By Steve Benson, Contributing Writer
For the past few months I’ve detailed a new ay of predicting the inside bend radius. In- stead of the traditional terms such as radius
and profound-radius bends, we’re using some new
terminology. I’m calling this the “Grand Unifying
Theory” of radius, bend deduction, and die selection, and ultimately it may help you make your
bend calculations more accurate than ever.
For this final installment, I’ll review the definitions and bring you through the entire process,
from calculating springback to incorporating the
bend function calculations. Note that all measurement values—including material thickness, radius,
punch nose diameter, and die width—are in inches
unless otherwise specified.
Types of Bends
Sharp bend: Specified inside bend radius that is
generally 63 percent or less of the material thickness of 60-KSI cold-rolled steel. (For more on this,
see “How an air bend turns sharp” on www.thefab
Perfect bend: Specified inside bend radius that’s
between 63 percent and 125 percent of the material
Radius bend: Specified inside bend radius that’s
greater than 125 percent of the material thickness.
Air forming: Three-point bending in which the
material contacts the punch nose and the radius
of the die shoulders during the forming cycle. The
floated inside radius forms as a percentage of the
Bottoming: Forced bends, usually at 90 degrees,
with angular clearance between the punch and die;
the punch descends until the material wraps around
the punch nose, after which the ram continues to
apply pressure, forcing material against the die face
to the desired bend angle. This produces a radius
that’s slightly larger than the punch nose, which is
why a springback factor still needs to be taken into
account. Bottoming occurs at about 20 percent of
the material thickness, as measured from the bottom of the V die.
Coining: Occurs when the tool geometry is
stamped into the material at less than the material
thickness. It is rarely performed.
Wiping: Occurs in aircraft tooling during the forming of complementary angles greater than 90 degrees.
Bend angle: Angle after springback.
Read more from Steve Benson at www.thefabricator.com/author/steve-benson
A grand unifying theory of bending: Part IV
Tying it all together
This chart —Deductio de Diabolus, or Deductions of the Devil—summarizes the concepts discussed in this series. A perfect bend (in gold) is a bend where the inside bend radius
and material thickness are the same. As long as you use the tooling in the manner described in the calculations, a perfect bend will be the result. To the left of the gold spot, the
radius-to-material-thickness ratio is less than 1-to-1; to the right, the ratio is greater than 1-to-1.
V dies, acute and aircraft
Relieved and channel
curve Springback curve
4 mm thick
1 mm thick
as perfect Calculate BD
(575 x Mt2)/Width
63% of Mt
60 KSI tensile
1-to-1 Tonnage curve, air form
Material thickness to die width
Deductio de Diabolus
Data based on 60-KSI AISI-1035
mild cold-rolled steel
Radius 1-to-1 with material thickness
90° of bend angle
Die width = Mt x 6.858 Die width = Outside radius x 5