ArticlesTemp

reproducible and reliable press-
brake process relies on the combi-
nation of the press brake and its

tools. A press brake consists of two
robust C frames forming the sides of the
machine, connected on the bottom by a
massive table and on the top by a move-
able upper beam, though the opposite
configuration is possible. The bottom
tool rests on the table while the top
tool attaches to the upper beam. With
hydraulic press brakes—the majority
of machines produced these days—the
upper beam moves via two synchro-
nized hydraulic cylinders attached to
the C frames.

Characteristics that define press-

brake capabilities include pressure or
tonnage, working length, distance to
the backgauge, work height and stroke.
The speed at which the upper beam
operates usually ranges from 1 to 15
mm/sec.

Increasingly, press brakes feature

multi-axis computer-controlled back-
gauges, and, to make adjustments dur-
ing the bending process, mechanical
and optical sensors. These sensors meas-
ure bending angle during the bend cycle

and transmit data real-time to machine
controls, which in turn adjust process
parameters.

Ultimately, press-brake bending is a

combination involving the geometry
of the top tool (with the punch angle
and punch-tip radius the most impor-
tant parameters), the geometry of the
bottom tool (the width of the V open-
ing, the V angle and the bending radii of
the V opening in particular), and the
pressing force and speed of the press
brake.

Types of Bending

Folding

—When folding, the longest

leg of the sheet clamps between clamp-
ing beams, then the bend beam rises and
folds the extending sheet part around a
bend profile (Fig. 1). In today’s bending
machines, the bend beam can form
upward and downward, a significant
advantage when creating complex parts
with positive and negative bend angles.
The resulting bend angle is determined
by the folding angle of the bending
beam, tool geometry and material prop-
erties. Bending via folding offers a sig-
nificant advantage in that large sheets

can be handled relatively easily, making
this technique simple to automate. Also,
with folding, the risk of damage to the
sheetmetal surface is minimal. One lim-
iting factor of folding: The movement of
the bend beam requires the necessary
space and throughput time.

Wiping

—When wiping, the sheet

again clamps between the clamping
beams, after which the tool bends the
protruding part of the sheet around
the bend profile by moving up and

Bending

Methods and

Challenges

Press brakes can do a lot, though challenges

abound in creating top-quality parts. Here we

discuss the types of bending, and factors

that affect machine performance.

Folding

Fig. 1

Press-Brake

METALFORMING / AUGUST 2008

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down (Fig. 2). Wiping, though faster
than folding, increases the risk of
scratches or other damage to the sheet
as the tool moves over the sheet surface.
This is especially true if bending
involves sharp angles. This technique
finds use for making panel-type prod-
ucts with small profiled edges. Using
special tools, wiping can be readily
accomplished on press brakes.

Bending Variations

In bending, a distinction can be

made between four variations: air bend-
ing, bottoming, coining and three-point
bending. Characteristic of these: The
sheet is pressed by a top tool into the
opening of the bottom tool (Fig. 3). As
a result, sheetmetal on each side of the
bend is lifted, causing problems such as

sagging and folding with large sheets. In
that case, folding or wiping is preferred,
although sheet-follow supports also can
be used with the press brake to alleviate
this. Where bending involves positive
and negative angles, folding offers more
flexibility. The significant advantages
offered by press brakes are increased
speed and flexibility.

Air Bending

—With air bending, the

top tool presses a sheet into the V open-
ing in the bottom tool to a predeter-
mined depth, but without touching the
bottom of the tool (Fig. 4). This is a type
of three-point bending, where only the
bending radii of the top and bottom

tools contact with the sheet. The punch
radius of the top tool and the V angle of
the bottom tool need not be the same.
In some cases, a square opening replaces
the V opening in the bottom tool—
especially given today’s adjustable bot-
tom tools. The combination of top and
bottom tools, therefore, can be applied
universally, meaning that with a single
combination, various products and pro-
file shapes can be produced simply by
adjusting the press-stroke depth. In
other words, with a single combination
of tools, multiple materials and thick-
nesses can be bent in a range of bend
angles. This makes air bending a high-
ly flexible technique. It also means that
the number of tool changes can be lim-
ited considerably, enhancing produc-
tivity. Another advantage: Less bend
force is required, meaning less bulky

tools and resulting in extra allowance in
product design.

One limitation of air bending: It is

less precise than processes where sheet
fully maintains contact with tooling.
The stroke depth must maintain high
accuracy, and variations in sheet thick-
ness and local wear on the top and bot-
tom tools can result in unacceptable
deviations. Variations in material prop-
erties also affect the resulting bend angle
due to springback. To achieve maxi-
mum angle accuracy with air bending,
a value is applied to the width of the V
opening, ranging from 6S (six times
material thickness) for sheets to 3 mm
thick to 12S for sheets more than 10 mm
thick. A rule of thumb: V=8S.

