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Design for Stamping. Terry Sizemore University of Detroit-Mercy MPD Cohort 5. References. Eary and Reed: Techniques of Pressworking Sheet Metal, 2 nd ed. Prentice Hall Boothroyd, Dewhurst, Knight: Product Design for Manufacture and Assembly, 2 nd ed. Marcel Decker

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design for stamping

Design for Stamping

Terry Sizemore

University of Detroit-Mercy

MPD Cohort 5

references
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
design for stamping dfs
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
product development process ulrich and eppenger 1995
Product Development ProcessUlrich and Eppenger, 1995

Mission

Statement

Design for Stamping

Concept

Development

System

Design

Detail

Design

Product

Launch

Testing/

Refinement

Production

Ramp up

agenda
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
agenda1
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
theory of cutting
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
analysis of cutting
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)
characteristics of a die cut edge
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
Penetration

Roll Over + Burnish = Penetration

percent penetrations
Percent Penetrations

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

die and punch clearance
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 clearance1
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???)
other characteristics
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
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 cutting1
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
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
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
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
agenda2
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
stress strain curves
Stress/Strain Curves

Insert Curve with details

geology of stress strain curve
Geology of Stress Strain Curve
  • Elastic Region
  • Yield Point
  • Necking Region
  • Ultimate Point
  • Elongation
  • Spring Back
agenda3
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
agenda4
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
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

Transfer Dies
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

Hydro-forming - Bladder press
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

Progressive Dies
agenda5
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
stamping applications
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
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
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
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
design recommendations1
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
design recommendations2
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
dimensional considerations
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
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