The red numbers in the figure correspond to the
bold numbers in the text that follows.
Find where the two bend centerlines intersect (1).
From that vertical outside mold line you subtract
the outside flange dimension. In our example, the
outside flange dimension is 0.750 in. So in the X direction, we measure 0.750 in. from the vertical outside mold line ( 2). That value is the coordinate of the
notch corner closest to the zero-zero point ( 3).
Return to the perpendicular inside mold line and
add the 0.750-in. flange dimension to that value ( 4).
Now you have the coordinates for the notch corner
farthest from zero-zero ( 5). With this, you can program or lay out the part and cut the notch, taking
elongation during forming into account.
If you’re notching by hand, you might find it difficult to get the notch perfect, so you may need to file
the edge to get the notch to close up correctly. Don’t
overdo your filing, though, or you’ll end up with significant gaps where the edges meet.
Also, notching material with large amounts of
springback may require you to move the outside-edge notch coordinates apart just a little bit—a
degree or two on each side for the notch angles.
Opening each of those angles to 46 degrees each (1
more degree each side, 2 degrees total) would accommodate for 2 degrees of springback, giving us
the needed extra clearance for the bend and the
Another Level: More Than 90 Degrees
Notches cut at 45 degrees work for 90-degree
bending. But ho w exactly do you calculate the notch
dimensions for bends that are not 90 degrees? Here
is where some right-angle trigonometry comes
Consider Figure 4, which shows a notch that allows us to bend past 90 degrees. The 0.500-in. side
flanges are bent at the horizontal mold lines to 90
degrees; the bend deduction (and distance between
the mold lines) is 0.100 in. Meanwhile, the perpendicular bend is 120 degrees complementary ( 60
degrees included), with a bend deduction of 0.250
in. That 120-degree-complementary bend changes
your notch dimensions.
How do we find these dimensions? First, we need
to define a right triangle at the intersection of the
notch, based on what we know. As shown by the
side view in Figure 4, the notch is being bent to a
angle. We draw a triangle where the notch dimension will be. The triangle splits that 60-degree-in-
cluded bend angle in half, so we know that angle C
has to be 30 degrees. We also know that side c is the
same dimension as the side flange: 0.500 in.
So now we have enough information to solve for
the missing side using right-angle trigonometry.
Specifically, we need to find side b, which will give
us the dimension “L” shown in Figure 4:
b = c/tan(C)
b = 0.500/tan( 30)
b = 0.866 in.
This 0.866-in. dimension is the adjacent side of
the triangle and the required dimension needed
to lay out the notch. As before, you start by finding
where the two centerlines intersect. From the vertical mold lines you measure 0.866 in. to the right
and left, as shown in Figure 4. All this corresponds
to Steps 1 through 5 shown on Figure 2.
Another Level: Less Than 90 Degrees
Now let’s look at a notch bent to less than 90 degrees
complementary, to 60 degrees. This time we need to
define the right triangle shown in red in Figure 5.
Our bend deduction for the 90-degree bend remains
This shows a notch bent to 60 degrees complementary
with equal side flanges. The bend deduction for the
90-degree bend is 0.100 in., while the 60-degree bend
has a bend deduction of 0.050 in. The side b dimension
in the right triangle is the same as the dimension between the vertical mold line and notch corner in the
flat pattern, as shown.
0.100 Bend Deduction
This simple part has two 90-degree flanges that are
both 0.750 in. The red numbers on the flat pattern correspond to the description in this article. (IML = inside
mold line; OML = outside mold line; C/L = centerline.)
L = 0.866
This shows a side view (top) and flat pattern for a notch
bent to 120 degrees complementary ( 60 degrees included). Side b of the right triangle gives us the 0.866-
in. distance between the vertical mold lines and the
bottom of the notch, shown on the bottom.
0.039 Inside Radius
0.100 90 Degrees
0.050 60 Degrees 0.750
0.100 in., but our bend deduction changes to 0.050
in. for the 60-degree-complementary bend.
Again, the right triangle splits the 120-degree-in-
cluded angle in two, so the angle at C is 60 degrees.
And we know side c is the 0.750-in. flange dimension. From here we solve for our missing value: b.
b = c/tan(C)
b = 0.750 in./tan( 60)
b = 0.433 in.
We then directly apply the 0.433-in. dimension to
the appropriate mold line and in the proper direction to find the notch location points on the edge
of the workpiece, just as we did earlier (again, as
outlined in the steps described in Figure 3, but with
dimensions shown in Figure 5).
No Longer a Manual Process
We’ve looked at only a few of the notching possibilities your CAD system is capable of making. For
even more, check out “Press brake bending and the
notch: a deeper dive” from March 2016, archived at
True, with the exception of some prototype work,
you probably won’t be laying out your notches by
hand. It’s just too time-consuming to manufacture
products this way. Regardless, by taking a little time
to learn how notching works and then applying that
knowledge to your corner selection on your CAD
system, you can’t help but build better parts.
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.
The author’s latest book, Bending Basics, is now available at the FMA bookstore, www.fmanet.org/store.