Page 31 - MetalForming July 2014

operations. Unfortunately, both can be difficult to remove.
Strive to carefully control blankholding force. Sufficient pres- sure is required to keep the alu- minum sheet from wrinkling, but excessive pressure can force the aluminum to stretch in areas that should draw. Use draw beads (Fig. 3) and lock beads (Fig. 4) to provide additional restraining forces to avoid excessive blankholder forces.
Large depths of draw are required for curved parts if drawn off of a flat blankholder. The long drawing distances combined with pressure profiles—the change in pressure vs. distance—generated by gas springs can limit the forma- bility of aluminum panels.
Developed—or curved — blankholders minimize the required depth of draw by pre- shaping the blank to conform to the general shape of the finished stamping as the die closes (Fig. 5). The objective of a developed blankholder, in combination with properly designed adden- dum: Provide a uniform draw depth, and evenly distribute strains, during forming to pre-
Draw beads restrict material flow into a die cavity, by acting like speed bumps to slow materi-
al flow into areas where more stretching is required. This pro- duces a tighter appearance in the stamping. The Aluminum Auto- motive Manual (from the Euro- pean Aluminum Association) recom- mends the following general design rules for draw-bead profiles:
R1=3to6t;R2=6t;B1=12t;B2=15t h = 6 to 10t (t = sheet thickness)
The center of the draw bead tradi- tionally is placed about 25 times the sheet thickness from the side of the draw die (punch opening line).
Standoffs will control the gap between the die face and the blankholder surface and aid material flow.
Fig. 5—A Curved Blankholder (Courtesy of Altair Engi- neering)
Lock beads will completely stop material from flowing into the die cav- ity. All deformation will be stretching, ensuring a panel that is as tight as pos- sible. The Aluminum Automotive Man- ual recommends the following general design rules for lock-bead profiles:
R1=3t;R2=2t;B1=12t;B2=15t; h = 6t (t = sheet thickness)
The center of the lock bead is placed about 24 times sheet thickness from the side of the draw die (punch open- ing line).
vent excessive stretching and fractur- ing of the stamping.
Bending and Forming
Total elongation values are impor- tant in bending because bending strains relate to the tensile elongation generated along the outside surface of the bend. Aluminum alloys with nega- tive m-values will have low values of total elongation when measured during the tensile test. Instead of deforming all the way to normal fracture strains, the
RESISTANCE WELDING ONE-SPOT-SHOP
Hotoff the Press
Your daily
source for
news of the metalforming industry
www.metalformingmagazine.com
sign • New & Used • Parts & Supplies • 24/7 Service • Process Training • Repairs • Control Upgrades • Water Chillers
DIVERSE CONSUMABLES INVENTORY STANDARD & SPECIAL DESIGN RESISTANCE WELDERS
Resistance Welding M
Ma
ac
ch
hi
in
ne
er
ry
y•
•S
St
ta
an
nd
da
ar
rd
d&
&S
Sp
pe
ec
ci
ia
al
lD
De
es
s
www.metalformingmagazine.com
MetalForming/July 2014 29
Helping customers succeed with resistance welding for 50 years
www.tjsnow.com | 1-800-NOW-SNOW | welders@tjsnow.com