Enhancing and Restoring Safety & Quality Cultures - Dave Litwiller - May 2024...
Gp 2012 part 2
1. 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.
2. 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)
• Take caution in using value of shear strength. 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
4. Work and Energy
• In terms of metal cutting:
Energy of cutting =average force*Penetration
• Force: Since the force/displacement curve for cutting sheet metal
is nearly rectangular use the maximum force prior to fracture as
the average force
5. 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
17. 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
18. Tool Design
• It is a specialized area of manufacturing engineering which
comprises the analysis, planning, design, construction and
application of tools, methods and procedures necessary to
increase manufacturing productivity.
• Work holding tools – Jigs and Fixtures
• Cutting tools
• Sheet metal dies
• Forging dies
• Extrusion dies
• Welding and inspection fixtures
• Injection molds
31. Bending
• FIGURE 7.15 (a) Bending terminology. The bend radius is measured to the
inner surface of the bend. Note that the length of the bend is the width of the
sheet. Also note that the bend angle and the bend radius (sharpness of the
bend) are two different variables. (b) Relationship between the ratio of bend
radius to sheet thickness and tensile reduction of area for various materials.
Note that sheet metal with a reduction of area of about 50% can be bent and
flattened over itself without crackling. Source: After J. Datsko and C. T. Yang.
32. Methods of Reducing or Eliminating
Springback
• FIGURE 7.21 Methods of reducing or eliminating
springback in bending operations..
39. Bending force :
Maximum bending force, P = KLSt2
W
K – constant ranges from 0.33(wiping die) – 0.66(u-die)-1.32(V-die)
S – yield stress
L- length of the bend
t- thickness of sheet
For a V-die
Max bending force, P = (UTS)LT 2
W
UTS – Ultimate tensile strength
40.
41.
42. Beading :
• The periphery if the sheet metal is bent into the
cavity of a die
Fig 16.24 (a) Bead forming with a single die (b) Bead forming with two
dies,in a press brake
43. Dimpling :
• First hole is punched and expanded into a flange
• Flanges can be produced by piercing with shaped
punch
• When bend angle < 90 degrees as in fitting conical
ends its called flanging
49. Explosive forming :
• Explosive energy used s metal forming
• Sheet-metal blank is clamped over a die
• Assembly is immersed in a tank with water
• Rapid conversion of explosive charge into gas generates a shock wave
.the pressure of this wave is sufficient to form sheet metals
