1. Terry Sizemore University of Detroit-Mercy MPD Cohort 5
Design for Stamping
Terry Sizemore
University of Detroit-Mercy
MPD Cohort 5

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 Eppenger

3. Design for Stamping (DFS)
Assumptions
DFS will be “Design for Stamping” in this lecture
DFS applies to sheet materials from .035 to .1875
Successful use of DFS is measured by:
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

4. Product Development Process Ulrich 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
Design Practices

6. 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

7. Theory of Cutting Assumptions
Theory of Cutting also applies to the trimming of forgings, extrusions and castings and the cutting of bar stock
Sheet metal is anything <.125, Plate is anything >.125
These rules do not apply to very brittle materials such as magnesium

8. 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 (insert figures from pg 3)

9. 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

10. Penetration
Roll Over + Burnish = Penetration

11. 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

12. 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.

13. Die and Punch Clearance
Force Curves – Using strain gages or other transducers to create force vs. displacement curves is a common tool for analyzing various clearance conditions. Poor clearance conditions result in less than ideal force curves (may put in curves???)

14. 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)

15. Forces for Cutting For Cutting:
In general 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

16. 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

17. 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.

18. 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

19. Cutting Operations Blanking – Material removed is the work-piece
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

20. Blanking

21. Piercing

22. Lancing

23. Cut-Off/Parting

24. Notching

25. Shaving

26. Trimming

27. 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

28. Stress/Strain Curves
Insert Curve with details

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

30. Spring Back

31. 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

32. Forming Limit Diagram

33. Bending

34. Embossing

35. Drawing

36. Hydro-forming

37. 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

38. 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

39. 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

40. Progressive Dies Dies fed directly from steel coil
No need for blanking operation
Scrap get cut away as part gets formed
Restricted to simple parts

41. Flexible Forming Dies

42. Rubber Pad Dies

43. Tooling Materials

44. 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

45. 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

46. 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

47. 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

48. 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 then 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

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

50. 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)
Set-outs – used for location, rivets, etc.
Height to be .5T
Be aware of the burr

51. 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