Air bending boasts angle accuracy of

approximately ±0.5 deg. Unlike with
bottoming and coining, bend radius is
not determined by tool shape, but
depends on material elasticity (Fig. 4).
Normally, the bend radius resides
between 1S and 2S. Based on its flexi-
bility and relatively low tonnage require-
ments, fabricators are moving more
toward air bending as the preferred
forming technique. The disadvantages
of this technique related to quality are
remedied by taking special measures—
angle-measuring systems, clamps and
crowning systems that are adjustable
along the x and y axes, and wear-resist-
ant tools.

Bottoming

—Bottoming, a variation

of air bending, presses the sheet against
the slopes of the V opening in the bot-
tom tool (Fig. 5), with air between the

Wiping

Top
tool

Bottom tool

Characteristics of Bending

Air Bending

Fig. 2

Fig. 4

Fig. 3

Rmax

Top
tool

Bottom tool

Bottoming

Fig. 5

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METALFORMING / AUGUST 2008

sheet and the bottom of the V opening.
In this case, the punch radius and the V-
opening angle are directly linked, mean-
ing that bottoming does not offer the
same flexibility as air bending. Every
bend angle and every sheet thickness
requires a separate tool set, and the
same often applies for different materi-
als due to springback differences and
compensation required in the tool. For
bottoming, the optimum width of the V
opening (U-shaped openings cannot
be used) is 6S for sheets to thicknesses
of about 3 mm, increasing to 12S for
sheets more than 12 mm thick. Again,
the rule of thumb: V=8S. The mini-
mum acceptable bending radius for
sheet steel ranges from 0.8S to 2S,
although material quality plays a role.
And with soft materials such as copper
alloys, the radius of the bend angle may
be much smaller—a lower limit of 0.25S
is possible.

For larger bend radii, bottoming

requires tonnage roughly the same as for
air bending for larger bend radii. Small-

er radii require force as much as five
times greater when bottoming. This
brings the advantage of greater accura-
cy. The resulting bend angle is wholly
determined by the tool, with the excep-
tion of springback, for which a correc-
tion can be made. Note that bottoming
results in less springback than when
employing air bending. Theoretically,
angle accuracies with bottoming
approach ±0.25 deg. But because con-
trol and adjustment possibilities on
press brakes have increased consider-
ably, even on less-expensive machines,
air bending increasingly is preferred to
bottoming.

Coining

—With coining, the top tool

crushes sheet into the opening of the
bottom tool, down to the bottom of
the V opening (Fig. 6). Coining requires
many times the bend force of air bend-
ing and bottoming—normally, five to 10
times higher tonnage, and in some
instances, 25 to 30 times higher. But
coining offers the advantage of a high
level of precision. Because of the

extremely high pressure exerted on the
punch tip into the material, permanent
deformation occurs throughout the
entire cross-section of the sheet, with
springback reduced to virtually zero.
As the punch and V-die angle are iden-
tical, the desired bend angle can be eas-
ily selected, and variations in sheet
thickness and material properties have
little or no effect on coining results.

Coining

Fig. 6

Press-Brake Bending

The high level of force and the perma-
nent deformation mean that the mini-
mum achievable inside radius—starting
at 0.4S—is less than with air and bot-
toming, with the width of the V open-
ing required usually about 5S. A wider
V opening would mean that depth must
be greater in order to achieve the same
bend angle. In general, coining costs
more than air bending and bottoming,
therefore, it is sporadically applied, and
even then only for thin sheets.

Three-Point Bending

—A relatively

new bending technique, three-point
bending is considered by some to be a
special variation of air bending. This
technique employs a special die where
its bottom tool can be precisely adjust-
ed in height via a servo motor. The
sheet bends over the bend radii of the
die until it touches bottom, with the
bend angle decreasing as the depth of
the die bottom increases. The bottom
height of the die, as already indicated,
can be determined very precisely (±0.01
mm), with corrections made between
the ram and the upper tool using a
hydraulic cushion to compensate for
deviations in sheet thickness. As a result,
the process can achieve bend angles
with precision of less than 0.25 deg.
Advantages of three-point bending
include high flexibility combined with
high bending precision. Obstacles
include high costs and a limited range of
available tools. As a result, this tech-
nique, for the time being, is limited to
highly demanding niche markets where
the additional costs are outweighed by
the stated advantages.

Difficulties in
Press-Brake Bending

Anisotropy.

