Design for Stamping Terry Sizemore University of

1
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
  (Taguchis 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 ProcessUlrich and Eppenger,
1995
Mission Statement
Design for Stamping
Concept Development
Detail Design
System Design
Product Launch
Testing/ Refinement
Production Ramp up
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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 lt.125, Plate is anything
  gt.125
• These rules do not apply to very brittle
  materials such as magnesium

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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

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Penetration

• Roll Over Burnish Penetration

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

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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)

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Forces for Cutting

• For Cutting
• In general ferrous stamping materials, shear
  strength is 70-80 ultimate tensile strength
• ForceShear StrengthPerimeter of CutThickness
• When calculating tonnage required it is
  recommended that ultimate tensile strength be
  used instead of shear strength to compensate for
  die wear.
• Tonnage(UTSPerimeterThickness)/2000

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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

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Work and Energy

• In terms of metal cutting
• Workaverage forcedistance
• 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.

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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
• Tonnage11053/20005.5 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

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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
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Trimming
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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
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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
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Embossing
35
Drawing
36
Hydro-forming
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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

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Production

• 35 to 500 parts per minute
• 250000 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

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Design Recommendations

• Avoid sharp corners
• Improves tool wear
• Increases bur size
• Lowers stress
• Minimum radius of .5T or .03125
• 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