ME 350 – Lecture 18 – Chapter 20

1. ME 350 – Lecture 18 – Chapter 20 SHEET METALWORKING • Cutting Operations • Bending Operations • Drawing or Forming • Other Sheet Metal Forming Operations

2. Advantages of Sheet Metal Parts • High strength • Good dimensional accuracy • Good surface finish • Relatively low cost • Economical mass production for large quantities

3. Basic Types of Sheet Metal Processes • Cutting • Shearing to separate large sheets • Blanking to cut part perimeters out of sheet metal • Punching to make holes in sheet metal • Bending • Straining sheet around a straight axis • Drawing or Forming • Forming of sheet into convex or concave shapes

4. Soft Tooled v. Hard Tooled Processes • Soft Tooled (Programmable - Expendable tooling – medium to low volume applications) • Laser, Plasma, and Oxy-fuel Cutting • Bend Brake • Turret Press • Hard Tooled (Stamping Dies – high capital investment) • Standard Press Brake (manual batch) • Stage Tooling (manual line transfer) • Progressive Die (coil/strip transfer) • Transfer Presses (Robotic/automated transfer)

5. Sheet metal cutting examples: Shearing, Blanking, & Punching Sheet Metal Cutting (1) punch before contact, clearance ‘c’ between punch and die (2) punch causes material to plastically deform (3) smooth cut surface is formed (4) fracture initiated at the opposing cutting edges which separates the sheet.

6. 1. Shearing Sheet metal cutting operation along a straight line between two cutting edges • Typically used to cut large sheets (a) side view of the shearing operation; (b) front view of power shears equipped with inclined upper cutting blade.

7. 2. Blanking and Punching Blanking - sheet metal cutting to separate piece (called a blank) from surrounding stock Punching - similar to blanking except cut piece is scrap, called a slug (a) Blanking and (b) punching.

8. Clearance in Sheet Metal Cutting • Distance between punch cutting edge and die cutting edge (typically 4 - 8% of thickness) • If too small (a), fracture lines pass each other, causing double burnishing and larger force • If too large (b), metal is pinched between cutting edges and excessive burr results

9. Clearance in Sheet Metal Cutting • Recommended clearance is calculated by: c = a∙t where c = clearance; a = allowance; t = stock thickness • Allowance is determined according to type of metal: Metal group a _ aluminum alloys (1100, 5052) 0.045 aluminum alloys (2024 and 6061); brass, 0.060 soft cold rolled steel, soft stainless steel cold rolled steel, stainless steel, (hard & half-hard) 0.075

10. Punch and Die Sizes • For a round blank of dia. Db and clearance c: • Blanking punch diameter = Db ‑ 2c • Blanking die diameter = Db • For a round hole of dia. Dh and clearance c: • Hole punch diameter = Dh • Hole die diameter = Dh + 2c

11. Angular Clearance Purpose: allows slug or blank to drop through die • Typical values: 0.25 to 1.5 on each side Cutting Force: F = S t L where S = shear strength; t = stock thickness, L = length of cut edge (disregard ‘c’) 1 ton ≈ 8896 N

12. Progressive Die

13. Types of Sheet Metal Bending • V‑bending - performed with a V‑shaped die • Performed on a press brake • V-dies are simple and inexpensive • Edge bending - performed with a wiping die • Pressure pad required • Dies are more complicated and costly

14. Stretching during Bending • If bend radius is small relative to stock thickness, metal tends to stretch during bending • Important to estimate amount of stretching, so final part length = specified dimension • Problem: to determine the length of neutral axis of the part before bending

15. Bend Allowance Formula L1 where Ab = bend allowance;  = bend angle; R= bend radius; t = stock thickness; Kba is factor to estimate stretching If R < 2t, Kba = 0.33 If R 2t, Kba = 0.50 L = L1 + Ab + L2 L2

16. Springback Increase in included angle of bent part relative to included angle of forming tool after tool is removed • Reason for springback: • When bending pressure is removed, elastic energy remains in bent part, (1) during bending, the work is forced to take radius Rb and included angle b' of the bending tool, (2) after punch is removed, the work springs back to radius R and angle ‘.

17. Bending Force Maximum bending force estimated as follows: where F = bending force; TS = tensile strength; w = part width; t = stock thickness. D = die opening For V- bending, Kbf = 1.33; For edge bending, Kbf = 0.33

18. Drawing • Sheet metal forming to make cup‑shaped, box‑shaped, or other hollow‑shaped parts • clearance c = 1.1 t • where t = stock thickness • In other words, clearance is about 10% greater than stock thickness

19. Tests of Drawing Feasibility • Drawing ratio • where Db = blank diameter; and Dp = punch diameter • Indicates severity of a given drawing operation • Upper limit: DR  2.0 • Reduction • Value of r should be less than 0.50 for a cylinder • Thickness-to-diameter ratio • Thickness of starting blank divided by blank diameter • Desirable for t/Db ratio to be greater than 1% • As t/Db decreases, tendency for wrinkling increases

20. Blank Size Determination • For final dimensions of drawn shape to be correct, starting blank diameter Db must be right • Solve for Db by setting starting sheet metal blank volume = final product volume • To facilitate calculation, assume negligible thinning of part wall

21. Other Sheet Metal Forming • Ironing • Embossing • The Guerin process • Stretch forming • Roll bending • Roll forming • Spinning

22. Ironing • Makes wall thickness of cylindrical cup more uniform Ironing to achieve more uniform wall thickness in a drawn cup: (1) start of process; (2) during process. Note thinning and elongation of walls.

23. Embossing Creates indentations in sheet, such as raised (or indented) lettering, or strengthening ribs Embossing: (a) cross‑section of punch and die configuration during pressing; (b) finished part with embossed ribs.

24. Guerin Process Advantages: • Low tooling cost • Rubber pad can be used with different form blocks • Suited for prototype production

25. Stretch Forming Sheet metal is stretched and simultaneously bent: φ • F = stretching force; L = length in direction perpendicular to stretching (e.g. width ); t = thickness; Yf = flow stress • Initially assume ε = 0.002, calculate Yf = K εn • As ‘length’ is increased ‘t’ is decreased • Find tf by conservation of volume: wotoLo= wf ∙tf ∙Lf • Die force Fdie can be determined by balancing vertical force components: Fdie = 2F sin φ

26. Roll Bending Large metal sheets and plates are formed into curved sections using rolls

27. Roll Forming Continuous bending process in which opposing rolls produce long sections of formed shapes from coil or strip stock

28. Spinning Metal forming process in which an axially symmetric part is gradually shaped over a rotating mandrel using a rounded tool or roller α Conventional spinning to = tf, Final diameter? Shear spinning tf = to sin α Final diameter?