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Design for Stamping (DFS)
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  1. Design for Stamping (DFS) Terry Sizemore Edits from Mark Courtright, Dwayne Mattison, Ravi Ranganathan, Mac Lunn, Rolf Glaser

  2. References • Eary and Reed: Techniques of Pressworking Sheet Metal, 2nd ed. Prentice Hall • Boothroyd, Dewhurst, Knight: Product Design for Manufacture and Assembly, 2nd ed. Marcel Decker • Brallia: Design for Manufacturability Handbook, 2nd ed., McGraw Hill • Sizemore: EMU MFG 316 Lecture Notes • Ulrich and Eppinger • SME Journal of Manufacturing Systems Vol23, No3, 2004 (reference 1)

  3. Design for Stamping (DFS) • Assumptions • DFS will be “Design for Stamping” in this lecture • DFS applies to sheet materials from 0.026 to 0.1875 inches in thickness (0.88-4.76mm) • Successful use of DFS is measured by: • Material utilization percentage • Improvement in quality by decreasing Quality Loss (Taguchi’s quality loss function) • $$$’s of Die Cost Avoidance • Number of processes eliminated • Number reduced parts due to adding “Free” features • Number of re-orientations eliminated • This list of metrics can be applied, not all are equal and you have to consider compromise for your design

  4. Product Development ProcessUlrich and Eppenger, 1995 Mission Statement Design for Stamping Concept Development System Design Detail Design Product Launch Testing/ Refinement Production Ramp up

  5. Agenda • Cutting • Theory of Cutting Sheet Metal • Forces for Cutting • Die Cutting Operations • Properties of Metals (stress strain curve, spring back, etc) • Forming • Bending • Embossing and Miscellaneous Forming • Drawing • Tooling • Assembly • Design Practices

  6. Theory of Cutting • Assumptions • Theory of Cutting applies to the trimming of forgings, extrusions and castings and the cutting of bar stock • Sheet metal is material <0.125” thick Plate is material >0.125” thick • Does not apply to brittle materials (i.e. magnesium)

  7. Analysis of Cutting • Forces applied by the punch and die are shearing forces, which apply a shearing stress to the material until fracture • Material deformation occurs in the plane of shear • As the tool wears and the clearance between the punch and die grow the material will begin to experience more tensile deformation and less shear deformation prior to fracture

  8. Characteristics of a Die Cut Edge • Roll Over – Flow of material around the punch and die • The larger the clearance the greater the roll over • Burnish – The rubbed or “cut” portion of the edge • The sharper the punch the wider the burnish • Fracture – The angled surface where the material separates from the parent material • Burr – The very sharp projection caused by a dull cutting on the punch or die. General Rules: The more dull the tool the greater the burr. The softer the material the greater the burr. *These characteristics are evident on both the hole and slug

  9. Penetration Roll Over + Burnish = Penetration

  10. Percent Penetrations E.V. crane, Plastic Working in Presses, John Wiley and Sons, Inc., New York, 1948, p. 36

  11. Die and Punch Clearance Proper Clearance • Too Big – Blank ends up with roll-over and/or a crown effect. • Too Small – Results in large stripping force and secondary shear. Secondary shear is when the fracture propagating from the punch misses the fracture propagating from the die. • When proper clearance exists, the fractures meetwhich yields a preferable break edge.

  12. Die and Punch Clearance • Force Curves – A common tool for analyzing various clearance conditions is by using strain gages or other transducers to create force vs. displacement curves. Poor clearance conditions result in less than ideal force curves.

  13. Other Characteristics • Dish Distortion • Spacing Distortion – When holes are punched next to each other in sequence distortion in the circularity and position of the first hole will occur. If possible punch closely proximate holes simultaneously. See attached table for recommended design practices. (insert figure and chart from page 20)

  14. Forces for Cutting For Cutting: • Ferrous stamping materials shear strength is 70-80% ultimate tensile strength • Force=Shear Strength*Perimeter of Cut*Thickness • When calculating tonnage required it is recommended that ultimate tensile strength be used instead of shear strength to compensate for die wear. Tonnage=(UTS*Perimeter*Thickness)/2000

  15. Forces for Cutting • Take caution in what number is used for shear strength or UTS. Consideration must be made for prior operations that may affect the material properties. • Work Hardening • Annealing or Tempering • Other processes that affect the mechanical properties of the material

  16. Work and Energy • In terms of metal cutting: Work=average force*distance • Force: Since the force/displacement curve for cutting sheet metal is nearly rectangular use the maximum force prior to fracture as the average force • Distance: The distance used in this calculation is percent penetration (see earlier slide) multiplied by material thickness. • This calculation assumes no secondary shear, which will require additional energy during cutting.

