By Steve Benson
So you’ve added a state-of-the-art press brake, and you’ve spent a small fortune on precision-ground tooling. The press
brake manufacturer promised that this new
machine would repeat in microns, but for all
your troubles, research, and due diligence,
you’re still having the same issues that you’ve
always had: inconsistent bend angles and part
dimensions. Your operators struggle as they
experience part-to-part variations, and your
expected production goals are still just a pipe
dream. You’re now second-guessing your decision to purchase all that new equipment.
These modern machines will do what the brochures say they do, and the precision-ground
tooling is so precise that you’re not experiencing a single height or tool-center issue. They
easily hold the TX and TY axes. So why are you
still having problems? We could point to various
reasons, but the biggest reason comes down to
a single word: tolerance.
Materials Have Tolerances Too
Every day your operators know to keep the bend
angles and dimensions within the tolerances
found in the title block on the print. But have
you ever considered that the material you’re
working with also has tolerances? And they involve more than just thickness variation.
Sheet metal behavior is at best unpredictable.
Quite often many material variables are not
even considered when designing, programming,
or even building the part. But knowing these
variables, their tolerances, and compensating
for them is necessary to increase production
and quality. And it requires much more than just
relying on the manufacturer’s certification or
ASTM International, formerly known as the
American Society for Testing and Materials,
came into existence in 1898 as an organization
that develops and publishes technical standards
for many materials, including all types of steel,
aluminum, and stainless.
Just like your products, sheet metal from the
mill has allowable variations, these being set by
ASTM International rather than the tolerance
block on the print. Nonetheless, not all sheet
metal and plate are created equal. Variabilities
include thickness, yield strength, and chemical
Even within a single material group, sheet metal makeup and gauges are not fabricated equally
at the mill or between mills. You need to consider imperfections induced by the manufacturing process and impurities and contamination
within the material itself.
Consider one of the most common materials
used in the modern shop, A36 steel. ASTM standards require the steel manufacturer to certify
the minimum yield strength of its product. This
means that any steel that can meet the requirement for a minimum yield strength of 36,000
PSI can be sold as A36 steel. Sheet metal with an
average yield strength of 41,000 PSI can be sold
as A36 steel, even though it is 13 percent harder
than 36,000-PSI steel.
This small variation might not seem like much,
but the increased resistance to force requires
more tonnage from the press brake in order to
break the yield in the material and make it bend.
When your material changes from a 36,000-PSI
to a 41,000-PSI yield strength, you’ll find that
your press brake’s depth of penetration changes
approximately 0.002 inch. Depending on the
width of the die opening, this can create several
degrees of bend-angle variation.
For example, braking a material over a 0.315-in.
die opening will result in a 1-degree difference
just from changing the material yield strength
from 36,000 to 41,000 PSI. The smaller the die
opening is relative to material thickness, the
greater the effect.
Now consider 16-gauge mild steel. The sheet
has a nominal thickness of 0.059 in., an upper
limit of 0.0648 in., and a lower limit of 0.0548
in., giving us a range of 0.010 in. Add this to the
0.002-in. depth-of-penetration variation noted
earlier, and you can see how the variabilities really add up.
Read more from Steve Benson at
The challenge of tolerance
It’s more than just the tolerance block on the print
EXPERTISE » BENDING BASICS
» Figure 3
This antideflection device has adjustable wedges.
Depending on the device, wedges can be moved with a
mechanical or hydraulic system or a combination of both.
» Figure 1
A material’s grain direction adds yet another variable to
a bending process.
» Figure 2
Ram deflection causes a canoe effect, when the part’s
internal bend angle is greater in the center than at the