error [x 0. 001 in. ] 60 [x 0. 001 in. ] MAXIMAL SHAPE ERROR Cylindrical Part Error Reduction

Large Scale Tool 6 feet

Stretch Forming with Reconfigurable Tool @ Northrop Grumman

Stamping and TPS: Quick Exchange of Dies Ref. Shigeo Shingo, “A Revolution in Manufacturing: The SMED System” Productivity Press. 1985 • Simplify, Organize, Standardize, • Eliminate Adjustments, • Convert Internal to External Set-Ups

Standard fixtures

Alternative materials for auto body panels

Comparison Steel Vs $0. 35/lb 0. 03 thick 7. 6 lb 40% scrap $4. 25 mat’l cost 400/hr 5 workers $18. 90/hr (Union) $0. 24 labor cost $5, 000 equipment $900, 000 tools $7. 71 unit cost at 100, 000 units Ref John Busch SMC $0. 65/lb. 0. 12 thick 7. 0 lb 6% scrap $4. 84 mat’l cost 40/hr $12. 50/hr (non-Union) $0. 63 labor cost $1, 200, 000 eqipment $250, 000 tools $7. 75 unit cost at 100, 000 units

Cost comparison between sheet steel and plastics and composites for automotive panels ref John Busch

Environment punching Vs machining hydraulic fluids and lubricants scrap energy painting, cleaning

Steel can production at Toyo Seikan See Appendix D; http: //itri. loyola. edu/ebm/

Summary Note on Historical Development Materials and Basic Mechanics Aerospace and Automotive Forming New Developments Environmental Issues Solidworks and Metal Forming your Chassis

Readings 1. “Sheet Metal Forming” Ch. 16 Kalpakjian (3 rd ed. ) 2. “Economic Criteria for Sensible Selection of Body 3. 4. 5. Panel Materials” John Busch and Jeff Dieffenbach Handout from Shigeo Shingo, The SMED System “Steps to Building a Sheet Metal Chassis for your 2. 810 Car Using Solidworks”, by Eddy Reif “Design for Sheetmetal Working”, Ch. 9 Boothroyd, Dewhurst and Knight