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10 tips for bottom
Bottom bending isn’t as common as it once was,
but a knowledgeable operator should still be able
to perform it efficiently and safely
By Steve Benson, Contributing Writer
Question: Our team still has some problems with
overbending, at least in some materials and parts.
We recently started bottom bending certain parts to
make sure the bend angle is exactly 90 degrees. As
a result, we’re thinking about bottoming all aluminum. There is good discussion going back and forth
on both sides. Is this a good idea or bad idea? What
are the advantages and disadvantages in regards to
tooling, machinery, safety, and productivity?
Answer: Bottom bending can be done with softer
grades of aluminum, such as the H series. Assuming
the press brake operators have a full grasp of bottoming and are capable of performing the process
safely, here is a list of best practices that should help.
1. Pay attention to the grade. For aluminum, the
harder the grade, the greater the amount of springback that you will need to deal with; very soft aluminum may exhibit no springback at all.
2. Watch out for creasing along the bend line.
Aluminum generally loses its integrity if the material
is creased. If you make aircraft parts with that crease
along the bend line, that alone will make the parts
Make sure the piercing tonnage—that is, the force
it takes for the punch to pierce the material surface—
exceeds the forming tonnage. It sounds as if your
operators work with high tonnages, which increases
the likelihood of damage from bottom bending.
For more on this, I’d recommend three articles
available on thefabricator.com archive, which you
can find by typing the title into the search bar:
“Forming aluminum on the press brake: Bending
sharp, not soft,” “How an air bend turns sharp,” and
“The 4 pillars of press brake tonnage limits.”
I highly recommend that you review the mate-
rial’s specifications for the minimum acceptable
inside bend radius. For the most part, a 1-to-1 ma-
is recommended, and in some cases a sharp radius
may invalidate the material’s warranty.
3. Avoid a punch tip that’s too narrow. Use a
punch tip radius that’s as close as possible to the
material thickness. Be careful to avoid coining,
which occurs when the punch nose penetrates the
neutral axis of the material thickness—a depth of
penetration at less than material thickness.
4. Watch out for spreading or cracking. Bottoming, especially if the bend is sharp, makes the
grain in aluminum more susceptible to spreading or
cracking. The closer you are to that 1-to-1 relationship—with the inside bend radius, material thickness, and punch tip radius as close as possible to
equal—the less likely this cracking will occur on the
outside of the bend. Regardless, you still may need
to pay attention to the grain direction. Also, if the
part has a “grained finish,” be sure to grain it opposite of the material grain direction.
5. You can’t use the crease to compensate for
springback. Because you should not crease the
bend at its center, as you would with a coined or
sharp-bend radius, you will not be able to use that
crease as a means of compensating for springback
as you could if you were working with mild steel.
The only way to bottom-bend aluminum is to use
the 88-degree punch. An 85-degree punch will
take the bend too far. This means that you cannot
achieve enough negative springback (
springfor-ward) to force the bend back to the 90-degree angle.
This means two things: First, unless the operator
is very skilled and knowledgeable, he won’t be able
to bottom-bend a piece of material in which the
springback exceeds 2 degrees. There are ways to exceed that 2-degree limit with an 88-degree punch,
but not by much. Second, bottoming is best used for
light-gauge materials and bends of 90 degrees.
6. Take angular clearance into account.
Assuming you use a 90-degree die and an 88-degree
punch, you will have 2 degrees of angular clearance.
In bottoming, we try to match the punch angle to
the material’s springback.
To deal with bends having different amounts
of springback than the angular clearance allows
for—say, only 1 degree of springback instead of 2
degrees— you will need to cheat a little. Purchase a
roll of 1⁄8-in.-wide vinyl pinstriping tape, the cheaper
the better. Place a piece high on the punch face. This
has the effect of changing the punch angle.
Most will use masking tape, which works for a
while but needs to be replaced regularly. Pinstriping tape, which is made from vinyl, won’t need to be
replaced often, if at all.
Note that rarely will you be able to use tape on
both sides of the punch. This usually does not work.
Also, never cover the entire face of the punch with
tape; that changes the center of the tool, not the
7. Select your die width as a “perfect” bend.
This refers to a bend in which the die width is determined from a 1-to-1 relationship between the inside
bend radius and the material thickness. The formula for a perfect die width is: ( 2 × Material thickness) ×
3.429435. Even if the punch radius is less than 1-to-
1, use this formula.
Inside radii greater than 120 percent of the material thickness generally are not suited for bottoming. The exception would be when forming larger-radius bends into urethane, which can have similar
effects to those of true bottom bending.
8. Take your time setting up the press brake.
Make sure everything is clean before installing the
tooling. Most important, be sure to center the punch
into the die, even if you need to walk back and forth
In bottoming, there is angular clearance between the punch and die. The punch descends and the material wraps around the punch nose. As the punch continues to apply
pressure, the material is forced down to conform with the die angle.