# Bending Sheet Metal Part 1

This is the Rainbow Aviation Video
Channel, and I’m your host Brian Carpenter. In today’s episode we’re going
to be looking at bending aluminum sheet metal for aircraft, in particular we will
take an in-depth look at the theory for making a flat lay out, calculating Bend
allowance, and determining setback. This is a companion video for the “Technically
Speaking” article published in the May 2017 Sport Aiation Magazine. In this
video we’re going to back up a bit and provide some of the theory necessary to
understanding how we go about the process of converting a flat piece of
sheet metal into a complex sheet metal component. Learning to accurately layout
and Bend sheet metal is a very useful exercise. Once you’ve mastered the
process, you will find that it not only saves you a great deal, of time but can
also save you a great deal of wasted material. To start with, let’s examine
some of the properties of aluminum sheet metal used in aircraft. The two most
common alloys used are 6061-t6 and 20 20 43. 6061-t6 is one of the least expensive
and most versatile of the heat treatable aluminum alloys. 6061-t6 has a tensile
strength of approximately 40,000 psi it has good corrosion resistance in
comparison to 2024 T3. And the current cost per square foot for a piece of
.040″ thick material is about \$2.53 on the other hand
2024 is one of the best known of the high-strength aluminum alloys with its
high strength about 50,000 psi tensile strength in the t3 condition. It is used
on structures and parts where a good strength to weight ratio is desired.
Since corrosion resistance is relatively low 2024 is commonly used in the clad
or alclad form. This provides a thin surface layer of high purity aluminum on
the surface of the alloy aluminum. The cost of 2020 T3 is about 50% more
than that of 6061-T6 aluminum, costing about \$3.94 cents per square foot
for .040″ thick aluminum sheet. Understanding the properties of
each of the aluminum alloys becomes very important during the sheet metal layout
and bending process. In particular the malleability and ductility. By definition,
ductility is the solid materials ability to deform under tensile stress. And
malleability is the ability of the material to deform under compressive
stress. The ductility of 2020 T3 is about 18%. When bending the
aluminum around a radius, we can see that we are both stretching one side of the
aluminum, this is ductility. And compressing the other side of the
aluminum, this is called malleability. Extensive testing has shown that the
neutral axis during the bending process is about .445 times the
thickness of the material. Now the neutral axis is the section of
aluminum that is neither under compression,
nor is it under tension. During the bending process the smaller the radius
that the metal is bent around the greater the differential between the
neutral axis and the outside arc of the skin.
Additionally, the greater the thickness of material, the greater the differential
between the neutral axis and the outside arc of the skin. Stretching the outer
skin beyond its limits will normally result in cracking of the aluminum. Of
course there isn’t a necessity for calculating minimum Bend radius because
most sheet metal manuals, including FAA advisory circular AC 43.13 – 1b have a
minimum bend radius chart available for quick reference. The tool which we use
for bending sheet metal is called a brake. A sheet metal brake use for
aircraft aluminum has either a fixed or interchangeable jaws with a very
specific radius built into the jaws. In our shop we use a 1/8 inch radius.
This allows us to bend up to 0.063″ thick 6061 aluminum in the T6 condition. Tt
also allows us the ability to bend the majority of sheet metal sizes used in
small experimental aircraft. Understanding the necessity for
utilizing a radius during the bending process, will now help us to understand
how to calculate Bend allowance. Bend allowance is nothing more than the
amount of material that is used for the bent portion of sheet metal. The radius
of the bend at the neutral axis is the tooling radius + 0.445) x the
thickness of the sheet metal. Multiplying the radius X 2 will give us the
diameter, and multiplying that times pi 3.1415 Taking a circumference and dividing by
360 degrees will leave us with a dimension per one degree of bend.
Multiplying that times 90 will give us the Bend allowance for a 90 degree bend.
Although the process of calculating bent allowance is relatively simple, it’s made
even easier by the use of a bend allowance table. A bend allowance table
has a matrix of the most common sheet metal sizes and the standard bending
radius already used and calculated for both a 1 degree Bend, as well as the most
common, 90 degree bend. When we prepare a piece of sheet metal
for bending we are doing what we call a flat layout. All of the sheet metal
components are simply a series of flat sections, and bends. Prior to bending up a
sheet metal part, we will simply get out a piece of scratch paper and formulate
the layout similar to the part that we’re going to manufacture. We will lay
out each flat section with the bend allowance required for each of the bends.
And in this case, because the bends are 90 degrees, the material thickness is the
same and the radius for each of the bends is also the same. We will only need
to calculate or look up the bend allowance one time. The amount of
material or Bend allowance used for each of the bends is identical. Next we simply
need to calculate the length of each one of the flat section. The normal formula
for calculating the flat section is the given dimension – setback. Setback by
definition, is the radius + the thickness used during the bend. If all of
the dimensions were given from the outside of the material to the end of
the flat section, this formula would work great. However, there are many cases where
you’re going to have to extrapolate on this formula in order to calculate the
flat section. For example, in order to calculate the length of flat “A”, the given
dimension is from the inside of the bend. In this case flat “A” =the given
dimension of 0.375 – Bend radius of 0.125 and that=0.25. In our
classes, in order to keep comprehension to a higher level, we normally start by
teaching Bend allowance as we’ve shown here with all of the bends conducted at
90 degrees. Once we’ve mastered the process of calculating for 90 degrees, we
can now venture into the calculations necessary for bends that are more acute
or obtuse. We still use setback which is radius + thickness, however, this time
we multiply X a “K” factor. A “K” factor chart is available in FAA advisory
circular AC 43:13 – 1b. This is simply another complex mathematical calculation
distilled into a matrix which correlates the correction factor to the angle of
the bend to calculate the length of each flat. Using the same procedures we used in
calculating for a 90 degree bend, simply take the given dimension and subtract
the setback. When calculating the bend allowance for bends that are other than
90 degrees simply multiply the bend allowance for one degree times the
number of degrees that the metal is bent. This is the same number of degrees used
when calculating setback utilizing K factor. You may have become very
proficient at bending aluminum using the old standby method where you start with
an extra large sheet bend it to the appropriate angle then cut off excess
material to come up with your final dimension. Well, there’s nothing
particularly wrong with utilizing this method. However, if you have more than one
bend, you’re gonna be in big trouble. This is where I see individuals getting
fairly creative by guessing at the dimension, bending the metal, and re
measuring to see how far they are off and then changing their original
dimension by the amount of error in the original part and rebending a new piece.
After about three or four tries, they can get typically pretty close to what you
might want, but as you can imagine this can be quite time-consuming expensive
and frustrating. If you find yourself working on aluminum aircraft on a
regular basis the amount of effort required to learn to do the sheet metal
layout is really quite minimal. Once you practice a bit, you can develop
confidence and accuracy worthy of a professional. It’s very rewarding to go
through the process of laying out a fairly complex part with multiple bends
and have it fit into the aircraft on the first shot. In part 2 of this video we
will address some of the more practical aspects of bending aluminum such as how
to place the metal into and setup the sheetmetal brake.
Establishing a sight line, and some other tricks and tips that will help you get
well on your way to becoming a sheetmetal Wiz. Well we’ve come to the
end of this episode on bending sheetmetal part 1. We put a lot of effort
into bringing you only high quality useful videos for the sport aviation
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