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with notch layout
Such knowledge shouldn’t be lost
By Steve Benson
Question: I have an odd question. It is difficult to
explain without a picture, so I drew a crude one (see
I have a piece of sheet metal with a 150-degree
bend in it. I want to keep the top part of the bend
continuous and keep the bottom part flat. I need to
make a few slight bends so that the bottom part is no
longer straight. If I cut out a wedge (notch) from the
bottom part and bring the two edges together, what
should the angle of the wedge be to maintain the 150
Answer: The answer to your question is not quite
as straightforward as it may first seem to be. First,
there is no simple formula for solving this notch. By
the same token, it is not that difficult to calculate
either, but it does take a little bit of that esoteric
knowledge I spoke about in my Bending Basics column back in September.
The information you provided is a little slim on details, so I cannot give an exact answer. I can, however, take you through the process so that you can
apply your numbers to your products. That is probably all for the best as you will then learn to solve
this and future notching problems.
The information presented here will only touch
on the basics of notching. Beyond the basics it can
get pretty deep in the weeds, with odd angles, uneven flange lengths, center shifting, and multiple
We are in some ways fortunate that CAD systems
now make these calculations for us. At the same
time, few tradespeople today can do these calculations manually. Software is efficient, but it can come
at the cost of knowledge lost.
If you are interested in this topic, you may want
to check out Bending Basics, my new textbook pub-
lished by the Fabricators & Manufacturers Associa-
tion Intl. I dive quite deeply into the subject. Also,
keep an eye out for old pattern drafting books from
the ’40s, ’50s, and ’60s. You can sometimes pick
these up at used bookstores for free, or next to free.
Few people realize their value.
Nonetheless, if you spend a little time studying
notching and learning how it works, you will be able
to make more informed decisions as to which notch
is the right one to select from the many available in
your CAD menu. It’s worth the effort.
First, you need to know about mold lines. On the flat
pattern or drawing, the area between two mold lines
represents the area the radius will be after forming.
There are two of them, an inside mold line and an
outside mold line. Which line is outside and which is
inside? That depends on which end of the part you
are working from. Generally, the outside mold line
location determines the flange’s outside dimension.
The inside mold line is one bend deduction less.
That is, subtract the value of one bend deduction
from the outside mold line location and you find the
location of the inside mold line.
Figure 2 shows a simple part with two 0.750-
inch outside flange dimensions at 90 degrees, and
an overall outside dimension of 2.000 in. To make
things simple, we’ll assume the bend deduction is
0.100 in. and the material is 0.060-in.-thick A36 mild
Again, the area between the inside and outside
mold lines will be the radius after forming. Knowing this, we can design a notch that allows for the
elongation that occurs at each bend. Working from
the zero-zero point (the bottom right in Figure 2), we
find our first outside mold line dimension is 0.750
in., the same as our outside flange dimension. We
then subtract one bend deduction, 0.100 in., to determine the location of the inside mold line at 0.650
in. (0.750 in. – 0.100 in. = 0.650 in.).
To find the second set of mold lines for our second
flange (the one on the left in Figure 2), we start at the
inside mold line at 0.650 in. and then add the overall outside dimension of 2.000 in. This gives us the
location of the second outside mold line at 2.650 in.
(0.650 + 2.000 = 2.650). From the outside mold line
at 2.650 in., we subtract one bend deduction (0.100
in.) to find the location of the second inside mold
line, at 2.550 in. ( 2.650 – 0.100 = 2.550 in.). Finally,
from the second inside mold line (at 2.550 in.) we
add another 0.750 in. for the outside flange dimension, giving us our complete flat-blank dimension of
3.300 in. ( 2.550 + 0.750 = 3.300).
Cut a wedge out
Pull the “legs” together so
they touch. Maintain same
150° angle on the top half.
How much wedge do I cut out?
What angle should the notch be? It’s not as straightforward as you might think.
To double-check your numbers, add the two outside flange dimensions (0.750 in.) to the overall outside dimension ( 2.000 in.) and subtract two bend
deductions from the total: (0.750 + 0.750 + 2.000) –
0.100 – 0.100 = 3.300 in.
Once placed on the flat pattern, mold lines help
reveal any features that lie on the radius and, therefore, distort during forming, assuming you achieved
the predicted radius in the workpiece. For more information on predicting the inside radius and the
corresponding bend deduction calculations, check
out the four-part “Grand unifying theory of bending”
series from 2015, archived at thefabricator.com.
The Next Level:
Two 90-degree Bends, Two Axes
The previous example was simple, with two bends
on the same axis, parallel to one another. The next
stop on this journey starts with the workpiece in
Figure 3, which has bends on two axes; one bend
is perpendicular to the other. The part has two side
flanges of equal length bent to 90 degrees, and a
single perpendicular flange also bent to 90 degrees.
To lay out this notch requires us to use those
mold lines again. After we find the outside and inside mold lines for both bends, we define the
centerline for both by subtracting half a bend deduction.
Again, the distance between the inside and outside
mold lines is one bend deduction, and that centerline splits the distance—half a bend deduction
to one mold line and half a bend deduction to the
other mold line.
With the centerlines defined, we then locate the
X-Y coordinates for each of the outside notch vertices. The point at which those centerlines intersect
becomes the innermost location, or top-center, of
In this example, we’ll use an H-series aluminum,
5052 H32, with an inside radius and thickness of
0.063 in. and a bend deduction of 0.100 in. To see
how it’s done, refer to the flat pattern in Figure 3.
2.000 0.100 Bend Deduction
Outside Mold Lines (OML)
Inside Mold Lines (IML)
This simple part has two 0.750-in. flanges and an overall outside dimension of 2.000 in.