At the mill the material is first rolled hot at a temperature above the point
of recrystallization, meaning the grains have no directional preference.
Cold-rolled is rerolled cold, causing the grains to show a directional preference in the direction of rolling.
This makes it anisotropic. When a material is anisotropic, the results of
bending will change in relation to the grain. Bending with, against, or diagonal to the grain direction will produce different bend angles and inside
bend radii, all requiring different bend deductions. This adds yet another
variable to your formed products (see Figure 1).
The age of the material will also affect your results. Aged or oxidized
sheet is softer than a recently pickled hot-rolled steel. You also need to
consider the temperature at which the sheet is being bent. A hot piece of
steel will form differently than a sheet that is formed at room temperature.
Bending results are also affected by the configuration of the part and
how the part has been processed within your facility. Holes and features
can induce internal stresses into the workpiece and, when placed on or too
close to the bend line, weaken the bend.
Also consider the heat-affected zone created when laser or plasma cutting the flat. Thermal shock hardens the material in the area of the cut,
which again affects how the workpiece will form because of the change in
material temper across a bend—softer in some places, harder in others.
Back to the Machine
Whether it’s a change in temper, thickness, or yield strength, all these factors change how the press brake itself responds and how the change in
bending forces is manifested.
Of course, the machine itself has its own set of variables. One of the
most significant and unavoidable issues for the press brake is that its
frames deflect or deform under load. This occurs because the power
flow of a press brake is typically produced at the ends of the bed while we
generally do our forming in the center. This causes plenty of deflection
in the bed and ram, where the forming is taking place. The deflection is
amplified by the flexing back of the side frames under load, a phenomenon
known as yaw. When you change the material yield strength or thickness,
the deflection will increase or decrease accordingly.
What will manifest in the part is known as the canoe effect (see Figure
2), where the internal bend angle is greater in the center than at the
sides. This occurs because the punch and die are farther apart at center
than at the ends.
You can compensate for this deformation with a crowning system (see
Figure 3), which your new brake no doubt has, but how much crowning is
necessary? Many press brake manufacturers use databases calculated based
on a given press brake design and standard sheet metal characteristics as
dictated by the mill’s ASTM requirements and published certifications. Of
course, as described previously, material characteristics vary, which is why
some machines make crowning and angle adjustments in real time.
Nothing Is a Panacea
To think that sheet metal will behave in an unchangeable manner is merely
an illusion. Various press brake technologies help deal with material variation, but of course no one technological feature is a panacea, at times
leaving press brake operators still struggling with part-to-part consistency.
And again, they do so for a straightforward reason: sheet metal behavior
is unpredictable at best.
With all the variables involved within sheet metal, operators still can find
it difficult to produce parts, even with a state-of-the-art press brake. They
need to know what the sheet metal tolerance variables are and how to
compensate for them, especially when single-piece jobs or small batches
require operators to achieve the correct bend angles and dimensions on
the first try. They should be able to compensate for some of these variables
using functions incorporated into the press brake and its controller—but
as for other variables, not so much.
Still, don’t lose your faith in your purchase of that new press brake with
all its bells and whistles and precision-ground tooling. Compared to the
alternatives, they are making your operation far more efficient.
Steve Benson is a member and former chair of the Precision Sheet Metal Technology Council of
the Fabricators & Manufacturers Association International®. He is the president of ASMA LLC,
email@example.com. Benson also conducts FMA’s Precision Press Brake Certificate
Program, which is held at locations across the country. For more information, visit www.fmanet.
org/training, or call 888-394-4362. The author’s latest book, Bending Basics, is now available at the
FMA bookstore, www.fmanet.org/store.