Read more from Steve Benson at
from around the globe
Reader questions from Australia,
India, Mexico, and the U.S.
By Steve Benson
The FABRICATOR is a U.S. publication, but hanks to the Internet, it’s a global one too. Over the years I’ve received many reader
questions, and a lot of them come from elsewhere
in the world. This month I thought I would share
a few of them, with a final question from the U.S.
thrown in for good measure.
In some of the answers, I refer the reader to articles archived on thefabricator.com. To access them,
simply visit the website and type the article headline in the search bar.
Thanks to all for the continued feedback, and
keep the questions coming!
Bending Thick in Australia
Question: I read your great article, “Calculating
bend allowance, outside setback, and bend deductions,” from November 2012. Can these calculations
be applied to the bending of mild steel plates 10 to 20
mm (0.394 to 0.787 in.) thick, or does it apply only to
sheet metal? I’m bending steel plates for pump bases
and have to work out the flat pattern. Typically, the
bends are 90 degrees.
Answer: Mathematically and practically, the formulas apply. The real trick is knowing what the inside
radius is going to be; get that correct and the math
Specifically, you need to correctly estimate what
the inside radius is going to be and use that value
in the calculations. If you are air forming, the radius
is produced as a percentage of the die opening. The
results you attain are affected greatly by the punch
A perfect bend is one in which the radius and material thickness are one and the same. If the punch
nose radius is significantly less than the material
thickness, the force the punch exerts may be so
great that it creates a crease along the bend line,
creating a sharp bend. You will need to have a good
understanding of what makes a bend sharp.
To ensure you always work within the tonnage
limits of your press brake and tooling, first calculate
how much tonnage you need. Second, identify your
tooling load limits. Third, calculate the sinking tonnage limit, which, if exceeded, can embed tooling
into the ram or bed. Fourth, determine your press
brake’s centerline load limits.
For complete information on this topic, check out
several articles archived on thefabricator.com: “The
four pillars of press brake tonnage limits” (April
2015); “How an air bend turns sharp” (May 2015);
and “Forming aluminum on a press brake: Bending
soft, not sharp” (June 2015).
I also invite you to review the four-part series, “A
grand unifying theory of bending,” which ran from
September through December 2015. It’s a new
theory of bending, altering some long-held labels
and definitions and introducing new formulas. The
traditional definitions worked well, but these new
definitions and formulas may help a press brake
technician be even more accurate when predicting
how a part will form.
For this new theory, we keep our definition of a
sharp bend, which on average has a radius that’s 63
percent or less of the material thickness. We then
add two new terms. A perfect bend starts at an inside
radius larger than 63 percent of the material thickness and extends up to 125 percent. At the median,
the perfect bend has the ideal 1-to-1 inside bend
radius-to-material thickness ratio. That is, the material thickness and inside bend radius are the same.
A radius bend is defined as any bend with an inside radius greater than 125 percent of the material
thickness. It requires careful calculation, not only
because of significant springback, but also because
of the tooling it requires. This includes the use of relieved dies.
Tonnage Considerations in India
Question: While searching on the net for information
on press brakes, I came across your article on bending radius and force calculations.
We are considering the purchase of a 300-ton press
brake, and I am not clear about the tonnage requirement for a bending job that involves ST46 grade,
10-mm-thick HR material with a bend that’s 2,760
mm (109 in.) long. We need to form 90-degree bends
with a V-die width of 80 mm ( 3. 15 in.).
As per the bending force calculations given by the
machine supplier, the pressure requirement works
out to be 277 tons, which gets us close to the machine’s capacity. For most jobs, however, this machine will be processing material that’s only 1.5 mm
(0.059 in.) thick.
Will the machine’s 300-ton capacity suffice for our
application, and how will such a machine perform
over the long term? If we go for a 400-ton machine,
will it affect the bending accuracy of thinner components, such as the 1.5-mm-thick material? Also, do
you recommend hydraulic dynamic crowning of the
lower beam, and is it really effective compared to
Answer: First, I recommend you consider a 400-ton
machine. The 277 tons of required force would seriously approach the limits of the 300-ton machine.
Plus, having the extra tonnage gives you the ability
to increase the tonnage load if needed. You never
want to run any machine near, at, or over the rated
I would also seriously consider the dynamic
crowning feature if the parts you are producing are
greater than 60 percent of the distance between the
Make sure whichever brand of machine you purchase that the tonnage requirements do not exceed
the rated centerline load limit of the ram. If you exceed that limit, you will permanently bend the ram
and bed; this ram upset will ruin your new press
In summary, I would recommend the 400-ton machine with dynamic crowning.
Forming Offsets in Mexico
Question: I’m from Torreon City in Mexico, and I just
finished reading the article “Strategies for forming
offsets.” We manufacture a part with a forming offset, but we don’t know how to calculate the force. We
have a 350-ton press brake. Are the force calculations
the same as they are for air bending, or are there different calculations to consider?
Answer: Required tonnages for offsets vary greatly
when using offset tools, mainly because they can
involve either air forming or bottoming. Bottoming
produces the best results but requires a lot of tonnage. Air forming requires much less tonnage, but
the final bend will take on more of a Z shape rather
than a true 90-degree offset. Especially when bottoming with offset tools, your tonnage calculations
need to take extra force requirements into account.
Start by calculating for a single bend in mild cold-rolled steel like A36, with a 60-KSI tensile strength.
Factor that value by material type—for example, 1.4
for 304 stainless. Determine the material factor by
comparing your material’s tensile strength with the
60,000-PSI tensile strength of the baseline material.
If the material has a 120,000-PSI tensile strength,
your material factor would be 2.
Next comes the method factor. If you’re air bending, you don’t need to incorporate a method factor.
If you are bottoming with your offset tool, you need
to multiply by a factor of 5.
Next comes the multiple-bend-tooling factor. If
your material is less than 0.250 in., multiply the tonnage calculations by 5; if thicker, multiply by a factor
You need the die opening value to calculate the
tonnage; this is not the size of the offset—that is, its
depth. For the tonnage calculations, it’s the width
of the opening that matters, and you measure it the
same as you would any other die opening, from one
top corner to the other.
All this gives us the variables we need to plug into
the following formula (all dimension measurements
are in inches):