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, (reference 1)

3. Design for Stamping (DFS)
Assumptions
DFS will be “Design for Stamping” in this lecture
DFS applies to sheet materials from to inches in thickness ( mm)
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 Material % Penetration Silicon Steel 30 Aluminum60
.10 C Steel Annealed 50
.10 C Steel Cold Rolled 38
.20 C Steel Annealed 40
.20 C Steel Cold Rolled 28
.30 C Steel Annealed 33
.30 C Cold Rolled 22
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 meet which 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 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

51. Transfer Dies Most automotive stampings created by transfer press
Automation “transfers” part from die to die
First picture shows stampings transferred from the side
Second picture shows stampings transferred from the front and back

52. Hydro-forming - Bladder press
Create only bottom half of the die (cheaper and faster)
Sheet metal placed over die
Rubber-like material placed over sheet metal
High pressure water forms part
The dies are less expensive than a transfer press but variable cost will be much higher due to significantly slower cycle times
More appropriate for low volume stampings
Can form more aggressive shapes than traditional draw forming

53. Progressive Dies Dies fed directly from steel coil
No need for blanking operation
Scrap gets cut away as part gets formed
Surfaces must be flanged instead of drawn home. This requires notches which reduce the strength of the part

54. YouTube Videos on Progresisve Dies
ture=fvwrel

55. Rubber Pad Dies

56. Forming Process Selection Chart
Transfer Dies
Progressive Dies
Sheet Hydro-forming
Investment Required H M L
Process Cycle Time
Part Variable Cost
Suitable Part Design
Class “A” Panels
Closure Inner Panel
Underbody Cross Members
Tire Tubs
Reinforcement Brackets
Nut Plates
Hinges
Similar to transfer dies
More aggressive shapes possible

57. 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

58. Design for Stampings
Assembly Process is considered during the component design
Assembly sequence and weld placement
Process Flow
Assembly equipment layout
Equipment used and types of part joining
Process to control variation
Part variation
Fixture variation
Weld gun variation

59. Welding Assembly
Design for welding includes manufacturing and assembly considerations during the component development stage
Weld points locations and access are considered during the component design

60. Process Flow
High level process flow development provides considerations for :
Design features for needed for assembly
Assembly sequence
Preparation for floor plan layout

61. Assembly Floor plan - Process Flow
Assembly equipment layout
Floor plan process flow provides:
Process Sequence
Welding times analysis
Fixture layout
Equipment placement
Robot programming
Part delivery and removal
Piece cost development
Capital expense evaluation
Floor plan layouts are a critical step during the design for welding process

62. Assembly Floor plan
Assembly equipment layout
Floor plan shows sequence of operation of this assembly

63. Assembly Equipment Welding Type by equipment Arc welding
Robot with EOAT
Pedestal Welder
Holding fixture
Robot EOAT
Weld Nuts
MIG Welding
Robot with EOAT
Laser Welding
Robot
Ceiling
Tape
Dispensable Sealer
Fixture
Holding
Part Pass
Pedestal welder holding fixture
Joining
Clinching
Riveting

64. Fixture Design Considerations
Design for process Variation
Figure 2
1. Parts are loaded in the assembly station (Figure
2a)
2. Tooling is closed, deforming the parts to a nominal position (Figure 2b)
3. Parts are assembled / joined together (Figure 2c)
4. Tooling and extra locators are released and the
assembly springs back (Figure 2d)
Reference 1
The variation within the process needs to be controlled at each station

65. Sources of Variation
Part Variation: In the absence of tooling variation, fixture position has no major impact on assembly variation in the presence of part variation. The final assembly variation is only a function of part deviation. The spring-back effect is totally compensated by the relocation effect when fixtures are moved to different positions.
Reference 1

66. Sources of Variation
Fixture Variation: In the presence of fixture variation, assembly variation depends on fixture positions. A general rule for variation reduction is to avoid locating non-nominal fixtures close to welding locations and other fixtures. An optimal fixture position can be found.
Reference 1

67. Sources of Variation
Welding Gun Variation: In the presence of welding gun variation, assembly variation depends on the fixture positions. The guideline for fixture design is to move fixtures as far as possible from the locations of faulty welding gun. This minimizes any restraint to part deformation. In general, the optimal solution locates the fixtures such that they do not provide any support to the parts during the assembly process. However, this general solution is not feasible. Parts must be held or supported at a specific position before assembly.
Reference 1

68. 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

69. Stamping Applications
Can accommodate many functional features and attachment features
Natural uniform wall thickness
Can incorporate
Springs
Snap fit
Tabs
Spot welding
Material Thickness from .001 in to .790 in

70. Production 35 to 500 parts per minute
per year minimum to justify using progressive die
Progressive Die should eliminate at least two secondary operations before consideration
Short run press tooling – Short run is when the cost of the tool exceeds the cost of the parts
Punch presses should be used for low volume parts when possible

71. Materials
Any material that can be produced in sheet can be press-worked
Deep drawn parts require “Draw Quality” steels
Non-ferrous metals may require modified processing or additional processing steps

72. Design Recommendations
Shaping and nesting on strip
Stamp multiple parts on same strip to increase strip utilization
Design part/strip so part can be “cut-off”, not “blanked”
Holes
Diameter not less than T, spacing should be 2T to 3T
1.5 to 2T between a hole and edge
1.5T + bending radius spacing between surface and hole
Use pilot holes

73. Design Recommendations
Avoid sharp corners
Improves tool wear
Increases bur size
Lowers stress
Minimum radius of .5T or
Be aware of grain direction and how it may change from blank to blank
Long sections should greater than 1.5T wide to avoid distortion and a weak problematic tool design

74. Design Recommendations
Use stiffening ribs or darts when more strength is needed
Use extruded holes when threaded fasteners must be used (1.5 T is the max thread contact you can achieve; progressive dies can do better)
Set-outs – used for location, rivets, etc.
Height to be .5T
Be aware of the burr direction and how the mating part is installed in the hole

75. Dimensional Considerations
Spring-back, die wear, material variation (temper, thickness, content) are sources of variation
Short run prototype stampings should represent the dimensional population of the production tooled parts to prevent system failures when part goes into production

76. Material Utilization There are two types of material utilization (MUD)
Engineering MUD and Process MUD
Engineering MUD is the part weight divided by the weight of the minimum amount of material required to make ONLY the outline of the part. This number will run between 80-90% for the average body structure stamping
Process MUD is the part weight divided by the weight of the blank that is used to make the part. This is what defines the cost for the part, as you have to purchase all of the material required to make the part. This number is much lower than the engineering MUD number and typically runs between 55-65% for the average body structure stamping