  17. Example 10 inch diameter aluminum blank made from .032 inch 3003 aluminum (3003 UTS is 11000 psi) Force=(11000)(3.14)(10)(.032) =11053 lbs Tonnage=11053/2000=5.5 tons Work=(5.500)(.600)(.032)=.1056 inch tons* (Need to insert penetration chart page 10) *Most press flywheels are rated in inch ton capacity

  18. Cutting Operations • Blanking – Material removed is the work-piece • Perforating – Material removed is scrap • Piercing – Material removed is scrap • Lancing – No metal removed, bending and cutting • Cut-off/Parting- Separating parts or reducing scrap strip size • Notching – Removing material from the outer edges of the strip • Shaving – Removing the break edge • Trimming – Removing “Flash” from drawn parts

  19. Blanking

  20. Bending Bending- a metal forming process in which a force is applied to a piece of sheet metal, causing it to bend at an angle and form the desired shape. A bending operation causes deformation along one axis, but a sequence of several different operations can be performed to create a complex part.

  21. Perforating *

  22. Piercing

  23. Lancing

  24. Cut-Off/Parting

  25. Notching

  26. Shaving

  27. Shaving The shaving process is a finish operation where a small amount of metal is sheared away from an already blanked part. Its main purpose is to obtain better dimensional accuracy,

  28. Trimming Punching away excess material from the perimeter of a part, such as trimming the flange from a drawn cup.

  29. Slitting Cutting straight lines in the sheet. No scrap material is produced.

  30. Perforating Punching a close arrangement of a large number of holes in a single operation.

  31. Dinking A specialized form of piercing used for punching soft metals. A hollow punch, called a dinking die, with beveled, sharpened edges presses the sheet into a block of wood or soft metal.

  32. Parting Separating a part from the remaining sheet, by punching away the material between parts.

  33. Embossing Embossing is a metal forming process for producing raised or sunken designs or relief in sheet material by means of matched male and female roller dies.

  34. Drawing Deep drawing is a metal forming process in which sheet metal is stretched into the desired part shape. A tool pushes downward on the sheet metal, forcing it into a die cavity in the shape of the desired part.

  35. Hydro-forming Hydro-forming is a manufacturing process where fluid pressure is applied to a ductile metallic blank to form a desired component shape

  36. Agenda • Cutting • Theory of Cutting Sheet Metal • Forces for Cutting • Die Cutting Operations • Properties of Metals (stress strain curve, spring back, etc) • Forming • Bending • Embossing and Miscellaneous Forming • Drawing • Tooling • Assembly • Design Practices

  37. Stress/Strain Curves

  38. Geology of Stress Strain Curve • Elastic Region • Yield Point • Necking Region • Ultimate Point • Elongation • Spring Back

  39. Spring Back Spring-back is the material’s tendency to return to its original shape after forming. This must be anticipated in both the tooling and part design. Darts can be added in bend Radii to help the panel retain its shape. Designer should also anticipate that 90° flanges Will not be possible due to spring back of at least 3°. If 90° is required then additional process will be necessary.

  40. Stress/Strain Curves Springback or the elastic strain, is then simply the amount of strain returned to the part as the stress returns to zero

  41. Agenda • Cutting • Theory of Cutting Sheet Metal • Forces for Cutting • Die Cutting Operations • Properties of Metals (stress strain curve, spring back, etc) • Forming • Bending • Embossing and Miscellaneous Forming • Drawing • Tooling • Design Practices

  42. Forming Limit Diagram

  43. Embossing

  44. Drawing

  45. Bending *

  46. Coining *

  47. Embossing *

  48. Projection *

  49. Hydro-forming

  50. Agenda • Cutting • Theory of Cutting Sheet Metal • Forces for Cutting • Die Cutting Operations • Properties of Metals (stress strain curve, spring back, etc) • Forming • Bending • Embossing and Miscellaneous Forming • Drawing • Tooling • Assembly • Design Practices