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Time for new tooling
How dies and punches affect
the inside bend radius—Part I
By Steve Benson
Question: I am an intern at a fabricator, and I’m
working with our new press brake operator and optimizing our bending processes. When I started a few
months ago, neither the operator nor I had much (or
any, in my case) experience with press brakes.
I absorbed as much information as I could from
thefabricator.com. Your articles have helped me
understand how bending sheet metal works. Using
equations from your articles, I was even able to come
up with a way to determine the bend line on some of
the round stock we bend. But I can tell I have a long
way to go yet, and I’m still confused about a couple
I was told we have 0.125-in. punches and dies
along with 0.25-in. punches and dies, and this means
we can do either a 0.125- or 0.25-in. inside-radius
bend. We achieve the dimensions we are seeking, for
the most part, but our tooling is very old, and we had
two of our 0.125-in. dies split open (see Figure 1).
We are looking for new tooling, but I want to be sure
of what we need.
We mostly bend to 90 degrees, and to achieve this
we bottom-bend the part. I understand this takes
about five times more tonnage than air bending and
is much harder on the punch and die, so I would like
to avoid this if possible.
If we move to air bending, should we acquire dies
with an angle of 85 degrees? And is the bend radius
of the die unimportant when air bending, and all that
matters is the punch and the size of the V opening?
Also, what is the best way for us to get our 90-de-
gree bends while maintaining the inside radius dimension so that our postbending dimensions come
out the way we want?
Answer: To start, let me comment on the die photograph you sent. I can’t begin to tell you the number
of shops I have visited over the years where the tools
they were using looked like these. Even without the
splitting damage, it is obvious these dies have seen
better days; by anyone’s standard, they’re completely worn out.
Just from the picture I can see seriously worn
spots that are inches long, and areas where the die
opening is wider than it should be when compared
to the rest of the die. This damage occurs when
tools are overloaded. If you don’t bottom-bend or
coin, your chances of getting a consistent bend are
small, if not outright impossible.
From the looks of your two dies, there’s a good
chance the ram of the press brake su;ers from ram
upset, a condition where the ram and/or the bed
of the press brake are permanently deflected (see
Because you have been bottoming for some
time, it may be in your best interest to run a magnetic base with a dial indicator down the length of
the ram and bed. See if the indicator climbs as you
reach the center, and check for ram upset. If you find
it in either the ram or bed, seek medical treatment
for your press brake, or have the machine put down.
It’s not worth the fight. So yes, you certainly need
new tooling, and, depending on the damage done,
you may also need new press brakes.
Radius Does Matter
Regardless of method—air bending, bottoming, or
coining—the inside bend radius is at the heart of precision sheet metal bending. Without it we will not be
able to calculate the values for K factors, bend allowance, outside setbacks, and bend deductions. The
inside radius of a workpiece matters—big time.
First, let’s clarify some terminology. The punch radius is obvious: It’s the radius of the punch tip. Die
radius usually refers to the radius of the die shoulder on either side of the die opening. Based on your
question, I’m going to assume that you’re referring
to the radius at the bottom of the V die.
When air bending or bottoming, a sharp corner
at the bottom of the V is still technically a “radius.”
However, you may be coining, in which case you
would be using a die with a noticeable radius at
the bottom of the V. That radius should match the
outside radius of the bend, like how a stamping die
You stated that you have a 0.125-in.-radius punch
and a 0.125-in. radius at the bottom of the die. With
this, you are trying to produce a 0.125-in. inside radius in materials from 0.108 to 0.375 in. thick. You’re
also trying to produce a 0.25-in. radius with a 0.25-
in. punch nose radius and 0.25-in. radius at the bottom of the die.
We need to consider one more factor here: your
outside bend radius. You calculate the outside bend
radius by adding the inside bend radius to the material thickness. A 0.125-in. inside bend radius in
0.125-in.-thick material will have an outside radius
of 0.25 in. (0.125 + 0.125 = 0.25).
Let’s consider your application again (see Figure
3). If you bend 0.125-in.-thick material with a 0.125-
in. punch nose, you’ll achieve a 0.125-in. inside bend
radius and a 0.25-in. outside bend radius—which
you’ll be pushing into a die with only a 0.125-in. radius at the bottom. How’s that working out?
This is an extreme example of what happens when
the die width and radius at the bottom of the V are
too small for the job. I’d be willing to bet that the
look on the outside of your bends gets a little ugly.
How a Radius Forms
Just how the radius is created varies greatly with the
bending method. If I use a punch with a nose radius
equal to the inside radius I want to achieve, then I
can bottom-bend or air-form the part. This is a “
In an air bend, what matters most is the width of
the die opening, as the radius will be floated as a
percentage of that width. The percentages vary by
material type. The correct die width will give you the
bend radius you need. (For more on this topic, you
can review “How the inside bend radius forms,” archived on thefabricator.com.)
These split dies have seen better days.
Ram upset can permanently deflect the bed and ram of
the press brake.