Design for Stamping
Terry Sizemore
University of Detroit-Mercy
MPD Cohort 5
References
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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
Design for Stamping (DFS)
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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
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Product Development Process
Ulrich and Eppenger, 1995
Mission
Statement
Design for Stamping
Concept
Development
System
Design
Detail
Design
Testing/
Refinement
Production
Ramp up
Product
Launch
Agenda
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Cutting
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Properties of Metals (stress strain curve, spring back, etc)
Forming
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Theory of Cutting Sheet Metal
Forces for Cutting
Die Cutting Operations
Bending
Embossing and Miscellaneous Forming
Drawing
Tooling
Design Practices
Agenda
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Cutting
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Properties of Metals (stress strain curve, spring back, etc)
Forming
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Theory of Cutting Sheet Metal
Forces for Cutting
Die Cutting Operations
Bending
Embossing and Miscellaneous Forming
Drawing
Tooling
Design Practices
Theory of Cutting
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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
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Analysis of Cutting
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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)
Characteristics of a Die Cut Edge
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Roll Over – Flow of material around the punch and die
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The larger the clearance the greater the roll over
Burnish – The rubbed or “cut” portion of the edge
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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
Percent Penetrations
Material
Silicon Steel
Aluminum
% Penetration
30
60
.10 C Steel Annealed
.10 C Steel Cold Rolled
.20 C Steel Annealed
.20 C Steel Cold Rolled
.30 C Steel Annealed
.30 C Cold Rolled
50
38
40
28
33
22
E.V. crane, Plastic Working in Presses, John Wiley and Sons, Inc., New
York, 1948, p. 36
Die and Punch Clearance
Proper Clearance
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Too Big – Blank ends up with rollover 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.
Die and Punch Clearance
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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???)
Other Characteristics
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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)
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
Forces for Cutting
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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
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In terms of metal cutting:
Work=average force*distance
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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.
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
Cutting Operations
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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
Blanking
Piercing
Lancing
Cut-Off/Parting
Notching
Shaving
Trimming
Agenda
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Cutting
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Properties of Metals (stress strain curve, spring
back, etc)
Forming
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Theory of Cutting Sheet Metal
Forces for Cutting
Die Cutting Operations
Bending
Embossing and Miscellaneous Forming
Drawing
Tooling
Design Practices
Stress/Strain Curves
Insert Curve with details
Geology of Stress Strain Curve
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Elastic Region
Yield Point
Necking Region
Ultimate Point
Elongation
Spring Back
Spring Back
Agenda
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Cutting
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Properties of Metals (stress strain curve, spring back, etc)
Forming
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Theory of Cutting Sheet Metal
Forces for Cutting
Die Cutting Operations
Bending
Embossing and Miscellaneous Forming
Drawing
Tooling
Design Practices
Forming Limit Diagram
Bending
Embossing
Drawing
Hydro-forming
Agenda
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Cutting
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Properties of Metals (stress strain curve, spring back, etc)
Forming
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Theory of Cutting Sheet Metal
Forces for Cutting
Die Cutting Operations
Bending
Embossing and Miscellaneous Forming
Drawing
Tooling
Design Practices
Transfer Dies
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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
Hydro-forming - Bladder press
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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
Progressive Dies
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Dies fed directly from
steel coil
No need for blanking
operation
Scrap get cut away as
part gets formed
Restricted to simple parts
Flexible Forming Dies
Rubber Pad Dies
Tooling Materials
Agenda
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Cutting
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Properties of Metals (stress strain curve, spring back, etc)
Forming
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Theory of Cutting Sheet Metal
Forces for Cutting
Die Cutting Operations
Bending
Embossing and Miscellaneous Forming
Drawing
Tooling
Design Practices
Stamping Applications
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Can accommodate many functional features and
attachment features
Natural uniform wall thickness
Can incorporate
Springs
 Snap fit
 Tabs
 Spot welding
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Material Thickness from .001 in to .790 in
Production
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35 to 500 parts per minute
250000 per year minimum to justify using
progressive die
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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
Materials
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Any material that can be produced in sheet can
be press-worked
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Deep drawn parts require “Draw Quality” steels
Non-ferrous metals may require modified
processing or additional processing steps
Design Recommendations
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Shaping and nesting on strip
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Stamp multiple parts on same strip to increase strip
utilization
Design part/strip so part can be “cut-off ”, not “blanked”
Holes
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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
Design Recommendations
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Avoid sharp corners
Improves tool wear
 Increases bur size
 Lowers stress
 Minimum radius of .5T or .03125
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Be aware of grain direction
Long sections should greater than 1.5T wide to
avoid distortion and a weak problematic tool
design
Design Recommendations
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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.
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Height to be .5T
Be aware of the burr
Dimensional Considerations
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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