Design for Stamping

(DFS)Terry Sizemore

Edits from Mark Courtright, Dwayne Mattison, Ravi

Ranganathan, Mac Lunn, Rolf Glaser

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, 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 (0.88-4.76mm)■ 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

Testing/Refinement

ConceptDevelopment

DetailDesign

SystemDesign

ProductionRamp up

ProductLaunch

MissionStatement Design for Stamping

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

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)

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

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

Penetration

Roll Over + Burnish = Penetration

Percent PenetrationsMaterial % Penetration

Silicon Steel 30

Aluminum 60

.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 22E.V. crane, Plastic Working in Presses, John Wiley and Sons, Inc., New York, 1948, p. 36

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.

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.

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)

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

■ 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

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 tonsWork=(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

■ 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.

EmbossingEmbossing 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.

DrawingDeep 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-formingHydro-forming is a manufacturing process where fluid pressure is applied to a ductile metallic blank to form a desired component shape

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

Stress/Strain Curves

Geology of Stress Strain Curve

■ 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 bendRadii to help the panel retain its shape. Designer should also anticipate that 90° flangesWill 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

■ 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

Forming Limit Diagram

Embossing

Drawing

Bending

*

Coining

*

Embossing

*

Projection

*

Hydro-forming

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

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

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

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

YouTube Videos on Progresisve Dies

■ http://www.youtube.com/watch?v=10vNgC4LpkQ■ 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 Hydro-forming

Investment Required

H M L

Process Cycle Time

M H L

Part Variable Cost

M L H

Suitable Part Design

Class “A” PanelsClosure Inner

PanelUnderbody Cross

MembersTire Tubs

Reinforcement Brackets

Nut PlatesHinges

Similar to transfer dies

More aggressive shapes possible

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

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

• 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 equipment layout

Assembly Floor plan

Floor plan shows sequence of operation of this assembly

Assembly equipment layout

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

1. Parts are loaded in the assembly station (Figure2a)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 theassembly springs back (Figure 2d)

Figure 2

Design for process Variation

The variation within the process needs to be controlled at each station

Reference 1

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

■ 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

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

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

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

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

■ Avoid sharp corners■ 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

■ 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

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

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