Design for Stamping
(DFS)
Terry Sizemore
Edits from Mark Courtright, Dwayne
Mattison, Ravi Ranganathan, Mac
Lunn, Rolf Glaser
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 Eppinger
SME Journal of Manufacturing Systems Vol23, No3, 2004
(reference 1)
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 (0.884.76mm)
■ 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
Product Development ProcessUlrich
and Eppenger, 1995
Mission
Statement
Concept
Development
Testing/
Refinement
Design for Stamping
System
Design
Production
Ramp up
Detail
Design
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
Assembly
Design Practices
Theory of Cutting
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Assumptions
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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)
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
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 – 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.
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:
■ 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.
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Work Hardening
Annealing or Tempering
Other processes that affect the mechanical
properties of the material
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.
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
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
Blanking
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.
Perforating
*
Piercing
Lancing
Cut-Off/Parting
Notching
Shaving
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,
Trimming
Punching away excess material from the
perimeter of a part, such as trimming the
flange from a drawn cup.
Slitting
Cutting straight lines in the sheet. No scrap
material is produced.
Perforating
Punching a close arrangement of a large
number of holes in a single operation.
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.
Parting
Separating a part from the remaining sheet, by
punching away the material between parts.
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.
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.
Hydro-forming
Hydro-forming is a manufacturing process where fluid
pressure is applied to a ductile metallic blank to form a desired
component shape
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Cutting
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Theory of Cutting Sheet Metal
Forces for Cutting
Die Cutting Operations
Properties of Metals (stress strain curve, spring
back, etc)
Forming
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Agenda
Bending
Embossing and Miscellaneous Forming
Drawing
Tooling
Assembly
Design Practices
Stress/Strain Curves
Geology of Stress Strain Curve
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Elastic Region
Yield Point
Necking Region
Ultimate Point
Elongation
Spring Back
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.
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
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
Embossing
Drawing
Bending
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Coining
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Embossing
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Projection
*
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
Assembly
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
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
Progressive Dies
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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
YouTube Videos
on Progresisve Dies
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■ http://www.youtube.com/watch?v=10vNgC4LpkQ
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http://www.youtube.com/watch?v=GKTDgBeEFik&fea
ture=fvwrel
http://www.youtube.com/watch?v=IgEIt7fnHH4
Rubber Pad Dies
Forming Process Selection Chart
Transfer Dies
Progressive Dies
Sheet Hydroforming
Investment Required
H
M
L
Process Cycle Time
M
H
L
Part Variable Cost
M
L
H
Class “A” Panels
Closure Inner Panel
Underbody Cross
Members
Tire Tubs
Reinforcement
Brackets
Nut Plates
Hinges
Similar to transfer dies
More aggressive
shapes possible
Suitable Part Design
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
Assembly
Design Practices
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
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
Process Flow
•High level process flow development
provides considerations for :
•Design features for needed for assembly
•Assembly sequence
•Preparation for floor plan layout
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
Assembly Floor plan
Assembly equipment layout
Floor plan shows sequence of
operation of this assembly
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
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
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
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
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
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
Assembly
Design Practices
Stamping Applications
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Can accommodate many functional features and
attachment features
Natural uniform wall thickness
Can incorporate
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Springs
Snap fit
Tabs
Spot welding
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
■ 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
Design Recommendations
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Avoid sharp corners
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Improves tool wear
Increases bur size
Lowers stress
Minimum radius of .5T or .03125
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
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; progressive dies can do better)
Set-outs – used for location, rivets, etc.
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Height to be .5T
Be aware of the burr direction and how the
mating part is installed in the hole
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
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