Sheet material itself, its

properties and especially the variations
in these properties, can influence the
press-brake-bending process. Sheet-
metal, produced on large rolling mills,
undergoes hot or cold rolling to reach
final thickness: hot rolling typically for
thicker sheet and cold rolling for thin-
ner sheets due to the high loss of heat
and difficulty in maintaining constant
temperature in thin material. Also, cold
rolling better controls thickness toler-

ances and causes hardening of the sur-
face layer.

Rolling stretches the crystal struc-

ture, causing material to acquire differ-
ent mechanical properties across its
length than across its width. In other
words, the material becomes anisotrop-
ic, and this affects the subsequent pro-
cessing. During bending, this can lead to
variations in the bend angle. Apart from
this anisotropic nature, unavoidable

variations occur in material properties
as a result of minute differences in mate-
rial composition and rolling conditions.
This also results in variations in stress/
strain curves, not only between differ-
ent batches of sheet materials, but even
within a single batch.

Springback.

Springback is the phe-

nomenon by which sheet rebounds on
either side of the bend after the bending
tool has been removed. Why? In the

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AUGUST 2008

center of the sheet—not exactly the
geometrical center, but close to it—
resides a zone with low stress in which,
even under large bend forces, only elas-
tic deformation occurs. This part of the
sheet’s cross-section, therefore, wants to
return to its original shape after bend
force is lifted. The extent to which
springback occurs depends on the
nature of the sheet material: The stiffer
the material, the greater the spring-

back. Soft materials exhibit springback
limited to no more than 0.5 deg., and
steel to 1 deg., but springback in stain-
less steel can amount to as much as
3 deg.

Bend angle also is a determining fac-

tor. The smaller the relative effect on the
elastic area in the neutral zone, the
smaller the springback. This is the case
with small bend angles and small bend
radii (meaning a sharp tool). For exam-

ple, a steel sheet 0.8 mm thick bent
with a bend radius of 1S exhibits spring-
back of 0.5 to 1 deg. The same sheet
bent with a bending radius of 77S
results in springback of as much as 30
deg., according to Steve Benson in his
book, Press Brake Technology: A Guide
to Precision Sheet Metal (published by
the Society of Manufacturing Engi-
neers). With a leg length of 100 mm,
each degree of deviation will mean that
the end of the sheet will have a spatial
deviation of 1.7 mm. For post-process-
ing, such as robotic welding, a deviation
of this size will soon exceed acceptable
tolerance limits. In practice, it is rela-
tively easy to correct for springback
when bending a sheet, providing that
influential parameters are known. For
calculating springback for cold-rolled
steel, a formula offered by Benson is

D = R / (2.1 x S)
where R is the radius of the angle in

mm and S is the sheet thickness in mm.
Using this formula, a steel sheet 0.8 mm
thick, and given a bend radius of 20 mm
and a bend angle of 90 deg., has a
springback value of 11.9 deg. To calcu-
late springback for other materials, Ben-
son uses a correction factor (0.5 for
copper, 0.75 for hot-rolled steel and 2.0
for stainless steel).

Keep in mind that under certain air-

bending conditions, negative spring-
back can occur, particularly when
employing dull tools in combination
with a large punch angle as deforma-
tions then can occur in the sheet
between the punch and die surface.
When coining, given high pressing pres-
sure and a sharp top tool, this tool can
press into the sheet past the neutral
zone. In that case, the plastic phase is
achieved everywhere and springback is
reduced to virtually zero.

Galling.

Galling of the bend tool—

particles of material or part flakes cling
to spots on the tool during bending—is
especially a concern with the bend radii
in the bottom tool. Galling can result in
damage to tools and to the sheet surface.
This problem can be minimized by
selecting an optimum bend radius for
the V-die (Fig. 7) and by hardening the
relevant bend radius. Hardened sur-

Press-Brake Bending

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faces are much less sensitive to galling.

Machine Deflection.

When high

tonnage is exerted, deflection unavoid-
ably occurs lengthwise in the top and
bottom tools. As a result, the top and
bottom tools no longer remain parallel
during the bend process, bringing vari-
ations in the bend angle over the length

of the product (Fig. 8). This adversely
affects post-bend processes such as
robotic welding. In the past, this prob-
lem often was remedied by shimming
the bottom tool to acquire a crowning
that compensated for the deflection.
Today, computer-controlled or central-
ly adjustable crowning systems quickly
and accurately compensate for deflec-
tion over the entire machine length.

Information for this article excerpted

from the Press Brake Productivity Guide,
published by Wila USA, Hanover, MD.
Tel. 888/696-9452; www.wilausa.com.

Fig. 8

Bottom tool

R bending

Optimum Bend Radius
Minimizes Galling

Fig. 7

Machine Deflection Causes
Bend-Angle Variation

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AUGUST 2008