Die Design for Sheet Metal Forming & Punching
Die Sets are Tools for Working Sheet Metal
Shortcut to punching several holes
Unipunch
Single Station Dies
Transfer Dies
Workpieces are separate units that are transferred from one die station (or press) to another)
Progressive Dies
Work remains on carrier strip through multiple stations.
Progressive Die Wizard (NX)
Die Components
http://commons.wikimedia.org/wiki/File:ProgressiveDieToyota-strip-scrap.jpg
Simplest Case...
See: The Fabricator “Die Basics”
Die Set
See: ANSI B5.25
Posts
Bearings
Die Shoe
Punch Holder
Pin
Die set model: http://grabcad.com/stan.wile
Die Set Anatomy
Die Shoe
Guide Posts
Pins
Die Block
Strip Guide
Stops
Stripper Plate (fixed) & Perforator Punches
Blanking punch, Punch retainers, Internal Pilots (stripper removed)
(stripper in place)
Punch retainer hardware in place
Punch Holder in place
Die Components
Springs
Spring Color Codes: ISO, JIS, American
Spring Principles
Hooke's Law
Fatigue/endurance limit
Less Travel = Longer Life
Protect surface finish of springs
Use spring cages
Do not alter springs
Keep springs pre-loaded, reduces shock
1/16 of travel or .125 inches minimum
Replace all springs at the same time
Punches and Retainers
Headed (http://www.daytonprogress.com/catalogs/pdf/29.pdf)
Punches and Retainers
Ball Lock Retainer
http://www.anchorlamina.com/pdf.d/Punchrite-Retainers.pdf
http://www.daytonprogress.com/catalogs/pdf/28.pdf
Punches and Retainers
Pilots
http://www.daytonprogress.com/catalogs/pdf/28.pdf
Die Buttons
http://www.daytonprogress.com/catalogs/pdf/19.pdf
Die Design Example
Mounting Bracket
Design Steps
Develop Blank Pattern
Bend allowance
Neutral Axis
Blank Development
If r < t, K = 1/4
If r is between one and two t, K = 1/3
If r is greater than 2t, K = 1/2
Bending
Air Bending
Springback
Coining
Wipe Bending
Rotary-Action Bending
Force Required to Bend
Design Steps
Determine Scrap Strip Layout
Pilot Method
Strip Development
Minimize scrap/drop
Design Steps
Work Out Die Block, Bending sequences
Design Steps
Place Piercing Punches, Retainers & Die Buttons
Design
Shearing Force
Typical shear force is 60-80% of UTS (FTD, p. 216)
Use UTS in calculation to get a satisfactory estimation of shearing force + factor of safety.
Fs = LtSs
Fs = Shearing force
L = Length of cut perimeter
t = Material thickness
Ss = Shear strength
Note: Force from springs works against this, you need to add the force from the springs to your calculated value.
Stress Developed in Punch
Stress is force/area
Punches experience stress in compression (and also tension).
See
A2 Cold Work Tool Steel
Design Steps
Determine blanking,shaped punches & Dies
Die Clearance
Die Clearance (see SME TMEH Vol 2, Ch6-22)
Mild Steel 5-12%
Example design .062 galvanized steel, using .006 clearance (9.7% ≅ 10%)
Die Clearance
Design Strippers/Shedders
Pull blank off of punch body or shed blanks from punch faces.
Shoulder screw
Design
Stripping Force (rule of thumb)
F = (1.5)LT
F = Stripping force in tons
L = Length in inches
T = Material thickness in inches
OR….10-15% of punching force
Stripping force can be reduced by staggering punch lengths
Bill of Material
Design
Reducing Cutting Force
Design
Center of Pressure
Center of gravity of the lines that form the perimeter of the cutout.
Useful for balancing the die set pressure, locating it centrally, minimizing lateral forces.
For semicircular arc: 2r/π (from center)
Sum CG distances * lengths from reference axis, divide by total length = distance to CP
Can check with Region, MassProp in AutoCAD
Remember Centroids and Centers of Gravity?
Example
Location of COP of semicircular arc:
2r/π (from center)
Forming
Solid form dies
Coining dies
Hole Flanging or Extruding
Stamping Analysis
Circle Grid Analysis
Small etched circles on the workpiece show magnitude and direction of strain.
(see also: multipoint forming)
Deep Drawing
Blank holder irons material
Use die cushions, double action presses
Sometimes use multiple draws
Flat Pattern Development
Cones
Square Hopper
Cut Cylinder/Cylindrical Intersection
See also Metalgeek.com
Flat Pattern Development
Square/Rectangular to Round Transition
Die Design/Build Example
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